Analysis of a food may indicate that it is a rich source of antioxidants, but they may be destroyed by incorrect cooking practices or present in an indigestible form and will be excreted (see biochemical note). Certain antioxidants may be shown in vitro to be chemically effective, but may be poorly absorbed by the body. Culinary practices - and according to recent observations - storage, can improve the availability (digestibility and absorbance from the gut) or reduce the losses of some antioxidants. Unfortunately, it may be necessary to decide what it is in the food that you want to optimise.
Most plant foods are better eaten fresh, without long preparation times or high temperatures being involved. The vitamins, especially vitamin C will be optimised by these practices.

Food storage

Recent studies have produced a controversial result. Not all antioxidants are oxidised (degraded) during storage. Polyphenols are generally resistant to aerial oxidation and therefore, less than totally fresh fruit and vegetables may contain as much (if not more of certain polyphenol antioxidants) than when purchased. This observation fits with the general notion that the chemical conversion to more digestible forms can be effected by processing (in this case, the thermal or chemical ageing reactions). For example, the preparation of pastes and purees is recommended to optimise carotenoid release from tomatoes. It is worth noting that "sell by" dates are set primarily to protect the industry from legal claims arising from possible food poisoning cases, and to increase sales. However, as always, decomposing food should be rejected based on the appearance, smell and if necessary, taste of a small sample.
The quick freezing of peas within an hour of being picked in the field means that frozen peas often contain more vitamins than their fresh equivalents, even after two or three months of (correct) frozen storage.

Food preparation

1. The acts of chopping, dicing, shredding, grating, etc. aid the release of cetain antioxidants from the disrupted cells. This is especially true when enzyme release is required to degrade the cellular constituents, e.g. conversion of glucosinolates to therapeutic isothiocyanates and nitriles. A suggestion based on observations made during the identification of the isothiocyanates produced from brassica varieties, was that if the cooking water was raised to almost 60 degrees centigrade and left for several minutes at that temperature (i,e, the optimum temperature for enzyme activity) before bringing to the boil, more isothicyanates were detected in the vapour and liquid phases than if the food was simply "brought to the boil". However, leaching into cooking water may lead to losses. Therefore, the practice of using the cooking liquor to make soups, for example, is an effective way of optimising ingestion of leachable components. Steaming often reduces losses incurred by leaching.

2. Fat soluble carotenoid antioxidants e.g. in tomatoes, carrots, etc. can be released more easily from foods by cooking and especially cooking in oils (roasting, frying). As stated above, the lipophilic carotenoids are likely to be more available from processed products such as puree, sauce, etc. And the practice of dressing salads with oil will increase the absorption of lipophilic antioxidants.

3. An interesting paper [S. Wachtel-Galor, et al., Food Chem., 110, 706-710, 2008] on the effect of cooking on the loss of antioxidants from brassica vegetables found that steaming < boiling < microwaving, and that 5 min was better than 10 min. Cooked cauliflower and broccoli contained more antioxidants than raw, but cooked cabbage contained less. The article makes the point made in 1 above, that the cooking water is a rich source of antioxidants.

Therapeutic food combinations

Examples of synergistic food combinations are emerging in the scientific literature. The preparation of meals containing both chicken and broccoli, which contain the antioxidant mineral, selenium and the isothiocyanate, sulforaphane, respectively, have been found to be 13 times more powerful in attacking cancer than when the two ingredients are eaten alone, say scientists at the Institute of Food Research, Norwich, UK. Go to >>>>Food combining fights cancer. From the same science group comes advice that the non-haem iron in spinach can be made more bioavailable by drinking orange juice at the same meal.

Biochemical observation

Also, recent work has shown that indigestible polyphenols (tannins in particular) entering the large colon provide protection against ROS in this region. This is a natural consequence of a diet rich in fibre with the attendant tannin antioxidants, etc. Epidemiological evidence links diets rich in antioxidants with protection from various cancers of the lower intestine, colon and rectum. A paper in 2005 showed the survival in high concentrations of phenolic acids into the colon.

Reference A.M. Jenner et al., Free Rad. Biol. Med., 38 (6) 763-772, 2005.

For general advice on getting the best out of your food, go to >>>>BBC notes on food preparation