Both glucose and fructose are six-carbon sugars but three-, four-, five- and seven-carbon sugars are also present in the grape.
en.wikipedia.org Lactic acid is produced actively without liberation of gas from glucose, fructose, mannose, sucrose, maltose, trehalose, raffinose, inulin, mannitol, sorbitol and alpha-methyl glucoside.
en.wikipedia.org Fructose does essentially the same thing, and it is created even further on in the chemical pathway.
en.wikipedia.org For example, glucose isomerase, which is used industrially to convert glucose into the sweetener fructose, is a xylose isomerase "in vivo" (within the body).
en.wikipedia.org However, honey contains fructose, which caramelizes at lower temperatures than the glucose.
en.wikipedia.org