Syrup enzyme conversion

P-amylase, and debranching enzymes. Conversion of D-glucose to D-fmctose is mediated by glucose isomerase, mosdy in its immobilized form in columns. Enzymic degradation of starch to symps has been well reviewed (116—118), and enzymic isomerization, especially by immobilized glucose isomerase, has been fiiUy described (119) (see Syrups). 

At completion of the enzyme conversion, the tanks are emptied in succession and the liquor is processed through filtration and carbon bleaching, as previously described, and evaporated to the proper solids level. The advent of enzyme-converted syrups lessens the importance of traditional methods of measurement, such as determination of DE. It is possible to have two syrups with the same DE and completely different carbohydrate profiles and performance characteristics, as shown in Table 21.4.31... 

Enzyme conversions are more specific and controlled compared to acid hydrolysis. This is dne to the specificity of the enzymes for certain substrates or chemical bonds. The final result is that enzyme-based processes yield better-quaUty syrups. The most common and widely used enzymes with their specific activity are a-amylase, P-amylase, pullulanase, amyloglucosidase, cyclodextrin glycosyl-transferase, and glucose isomerase (Guzman Maldonado and Paredes Lopez 1995, Hobbs 2003, Kruger et al. 1987, Teague and Brumm 1992, Woods and Swinton 1995). 

Interest in the bacterial ens me xylose/glucose isomerase has been driven by its use in the isomerization of ucose to fructose to produce high>fructose corn syrups, and in the isomerization of xylose to xylulose for the conversion of the more fermentable xylulose to ethanol In this work, a brief historical perspective is presented, followed by a summary of the current understanding of the enzyme s major features. Also, a useful compilation of available xylose isomerase DNA sequences is presented with annotation of some of the major areas identified as being of functional significance. The extent of homology between the xylose isomerases is discussed with reference to differences in their function. 

Thirdly there is the conversion of the glucose syrups into HFCS using iimnobilised glucose isomerase. Use of soluble enzyme is not possible because of its high cost, and because it is an intracellular enzyme and is only stable when used still associated with its parent cell. The activities and costs of these enzymes are given in Table 4.5. These starch enzymes rank with some of the largest enzymes in world market. 

The Immobilized Enzyme System. The glucose isomerases used are immobilized and granulated to a particle size between 0.3 and 1.0 mm. The enzyme granulates must be rigid enough to withstand compaction when they are packed into the column. Ca2+ acts as an inhibitor in the system, and therefore calcium salts need to be removed from the feed syrup. Conversely, Mg2+ acts as an activator, and magnesium salts are added to the feed syrup. 

Physical properties of a syrup depend heavily on its carbohydrate profile. The carbohydrate profile, in turn, is determined by the type of conversion and the nature of the enzyme treatment (previously discussed). Table 21.2 gives typical DE and carbohydrate profiles of syrups in common production today. Because enzyme treatments can provide sweeteners with different carbohydrate profiles but the same DE value, it is usual to refer to a product using more than one descriptor, e.g. a 43 DE, high-maltose syrup. This issue becomes particularly important when addressing functional differences and applications of starch-derived sweeteners. 

The com industry makes widespread use of enzymes for carbohydrate conversion. The advent of enzyme technology in the corn industry in the 1960s dramatically changed the starch industry and allowed the development of new products. Today, enzyme hydrolysis of starch has largely replaced acid hydrolysis, which is used as an adjunct in starch conversion. Enzymes used to make corn syrups and HFCS include the following. 

Starch conversion refers to the process of converting starch into other products. It involves gelatinization, liquefaction, and saccharification. Liquefaction refers to the acid-or enzyme-catalyzed conversion of starch into maltodextrin. Starch, usually from wet milling of com, is pumped in a slurry to the conversion plant, where it undergoes one or more hydrolytic processes to yield mixtures of various carbohydrates in the form of syrups. The kind and amount of the various carbohydrates obtained depend upon the type of hydrolysis system used (acid, acid-enzyme, or enzyme-enzyme), the extent to which the hydrolytic reaction is allowed to proceed, and the type of enzyme(s) used. The fact that most starches consist of two different kinds of polymers... 

Debranching Enzymes. One major source of yield loss in the production of high fructose corn syrup is the loss due to incomplete conversion of starch to glucose. When a need exists with a significant dollar value associated with it, someone will develop a cost effective solution if given enough time. 

Figure 4-18 Major Steps in Enzymic Starch Conversion. Source Reprinted from H.S. Olsen, Enzymic Production of Glucose Syrups, in Handbook of Starch Hydrolysis Products and Their Derivatives, M.W. Kearsley and S.Z. Dziedzic, eds., p. 30, 1995, Aspen Publishers, Inc.

The isomerization of glucose to fructose opened the way for starch hydrolyzates to replace cane or beet sugar (Dziezak 1987). This process is done with glucose isomerase in immobilized enzyme reactors. The conversion is reversible and the equilibrium is at 50 percent conversion. High-fructose com syrups are produced with 42 or 55 percent fructose. These sweeteners have taken over one-third of the sugar market in the United States (Olsen 1995). 

The most widely used immobilized enzyme process involves the use of the enzyme glucose isomerase for the conversion of glucose to fructose in com syrup (Carasik and Carroll 1983). The organism Bacillus coagu-lans has been selected for the production of glucose isomerase. The development of the immobilized cell slurry has not proceeded to the point where half-lives of the enzyme are more than 75 days. A half-life is defined as the time taken for a 50 percent decrease in activity. Such immobilized enzyme columns can be operated for periods of over three half-lives. 

While glucose syrups were made by acid conversion, the DE gave a complete specification of the product. The ready availability of suitable enzymes has widened enormously the types of glucose syrups available. 

Fig. 5.—Process and Conditions for the Manufacture of Dual-conversion (Acid- and Enzyme-Catalyzed Hydrolyses) Com Syrups.

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