Classification of food antioxidants: primary antioxidants

As we have already commented on in other occasions, antioxidants are any substance capable of inhibiting, delayingor preventing the development of rancidity in food or other deterioration of aromas due to oxidation. According to this definition, antioxidants do not improve the quality of food, but their use simply aims to maintain food quality.

To inhibit, reduce or delay the oxidation of lipids it is necessary to act against one or more of the factors that favor their development. In a broad sense, according to the above definition, an antioxidant is considered to be any substance or action procedure that helps to limit the speed and/or extension of oxidative processes, so it can be considered as such not only the chemical compounds that can be added to the product, but also vacuum packaging in an inert gas atmosphere or even freezing.

According to what has been said and to their mechanism of action, three types of antioxidants could be considered. Two of them are associated with the addition of chemical compounds, that are, in addition, those that we will consider in this article. The third type of antioxidant owes its action to modifications of certain factors in the food and/or its processing and will not be analyzed here by us.

Types of primary and secondary antioxidants

For its mechanism of action  two main types of antioxidants can be considered: I and II. The primary antioxidants (type I) are those that break the chain reaction of oxidation through the donation of hydrogen and the generation of more stable radicals. In contrast, secondary antioxidants (type II) are those that delay oxidation through other mechanisms, such as metal chelation, the regeneration of primary antioxidants, the decomposition of hydroperoxides and the elimination of oxygen, among others. This mechanism of antioxidant activity has been studied by numerous researchers (Johnson, 1971, Labuza, 1971 and Gordon, 1990).

Primary antioxidants or type I

Primary antioxidants are those which break the chain reaction of oxidation through the donation of hydrogen and the generation of more stable radicals.

The addition of these compounds to food should, by itself, imply an increase in the induction period. This increase is directly related to the amount of antioxidant added up to a certain concentration, since sometimes with higher proportions an opposite effect is achieved.

The effectiveness of the activity of these antioxidants depends both on the antioxidant itself and on the medium in which it acts. Thus, it has been proven that in phenolic antioxidants its activity is favored when vacuum packaging is done, since the level of available oxygen is very low. However, this offer little protection when the concentration of metals is very high.

It is necessary to know the exact moment of the incorporation of the antioxidant, since if the oxidation process is advanced the antioxidant loses its capacity of action.

Among the main primary antioxidants include phenolic antioxidants and breakthrough phenols.

Phenolic antioxidants

In this type antioxidants are phenolic type hydrogen donors, and are able to effectively move an unpaired electron. The main antioxidants of this type are:

  • Propyl gallate (E-310): white crystalline powder used in food when other synthetic fat-soluble antioxidants are not suitable. It is not very soluble in water, and, in the presence of traces of iron derived from food or from the equipment used in the processing, it gives rise to the appearance of unattractive, dark blue colors. Occasionally, Propyl Galato acts together with natural and synthetic antioxidants. It is important to bear in mind that it is a substance that is sensitive to high preparation temperatures.
  • Octyl gallate (E-311): used as a synthetic antioxidant in fats and water, where it is sometimes included to prevent rancidity in oils.
  • Dodecyl gallate (E-312): used as a synthetic antioxidant in fats and beverages, particularly to prevent rancidity in oils.

The most important technological property is the low resistance to heating. They are not very useful for protecting frying oils or sometimes foods that are subject to high cooking temperatures strong foods during their manufacturing, such as confectionery products or cookies. The low resistance to heat can be avoided by adding citric acid to the product. They are used, mixed with BHA (E 320) and BHT (E 321), for the protection of edible fats and oils.

Galactose, BHA and BHT were used together in oils with the exception of olive oil. They are also used in canned and semi-preserved fish and processed cheese, pastry or confectionery and cookies.

Breakthrough phenols

The main antioxidants of this type are:

  • Butyl-hydroxy-anisole (BHA, E-320): it is one of the most common antioxidants in human nutrition. Chemically, BHA is a mixture of two isomers 2-tert-butyl-4-hydroxyanisole and 3-tert-butyl-4-hydroxyanisole. The second one is generally considered as a better antioxidant and represents 90% of commercial BHA.

This antioxidant is effective primarily in animal fats and more discreetly in vegetable fats and oils. However, due to the chemical structure they present, they are extremely volatile at baking and frying temperatures.

  • Butyl-hydroxy-toluol (BHT, E-321): together with BHA, they are the most used antioxidants in human nutrition. BHT (3,5-di-tert-butyl-4-hydroxytoluene) is an appropriate antioxidant for heat treatment, although it is not so stable.

It is commonly used in combination with BHA to provide increased antioxidant activity. It is also commonly used together with other antioxidants, such as propyl gallate and citric acid, for the stabilization of oils and high-fat foods.

Both BHA and BHT have a slight phenolic odor when used at high temperature for a prolonged period of time.

  • Terbutil hydroquinone (TBHQ, E-319): TBHQ is a white or beige powder that is frequently used in vegetable oils and animal fats. As an antioxidant, TBHQ is more effective in vegetable oils than BHA and BHT. It is heat-stable and very useful in the prevention of oxidation of frying oils. Like the BHA and the BHT, there are indications that in high doses it can be harmful to health, which is why the European Food Safety Authority (EFSA) has banned its use in Europe and the Food and Drug Administration of the United States (FDA) has established certain limits for its use in human nutrition.
  • Tocopherols (E-306): is the antioxidant extracted from nature most common in the food industry. It is completely fat-soluble and does not alter the organoleptic properties of the food. It is also safe, effective and easy to incorporate. The tocopherols are formed by four isomers (Alpha, Beta, Gamma and Delta) with different antioxidant and vitamin activities. After several exhaustive studies, it has been shown that the main antioxidant activity is produced by the Gamma and Delta isomers. In products formed by structures with double bonds more input of tocopherols is needed, since the unsaturated substances are more sensitive to oxidation.

So far we have analyzed the primary antioxidants (type I). In another article we will complete this analysis with the development of secondary antioxidants (type II).

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