Because of the moisture contained in foods, atmospheric oxygen and high temperatures (160-200 ºC) reactions such as hydrolysis, oxidation and polymerization are produced. These reactions change and modify the chemical composition of the used frying oils, accelerating its degradation.
During deep fat frying, the reaction between water and oil leads to the release of fatty acids and partial ester of glycerol from triacylglycerides. The triacylglycerides with short chain fatty acids (SCFA) are more sensitive to hydrolysis than those presented by long-chain fatty acids (LCFA). The hydrolysis derivatives, free fatty acids, mono and diglycerides, present a speed of oxidation and reactivity higher than the original triacylglycerides, promoting alterations. The intensity of this process depends on different factors:
- The quantity of water interacting with the frying oil or fat (the relative moisture of the food is a key factor, frozen foods or foods rich in water).
- The surface/volume proportion between the food being fried (the greater the value of this proportion the greater the contact between the oil and the water from the product).
- The excessive presence of residual solid particles in frying oils accelerates the formation of free fatty acids.
- At temperatures of 180-190 ºC, the hydrolysis process is insignificant, since the moisture is eliminated in the form of steam. Nevertheless, the greatest modifications are produced if there is moisture at the moment of heating or cooling the oil (<100 ºC), and during its storage (time periods between deep-frying processes), since the water does not evaporate.
The development of the free acidity in oil is produced at the same time as the other alteration reactions that take place during the frying process. In addition, the free fatty acids are a negative factor, as they help the formation of smoke. Also, undesirable smells and flavors can appear.
Auto-oxidation is the most frequent modification during the deep-frying process. It is a non-enzymatic oxidative process characterized by the oxidation of fatty acids (especially polyunsaturated fatty acids which are more sensitive than saturated fatty acids) and other unsaturated molecules when exposed to atmospheric oxygen, generating intermediate unstable compounds called hydroperoxides or peroxides which will result in the formation of free radicals. Because of the unstable nature the hydroperoxides and the free radical possess we can only observe their role as intermediates of reactions; however, they are the starting point of the formation of numerous alteration compounds (dimers, cyclic monomers, carbonyl compounds, alcohols, hydrocarbons, etc.). When reaching the temperature necessary to deep fry, the availability of oxygen is a constraint factor. This process is favored and fostered by the incidence of light, which acts as a catalyst, and by the presence of photosensitive substances.
Three phases can be distinguished in this process :
1. Initiation phase or induction
During this phase, the free radicals (R*), are formed, either from a hydroperoxide (ROOH), which is favored by high temperature and light, or from a fatty acid (RH) because it is found in the metallic frying unit and has the ease to change valence.
2. Propagation or continuation phase
Propagation or continuation phase: The free radicals formed in the previous phase because of their great reactivity, in conjunction with the oxygen or other chains of fatty acids generating a chain reaction (the grade of oxidation increases as the reaction progresses).
3. Termination or end phase
- Termination or end phase: When two free radicals find each other they can interact, producing a new compound, generally an aldehyde or a ketone. When there are no free radicals left to react with the oxygen, a new phase and initiation is required for the oxidation to continue.
During the process of auto-oxidation, new changes appear such as organoleptic changes (flavor alteration, palatability, darkening) physical changes (increase in viscosity and formation of scum) and chemical changes (formation of polymers, volatile compounds). The non-volatile compounds are retained in the oil, and therefore, the fried products absorb them, reaching the consumer. This is the reason why the levels of these derivatives in frying fats and in a fried product should be known.
The thermal oxidation is produced by the effect of high temperatures, which promotes oxidative alteration even more.
Excessive consumption of toxic substances originating from oxidized oils may cause, among other conditions, different types of gastrointestinal disorders.
Free radicals tend to combine with each other or with other fatty acids, and tend to form long and ramified lineal compounds, or cyclic compounds, especially when there are double bonds. The cyclization of fatty acid molecules is one of the main reactions produced due to the intense thermal treatment of the deep fat frying process.
However, from a quantitative point of view, the most important group of alteration compounds are the dimers and the polymers of the triacylglycerides, and their formation is also catalyzed by the high temperatures reached during the process.
Dimers, trimers and polymers can be of two types, depending on whether they are combined with oxidized radicals or with non-oxidized radicals:
- Non-polar dimers and polymers (non-oxidized).
- Polar dimers and polymers (oxidized).
Because these polymers have a greater molecular size and weight, they tend to increase the viscosity of the oil contributing, on one hand, to the formation of foam, and, therefore, easing the oxidation; but on the other hand, having the product soaking in a greater quantity of oil. On the oil surface or in the sides of the deep fat fryer, the polymers form a layer of plastic consistency that is very adhesive and difficult to remove.
From a nutritional point of view, it seems to be that the polymers of high molecular weight are indigestible, and consequently, have little importance with regard to nutrition and health; but the shortest compounds, monomers and dimers, are indeed absorbed by the intestinal walls, having an effect on the consumer’s health. Many of these substances are recognized as toxic or potentially carcinogenic, for instance, the benzopyrene produced by cholesterol cyclization.