Starch, acid conversion hydrolysis

As in the case of acid-catalyzed hydrolysis, the starch molecule is hydrolyzed to the desired starting DE in a converter, but further conversion is carried out with enzymes until the final DE or carbohydrate profile is reached. This is done by adding the appropriate enzymes to the acid-converted slurry and allowing them to react in a holding vessel called an enzyme tank. Several enzymes may be used to achieve the desired carbohydrate profile. 

Acid conversions when starch is treated with an acid such as HCI or H2SO4 at 40-60°C (commonly a starch slurry with 35-40% solids is treated with 0.2-0.5 N HCI for a few hours). Following the acid conversion, the acid is neutralized. In this process, the DP value decreases (and several physical properties are modified). Although chemical studies show that the a-D-(1- 4) links are more sensitive to hydrolysis than a-D-(1->6) links, it was determined that due to starch crystallinity more a-D-(1->6) links are hydrolyzed [55]. This happens because the a-D-(1 6) links are in the amorphous regions and they are more accessible to the reagent (less hydrogen bonds). This explains why the DP of amylose is reduced less than that of the amylopectin component. 

The conversion of starch [11] to glucose syrups was originally based on acid catalyzed hydrolysis, giving rise to large salt loads upon neutralization during work-up and the formation of imdesired coloured degradation products. 

Acid Conversion.—The theory of the acid conversion of starch into sugars was discussed in Chapter I. Practically, this method finds some use in the preparation of cereal raw materials prior to fermentation although probably less in this country than abroad. In the United States the hydrolysis of starch for fermentation is almost invariably accomplished by the diastatic action of malts. These malts are mixed with unmalted grain (the starch of which has been pastified by prior cooking) and the conversion to fermentable sugar carried to completion by a subsequent operation called mashing. ... 

The enzymatic degradation of starch is applied on an industrial scale to the production of corn syrup (a solution of oligomers) and dextrose (glucose) by utilizing thermally stable endoenzymes that can operate in an aqueous medium at temperatures up to 70-90 ° [14]. These temperatures are desired because the starch granules rupture in water at about 60 °C. The use of enzymatic hydrolysis for the conversion of starch to sugars is a much cleaner (more selective) process than is an acid-catalyzed hydrolysis. 

There are several examples of one-pot reactions with bifunctional catalysts. Thus, using a bifunctional Ru/HY catalyst, water solutions of corn starch (25 wt.%) have been hydrolyzed on acidic sites of the Y-type zeolite, and glucose formed transiently was hydrogenated on ruthenium to a mixture of sorbitol (96%), mannitol (1%), and xylitol (2%) [68]. Similarly a one-pot process for the hydrolysis and hydrogenation of inulin to sorbitol and mannitol has been achieved with Ru/C catalysts where the carbon support was preoxidized to generate acidic sites [69]. Ribeiro and Schuchardt [70] have succeeded in converting fructose into furan-2,5-dicarboxylic acid with 99% selectivity at 72% conversion in a one-pot reaction... 

Hydrolysis. The chemical reaction of a substance with water to form one or more new substances. Examples of hydrolysis are the catalytic conversion of starch into glucose the catalytic or enzymatic conversion of sucrose into glucose and fructose the conversion of natural fats into fatty acids and glycerin... 

During hydrolysis, both 1,4 and 1,6 linkages are cleaved, converting the starch molecule to increasingly lower molecular weight products. A typical residence time in the converter is 5-10 minutes for low DE syrups high DE syrups may require 15-20 minutes at the necessary temperature. It is important to keep the conversion time short to prevent unnecessary color development. A typical commercial acid converter system is shown in Figure 21 A.22... 

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... 

Enzyme-enzyme conversion employs heat and an enzyme for starch liquefaction in place of acid. This is the most common form of com processing today Subsequent hydrolysis is by enzymes, as above. The choice of hydrolytic system depends upon economics and the kind of endproduct desired. Enzymes are usually inactivated by heating the symp to 75-80°C, with the exception of the heat-stable a-amylases that have come on the market in the last 10 to 15 years. 

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