CHEMICAL COMPONENTS products. Some hydrolytic enzymes involved in breakdown of starch and protein stored in the pericarp are found before maturity and may persist(Fretzdorf and Weipert, 1990) In the mature grain the enzyme relatively low if the grain is sound ar damaged, as in milling, lipids become lipase. This is particularly relevant to oats; and to germ and bran fractions of other grains On adequate damping germination begins o amyloglucosidase and enzymes concerned with solubilization are produced Cell walls are hydrolyzed, permitting grammatic representation of hydrolytic cleavage reater access to storage products by enzymes by alpha-amylase, beta-amylase amd. amylogluco that catalyze hydrolysis of starch and protein( see ly From D. H. Simmonds( 1989). Reproduce Ch.2) Technologically the highest profile enzyme is alpha-amylase, as large quantities are essential in capacity of the solution increases rapidly. When uccessful malting and brewing and small quanti- the substrate is amylose the iodine staining reaction ties are necessary in breadmaking. Excessive is reduced only slowly as the chain lengths, on alpha-amylase in milling wheats is disastrous, which iodine binding depends, are slowly reduced dextrin production during baking By contrast endo-action of alpha-amylolysis making the crumb sticky. polyphenol oxidases through random fragmentation reduces iodine can lead to production of dark specks in stored staining relatively quickly in relation to the four products. Other classes of enzymes of tech- increase in reducing power. A further conse- nological importance, found in cereals are quence of the rapid reduction in molecular size beta-amylases, proteases, beta-glucanases, lipases, resulting from alpha-amylolysis marked lipoxygenase and phytase reduction in viscosity of a starch suspension. This is exploited in laboratory tests for the enzyme Assaying for beta-amylase is more difficult because the rate of maltose production is influenced by the Both alpha- and beta-amylases are a-(1-4)-D. presence of alpha-amylase and the enzymes are glucanases; by definition catalyzing the hydro- almost always present together. Even in well lysis of the same bonds within starch molecules. washed starch they are absorbed on the granule Their action is synergistic because beta-amylase surface(Bowler et al., 1985) gains greater access to the substrate through the grain quality is more influenced by the alpha tivity of alpha-amylase. As this last observation enzyme as its amount is more variable according implies, their modes of action are quite different: to the condition of the grain beta-amylase is alpha-amylase is endo-acting while beta-amylase present in resting grain and increases only a few is exo-acting(Fig. 2) fold on germination through release of a bound Exo-enzymes catalyze removal of successive form low molecular weight products from the non Alpha-amylase is actually synthesized during reducing chain-end, the product removed through germination and activity increases progressively eta-amylase activity is maltose due to the hydro- as germination proceeds, by several hundred fold lysis of alternate a-(1-4)-glycosidic bonds. beta- In different cereals the site of synthesis of alpha Amylase is inactive on granular starch but is capable amylase varies; in wheat, rye and barley it of rapid action when the substrate is in solution. first in the scutellum and later in the al As the exo-action produces a large number of in maize only the scutellum is involved small sugars with reducing power, the reducing isoenzymes of the alpha-amylase typeCHEMICAL COMPONENTS 67 1 0 A m y I o s e f products. Some hydrolytic enzymes involved in breakdown of starch and protein stored in the pericarp are found before maturity and may persist (Fretzdorf and Weipert, 1990). In the mature grain the enzyme levels are relatively low if the grain is sound and dry. If damaged, as in milling, lipids become exposed to lipase. This is particularly relevant to oats; and to germ and bran fractions of other grains. On adequate damping, germination begins and enzymes concerned with solubilization are produced. Cell walls are hydrolyzed, permitting greater access to storage products by enzymes that catalyze hydrolysis of starch and protein (see Ch. 2). Technologically the highest profile enzyme is alpha-amylase, as large quantities are essential in capacity of the solution increases rapidly. When successful malting and brewing and small quanti- the substrate is amylose the iodine staining reaction ties are necessary in breadmaking. Excessive is reduced only slowly as the chain lengths, on alpha-amylase in milling wheats is disastrous, which iodine binding depends, are slowly reduced. leading to dextrin production during baking, By contrast endo-action of alpha-amylolysis making the crumb sticky. Polyphenol oxidases through random fragmentation reduces iodine can lead to production of dark specks in stored staining relatively quickly in relation to the flour products. Other classes of enzymes of tech- increase in reducing power. A further conse￾nological importance, found in cereals are quence of the rapid reduction in molecular size beta-amylases, proteases, beta-glucanases, lipases, resulting from alpha-amylolysis is a marked lipoxygenase and phytase. reduction in viscosity of a starch suspension. This is exploited in laboratory tests for the enzyme. Assaying for beta-amylase is more difficult because the rate of maltose production is influenced by the Amylases Both alpha- and beta-amylases are a-( 1+4)-~- presence of alpha-amylase, and the enzymes are glucanases; by definition catalyzing the hydro- almost always present together. Even in well lysis of the same bonds within starch molecules. washed starch they are absorbed on the granule Their action is synergistic because beta-amylase surface (Bowler et al. , 1985). gains greater access to the substrate through the Grain quality is more influenced by the alpha￾activity of alpha-amylase. As this last observation enzyme ss its amount is more variable according implies, their modes of action are quite different: to the condition of the grain. beta-Amylase is alpha-amylase is endo-acting while beta-amylase present in resting grain and increases only a few is exo-acting (Fig. 3.12). fold on germination through release of a bound Exo-enzymes catalyze removal of successive form. low molecular weight products from the non- Alpha-amylase is actually synthesized during reducing chain-end, the product removed through germination and activity increases progressively, beta-amylase activity is maltose due to the hydro- as germination proceeds, by several hundred fold. lysis of alternate a-( 1-+4)-glycosidic bonds. beta- In different cereals the site of synthesis of alpha￾Amylase is inactive on granular starch but is capable amylase varies; in wheat, rye and barley it occurs of rapid action when the substrate is in solution. first in the scutellum and later in the aleurone, As the exo-action produces a large number of in maize only the scutellum is involved. Several small sugars with reducing power, the reducing isoenzymes of the alpha-amylase type exist in ttt Amylopectin x--) alpha-amylase T + befa-amylase w amyloglucosidase FIG 3.12 Diagrammatic representation of hydrolytic cleavage catalyzed by alpha-amylase, beta-amylase amd amylogluco- sidase respectively. From D. H. Simmonds (1989). Reproduced by courtesy of CSIRO