BIOCHEMISTRY OF FOOD CARBOHYDRATES
‘Carbohydrate’ literally means ‘carbon hydrate,’ which is reflected in the basic building block unit of simple carbohydrates, i.e. (CH2O)n. Carbohydrates make up the majority of organic mass on earth having the biologically important roles of energy storage (e.g. plant starch, animal glycogen), energy transmission (e.g. ATP, many metabolic intermediates), structural components (e.g. plant cellulose, arthropod chitin), and intra- and extracellular communication (e.g. egg-sperm binding, immune system recognition). Critical for the food industry, carbohydrates serve as the primary nutritive energy sources from foods like grains, fruits and vegetables, as well as being important ingredients for many formulated or processed foods. Carbohydrates are used to sweeten, gel, emulsify, encapsulate or bind flavors, can be altered to produce color and flavor via various browning reactions, and are used to control humidity and water activity
The basic unit of a carbohydrate is a monosaccharide; 2 monosaccharide’s bound together are called a disaccharide; 3 are called a disaccharide, 2–10 monosaccharide’s in a chain are termed an oligosaccharide, and 10 or more are termed a polysaccharide. The simplest food-related carbohydrates, monosaccharide’s, are glucose, mannose, GA lactose and fructose.
Carbohydrate Browning Reactions
There are three general categories of browning reactions in foods: oxidative/enzymatic browning, caramelisation and no oxidative/non-enzymatic/Mallard browning. Oxidative browning is discussed later in the section on proteins. The latter two types of browning involve carbohydrate reactions. Caramelisation involves a complex group of reactions that are the result of direct heating of carbohydrates, particularly sugars.
Metabolism of Carbohydrates
The characteristics of carbohydrates in both their natural states and as processed food ingredients determine the properties of many foods as well as their utilization as nutrients. Glycolysis is a fundamental pathway of metabolism consisting of a series of reactions in the cytosol, where glucose is converted to pyruvate via nine enzymatically catalyzed reactions.
Pectin substances include polymers composed mainly of α-(1, 4)-d-galacturonopyranosyl units and constitute the middle lamella of plant cells. Pectin’s exist in the prospecting form in unripe (green) fruit, contributing to firm, hard structures. Upon ripening, projections are metabolized into smaller molecules, giving ripe fruits a soft texture. Controlling enzymatic activity against prospecting is commercially important in fruits such as tomatoes, apples and persimmons.