While enzymes generally exert positive influence on food quality, the activities of some enzymes need to be controlled postharvest or postprocessing to avoid compromising the quality of food products. For instance, protease activity is desirable for meat tenderization during ageing, but left uncontrolled, meats become mushy and overtenderized.
Similarly, excessive lipase and oxidase activities in foods during storage, especially under temperature-abuse conditions, may cause rancidity and impact quality attributes such as color, flavor, and texture. Therefore, a number of preservation techniques developed through the years have been aimed at controlling such undesirable postharvest enzymatic as well as microbial activities to ensure extended shelf life of food products.
The activities of a number of enzymes as well as the content of certain food components and secondary metabolites resulting from postharvest or postprocessing biochemical and microbial activities have been recognized as quality indices for various foods and consequently monitored. An example is the milk endogenous alkaline phosphatase, which is inactivated following heat treatment at 60◦C for 5 seconds. With milk generally subjected to HTST/flash pasteurization (i.e., 71.5–74◦C/160–165◦F for 15–30 seconds) or ultra-high-temperature treatment (i.e., 135◦C/275◦F for 1–2 seconds), residual alkaline phosphatase activity following such treatment provides a good indication of efficacy of the treatment.
The phosphatase test is based on hydrolysis of disodium phenyl phosphate to liberate phenol, which reacts with dichloroquinonechloroimide to form a blue indophenol that is measured colorimetrically at 650 nm (Murphy et al. 1992). An alternative fluorimetric method for alkaline phosphatase analysis has also been developed and commercialized (Rocco 1990). The adequacy of blanching, an essential step in vegetable processing, is also established by measuring residual lipoxygenase or peroxidase activities (Powers 1998).
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Amylase activity in malt, also referred to as its diastatic power, is a very essential quality parameter that influences dextrinizing time and, therefore, closely monitored (Powers 1998). In most seafoods, freshness is rapidly compromised postharvest, particularly when handled under temperature-abuse conditions that promote endogenous enzyme activity.
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