Enzymes
Pages 73-79
DOI: https://doi.org/10.1094/0913250961.007
ISBN: 0-913250-96-1
Abstract
Topics Covered
- Commercial Sweetener Production
- Dairy Applications
- Cheesemaking
- Surface-Ripened Cheeses
- Enzyme-Modified Cheeses
- Protein Modification
- Endogenous Enzymes as Processing Indicators
Introduction to Chapter
Chapter 3 discussed enzymes that convert starch to dextrins and simple sugars, particularly glucose and maltose. The present chapter considers the conversion of starch to sugars, primarily glucose and fructose, on an industrial level. The starch can be derived from a number of sources such as wheat, corn, rice, potatoes, and barley. By far the most common starting material is corn starch. The major classes of sweeteners produced by conversion of corn starch are corn syrups, dextrose, high-fructose corn syrup, and fructose.
The conversion of starch to sugar is generally described by referring to a unit called a “dextrose equivalent” (DE). (The terms dextrose and glucose both refer to the same sugar molecule.) A “DE” is a measure of the degree of hydrolysis of the syrup product. Starch itself has no free dextrose and so has a DE of zero. Dextrose has a DE of 100; i.e., it is 100% dextrose. All DE values are therefore between 0 and 100.
The initial (acid/heat) method of conversion of starch to sugar had a number of drawbacks: low efficiency, e.g., low rate of conversion and incomplete hydrolysis (DE of 35–45); undesirable by-products; and the need for specialized equipment to withstand the high heat under acid conditions. These negative aspects led to the search for a better conversion process.
Over the past 25–30 years, a new approach has evolved that involves the enzymatic conversion of starch to sugar. Initially, the enzymes were used in conjunction with the acid conversion. The starch slurry was gelatinized and treated with acid and heat. Then, the enzymes, usually a mixture of α-amylase and glucoamylase (amyloglucosidase) enzymes, were added to break the starch dextrins down further. It was determined that the acid pretreatment facilitated the subsequent enzyme treatment of the starch dextrins. The primary limitation with the enzymatic conversion was that, at the time, the α-amylase enzymes were still not very stable under the relatively high heat conditions used. The DE values of the sugar syrups were in the range of 60–80.
Procedures were developed that used only enzymatic starch conversion, but the rates of conversion were still not as high as desired, again because the bacterial α-amylases then in use did not have adequate heat stability. In the early 1970s, bacterial α-amylases were found that were unusually stable to heat; an example is the α-amylase from Bacillus licheniformis. The higher conversion rates led to an allenzyme conversion process, resulting in sugar syrups with DE values in the range of 95–98 DE.
This all-enzymatic conversion of starch to sugars is basically a two-step process. First comes the liquefaction step, followed by a saccharification step (Fig. 7-1).
