Blackening of Pepper

A preliminary report on the blackening of pepper by Lewis et al. (1976) suggested that it was due to enzymatic oxidation of polyphenolic substrates present in the skin of green pepper. Based on this, Lewis et al. developed a process for dehydrated green pepper in which green pepper was blanched to arrest the enzyme action and subsequently dried in a cross flow drier. The green pepper could also be preserved in brine containing acetic acid or citric acid (Pruthi 1976).

Browning of raw fruits,vegetables and beverages is a major problem in food industry and is believed to be one of the main cause of quality loss during post harvest handling and processing. The pathway of browning in food is well characterised and can be enzymatic or non enzymatic in origin. The formation of pigments via enzymatic browning is initiated by the enzyme polyphenol oxidase (PPO) also known as tyrosinase, phenol oxidase or cresolase. Endogenous PPO activity is present in foods like potato, apples, mushrooms, banana, peaches, fruit juices and wines.

Enzymatic browning is the result of PPO catalysed oxidation of phenols or diphenols to o-quinones. PPO is a mixed function oxidase that catalyses both the

Scheme 2 Action of PPO on phenolie substrates.

hydroxylation of monophenols to diphenols and the subsequent oxidation to o-quinones. The o-quinones are highly reactive and can polymerise to form high molecular weight compounds or brown pigments (melanin) or react with amino acids and proteins that enhance the brown colour produced (Scheme 2).

Variyar et al. (1988) isolated the crude enzyme from green pepper and used it to pinpoint the active phenolic substrates of the enzyme. They isolated 2,4-dihydroxy phenyl ethanol glycoside (14) and its aglycone and found to be the active substrates for this o-diphenol oxidase enzyme. They also isolated another efficient substrate for polyphenol oxidase and characterised as a 3,4-dihydroxy-6-(n-ethyl amino) benzamide (15) (Bandyopadhyay 1990).

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