The brown color produced during the heating of many different foods comes, in part, from the Maillard reaction. This reaction contributes to the golden crust of baked products, the browning of meats, and the dark color of roasted coffee. Temperatures most conducive to the Maillard reaction are those reaching at least 194°F (90°C), but browning can occur at lower temperatures, as seen in dried milk that has been stored too long.
Enzymatic browning is the result of an entirely different mechanism than the Maillard reaction and is familiar to anyone who has ever taken a bite out of an apple or pear and then let it sit. Enzymatic browning is responsible for this discoloration seen in certain cut fruits and vegetables. It requires the presence of three substances: oxygen, an enzyme (polyphenolase), and a phenolic compound. Normally, the cell structure separates the enzymes from the phenolic compounds, but when the vegetable or fruit is cut or bruised, the phenols and enzymes, thus exposed to oxygen, react in its presence to produce brown-colored products. Not all fruits and vegetables contain phenolic compounds, but sliced apples, pears, bananas, and eggplants turn brown rather rapidly after cutting. Potatoes turn slightly pink or gray:
Another type of enzymatic browning occurs when the enzyme tyrosinase oxidizes the amino acid tyrosine to result in dark-colored melanin compounds such as that observed in browning mushrooms. Although the browning from either phenolase or tyrosinase is unappealing in itself, it is harmless.