Unit 4: Lipids - Rxns in Lipids, Lipolysis & Oxidation

the oxidation products of an unsaturated system produce what?

a colour reaction with TBA based on the formation of chromatin from the condensation of two molecules of TBA with one molecule of malonaldehyde.

what two reactions happen when saturated and unsaturated fatty acids undergo chemical decomposition?

thermal non-oxidative and thermal oxidative reaction.

how are free fatty acids produced during frying?

these arise from hydrolysis of triacylglycerols in the presence of heat and water. free fatty acids are very susceptible to oxidation resulting in volatile formation. one of the common tests for determining the quality of frying oil is the free fatty acid test.

How are nonpolymeric compounds produced during frying?

these compounds are produced from the various pathways involving alkyl radicals.

how are volatiles produced from oil during frying?

these compounds result from the decomposition if hydroperoxides and include saturated and unsaturated aldehydes, ketones, hydrocarbons, lactones, alcohols, acids and esters.

how are dimeric and polymeric acid/glycerids produced during frying?

these compounds result from thermal and oxidative free radical combinations. polymerization results in an increase in the viscosity of the frying oil.

what is the thermal oxidative reaction of Unsaturated fatty acids?

this reaction is most often encountered in foods during the heating of fats and oils. at high temperatures, oxidative decomposition occurs very rapidly, producing major compounds that are similar to those produced as a result of room temperature autooxiation.

What are the steps in lipase hydolysis?

triacylglycerol -> 1,2 diacylgylcerol OR 2,3 diacylglycerol -> 2-monoacylglycerol.

What happens to foods during frying?

water is continually released into the oil, which hastens hydrolysis to yield increased amounts of free fatty acids. volatiles may develop in the food itself or from the interaction between food and the oil. food absorbs varying amounts of oil from deep fat fryers and therefore, oil needs to be continuously added. food releases some of its endogenous lipids into the fryer.

What is oxidative rancidity?

Undesirable reaction that affects fats, oil, and fatty parts of foods caused by reactions involving oxygen.

The rate of oxidation greatly increases with higher levels of what?

Unsaturation.

Initiation may take place by:

1. Hydroperoxide decomposition. 2. metal catalysis 3. Exposure to light - creates singlet oxygen.

What are the 3 phases of lipid oxidation and what do they do?

1. Initiation: the formation of free radicals. 2. Propagation: the free-radical chain reactions. 3. Termination: the formation of non-radical products.

What are the 10 factors that influence the rate of lipid oxidation?

1. fatty acid composition. 2. free fatty acids vs. corresponding acylglycerols. 3. oxygen. 4. temperature. 5. surface area. 6. moisture. 7. pro-oxidants. 8.radiant energy. 9. enzymes. 10. antioxidants.

what are some of the changes that occur in oil during frying? (4)

1. increases in frying oil viscosity (as a result of polymer formation). 2. increase in fatty acid content. 3. development of a dark colour (due to polymeric compounds). 4. an increased tendency of the oil to form (due to compounds which migrate from the food into the oil).

What are antioxidants mode of action? (2)

1. inhibition of free radicals in the initiation step. 2. interrupting propagation of the free radical chain through donation of hydrogen atoms.

what are the 8 techniques to measure lipid properties?

1. peroxide value 2. thiobarbituric acid test 3. iodine value 4. chromatographic 5. refractive index 6. saponification value 7. free fatty acid value 8. solid fat index/content

Hydrolysis occurs as a result to what? (3)

1. thermal stress (heating of oils) 2. chemical reactions (saponification of NaOH) 3. enzymatic action

What are some control methods for frying oil? (6)

1. use of good quality oil with consistent stability. 2. use lowest frying temperature possible. 3. filter oil to remove food products. 4. replace oil. 5. use of antioxidants. 6. frequent testing of oil throughout the frying process.

Characteristic of the antioxidant tocopherols (4)

1. vitamin E derivatives 2. primary antioxidant in vegetable oils. 3. work best at low levels 4. at high levels they may act as pro-oxidants.

What are the classes of compounds produced from oil during drying? (4)

1. volatiles: 2. nonpolymeric polar compounds 3. dimeric and polymeric acids and dimeric and polymeric glycerides. 4. free fatty acids.

What are lipoxygenases (LOX)?

A group of enzymes that can cause the oxidation of lipids.

In the first step of lipid oxidation (initiation) what takes place?

A hydrogen is abstracted from a fatty acid chain, producing free radicals.

What does the major pathway for the oxidation of unsaturated fatty acids involve?

A self-catalytic free radical mechanism that accounts for the chain reaction of hydroperoide formation and decomposition.

Free fatty acids are absent in what? But can be formed by what?

Absent in the fat of living animals but can be formed by enzymatic action after an animal is killed.

What major class of compounds result from autoxidation?

Aldehydes.

What does the primary antioxidant act as? What does the synthergist act as?

Antioxidant acts as a free radical acceptor. The synergist acts as a metal sequestering agent.

Hydroperoxies begin to decompose when?

As soon as they are formed.

During the initiation phase in an unsaturated fatty acid, where does the abstraction of the H atoms take place?

At positions next to double bonds known as a-methylenic carbon.

Where does phospholipase B hydrolyze?

At the sn-1 of the corresponding lysolecithin to result in glycerophosphocholine.

Where does phospholipase A1 hydrolyze?

At the sn-1 position.

Where does phospholipase A2 hydrolyze?

At the sn-2 position.

Where does phospholipase C hydrolyze?

At the sn-3 position resulting in diacylglycerol and phosphorylcholine.

Antioxidants.

Both natural and synthetic can delay the onset or slow the rate of oxidation of autoxidizable materials.

Disruption of the cellular structure is referred to as what?

Decompartmentalization

The development of high levels of free fatty acids is associated with what in oil/food? (3)

Decreases the smoke point and surface tension of the oil and a reduction in the quality of fried food.

Endogenous lipases are important for what?

Desirable flavours (cheese) and undesirable changes.

The antioxidant propyl gallate is effective due to what?

Due its phenolic structure and three OH groups.

Lipolysis is the major reaction during what? And what is it due to?

During deep fat frying. It is due to the large amounts of water introduced from the food and the relatively high temperatures at which the oil is maintained.

What are lipases?

Enzymes that hydrolyze ester linkages of emulsified triaclyglycerols at an oil/water interface.

Lipid oxidation is one of the major causes of what?

Food spoilage.

During the final stage of lipid oxidation (termination) what takes place?

Free radicals combine to form non-radical products.

the saponified sample is back titrated with what?

HCl

Autoxidation for oleate

Hydrogen abstractions occur at C8 & C11 to give to allylic racial intermediates. Oxygen attacks then results in an isomeric mixture of 8,9,10 and 11-allylic hydroperoxides. The 8 and 11 hydroperoxides are formed slightly more than the 9, and 10 isomers.

What is the primary oxidation products of autoxidation?

Hydroperoxides.

Oxygen.

If the supply of oxygen is unlimited, the rate of oxidation is independent of oxygen pressure, but at very low oxygen pressure, the rate is approximately proportional to oxygen pressure.

what is autoxidation of fats? and what is it characterized by?

It is a free radical mediated process that is characterized by: 1. catalysis of light and by free radicals producing substances. 2. high yields of hydroperoxide 3. marked inhibition in rate by chemical species known to interfere with other well-established free radical reactions.

What is phospholipase D and where does it hydrolyze?

It is choline-hydrolyzing enzyme with hydrolyzes at sn-4 to give phosphatidic acid.

what is lipid oxidation?

It is the process where lipid molecules degrade after oxidation reaction.

What happens when the water activity is increased to 0.3?

It retards oxidation to a minimum by reding the catalytic activity of metal catalysts (metal catalysts become hydrated, and therefore less active), by quenching free radicals (quenched free radicals cannot participate in oxidation easily), by promoting non-enzymatic browning (has antioxidant activities) and/or impeding the access of oxygen to food.

Where are lipoxygenases present in?

Legumes, soybeans, mung beans, navy beans, green beans, peas and peanuts.

Rancidity

Lipid deterioration that produces off-flavours and off-odors

Moisture.

Lipid oxidation depends strongly on water activity. In dried foods, with very low water activity, oxidation proceeds very rapidly. When the water activity is higher, the rate increases again as a result of increased mobilization of catalysts.

Enzymes.

Lipoxygenase is an important catalyst in lipid oxidation.

The resulting compound with a hydroxyl at the sn-2 position is known as what?

Lysolecithin.

Pro-oxidants.

Major pro-oxidants include transition metals, particularly those possessing two or more valency states (Ca2+, Fe2+ Cu+) with a suitable oxidation/reduction potential between them (Cobalt, copper, iron, manganese and nickel).

In the second step of lipid oxidation (propagation) what takes place?

Molecular oxygen (O2), combines with free radicals to form proxy radicals. These in turn, abstract H from a-methylenic groups of RH to form hydroperoxides (ROOH) and free radicals. The new R groups then react with molecular O2 and the sequence is repeated.

The majority of antioxidants are what?

Monohydric or polyhydric phenols with various ring substitutions.

The decomposition products of hydroperoxides are dependent on what?

On the specific hydroperoxide and its position in the parent molecule.

For maximum efficiency, the primary antioxidant is used in combination with what?

Other phenolic antioxidants and, or with various synergists.

Temperature.

Oxidation rate increases as temperature increases.

Free fatty acids are more susceptible to what?

Oxidation.

What are the type types of rancidity?

Oxidative and hydrolytic.

Autoxidation for linolenates

Two 1,4-pentadiene structures are present. Hydrogen abstraction at the two active methylene groups of carbons 11 and 14 produce two pentadienyl radicals. Attack of oxygen at the carbon end of each radical results in the formation of a mixture of isomeric 9,12,13 and 16 hydroperoxides.

What is one way to to monitor the extent of lipid oxidation?

Peroxide value determination. However, with time, the level of peroxides decreases as the peroxides decompose into things like formic acid, which increases in an oxidized fat.

What is lipolysis?

Refers to hydrolysis of the ester linkage between glycerol and a fatty acid in the triacylglycerol molecule.

What is propyl gallate effective in doing?

Retarding lipoxygenase oxidation of linoleic acid.

What is believed to be the active species in photo-oxidative deterioration?

Singlet oxygen. This is because it is more electrophilic and reacts rapidly with moieties of high density.

What does frying refer to?

Situations where foods are fried in hot oil in the presence of air. the finished product usually contains 5-40% oil.

what compounds interfere with the TBA test?

Sucrose and some compounds of wood smoke react with TBA. Malonaldehyde reacts with proteins.

In the food industry, lipid oxidation is of major concern since it leads to what?

The development of various off-flavours and off-odours in edible oils and fat containing foods.

What is involved in the first step in hydroperoxide decomposition?

The formation of alkoxy and hydroxy free radicals.

Surface area.

The rate of oxidation increases in proportion to the surface area of lipid exposed to air.

What is responsible for the development of rancid flavours in milk?

The release of SCFAs by hydrolysis.

Autoxidation for linoleate

The structure of linoleate is 20 times more susceptible to oxidation than oleate. H abstraction produces a pentadienyl radical intermediate upon reaction with oxygen to give an unequal mixture of conjugated 9, 13-diene hydroperoxides.

during frying and other processes, high temperatures can lead to what?

Thermal decomposition of lipids.

Why are the lipoprotein lipases of milk important?

They are important since they release SCFAs from milk fat that leads to undesirable hydrolytic rancidity.

Peroxides do not contribute to what?

They do not contribute to flavour deterioration. However, they are unstable and decompose into secondary oxidation products.

what is the thermal non-oxidative reaction with saturated fatty acids?

Very high temperatures are required to produce substantial non-oxidative decomposition of saturated fatty acids and detectable products consist mostly of hydrocarbons, acids and ketones.

Radiant energy

Visible and ultraviolet radiation can promote the generation of free radicals, thereby leading to autoxidation.

Hydroperoxides decompose into what?

Volatile flavour molecules.

Butylated hydroxyanisole (BHA) and butylated hydroxytoluene (BHT) are what?

Weak antioxidants in vegetable oils. BHA has a phenolic odour that becomes noticeable when the oil is subjected to high heat.

When does synergy exist?

When a mixture produces a more pronounced activity than the sum of the individual parts.

How does LOX work in regards to the aroma of a tomato?

When the tomato is cut, the cellular structure is broken and this allows for the LOX and the substrate to come together, allowing LOX to oxidize the fatty acid.

Controlled and selective lipolysis is also used in the manufacture of other foods such as what?

Yogurt and bread.

alkanals, alkenals and 2,4-dienals produce what?

a yellow pigment at 450nm

specific gravity increases with what? and decreases with what?

an increasing proportion of unsaturated fatty acid triacylglycerols, and decreases with an increasing proportion of long-chain fatty acid triacylglycerols.

hydrogenation results in the refractive index to what ? as a result of what?

decrease, as a result of a decrease in unsaturation.

In the presence of iron, propyl gallate does what?

discolours at alkaline pH, but the colour is lost during baking or frying.

Free fatty acids vs. corresponding acylglycerols

fatty acids oxidize at a slightly greater rate when free, than when esterified to glycerol.

flavour reversion is characterized by what?

grassy, painty, and fishy off-flavours. these flavour originante from volatile oxidation products that result from the terminal pentane radical of linolenic acid.

Conjugated unsaturation results in what kind of refractive index than unconjugated unsaturation?

higher.

increases in molecular weights and degree of unsaturation causes what in the refractive index?

increase.

What is the iodine value related to?

iodine value is related to melting point or hardness and to unsaturated fatty acid triacylglycerols in fats.

what is a flavour reversion?

it is a type of oxidative rancidity that can occur in fats, like soybean oil, which contain linolenic acid.

what is the solid fat index/content?

it is an empirical measure of the solid fat content and is actually a measurement of specific gravity and density.

in the saponification test how is the sample prepared?

it is prepared in alcoholic alkali under reflux conditions and alkali is consumed not only for the saponification of all the acylglycerols but also for the neutralization for all free fatty acids in the sample.

What is the iodine value test?

it measures unsaturated linkages in a fat and is expressed in terms of % iodine absorbed.

what are the different types of chromatographic tests? (4)

liquid, thin-layer, high performance liquid, and gas chromatography.

saponification value is an index of what?

mean molecular weight of the acylglycerols comprising of a fat.

Thiobarabituric acid test (TBA)

most widely used test for evaluating lipid oxidation.

what is the peroxide value test?

peroxides are the main initial products of autoxidation. the test is based on their ability to liberate iodine from Kr or to oxidize ferrous iron to ferric ion. results are expressed as milliequivalent 0.2 fat.

The most important way singlet oxygen is generated is by what?

photosensitization by the natural pigments of food.

what is the thermal non-oxidative reaction of Unsaturated fatty acids?

predominant reaction is the formation of dimeric compounds which requires severe temperatures. in addition, other substances of low molecular weights are formed.

Dicnals produce what kind of pigment?

red (530nm)

if a fat contains large amounts of unsaponifiables it would have a low what?

saponification value.

what is the thermal oxidative reaction of saturated fatty acids?

saturated acids and esters are more stable, but when heated in air at temperatures > 150C, saturated lipids undergo oxidation. the decomposition pattern is complex, with major breakdown products including carboxylic acids, 2-alaknones, n-alkanals, lactones, n-alkanes, and 1-aklenes.

what is the refraction index related to?

the density, molecular weight, and internal structural arrangement of fatty acids.

what is the fatty acid value defined as?

the milligrams of KOH required to neutralize the free fatty acids in a gram of fat. assay involves titration with potassium hydroxide and is a measurement of the amount of fatty acids that have been freed by hydrolysis.

what is the saponification value defined as?

the milligrams of KOH required to saponify 1g of fat.

Fatty acid composition

the number, position and geometry of double bonds affect oxidation rate.

Oxidative reactions can also decrease what?

the nutritional quality of a food.

Autoxidation is the main reaction involved in what?

the oxidative deterioration of lipids.

the free fatty acid value is is dependent on what? (4)

the quality of fat in terms of purity, age, extent of hydrolysis and degree of oxidation.

the different methods of chromatography are based on?

the separation and quantitative measurement of certain fractions that are volatile, polar, or polymeric components associated with autoxidation.