A kinetic model for enzymatic wheat starch saccharification

Abstract

A kinetic model describing the enzymatic sacchari®cation of wheat starch by a mixture of a-amylase and amyloglucosidase has been developed. The model describes the in¯uence of pH, glucose inhibition and starch and enzyme concentration. The results of experimental sacchari®cation under different physical conditions, eg pH and temperature, were used to determine the parameters in the model. The dominant enzyme in the mixture was amyloglucosidase and the maximum rate of sacchari®cation due to this enzyme was found to be optimal at pH 5, and increased Five-Fold when the temperature was increased from 30 to 55°C. Sacchari®cation due to the action of amyloglucosidase was inhibited by the glucose produced and simulation showed that the maximum rate of sacchari®cation decreased by 58% at a starch concentration of 140 g dm À3 compared with a starch concentration much less than 110 g dm À3 where the effect of glucose inhibition was negligible. # 2000 Society of Chemical Industry Keywords: kinetics; model; sacchari®cation; starch; wheat; enzyme catalysisGlucose concentration (g glucose (100 g of solution)Maximum glucose concentration (g glucose (100 g of solution)Concentration of an oligosaccharide with n glucose units (g dmTotal concentration of oligosaccharides and polysaccharides consisting of more than seven glucose units (g dmParameter in velocity expression (g dm À3 ) (eqns (2) and (11))Michaelis±Menten constant for an n-mer oligosaccharide (g oligosaccharide dmMaximal velocity for digestion rate of an n-mer oligosaccharide (h À1 ) (eqn (2)) k n Proportional parameter for the formation of glucose or an n-mer oligosaccharide from larger oligosaccharides (h À1 ) (eqns (10) and (12) INTRODUCTIONStarch hydrolysates are often used as substrates in the fermentative production of alcohols, amino acids, biopolymers, carboxylic acids and enzymes. 1 In the present study, the sacchari®cation of wheat starch to glucose was investigated with the intention of fermenting the glucose to lactic acid.2,3 The sacchari®cation and fermentation steps are often integrated to reduce the effect of glucose inhibition in the two process steps and a kinetic model is useful in evaluating whether the sacchari®cation and the fermentation steps should be performed simultaneously or separately. It is important to determine the model parameters for each step individually, and in this study an empirical kinetic model was developed which described enzymatic wheat starch sacchari®cation. The model will be used for simulation and optimisation of the total process of fermentative production of lactic acid from wheat starch.In the present study, starch was hydrolysed in two separate steps: liquefaction and sacchari®cation. In the liquefaction step the starch was hydrolysed to soluble oligosaccharides using a thermostable a-amylase. This enzyme is an endoamylase which randomly attacks the a(1→4) glucosidic linkages of starch to reduce the length of the starch chains. In the subsequent sacchari®cation step, the oligosaccharides were further hydroly...

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