This approach, however, may not be practical for the home "culinary scientist." One approach the food industry employs to prevent enzymatic browning is to select fruit varieties that are less susceptible to discoloration-either due to lower PPO activity or lower substrate concentration. In addition, a tissue's PPO level can vary depending on growing conditions and fruit maturity. Well, nearly all plant tissues contain PPO, however, the level of PPO activity and concentration of substrate-here, the phenolic compounds-can vary between varieties of fruits (say, Granny Smith versus Red Delicious). One question that often accompanies yours is, "Why do some apples seem to brown faster than others?" O-quinones then produce the well documented brown color by reacting to form compounds with amino acids or proteins, or they self-assemble to make polymers. When oxygen is present in cells, polyphenol oxidase (PPO) enzymes in the chloroplasts rapidly oxidize phenolic compounds naturally present in the apple tissues to o-quinones, colorless precursors to brown-colored secondary products. When an apple is cut (or bruised), oxygen is introduced into the injured plant tissue. Lynne McLandsborough, a professor of food science at the University of Massachusetts Amherst, explains this oft-observed kitchen conundrum.
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