Continuing from the previously discussed topic of primary food antioxidants, we will talk about secondary antioxidants.
For some context, we will say that food antioxidants act by exerting an inhibitory action on peroxidation, transforming free radicals into more stable compounds.
In this case, secondary antioxidants delay oxidation by chelating metals, regenerating primary antioxidants, decomposing hydroperoxides or eliminating oxygen (Johnson 1971, Labuza 1971 and Gordon 1991).
Secondary antioxidants can be classified into the following groups:
A) Antioxidant oxygen receptors
Ascorbic acid and ascorbates
E-300 Ascorbic acid
E-301 Ascorbate sodium ascorbic acid sodium salt (L +)
E-302 Calcium ascorbate Calcium salt of ascorbic acid (L +)
E-303 Ascorbyl Diacetate
The L-ascorbic or vitamin C, is a white solid which is odorless, highly soluble in water and insoluble in fats and oils. It can act as an oxygen receptor, although its form of action depends on the concentration and the product in which it is used. Accordingly, ascorbic acid can be used:
- For chelating, when there is low water activity.
- As a receiver or oxygen eliminator of the medium. In the presence of oxygen and metal ions, in an aqueous medium, it is oxidized to dehydroascorbic acid, being more effective at low oxygen levels.
- As asynergistic of type l antioxidants.
- As an agent that helps the formation of radicals and act, therefore, as prooxidant.
The food industry uses ascorbic acid for the production of a wide variety of food products, including canned or bottled products, beverages, vegetable oils, fruits, butter, vegetables, cured meat and canned fish.
Erythorbic acid (isoascorbic acid)
E-315 Erythorbic acid (isoascorbic)
E-316 Sodium erythorbate (sodium isoascorbate)
Erythorbic acid is the D- isomer of ascorbic acid. It has no vitamin activity and is found naturally in food. Erythorbic acid as well as its sodium salt are used in the stabilization of nitrates and nitrites in cured meat products, dehydrated fruits and vegetables, and as synergists of tocopherols in fats and oils (Nakao, et al., 1972, Movaghar, 1990) .
Erythorbic acid, in combination with citric acid, can be used, as an alternative to sulphites in frozen seaoods, vegetable salads and apples.
B) Chelating agents
In this group we include substances that have antioxidant action through a specific mechanism, which consists of the sequestration of traces of metals present in food. The chelating agents have the mission of capturing the metal ions, forming complexes that remain soluble and innocuous, which is of great importance in the food industry, avoiding undesired effects in the production processes or in the final product.
In the process of selecting a chelating agent, in addition to taking into account toxicological and sensory aspects (color and taste), other aspects of interest must be assessed, such as the characteristics of the medium (pH) since they significantly influence the effectiveness of the chelation and solubility.
E-338 Phosphoric acid
E-339 Orthophosphoric salts
E-340 potassium dihydrogen phosphate
E-341 Calcium orthophosphate
E-341iii) Tricalcium Orthophosphate
Phosphoric acid and its salts are used in the food industry as chelating agents, as stabilizers of emulsions and as anti-hardening agents.
The excretion of phosphates takes place, especially in faeces, in the form of calcium phosphate. Because of this, an excessive intake of phosphates can produce bone mass and decrease in calcium.
E-334 Tartaric acid
E-335 Sodium tartrate
E-336 Potassium Tartrate
E-337 Mixed Tartrate of Potassium and Sodium / Salt of Seignett
Tartaric acid is found naturally in many fruits, and is also a byproduct of winemaking. Tartaric acid is absorbed almost completely in the intestinal tract, being metabolized significantly, in body tissues, giving rise to CO2.
E-330 Citric acid
E-331 Sodium Citrate / Sodium Citrate
E-332 potassium citrate
E-333 Calcium citrate
Citric acid and its salts are used as chelating agents. They are used as synergists with primary antioxidants and with oxygen receptors at levels of 0.1-0.3%. In fats and oils, citric acid forms chelates with metal ions at concentrations of 0.005-0.2% (DziezaK, 1986).
Lecithin (phosphatidyl choline) is a phospholipid found in natural products, with a percentage of 1-2% in many vegetable oils and animal fats. The main source is soy. Commercial lecithin is formed by a mixture of phospholipids.
Lecithin acts as a powerful synergist in fats and oils, along with primary antioxidants and oxygen receptors at temperatures above 80 ° C. When there are low concentrations of antioxidants, lecithin is more effective. It is also very effective when forming ternary mixtures with vitamins E and C, to such an extent that the induction times in the oils increased about 25 times when adding 500 ppm of vitamin E and 1000 ppm of vitamin C (Loliger, 1991). Similar effects are found in mixtures containing ascorbyl palmitate, lecithin and α-tocopherol (Hudson and Ghavami, 1984).
C) Eventual antioxidants
Amino acids are effective both as primary antioxidants and as synergists (Bishov and Henick, 1975). The antioxidant activity of many amino acids is dependent on concentration and pH. At high concentrations and low pH they act as pro-oxidants, while at low concentrations and high pH they have antioxidant properties.
Methionine, histidine, proline, tryptophan, glycine and lysine are effective in fats and oils.
Spice extracts are a potential source of natural antioxidants. They are effective in fat, meat and bakery products. Rosemary and sage bring good antioxidant properties to lard. Eugenol is the main component of clove and curcumin, the main pigment in turmeric, all three of which have antioxidant properties (Cort, 1974b). Spice extracts have a strong smell, color and flavor, so they can only be used in foods that are compatible with these characteristics.
Vitamin A has a very limited use as an antioxidant due to its tendency to oxidize when exposed to light or air, conditions under which the vitamin becomes pro-oxidant.
Retinol is a form of vitamin A. It belongs to the group of retinoids and is widely used for its high effectiveness in fats and oils when stored in the dark. In addition, this substance inhibits the formation of free acids in vegetable oils. Retinol is found in all animal tissues, mainly in the liver, as well as in eggs and milk. The liver is the primary storage site for vitamin A. The recommended daily intake is 750 mg / kg-pc (FAO / WHO, 1967).
Incorporation of antioxidants
One of the main problems that arise when using antioxidants is the achievement of an effective and complete distribution in the food, in such a way that they can come into contact with the lipids and act effectively. This problem worsens when the antioxidant has to be added in foods with a low lipid content and with a defined structure.
The addition of antioxidants is easier in more or less viscous products (oils and fats), or have physical structures that allow homogenization (chopped products, emulsions, etc.).