Chemists determine the most effective type of processing treatment for each food product. Foods are complex, heterogeneous biological materials. Food chemists also deal with milk, a biological fluid, and single-cell sources of food such as eggs, yeast, and other microorganisms. Food products are often highly hydrated, but their moisture content may vary from more than 95 percent to less than 4 percent. Appearance, color, flavor, and texture are important aspects of the organoleptic quality of foods.
Texture is the response to forces involved in chewing, swallowing, cutting, or spreading.
Instability of foods, which limits their shelf life, can be caused by chemical reactions, biochemical reactions, or physical changes. Changes in texture may be such things as either a toughening or a softening, a loss or gain in solubility, or a loss or gain in water-holding capacity. Typical flavor changes may be caused by the development of hydrolytic or oxidative rancidity, other off-flavors, cooked or caramel flavors, or other desirable flavors. Typical color changes include darkening or bleaching and the development of desirable colors and off-colors. Changes in nutritive value may arise from either increases in bioavailability or losses of proteins, starch, vitamins, lipids, or minerals.