What is sugar?
Sweetness is always the first thing that comes to mind in responding to the question or in contemplating what is sugar. In ordinary usage, sugar is the common term for sucrose, those brownish granules or fine white crystals that bear the name brown sugar or refined sugar with which coffee, milk and other drinks are sweetened. Being processed from raw materials, it is likewise described as an industrial product with varied uses particularly in food science including as preservative.
In studying basic physiology in plants, sugar is commonly used interchangeably with carbohydrate. Both words are used to refer to the product of photosynthesis which stores the energy obtained from the sun and the carbon fixed from the atmosphere. However, the sugars are only a subset of carbohydrates. The question What is sugar? therefore ought to settle what is carbohydrate.
So first, what are carbohydrates?
According to Carey (1992), carbohydrates used to be considered as "hydrates of carbon" because many of them (not all) have molecular formulas that correspond to Cn(H2O)m. Carbohydrates are one of major classes of substances that are common to living systems, the others being lipids, proteins, and nucleic acids. Among others, they are structural components of the cell wall and the wood of trees. They provide most of the energy and the nourishment that we obtain from the food that we eat.
Saccharon is the Latin word for sugar (Kidd 1957), from which the words saccharum and saccharide are derived. Carbohydrates are also called saccharides.
Based on structural complexity, carbohydrates are divided into the monosaccharides, oligosaccharides (including disaccharides), and polysaccharides. Carbohydrate names generally end with ose.
Combining human nutrition (Wardlaw et al. 2004), organic chemistry and biochemistry, sugar is used as the general term for the simple carbohydrates, that is, the monosaccharides and disaccharides. Most of these carbohydrates are sweet, the reason why they are called sugar (Bettelheim and March 1998). In contrast, polysaccharides in general lack sweetness (Devlin 1975).
The pentoses, referring to the five-carbon monosaccharides, and the hexoses, which contain a chain of six carbons, are of particular importance in plant physiology.
The pentoses consist of the ribose, deoxyribose, arabinose, xylose, lyxose, and erythrupentulose (Mathews and Van Holde 1990; Carey 1992). The sugar 2-deoxy-D-ribose is a component of the DNA molecule.
The hexoses which are of common occurence in most plants are the glucose, fructose, mannose, and galactose. They are either dissolved in the cell in the free form (i.e., glucose and fructose) or, as in the pentoses in general, joined as a component of some complex carbohydrate (Devlin 1975).
Glucose (C6H12O6) is the first product of photosynthesis as well as the end product of hydrolysis of starch, cellulose, and other complex carbohydrates. It is the main source of energy for all living organisms and is one among the few organic compounds that may be supplied as food into the bloodstream by injection (Linstromberg and Baumgarten 1987).
Sugars also differ as to common usage, sweetness, and commercial methods of production. The distinctions will further help in clarifying what is sugar particularly in relation to sugar crops.
Sucrose, also known as table sugar or cane sugar, is the most widely used sweetener in the world. Upon hydrolysis by the enzyme invertase, sucrose yields glucose and fructose in equal proportion. The mixture is called invert sugar (Linstromberg and Baumgarten 1987; Bettelheim and March 1998). This process of transforming sucrose to invert sugar is also accomplished by bees in producing honey. Invert sugar is largely preferred in the manufacture of candy because of lesser tendency to crystallize than sucrose (Linstromberg and Baumgarten 1987).
Note: According to Wardlaw et al. (2004), honey should be avoided as food to infants and children below 1 year old because it can contain spores of Clostridium botulinum, a bacterium that causes the fatal foodborne illness known as botulism. Unlike infants, however, an adult's stomach produces much acid which inhibits the growth of the bacteria.
Glucose is a type of sugar that is referred to as dextrose, blood sugar, or grape sugar. It was isolated from raisins in 1747 (Carey 1992). Commercially, it is produced from starch, a polysaccharide that is considered as a major storage product of photosynthesis in many plants, as in cereals and starchy root and tuber crops.
Fructose, also called levulose or fruit sugar, occurs with glucose in the juices of many ripe fruits. It is also prepared commercially from inulin, a polysaccharide that is found in the roots of dahlia and in the tubers of Jerusalem artichoke (Hill 1972; Linstromberg and Baumgarten 1987).The high-fructose corn syrup (HFCS) is widely used in making soft drinks, frozen desserts, and confections (Wardlaw et al. 2004).
Mannose is used largely in medicine but it does not occur naturally. It is prepared by hydrolysis of polysaccharides. Similarly, maltose or malt sugar is not readily available from plants. It is produced from starch through enzymatic degradation. Maltose is largely used in the brewing industy (Hill 1972). The chemical catalyst, an enzyme called malt diastate, is found in sprouting barley (Linstromberg and Baumgarten 1987). D-xylose can be isolated through hydrolysis of the polysaccharides in corncobs and wood of trees. Arabinose is also obtained from the polysaccharide in mesquite gum through acid hydrolysis (Carey 1992).
Further, it has become possible to elucidate what is sugar based on sweetness. Wardlaw et al. (2004) provides the following relative sweetness of different types of sugar on a per gram basis in comparison with sucrose (1.0): maltose- 0.4, glucose- 0.7, invert sugar- 1.3; and fructose- 1.2-1.8. This means that fructose is about 1.2 to 1.8 times (nearly double) more sweeter than table sugar. The values given by Bettelheim and March (1998) slightly differ but nevertheless show that galactose has the least sweetness with a score of 0.32.
(Ben G. Bareja December 2011)