Reactions and Behavior in Deep Frying of Food

 
BINITA RANI
ASSOCIATE PROFESSOR (DAIRY CHEMISTRY)
FACULTY OF DAIRY TECHNOLOGY
S.G.I.D.T., BVC CAMPUS,
P.O.- BVC, DIST.-PATNA-800014
 
FOOD CHEMISTRY
DTC-321     Credit hours-3(2+1)
 
Reactions involved during
Deep frying of food
 
Deep frying 
 
food preparation 
 in home and in industry.
Food products 
 potato chips, meat, fish, 
 prepared by frying 
 into fat/oil heated to
about 180 °C.
frying process 
 food article 
 
sufficiently cooked 
 
consumed.
Prolonged heating 
 
high temperature 
 
substantial changes  
 chemical and physical
properties 
 fat or oil 
 
affects the 
quality
 
of fat/oil 
& 
quality of finished foods
.
 
Short  frying period 
 production of 
desirable
 flavour and aroma 
       organoleptic
quality of product.
 
 In 
continuous deep fat frying 
 large quantities of fat 
 absorbed 
by food 
replenished
 
by fresh frying oil 
 
steady state condition wherein  
 
it is unlikely that the oil
deteriorates
 beyond a certain point.
 
 In 
intermittent frying 
 fats 
remain hot for long periods 
 
many heating and cooling
cycles 
 
more 
rapid
 
destruction of fat 
 due to increase in 
hydroperoxides
 upon cooling
 their 
decomposition  
  when fat is 
reheated
.
 
BEHAVIOUR OF FRYING 
OIL
 
Deep fat frying
 different classes of 
compounds 
:
1. Volatiles
 Oxidative reactions 
lead to production of:
alcohols,
 acids,
esters,
hydrocarbons,
Ketones,
lactones  and
Saturated 
&
 unsaturated aldehydes,
 
 
A
mounts
 of volatiles produced vary 
:
 
type of oil,
 type of food and
 extent of heat treatment
 
2. Nonpolymeric polar compounds of moderate volatility
e.g. hydroxyl & epoxy acids produced 
 through 
oxidative pathways
3. Dimeric and polymeric acids and glycerides
Thermal and oxidative free radicals 
 polymerization 
 
 substantial increase
in viscosity of the frying oil
4. Free fatty acids
Hydrolysis of triacylglycerol 
 
 in presence of heat and water.
 
These reactions 
 responsible 
 
physical and chemical changes 
 
frying oil
Increase in 
foaming tendency 
and 
viscosity
Changes in 
flavor
 and 
colour (dark)
Decrease in 
surface tension 
and 
iodine value
Changes in 
refractive indices
 
    
Behaviour of 
food
 during frying
 
Water
 
 food 
 hot oil 
 
steam distillation
 
effect
 
 sweeping volatile
oxidative products from oil.
Released moisture 
 
agitates the oil 
 
hastens hydrolysis
.
 
Blanket of steam 
above the surface of oil tends to  
   the amount of oxygen 
oxidation
Volatiles
 
develop
 in food 
itself 
and/or from interactions between 
food and oil
.
 Food 
absorbs
 varying amounts of oil – 
5 to 40% 
e.g., potato chips 
 final
fat content 35%
Food 
release
 its 
endogenous lipids
 
 frying oil/fat e.g., fat from chicken
oxidative stability 
of new mixture 
 
different
 from original frying oil/fat
 
CHANGES IN FRYING MEDIUM
 
During deep fat frying :
 
Hydrolysis,
 oxidation and
 polymerization
 
Factors
 
 
proportions of breakdown components 
in vegetable oils :
Temperature 
 
                Method of heat transfer
Presence of O2 
 
       Metals in contact with oil
Heating time
 
                Turnover
Frying capacity
 
       Nature of food being fried
 
commonly observed chemical changes are
:
 
OXIDATION AND DECOMPOSITION
 
Moisture
, 
high
 
temperature
 and 
atmospheric O2 
 
favours
 the
 oxidation 
of
frying medium.
oxygen introduced into the 
oil with food 
 
oxidative
 changes.
 
peroxide content
 
of food begins to 
increase 
after an initial induction period
and 
finally decreases
.
Major reactions during autoxidation 
 
degradation
 reactions 
 
formation of
volatile compounds.
 
Autoxidation
 of PUFA 
 
formation of conjugated 
hydroperoxides
 and
peroxides
 
 
decompose
 
 
volatile acids, aldehydes
, 
alcohols, ketones 
 and
hydrocarbons.
 
     
THERMAL OXIDATION
In the presence of 
O2 
when oil is heated at 
high temperature 
:
 
Formation of 
free fatty acids
Formation of 
hydroperoxides
 
which may undergo :
 
1. Fission
 
 form 
alcohol, aldehydes 
and 
acids
 
 
darkening
 of frying
medium and 
flavour
 changes.
2. 
Dehydration
 
 form 
ketones
3. 
Formation of 
free radicals 
 
their
 combination 
to form 
dimers,
trimers, epoxides, alcohols 
and 
hydrocarbons 
 
increase viscosity
 
of the
oil.
 
During deep frying 
 
thermal and oxidative decomposition 
of oil 
 volatile and non-
volatile products:
 
     1. 
Volatile decomposition products:
 
Most of 
them (generated during frying) 
 
removed by steam
.
 
They (e.g. unsaturated lactones) 
 
flavour
 
     2. 
Non volatile decomposition products:
F
ormed 
 
thermal oxidation 
and
 polymerization 
of 
unsaturated fatty acids
  
 (frying
medium).
Include 
cyclic acids, fatty acids,
 
polymeric triglycerides, 
and 
other oxidative products
.
Accumulation
 of these products 
 
changes:
 
increase 
in carbonyl value
, FFA content, 
-OH content
 and 
saponification value 
and
decrease
 in 
unsaturation
 
  
 
Iodine value
.
Also 
increase in viscosity 
and 
refractive index
.
 
POLYMERIZATION
 
Oxidation and thermal alteration
 products 
 undergo polymerization 
 forming 
gums
and 
residues
.
 Rks b/w 
FA 
of same/different triglycerides 
 
cyclic and non-cyclic dimers 
and other
polymeric compounds
.
Thermal polymerization of 
PUFA 
 
cyclic monomers, dimers, trimers 
and
 higher
polymers.
Rate of polymerization     with 
increase in 
unsaturation
 
of  TG
  and 
frying time
.
 This results 
 
changes 
 darkening of colour, foaming, heat transfer rate, viscosity,
molecular weight, 
and
 gum accumulation.
 
Polymerization 
 
increased 
fat
 
 absorption
 by food 
 
unpalatable and greasy.
 
HYDROLYSIS 
:
 
Moisture
 from food during frying 
 
hydrolysis
 
of fat 
 
increase in acidity
 
initial formation of FFAs, mono and diglycerides and glycerols
.
Soaps
 of some fatty acids 
 
formed
 
 accelerate 
deterioration of frying
medium
.
 Accumulation of 
alkaline material   
the interfacial tension 
b/w product and
frying medium 
 
decreases the food quality
.
 Liberation of 
FFAs
 
 decrease in 
smoke point 
of oil.
Colour , iodine 
value and viscosity
 of hydrogenated oils 
 changes more
rapidly at 
FFA levels of ~1.5%.
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The process of deep frying involves reactions that lead to changes in the chemical and physical properties of fats or oils, influencing the quality of the finished food products. Factors such as heat treatment, type of oil, and food impact the production of volatile compounds during frying. The behavior of frying oil changes due to oxidative reactions, leading to the production of various compounds. Food interactions with hot oil result in the absorption of oil, affecting the final fat content. Understanding the reactions and behaviors in deep frying is crucial for maintaining the quality of fried foods.


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  1. FOOD CHEMISTRY DTC-321 Credit hours-3(2+1) Reactions involved during Deep frying of food BINITA RANI ASSOCIATE PROFESSOR (DAIRY CHEMISTRY) FACULTY OF DAIRY TECHNOLOGY S.G.I.D.T., BVC CAMPUS, P.O.- BVC, DIST.-PATNA-800014

  2. Deep frying food preparation in home and in industry. Food products potato chips, meat, fish, prepared by frying into fat/oil heated to about 180 C. frying process food article sufficiently cooked consumed. Prolonged heating high temperature substantial changes chemical and physical properties fat or oil affects the quality of fat/oil & quality of finished foods. Short frying period production of desirable flavour and aroma organoleptic quality of product. In continuous deep fat frying large quantities of fat absorbed by food replenished by fresh frying oil steady state condition wherein it is unlikely that the oil deteriorates beyond a certain point. In intermittent frying cycles more rapid destruction of fat due to increase in hydroperoxides upon cooling their decomposition when fat is reheated. fats remain hot for long periods many heating and cooling

  3. BEHAVIOUR OF FRYING OIL Deep fat frying different classes of compounds : 1. Volatiles Oxidative reactions lead to production of: alcohols, acids, esters, hydrocarbons, Ketones, lactones and Saturated & unsaturated aldehydes, Amounts of volatiles produced vary : type of oil, type of food and extent of heat treatment

  4. 2. Nonpolymeric polar compounds of moderate volatility e.g. hydroxyl & epoxy acids produced through oxidative pathways 3. Dimeric and polymeric acids and glycerides Thermal and oxidative free radicals polymerization substantial increase in viscosity of the frying oil 4. Free fatty acids Hydrolysis of triacylglycerol in presence of heat and water. These reactions responsible physical and chemical changes frying oil Increase in foaming tendency and viscosity Changes in flavor and colour (dark) Decrease in surface tension and iodine value Changes in refractive indices

  5. Behaviour of food during frying Water food hot oil steam distillation effect sweeping volatile oxidative products from oil. Released moisture agitates the oil hastens hydrolysis. Blanket of steam above the surface of oil tends to the amount of oxygen oxidation Volatilesdevelop in food itself and/or from interactions between food and oil. Food absorbs varying amounts of oil 5 to 40% e.g., potato chips final fat content 35% Food release its endogenous lipids frying oil/fat e.g., fat from chicken oxidative stability of new mixture different from original frying oil/fat

  6. CHANGES IN FRYING MEDIUM During deep fat frying : Hydrolysis, oxidation and polymerization Factors Temperature Method of heat transfer Presence of O2 Metals in contact with oil Heating time Turnover Frying capacity Nature of food being fried proportions of breakdown components in vegetable oils :

  7. commonly observed chemical changes are: OXIDATION AND DECOMPOSITION Moisture, high temperature and atmospheric O2 favours the oxidation of frying medium. oxygen introduced into the oil with food oxidative changes. peroxide content of food begins to increase after an initial induction period and finally decreases. Major reactions during autoxidation degradation reactions formation of volatile compounds. Autoxidation of PUFA formation of conjugated hydroperoxides and peroxides decompose volatile acids, aldehydes, alcohols, ketones and hydrocarbons.

  8. THERMAL OXIDATION In the presence of O2 when oil is heated at high temperature : Formation of free fatty acids Formation of hydroperoxides which may undergo : 1. Fission form alcohol, aldehydes and acids darkening of frying medium and flavour changes. 2. Dehydration form ketones 3. Formation of free radicals their combination to form dimers, trimers, epoxides, alcohols and hydrocarbons increase viscosity of the oil.

  9. During deep frying thermal and oxidative decomposition of oil volatile and non- volatile products: 1. Volatile decomposition products: Most of them (generated during frying) removed by steam. They (e.g. unsaturated lactones) flavour 2. Non volatile decomposition products: Formed thermal oxidation and polymerization of unsaturated fatty acids (frying medium). Include cyclic acids, fatty acids, polymeric triglycerides, and other oxidative products. Accumulation of these products changes: increase in carbonyl value, FFA content, -OH content and saponification value and decrease in unsaturation Iodine value. Also increase in viscosity and refractive index.

  10. POLYMERIZATION Oxidation and thermal alteration products undergo polymerization forming gums and residues. Rks b/w FA of same/different triglycerides cyclic and non-cyclic dimers and other polymeric compounds. Thermal polymerization of PUFA cyclic monomers, dimers, trimers and higher polymers. Rate of polymerization with increase in unsaturation of TG and frying time. This results changes darkening of colour, foaming, heat transfer rate, viscosity, molecular weight, and gum accumulation. Polymerization increased fatabsorption by food unpalatable and greasy.

  11. HYDROLYSIS : Moisture from food during frying hydrolysis of fat increase in acidity initial formation of FFAs, mono and diglycerides and glycerols. Soaps of some fatty acids formed accelerate deterioration of frying medium. Accumulation of alkaline material the interfacial tension b/w product and frying medium decreases the food quality. Liberation of FFAs decrease in smoke point of oil. Colour , iodine value and viscosity of hydrogenated oils changes more rapidly at FFA levels of ~1.5%.

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