Lipids VI Flashcards
What is saponification?
Hydrolysis of ester under alkaline conditions
What is the saponification value?
- The number of milligrams (mg) of potassium hydroxide (KOH) required to neutralize the fatty acids resulting from the complete hydrolysis of 1 g of the sample.
- mg of KOH required to saponify 1 gram of fat
How do you determine the saponification value experimentally?
- Weigh 5 g of fat in 250-mL Erlenmeyer.
- Add 50 ml KOH (0.5 N) to Erlenmeyer.
- Boil to saponify fat.
- Titrate with HCl (0.5 N) using phenolphthalein.
- Conduct blank determination.
What is the calculation for the saponification value?
Saponification value = 56.1 × (B – S) × normality of HCl / g of sample
- B = mL of HCl required by blank to reach phenolphthalein endpoint
- S = mL of HCl required by sample to reach phenolphthalein endpoint
- Molecular Weight of KOH = 56.1
During saponification, what does a TG become?
- Glycerol
- 3 Potassium salts
1 mol of TG requires how many moles of KOH for saponification?
3 moles
How many mg of KOH does it require to saponify 1 mol of TG? How do you calculate the SV?
- 168 000 mg
- SN = 168000/MW(TG)
What are the steps for GC analysis of fatty acids.
- Extract fat.
- Saponify (hydrolysis under basic condition).
- Prepare methyl esters (CH3ONa).
- Chromatograph methyl esters.
- Determine peak areas of fatty acids.
- Compare with response curve of fatty acid standards.
How are fatty acids identified in GC analysis?
By retention time
What is the iodine value?
mass of iodine absorbed by 100 g of the sample.
What does the iodine value indicate?
- Unsaturated fatty acid residues of the glycerides react with iodine
- Indicates the degree of unsaturation
- Identifying the source of an oil
What do higher iodine values indicate? What about lower?
- Higher: oils
- Lower: fats
What are the two methods to determine the iodine value?
Wijis or Hanus
What is the fat first reacted with in the Wijis method?
With iodine chloride
What is used to convert excess unreacted ICl in the Wijis method? What is it transformed into?
ICl + KI -> KCl + I2
What is I2 titrated with in the Wijis method?
I2 + 2 Na2S2O3 -> Na2S4O6 + 2 NaI
Which oils prefer the Beta-type crystals?
- Coconut
- Corn oil
- Olive oil
- Lard
- Palm kernal oil
Which oils prefer the Beta-Prime type crystals?
- Cottonseed oil
- Herring oil
- Menhaden oil
- Milk fat
- Palm oil
What are the important characteristics of shortenings?
- Incorporation of air, plasticity, and consistency
- Solid-liquid ratio
What do plasticity, consistency, and solid-liquid ratio depend on?
- Melting range
- Proper tempering is required to form the mixed crystals required for a broad melting range
Which crystal does natural lard prefer? Which crystal does randomized lard prefer?
Natural lard: beta
Randomized lard: beta-prime
In shortenings, how does the proportion of palmitic acid change? Which position?
The proportion of palmitic acid in the 2-position is reduced from about 64% to 24% on random interesterification.
What does the randomization of lard do? How does it compared to natural lard?
- Randomization of lard improves its plastic range
- Makes it a better shortening than natural lard.
What is plasticity?
The changes in consistency as a function of temperature.
Which crystal does natural lard prefer? Which crystal does randomized lard prefer?
Natural lard: beta
Randomized lard: beta-prime (better shortening, smaller crystals)
How are high polyunsaturated content and low-to-zero trans-acid containing margarines produced?
By interesterifying a blend of liquid oil and a fully hydrogenated oil
Why is interesterification bad?
When you randomize fats, you have new sets of FA
in the 2 position; liver will take whatever is in the 2
position and incorporates in the cells
Can increase cholesterol
What are the characteristics of hydrogenated palm kernal oil?
- Hard butter melting at 46oC
- Produces a waxy feel in the mouth
How are high polyunsaturated content and low-to-zero trans-acid containing margarines produced?
- By interesterifying a blend of liquid oil and a fully hydrogenated oil
- Nutritional margarine
What happens when you blend hydrogenated palm kernal oil and its randomized product?
A whole series of hard butters with highly desirable melting qualities (rapid melt in mouth) are obtained
What is the effect of randomization of a lightly hydrogenated soybean oil?
Melting point lowers
What is the effect of interesterification of cocoa butter?
Cocoa butter has a less sharp, higher melting point
What are the three major confectionary applications of fat?
- Shortenings
- Margarines
- Confectionary fats
What are the functions of shortenings in baking?
Have the capability to reduce the elasticity of a dough and to make pastry more tender/flaky.
Which fats have greater shortening abilities? How are they prepared?
- Fats with higher levels of unsaturated fatty acids
- Often prepared by hydrogenation of vegetable oils
Which fat is a natural shortening? What does its quality depend on?
- Lard
- Quality depends on the dominant crystal form and its solid fat content
What do plasticity and consistency depend on?
These physical characteristics in turn depend upon the polymorphic forms of the fats used and the methods of preparation.
How do beta-prime crystals incorporate air?
Large amount of small air bubbles; creamier, uniform and glossy shortenings
How do beta crystals incorporate air?
Small amount of large air bubbles; waxy or grainy texture
What is margarine fat composed of? (4)
- hydrogenated fats
- mixture of hydrogenated fat(s) and unhydrogenated oil(s)
- mixture of interesterified fat(s) and unhydrogenated oil(s), or
- mixture of unhydrogenated fats
What are the three high-stability margarine blends?
- 75 % co-randomized 40 % coconut oil/60 % palm oil),
- 10 % co-randomized 50% coconut oil/50% hydrogenated canola oil
- 15% hydrogenated soybean oil
Good spreadability, high temperature stability, and good eating qualities
How is margarine produced?
- Mixture is fed into a scraped-surface heat exchanger, which partially solidifies the material
How can margarine allow for crystallization in a manner that gives good plasticity and spreadability?
By tempering
What is tempering?
Holding a fat at a temperature and for an amount of time that permits transformation to the desired polymorphic form.
What must be removed during tempering? Why?
The heat of transformation must be removed to avoid melting and a later conversion into large beta crystals.
Which type of fats are used to enrobe confectionary products?
- Enrobing fats
- Cocoa butter
- Melts in your mouth (narrow melting point range)
What are enrobing fats composed of?
- 80% disaturated triglycerides
20% SOS - stearic, oleic and stearic 55% POS - palmitic, oleic and stearic
5% POP - palmitic, oleic and palmitic
What is the advantage of enrobing fats?
They solidify rapidly, enabling them as enrobing fats, but melt at body temperature
What determines characteristic cocoa butter texture?
POS
What happens when POS (palmitic, oleic, stearic) is converted from beta-prime to beta?
Chocolate “bloom” - white spots and dull surface appearance
What can be used to produce cheaper enrobing fats?
Selective interesterification and hydrogenation
Why are lipids required?
- Lipids are an integral constituent of cell membranes and hence are required in adequate amounts
- Fat-soluble vitamins
- Essential fatty acids
- Feeling of satiety or fullness
- Heat transfer agent
Why are fats used in baked goods?
To shorten texture; reduces the interaction of the wheat proteins to produce a desirable “shortened” or crumbly texture required for cakes
What are emulsions?
Intimate mixtures of water and oil that normally break (separate)
What are emulsifiers?
- Emulsifiers stabilize and reduce the interfacial tension between the hydrophilic and hydrophobic constituents
- To reduce interfacial surface tension, an emulsifier must have both hydrophilic and hydrophobic groups in its molecular structure.
Which two types of lipids have good emulsifying abilities?
Monoglycerides and phospholipids
What happens to the properties of a lipid once an emulsion is formed and stabilized?
Becomes more viscous, opaque, and the fatty sensation is masked
What is autoxidation? What is the overall result?
- It is a free radical reaction of unsaturated fatty acids with MOLECULAR OXYGEN
- Result: development of “rancidity”
What is rancidity?
Nebulous term, describing a wide variety of undesirable off-flavors and odors associated with oxidized fats and oils
What does extensive autoxidation lead to?
- Losses in functionality and a decrease in nutritional value
- The oil may eventually become toxic
- Spoilage of edible fats and oils, usually becomes inedible
What are the key steps of the autoxidation reaction?
1) Initiation
2) Propagation
3) Termination
What are the two actions that autoxidation requires?
1) A hydrogen must be abstracted from the fatty acid chain
2) Molecular oxygen must be present
How can hydrogen be abstracted from the fatty acid chain?
• Light or ionizing radiation (h )
• Thermal energy (heat)
• Presence of metallic cations: Cu2+, Fe2+
• Enzymatic catalysis - i.e., lipoxygenase
• Reactive singlet oxygen produced by photosensitizers such as
chlorophyll or myoglobin
or combinations/permutations of the above
Are free radicals reactive? How long do they survive?
- Extremely reactive
- 10^-9 (long-lived)
- 10^-12 (short-lived)
What happens in the initiation step of autoxidation?
Homolytic cleavage of a covalent bond,
and the products formed are free radicals.
What happens to free radicals in the absence of O2?
The free radicals quickly recombine
What does a fatty acid free radical form in the presence of oxygen during the propagation step?
Forms a peroxy radical
What is the key to the self-propagating nature of the autoxidation reactions?
Peroxy radical has a preference for terminating its radical state by abstracting a hydrogen from another fatty acid
What happens when a peroxy radical abstracts a hydrogen from another FA?
Formation of a hydroperoxide (ROOH) and a new fatty acid radical (R.), which in turn can react with molecular oxygen
How is autoxidation terminated?
- When the peroxy radical reacts with other free radicals in the vicinity
- A neutral, unreactive species is formed and the free radical chain reaction terminates (or is interruptive
Since there is a greater probability of another hydrogen being abstracted during propagation of autoxidation, what is the net effect?
An accumulation of hydroperoxides with time
What happens to an oil when there is an accumulation of hydroperoxides?
- Nothing, they are bland
- BUT, they are precursors to other molecules that produce taste
What are the primary oxidation products? (2)
- Hydroperoxides (R-O-O-H)
- Peroxides (R-O-O-R)
What are the compounds responsible for off-flavours?
Low-molecular weight aldehydes, ketones, alcohols, and short-chain FA
Are hydroperoxides stable?
No, they will breakdown
What are the two basic hydroperoxide breakdown mechniams?
- Monomolecular reaction
- Bimolecular reaction
When does the monomolecular hydroperoxide breakdown predominate? When does the bimolecular predominate?
Monomolecular: when hydroperoxide concentration is LOW
Bimolecualr: when hydorperoxide concentration is HIGH
What does the monomolecular breakdown of hydroperoxide produce?
- ROOH -> RO. + OH.
- Alkoxy radical (RO.)
- Hydroxy radical (OH.)
How can the alkoxy radical be decomposed?
- Aldehyde generation
- Unstable alkoxy radical steals an electron from the R2 portion, providing the electron to form the double bond
- Aldehyde is formed
- Release of a free radical (R2.)
What is the R2 radical produced from aldehyde generation going to do?
Capable of perpetuating the autoxidation reaction
What is the aldehyde formed from aldehyde generation?
- Not necessarily stable
- Can be oxidized or reduced, depending on the environmental conditions
What happens when an aldehyde is oxidized? What happens when an aldehyde is reduced?
Oxidation: to a carboxylic acid
Reduction: to an alcohol
How does the alkoxy radical decompose? (3)
- Aldehyde generation
- Alcohol generation (direct)
- Formation of a ketone
What happens to the alkoxy radical during alcohol generation?
- The alkoxy radical steals a hydrogen from R3H to form the alcohol
- Free radical is formed to carry on the reaction
What happens to the alkoxy radical during the formation of a ketone?
Hydrogen is abstracted from the radical chain and the reaction is a termination step
What does a bimolecular reaction require? What does it form?
- Requires two hydroperoxides (2 ROOH)
- Forms an alkoxy radical (RO.)
- Forms a peroxy radical (ROO.)
- Forms water
How do monomolecular and bimolecular reactions differ in terms of the reactive species produced?
Mono: 2 very reactive species (alkoxy and hydroxy radicals)
Bi: less reactive species (alkoxy and peroxy radicals)
What is the fate of alkoxy and peroxy radicals?
Can participate in the ongoing autoxidation reaction
What does the peroxy radical prefer to do?
Prefer to abstract a hydrogen from a fatty acid (propagation radicals)
What does the alkoxy radical prefer to do? Why? What does it form?
- It is unstable
- Decompose, leading to chain scission and the formation of aldehydes, ketones, alcohols, or carboxylic acids.
- Short-chain aldehydes creates a bad smell
What is a common test to evaluate the oxidative status of an oil?
The Peroxide Value
What does the Peroxide Value measure? Why?
- Hydroperoxides
- They are considered the primary oxidation products
What is the definition of the peroxide value?
milli-equivalents (mEq) of peroxide oxygen per 1000 grams of fat.
How do we measure the peroxide value experimentally?
- Hydroperoxides are capable of releasing molecular iodine (I2) from potassium iodide (KI) in an acidic environment
- The amount of iodine released is measured by titration with standardized sodium thiosulfate.
How would the peroxide value change over time?
- Would go up exponentially
- Plateau
- Then go down
What is the induction period?
- Slow initial accumulation of hydroperoxides
- Predominantly the monomolecular reaction
What is the end of the induction period signaled by?
By the rapid rise in the PV; the point at which the bimolecular reaction comes into play
Why does PV fall off later on?
Due to the fact that the decomposition of hydroperoxides is outpacing their formation
What is the limitation of the PV test?
One can obtain a low peroxide value when the fat is rancid organoleptically since PV falls when the decomposition of hydroperoxides outpaces their formation
What can PV be correlated with to provide an indicator of acceptability?
With sensory data (taste panel)
Is there off-flavour associated with the induction period? Why/Why not?
- No off-flavour associated
- Because the hydroperoxides are mainly being formed, not broken down
What does the TBA test analyze? How is it different from PV?
- PV measures the initial stages of the autoxidation process (primary reaction products)
- TBA test looks at one of the many secondary breakdown products
What does TBA stand for?
Thiobarbituric Acid Test (TBA)
What is the assumption made in the TBA test?
- The quantities of malonaldehyde produced are directly proportional to the degree of oxidation
- This measure can be correlated to the flavour defects found in a fat or oil
What does malonaldehyde react with in the TBA test? What does it produce?
- Reacts with 2 molecules of TBA
- Produces a red complex, which can be readily assessed spectrophotometrically
What is malonaldehyde often found attached to? How is it released?
- Often found attached to proteins due to its high reactivity
- Has to be released using acids
What happens if the TBA test produces a red complex?
If its red, you know you have a lot of aldehydes present and the oil is rancid
What is another way to quantify malonaldehyde?
By HPLC
What can you do with the red complex produced from the TBA test?
- Measured spectrophotometrically
- Distilled off directly and reacted with TBA
Why do you need to carry out a sensory evaluation with a taste panel following the TBA test?
Since malonaldehyde formation is a function of the lipid makeup
What are other secondary oxidation product tests? (2)
- Anisidine value (AV): reaction of p-anisidine with aldehydes
- Carbonyl number
The rate of oxidation is a function of the ___________ of the fatty acids in the lipid system
degree of unsaturation
How does the autoxidation of linolenic acid compare to stearic acid?
2500 faster in linolenic acid
Why are molecules with double bonds more susceptible to abstraction of a hydrogen?
- Because the LIFETIME of the free radical formed is extended significantly
- Electrons from the double bonds can resonate and stabilize the free radical structure
Where are hydrogens abstracted from in unsaturated fatty acids during oxidation? Why?
- From the alpha position
- As they are more labile
What is resonance? What does it increase?
- The electron density is shuffled in between the carbons
- Increases
the lifetime of the radical - If the lifetime of the radical is increased, it has a chance to meet an oxygen
What is an important consequence of resonance concerning hydroperoxides?
Isomeric hydroperoxides can be formed, depending on which free radical form exists at the time oxygen attacks
How many positional hydroperoxide isomers can be formed in oleic acid?
Four
What can take place during molecular rotation while the molecule is in the resonant state?
Cis/Trans isomerization
Positional cis/trans isomers occur as ________ form and decompose.
Peroxy radicals
What kind of compound has an even more labile methylene group between the double bonds once resonance is initiated?
Unconjugated double bonds
Which oil is found naturally in the unconjugated form, but is readily converted into a mixed bonding system?
Linolenic acid
Where is the most unstable hydrogen in an oil?
The hydrogen from the methylene carbon
How do triplet oxygen and singlet oxygen differ in terms of reaction?
Triplet oxygen: waits for a radical to bind
Singlet oxygen: attacks a double bond (not energetically favored)
What does the mixed bonding configuration produce?
Produces an active methylene group, which is very readily abstracted to form a free radical and has many stable resonance forms
What happens to the linolenic hydroperoxides when the free radical has reacted with molecular oxygen?
Hydroperoxides are very unstable and will undergo almost immediate decomposition
Which kinds of oils will rapidly form low molecular weight aldehydes, ketones, alcohols, and carboxylic acids; even before detectable peroxide levels develop?
Linolenic acid or fatty acids with a higher degree of unsaturation
What explains reversion?
High reactivity and rapid decomposition of polyunsaturates
What is reversion?
- Rapid onset of rancidity without an apparent rise in the peroxide value
- An accelerated oxidation reaction, which does not allow hydroperoxides to linger
Why do soybean and fish oils undergo reversion?
if linolenic and higher unsaturated fat are NOT MODIFIED by hydrogenation
