Influence of Thermal Processing on Phenolic Content
The antioxidant activity of fruits and vegetables appreciably rely on the presence of bioactive phenolic compounds. This has led to a growing interest in researching on intrinsic polyphenolic content in fresh and processed foods. In this regard, many studies have emphasized the influence of post-harvest activities on decreasing or even increasing biophenols that can crucially result in modifying the physiological properties of the end products.
The harvested crops are generally subjected to various industrial operations such as technological processing, handling, storage and transportation. The processing methods include cooking/heat treatments such as blanching, steaming, frying, boiling and baking. During water-based boiling of vegetables for example, some physiochemical changes are likely to occur. These changes primarily arise from the disruption of the cell structure and breaking down the cell components like phenols. In other words, it reflects the ability of cooking water (at high temperature within sufficient cooking time) in exerting the liberation of phenolics components that are greatly water-soluble, from the tissue matrix. On this account, the loss/reduction of natural phenols may be significant in the boiled vegetables compared to the corresponding proportions in the fresh vegetables.
However, the action of cell disruption in some plants is likely to provide favourable results in antioxidant profile of the end products. It has been indicated that the thermal processing such as air-drying on tomatoes has the potential to enhance antioxidant capacity due to the extractability of bound phenolics from the matrix. This can result in greater bioavailability of the released compounds in the hydrophilic extract of tomato puree. Moreover, heat treatment of apples (using microwave to produce apple puree) has been found to be effective in the retention/increment of phenolic substances.
There are extensive variations in phenolic characteristics between various groups of plants. Some specific types of phenolic constituents are more susceptible to degradation. For example, some components like kaempferol and myricetin are more liable to losses during heat treatment and storage compared to those like quercetin that are less sensitive to the processing operation. Furthermore, application of various types of processing techniques is highly influential on the extent of phenolic concentration. In general, heat treatments such water boiling brings about significant reduction of antioxidative compounds while other cooking methods like steam treatments, microwave and stir-frying may cause less reduction.
It is noteworthy that the effect of the same cooking process on changing the extent of bioavailability varies in different groups of phytochemicals. For instance, stir-frying can have slight/insignificant influence on reducing phenolic concentration while it may cause significant loss of carotenoids in vegetables such as carrots and broccoli. The substantial decrease of carotenoids (plant pigments responsible for yellow, orange and red compounds) in frying operation, reflects the lipophilic properties of carotenoids; i.e., they dissolve in fats and lipids.
In summary, the impact of processing on polyphenolic content varies widely depending on some key factors including plant variety, chemical structure, molecular complexity, characteristics of each functional components, interaction of phenols with other components and conditions/nature of processing treatments. Moreover, quantitative and qualitative evaluation of individual phenolic components is of great value to obtain more clarified conclusion on the bioactivity of phenolic-rich foods after processing.