During food processing, interactions of antioxidants — either as antioxidant preparations or natural constituents — with proteins and other food constituents take place, and the activity of some antioxidants may change. Jan Pokorny and Stefan Schmidt explain what this means for food formulators
Nutritional factors are widely considered to be critical for human health. Overwhelming evidence from epidemiological studies indicates that diets rich in fruits and vegetables are associated with a lower risk of several degenerative diseases.
However, the health-promoting capacity of such food is wholly dependent on processing history. This aspect has generally been neglected or scarcely considered in current nutritional and epidemiological studies.1 Processing is expected to affect the content, activity and availability of bioactive compounds, so the requirement to better understand the role and fate of natural and process-induced antioxidants on both food stability and human health leads to a need for further research.
Nowadays, consumers would like those antioxidants present in food products not only to stabilise food lipids, but also to be absorbed through the intestinal wall and protect the lipids of blood plasma against oxidation.
This effect is relatively evident in the case of tocopherols (which are liposoluble) or ascorbic acid (which is hydrophilic), but much less evidence is available on antioxidants of medium polarity, such as flavonoids, rosemary oleoresins, or green or black tea catechins.
If the antioxidant content is too low, other natural antioxidants could be added, either in the form of natural food constituents rich in antioxidants or as antioxidant preparations.
In addition to phenolic substances, there are other components present in foods that have no antioxidant activity of their own, but which increase that of phenolic antioxidants. They are called synergists, and they should be accounted for in any discussion of antioxidant activity. Polyvalent organic acids, amino acids, phospholipids (lecithin) and various chelating agents belong to this group. Proteins may modify the efficiency of antioxidants as they react with the reaction products of both antioxidants and synergists.
Mechanism of changes
During food processing, interactions of antioxidants with proteins and other food constituents take place, and the activity of some antioxidants may change as a result of hydrolytical processes because glycosides and esters are converted into free phenolic derivatives. Formation of complexes by reaction of phenolic substances with metals is also important. The most important reactions of phenolic antioxidants are, however, different oxidation reactions as they affect their functionality to a pronounced degree.
Metals of transient valency, particularly copper and iron, catalyse the lipid oxidation because they decompose lipid hydroperoxides with formation of free radicals. The valency of the metal ion changes in every step so that a single atom of heavy metal may produce many free radicals. Metal chelating compounds, such as citric, tartaric or phosphoric acids, ascorbic acid, or phytin or phosphatidic acids, combine with metals to form nonreactive compounds so that the oxidation reactions are inhibited and natural food antioxidants are saved.
In most foods, both aqueous and lipid phases are present. Polar antioxidants, such as ascorbic acid, are dissolved in the aqueous phase, and react with hydrophilic free radicals. In contrast, lipophilic antioxidants, such as tocopherols, are dissolved in the lipidic phase, reacting with liposoluble free radicals produced during lipid oxidation or decomposition of lipid hydroperoxides.
At the water-oil interface, antioxidants can accumulate, forming an oriented mono-molecular layer, according to their polarity. This layer protects the lipidic phase against oxidation by oxygen dissolved in the aqueous phase.2 Therefore, the activity of antioxidants is very different in bulk fats and oils and in lipid emulsions. This behaviour should be taken into account when considering changes of antioxidant functionality.
Changes during heating
Water as the heat transfer
Changes of antioxidant functionality during food processing, storage and meal preparation depend on processing conditions. Energy is applied to the food material in many processes, while others do not require energy input. Changes in the antioxidant functionality depend not only on the energy requirement, but also on other factors such as air access, temperature, food composition, time and light access.
Some phenolic antioxidants, especially flavonoids, are present as esters or glycosides. They are partially hydrolysed during boiling, and these hydrolytical changes influence both their distribution between the lipidic and aqueous phases and their reaction with lipidic free radicals. The nutritional value is partially lost at the same time.3
Another important food processing technology is pasteurisation. It consists of rapid heating to temperatures between 60 and 65°C in order to destroy micro-organisms.
Oxidoreductases are inactivated at the same time. As the heating is short, the destruction of antioxidants is only moderate. Losses of ascorbic acid are a good indicator of the destructive changes. Losses of ascorbic acid and carotenes are minimised by deaeration.
Evaporation is the oldest process for the concentration of liquid foods. Temperatures are higher compared to those of the more modern membrane filtration or freeze concentration processes. Tocopherols, carotenes, ascorbic acid, flavonoids and other phenolic antioxidants are partially destroyed by heating. Therefore, it is necessary to minimise the time needed for evaporation, and heating to the evaporation temperature should be carried out very rapidly. The temperature may be decreased if the pressure is reduced. The process is then more expensive, but losses of antioxidants become substantially lower.
Air as the heat transfer medium
Heat is transferred more slowly by hot air than by hot water because of differences in heat conductivity. Therefore, in most circumstances, higher temperatures and longer processing times should be used than is the case with boiling.
Hot air reaches the surface of food, which is then changed more intensively than the inner layers, where the temperature does not exceed about 100°C. Losses of antioxidants are, of course, also higher on the surface than in the interior of heated food.
Energy transferred in waves
Microwave and infrared energy are both transmitted as waves, which penetrate food and are converted into heat. Tocopherols and other liposoluble antioxidants are partially destroyed during microwave cooking of oil seeds.4 About 10 per cent are destroyed during the first six minutes of microwave heating, and the losses of tocopherols increase up to 40 per cent during the next six minutes of microwave application.5
Oil as the heat transfer medium
Frying is a process in which food is heated in contact with hot oil. Changes of antioxidants in frying oil are usually very pronounced, as it is used repeatedly for frying, sometimes for several days or even weeks. In the case of intermittent frying, hot frying oil is left to cool without further heating after the operation, and the heating is resumed at the next frying process, usually on the next day. Frying oil usually contains tocopherols and in some cases other antioxidants as well.
Some antioxidants, especially BHT and other relatively nonpolar synthetic antioxidants or essential oils present in natural antioxidants (which also possess some antioxidant activities), evaporate with water vapour from the frying medium. Therefore, nonvolatile antioxidants such as tocopherols, or rosemary or sage resins or extracts should be used to protect frying oils.
Changes in antioxidants during storage
Storage occurs at ambient or still lower temperatures so that the extent of oxidation and the subsequent antioxidant damage are slow. Nevertheless, after long storage times of several months or even years, they may become quite considerable.
The most frequent use of antioxidants is to improve the stability of fats, oils and emulsified fat products. They usually contain natural antioxidants, especially tocopherols, and additional antioxidants are sometimes added, especially to lard. They are mostly stored at 15°C or at even lower temperatures. In our experiments, the content of tocopherols did not substantially decrease during storage for a year in the refrigerator.
Natural ascorbic acid is rapidly destroyed on processing and storage, but it is often added after the processing is complete. Flavonoids are more stable.
Synthetic antioxidants are cheaper and purer than natural antioxidants but, nevertheless, the majority of consumers still prefer natural antioxidants. This trend will surely persist in the near future.
The most common natural antioxidants are tocopherols, ascorbic acid and beta-carotene (more often synthetic nature-identical compounds than natural products). Their changes were studied in detail in model systems, fats and oils, but experimental evidence is mainly lacking on more complicated systems, such as natural foods and ready dishes.
Still less is known on different antioxidants from spices and from essential oils. These data will probably be obtained gradually. Very little is known about synergism of antioxidants in food products other than edible fats and oils, or their regeneration from the respective free radicals and quinones.
The protection of foods from oxygen is the basic principle upon which antioxidant protective technologies are now based. The contribution of food technology, both to food safety and to the maintenance of high nutritional and sensory value, should not be underestimated.
The mechanisms of antioxidant destruction and the composition of reaction products during miscellaneous technological steps are different, depending on the concentration of free radicals and on oxygen pressure and process temperature.
This point of view needs further research; however, it is clear that monitoring for the retention of antioxidants throughout processing and storage is needed. To do this, it may be necessary either to develop rapid methods of monitoring the survival of the antioxidants themselves, or to measure secondary effects such as, in the case of oils and fats, peroxide values.6
Jan Pokorny is professor at Prague Institute of Chemical Technology in the Czech Republic. Stefan Schmidt is professor at Slovak Technical University in the Slovak Republic. Excerpted from Phytochemical Functional Foods, Johnson I, Williamson G, editors. ISBN 1 85573 672 1. Published by Woodhead Publishing Ltd, England. www.woodheadpublishing.com
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2. Frankel et al. Interfacial phenomena in the evaluation of antioxidants. J Agric Food Chem 1993; 42:1054-8.
3. Price et al. Composition and content of flavonol glycosides in broccoli florets (Brassica olearacea) and their fate during cooking. J Sci Food Agric 1998; 77(4):468-72.
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