Hoa_sinh_II

5/28/2018 Hoa_sinh_II

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I HC NNG

TRNG I HC BCH KHOA

KHOA HA

HA SINH IIBi ging

Bin son: TS. ng Minh Nht

Nng, 2007


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CHNG 1 - KH NNG CHUYN HO CA PROTEIN, GLUXIT, LIPIT

1.1 CC BIN I CA PROTEIN TRONG QU TRNH SN XUT V

BO QUN

1.1.1 BIN I DO NHIT

Cc qu trnh ch bin thc phm bao gm un nng, lm lnh, sy, s lho cht, ln men, chiu bc x v cc qu trnh x l khc. Trong , un nng l

qu trnh ch bin c s dng ph bin nht.

Thng thng, un nng c thc hin nhm mc ch v hot vi sinh vt

v cc enzym ni sinh (endogenous enzyme) gy cc phn ng oxy ho hoc thuphn khng mong mun khi bo qun hoc bin i hn hp cc thnh phn

nguyn liu thc phm khng hp dn thnh sn phm thc phm c kt cu ng

nht v c sc li cun v mt cm quan. Hn na, mt s protein nh -

lactoglobulin v -lactalbumin t sa b v protein t u nnh c th gy d ng,

tr thnh v hi nh qu trnh ny.

Tuy nhin, cc nh hng c li nh qu trnh un nng thc phm giu

protein ni chung thng i km vi mt s nh hng xu, lm gim gi tr dinhdng v cc tnh cht chc nng ca protein.

Gia nhit va phi

Gia nhit va phi ch lm protein bin tnh v gip ci thin gi tr dinhdng ca sn phm, do:

- lm mt c tnh ca cc c t c bn cht protein (nh enterotoxin cu

Staphyloccocus aureus) hoc ca cc cht km hm cc enzym ng tiu ho (nhantitripxin Kunitz v Bowman c trong ht u tng).

- lm v hot cc enzym (protease, polyphenoloxidase, lipoxydase) vnxc tc phn ng ph hu cc vitamin.

- lm tng kh nng tiu ho ca mt s protein do lm dui mch peptit cachng, gii to cc gc axit amin trc y b vi trong phn t, do to iu kin

cho protease tc dng c thun li hn.Vd.: colagen, glixinin u tng,ovalbumin

Thanh trngGia nhit kiu thanh trng nhit ln hn 110-115 oC cc sn phm giu

protein nh tht, c, sa s gy ph hu mt phn cc gc xistin, xistein hnh thnhnn H2S, dimetylsulfua, axit xisteic v hp cht bay hi khc khin cho cc snphm ny c mi c trng.


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Gia nhit khan

Gia nhit khan protein nhit trn 200 oC (nhit rn tht v c) to ra

, hoc cacbolin do phn ng vng ho ca tryptophan:

NH

R

NH2

-cacbolin

(R=H hay CH3)

NH

N

R

R NH

NH2

CH3

R

-cacbolin

(R=H hay CH3)

-cacbolin

(R=H hay CH3)

Gia nhit nhit cao

Gia nhit nhit cao (hn 200 oC) thc phm giu protein c pH trungtnh hoc kim tnh s xy ra:

Thu phn cc lin kt peptit v ng phn ho cc gc axit amin, to ra hnhp raxemic do lm gim gi tr dinh dng i 50%, do cc ng phn D cto thnh kh b tiu ho hn;

Ph hu mt s axit amin: nh arginin s b chuyn thnh ornitin, ure,xitrulin v amoniac. Xistein b chuyn thnh dehydroalanin. Gia nhit mi trng

kim, serin, treonin v lizin cng b ph hu;

To cu ni ng ho tr kiu lyzinoalanin, ornitinoalanin hoc lantionin linkt gc dehydroalanin (DHA) chui polypeptit ny vi cc gc lyzin, ornitin hoc

gc xistin chui polypeptit khc.

Gc DHA c to ra do phn ng loi gc xistein hoc phosphoserin:

NH HC

CH2

X

C

OOH-

NH C-

CH2

X

C

O

NH C

CH2

C

O

+ X-

(X l SH hoc PO3H2) gc DHA

Mch polypeptit c ch gc lizin, gc ornitin hoc gc xistein s kt hpvi mch polypeptit c ch gc DHA theo s sau:


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NH CH C

O

(CH2)4

NH2

NH CH C

O

(CH2)3

NH2

NH CH C

O

CH2

SH

NH CH C

O

CH2

NH CH C

O

CH2

NH CH C

O

CH2

NH CHC

O

(CH2)4

NH

NH

CH C

O

(CH2)3

NH

NH

CH C

O

CH2

S

NH CH C

O

CH2

NH CH C

O

H2C

NH CH C

O

H2C

Khi x l nhit tht hoc c nhit cao hn nhit thanh trng th cn

to ra cu ng ho tr kiu izopeptit gi gc lizin cu chui peptit ny vi gc

glutamic hoc gc aspartic cu chui peptit khc:

CH2

NH

C

(CH2)4 NH C

O

(CH2)2 HC

NH

C OO

Cu izopeptit

V phng din dinh dng, vic to ra cu ng ho tr izopeptit trong cngmt phn t hoc gi cc phn t s lm gim tiu ho cu nit, gim h s sdng protein v gi tr sinh hc cu protein. Hn n do hiu ng khng gian cccu ng ho tr (glutaminlizyl hoc aspartyl-lizyl) s ngn cn khng cho cc

protease n c vng c hiu thu phn do lm cho qu trnh tiu hoprotein b chm li.

1.1.2 BIN I DO ENZYMTrong qu trnh bo qun cc thc phm giu protein thng xy ra hin

tng i thi lm mt gi tr dinh dng cu thc phm. Nguyn nhn l do tcdng cu enzym c sn trong thc phm, cng nh cu vi sinh vt xm nhp t mi

trng ngoi vo. Khi protein b chuyn ho theo nhng phn ng c bn sau:


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a) Phn ng kh amin:

R CH COOH

NH2

+ OH2 R CH COOH

OH

+ NH3

R CH COOH

NH2

+ H2 R CH2 COOH + NH3enzym cuavi sinh vat hieu khi

b) Phn ng kh nhm carboxyl:

R CH COOH

NH2

R CH2 NH2 + CO2

Phn ng ny to thnh cc amin khc nhau. T lizin to thnh cardaverin, t

histidin to thnh histamin, l nhng cht c.c) Phn ng kh amin kh carboxyl:

R CH COOH

NH2

+ 1/2 O2 R CH COOH

O

+ NH3

R CH COOH

O

R CH O + CO2

R CH COOH

NH2+ OH2 R CH COOH

OH+ NH3

R CH COOH

OH

R CH OH + CO2

decarboxylase

d) Phn ng to thnh mercaptan. Thng xy ra i v cc axit aminc ch lu hunh:

CH2 CH COOH

NH2

SH CH2 CH3SH

+ +CO2 NH

3

+2H

etylmercaptit

e) Phn ng to thnh scatol, crezol, phenol: do cc vi sinh vt gy thi r

thng gp trong ng rut v trong qu trnh ct gi protein chuyn ho cc axitamin vng thnh cc sn phm c c mi kh chu nh: scatol, indol, crezol,

phenol.


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- Phn ng to scatol, indol:

COOHCHCH2

NH2NH

+ OH2COOHCHCH2

OHNH

+ NH3

tryptophan axit indoloxypsopinic

COOHCHCH2

OHNH

+ O2 COOHCH2

NH

+ OH2 + CO2

axit indolaxetic

COOHCH2

NH

CH3

NH

+ CO2

CH3

NH

scatol

+ 3/2 O2

NH

indol

+ +OH2 CO2

- Phn ng to thnh crezol, phenol:

COOHCHCH2

NH2

OH

CH2

OH OH

crezol phenoltyrosine

f) Phn ng to thnh di- trimethylamin t cc lipoprotein.

Phn lipit sau khi tc t lipoprotein s b chuyn ho thnh cc di-trimethylamin:

V d:


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CH3 N

CH3

CH3

CH2 CH2 OH

OH

CH3 N

CH3

CH3

CH3N

CH3

CH3

O

colin trimetyl amin oxytrimetylamin (mui tanh)

[O]

g) Phn ng to thnh phosphin: Xy ra vi phosphoprotein vnucleoprotein. Ngun to thnh phosphin l cc axit phosphoric c trong protein

c gii phng ra khi phn hu. Phosphin l kh khng mu, mi thi, rt c

1.2 KH NNG CHUYN HO CU CARBOHYDRAT

1.2.1 S PHN GII CU DISACARIT V POLYSACARIT

S phn gii cu disacarit v polysacarit thc hin theo hai kiu: thu phn

v phosphoryl phn.a) Thu phn:l qu trnh phn gii c s tham gia cu nc.

Di tc dng cu nhiu loi enzym:

Polysaccarit oligosaccaritmonosaccarit

V d: tinh bt, glycogendextrinmaltoseglucose; qu trnh phn gii ny

c thc hin nh cc enzym -amylase, -amylase v -amylase (glucoamylase).

b) Phosphoryl phn:qu trnh phn gii, trong axit phosphoric ng vaitr cu nc trong qu trnh thu phn, xy ra di tc dng cu enzym

phosphorylase.

Phosphorylase ch tc dng vo lin kt 1,4 cu phn t tinh bt v s dngli khi ti lin kt 1,6. Tc dng cu n ch tip tc c sau khi lin kt 1,6 cgii phng nh enzym amylo-1,6-glucosidase. C th coi phosphorylase l enzym

glucosyl transferase, v n xc tc cho s chuyn gc glucose ti gc axitphosphoric. Qu trnh phn ng c tip din ln lt v to nn hng loi phn tglucose-1-phosphat.

S phn gii glycogen cng tin hnh tng t v cng to nn glucose-1-

phosphat. Cc disacarit cng c th b tc dng cu enzym phosphorylase tng

ng v to nn dn xut phosphat cu monosacarit ng thi gii phng mtmonosacarit th hai.

V d enzym maltosephosphorylase xc tc cho qu trnh phn gii maltosethnh glucose-1-phosphat v glucose nh s tham gia cu H3PO3.

Phn ng phosphorylase ni trn l thun nghch nn c th xc tc cho sphn gii cc polysacarit v oligosacarit cng nh s ti t hp chng.


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1.2.2 S CHUYN HO CU CC MONOSACARIT

V bn cht qu trnh ny bao gm cc phn ng oxy ho kh sinh hc cthc hin bi mt lot cc enzym c trong ti th. Kt qu l phn t hexose (chyu l glucose) b oxy ho hon ton to ra cc sn phm v c l kh CO2v nc,

cn nng lng m cht hu c gii phng ra c tch lu trong lin kt cao nngATP.

Tu thuc vo iu kin khng c hay c oxy m s phn gii glucose xy ratheo hai hng chnh:

- Phn gii k kh: bao gm giai on ng phn v s ln men k kh.

- Phn gii ho kh (cn gi l qu trnh h hp): bao gm c giai on

ng phn, s oxy ho axit pyruvic v chu trnh Crep.

Nh vy hai qu trnh trn u c chung chui phn ng t glucose n axit

pyruvic.

1.2.2.1 S ng phn

Qu trnh ny c pht hin bi Embden, Meyerhof v Parnas. y l qutrnh phc tp, c xc tc bi nhiu enzym v khng c s tham gia cu oxy.Phn t ng ln lt tri qua cc giai on: hot ho, ct i phn t hexose (6C)to thnh 2 phn t ng triose (3C), loi hydro cu triose-phosphat to thnh

phospho glycerat, chuyn sn phm trn thnh axit pyruvic.

a) Giai on hot ho phn t hexose: gm 3 phn ng- phn ng to thnh glucose-6-phosphat nh enzym phospho-gluco-kinase

vi s tham gia cu Mg2+

:

O

OH

HH

H

OH

OH

H OH

H

H2C OH

ATP ADP O

OH

HH

H

OH

OH

H OH

H

H2C O P

Glucose Glucose-6-phosphat

Mg2+

- Phn ng ng phn ho do enzym isomerase xc tc:

O

OH

HH

H

OH

OH

H OH

H

H2C O P

Glucose-6-phosphat

O CH2OH

H

OH

OH

H

H

O

OH

P

Fructose-6-phosphat


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Cu to dng vng furan c lin kt km bn vng hn, do mch cacbond b ct hn.

- Fructose-6-phosphat tip tc b phosphoryl ho ln th hai nh enzymphospho fructose kinase vi s tham gia cu phn t ATP (adenosin triphosphat)

Fructose-1,6-di-phosphat

O CH2OH

H

OH

OH

H

H

O

OH

P

Fructose-6-phosphat

O P

OH

2C

H

OH

OH

H

CH2

H

O

OH

PADPATP

Mg2+

Do cu to i xng, n d b ct mch cacbon im gia.

b) Giai on ct mch cacbon:

Fructose-1,6-di-phosphat

O P

OH

2C

H

OH

OH

H

CH2

H

O

OH

P

H2C

C

H2C

O

O

OH

P

O P

CH

HC

CH2

OH

O

Aldolase l enzym xc tc s phn ly fructose-1,6-diphosphat thnh hai phnt triose phosphat (dihydroxy axeton phosphat v glyxeraldehyd-3-phosphat). Haitriose phosphat ny c th chuyn ho ln nhau nh enzym ng phn (triose-

phosphat-isomerase). Cn bng cu phn ng ny lch v pha to thnhglyxeraldehyt-3-phosphat. Bi vy, t 1 phn t fructose-1,6-diphosphat s to

thnh 2 phn t glyxeraldehyd-3-phosphat.

c) Giai on oxy ho:c thc hin nh glyxeraldehyt-3-phosphat dehydrogenase. Enzym ny c

coenzym l NAD+, trong trung tm hot ng c nhm -SH.

Trong phn ng ny gc phosphat cao nng cu axit 1,3-diphosphoglyxeric

chuyn cho ADP. cng l kiu phosphoryl ho ADP thnh ATP. Phn ng nyrt c ngha v nng lng gii phng trong qu trnh oxy ho c tch ly trongphn t ATP, bo m cung cp nng lng cho hot ng t bo trong iu kin


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khng c oxy. Phn ng do enzym 3-phospho glyxerat kinase xc tc v to thnhaxit 3-phospho glyxeric:

Axit 3-phospho glyxeric bin i thnh axit 2-phospho glyxeric nh enzym

phospho glyxerat mutase:

d)S to thnh axit pyruvic: bao gm- qu trnh loi nc nh enolase xc tc to thnh axit phospho

enolpyruvic, c ch lin kt cao nng.

- Qu trnh chuyn gc phosphat cao nng cho ADP to thnh axit enol

pyruvic v ATP:

Dng enol cu axit pyruvic khng bn v n s chuyn thnh dng xeton bnhn.

e) S chuyn ho cc sn phm cui cng cu qu trnh ng phn.Nh vy, sau qu trnh ng phn, ngoi ATP sn phm to thnh s l

NADH v pyruvat. S chuyn ho tip theo cu chng ph thuc vo cc conng chuyn ho khc nhau t bo. NADH phi c chuyn ho tr li v dng

NAD+, nu khng s lm hn ch qu trnh ng phn. NADH c th quay li

dng NAD+tr li bng con ng hiu kh v k kh. C hai u con ng ugn vi s chuyn ho tip theo cu pyruvat. S phn cu pyruvat trong t bo sph thuc vo s c mt cu oxy.

Trong iu kin hiu kh, pyruvat b oxy ho trong chu trnh axit xitric, tothnh CO2v ng thi to ra thm NADH (v FADH2) v trong iu kin hiu kh

cc phn t NADH c to ra t qu trnh ng phn v chu trnh axit xitric s boxy ho tr li thnh NADH+qua chui vn chuyn electron ty th.

Trong iu kin k kh, pyruvat s c chuyn ho theo cch khc.

- S to thnh axit lactic: di tc dng cu lactat dehydrogenase, axitpyruvic b kh thnh axit lactic:

Phn ng xy ra trong m c ng vt th to thnh axit L-lactic, cn trong

qu trnh ln men lactic do cc vi sinh vt gy ra th to thnh D-lactic.

- Ln men ru: mt s vi sinh vt nh nm men c kh nng bin i axit

pyruvic thnh ru etylic. Qu trnh xy ra theo hai bc. Bc u axit


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pyruvic b kh nhm carboxyl thnh CO2v axetaldehyt nh enzym pyruvatdecarboxylase c coenzym l thiamin pyrophosphat (cha vitamin B1) v

Zn2+. Bc hai, axetaldehyt b kh thnh etanol. Phn ng c xc tc bialcohol dehydrogenase:

- S to thnh -glyxerophosphat v glyxerin

S ln men glucose cu nm men lun km theo to thnh mt lng nhglyxerin:

Tt c cc phn ng kh axit pyruvic trn u gip NADH chuyn ho trli dng NAD+cn thit cho qu trnh ng phn.

Qu trnh ng phn v ln men c tm tt bng sau:

Qu trnh oxy ho k kh 1 phn t glucose to thnh 2 phn t axit pyruvic s dng 2 ATP v to ra c 4 ATP, nh vy tch lu c 2 ATP. So vi nng

lng d tr cu phn t glucose, qu trnh ny ch gii phng mt phn nh nnglng. Phn ln nng lng vn cn tng tr trong sn phm cui cng (axit lactic,etanol v.v.). Nh vy i vi cc vi sinh vt k kh, bo m nng lng chocc hot ng sng, chng phi phn gii mt lng ln ng, to thnh mt

lng ln sn phm cui cng l nhng hp cht hu c (ru, axit hu c v.v.).Ngi ta li dng iu sn xut ru, bia, cn, axit hu c v.v.

f) S tham gia cu cc c cht khc vo qu trnh ng phn


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1.2.2.2 Chu trnh Crep

Qu trnh phn gii ho kh glucose xy ra theo 4 giai on chnh:

- T glucose n axit pyruvic: cc phn ng ging vi s ng phn kkh

- T axit pyruvic n axetyl-CoA- Oxy ho axetyl-CoA qua chu trnh Crep

- Oxy ho cc coenzym kh qua chui h hp

Trong iu kin c oxy, axit pyruvic b oxy ho hon ton n CO2 vH2O v gii phng ton b nng lng cn li. Cc phn ng cu qu trnh ny xyra theo mt chu trnh, mang tn nh bc hc pht hin ra n gi l chu trnh Crep

(Krebs).

a)S kh carboxyl oxy ho axit pyruvic thnh axetyl-CoA:Trc khi tham gia vo chu trnh, axit pyruvic b kh nhm carboxylng thi b oxy ho di tc dng cu mt phc h a enzym c tn l pyruvat

oxydase hay pyruvat decarboxylase.

Phng trnh tng qut cu giai on ny nh sau:

b) Cc phn ng trong chu trnh Crep- Giai on 1:nh xc tc cu enzym citrat syntase, nhm axetyl cu axetyl-

CoA s c chuyn cho axit oxaloaxetic to thnh axit axetic, ng thi gii

phng coenzym A.

- Giai on 2: axit xitric b bin i thnh dng ng phn cu n l axit

izoxitric, qu trnh ny c xc tc bi enzym aconitase.

- Giai on 3: xy ra s oxy ho axit izoxitric di tc dng cu enzymizoxitrat dehydrogenase. Axit izoxitric b kh hydro, 2 nguyn t hydro cu nc chuyn cho coenzym NADP+. Kt qu to thnh NADPH+H+ v axit

oxaloxucxinic. Ngay sau , axit oxaloxucxinic li b kh carboxyl bin thnh

axit -xetoglutaric. Mt nguyn t carbon li c tch ra khi c cht di dng

CO2.

- Giai on 4: axit -xetoglutaric li b kh carboxyl oxy ho do phc henzym -xetoglutarat dehydrogenase xc tc. Phn ng ny cng tng t nh s

oxy ho axit pyruvic giai on trn v cn cc coenzym nh tiamin pyrophosphat,axit lipoic, coenzym A v NAD+ tham gia. Sn phm cu phn ng l axitdicarboxylic dng hot ng, l xucxinyl coenzym A (c ch lin kt tio-este

giu nng lng) v mt nguyn t cacbon b tch ra giai on ny l phn t

CO2.


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- Giai on 5:Nng lng trong lin kt cao nng cu xucxinyl-coenzym Ac chuyn thnh lin kt cao nng cu GTP, nh tc dng cu enzym xucxinat-

tiokinase, cui cng nng lng c chuyn t GTP cho DP tng hp nnATP. y l phn ng duy nht trong chu trnh Crep xy ra s tch lu nng lng

trong ATP.

- Giai on 6:axit xucxinic b oxy ho thnh axit fumaric nh tc dng cuxucxinat dehydrogenase. Enzym ny c coenzym l FAD, khi nhn hydro t c chts tr thnh FADH2. Hydro cu FADH2 c tip tc chuyn cho h thng vn

chuyn in t to thnh nc v tng hp c 2 ATP.

- Giai on 7: axit fumaric c hydrat ho s to thnh axit malic nh

enzym fumarat hydratase.

- Giai on 8: axit malic v c to thnh s b oxy ho thnh axitoxaloaxetic di tc dng cu enzym malat dehydrogenase c coenzym l NAD+.

Khi phn t axit oxaloaxetic ban u tham gia vo chu trnh c ti to th chutrnh khp kn. y khng phi hon ton l phn t axit oxaloaxetic ban u,bi v trong cc phn ng n nhng 2 nguyn t cacbon dng CO2v c bsung bng hai nguyn t cacbon mi t axetyl-coenzym A. Phn t oxaloaxetic miny li tip tc ngng t vi phn t axetyl-coenzym A v chu trnh c lp li.

Nh vy qua mt vng cu chu trnh Crep, gc axetyl hot ng to ra 2phn t CO2. T axit pyruvic th to ra c 3 phn t CO2. Cc phn t CO2c

to thnh t chu trnh Crep c th c s dng vo cc qu trnh khc nhau trongc th (s carboxyl ho, tng hp ure, tng hp nucleotit. (hnh v)

Qua s trn c th thy rng, khi oxy ho hon ton 1 phn t axitpyruvic, c 5 cp nguyn t hydro c tch khi c cht to thnh cc coenzym

kh: 1 [NADPH+H+], 3[NADH+H+] v 1 FADH2. Trong thnh phn, phn t axitpyruvic ch ch 4 nguyn t hydro, nh vy cc nguyn t hydro cn li l cuphn t nc trong mi trng t bo tham gia vo. Phng trnh tng qut cu soxy ho axit pyruvic nh sau:

C3H4O3 + 3 H2O 3 CO2 + 5 H2O

S to thnh 5 phn t H2O trong phng trnh trn l kt qu cu s oxyho cc coenzym kh nh h thng vn chuyn in t, ng thi thng qua qu

trnh ny cng tng hp c mt s ln lin kt cao nng.

c) Tng kt nng lng khi oxy ho hon ton mt phn t glucoseNng lng trong phn t glucose ch yu c gii phng giai on phn

gii ho kh. C 1 vng cu chu trnh Crep gii phng c 12 ATP cc giai on

phn ng.


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Tuy nhin, trc khi i vo chu trnh Crep, phn t glucose phi tri qua ccphn ng cu giai on ng phn v phn ng kh carboxyl oxy ho axit pyruvic.

Ho nng cu glucose gii phng cac giai on phn ng c tng kt nh sau:

Glucose

-ATPGlucose-6-phosphat

Fructose-6-phosphat

-ATP

Fructose-1,6-diphosphat

2 Glyxeraldehyt-3-phosphat

2NADH+H+ +6 ATP2 1,3-diphosphoglyxerat + O2 H2O

+2 ATP

2 3-phosphoglyxerat

2 2-phosphoenol pyruvat

+2 ATP

2 pyruvat

2 CO2 2 NADH+H+ +6 ATP

2 axetyl-CoA + O2 H2O

+ O2 Chu trnh Crep v +24 ATP

chui h hp

4 CO2 + 4H2O Tng cng: 38 ATP

Khi oxy ho hon ton 1 phn t gam glucose s thu c 686 kcal. Nhng tng hp ATP t ADP v Picn cung cp 8-10 kcal/mol. Bi vy nu ton bnng lng c phn gii mt mol glucose u c dng tng hp ATP th sthu c 686/976 mol ATP.

Thc t ch to c 38 mol ATP, chin 50% tng s nng lng. Nh vy

cn 50% c gii phng dng nhit nng

chui h hp

chui h hp


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d) nghi cu chu trnh CrepThng qua chu trnh ny, phn t glucose b oxy ho hon ton n CO2,

H2O v gii phng ton b nng lng mt phn di dng ho nng trong ATP vmt phn dng nhit nng c tc dng gi m t bo. T ATP c th tng hp nn

GTP, XTP, UTP l ngun ngn lng cn thit cho cc qu trnh sinh tng hp cccht khc nahu tng c th.

Chu trnh Crep to ra nhiu coenzym kh. Cc coenzym kh ngoi vai trto thnh ATP cn dng kh cc lin kt kp, cc nhm carbonyl, nhm imin

trong cc phn ng cn thit cho t bo.

Chu trnh Crep cn l ngun cacbon cho cc qu trnh tng hp khc nhau.

V d axetyl-CoA c th dng tng hp cc axit bo, steroit, axetoaxetat hoc

axetyl colin. CO2dng tng hp e, tng hp mi AMP v trong cc phn ng

carboxyl ho khc. Mt s xetoaxit trong chu trnh nh -xetoglutaric v

oxaloaxetic thng l nguyn liu tng hp cc axit amin glutamic v aspartic.Cng v vy chu trnh Crep l mt xch lin hp, l im giao lu cu nhiu

ng hng phn gii v tng hp cc cht khc nhau trong t bo, ng thi cngl ng hng chnh phn gii cc hp cht hu c.

1.2.2.3S oxy ho trc tip glucose. Chu trnh pentose phosphat

c th sinh vt cn c mt kim phn gii glucose thnh CO 2 v H2Onhng khng tri qua qu trnh ng phn v chu trnh Crep. Trong qu trnh ny,cc phn ng cng to thnh mt chu trnh m cc sn phm trung gian ch yu l

cc pentose phosphat. V vy ngi ta gi l chu trnh pentose phosphat. Ngoi

ra, cn c vo cht ca ng cu chu trnh l glucose-6-phosphat, ngi ta cn gil chu trnh hexose monophosphat. y l qu trnh oxy ho trc tip glucose v soxy ho xy ra ngay giai on hexose ch khng phi qua triose nh qu trnh

ng phn. Chu trnh ny do Dickens-Horecker nghin cu v pht hin.

a) Cc phn ng: gm 2 giai on phn ng

- Giai on oxy ho glucose glucose-6-P

Glucose-6-P l cht u tin gia nhp vo chu trnh. N b oxy ho di tc

dng cu enzym glucose-6-P dehydrogenase (c coenzym l NADP+), bin thnh

axit 6-P-gluconic. Tuy nhin, giai on ny c to thnh mt sn phm trung gian l6-P-glucono--lacton, cht ny sau b thu phn thnh axit 6-P-gluconic.

Qu trnh oxy ho ln th 2 di tc dng cu enzym 6-P-gluconatdehydrogenase (c coenzym NADP+) to thnh axit 3-xeto-6-P-gluconic, ri sau

cht ny b kh carboxyl to thnh D-ribulose-5-P.


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Trong ton b chu trnh ch xy ra 2 lt oxy ho (phn ng 1 v 3) nh nu trn. Kt qu l t 1 hexose phosphat qua 1 vng cu chu trnh to ra 1

pentose phosphat, 1 phn t CO2v 2 (NADP+H+)

- Giai on ti to hexose phosphat

s oxy ho c tip tc, pentose phosphat va to thnh cn tri quanhng bin i ti to li hexose phosphat, khp kn chu trnh. Nhng bin i

ny c th chia thnh 2 bc sau:

Bc 1:S ng phn ho cu ribulose-5-P

Ribulose-5-P l mt xetopentose, n c th chuyn thnh dng aldose lribose-5-P nh tc dng cu phosphoribo isomerase. Mt khc, ribulose-5-P c th

bin i thnh dng ng phn epime ( v tr cacbon th 3) l xilulose-5-P nh tcdng cu enzym phospho-xetopento-epimerase. Hai phn ng ny c v tr quan

trng, n c coi l im ni gia giai on u v giai on tip theo cu chu

trnh.Bc 2:S bin i tng h gia cc pentose-P v cc hexose-P

C hai kiu phn ng quan trng tng giai on ny. l s chuyn aldol

ho v s chuyn xetol ho.

S chuyn aldol ho l s chuyn mt nhm 3 nguyn t ccbon (dihydroxy

axeton) ca mt ketose phosphat (v d: fructose-6-P hoc xetoheptulose-7-P) chocht nhn l mt aldose phosphat (v d, glyxeraldehyt-3-P hoc erytrose-4-P).

S chuyn ho xetol ho l s chuyn mt nhm 2 nguyn t cacbon (xetol

hoc glycolaldehyt) cu cht cho l xetose phosphat cho cht nhn l mt aldosephosphat.

C 6 phn t glucose-6-P tham gia vo chu trnh th 5 phn t c ti to, 1

phn t b oxy ho hon ton n CO2v to thnh 12 coenzym kh (NADPH+H+).

Qu trnh oxy ho glucose-6-P qua chu trnh pentose phosphat c trnh by hnh v.

b) Tng kt nng lng: phng trnh tng qut mt vng chu trnh nhsau:

6 glucose-P + 12 NADP+ 5 glucose-6-P + 6 CO2+ Pv+ 12 NADPH + 12 H+

Hoc vit dng n gin:1-glucose-6-P + 12 NADP+ 6 CO2 + Pv + 12 (NADPH + H

+)

C mi phn t glucose-6-P b oxy ho hon ton trong chu trnh to ra c

12 phn t NADPH + H+. Nu hydro c chuyn qua h thng vn chuyn int, cho php to thnh 36 ATP. Do cn 1 ATP phosphoryl ho glucose ban u

thnh glucose-6-P, nn cn thu c 35 ATP. l s nng lng tng ng vi


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s oxy ho hon ton 1 phn t glucose qua s ng phn v chu trnh Crep (38ATP). ng thi, cng hn hn mc nng lng do s phn gii glucose trong iu

kin k kh (qu trnh ng phn ch to ra 2 ATP).

c) nghi cu chu trnh pentose phosphat:

- Chu trnh cung cp cc pentose phosphat l nguyn liu cn thit cho ssinh tng hp nucleotit, c trong thnh phn cu to cu cc coenzym v axit

nucleic.

- Cung cp NADPH cn thit cho cc phn ng khc nhau, nht l trong stng hp axit bo v steroit.

Trong iu kin cc NADPH b oxy ho qua chui h hp s to thnh mt

lng ng k ATP.

- Chu trnh cho php cc c th quang hp tng hp c xacarit v cc cht

hu c khc t CO2.

Tm li, trong c th sinh vt, s phn gii ng c th tin hnh theonhiu kiu khc nhau. Cc kiu ny b sung cho nhau c th xy ra song song cngnhau hoc mt vi kiu chim u th nhng iu kin nht nh. iu cng ni

ln s phong ph cu cc qu trnh thch nghi cu vi sinh vt i vi mi trngsng, v qu trnh phn gii ng cung cp nng lng ch yu cho mi hot ng

sinh sng cu c th.

1.3 CHUYN HO CA LIPIT

1.3.1 Kh nng chuyn ho cu lipitTrong iu kin bo qun cng nh trong c th, lipit c th b chuyn ho

to thnh nhng phm vt khc nhau. Cc qu trnh chuyn ho ny rt phc tp,di tc dng cu mt lot cc nhn t v c enzym hoc khng c enzym xc tc.

1.3.1.1 S i ho

Khi bo qun lu, di tc dng cu nhiu nhn t nh nh sng, khng kh,nhit , nc v vi sinh vt..., lipit b thay i trng thi, mu sc v c mi v kh

chu. Thng ngi ta gi qu trnh tng hp ny l s i ho. Thc cht cu s i

ho l qu trnh oxy ho. Tuy vy, nu da vo c ch phn ng th c th phn rai ho do thu phn v i ho do oxy ho.

a) i ho do phn ng thu phn

Phn ng thu phn lipit cng c th xy ra, khi c enzym cng nh khngc enzym xc tc.

Trng hp th nht xy ra trong pha bo v ch c nc ho tan tronglipit (du, m) mi tham gia phn ng, ngha l phn ng tin hnh trong mi


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trng ng th. Khi trong lipit c mt nc vi mt lng ng k, nhng nhit thng, th vn tc phn ng cng rt b. V l nhit thng vn tc phn

ng thu phn rt chm. Hn na kh nng khng ho tan cu pha nc trong phabo cng ngn cn s tip xc cn thit gia chng.

Trng hp thu phn do enzym xc tc thng xy ra trn b mt tipxc gia lipit v nc. Enzym lipase xc tc phn ng thu phn c th c trongnguyn liu cng nh do vi sinh vt mang vo.

Chng ta u bit lipase l mt globulin. N xc tc khng nhng phn ng

thu phn m c phn ng tng hp na. Do cn bng phn ng ph thuc votng iu kin c th m c th chuyn dch v phi cng nh v tri. Khi hm m

cao th s thu phn u th hn s tng hp. Ngc li, trong ht kh th s tnghp li u th hn. Lipase c trong ht v trong cc c quan dinh dng cu cy.

Lipase c nhiu trong ht thu du. Tc dng ti u cu lipase cu ht l 35-38 oC.

Hot cu n gim xung mt cch ng k khi ch bin thu nhit cc ht cngnh khi p nng cc ht c du. Phn ng thu phn lipit s tng mnh khi ht bnghin c hc hoc b tn thng do cn trng. Lipase c c hiu thp so vi cu

trc cu lipit nhng li c c hiu quang hc cao i vi nhng c cht c hotquang. Ni chung i vi nhng c cht khng mang in tch th di cu mch

cacbon cu axit bo tham gia trong lipit c nghi ln i vi hot cu lipase.

Trng hp n gin nht cu s i ho do phn ng thy phn thng thykhi bo qun b v margarin. Khi s gii phng ra axit butyric l axit c mi rtkh chu.

Trong ch bin lng thc, bo qun c ht, bt v tm, vai tr quytnh l nhit v thy phn. Chng hn bt khi nhit di 5 oC vn bo qun

c tt ngay c khi c hm m cao. Ngi ta nhn thy rng khi bo qun bt chm m cao hn 15% th phn ng thu phn ch yu l do emzym ca nm mc.

Khi axit cng tng lin do tch t cac axit hu c ho tan v bt s c mi v

kh chu. Ngi ta cn nhn thy i vi bt kh, nm mc khng pht trin c, axit cng tng nhng khng c mi v axit, v ch yu l to ra nhng axit bokhng ho tan trong nc.

b) i ho do phn ng oxy ho kh

i ho theo kiu ny l ph bin nht trong bo qun cc lipit. Thng ngita phn bit hai loi: i ho ho hc v i ho sinh hc.

i ho ho hc l qu trnh t oxy ho. Khi xy ra s tn cng cc gcaxit bo t do cng nh kt hp bi oxy phn t. p sut ca oxy v lng ni i

trong phn t axit bo c nh hng n tin trnh phn ng. Sn phm u tin lhydro peroxit. T to nn aldehyt no v khng no, xeton, axit mono- v


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dicarboxylic, aldoaxit, xetoaxit, epoxyt v.v. Ngoi ra cn c th trng hp ho ccsn phm oxy ho na.

a s nhng cht ny c vai tr quyt nh trong vic pht trin mi v.

i ho sinh ho hc li bao gm s i ho do enzym lipoxygenase v s i

ho xeton. Kiu i ho th hai ny thng c trng i vi lipit c cha axit bono vi phn t lng trung bnh v thp, khi c hm m ng k. Khi axit bo b

-oxy ho v decarboxyl ho, kt qu s l tch t cc alkylmetylxeton c mi kh

chu.

Khi lipit b i ho, thng b mt hot tnh vitamin. V l khi cc axit bokhng no cao phn t cng nh cc vitamin u b ph hu bi cc phm vt oxytch t trong lipit.

Cc sn phm oxy ho ca lipit thng lm v hot enzym v c bit lm

gim hot ca suxinoxydase, xitocromoxydase v colinoxydase.

Sn phm oxy ho cn c kh nng phn ng cao vi protein. Hp cht tothnh ny bn vng, khng ho tan trong nc cng nh trong dung mi hu c v

cng khng b phn ly bi enzym.

Ngi ta cn cho rng s pht trin ca bnh va x ng mch l do s to

thnh phc gia sn phm oxy ho ca lipit vi protein c trong thnh phn cathnh huyt qun.

Lipit b oxy ho cn km hm s pht trin ca ng vt.

C ch ca qu trnh i ho ho hc

Qu trnh i ho ho hc l phn ng chui nn thng c 3 thi k sau:- Pht sinh:

u tin, phn ng c khi mo bng vic mt vi phn t lipit (RH) boxy ho to thnh gc t do:

R : gc ca axit bo no hoc khng no t do hoc gc ca axit bo trong

phn t glyxerit;

H: nguyn t H Cso vi ni i, hoc nguyn t H ca nhm metylen

bt k trong axit bo no. to thnh gc, ngha l t lin kt C-H trong phn ng trn i hi

nng lng l 70-100 kcal/mol.

Tuy nhin khi c oxy ho tan th tng tc gia RH ban u vi oxy s xyra mt cch mnh m hn. V l s to thnh gc theo phn ng lng phn di

y i hi nng lng ch l 47 kcal/mol.

RH + O2 R + HO2


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Trng hp khi nng RH cao th c th xy ra phn ng tam phn:

RH + O2 + HR1 R + H2O2 + R1

Phn ng ny i hi nng lng cn b hn phn ng lng phn.

Gc t do cng c th pht sinh khi c ion kim loi giao chuyn:

M3+ + RH M2+ + R + HM2+ + ROOH M3+ + RO + OH

Pht trin

Gc t do R hoc RO c to thnh do qu trnh khi mo s bt u

chui chuyn oxy ho:

R + O2 RO2 (1)

RO2 + RH ROOH + R (2)

Phn ng (1) c bit nhanh, thc t khng cn nng lng hot ho. Cn

nng lng hot ho ca phn ng (2) l 4-12 kcal/mol. T y ta thy gc RO2l

gc ch o trong mch oxy ho v phn ng c ngha quyt nh vn tc pht

trin ca qu trnh t oxy ho l phn ng tng tc gia gc RO2vi phn t lipit

(phn ng (2)).

T hydroperoxit s phn mch cho nhng gc t do theo ng hngsau:

ROOH RO2 + OH (3)

Hoc 2 ROOH RO2 + H2O + RO

(4)

Hoc ROOH + RH RO + H2O + R (5)

Vic t lin kt O-O to thnh hai gc t do theo phn ng (3) i hinng lng hot ho 30-35 kcal/mol.

S lin hp cc hydroperoxit thnh dime theo phn ng (4) s thun li khinng hydroperoxit ln. Vic to nn dime (do lin kt hydro) s lm yu lin kt

O-H v O-O, do lm gim bi nng lng phn gii dime thnh cc gc.

Hydroperoxit c th phn gii theo phn ng (5) i hi nng lng hot ho

b hn phn ng n phn (3).

Cn ch rng gc peroxit c xu hng phn ng vi sn phm b oxyho hn l vi phn t cha b oxy ho do ch trong sn phm b oxy ho thngcha cc lin kt C-H c kh nng phn ng hn.

Gc peroxit cng c th t nguyn t H - c bit v tr do qu trnh ni

phn v cng c th kp hp vi ni i olefin. V vy vic chuyn ho cc gcperoxit s a ti to nn khng nhng hydroperoxit m c peroxit polyme, peroxitvng, oxit, aldehyt v cc sn phm khc.


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Do qu trnh pht trin ca chui phn ng oxy ho vi cc phn ng phn

mch (3), (4), (5) nn tch t gc alcoxyl RO, peroxit ROOv hydroxyl OH.

T gc alcoxy c th to nn nhng sn phm th cp nh ru, xeton,

aldehyt v.v.: RO + RH ROH + RO

ru gc t doTng tc ca hai gc alcoxyl:

Tng tc ca gc alcoxyl vi gc alkyl:

Hoc

Hoc t hydroperoxit v gc alkyl cng to ra xeton:

Aldehyt cng c th c to thnh t xeton bng cch oxy ho chng n

-xetohydroperoxit ri n aldehyt v axit:

Ngoi ra, trong lipit b oxy ho cn c th to thnh cc sn phm trng hp

cao phn t:

HocKt thc:

Vic t mch, ngha l mt cc gc t do xy ra ch yu do kt qu tngtc ca cc gc theo c ch lng phn:

R + R

R + RO2 sn phm phn t

RO2 + RO2

Nng lng hot ho ca cac phn ng ny rt khng ng k (1-2 kcal/mol)

do vn tc phn ng rt ln.

Cc yu t nh hng n vn tc oxy ho

Cc yu t kch thch

nh hng ca axit bo t do: axit bo t do c tc dng xc tc qu trnhoxy ho, lm tng nhanh s phn gii hydroperoxit thnh cc gc.


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nh hng ca oxy:vn tc t oxy ho ph thuc vo lng oxy trong mitrng. V s pht sinh chui phn ng ch yu ph thuc vo RH + O 2.

nh hng ca nhit :nhit c nh hng ln n vn tc phn ngho hc. Tng nhit , lng peroxit to thnh tng. nhit 40-45 oC v cao

hn, s phn gii hydroperoxit s tng.

nh hng ca trng thi lipit:b mt tip xc ca cht bo vi khng kh

cng ln th vn tc oxy ho cng cao. Vn tc t oxy ho cn ph thuc vo chiudy ca lp. Khi lp dy th oxy xm nhp kh. Gim nhit n lc cht bo

chuyn sang trng thi rn th vn tc oxy ho cng gim.

nh hng ca ion kim loi chuyn tip:cc ion cng nh cc hp cht ca

kim loi chuyn tip c tc dng xc tc s oxy ho lipit. Thc nghim cho thy

rng khi c mt st trong du vi lng 0,2-10 mg/kg st s lm gim bn cadu i vi oxy ho 20-50%. Vi Cu ch cn 0,05-0,2 mg/kg s lm gim bn

ca du i vi oxy ho 20-50%.Ion kim loi chuyn tip c cc electron c thn v thc cht l gc. Nhng

tnh cht gc chng th hin yu do chng c th tn ti trong dung dch vi

nng ln. Tuy nhin chng c th chuyn hoc ly electron t mt ht bo hoho tr no to thnh gc t do hot ng hoc gc ion.

Mn+ + RO M(n+1)+ + RO-

Mn+ + RO2 M(n+1)+ + RO2

-

M(n+1) + RO2 Mn+ + R+ + O2

nh hng ca nng lng mt tri v tia ion: nng lng nh sng c bitl tia cc tm v tia ngn ca quang ph nhn thy c tc dng xc tc qu trnh oxyha lipit. C l nng lng nh sng s quang phn hydroperoxit thnh cc gc tdo.

nh hng ca nc: ngi ta nhn thy rng nc c tc dng lm tngtnh n nh ca cht bo sa i vi s oxy ha nhit 95 oC v khng c nhhng g n m ln 50 oC.

nh hng ca yu t km hm

Vn tc phn ng RO2+ RH c th c lm gim bng cch hng cc

phn ng ca gc peroxit theo chiu khc.

iu t c bng cch a vo trong lipit cht chng oxy ha InH 2,

ngha l cht c kh nng phn ng d dng vi gc RO2hn:

InH2 ROOH +InH

RO2

RH


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Kt qu l thay gc peroxit RO2bng gc ca cht chng oxy ha Inl gc

yu khng th tng tc vi phn t lipit. Cc gc Ins b v hot do s t hp:

RO2 + InH2 In

In + In In-In

RO2 + In ROOIniu kin tha mn c bng cch a vo nhng cht chng oxy ha

c bn cht phenol hoc amin v.v. Phenol c nng lng lin kt ca nguyn thydro trong nhm hydroxyl rt b (60-70 kcal). Mt cch khc gim nng

RO2l lm gim vn tc phn ng pht sinh ra chng.

Km hm s oxy ha bng cch lm t mch

C ch tc dng ca nhng cht chng oxy ha loi ny (nh phenol, amin)

c th nh sau:

RO2

+ InH2 ROOH +

InHInH + InH In + InH2

gc khng hot ng

hoc RO2 + InH2

[RO2InH2][RO2InH2] + RO2

sn phm bn

hoc RO2 + InH2 ROOH +

InHInH + RO2

sn phm bn

Km hm s oxy ho bng cch lm gim vn tc pht sinh mch

V d, cc sulfua c kh nng ph hu hydroperoxit:

ROOH + R1SR2 ROH + R1SOR2

ROOH + R1SOR2 ROH + RSO2R2

Cc sulfua kiu ny nh tiuam (tetration, tetrametyltiuramdisulfua)

V hot cc hp cht ch kim loi c hot ng xc tc

Cc kim loi chuyn tip c th tng tc vi hydroperoxit to thnh gct do, ng thi kim loi chuyn sang ho tr khc:

Fe2+ + ROOH Fe3+ + RO + OH

Fe3+ + ROOH Fe2+ + RO2 + H+

V vy phi chn nhng cht chng oxy ho c kh nng to phc vi ion

kim loi qu m phong to c s chuyn electron t mc N ti mc M,nghi l loi tr c kh nng chuyn ha tr. Nhng cht chng oxy ho loi ny

nh axit xitric, axit malic, axit fitinic...


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Axit carboxymetylmercaptosucxinic c tnh nng kh mnh thng cdng bo qun du u, du bng.

Trong bo qun lipit, ngi ta c th dng cng tnh cu hai cht lm tng

hiu qu cu cht chng oxy ho. Cht lm tng tnh cht chng oxy ho cu chtkia gi l cht hip tr. Cht hip tr ny c hoc khng c tnh cht chng oxy

ho. Bn cht cu chng c th l v c hay hu c. Axit phosphoric, axit ascorbic,axit xitric v mui cu chng, polyphosphat, axit amin, xephalin l nhng cht

thuc loi . C ch hip tr cu chng l ch chng l nhng cht cho hydro kh cc dng cht km hm b oxy ho. Chng hn km hm s oxy ho cht bo

bng hn hp axit ascorbic v quinon. Axit ascorbic chuyn hydro n quinon cho cht ny c tc dng c vi gc peroxit. Qu trnh chuyn hydro cng c

coi l s hip tr cu axit ascorbic vi tocoferol. Tc dng hip tr cu -alanin n

hydroquinon trong chng oxy ho cho du hng dng l ch khi b deamin hon axit pyruvic th ng thi cng kh c hydroquinon b oxy ho.

Cc cht nh tocoferol, xemazol, phosphatit, gossipol, quercetin...l nhngcht chng oxy ho t nhin, v thng ch trong trong cc ngun lipit t nhin.

Trong qu trnh khai thc v tinh luyn, nhng cht ny c th b mt i, do nhng ngun lipit ny d b i ho. Mt s cht chng oxy ho cng c tn to

khi gia cng cc ht c du nh melanoidin, gossiphosphatit,melanophosphatit...Hin nay ngi ta cn s dng nhiu cht chng oxy ho tng

hp nh butyloxyanizol, butyloxytoluen, dodexigalat v.v. trong bo qun du m.

c) C ch cu qu trnh i ho sinh hci ho do enzym lipoxydase

Trong qu trnh khai thc cng nh bo qun, lipit v c bit l du m, c

th b oxy ho di tc dng xc tc cu enzym lipoxydase to thnh nhng snphm khc nhau.

Lipoxydase hay lipoxygenase l enzym oxyhoas kh, thng c tn h thngl linoleat:O2oxydoreductase (1.13.1.13). Enzym lipoxygenase xc tc s oxy ho

cc axit bo khng no ch 2-3 ni kp (hoc nhiu hn) nh axit linoleic,

linolenic, axit arachidic. im c bit l lipoxygenase ch oxy ho dng cis-cis cndng cis-trans hoc trans-trans th hon ton khng c tc dng. C 3 axit ny ub oxy ho vi vn tc nh nhau. im khc l cc hydro peroxit c to thnh

y c hnh th cis-trans v c hot ng quang hc. C ch tc dng culipoxygenase nh sau:


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T hydroperoxit s chuyn ho mt cch bnh thng to ra epoxit,aldehyt, xeton v nhng phm vt oxy ho khc.

Nhiu hp cht hu c l nhng cht km hm cu lipoxygenase.Propylgalat, dephenylamin v oxyquinolin l nhng cht km hm lipoxygenase.

Tc dng km hm cu nhng cht ny l ch lm gim vn tc oxy ho linoleatv hp th oxy.

Lipoxygenase c nhiu trong ht u tng v trong ht ho tho. pH ti ucu lipoxygenase cu u tng l 9,0 v cu cc cy khc l 7,0. Lipoxygenase cu

ng vt c pH ti u t 4-7.

i ho xeton

i ho xeton thng c trng i vi lipit ch axit bo no, phn t lng

trung bnh, thp. Chng hn, trong bo qun b v margarin khi b nhim nm mc(Aspergillus, Penicillium) thng xy ra kiu i ho ny. Di tc dng cu cc

enzym cu vi sinh vt, axit bo b -oxy ho v decarboxyl ho m kt qu l to ra

cc alkylmetylxeton c mi kh chu.

S phn ng nh sau:

d) S oxy ho cc axit bo trong c th sinh vt

Trong m ng vt cng nh thc vt, cac axit bo cn b chuyn ho oxy

ho theo mt ng hng ring, thc hin theo tng nc, mi nc axit bo li bct ngn i mt mnh hai cacbon ra khi phn t. Cc axit bo cha no, trc tin

phi c kh thnh axit bo no, sau mi b oxy ho theo con ng . Qu

trnh oxy ho axit bo theo ng hng ny cng c tn l -oxy ho. Lc cc

phn ng cu qu trnh -oxy ho axit bo bao gm 5 phn ng:

- Phn ng 1: l phn ng hot ho axit bo nh axyl coenzym A CoA-SH,ATP v s xc tc cu enzym axyl-CoA-syntetase:

Pyrophosphat v to thnh s b phn gii thnh axit phosphoric (khi cenzym pyrophosphatase xc tc) do cn bn g ph n gs chuyn dch v phi

phi.

- Phn ng 2: l phn ng oxy ho axyl CoA nh enzym axyl CoA

dehydrogenase. Enzym ny l mt flavoprotein c nhm ngoi l FAD:


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Cc enzym ny khc nhau v tnh cht l hc v v tnh cht c hiu theo di cu mch carbon cu c cht.

- Phn ng 3 do enzym enoyl-CoA-hydratase xc tc:

- Phn ng 4 do enzym -oxyaxylCoA dehydrogenase ch nhm ngoi lNAD xc tc:

- Phn ng 5 bao gm vic chuyn nhm axyl cu phn t -xetoaxylCoA

ln mt phn t coenzym A mi. Enzym xc tc phn ng ny l -xetoaxylCoA

thiolase

Nh vy c qua 5 phn ng th phn t axit bo li ngn i hai nguyn tcacbon to thnh axetyl CoA. Qu trnh c th c lp i lp li nhiu ln cho

n khi ton b sn phm cui cng l axetyl CoA. T Axetyl CoA s i vo chutrnh Krebs (hoc chu trnh glyoxylic) cung cp nng lng cho c th sinh vt.

Khi oxy ho mt phn t lipit n gin (v d tristearin) th s cung cp cho

c th 458 mol ATP.

Nh vy cng vi carbohydrat, lipit l ngun nng lng d tr to ln cu c

th.


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CHNG 2: CC CHT MU

2.1 NGHA CA CC CHT MU TRONG SN XUT THC PHM

Cht lng cu cc sn phm thc phm khng nhng bao hm gi tr dinh

dng m cn bao hm c gi tr cm quan cu chng n. Mu sc l mt ch s

quan trng cu gi tr cm quan. Mu sc cu cc sn phm thc phm khng ch cgi tr v mt hnh thc m cn c tc dng sinh l rt r rt. Mu sc thch hp sgip cho c th ng ho thc phm d dng. V vy trong k thut sn xut thc

phm ngi ta khng ch bo v mu sc t nhin m cn cho thm cht mu mi,to ra nhng mu sc thch hp vi tnh cht v trng thi cu sn phm.

C th thc hin c iu bng nhng cch sau:

- Xy dng mt quy trnh gia cng nguyn liu, bn thnh phn bo ton

c ti a cc cht mu c sn trong nguyn liu;

- Tch ra, c c v bo qun cc cht mu t chnh nguyn liu thc vt

hoc t cc nguyn liu khc giu mu sc y. Sau t cc cht mu t nhin c c ny c th dng nhum mu cho chnh nguyn liu m t ta thuc cht mu hoc cho nhng dng nguyn liu hon ton khc.

- Tng hp nhn to cc cht mu ging nh cc mu t nhin cu sn phm

thc phm ri dng chng nhum mu cho cc sn phm khc m dng tnhin khng mnh hoc b mt mu ban u do qu trnh ch bin.

- Dng cc bin php k thut thch hp iu chnh cc phn ng theochiu to ra nhng cht mu mi t nhng hp phn c trong nguyn liu.

Da vo qu trnh hnh thnh c th chia cht mu thnh hai loi:- cc cht mu t nhin

- cc cht mu hnh thnh trong qu trnh gia cng k thut.

2.2 Cc cht mu t nhin

Cc cht mu t nhin thng gp ch yu trong cc nguyn liu thc vt,c th chia lm ba nhm chnh:

- Clorofil, dip lc hay cht mu xanh l cy;

- Cc carotenoit c trong lc lp, trong qu v rau mu da cam, mu vng

v i khi mu - Cc cht mu flavonoit c trong cc khng bo, c mu , xanh v vng,

2.2.1 Clorofil

Mu xanh l cy cu thc vt l do c mt clorofil. Cht mu ny ng rtquan trng trong qu trnh quang hp. N khng nhng cho mu xanh m cn che

m cc cht mu khc. Clorofil c trong lc lp hoc ht dip lc, phn tn nguyn sinh cht. N chim khong 1% cht kh.


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Clorofil c hai dng: clorofil a v clorofil b. Clorofil b c mu nht hn. Tln gia clorofil a v clorofil b trong thc vt khong 3:1.

Mg+

N N

NN

X

CH2 CH2

CH3

OCOOCH3

CH2

CH2

COOC20H39

CH3

CH3

CHCH2

Hnh 7.1 Cu to ca clorofil a v b:a X l CH3; b X l CHO

Tnh cht

a) Di tc dng ca nhit v axit cu dch bo mu xanh b mt i, mtmt l do protein ng t lm v t bo b ph hu, mt khc l do lin kt giclorofil v protein b t lm cho clorofil d dng tham gia phn ng 7.1 takp ra

feofytin c mu xanh oliu:

Clorofil + 2 HX Feofytin + MgX2 (7.1)

b) Khi cho tc dng vi kim nh nh cacbonat kim, kim th th chng strung ho axit v mui axit cu dch t bo v to nn mi trng kim lm choclorofil b x phng ho cho ru phytol, metanol v axit clorofilinic.

Clorofil a + kim (C32H30ON4Mg)(COONa)2 + CH3OH + ru phytol

Clorofil b + kim (C32H28O2N4Mg)(COONa)2 + CH3OH + ru phytol

Cc axit (C32H30ON4Mg)(COONa)2v (C32H28O2N4Mg)(COONa)2thu c

do x phng ho clorofil a v b uwojc gi l clorofilin hoc clorofilit. Cc axitcng nh mui cu chng u cho sn phm mu xanh m.

Trong mt s cy cn c enzym chlorophyllase cng c th thu phn c

cc lin kt este ny gii phng ra phytol v metanol. Enzym ny thng cnh v trong cc sc lp, kh bn vi nhit v ch c hot ho trong thi gianchn.

c) Clorofil cng c th b oxy ho do oxy v nh sng (quang oxy ho), dotip xc vi cc lipit b oxy ho hoc do tc dng ca enzym lipoxydase. Cc qu

trnh oxy ho ny c th xy ra trong cc rau (nht l rau sy) bo qun m


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tng i di 30%; cn khi m tng i cu khng kh cao hn th clorofil lib bin i thnh feofytin (phn ng 7.1). Mt s hp cht bay hi c th lm tng

(etylen) hay lm chm (kh CO2) s bin ho cu clorofil.

d) Di tc dng cu Fe, Sn, Al, Cu th Mg trong clorofil s b thay th v s

cho cc mu khc nhau sau:

- Vi Fe s cho mu nu;

- Vi Sn v Al cho mu xm;

- Vi Cu s cho mu xanh sng.

Trong sn xut thc phm, c bit l trong sn xut hp rau, ngi tathng dng cc bin php sau y bo v c mu xanh dip lc:

- Gia nhit nhanh trong mt lng nc si ln (3-4 l/kg) gim hmlng axit. Axit lc ny s b bay i cng vi hi nc;

- Gia nhit rau xanh trong nc cng, cacbonat kim th s trung ho mtphn axit cu dch bo.

Trong thc t, bo v mu xanh cu u ng hp, ngi ta cho vo hp

mt t dinatri glutamat hoc nhum mu xanh cho u vng, ngi ta dngclorofylin (clorofil + kim). Clorofil d b hp th trn b mt cu ht u v gic bn mu trn b mt lm cho mu ht rt p.

2.2.2 Carotenoit

Carotenoit l nhm cht mu ho tan trong cht bo lm cho rau v qu c

mu da cam, mu vng v mu . Nhm ny gm t 65-70 cht mu t nhin, tiubiu l caroten, licopen, xantofil, capxantin v xitroxantin. Carotenoit c trong a scy (tr mt s nm) v hu nh c trong tt c c th ng vt. Hm lngcarotenoit trong l xanh chim khong 0,07-0,2% cht kh, tng trng hp hnhu ngi ta thy nng carotenoit rt cao, v d trong bao phn cu nhiu loi

hoa loa kn c nhiu xantofil v anteracxantin.

Tt c carotenoit khng ho tan trong nc, rt nhy i vi axit v chtoxy ho, nhng li bn vng v kim. Mt trong nhng c im cu carotenoit lc nhiu ni i lun hp to nn nhng nhm mang mu cu chng. Mu cu

chng ph thuc vo nhng nhm ny.Tt c carotenoit t nhin c th xem nh dn xut cu licopen.

2.2.2.1 Licopen

Licopen c trong qu c chua v mt s qu khc. Mu cu c chua chnch yu do c mt licopen mc d trong c chua cng mt lot cc cait khac n

nh: fitoflutin, -caroten, -caroten, -caroten, licopen. Trong qu trnh chn, hm


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lng licopen trong c chua tng ln 10 ln. Tuy nhin, cht mu ny khng c hottnh vitamin. Licopen c cu to nh sau:

CH3

CH3CH3 CH3 CH3

CH3

CH3 CH3CH3CH3

licopen

Bng cch to thnh vng mt u hoc c hai u cu phn t licopen th

s c cc ng phn . , -caroten. Mu da cam cu c rt, m ch yu l do

caroten. Caroten c cu to nh sau:

CH3

CH3CH3 CH3 CH3

CH3

CH3 CH3CH3CH3

-caroten

CH3CH3 CH3 CH3

CH3

CH3 CH3CH3CH3

-caroten

CH3CH3

CH3 CH3

CH3

CH3 CH3CH3CH3CH3

-caroten

Tt c nhng carotenoit t nhin khc u l dn xut cu licopen v caroten.Chng c to thnh bng cch a nhm hydroxyl, carbonyl hot metoxy vo

mch, nh phn ng hydro ho hoc oxy ho.2.2.2.2 Xantofil

Xantofil c cng thc C40H56O2v c c bng cch gn thm hai nhm

hydroxyl vo phn t -caroten, do c tn 3, 3-dihydroxy -caroten v 3, 3-

dihydroxy -caroten vi t l 2:1. Xantofil cng vi caroten c ch trong rau xanh

v cng vi caroten v licopen cu c chua.


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CH3CH3

CH3 CH3

CH3

CH3 CH3CH3

CH3CH3

O

OH

O

OH

Xantofil

2.2.2.3 Capxantin

Capxantin c cng thc C40H58O3 l cht mu vng c trong t .Capxantin chim 7/8 tt c cht mu cu t. Capxantin l dn xut cu caroten,nhng c mu mnh hn cc carotenoit khc 10 ln. Trong t c cc carotenoitnhiu hn trong t xanh 35 ln.

2.2.2.4 Criptoxantin

Criptoxantin c cng thc nguyn C40H58O3(3- hoc 4- hydroxy -caroten).

Mu da cam cu qut, cam ch yu l do criptoxantin.

2.2.2.5 Birxin

Birxin l cht mu c trong qu cy nhit i tn l Bixa orellana, birxin

c dng nhum mu du, margarin v cc sn phm thc phm khc. Birxin lsn phm oxy ho cu cc carotenoit c trong 40 nguyn t cacbon.

2.2.2.6 Xitroxantin

Xitroxantin c cng thc C40H50O. Xitroxantin c trong v qu chanh.

Xitroxantin c c khi kt hp vo phn t -caroten mt nguyn t oxy to

thnh cu trc furanoit.

2.2.2.7 Astarxantin

Astarxantin cng l dn xut cu caroten v c tn gi l 3, 3-dihydroxy -

4,4-dixeto--caroten.

Trong trng cu loi gip xc c cht mu xanh ve gi l ovoverdin c thcoi nh l mui dng endil cu astarxantin v nhm amin cu protein (kt hp cu

astarxatin v protein).

Trong mai v gip cu cua, tm astarxantin cng tham gia vo thnh phncu lipoprotein gi l xianin.

Trong qu trnh gia nhit do protein b bin tnh v astarxantin b tch ra

di dng cht mu .


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2.2.3 Cc cht mu nhm flavonoit

Cc chtmu ny ho tan trong nc v ch trong cc khng bo. Trongrau, qu v hoa, s lng cng nh t l cac flavonoit khc nhau, do lm chochng c nhiu mu sc khc nhau: t mu n mu tm. flavonoit l nhng dn

xut cu croman v cromon. Croman v cromon l nhng phenylpropan v c chb khung cacbon C6-C3. Khi croman hoc cromon ngng t vi mt vng phenoln th c dn xut c tn l flavan.

O O O

croman cromon flavan

Da vo mc oxy ho (hoc kh) ca d vng, c th chia flavonoit ra cc

nhm c dng cu trc ch yu nh sau:O

OH

catesin (flavanol-3)

O

OH

OH

leucoantoxianidol(flavandiol-3,4)

O+

OH

antoxianidol

O

O

flavanon

O

O

flavanon

O

OH

O

flavanodol

O

O

OH

flavonol izoflavonO

chalcon

O

O


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O

CH2

O

auron

2.2.3.1Antoxian

Antoxian hay l antoxianozit l mono- diglucozit do gc ng glucose,

galactose hoc rhamnose kt hp vi gc aglucon c mu gi l antoxianidin. Do, khi thu phn antoxian th c ng v antoxianidin (antoxianidol). Antoxianho tan trong nc, cn antoxianidol th khng ho tan trong nc. Ccantoxianidol v cc antoxian l nhng cht to nn mu sc cho hoa v qu. Cc

antoxianidol c mu , xanh, tm hoc nhng gam mu trung gian.

Tt c antoxianidol u c ch trong vng pyran oxy ho tr t do. Tuy

nhin ngi ta cha bin c chc chn nguyn t no: oxy hay cacbon mang intch dng t do. Do antoxianidol thng c biu din di dng cng thc

trung tnh:

O+

OH

O

OH

Cl-

Cng thc thng Cng thc trung tnh

Nh in tch dng t do ny m cac antoxianidol trong dung dch axit tcdng nh nhng cation v to mui c vi cc axit. Cn trong dung dch kim thantoxianidol li tc dng nh anion v to mui vi baz.

Thng gp ba antoxianidol chnh l: pelargonidol, xianidol v delfinidol.

Cn apigenidol th t gp hn.

O+

OH

OH

OH

OH O

+OH

OH

OH

pelargonidol apigenidol


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O+

OH

OH

OH

OH

OH

O+

OH

OH

OH

OH

OHOH

xianidol delfinidol

O+

OH

OH

OH

OH

OCH3

O+

OH

OH

OH

OCH3

OCH3OH

xianidol xirinhidol (malvidol)

Tnh cht cu antoxian

Ni chung cc antoxian ho tan tt trong nc v trong dch bo ho. Khikp hp vi ng lm cho phn t antoxian cng ho tan hn.

Mu sc cu cc antoxian lun lun thay i ph thuc vo nhit , cc chtmu c mt v nhiu yu t khc,...Khi tng s lng nhm OH trong vng benzenth mu cng xanh m (trong vng benzen c th c 1-2 hoc 3 nhm OH)

Mc metyl ho cc nhm OH trong vng benzen cng cao th mu cng. Nu nhm OH v tr th 3 kt hp vi cc gc ng th mu sc cng s thayi theo s lng cc gc ng c nh vo nhiu hay t.

Cc antoxian cng c th to phc vi cc ion kim loi cho cc mu khcnhau: chng hn mui kali s cho vi antoxian mu mu, cn mui canxi v

magie s cho vi antoxian phc c mu xanh ve. Hoc ngi ta cng thy phc bn

t en s chuyn sang mu xanh, cn anh o th chuyn sang mu tm khi c mtthic, anh o cng s c mu tm khi c mt nhm, nhng nhm li khng nhhng n mu cu nho . Cc antoxian cu nho ch thay i ng k khi c Fe,

Sn hoc Cu. im ng ch l mu sc cu cc antoxian ph thuc rt mnh vopH mi trng.

Thng thng, khi pH>7, cc antoxian cho mu xanh v khi pH


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Khi pH=2,4-4,0 th c mu thm; pH=4-6 th c mu tm; pH=6 th cmu xanh lam; pH l kim th c mu xanh l cy.

Hoc nh xianin c trong hoa hng, mu sc s thay i khi pH cu dch bothay i.

Mu sc cu antoxian cn c th thay i do hp th trn polysacarit.Khi un nng lu di cc antoxian c th ph hu v mt mu, c bit l cc

antoxian cu du ty, anh o, c ci. Ngc li, cc antoxian cu phc bn t encng trong iu kin li khng b thay i. Nhn chung khi gia nhit, cac chtmu d dng b ph hu, cn cht mu vng th kh hn.

Cc mu sc khc nhau cu hoa c c l do t hp cc antoxianidin v cac

este metylic cu chng vi axit v baz.

Tm li, trong mi trng axit, cc antoxian l nhng baz mnh (oxonium)

v c th to mui bn vng vi axit. Antoxian cng c kh nng cho mui vibaz. Nh vy chng c tnh cht lng tnh. Mui vi axit th c mu , cnmui vi kim c mu xanh.

Trong sn xut hp, bo v mu sc t nhien, ngi ta c th cho thmcht chng oxy ho, v d, thm rutin bo v mu cu anh o v mn, thm

tanin, axit ascorbic bo v mu cu du v anh o. Mt bin php khc c thbo v c mu t nhin cu cc hp rau qu l thm enzym glucooxidase,chng hn nh thm gluco oxidase gi mu t nhin cu nc qu c...

Cc dn xut cromon ng ch hn c l flavanon v flavonol2.2.3.2 Flavanon

So vi cc flavonoit khc th cc flavanon t gp hn. Hesperidin v naringinl hai flavanon c trong v cam qut bn cnh cc catesin c hot tnh vitamin P.Aglucon cu cc glucozit ny l hesperetol v naringenol:

O+

OH

OH

OCH3

OH

O

O+

OH

OH

OH

O

hesperetol naringenol

Naringin l diholozit (vi glucose v rhamnose) thng gy ra v ng cubi nht l bi trc khi chn.


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OOH

OH

OH

O

OH R'

R

OO

OH

OH

O

O

HH

H

OH

OH

H O

H

CH2OH

O

CH3

OH

OH OH

naringin

Naringin l mt flavonoit khng mu, t tan trong nc, d dng kt tu di

dng cc tinh th nh do gy kh khn cho sn xut nc qu cng nh puree

cam qut. Khi qu chn, enzym nariginase phn ct lin kt glucose-rhamnose do mt v ng.

2.2.3.3 FlavonolFlavonol l glucozit lm cho rau qu v hoa c mu vng v da cam. Khi

flavonol b thu phn th gii phng ra aglucon mu vng. Cc glucozit nhmflavonol rt nhiu, nhng thuwofng gp hn c l nhng aglucon dn xut sau y:

Kempherol R=R=H

Querxetol R=OH, R=H

Mirixetin R=R=OH

Astragalin l 3-glucozit cu kempherol, c trong hoa t vn anh, hoa dnga, trong l ch v trong hoa hng. Querxetol c trong v si, l ch, to, nho,thuc l, ho huplng (hoa bia).

Rutin l 3-rhamnoglucozit cu querxetol rt thng gp trong cy. Rutinc ng dng rng ri trong y hc lm thuc b mao mch.

Cc flavonol u ho tan trong nc. Cng mu cu chng ph thucvo v tr nhm OH: mu xanh nht khi OH v tr orto.

Flavonol tng tc vi st cho ta phc mu xanh l cy, sau chuyn sangmu nu. Phn ng ny thng xy ra khi gia nhit rau qu trong thit b bng st

hoc bng st trng men b dp.

Vi ch axetat, flavonol cho phc mu vng xm. Trong mi trng kim,

flavonol rt d b oxy ho v sau ngng t to thnh sn phm mu .


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2.3 Cc cht mu hnh thnh trong qu trnh gia cng k thut

Trong cc nguyn liu a vo ch bin thc phm, thng cha mt t hpcac cht khc nhau. Trong qu trnh gia cng c nhit, chng s c tng tc vinhau to thnh nhng cht mu mi c nh hng tt hoc xu n thnh phm.

Cc phn ng to mu trong thc phm thng phc tp v a dng. Chnghn nh mu sc cu nc ch l do phn ng oxy ho cc polyphenol bng enzym,

mu sc xut hin khi sy malt, khi nng bnh m hoc trong cc sn phm chbin ht khc li hu nh ch yu do phn ng ozamin. Nu nh trong cc sn

phm bnh ko, mu c to nn ch yu l do phn ng caramen ho cc ngth trong ch bin rau qu mu c hnh thnh nn cn do mt lot cc phn ng

khc na.

Chung quy li, cc phn ng to mu l nhng phn ng oxy ho v nhngphn ng c enzym xc tc khng c enzym xc tc. l phn ng trng hp ho

cc phm vt oxy ho cu polyphenol v phn ng gi bisflavonoit vi axit amin.Chng gm c:

- phn ng gia ng v axit amin;

- phn ng dehydrat ho cc ng hay l phn ng caramen ho;

- phn ng phn hu axit ascorbic, axit limonic, axit maleic, axit tartric v

mt s axit hu c khc;

- phn ng oxy ha cc hp cht cu st v to thnh phc c mu;

- phn ng to nn cc sulfua kim loi c mu.

2.3.1 To mu mi do phn ng caramenPhn ng caramen ho c nh hng ln n mu sc cu cc sn phm giu

ng nh bnh, ko, mt...Phn ng xy ra mnh m nhit nng chy cung. Chng hn, vi glucose 146-150 oC, fructose 95-100 oC, saccharose 160-180 oC, lactose 223-252 oC. Tuy nhin, ph thuc vo nng ng, thnh

phn pH cu mi trng, thi gian un nng,...ngi ta vn tm thy cc sn phmcu s caramen nhit thp hn im nng chy cu ng. V d, saccharosec th bt u bin i ny khi nhit 135 oC.

Giai on u cu phn ng to nn cc anhydrit cu glucose, fructose,

saccharose nh glucozan, fructozan, sacarozan l nhng hp cht khng mu. Sau, bn cnh s dehydrat ho cn xy ra s trng hp ho cc ng ngdehydrat ho to thnh cc phm vt c mu nu vng.

Chng hn, vi saccharose, s phn ng caramen nh sau:

C12H22O11 C6H10O5 + C6H10O5 + H2O

n 185-190 oC s to thnh izosacarozan:


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glucozan + levulozan izosacarozan

C6H10O5 + C6H10O5 C12H20O10

Khi nhit cao hn s mt i 10% nc v to thnh caramelan(C12H18O9hoc C24H36O18) c mu vng:

2C12H22O10 (C12H18O9) hoc C24H36O18 + 2H2Oizosacarozan caramelan

Khi mt i 14% nc s to thnh caramelen:

C12H22O10 + C24H36O18 C36H48O24.H2O + 3 H2O

V khi mt i 25% nc s to thnh caramelin c mu nu en.

Cn ch rng tt c cc sn phm caramen ho u c v ng.

2.3.2 S to mu mi do phn ng melanoidin

Phn ng ozamin, cacbonylamin, aminoza hay phn ng melanoidin l phn

ng c vai tr c bit qun trng trong k thut sn xut thc phm. Cc hp phntham gia phn ng l protein (hoc cc sn phm phn gii cu chng) v

carbohydrat.

iu kin phn ng xy ra c l cht tham gia phn ng phi c nhm

carbonyl. Khc vi phn ng caramen ho, phn ng ozamin i hi nng lnghot ho b hn, nhng tin hnh phn ng bt buc trong mi trng phn ng

phi c nhm amin hoc amoniac.

Da vo mc v mu sc cu cc sn phm c th chia thnh ba giai on

k tip nhau. Sn phm cu giai on u khng mu v khng hp th nh sng

cc tm. Giai on ny bao gm hai phn ng: phn ng ngng t carbonylamin vphn ng chuyn v Amadori. Sn phm cu giai on th hai khng mu hoc cmu vng, nhng hp th mnh nh sng cc tm. Giai on ny bao gm phn ng

kh nc cu ng, phn hu ng v cc hp cht amin. Sn phm cu giaion cui c mu m. Giai on ny gm c phnng ngng t aldol, trng hpho aldehytamin v to thnh hop cht d vng ch nit.

Thc t trong hn hp phn ng c ch ng thi tt c cc sn phm,nhng t lng cu sn phm ny hay sn phm khc chim u th l ph thuc

mc tin hnh cu phn ng.

2.3.2.1Cc giai on cu phn ng to melanoidin

a) Giai on u cu phn ng to melanoidin

Phn ng ngng t cacbonylamin

Giai on u tin ca s to thnh melanoidin l s ngng t ng vi axitamin. Trong iu kin nhit sinh l th to thnh baz Schiff (hp cht hu c cnhm -HC=N-) hoc glycoside:


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CH2OH

O

H OH

OH H

H OH

H OH

+ NH2 R-H2O

+H2O

C

CH2OH

N

H OH

OH H

H OH

H OH

R

C

CH2OH

N

H OH

OH H

H OH

H

R

O

Glucose bazo schiff N-glucoside

Trong iu kin sn xut khi nhit cao, phn ng c bt u khng

phi t N-glycoside m t sacaroamin (ng amin) theo s :

C

C

R'

O

H OH

H COOH

C

H

N R"H

H

+

C

C

R'

O

H

H COOH

C

H

N R"

H

ng hp cht amin phc ng amin

Phc ng amin c to thnh khng mu v khng c tnh cht no cumelanoidin c.

Phn ng ny c th tin hnh ch lnh v qua mt vi gi th t c cnbng. Nhng c th tng rt nhanh khi un nng hn hp phn ng.

Phn ng chuyn v AmadoriKhi nhit cao th phc ng amin b ng phn ho hay ngi ta cn

gi l b chuyn v ni phn Amadori. Kt qu l gia nguyn t cacbon th nht vth hai pht sinh ra ni kp v to thnh 1-amin-1dezoxy-2-ketose:

C1

C2

R'

O

H

H COOH

C

H

N R"

H

C2

R'

O

CH21

COOH

C

H

NH R" CR'

OH

CH2

COOH

C

H

NH R"

Phc ng amin 1-amin-1-desoxy-2-ketose dng enolca 1-amin-1-desoxy-2-

ketose

Phm vt cu s chuyn v Amadori l hpo cht c kh nng phn ng v l

cht khi u to thnh polyme c mu sm gi l melanoidin.


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b) Giai on trung gian cu phn ng melanoidin

giai on ny xy ra s kh nc cu 1-amin-1-desoxy-2-ketose to thnhcc sn phm phn ly khc nhau. Ph thuc vo iu kin cu mi trng v nhit m giai on trung gian c th tin hnh bng mt vi con ng. S kh to

thnh furfurol hoc hydroxymetylfurfurol l mt trong nhng con ng kh nccu desoxyketose. Mt con n gkhc l to thnh reducton mch h. Con ngthc ba l phn hu hp phn ng to thnh aldehyt, eton, diaxetyl. Conng th t l to thnh ozon.

To thnh furfurol v ozon

Nu cu t u tin cu s to thnh melanoidin l glucose, th sn phm

chuyn v Amadori khi un nng s chuyn sang dng furan v sau thnh baz

Schiff cu hydroxymetylfurfurol:

C

C

C

C

C

CH2OH

NH

OH

OH H

H OH

H OH

R

-H2O

O

CH

C

CH OHOH

CH NH RH2COH

-H2OC

O

HC

C

CH

CH NH H2COH

1-amin-1-desoxy-2-ketose baz shiff cahydroxymetyl

furfurol

Nu ng khi u khng phi l hexose m l pentose th to thnh baz

Schiff cu furfurol.

Sau ny khi b thu phn, baz Schiff cu hydroxymethylfurfurol hoc cufurfurol c th b phn hu to thnh hydroxymetylfurfurol hoc furfurol v axitamin t do theo s :

C

O

CH

C

CH

CH NH RH2COH

+H2OC

O

CH

C

CH

CH2COH

H

O

NH2 R+

hydroxymetylfurfurol axitamin

i khi baz Schiff cu hydroxymetylfurfurol v cu furfurol cng c th

ngng t to thnh melanoidin ch nit. Trong mi trng axit hoc trung tnhv nhit cao th furfurol v cc dn xut cu n s to thnh trc tin.

Mt s tc gi tm thy mt con ng kh nc khc cu desoxyxetose.Ban u desoxyketose phn ng vi phn t glucose to thnh difuranozamin.


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Sau difuranozamin b phn hu to thnh 3-dezoxyglucozon, cac ozon khngno dng cis v trans:

C

C

C

C

C

CH2OH

OH

O

H H

H OH

H

O

C

C

C

C

C

CH2OH

OH

O

H

H

H

O

C

CC

C

C

CH2OH

OH

OH

H

H

O

3-dezoxyglucozon glucozon khng no dng cis glucozon khng no dng trans

Hydroxymetylfurfurol cng c th c to thnh. Cc tc gi ny cho rng

cc sn phm v k trn l nhng cht tin thn c bn cu cc polyme c mu nu

sm.To thnh reducton c 6 cacbon

Cc aminodezoxyketose c to thnh khi chuyn v Amadori, c th bkh i hai phn t H2O to thnh cc reducton c 6 cacbon.

Reducton l nhng hp cht hu c c tnh cht chung l kh mnh do c

mt nhm endiol:

C C

OH OH Theo thng l, nhm endiol cu reducton c lin kt vi gc aldehyt

hoc gc axit. V d:

CH C C

OH OH H

O

Do , nhng hp cht ny rt nhy vi phn ng oxy ho v kh.

Chng hn, reducton n gin nht l triozoreducton:

CH C C

OH OH H

O

Cht ny thc cht l enol cu hydroxymetylglyoxal:

CH2 C C

OH O H

O

CH C C

OH OH H

O

Hydroxymetylglyoxal reducton hay l enolaldehyd


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ca axit tartric

S t hp cu nhm endiol vi nhm carbonyl lm cho cc cht ny c khnng nhy cm c bit vi phn ng oxy ha v kh.

Kh nng phn ng cu reducton c th dn n s kh hydro chuyn

nhm endiol thnh nhm carbonyl

CH C R2

OH OH

R1

2HC C R2

O O

R1

reducton dehydroreducton

D nhin, reducton c th d dng kt hp vi oxy v chnh l iu rtquan trng cho s bn vng cuc cvat.

Reducton khng kh dung dch Fehling li d dng kh iot v 2,6-diclophenolindophenol.

Axit dehydroascorbic c th coi l mt v d cu reducton c su cacbon.

Khng phi ch ring reducton m cac dng dehydro cu chng cng tham

gia vo vic to nn mu nu.

Phn hu ng

Mt trong nhng giai on trung gian cu phnng to melanoidin l s phnhu cu t ng cunphm chuyn v Amadori to thnh cc phm vt khc

nhauL triozoreducton, aldehyt pyruvic, axeton, axetoin,

diaxetyl.Mt s cht to thnh kh phn hy ng c mfui

vi v v d chu do quyt nh cht lng cu sn

phm.

Phn hu cc hp cht amin

Trong qu trnh to nn melanoidin th CO2, aldehyt v NH3l nhng phmvt cu phn ng khi nhit cao. Nguyn do cu hin tng ny l s phn hu

Strecker theo nhng hng khc nhau:

1/ Phm vt chuyn v Amadori c th kt hp vi axit amin thot ra CO 2v H2O:

CH NH2

COOH

R + CH CH2

O

R1

CHNH

COOH

R2

CH CH2

R1

CHNH

COOH

R2

CH2 NR

-CO2

-H2O

C

C

C

CC

CH2OH

O

O

OH

OO

H

Axit dehydroascorbic


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Baz Schiff c to thnh b phn ly thy phn thnh aldehyt v hp chtamin. Chnh hp cht amin ny s cho cc phm vt ch ni t v c mu nu sau

ny. Cn aldehyt c to thnh trong phn ng ny, so vi axit amin tham giatng tc vi aminodezoxyketose th c t hn 1 cacbon.

C

N

C

C

N

C

H

R

H

H

R1

H

H

R2

COOH

H

H2OR C

H

O

NH2

C

C

N

C

H

H

R1

H

H

R2

COOH

H

+

2/ Gi furfurol hoc hydroxymetylfurfurol v axit amin c th xy ra s

tng tc oxy ho:

C

O

CH

C

CH

C

H

O O2C

O

CH

C

CH

C

OOH

O

furfurol axit furfurolic

C

O

CH

C

CH

C

OOH

O

HC NH2

COOH

R

+ C NHCOOH

R

C

O

CH

C

CH

C

OH

O

+ + OH2

axit amin axitimin axit furfurolic

C NH

COOH

R

+ OH2 C OCOOH

R

+ CO2 + NH3

aldehyt

Kt qu l t axit amin to thnh aldehyt c t hn mt nguyn t cacbon.

3/ Aldehyt cng c th c to thnh do kt qu cu s chuyn amin gi

axit amin vi reducton

HC NH2

COOH

R

+ C C R2O O

R1 C O

H

R

+ CH C R2NH2 O

R1 + CO2


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Gi s, do s chuyn amin m dn n hnh thnh nn aldehyt l phn ngch yu, th r rng l do kt qu cu phn ng m ni t tham gia vo polyme

mu nu. Phn ln CO2thot ra trong qu trnh to mela

nu do axit amin b phn hy Strecker. Cc aldehyt to thnh ng vai tr

quan trng trong vic hnh thnh hng v cu sn phm thc phm khi ch binnhit.

c) Giai on cui cng cu phn ng to melanoidin

Giai on cui cng ny bao gm rt nhiu phn ng phc tp. C th chiathnh hai kiu phn ng:

- Phn ng ngng t aldol vi s to thnh polyme mu nu khng ch

nit;

- Phn ng trng hp ho aldehytamin vi s to thnh cc hp cht nito

d vng.

S ngng t aldehyt axetic c th cng l v d v s ngng t aldol:

H3C C

H

O

H2CH C

H

O

+ H3C COH

CH2 C

H

O

aldehyt axetic aldol

Khi nguyn t hydro cu nhm metyl cu mt phn t aldehyt ny clin kt vi nguyn t oxy cu nhm carbonyl cu phn t aldehyt th 2.

S trng hp ho v oxy ho ng thi cc aldehyt aminoaxetic dn n to

thnh pyrazin.NH2

CH2

CHO

+

NH2

CH2

CHO+O2

N

N

+ OH23

Pyrazin c mi d chu nhng yu, ho tan trong nc.

Khi trng hp ho dehydroreducton vi amoniac v aldehyt s to thnh hp

cht phc tp c ch vng imidazol:

C

CR2

O

O

R1 +

NH3

NH3

R3C

H

O

C

NH

C

C

N

R3

R2

R1

+ OH23

imidazol dn xut

Ngoi nhng hp cht k, do kt qu cu phn ng ngng t aldehyt,

pyridin v pyrol cng c th c hnh thnh.


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S ngng t aldol ch tin hnh c khi caramen ho dung dch ngthun khit. Khi c mt cc hp cht amin mc du ch l vt cng xy ra ng thi

c hai kiu phn ng.

Giai on cui cng cu phn ng melanoidin s to nn u tin l cc

polyme khng no ha tan c trong nc, sau n l cc polyme khng no vkhng ho tan c trong nc, nhng u c mu m v c cng tn gi chung lmelanoidin.

Cc melanoidin hp th mnh tia cc tm (


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nc l iu kin cn thit tin hnh phn ng. Nng cht tc dng cng cao,lng nc cng t th to thnh melanoidin cng mnh.

c) nh hng cu nhit v pH mi trng

0 oC v di 0 oC, phn ng melanoidin khng xy ra. Cng vi s tng

nhit , vn tc phn ng tng ln rt mnh m. cc nhit khc nhau, cc snphm to thnh cng khc nhau. Ngi ta nhn thy khi nhit t 95-100 oC,

phn ng s cho cc sn phm c tnh cht cm quan tt hn c. Khi nhit qucao th cc melanoidin to c s c v ng v mi kht. V vy trong sn xut

thu c malt mu, ngi ta thng khng ch phn ng melanoidin 160 oC (mcd lng cht mu s cc i khi nhit 190 oC). V l nhit trn 160 oC

cc melanoidin to thnh s khng ho tan trong nc, do lm gim kh nngcho mu v trch ly cu malt thu c.

Phn ng Maillard c th tin hnh trong mt khong pH kh rng, tuy nhin

trong moi trng kim phn ng nahnh hn. Trong khi mi trng axit (pH


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Axit sulfur cn th hin tc dng khng khun mnh m v l cht chngoxy ho. Axit sulfur n nh c axit ascorbic dng hydroascorbic trong cc

sn phm. Ngi ta nhn thy rng, trong mt s sn phm (nc chanh, nc nho,nc to) khi lng axit ascorbic b gim th cng sm mu tng ln, lng axit

amin v ng kh b gim xung cn hm lng furfurol li tng ln. C l do

tnh kh cu axit ascorbic v kh nng phn ng cu n vi cc sn phm thcphm s lm yu s to thnh cc melanoidin. Tuy cht km hm axit ascorbic nylm tng gi tr sinh hc cu sn phm, nhng nghi thc tin li hn ch, v axit

ascorbic cng nh nhng axit hu c khc, khi phn hu s cho hp cht c musm. Ngoi ra, axit ascorbic l mt reducton, nghi l mt sn phm trung gian cu

phn ng melanoidin m sau ny s tng tc vi axit amin to thnh cht cmu sm.

Cn thy rng, trong cc sn phm c mu m do antoxian truyn cho, thdi tc dng cu vitamin C, mu t nhin s b nht i. Nh vy dng axit ascorbickm hm s sm mu l khng c gi tr vn nng. Ngi ta ch c th ng dngn i vi tng sn phm c th khi nghin cu k thnh phn ho hc cu n.

Cht km hm phn ng melanoidin tt nht l dimedon(dimetyldihydroreorxin). N c kh nng lin kt cc furfurol v aldehyt do lm

ngng s to thnh melanoidin.

Dimedon trong nhng iu kin nht nh c th lm kt tu hon tonfurfurol bng cch to ra sn phm ngng t (furfuroldimedon) (hai phn t). Cccht oxy ho (KmnO4, H2O2, quinhydron) v cc cht kh (H2SO3, natri hyposulfit,

hydroquinon, hydro mi sinh) u km hm phn ng to melanoidin nhng vicng khc nhau:

Gn y, ngi ta cn dng este cu axit pyrocacbonic lm cht km hmphn ng melanoidin. Este c cu to nh sau:

R1v R2c th l gc alkyl, aryl hoc cc gc d vng (furfuryl).

Trong mt s sn phm thc phm ngi ta cn dng cc anhydrit cacbonic

cu cc -hydroaxit nh sau:

V phng din l thuyt, hiu qu km hm cu pyrocacbonat cng nh cu

anhydrit cacbonic th hin ch chng tng tc vi nhm amin t do. Do , axitamin tr nn khng c kh nng phn ng vi nhm cacbonyl cu ng.

Cc cht tng tc phn ng melanoidin c axit lactic v phosphat. Mui cuaxit lactic lm tng nhanh vn tc phn ng ny rt mnh. Dung dch m phosphat


49/66

khng nhng lm tng nhanh vn tc phn ng gi axit amin v glucose m cnlm tng phn ng nu ho dung dch glucose tinh khit.

2.3.2.1Tnh ph bin cu phn ng Maillard

Nh nu trn, phn ng gi axit amin v ng xy ra trong iu kin

rt d dng, do phn ng ny rt ph bin trong sn xut v bo qun cc snphm thc phm. Tu thuc vo yu cu v tnh cht cm quan cu tng sn phm

m ngi ta hoc to iu kin tng cng phn ng n ti a hoc km hmphn ng n mc ti thiu.

Mt tng nhng qu trnh to iu kin phn ng ny pht trin ti a lsn xut bnh m. Mu sc cu v bnh m hu nh do phn ng ny quyt nh. V

vy nhng bin php k thut tng ng trong quy trnh sn xut ln men to

axit amin t do, ng kh, iu chnh nhit tng giai on nng u nhmmc ch .

Trong sn xut bia (v cvat) mu sc v hng v cu bia phn ln do maltquyt nh. Cc bin php k thut tng sn xut malt nhm iu ho phn ngmelanoidin. thu c malt vng, ngi ta chomc mm trong mt thi gian

ngn, lm mt nc nhanh thu c lng axit amin v t ng lm kh khncho phn ng carbonylamin. Theo Manxev, do thu phn axit amin v protein m

tch t nhng hp cht c kh nng to thnh cht thm. Malt sy nhit thpqu s nh huworng xu n v cu bi, cn khi sy nhit qu cao, bia s c v kht.

V vy, khi sn xut bia vng cn phi kim tra s to thnh melanoidin v hn chthi gian un si dung dch ln men.

Tri li trong sn xut ru ngi ta tm cch km hm phn ng Maillardmt cch trit . V phn ng s gy tn tht tinh bt v ng, n gthi

melanoidin to thnh c tc dng km hm hot ng cu enzym.

Trong sn xut thuc l, qu trnh sy, trc khi ln men v ln men cng do

phn ng Maillard quyt nh. Vic lm gim m cu mi trng s to iukin thun li cho phn ng v tng cng mu sm. Nguyn nhn c bn cus t m ho thuc l l phn ng melanoidin c km theo thot nc. Khi t l

gi axit amin v ng bng 1/2 th lng m thot ra l cc i v lng cht khl cc tiu. Thay i t l ny s c nh hng nhanh chng n lng nc thot

ra trong phn ng.Trong sn xut ng, khi c c, ng b sm mu cng l do phn ng

melanoidin. Phn ng melanoidin cn c nh hng ln n vic ch bin rau qucng nh bo qun chng. Ngi ta thy rng do phn ng gi ng nghch

o v axit amin hoc mui amon m tt c sir v nc qu c c u b sm mukhi bo qun, nht l nhit cao. Trong hp qu v rau, ngi ta c khuynh

hng bo v mu t nhin v hng thm cu chng. Phn ng Maillard lun lun


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lm xu i khng nhng tnh cht cm quan m c gi tr thc phm cu sn phmn.

Tm li, phn ng melanoidin rt ph bin trong mi sn phm thc phmc lin quan vi gia nhit v thi gian bo qun di.

2.3.3 S to thnh cc cht mu do phn ng oxy ho polyphenolBn cnh phn ng melanoidin, cn c phn ng to mu do oxy ho cc

polyphenol c enzym v khng enzym tham d. Ngi ta cng gi l phn ng smmu do enzym.

2.3.3.1 Cc c cht phenol

Hp cht phenol l nhng cht m phn t c cha vng benzen, trong cmt, hai hoc nhiu hn hai nhm hydroxyl. Da vo c trng cu b xng

cacbon, cc hp cht phenol t nhin c th chia ra lm ba nhm chnh:

- nhm hp cht phenol C6-C1(axit galic)

- nhm hp cht phenol C6-C3(axit cafeic)

- nhm hp cht phenol C6-C3-C6(catechin v flavonoit)

Kh nng phn ng to mu mnh hay yu ph thuc vo cu trc cu

phenol cng nh vo ngun enzym xc tc phn ng oxy ho ny. Chng hn, ccmetadiphenol thng l nhng c cht xu.

Cc c cht phenol ch yu cu phn ng to mu do enzym xc tc nhsau:

1/ Pyrocatechol v cc dn xut (ngoi tr gaiacol hay o-metoxyphenol)

2/ 3,4-dihydroxyphenylalanin (DOPA)

3/ 3,4-dihydroxyphenyletylamin hay dopamin

l cht chnh lm cho chui b sm mu.

4/ Axit galic

l axit c nhn thm, khi b oxy ho th cho mu sm, ho tan c trong

nc.

5/ Axit clorogenic, c trong to, khoai ty v l, l cht c kh nng to racht mu xanh en khi phn ng vi st trong qu trnh nu khoai ty.


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Khi gim pH xung nh hn 4 hoc dng cc tc nhn to phc c vi Fenh phosphat hoc axit etylendiamintetraaxetic (EDTA, trong iu kin cho php)

th c th trnh c phn ng to mu ny.

6/ Cc flavonoit, trong c antoxian, flavon v flavanol l nhng hp cht

nguyn thu c mu.

Catechin v leucoantoxianidol l nhng hp cht khng mu, d dng b oxy

ho (nht l khi gia nhit trong mi trng axit), mt nc chuyn thnh nhngsn phm (antoxianidol) c mu khc nhau.

Catechin hay catechol l tn gi chung cho cc phenol c dng flavanol-3

Quan trng nht trong nhm ny c catechin v galocatechin ch nhiu

trong l ch:

- Catechin (3,5,7,3,4-pentahydroxyflavan)

- Galocatechin (3,5,7,3,4,5-hexahydroxyflavan)

Trong phn t catechin v galocatechin c hai nguyn t cacbon bt ixng v tr C2v C3do to ra nhiu ng phn quang hc (24 ng phn).

7/ Tanin khng nhng l c cht cu phn ng sm mu do enzym xc tcm cn gp phn to ra kt cu v to v cho cc t chc thc vt. Ngi ta thngphn bit hai nhm tanin: tanin thu phn v tanin ngng t:

- Tanin thu phn (tanin pyrogalic) l este cu ng glucose vi cc axitpolyphenolic khc nhau (axit galic, axit digalic, axit egalic, axit luteic). Este cuglucose vi axit galic l galotanin, cu glucose vi axit egalic l egalotanin. Di

tc dng cu axit, kim hoc enzym chng b thu phn thnh cc cht ban u.

Chng hn nh tanin Trung Quc l -pentadigaloylglucose.

Tanin c tnh cht thuc da l hp cht c khi lng phn t t 500-3000,

l este cu axit digalic v glucose.- Tanin ngng t (hay tanin catechic) l hp cht do cc catechin ngng t

vi nhau theo kiu u ni ui (C6-C8vi catechin) hoc kiu ui ni ui(C6-C6, vi galocatechin) hoc kiu u ni u (C4-C8vi flavandiol -3,4) to

thnh. Tan nin ngng t khng b phn gii bi axit v c cng nh bi enzym.

2.3.3.2 Tnh cht chung cu cc hp cht polyphenol


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a) Phn ng oxy ho kh

Di tc dng cu enzym polyphenoloxydase, cac polyphenol b oxy ho to thnh cc quinon, S phn ng nh hnh bn.

Bn thn cc ortoquinon cng l nhng cht oxyhoas. Nu c cc cht kh

khc nh vitamin C, glucose, chng s ngng t vi nhau v s oxy ho cc cht theo s sau:

b) Phn ng cng

Khi c mt cc axit amin th cc ortoquinon c to thnh s tin hnhphn ng cng hp vi axit main:

Cc sn phm c to thnh cu phn ng cng li c th b oxy ho tonn cc ortoquinon tng ng.

c) Phn ng ngng t

Cc ortoquinon cng d dng ngng t vi nhau to nn cc sn phm cmu gi chung l flobafen

Polyphenol + O2 ortoquinon (1)

Ortoquinon + glucose polyphenol + CO2 + H2O (2)

n (ortoquinon) flobafen (3)

Trong iu kin h hp bnh thng th phn ng (1) v (2) chim u th.

Tri li, trong iu kin sn xut th phn ng (1) v (3) chim u th.

2.3.3.3 Chc nng cu cc polyphenol

Cc polyphenol c tc dng c ch nm. Ngi ta thng dng phenol (axitcacbovic) lm cht kh trng cng vi nghi . Thc ra bnh thng trong

thc vt, tc dng th hin tng i yu, nhng s tng lnmt cch mnh mtrong qu trnh polyphenol b oxy ho bi enzym. Thm ch, mt s polyphenol vi

mt nng nht nh c tc dng kch thch s pht trin cu k sinh trng, cntc dng km hm ch th hin ra trong qu trnh oxy ho chng bi enzym. Tuynhin, ng vai tr quan trng trong hin tng min dch thc vt ny khng phi

ch mt mnh polyphenol m l h polyphenol-polyphenoloxydase. Ngi ta chng minh rng trong nhiu cy b dp c hc hay b nhim trng th hm lng

polyphenol v hot polyphenoloxydase u tng ln. ch m b dp nt btu to thnh hp cht polyphenol mi v km theo s ngng t oxy ho trong lpb mt. Sn phm ngng t to thnh lp bo v.

Nhiu polyphenol c hot tnh sinh t P, nghi l c kh nng lm tng

n hi v chun ho tnh thm cu vi ti huyt qun. Trong s , catechin c hot


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tnh vitamin P cao hn c. Cn ch rng, tt c flavonoit c hot tnh sinh hc ubt buc phi c hai nhm ortohydroxyl , trong hydro cu mt nhm c th c

thay th bng gc bt k. Vng chnh l florogluxin, vng ph l pyrocatechic.

Polyphenol l nhng cht chuyn hydro (electron) trong chui h hp bnh

thng nh c ty th. Ubichinon (coenzym Q) l mt trong nhng cht ny.Nhiu polyphenol l nhng cht chng oxyho, do chng c ng dng trongcng nghip thc phm lm bn cht bo. Hot tnh chng oxy ho cu chng cth do:

- phenol to phc bn vng vi cc kim loi nng, do lm mt tc dngxc tc cu chng.

- phenol l cht nhn cc gc t do tc l c kh nng dp tt cc qu trnh

to ra gc t do.

7.3.3.4 Enzym xc tc phn ng oxy ho phenol

Phn ng to mu do enzym thng gm cc giai on sau:

S hydroxyl ho cc monophenol v oxy ha cc diphenol l hai tc ngkhc nhau v tch bit nhau. Tuy nhin i khi cng mt enzym cng c th xc tcc hai phn ng. Ni chung, cc enzym c ngun gc khc nhau s c t s hot

lng hydroxyl hot ho lng oxy ho cng nh hm lng Cu + v Cu2+ khcnhau.

Enzym xc tc giai on u c tn l monophenolase hoc creosolase, cn

enzym xc tc giai on hai c tn l polyphenolase hoc catecholase. Tn gi hthng cu enzym m trch giai on oxy ho l orthodiphenoloxy oxydoreductase(E.C.1.10.3.1). y oxy phn t ng vai tr cht tip nhn hydro.

Polyphenoloxydase l metaloenzym c ch khong 0,2% Cu tng phnt. Chng hn nh polyphenoloxydase l ch c khi lng phn t bng 144 000Da v c ti 7 mol ng/mol.

Phng trnh phn ng oxy ho cc diphenol vi polyphenol oxydase c thnh sau:

Ta thy phng (7.1) to ra polyphenol oxydase dng kh (ch Cu+) do c th a tc ng hydroxyl ho monophenol vo phng trnh tng cho y hn:

Nh vy, theo (7.4), chnh diphenol c tc dng xc tc s hydryl ho cc

monophenol.


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Cc quinon to ra do hu qu tc dng cu polyphenoloxydase cng c thoxy ho cc cht c th oxy ho kh b hn quinon:

Cc cht nh vitamin C, NADH2, glutation dng kh, xistein, cac

antoxianidol...c th kh cc quinon thnh diphenol.Nh vy, polyphenol oxydase c th oxy ho c nhiu c cht khc nhau:

Do , v mt ny polyphenol oxydase cng c vai tr tng t nh enzym

xitocromooxydase (cng ch Cu) cui chui h hp.

7.3.3.5 C ch chung cu phn ng to mu do enzym

Cc cht mu c to thnh do phn ng ku ny thng c tn gi chung

l melanin. Thc ra thut ng melanin l phng theo tn gi cu mt sn phm cmu do s oxy ho tyrozin bi enzym to ra. Cc cht mu cu phn ng sm muenzym cn c tn n l flobafen.

Ni chung sn phm cui cng, thng c mu nu, mu en hoc cc gammu trung gian nh: hng, , nu, xanh en, v.v.Phn ng to thnh cc cht mu

c bt u t quinon v khng c enzym tham gia.

u tin quinon phn ng vi nc to ra trihydroxybenzen, tip cctrihydroxybenzen li phn ng vi cc quinon khc hnh thnh ra cc

hydroxyquinon v cui cng chnh cc hydroxyquinon ny li l ni ngng toxy ho ( giai on ny cn tiu th oxy) dn n to thnh cc polyme theo s trn.

- Cc quinon cng c th tc dng vi nhm SH v NH2 cu protein, axitamin v amin theo c ch sau:

Quinonimin l phc gi quinon v axit amin thng c mu nu . Khi tothnh phc ny, axit amin kt hp vi cacbon v tr para cu mt trong hai nhm

quinon. Nu c hai v tr para ca cc nhm quinon u b thay th th axit amin skhng kt hp vo v phc quinon s khng c to thnh. Nhiu th nghim

chng t mu c to thnh mt cch mnh m ch khi hp cht nit tham giavo phc c ch nhm amin bt hai (prolin). Nu hp cht c nhm amin bc 1 th

phc to thnh c mu da cam, t m hn.


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- Phc gi quinon v polypeptit hoc protein th c mu nu. Cht mu nuc to thnh trong l thuc khi sy kh trong khng kh l mt phc thuc kiu

ny.

- Phc quinoimin khng nhng c tc dng to mu m cn c tc dng oxy

ho lng axit amin tha (khng c enzym) tham gia, phn ly n n amoniac v -

xetoaxit hoc aldehyt tng ng theo s sau:

7.3.3.7 Mt s bin php phng nga phn ng sm mu do enzym xc tc

Phn ng sm mu do enzym xc tc l phn ng rt thng xy ra trongqu trnh ch bin cc sn phm thc phm c ngun gc thc vaajt nh rau qu,cac sn phm dod hp rau qu, cc sn phm ch, thuc l,v.v. C nhi unhn t

nh hng n pahrnng ny nh cc nhn t sau:

- nng c cht phenol;

- hm lng v trjang thi cu polyphenoloxydase

- nng oxy;

- nng quinon;

- pH v.v.

Trong thc t ngng ng phn ng ngi ta thung ahy dng nhngbin php k thut tong ng sau:

- Trnh ng dp gy tn hi cc m, renzym khng c iu kin tip xcvi c cht to mu;

- V hot enzym polyphenol oxydase bng nhit (chn, thanh trng) rt chiu qu, nhng c th lm thay i tnh cht cm quan cu sn phm do khng

th p dng cho mi trng hp;

- Thm cc ph gia c tnh kh chuyn ho qui non tahfnh phenol, do

cng lm chm hoc ngng cn s sm mu. Ngi ta hay dng vitamin C cho vaocc nc qu v cc sn phm qu, ct lt cng vi mc hc . Khi liu lngvitamin C cao (c th t 0,5-1% so vi khi lng sn phm) enzym polyphenoloxydase c th b v hot hon ton trc khi lng vitamin ny bin ht trong mi

trng;

- Nhng ngp qu sau khi ct lt hoc sau khi gt v va trong dung dicjh

nc mui njhat, dung dch saccharose hoc dung dch glucose hn ch oxy xmnhp vo qu cng nh ngn cn qu hp th oxy. Khi ng xm nhp vo m

cng c tc dunng lm qu chc li do tng p sut thm thu. Ni chung cc qu lnh ng thong c bao bn gsi r (c th dng mt phn si r c nn g 30 -


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50% saccharose cho 3-7 phn qu). Trong trng hp ny, ng v c tc dngnh mt cht bo v lanh v cht gi hng thm cho qu.

- Dng axit xitric lm gim pH lm chm phn ng sm mu;

- Ly oxy trong cc m cu qu bng chn khg hac bng cc cht c kh

nng s rudjng oxy nh axit ascorbic, hoc nh enzym glucooxydase v catalase:

Glucose + O2 axit gluconic + H2O2

H2O2 H2O + O2

Tuy nhin cn trnh lm ym kh cc m thc vt m bo hot ng sinh

l bnh thng cho chng. Cn ch l vic kh oxy phi i km ngay vi x l

bo qun v phi bao gi bng bao b khng thm oxy.

- SO2v bisulfit cng c tc dng c ch phn ng sm mu do enzym xctc. Hp cht ny chng nhng c tc dng st trng (nhng khng phi vi liulng dng chng phn ng sm mu) m cn c kh nng lin kt vi cc nhmcarbonyl cu cc ng cng nh phn ng vi cc quinon. Nhng c th SO2tc

dng trc tip n polyphenol oxydase. Ngi ta cn nhn thy rng axit ascorbicv tiamin c th lm gim liu lng bisulfit. SO2cn c tc dng bo v mu sct nhin cho qu, c bit l qu sy (chui sy).

Trong thc t, vi nhng qu nhm bo qun lnh ng, th u tin ngi ta

nhng (ngm) 45 giy trong dung dch NaHSO30,25%, tip ngm 5 pht trong

mt dung dch K2HPO40,2%. Chnh K2HPO4l tc nhn lm gim kh nng phnng cu polyphenol.

2.3.1 MT S CHT MU T NHIN V TNG HP DNG TRONG

THC PHM

Trong thc t, ngi ta c th dng cc cht mu t nhin hoc tng hp

to mu cho cc thc phm lm tng gi tr cm quan cu chng. Cc cht mu v mu vng thng c s dng nhiu hn. Carotenoit l nhm cht mu tnhin c s dng nhiu nht.

Cu trc v cch s dng cu mt s cc cht mu t nhin v tng hp quan

trng trong thc phm c trnh by bng 7.2:

glucooxydase

catalase


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CHNG III: CC CHT THM

3.1 NGHA CA CC CHT THM TRONG SN XUT THCPHM

Cng nh mu sc, hng thm l mt tnh cht cm quan quan trng ca

thc phm, v chng c nhng tc dng sinh l rt r rt. Cht thm c nh hngn h tun hon, n nhp p cu tim, n h hp, n nhp th, n s tiu ho,n thnh gic, th gic v c xc gic na. V vy trong sn xut thc phm, ngi

ta tm mi bin php k thut bo v nhng cht thm t nhin, mt khc ngita cn tm cch iu khin cc phn ng to ra nhng hng thm mi.

Thng thng, ngi ta thc hin mt trong ba bin php sau to cho snphm c hng thm:

- Cht thm vn d bay hi v thng khng bn; v vy ngi ta dng ccbin php k thut v thit b phc tp thu hi cac cht thm bi tc ra khi sn

phm trong qu trnh gia nhit (un hoc c c), to iu kin gi chng li, hpth tr li vo thnh phm cc cht thm t nhin vn c trong nguyn liu ban

u;

- Chng ct v c c cc cht thm t nhin t cc ngun giu cht thm,

sau dng cc cht thm ny cho vo cc sn phm thc phm khc nhau;

- Tng hp cc cht thm nhn to c mi thch ng cho vo cc sn

phm thc phm.

3.2 Vi nt v mi

Trong hn hai triu cht hu c, th 400.000 cht c mi nhiu hoc t. as cht thm c mi c trng ring cu mnh. Mi cu chng do nhng nhmnguyn t c bit gi l nhm mang mi quyt nh. Tuy nhin nu tng s nhmmang mi tng mt phn t ln th khng lm tng mi m li lm yu mi v i

khi cn lm tt mi hon ton.

Nhng nhm mang mi c bn thng l nguyn t O, S, N, P, As, Se. Ccnhm mang mi hu c l:

C-OH;

C th tng mnh hac lm yu mi ny bng mt mi khc, hoc cho mi

mi m i khi khng ging v hon ton bt ng. Chng hn, mi x hng cutrinitrobutyltoluen hon ton mt i khi thm vo mt lwng nh quinin sulfat l

cht vn khng c mi.

Mi cu cao su, iot, long no v g b hng khi phi hp s trit tiu lnnhau hon ton. Mi cu vani trong lnh s mt hon ton qua mt vi pht,nhng nu thm mt t cumarin th hng cu vani s cm thy c rt lu.


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Cc cht rt ging nhau v cu to v tnh cht c th c mi khc nhau.Chng hn, cng thc cu hai cht h hng eugenol v dihydroxyeugenol ch khcnhau mch bn.

Nhng eugenol l thnh phn ch yu cu du inh hng v c mi inhhng mnh, cn dihydroxyeugenol li hu nh khng c mi.

Vanilin v izovanilin ch khc nhau v tr cu nhm th. Nhng vanilin lhng liu ni ting nht c mi d chu, cn izovanilin ch bt u c mi khi unsi.

Ngc li, cc cht hon ton khc nhau c th c mi ging nhau. Chng

hn, cng thc cu x hng v cc cht thay th n khng ging nhau nhng licho mi ging nhau.

i khi mi cn ph thuc vo nng cu cht trong khng kh. Chng

hn, mi cu ionon m c ging b hng, nhng khi trng thi long th iononli c mi cy hoa tm.

Cn thy rng, nhiu hin tng lin quan n mi khng th gii thch c

mt cch chc chn, v cha c mt quan nim y v khu gic lm vic nhth no v ti sao mt cht no li c mi.

3.2.1 Thuyt ho hc v mi

3.2.2 Thuyt l hc v mi

3.3 Cc cht mi t nhin

3.3.1 Tinh du v nha

Cc cht mi thng gp trong t nhin l

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