Changes in safety may arise from the development or loss of substances that are protective of health or the generation or inactivation of toxic substances. The impact of chemistry on food products occurs at all points in the system - during postharvest storage, processing, postprocessing storage, transportation, supermarket and home storage, and preparation. Examples of desirable and undesirable reactions that must be controlled can be found in the formation of colors and flavors. The flavors produced by culturing milk to make yogurt and by frying when making French fries are desirable, whereas the rancid flavor resulting from oxidation of fats and oils, such as in roasted peanuts, is undesirable. Physical changes may be related to glass transitions and, sometimes, subsequent melting of partially crystalline biopolymers.
Crystallization is important in either a positive or negative sense in many products.
Controlling crystal size is important for forming the right texture in ice cream and chocolate. These reactions may be controlled by adding compounds such as antioxidants and sequestrants, maintaining a certain water activity, or designing proper packaging. In newer packaging, such as that used to prevent browning of precut vegetables and fruits used in supermarket salad bars, the atmosphere has been modified. Chemists must be concerned with the public's fear of food additives, which is tied to a fear of chemicals, even though additives are used to improve the quality, variety, convenience, and nutritive value of our food and have been determined to be safe. At the same time, chemists guard against adulteration of food.