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Polyphenol Oxidase in the Food Industry
Polyphenol Oxidase (PPO) is an enzyme commonly found throughout the plant and animal kingdom, and a vast majority of fruits and vegetables. PPO is of importance to the food industry because it catalyzes enzymatic browning due to tissue damage such as bruising, compression and indentations. This is highly undesirable as it lowers the marketability of commercial fruit and vegetable products. The collection of black and brown pigments resulting from PPO reactions in fruits and vegetables creates visually undesirable products to the public leading to economic loss for fruit and vegetable producers. Enzymatic browning due to PPO can also lead to loss of nutritional content in the fruits and vegetables further lowering their marketability. The substrates of these PPO reactions are located in the vacuoles of the plants so once this subcellular division is damaged or broken, often as a result of improper handling, PPO usually located in the chloroplasts is able to access its substrate and the chain of browning reaction will occur. Exposure to oxygen when sliced or pureed also leads to enzymatic browning by PPO catalyzed reactions in fruits and vegetables.

Instances in which the browning reactions catalyzed by PPO are desirable include prunes, sultana grapes and the production of black tea.

Polyphenol Oxidase in Mangos
In mangos PPO catalyzed enzymatic browning is mainly caused by sap burn. In mangos sap burn disrupts the skin cells bringing together the PPO and its substrates causing enzymatic browning of the skin. Catechol oxidase-type PPO is located in the chloroplasts of mango skin cells and its phenolic substrates in the vacuoles. Sap burn is therefore what initially activates the PPO in mango skin as it breaks down cell compartmentalization. Polyphenol oxidase is located in the skin, sap and pulp of the mango with the highest levels of activity in the skin. It has also has a pH optimum of 6.5-7.

A possible solution to PPO catalyzed browning would be to coat the mango skin with wax or oil in order to block the sap from entering the skin and therefore maintain cell structure and reduce PPO's ability to react.

Polyphenol Oxidase in Apples
Polyphenol oxidase is present in the chloroplasts and mitochondria of all parts of an apple. It is also the major enzyme responsible for the enzymatic browning in apples. Due to a recent increase in consumer demand for pre-prepared fruits and vegetables a solution for enzymatic browning has been a targeted area of research. For instance pre-sliced apples have become increasingly popular but the slicing of apples induces PPO activity which leads to browning of the cut surfaces lowering their esthetic quality. This has caused the industry to invest in the search for PPO inhibiting substances or procedures that can be used during the processing of fresh fruits and vegetables to reduce PPO induced browning. A potential way to lower browning levels in sliced apples is through ethrel treatments to reduce PPO activity on the oxygen exposed cut edges of the slices.

Polyphenol Oxidase in Potatoes
Polyphenol oxidase is found in high concentrations in potato tuber peel and 1-2mm of the outer cortex tissue. It is used by the potato as a defense against insect herbivory which however leads to enzymatic browning as a result of tissue damage. Damage in the skin tissue of potato tuber, by insects or human interactions, causes a disruption of the compartmentation of the cells resulting in PPO catalyzed enzymatic browning. The brown or black pigments are produced from the reaction of PPO quinone products with amino acid groups in the tuber.

In potatoes PPO genes are not only expressed in potato tubers but also in leaves, petioles, flowers and roots. It has a pH-optimum of 5.8 and sees a peak activity level at 22°C when reacting with catechol as its substrate.