Lipids

Lipids

A triglyceride

O

O

O

CH3

CH3

O

CH3

OO

Another view of a triglyceride

O

O

O

O

O

O

glycerolgroup

fatty acids

Nomenclature conventionsOxidative rancidity happens here

Hydrolytic rancidityhappens here

Nomenclature conventions

We will call the shaded part of the molecule R so that we can refer to the whole molecule as RH when talking aboutoxidative rancidity

Monoglycerides

OH

OH

OCH3

O

Polar head

Non-polar tailThis arrangement is characteristic ofgood emulsifiers.

Diglycerides

O

OH

OCH3

O

CH3O

The ester linkage

CH3O

CH3

O

Structure and Physical Properties

• Fatty acid and triglyceride physical properties depend strongly on their chemical structures, especially– Chain length

– Saturation/unsaturation

Chain length (APK)

CH3OH

O

CH3OH

O

Butyric acid, C4, 4:0

Stearic acid, C18, 18:0

Saturation/Unsaturation (APK)

Oleic acid 18:1

Linoleic acid 18:2

O

OH

O

OH

Cis vs. trans

H

HH H

cis trans

Natural Fatty Acids

Commonname

Carbonatoms

Doublebonds

Meltingpoint

Butyric 4 0 -7.9 CPalmitic 16 0 63.1 CStearic 18 0 69.6 COleic 18 1 13.4 C

Linoleic 18 2 -5.0 CLinolenic 18 3 -11.0 C

Melting point vs. chain length

Melting point vs. chain length

-20-10

01020304050607080

0 5 10 15 20

# of carbon atoms

Mel

ting

poin

t (C)

Melting point vs. unsaturation

Melting point vs. # of double bonds

-20-10

01020304050607080

0 1 2 3 4

# of double bonds

Mel

ting

poin

t (C)

Fat Classification

• Milk fat group– 30-40% oleic

– 25-32% palmitic

– 10-15% stearic

– 3-15% butyric (smelly as the free acid)

Fat Classification

• Lauric acid group– Palm, coconut

– 40-50% lauric acid (C12, 12:0)

– Relatively low (compared to C16 and C18 triglycerides) melting point, considering how saturated these molecules are

Fat Classification

• Vegetable butters– Derived from the seeds of tropical trees

– Narrow melting range

– Best example-- cocoa butter

Fat Classification

• Oleic-linoleic acid group– Largest and most varied group

– Vegetable origin

– Cottonseed, corn, sunflower, safflower, peanut, olive, sesame, canola (Canadian oil, low acid)

– Less than 20% saturated fatty acids

Fat Classification

• Linolenic acid group

– Contains substantial amounts of linolenic acid (18:3), for example, soybean

– Linolenic acid is responsible for the “flavor reversion” in soy containing products

Fat Classification

• Animal fat– Lard -- pork

– Tallow (suet) -- beef

– 30-40% C16 or C18 saturated

– High melting points (solid at room temperature)

Fat Classification

• Marine oils– Contain long chain polyunsaturated fatty

acids– C20, C22, C24, C26– Contain up to 6 double bonds per fatty acid

chain– Susceptible to oxidation– Naturally rich in vitamins A and D

Chemical and Physical Properties

• Solubility– Insoluble in water, soluble in organic

solvents• Specific gravity

– Less than 1.0– Increases with increasing unsaturation– Decreases with increasing chain length

Chemical and Physical Properties

• Refractive index– Increases with increasing chain length– Increases with increasing unsaturation

• Melting point– Increases with decreasing unsaturation– Increases with increasing chain length

Properties of crystalline forms of fats

Characteristic Alpha form

Beta prime form

Beta form

Density Least dense

Intermediate Most dense

Melting point Lowest Medium Highest

Crystal size Smallest Medium Largest

Fat bloom

• A grayish coating that often appears on chocolate which has been subjected to wide variations in temperature

• Caused by a change in fat crystal size from a smaller beta form to a larger beta form

• A serious quality (sensory appearance) defect in chocolate

Chocolate bloom

Fat bloom analogy

Kindergarden:Average height = 4’6”

Image courtesy of www.fas.pps.k12.or.us/fas2/kinder/kinder2.html

Fat bloom analogy

Image courtesy of www.stanthony.com/gradeschool/basketball/8basketball.htm.

Eighth grade:Average height =5’11”

Fat bloom analogy

College:Average height =6’6”

Image courtesy of www.geocities.com/Colosseum/Track/3787/kobe.html.

This is my guy--Joe Smith--the secondcoming of Jordan. Wow!!

Fat bloom analogy

6’6”

Joe Smith1st roundpoint guard

7’4”

Joe Smithvirtually useless to my team

3 months

later atrookiecamp

Smoke point

O

O

OH

H

H

O

OHH2+

glycerol

acrolein

heat

(lachrymatory factor)

Hydrolytic rancidity

O

O

O

CH3

CH3

CH3

O

O

O

OH2+

OH

OH

OH CH3OH

O

CH3OH

O

2+ +

lipase

These low molecular weight

acids are very smelly!!

This is mainly aproblem indairy products.

Oxidative rancidity

• Initiation

• Propagation

• Termination

Initiation

R H

Light, heatmetal ions

A reactive C-H bond in a fat,an allylic bond

Remember our naming convention

Initiation

R. .H

Overall reactionRH R. + H.

Note that there are free radicals only on the product sideof this equation.

Propagation

R. + O=O ROO.

ROO. + RH ROOH + R.

Note that there are free radicals on both sides of theseequations.

Termination

• R. + R. R-R• ROO. + ROO. ROOR + O2

• RO. + R. ROR• ROO. + R. ROOR• 2RO. + 2ROO. 2ROOR + O2

Note in these reactions that free radicals only appearon the reactant side of the equations.

Termination

Termination is (fundamentally) the opposite of initiation.

R. .R

Termination

Termination is (fundamentally) the opposite of initiation.

R R

Autooxidation

Line of stink

Antioxidant

Smelly stuff

Smelly stuff

Lipid oxidation

Oxidative Rancidity

A mechanistic look at this reaction

Go to Slide Show mode and click to begin

TBA (thiobarbituric acid) reaction

TBA Malonaldehyde,a product of fat oxidation

Red color, 530 nm

Phenolics and antioxidation

O

O

H

H

O

O

phenolic

(hydroquinone)

quinone

oxidation

-2H

Phenolic antioxidants

O

O

HO

H

Butylated hydroxyanisole, BHA

Butylated hydroxytoluene, BHT

Phenolic antioxidants

O

O

H

H

OO

OH

H

H

OOtertiary butylhydroquinone,TBHQ

Propyl gallate, PG

Antioxidant mechanism

Antioxidant terminates initiating step in autooxidation

High energy Low energy

Antioxidant mechanism

Antioxidant terminates propagation step in autooxidation

High energy Low energy

Antioxidant mechanism

Decay of antioxidant radicals to yield a quinone

Regenerated antioxidant

Antioxidants

Phenolic antioxidation

Go to Slide Show mode and click the big red button

Metal prooxidants

Metal(I) + ROOH Metal(II) + RO- + OH.

Metal(II) + ROO- ROO. + Metal(I)

Chelators block these reactions

Chelators

• EDTA (ethylenediaminetetraacetic acid)

• Inorganic phosphates

• Citric acid

• Vitamin C (ascorbic acid)

EDTA

Image courtesy of www.chem.purdue.edu/courses/chm116/chime/polys.html.

EDTA complex

Image courtesy of http://www.benbest.com/nutrceut/EDTA.html