Enzymes are complex chemical substances, which are present in all living organisms and tissues, which control essential metabolic processes. Different biochemical reactions in foods and plants tissues are catalysed by enzymes. Enzymatic spoilage is the greatest cause of food deterioration. They are responsible for certain undesirable or desirable changes in fruits, vegetables and other foods. Examples involving endogenous enzymes include:
the post-harvest senescence and spoilage of fruit and vegetables;
oxidation of phenols in plant tissues to orthoquinones by phenolases, peroxidases and polyphenol oxidases (PPO). These orthoquinones rapidly polymerize to form brown pigments known as melanin leading to enzymic browning;
sugar – starch conversion in plant tissues by amylases;
post-harvest demethylation of pectic substances in plant tissues (leading to softening of plant tissues during ripening, and firming of plant tissues during processing).
If enzymatic reactions are uncontrolled, the off-odours, and off-colours may develop in foods.
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