CHEMICAL REACTIONS of DEEP-FAT FRYING OF FOODS

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

Annual Sales of Deep Fat Fried Foods in U.S.

Food Industries Restaurants

5109 pounds 2109 pounds

$ 15 billion per year

Sales of fried foods in U.S. increased by 35% from 1987 to 1996.

(Snack Food Association, 1997).

Physical and Chemical Reactions during Deep-Fat Frying

Physical Changes of Deep-Fat Frying

• Viscosity Increase

• Thickening of oil

• Decrease of interfacial tension

• Increase of density

• Increase of the specific heat

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

Oil Contents in Deep-Fat Fried Foods

• Formation of flavor

• Flavor stability and quality changes

• Color and texture of the fried foods changes

• Nutritional changes

Chemical Changes of Deep-Fat Frying

• Hydrolysis

• Oxidation

• Polymerization

• Pyrolysis

Chemical Reactions in Frying Oil

Chemical Reactions in Deep-Fat Frying of Foods

Volatile Flavor Compounds:

220 volatile compounds have been identified.

Deep-Fat Fried Flavor

4-hydroxy-2-nonenoic acid, lactone4- hydroxy-3-nonenoic acid2,4-decadienalnutty, 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

Volatile Products from Deep-Fat Frying

Acids -- Saturated AcidsUnsaturated acids (cis, trans)Hydroxy acids

Hydrocarbons – Saturated hydrocarbons Unsaturated hydrocarbons

AlcoholsAldehydes - Saturated

UnsaturatedKetonesEstersAromatic CompoundsLactonesMiscellaneous: 2-Pentyl furan

1,4-Dioxane

Aromatic Compound Formation

C H3 C H C H C H C H C H2 (C H2)n C H3

O 2

C H3 C H C H C H C H C H (C H2)n C H3

O O H+ R

- ROH

+ OR

C H3 C H C H C H C H C H (C H2)n C H3

O

C H 3 C H C H C H C H C (C H 2)n C H3

O

CHC H

C (C H2)n C H3

C HC H

O

C H3

(C H 2)n

- H 2O

C H 3

Aromatic Compound Formation

C H3 (C H2)3 C H2 C H C H C H2 C H C H (C H2)7 C O O R

O 2

C H2

C (C H2)7 C O O R

O

C H2 (C H2)2 C H3

C H2

C H2

(C H2)6 C O O R

(C H2)2 C H3

- H O2

Formation of -2 Nonelactone

C H3 C H2 C H2 C H2 C H2 C H C H C H2 C H C H

C H3 C H2 C H2 C H2 C H2 C H C H

C H3 C H2 C H2 C H2 C H2 C H C H

C H3 C H2 C H2 C H 2 C H2 C H C H

.

C H C H C H

O O H

C H C H

C H C H O H

R

R

+ OH.

C H 3 C H2 C H 2 C H 2 C H 2 C H C H C H 2 C

O

H

C H3 C H2 C H2 C H2 C H2 C H

O H

C H

C H 3 C H2 C H 2 C H 2 C H 2 C H C H. CC H

O

H

CC H

O

.

+ .OH

C H3 C H2 C H2 C H2 C H2 C H

O H

C H CC H O H

O

-H 2O

-2-Nonelactone

O2

- H

C H3

C H2

C H2

C H2

C H2

C H C H C H C H O H

Effect of Methionine Analogs on Potato Chip-Flavor in Deep-Fat Frying.

Compounds Structures Flavor Characteristics

D-MethionineL-MethionineDL-Methionine

 CH3-S-CH2-CH2-CH(NH2)COOH

 Good potato chip-like

 S-Methyl-L-Cysteine

 CH3-S-CH2-CH(NH2)COOH

 Good potato chip-like

 Methionine Hydroxy Analog

 CH3-S-CH2-CH2-CH(OH)COOH

 Obnoxious(cooked turnip)

 S-Carboxymethyl-L-Cysteine

 HOOC-CH2-S-CH2-CH(NH2)COOH

 Obnoxious(cooked turnip)

Polymer Formation

Carbon-Carbon Bond: A. Vinyl Type

C

CC

C

C

B. Diels Alder Type

C C

H

C C C

H H H H

- .H

C

H

H

HHHH

CCCC

C C

H H

C

H

HH

H CC

C

H

C

HC C

Intermolecularly or Intramolecularly

Carbon-Carbon Bond:

Carbon-Oxygen Bond

Through peroxide group – formed by autoxidation.

This can be formed intermolecularly or intramolecularly.

Through ether linkage – formed at high temperature.

O

O

O

O

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%

Types of Polymers

Trilinolein Monocyclic, Nonpolar C-C Dimer 4.9%

Noncyclic, Polar C-C Dimer 2.8%

Trimers - C-C, 8.4%

Trimers - 2 C-O, or 1 C-C, 1 C-O 4.9%

COOR

COOR

COOR

COOR

OH

OH

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

C H2

C H

C H2

C H2

C H

C H2

Dimerization Between Two Acyl Groups in the Same Triglyceride or

Dimerization Between Two Acyl Groups in Two Triglycerides

Dimerization

Composition of Oxidized and Polymerized Materials Formed during Simulated Deep-Fat Frying at 185C for 74 Hrs.

 

 TRILINOLEIN

 TRIOLEIN

 TRISTEARIN

Cyclic Dimers Carbon-to-Carbon Linkages 4.9 0.0 0.0

 Noncyclic DimerCarbon-to-Carbon Linkages

2.8 3.4 0.7

 

Trimers Two Carbon-to-Carbon Linkages

8.4 0.3 0.4

4.9 1.2

 Dimers and Trimers Carbon-to-Carbon or Oxygen Linkages

6.2

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)

Safety

Under Good Practice of Deep-Fat Frying:

Fats are not nutritionally damaged

Frying Oil Analyses by Liquid Chromatography

Thermal Oxidation Effect on Linoleic Acid Concentration

Levels of Linoleic Acida

FAT FRESH OXIDIZED

Corn oil 61.0 1.1

Olive oil 7.7 Trace

Learb 21.7 1.1

Lard 10.7 1.4

a Expressed as % of total fatty acids.

b Lear = Low erucic acid rapeseed oil.

Toxicity Symptoms of Highly Heat-Abused Oils to Laboratory Animals

Irritation of the digestive tractOrgan enlargement (kidney & liver)Growth depression

Carcinogenic properties

Good Practice of Deep-Fat Frying

Fats are not nutritionally damaged.