Glucose

Glucose is a sugar with the molecular formula C₆H₁₂O₆. The name "glucose" (/ˈɡluːkoʊs/) comes from the Greek word γλυκος, meaning "sweet wine, must". The suffix "-ose" is a chemical classifier, denoting a carbohydrate. It is also known as grape sugar. With 6 carbon atoms, it is classed as a hexose, a sub-category of monosaccharides. α-D-glucose is one of the 16 aldose stereoisomers. The D-isomer (D-glucose), also known as dextrose, occurs widely in nature, but the L-isomer (L-glucose) does not. Glucose is made during photosynthesis from water and carbon dioxide, using energy from sunlight. The reverse of the photosynthesis reaction, which releases this energy, is a very important source of power for cellular respiration. Glucose is stored as a polymer, in plants as starch and in animals as glycogen, for times when the organism will need it. Glucose circulates in the blood of animals as blood sugar. Glucose can be obtained by hydrolysis of carbohydrates such as milk, cane sugar, maltose, cellulose, glycogen etc. It is however, manufactured by hydrolysis of cornstarch by steaming and diluting acid.

Sugars in wine

The sugars in wine grapes are what make winemaking possible. During the process of fermentation, sugars are broken down and converted by yeast into alcohol (ethanol) and carbon dioxide. Grapes accumulate sugars as they grow on the grapevine through the translocation of sucrose molecules that are produced by photosynthesis from the leaves. During ripening the sucrose molecules are hydrolyzed (separated) by the enzyme invertase into glucose and fructose. By the time of harvest, between 15 and 25% of the grape will be composed of simple sugars. Both glucose and fructose are six-carbon sugars but three-, four-, five- and seven-carbon sugars are also present in the grape. Not all sugars are fermentable with sugars like the five-carbon arabinose, rhamnose and xylose still being present in the wine after fermentation. Very high sugar content will effectively kill the yeast once a certain (high) alcohol content is reached. For these reasons, no wine is ever fermented completely "dry" (meaning without any residual sugar). Sugar's role in dictating the final alcohol content of the wine (and such its resulting body and "mouth-feel") sometimes encourages winemakers to add sugar (usually sucrose) during winemaking in a process known as chaptalization solely in order to boost the alcohol content - chaptalization does not increase the sweetness of a wine.

L-Glucose

L-Glucose is an organic compound with formula C₆H₁₂O₆ or H–(C=O)–(CHOH)₅–H, specifically one of the aldohexose monosaccharides. As the L-isomer of glucose, it is the enantiomer of the more common D-glucose.

L-Glucose does not occur naturally in higher living organisms, but can be synthesized in the laboratory. L-Glucose is indistinguishable in taste from D-glucose, but cannot be used by living organisms as source of energy because it cannot be phosphorylated by hexokinase, the first enzyme in the glycolysis pathway. One of the known exceptions is in Burkholderia caryophylli, a plant pathogenic bacterium, which contains the enzyme D-threo-aldose 1-dehydrogenase which is capable of oxidizing L-glucose

Like the 'D-' isomer, L-glucose usually occurs as one of four cyclic structural isomers — α- and β-L-glucopyranose (the most common, with a six-atom ring), and α- and β-L-glucofuranose (with a five-atom ring). In water solution, these isomers interconvert in matters of hours, with the open-chain form as an intermediate stage.