The synthesis of fructose from starch

HIGH-FRUCTOSE CORN SYRUP (42% fructose, 51% glucose, 7% oligosaccharides) 

The large-scale process for the production of glucose from corm starch uses 

For starch to be effectively degraded it must first be heated above 65 °C. The starch grains then burst and they form a viscous gel. This must be thinned so that the viscosity is low enough for the dispersed starch to be handled. The a-amylase from barley is fairly stable to heat, and it is active at 65 °C. However some a-amylases from the organisms which grow at temperatures up to 80° or 90 °C (e.g. Bacillus stearothermophilus) are stable for short periods at temperatures above 100°C, and these heat-stable enzymes are very useful for the controlled thinning of starch gels. 

In the process a starch suspension containing the heat-stable a-amylase is heated briefly to 140 °C so that it forms a gel. This is sufficiently hydrolysed before the a-amylase is destroyed to allow it to be pumped to a vessel where more a-amylase is added, and the hydrolysis continues for about 30 min at 100 °C. At the end of this period about 10% of the a-1,4 links in the starch are hydrolysed and the gel is thin enough to be cooled to 55 °C without setting solid. Amyloglucosidase and pullulanase are stable at this lower temperature, and enough of these two enzymes are added to catalyse the hydrolysis of the starch to glucose over a two- or three-day period. Only some 2 or 3% of the links between the glucose units remain, and this, in fact, represents the equilibrium position of the hydrolysis. 

If necessary, pure fructose can be separated from the syrup chromatographi-cally. The glucose is returned to the isomerization process, but only after being treated again with amyloglucosidase which is chemically bonded to a supporting matrix. This preparation hydrolyses the residual oligosaccharides which would otherwise build up in the process. 

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