Deterioration of fat

The unsaturated bonds of fats present active centres for

reaction with oxygen. This reaction leads to 1, 2, and

3oxidation which may make the fat-containing food

unsuitable for consumption.

The process of autoxidation and the resulting deterioration

in flavour is termed Rancidity.

Rancidity is the deterioration of fats and oils leading to

unpleasant odour

Rancidity occur in two ways:

Hydrolytic rancidity: reaction of fat with water with the liberation

of free fatty acids from glycerol (Fig.1)

The reaction is catalyzed by heat and enzymes known as lipases.

Hydrolytic rancidity is a problem with deep-fat frying where the

temperature is high and wet foods are introduced. The

continued use of rancid oil results in additional breakdown of the

oil

Butter contains lipase and if left on the counter on a warm day,

it will develop a characteristics rancid smell due to liberation of

short chain butyric acid.

To avoid hydrolytic rancidity, fats should be stored at cool places

or if possible deactivate lipases

Fig. 1: Hydrolytic rancidity

Oxidative rancidity: also referred to as autoxidation

It involves the oxidation and decomposition of fat into volatile

compounds with shorter carbon chains such as fatty acids,

aldehydes, ketones.

Unsaturated fatty acids are subjected to oxidative rancidity.

The more double bonds, the greater the opportunity for

addition of oxygen to double bonds, increasing the risk that the

fat or oil will become rancid.

Autoxidation is promoted by light, heat, certain metals (iron

and copper), heat, enzymes (lipoxygenases)

Flavour Reversion is oxidative rancidity of oils containing linolenic

acid. It requires less oxygen than the common oxidation, but

produces objectionable flavours in food.

Factors Affecting Rate of Oxidation

i. Amounts of O2 present

ii. Degree of unsaturation of the lipids

iii. Presence of antioxidants

iv. Presence of prooxidants, especially Cu and some organic

compounds e.g. haem-containing molecules and lipoxidase

v. Nature of packaging material

vi. Exposure to light

vii. Temperature of storage.

Stages of Autoxidation

• Initiation

• Propagation

• Termination

Initiation stage:

The formation of free radicals – a hydrogen on a carbon

atom adjacent to the one carrying a double bond is

displaced to give a free radical (Fig. 2)

The free radicals formed are very reactive and unstable

Fig. 2: Initiation stage of autoxidation

Propagation stage:

Follows the oxidation stage

Involves the oxidation of free radicals to yield activated

peroxide

This in turn displaces hydrogen from another unsaturated

fatty acid, thereby forming another free radical.

The liberated hydrogen unites with the peroxide to form

hydrogenperoxide and the free radicals can be oxidized

The reaction thus repeats and propagates (chain reaction),

i.e. that is formation of one free radical, leads to the

oxidation of many unsaturated fatty acids (Fig. 3)

Fig. 3: Reactions of the propagation stage of autoxidation

Hydroperoxides are very unstable and decompose

into compounds with shorter carbon chain, such as

volatile fatty acid, aldehydes ad ketones

These compounds are responsible for the

characteristics odour of rancid fats and oils

Termination

This stage involves the reaction of free radicals to form

nonradical products

Elimination of all free radicals halts the oxidation

reaction

R + R R-R

R + RO2 RO2R

nRO2● (RO2)n

Note

i. Hydroperoxides formed during propagation are very unstable

and break down into 2 oxidation products.

ii. Organoleptic changes are more closely related to the 2

oxidation products.

iii. Aldehydes are oxidised to FFAs, which may be considered

tertiary oxidation products.

iv. The peroxides have no importance from the standpoint of

flavour deterioration, which is wholly caused by the 2

oxidation products.

v. Although even saturated fatty acids can be oxidised, the rate of

oxidation of a fat depends on the degree of oxidation e.g. in

18C fatty acids 18:0, 18:1, 18:2, 18:3, the rate of oxidation has

been reported to be in the ratio 1:100:1200:2500.

Factors affecting rate of Autoxidation

i. Composition of fat in terms of degree of unsaturation and type of unsaturated fatty acid present

ii. Storage temperature

iii. Light and ionising radiation

iv. Removal of O2 from food.

v. Trace metals especially Cu, and to a lesser extent Fe, will catalysefat oxidation

vi. Metal deactivators (chelating agents) e.g. citric acid, EDTA (Ethylene diamine tetraacetic acid) –will reduce the effect of (v).

vii. Lipoxygenase (lipoxidase) and haeme compounds act as catalysts of oxidation.

viii. Antioxidants slow down fat oxidation.

ix. Concentration of natural antioxidants is higher in vegetable oil than in animal fat, hence vegetable oils are more stable.

Prevention of autoxidation

Fats and oils must be stored in a cool dark environment (temperate

and light change control), closed container (oxygen control)

Vacuum packaging controls oxygen exposure, colored glass or

wraps control fluctuation in light intensity

Fats must be stored away from metals that can catalyse the react

Lipoxygenases should be inactivated

Addition of sequestering agent to bind metals, thus preventing

them from catalyzing autoxidation. Examples include EDTA

(ethylenediamine-tetraacetic acid) and citric acid

Addition of antioxidants to prevent autoxidation with its formation

of fatty acid free radicals.

Antioxidants

Antioxidants help prevent autoxidation with its formation of fatty

acid free radicals.

Antioxidants prevent rancidity by donating a hydrogen atom to the

double bond in a fatty acid and preventing the oxidation of any

unsaturated bond.

They halt the chain reaction along the fatty acid, which leads to

rancidity.

They act by reacting with free radicals and thereby terminate the

chain reaction.

The antioxidant AH may react with the fatty acid free radical or

with the peroxy free radical

AH + R RH + A

AH + RO2 ROOH + A

The antioxidant free radical is deactivated by

further oxidation into quinines

A + RH AH + R

Only phenolic compounds which can easily produce

quinones are active as antioxidants

For the 2 competing reactions

RO2 + AH ROOH + A

RO2 + RH ROOH + R

The efficiency of the antioxidant (AH) increases with

decreasing A-H bond strength.

-Ideally, however, the resulting antioxidant free radical must

not itself initiate new free radicals or be subject to rapid

oxidation by a chain reaction.

Use of antioxidants in foods containing fat increases their

keeping quality and shelf life.

Examination of food labels reveals that antioxidants are used

widely in many food products, from potato chips to cereals.

Without them, the quality of fat-containing foods would not be

as good and off-flavors and odors due to oxidative rancidity

would be commonplace.

Most antioxidants are phenolic compounds.

Phenolic antioxidant are effective because:

i. They are excellent H2 or electron donors

ii. Their radical intermediates are relatively stable due to

resonance delocalisation, and to lack of position

suitable for attack by O2.

Hydroquinones for example react with hydroperoxy

radicals forming stable semiquinone resonance hybrids.

Most antioxidants are DIPHENOLS or related compounds.

OH OH

HO HO OH OH

Ortho meta para

Only the ortho and meta diphenols are effective.

Antioxidants may be Natural or Synthetic

Natural Antioxidants

NDGA

Gallic acid

-tocopherols (Vit. E).

NDGA –Nordihydroguairetic (Creosote bush)

An extremely effective antioxidant

Solubility in oil is limited, but can be increased with

heating.

Has a few carry-through properties

Tends to darken on storage in the presence of Fe or when

subjected to high temperatures.

Activity markedly influenced by pH. Destroyed under highly

alkaline conditions.

Very effective in the prevention of haematin-catalysed

oxidation in fat aqueous systems and in certain meats.

Properties:

i. Antioxidant activity due to phenolic structure and 3 OH

groups

ii. Soluble in water, but nearly insoluble in oil.

iii. Esterification of the carboxyl group with alcohols of varying

lengths produces alkyl gallates with increased oil solubility

iv. Of the gallates, propyl gallate is popularly used

v. It is effective in retarding lipoxygenase oxidation of

linoleate.

vi. In the presence of traces of iron, the gallates give rise to a

blue-black discolouration at alkaline conditions.

vii. Not effective for baking or frying since effectiveness is

rapidly lost during these operations.

Tocopherols

Tocopherols are naturally occurring antioxidants

that are present in vegetable oils. They can be

added to both animal and vegetable oils to prevent

oxidation.

The tocopherols are sources of essential nutrient

vitamin E.

Synthetic Antioxidants

Include: Butylated hydroxyanisole (BHA)

Butylated hydroxytoluene (BHT)

Tertiary butyl hydroxyquinone (TBHQ)

Propyl gallate (PG)

2,4,5 Trihydroxybutyrophenone (THBP)

The effectiveness of antioxidants may be increased if

they are used together. Propyl gallate and BHA are more

effective when combined than if used separately.

Both BHA (Butylated hydroxy anisole) and BHT (Butylated

hydroxytoluene) have found wide commercial use in the

food industry

BHA is a waxy white solid that survives processing to create

a stable product.

BHT is a white crystalline solid that may be combined with

BHA.

Both BHA and BHT are highly soluble in oil and exhibit weak

antioxidant activity in vegetable oil, particularly those rich

in antioxidants i.e. It is effective in preventing oxidation of

animal fats but not vegetable oils.

BHA and BHT are relatively effective when used in

combination with other 1 antioxidants.

BHA has a typical phenolic odour, which becomes

noticeable when the oil is heated to very high

temperatures

The antioxidant activity of NDGA is markedly influenced

by pH and is readily destroyed under highly alkaline

conditions.

TBHQ is a white-to-tan-colored powder that functions

best in frying processes rather than baking applications.

It is moderately soluble in oil and slightly soluble in

water

Usually more effective than other common antioxidants

in providing oxidative stability to crude and refined

polyunsaturated oils, without encountering problems of

colour or flavour stability

It has good carryover property

It is extensively used in oil for frying.

Modes of Application

Added directly to vegetable oils or to melted animal

fats after they are rendered.

Better results are achieved when the antioxidant is

added in a diluent e.g. monoglycerol and glycerol in

propyl glycol, monoglycerol-water emulsions, and

mixtures of antioxidants in volatile solvents.

Food sprayed or dipped in solutions or suspensions of

antioxidants, or

Packaged in films containing antioxidants.

END