About the Hydration of Non-Hydratable Phosphatidesasaga.org.ar/descargas/material/SESION_PRO-R/PRO-R1_Dijkstra.pdf · rate of hydration – Has been published in1986 (Sen Gupta, Fette

About the Hydration of Non-Hydratable Phosphatides

Albert J. Dijkstra

3 November 2015 World Congress on Oils & Fats and 31st ISF Lecture Series, Rosario

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Questions to be raised (answered ?)

• What are phosphatides? • Why are some phosphatides hydratable and

others, the NHP, apparently not? – Thermodynamic approach versus kinetic approach

• How to get rid of NHP during degumming? • How to get rid of NHP during neutralisation? • What is the role of phospholipase enzymes? • What lessons can we learn from all this?


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Phosphatides are a nuisance

• The hydratable phosphatides (HP) throw a deposit during storage and transport – So crude oil has to be water degummed before being sold

• The NHP removal constitutes a refining loss • The NHP have to be removed before steam refining

– What to do with the gum stream? • Residual phosphatides may foul the deodoriser • Phosphatides have been reported to shorten the shelf

life of fully refined oil – But this may be iron or copper phosphatidate rather just

‘phosphatides’


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Structural formulae of phosphatides

C R1

OH2C

CHO

CR2 H2C O P O X

O

HOH2C

H2C N

CH3

CH3

CH3choline

HOH2C

H2C NH2

ethanolamine

OOH

OH

OHOH

OH

inositol, link in 1-position

X = choline (phosphatidylcholine or PC)

X = ethanolamine (phosphatidylethanolamine, PE)

X = inositol (phosphatidylinositol or PI)

X = hydrogen (phosphatidic acid or PA)

O

O

O

D

A1

A2

C


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Which phosphatide goes where on water degumming?

Phosphatide Gums obtained by water degumming

Oil resulting from water degumming

PC Present None

PE Present Present

PI Present None

PA Present Present

Why do they behave differently? There are two schools of thought

• The kinetic approach – All phosphatides are hydratable but they differ in

rate of hydration – Has been published in1986 (Sen Gupta, Fette

Seifen Anstrichm. 88, 79-86) and goes on being quoted

• The thermodynamic approach – Hydratability is caused by the partition coefficient

between the oil phase and the water phase – This partition coefficient can be pH-dependent


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Relative rates of hydration

Phosphatide Rate PC 100 PI 44 PI (Ca salt) 24 PE 16 PE (Ca salt) 0.9 PA 8.5 PA (Ca salt) 0.6 Phytosphingolipids 8.5

• The ‘observation’ that all phosphatides have a finite rate of hydra-tion would mean that a ship that is loaded with crude soya been oil in Argentina would arrive in Rotterdam with fully degummed oil: P = 0 ppm

• Nice but unlikely


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Extended water degumming experiments (Desmet-Ballestra)

• Experimental conditions – Starting material: rapeseed oil that had been two

times 15 min. degummed with 3% water at 85°C – Parr reactor (to prevent water from evaporating) – Temperature 84 ± 2 °C – 5 wt% water – 500 rpm

• Samples separated by centrifugation – 15 min at 2000 g


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Results of extended degumming

Degumming time 0 hours 3.5 days 1 week 2 weeks Phosphorus content (ppm P) 175 111 57 15 Calcium content (ppm Ca) 162 109 60 18 Magnesium content (ppm Mg) 22 12 6 2


es

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So with time, the phosphorus content decreases, which could point to a kinetically determined hydration process But more things are happening

Further experimental results

Degumming time 0 hours 3.5 days 1 week 2 weeks Phosphorus content (ppm P) 175 111 57 15 Calcium content (ppm Ca) 162 109 60 18 Magnesium content (ppm Mg) 22 12 6 2 Free Fatty Acids (wt% as oleic) 1.28 2.06 2.80 4.86 Monoglycerides (wt%) 0.01 0.03 0.11 0.31 Diglycerides (wt%) 1.04 2.47 4.95 8.52


s

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Prolonged contact of oil containing NHP with water causes extensive hydrolysis of triglycerides and phosphatides


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Conclusions Kinetic approach is not substantiated

• The NHP are hydrolysed to lysophosphatides – Lysophosphatides are more hydrophilic – Lysophosphatides move into the water phase – P-, Ca- and Mg-contents of the oil decrease

• Prolonged exposure to water at lower temperature does not lead to hydrolysis – P-, Ca- and Mg-contents do not decrease

• Hydration of phosphatides is not rate-controlled – Instead, it is controlled by their hydrophilicity


12 3 November 2015 12

Which phosphatide goes where on water degumming?

Phosphatide Gums obtained by water degumming

Oil resulting from water degumming

PC Present None

PE Present Present

PI Present None

PA Present Present


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What determines hydrophilicity? Logarithmic dissociation constants

Phosphatide Dissociating group pKa

Phosphatidylcholine Phosphate < 3.5

Phosphatidyl- ethanolamine

Phosphate < 3.5

Amino 9.8-11.5

Phosphatidylinositol Phosphate < 3.5

Phosphatidic acid Undissociated 2.7-3.8

Once dissociated 7.9-8.7


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Conclusions drawn from pKa-values

• At neutral pH, the acids groups are mostly dissociated and carry a negative charge

• At neutral pH, the amino groups (PC and PE) are mostly protonated

• At neutral pH, PC and PE are present as zwitterions • At low pH, acid groups are not dissociated

– PA is non-hydratable – PI is still hydratable because of hydroxy groups – PE is no longer a zwitterion but carries a positive charge and

is readily hydratable • Degumming with acidified water lowers P-content


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Charges of phosphatides at different pH values

pH PC PE PI PA CaPA

2 + + 0 0 0

3 (+) (+) (0) (0) 0

4 (±) (±) (-) (-) 0

5-7 ± ± - - 0

8-9 ± ± - (2-) - (?)

>10 ± - - 2- 2- (?)


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Legend of previous table (just for the record)

• + Nearly all moieties have a positive charge • (+) About half the moieties have a positive charge • ± Nearly all moieties are Zwitterions • (±) About half the moieties are Zwitterions • 0 Hardly any moieties carry a charge • (-) About half the moieties carry a negative charge • - Nearly all moieties carry a single negative charge • 2- Nearly all moieties carry a double negative charge

3 November 2015 17 3 November 2015 World Congress on Oils & Fats and 31st ISF Lecture Series, Rosario

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Observations and explanations

• PC and PI are fully hydratable, because – PC has a large dipole moment because the positive amino

group and the negative oxygen atom are far apart – Both groups have their own counterions

– PI is hydrophilic because of the five hydroxyl groups in the inositol moiety and negative charge at neutral pH

• PE and PA can be hydratable and non-hydratable – PE has small dipole moment; zwitterion with finite partition

coefficient between water and oil – PA is hydratable as potassium salt – PA is non-hydratable as Ca- or Mg-salt or when the acid is

not dissociated (pH < 3)

3 November 2015 18 3 November 2015 World Congress on Oils & Fats and 31st ISF Lecture Series, Rosario

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Stereochemistry of PE and PC • In the PE molecule the

positive charge is on the NH3 group and one of the phos-phate oxygen atoms has a negative charge. The six-membered ring brings the charges close together. The zwitterion has no counterions

• In the PC molecule the three CH3 groups prevent the charges from being close to-gether. They are wide apart, have their own counterion and make the molecule strongly hydrophilic

P

O

O CH2

CH2

H3NO

OH2C

CH

CH2

O

O

R2

O

R1

O

P

O

O CH2

CH2

O

OH2C

CH

CH2

O

O

R2

O

R1

O

NC

CH3

CH3H

HH


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For phosphatidic acid, (PA), the counterion makes the difference

H2CCH

OC

OR1

H2C OP

OCR2

OO

O

OCa2+

H2CCH

OC

OR1

H2C OP

OCR2

OO

OH

O K+

• The calcium is closely bound to the phosphate group, like in calcium phosphate – Non-hydratable

• The potassium is just a counterion that is not bound, like in potassium phosphate – Hydratable

Industrial processes in which NHP are hydrated

• Dry degumming process (→ to steam refining) – Uses degumming acid and bleaching earth – Used for palm oil lauric oils and animal fats

• Alkali refining process (→ bleaching/deodorisation) – Long-Mix process uses no degumming acid – Short-Mix process uses degumming acid – Used for all vegetable and animal oils and fats

• Acid refining process ( → bleaching/steam refining) – Uses degumming acid and small amount of caustic soda – Used for seed oils


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Crude oil

Water degumming or enzymatic PLC degumming

Oil with NHP

Acid degumming Acid refining

Enzymatic PLA gum treatment

Alkali refining

Oil with < 30 ppm P

Oil with < 5 ppm P

Bleaching Bleaching

Deodorisation

Fully refined oil

Physical refining

Dry degumming

Refining routes


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Reagents used in various processes Process Water Acid Base Bleaching

earth Other

Water degumming X (X)

Acid degumming X X

Dry degumming X X

Acid refining X X X

Alkali refining X X X

Gum treatment (PLC) X PLC

Gum treatment (PLA) X (X) (X) PLA


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The reaction between NHP and phosphoric acid

• The stronger phosphoric acid dislodges the weaker phosphatidic acid from its salts

• Besides, the calcium ions form an insoluble salt with the phosphoric acid so that when the pH is raised and the H2PA dissociates, there is no free Ca2+ to regenerate the CaPA.

• The reaction only proceeds if the acid is very finely dispersed in the oil

3(CaPA) +2(H3PO4) 3 (H2PA) + 2 (Ca3(PO4)2


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The difference between acid degumming and acid refining

• In the SuperDegumming® process, citric acid reacts with the NHP at high temperature, water is added and the mixture is cooled and held to allow gums to separate

• In the acid refining process, degumming acid reacts with the NHP and lye is added to dissociate the PA and immediately pull it into the water phase – Acid degumming P < 30 ppm – Acid refining P < 5 ppm

• We can regard the acid refining process as obsolete – Unilever complemented it with Unidegumming, which is an

acid refining process

There are other ways to remove the alkaline earth ions from the NHP

• EDTA (ethylenediaminetetraacetate) builds a very strong chelate with calcium, magnesium and iron II and iron III

• The pK values below show the differences in formation constants (E. Deffense, Soft Degumming, Oils-Fats-Lipids 1995)


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Chelating agents Calcium Magnesium Phosphatidic acid 4.6 4.0 EDTA 10.7 8.7

Further reagents for alkaline earth ions (A Hvolby, J.Am.Oil Chem.Soc.,48, 503-509, 1971)

• There are other Ca/Mg binding agents: – Fluorides, sulphates, carbonates, phosphates,

pyrophosphates, oxalates, citrates, tartrates • Mixing those in oil, preferably at elevated pH,

leads to almost complete phosphatide removal – Literature does not explain role of elevated pH – Could be partial exposure of the calcium that is

being dislodged from its salt by stronger base


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Neutralisation (US refining) Short Mix (Europe) Long Mix (US) Crude oil

Heating

Mixing

Separating

Dilute lye 14-18°Bé Degumming acid

Separating

Mixing

Heating Dilute lye 20-28°Bé

Mixing

Hot washing water Heating

Separating/ drying

Separating/ drying

Neutral (refined) oil


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Differences between Long-Mix and Short-Mix processes

• Short-Mix uses a degumming acid ; Long-Mix can operate without a degumming acid

• Short-mix uses intense mixing of degumming acid; if Long-Mix uses an acid, it may be added to day tank

• Short-Mix is a high temperature process; Long-Mix starts at low temperature and raises the temperature during the process

• Lye used in Long-Mix process has been further diluted than lye used in Short-Mix process – Short-Mix uses a relative excess lye of 10% – Long-Mix uses an absolute excess of 0.10-0.15% on oil


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How does Long-Mix get rid of NHP? What does the literature say?

• “The phosphatides and gums absorb alkali and are coagulated through hydration and degradation” (Sullivan, 1968)

• “The sodium hydroxide …. will react with …. the phosphatidic material…. “ (Hvolby, 1971)

• “Soybean oil … requires longer effective mixing times … for contact with alkali in the refining sequence.” (Wiedermann, 1981)

• Phosphatide reduction is determined largely by the amount of water present in the lye.” (O’Brien & Wan, 2001)

Structure of calcium phosphatidate

• The oxygen atoms on the left are part of the glycerol moieties

• The salt has three mesomeric structures • The three oxygen atoms bonded to the

calcium ion could well be equivalent


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O P

O

O

O Ca2+ O P

O

O

O Ca2+ O P

O

O

O Ca2+

Reaction of calcium phosphatidate with sodium hydroxide

• The stronger sodium hydroxide dislodges the weaker calcium hydroxide from its salts – Position of equilibrium depends on [OH’], pH

• The resulting calcium hydroxide phosphatidate has a negative charge and will therefore be hydratable

• The calcium will be present in the gum phase


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O P

O

O

O Ca2+ NaOH O P

O

O

O Ca OH Na+

The reaction could go further

• At high pH, the calcium hydroxide phosphatidate may react with a further hydroxy anion to form twice dissociated disodium phosphatidate and non-dissociated calcium dihydroxide

• This reaction will take place in the water phase • The calcium will be present as free calcium 3 November 2015 World Congress on Oils & Fats and

31st ISF Lecture Series, Rosario

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O P

O

O

O Ca OH

Na

NaOH O P

O

O

O

Na

NaCa( OH) 2

Experimental support for these ideas (Literature and GEA-Westfalia)

• Ammonia can neutralise free fatty acids but leaves the NHP in the oil – pH is too low to break a calcium/phosphate

oxygen bond; [OH’] too low • Treating oil with NHP with low FFA content

with weak caustic soda removes some NHP – Incomplete decomposition to calcium hydroxide

phosphatidate because of equilibrium • Strong lye removes all NHP

3 November 2015 World Congress on Oils & Fats and 31st ISF Lecture Series

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Interim conclusions (theoretical)

• Mechanism of NHP removal by caustic has not yet been fully elucidated – We need to demonstrate the presence of calcium in

the water phase at high pH to prove complete dislodging of calcium from NHP

– We need more insight in partition coefficients of various phosphatides between oil and water phase

– Determining these partition coefficients will have to take into account that compounds may take part in pH-dependent equilibrium reactions in water phase


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Interim conclusions (practical)

• Pulling NHP into the water phase is one thing but what about: – Emulsion formation – Neutral oil entrainment in gums – Fouling of centrifugal separators

• Acid refining experience shows that not all oils are amenable for this process – Are there oil differences that affect the NHP

behaviour?


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What is the role of phospholipases ?


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C R1

OH2C

CHO

CR2 H2C O P O X

O

HOH2C

H2C N

CH3

CH3

CH3choline

HOH2C

H2C NH2

ethanolamine

OOH

OH

OHOH

OH

inositol, link in 1-position

X = choline (phosphatidylcholine or PC)

X = ethanolamine (phosphatidylethanolamine, PE)

X = inositol (phosphatidylinositol or PI)

X = hydrogen (phosphatidic acid or PA)

O

O

O

D

A1

A2

C

Enzymatic hydrolysis products

• Phospholipase C catalyses the hydrolysis to: – Diglycerides that move into the oil phase – Phosphate esters of choline etc that stay in the

water phase and do not retain any oil • Phospholipase A catalyses the hydrolysis to:

– Free fatty acids that move into the oil phase and are subsequently removed during refining

– Lysophosphatides that retain less oil • Both enzymes therefore increase the oil yield 3 November 2015

World Congress on Oils & Fats and 31st ISF Lecture Series, Rosario

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However,

• The PLCs that are commercially available do not act on phosphatidic acid

• Enzymes only act on hydrated phosphatides – To obtain a low residual phosphorus content, the

oil has to be acid refined first • Enzymes cost money so the yield increase

must be large enough to justify their use – Crude oils with high phosphatide content, or – Gums from oils with lower phosphatide content


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Take home messages

• Understanding the chemistry of our processes can lead to unexpected process improvements – This also holds for “established” processes

• Understanding originates from questioning established “truths” – They can be myths that people got used to

• Biased reporting will eventually be exposed – Then it may seriously backfire


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Also some outstanding questions

• Do we really need a degumming acid in the alkali refining process? – Doing away with it would save money during

refining, soapstock splitting and effluent treatment • Can we separate phosphatide removal from

FFA removal in alkali refining? – Would simplify soapstock treatment

• An EDTA wash for residual metal ions? – Would be effective and could be less costly


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DIXI

(I have spoken)

Documents

About the Hydration of Non-Hydratable Phosphatidesasaga.org.ar/descargas/material/SESION_PRO-R/PRO-R1_Dijkstra.pdf · rate of hydration – Has been published in1986 (Sen Gupta, Fette