Humans are the only animals known to cook their food. This processing stage before ingesting the food can alter the chemistry of human nutrition when compared to other species. It is known for example that cooking can have quite different effects on the bioavailability of phytonutrients present in different foods. Cooking spinach and carrots has been shown to increase the bioavailability of the β‑carotene and cooking tomatoes in oil increases the bioavailability of lycopene. This is because in many cases the nutrients are locked inside indigestible cell walls that are destroyed in the heating process. In contrast, other foods contain nutrient precursors that require enzymatic conversion to create new compounds with active biological effects. Enzymes are thermolabile, and as a result heating causes deactivation resulting in a decreased bioavailability of some of the nutrients within the food.
For example, research suggests that cooking garlic may lessen the biological effects. Heating garlic for as little as 60 seconds in a microwave, or 45 min of oven heating, can block the anticancer ability of garlic in rats1. However, if the crushed garlic was allowed to stand for 10 min before cooking, there was only a 30 % loss during microwaving. This is because the organosulphur compounds responsible for the biological activity of garlic are created by the enzyme alliinase upon crushing. Heating garlic causes a loss of alliinase activity because it is thermolabile. Cooking therefore prevents the formation of the important organosulphur compounds necessary for garlic’s biological activity (figure 1). However, crushing the garlic and allowing it to stand gives time for the formation of the organosulphur compounds, which are not subsequently completely destroyed in the heating process.
Figure 1. The effects of heating garlic1.
Upon crushing, alliinase rapidly converts alliin (S-alkyl-L-cysteine sulfoxide) to allicin (dialkyl thiosulfide), which is then subsequently converted to diallyl sulphide (DAS), diallyl disulphide (DADS) and diallyl trisulfide (DATS). Allicin and these other organosulphur compounds are responsible for the characteristic taste and smell of garlic. More importantly these metabolites are thought to be responsible for some of the biological effects attributed to garlic. However, the chemicals associated with crushing raw garlic are different to those associated with aged garlic extract (AGE). Aged garlic extract is produced by letting garlic naturally aged for around 20 months. This results in the conversion of γ-glutamylcysteine (an alliin precursor) to a number of less pungent compounds such as S-allylcysteine. These other compounds also appear to display important biological effects related to health and so both raw garlic and AGE are recommended.
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