Deep Fat Frying
Did you ever think of your kitchen as a place where you can learn chemistry? If not you’ll find some interesting facts about techniques that you use in the kitchen for food preparation. Chemistry is around us, whether we see it in that perspective or not. Most of the substances that we use in our day to day life have chemical basis. In the kitchen, we use number of substances, and when they are processed with food, they undergo many reactions producing various types of wanted and unwanted products. Some of these unwanted compounds will lead to a health risk.
As the first step I’ll take you on a journey where you can learn about the basic reactions and resulted compounds due to the frying of food.
These days people eat lot of fried food since the fast food industry has become very popular around the world (I think this is better than saying a large business). Fat and oil are used for frying, so first of all let’s have a better understanding about fat and oil. Fat and oil belong to a class of bio molecules known as lipids. Fats are solids and oils are liquids at room temperature. Both fats and oils are mainly composed of triglycerides. Other than that there can be diglycerides and monoglycerides as well. Most of the oils have plant origin while most of the fats have animal origin. The reaction between fatty acids and glycerol results these compounds and the reaction is known as esterification (Figure 1).
Figure 1: Esterification reaction
Fatty acids in oil are generally derived from triacyl glycerols and when fatty acids are not bound to any other molecules they are known as free fatty acids. These fatty acids can be saturated or unsaturated and unsaturated fatty acids can be in either cis or trans configurations.
Several physical changes occur during the frying process. Some of these include increase of viscosity, thickening of oil, decrease of interfacial tension, increase of density, and increase of specific heat (isn’t it specific heat capacity) etc.
Now let’s see what are the chemical reactions undergo when frying food. Frying is defined as the process of cooking and drying through contact with hot oil and it involves simultaneous heat and mass transfer. Oil, not only acts as a heat transfer medium, but it also gives a flavor to the food. Factors such as frying temperature, frying time, fryer type, frying oil, properties of the food, and food additives affect the frying process. Inherent stability to oxidation can be used to determine the suitability of an oil to be used for frying. Inherent stability means numbers related to relative reaction rates of unsaturated fatty acids with oxygen. Hence oils that have low inherent stability are less susceptible for oxidation during frying. Some of the oils and their inherent stability are mentioned below. (Nurhan Dunford, Ph.D., Robert M. Kerr)
Oil Inherent Stability
Safflower 7.6
Soybean 7.0
Sunflower 6.8
Corn 6.2
Rapeseed (Low Erucic Acid) 5.5
Cottonseed 5.4
Rapeseed (High Erucic Acid) 4.1
Peanut 3.7
Lard 1.7
Olive 1.5
Palm 1.3
Tallow 0.86
Palm kernel 0.27
Coconut 0.24
According to the list coconut and palm oil have the lowest inherent stability which means it is they are less susceptible for the oxidation. Palm oil consists of with palmatic acid as the major saturated fatty acid, which was early suspected on cholesterol raising effect. According to the early data, when compared to stearic acid, palmatic , lauric, and myristic acids have more pronounced cholesterol raising effect but modern data have a controversy with the previous data. However palm oil consists of tocopherol and tocotrienols (vitamin E) which are recommended on the treatment or the prevention of cardiovascular diseases. (S. Mackay, 2000)
When the food is deep fried, oxidation leads to the formation of compounds such as hydroperoxides. Since these primary oxidized products are unstable in high temperatures, they are converted in to various types of secondary oxidized products . These products consist of both non-volatile and volatile molecules where volatile molecules are rapidly removed from the oil during the process and those have implications in flavor of the food and the oil. Some of them are mentioned bellow.
- 4-hydroxy-2-nonenoic acid, lactone
- 4- hydroxy-3-nonenoic acid
- 2,4-decadienal
Some volatile compounds are toxic.
e.g.
- 1,4- Dioxane
- Benzene
- Toluene
- Hexyl-benzene
Compounds | Structures | Flavor Characteristics |
D-Methionine L-Methionine DL-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) |
Non volatile molecules are absorbed by the food. (Minamoto, S.,et al.,1988 & Kanazawa, K. and Ashida, 1991).
Other than oxidation there are several other reactions known as Hydrolysis, Polymerization, and Pyrolysis occur during frying. Oil is hydrolyzed by moisture to produce free fatty acids. Small molecules react with each other and undergo polymerization resulting large molecules.
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%
References
- Nurhan Dunford, Ph.D., Robert M. Deep-Fat Frying Basics for Food Services Kerr Food & Agricultural Products Center, Oklahoma Cooperative Extension Service.
- Minamoto, S., Kanazawa, K., Ashida, H. and Natake, M. The induction of lipid peroxidation in rat liver by oral intake of 9-oxononanoic acid contained in autoxidation linoleic acid. Biochim. Biophys. Acta, 958, 199-204 (1988).
- Kanazawa, K. and Ashida, H. Target enzymes on hepatic dysfunction caused by dietary products of lipid peroxidation. Arch. Biochem. Biophys., 288, 71-78 (1991).
