CHEMICAL REACTIONS of DEEPFAT FRYING OF FOODS

1
CHEMICAL REACTIONS of DEEP-FAT FRYING OF FOODS
2
Deep
-
Fat Frying

Deep
-
Fat Frying is a process of immersing food in hot
oil.

A process of cooking and drying produces unique
fried
foods by simultaneous heat and mass transfer .

Flavor compounds are formed and retained in a
crisp crust
of food
3
Annual Sales of Deep Fat Fried Foods in U.S.
Food Industries Restaurants 5?109
pounds 2?109 pounds 15 billion per year
Sales of fried foods in U.S. increased by 35
from 1987 to 1996.
(Snack Food Association, 1997).
4
Physical and Chemical Reactions during Deep-Fat
Frying
5
Physical Changes of Deep-Fat Frying

• Viscosity Increase
• Thickening of oil
• Decrease of interfacial tension
• Increase of density
• Increase of the specific heat

6
Oil Contents in Deep-Fat Fried Foods
Products Oil Contents () Potato
chips 33-38 Corn chips 30-38 Tortilla
chips 23-30 Doughnuts 20-25 Frozen
food 10-15 French fries 10-15
7
Chemical Changes of Deep-Fat Frying

• Formation of flavor
• Flavor stability and quality changes
• Color and texture of the fried foods changes
• Nutritional changes

8
Chemical Reactions in Frying Oil

• Hydrolysis
• Oxidation
• Polymerization
• Pyrolysis

9
Chemical Reactions in Deep-Fat Frying of Foods
Volatile Flavor Compounds 220 volatile
compounds have been identified.
10
Deep-Fat Fried Flavor
4-hydroxy-2-nonenoic acid, lactone 4-
hydroxy-3-nonenoic acid 2,4-decadienal nutty,
fried fat notes plus a butter-like note Some of
volatile compounds formed in deep-fat frying
condition are known as toxic compounds.
Example 1,4- Dioxane Benzene
Toluene Hexyl-benzene
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Volatile Products from Deep-Fat Frying
Acids -- Saturated Acids Unsaturated acids
(cis, trans) Hydroxy acids Hydrocarbons
Saturated hydrocarbons Unsaturated
hydrocarbons Alcohols Aldehydes -
Saturated Unsaturated Ketones Esters Aromatic
Compounds Lactones Miscellaneous 2-Pentyl
furan 1,4-Dioxane
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Aromatic Compound Formation
C
H
C
H
C
H
C
H
C
H
C
H
(
C
H
)
C
H
n
3
2
2
3
O
2
C
H
C
H
C
H
C
H
C
H
C
H
(
C
H
)
C
H
n
3
2
3
O
O
H

R

-
ROH
C
H
C
H
C
H
C
H
C
H
C
H
(
C
H
)
C
H
n
3
2
3
O


OR

13
C
H
C
H
C
H
C
H
C
H
C
n
(
C
H
)
C
H
3
2
3
O
-
O
H
2
(
C
H
)
C
H
n
2
3
14
Aromatic Compound Formation
C
H
(
C
H
)
C
H
C
H
C
H
C
H
C
H
C
H
(
C
H
)
C
O
O
R
3
2
3
2
2
2
7
O
2
O
C
(
C
H
)
C
O
O
R
2
7
C
H
C
H
(
C
H
)
C
H
2
2
2
2
3
D
-
O
H
2
C
H
(
C
H
)
C
O
O
R
2
2
6
C
H
(
C
H
)
C
H
2
2
2
3
15
Formation of ?-2 Nonelactone
R
C
H
C
H
C
H
C
H
C
H
C
H
C
H
C
H
C
H
C
H
3
2
2
2
2
2
H
C
H
C
H
C
C
H
C
H
C
H
C
H
C
H
C
H
C
H
R
3
2
2
2
2
O
O
H
.
C
H
C
H
C
H
C
H
C
H
C
H
C
H
C
H
C
H
3
2
2
2
2
.

OH
C
H
C
H
O
H
C
H
C
H
C
H
C
H
C
H
C
H
C
H
3
2
2
2
2
16
C
H
C
H
O
H
C
H
C
H
C
H
C
H
C
H
C
H
C
H
3
2
2
2
2
O
C
H
C
H
C
H
C
H
C
H
C
H
C
H
C
H
C
3
2
2
2
2
2
H
- H

O
.
C
C
H
C
H
C
H
C
H
C
H
C
H
C
H
C
H
3
2
2
2
2
H
O
2
O
.
C
C
H
C
H
C
H
C
H
C
H
C
H
C
H
C
H
3
2
2
2
2
O
H
.
OH

O
C
H
C
H
C
H
C
H
C
H
C
H
C
H
C
C
H
O
H
3
2
2
2
2
O
H
-H
O
2
g
-2-Nonelactone
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Effect of Methionine Analogs on Potato
Chip-Flavor in Deep-Fat Frying.
Structures
Flavor Characteristics
Compounds
D-Methionine L-Methionine DL-Methionine
  Good potato chip-like
  CH3-S-CH2-CH2-CH(NH2)COOH
  S-Methyl-L-Cysteine
  CH3-S-CH2-CH(NH2)COOH
  Good potato chip-like
  Obnoxious (cooked turnip)
  Methionine Hydroxy Analog
  CH3-S-CH2-CH2-CH(OH)COOH
  HOOC-CH2-S-CH2-CH(NH2)COOH
  Obnoxious (cooked turnip)
  S-Carboxymethyl-L-Cysteine
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Polymer Formation
Carbon-Carbon Bond
A.
Vinyl Type
C
C
C
C
C
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Carbon-Carbon Bond
H
H
H
H
H
C
C
C
C
C
- .H
H
H
H
H
H
C
C
C
C
C
H
H
H
C
C
H
H
C
C
H
H
H
C
C
C
H
C
C
Intermolecularly or Intramolecularly
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Carbon-Oxygen Bond
Through peroxide group formed by autoxidation.
O
O
This can be formed intermolecularly or
intramolecularly.
Through ether linkage formed at high
temperature.
O
O
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Polymers Formed during Deep-Fat Frying
The 74 hrs. deep-fat frying conditions
Trilinolenin 26.3 Trilinolein 10.0
Triolein 10.8 Tristearin 4.2
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Types of Polymers
Trilinolein
Monocyclic, Nonpolar C-C Dimer 4.9
C
O
O
R
C
O
O
R
Noncyclic, Polar C-C Dimer 2.8
COOR
OH
COOR
OH
Trimers
-
C-C, 8.4
Trimers
-
2 C-O, or 1 C-C, 1 C-O 4.9
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Diels-Alder Reaction
C
O
O
H
C
O
O
H
C
O
O
H
C
O
O
H
C
O
O
H
C
O
O
H
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Dimerization
C
H
2
C
H
C
H
2
C
H
2
C
H
C
H
2
or
Dimerization
Between Two
Acyl Groups in the Same
Triglyceride
Dimerization
Between Two
Acyl Groups in Two
Triglycerides
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Composition of Oxidized and Polymerized Materials
Formed during Simulated Deep-Fat Frying at 185?C
for 74 Hrs.
  TRILINOLEIN
  TRIOLEIN
Cyclic Dimers Carbon-to-Carbon Linkages
4.9
0.0
0.0
  Noncyclic Dimer Carbon-to-Carbon Linkages
2.8
3.4
0.7
  Trimers Two Carbon-to-Carbon Linkages
0.3
8.4
0.4
1.2
  Dimers and Trimers Carbon-to-Carbon or Oxygen
Linkages
6.2
4.9
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Biological Effects of Used Frying Oil

• A slight depression in growth to very poor
  growth
• Diminished feed efficiency
• Increased liver, kidney and heart sizes
• Fatty tissues of liver, kidney and heart organs
• Liver enzymes such as thiokinase and
  succinyldehydrogenase
• had lower activity
• The evidence of carcinogenicity (in highly
  abused frying oil)

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Safety
Under Good Practice of Deep-Fat Frying Fats are
not nutritionally damaged
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Frying Oil Analyses by Liquid Chromatography
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Thermal Oxidation Effect on Linoleic Acid
Concentration
a
Levels of Linoleic Acid
FAT
FRESH
OXIDIZED
Corn oil
61.0
1.1
Olive oil
7.7
Trace
b
Lear
21.7
1.1
Lard
10.7
1.4
a
Expressed as of total fatty acids.
b
Lear Low erucic acid rapeseed oil.
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Toxicity Symptoms of Highly Heat-Abused Oils to
Laboratory Animals
Irritation of the digestive tract Organ
enlargement (kidney liver) Growth
depression Carcinogenic properties
Good Practice of Deep-Fat Frying Fats are
not nutritionally damaged.