There are numerous articles describing the effect of various processing condition (including temperature) on the phenolic components of fruit and vegetable. Applying heat treatment however may not always degrade the phenolic compounds present in food.

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In most studies on the effects of heat treatment on the total phenolic content, the results are contradicting. Some researchers reported an increase in the phenolic content whilst others observed a decrease. In some researches an attempt was made to simulate the actual cooking conditions and as a result in some papers the cooking conditions were not explicitly specified. The data generated using the actual cooking conditions is beneficial when included in food composition databases as it will enable the users to evaluate the actual amounts of bioactive compounds consumed.Lima et al. (2009) observed a dramatic loss of phenolic content on convectional and organic grown food as a result of thermal treatment. Significant differences between foods obtained by the two cultivation procedures were not observed. Organic and minimally processed foods are mostly preferred because of the nominal changes to the nutrients and non-nutrient components of foods. Hence, the consumption of organically grown and minimally processed foods will entail food that is free from toxicants and nutritious.

Cooking asparagus was found to increase total phenols by 23% (Fanasca et al., 2009). The results also indicate that the effect of cooking process was significant and more pronounced than the effect of cultivars. The study was in agreement with that by Lima et al. (2009) in which the type of food did not have a profound effect on the phenolic content. Ascorbic acid of asparagus was greatly reduced by the cooking process (Fanasca et al., 2009). The vitamin contributes to the total phenols as it is capable of reducing the active reagent used in the analysis of phenols. Hence processes that affect ascorbic acid will ultimately reduce the total phenolic content.

Decreasing temperature of processing was also found to preserve 80-100% of phenolic content in some vegetables (Roy et al., 2007). Convectional and steam cooking caused significant reduction in total phenol content of red cabbage and decreasing cooking water and time by half led to better retention (Podsedek et al., 2008). The short cooking time used in steam cooking preserves the antioxidant components of the vegetables than convectional cooking.

Total phenolic content of selected vegetables (peas, carrot, spinach, cabbage, cauliflower, yellow turnip and white turnip) was found to be generally decreased by boiling, frying and microwave cooking (Sultana et al., 2008). Microwave cooking was found to cause more significant changes than the other methods of processing. Thus an appropriate method might be sought for the processing of such vegetables to retain their antioxidant components at maximum level.

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