The use of enzymes as catalysts is well known in a number of industries, such as baking or wine making. But punfied enzymes are soluble in water. It is, therefore, not easy to remove them from the final product. Further, it is difficult to re-use them. Thus, enzyme activity is lost in one cycle of the chemical reaction. These difficulties led to the development in the late 1960s of immobilised enzymes. The trick is to link an enzyme chemically to a large molecule, such as gelatin. It can then be used as a catalyst, and it can be extracted with the large molecule, for use once again. Immobilised enzymes have been successfully used in the production of semi-synthetic penicillin and in the large scale production of fructose from maize. Fructose is sweeter than glucose, yet it has the same calorific value and is used as a low calorie sweetener.