Transcript
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
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