CHAPTER 7
Enzymes
Enzymes are complex, protein-based biological catalysts that induce reactions between substances without being changed by the reaction or appearing in its end product. Enzymes may be constitutive, that is, normally present within the cell, or inducible, formed only in the presence of a particular substrate. They may be intracellular, operating only within the cell, or extracellular, excreted by the cell into solution.
During the malting and brewing cycle, the barley kernel is decomposed to soluble simple sugars and albuminoids by diastatic and proteolytic enzymes. These sugars are in turn fermented to carbon dioxide and ethyl alcohol by the zymase enzyme group, while other enzymes form organic acids, aldehydes, fusel alcohols, and esters.
The traditional decoction mash is constructed largely upon a series of conditions that reactivate enzyme activity that was prematurely checked by kilning the green malt. It completes the reduction of the native barley proteins and carbohydrates to a soluble extract. In the decoction mash, proteolytic enzymes associated with malting are employed to overcome flaws in the malt.
The proteolytic (peptonizing) group reduces proteins of high molecular complexity to simpler peptides and amino-acid constituents through a structured series of interdependent reactions that sever the peptide links (CO–NH) between protein coils and replace them with a water molecule. This restores the amine (NH2) and carboxyl (COOH) groups of the amino acid
Protease and proteinase (optimum range 122 to 140 degrees F [50 to 60 degrees C] pH 4.6 to 5.0), and then peptase and peptidase (optimum range 113 to 122 degrees F [45 to 50 degrees C] pH below 5.3) solubilize protein and sequentially reduce it to proteose, peptones, polypetides, peptides, and amino acids.
Phytase and phosphatase acidify the malt by forming phytic acid, and they are primarily responsible for the acidulation of the mash at 95 to 122 degrees F (35 to 50 degrees C). They also increase the soluble mineral content of the wort. Cellulase, hemicellulase, collagenase, and pectinase are active within the same temperature range, dissolving the cell walls, endosperm case, gelatin, and pectins.
The diastatic enzymes reduce starch to fractions. Primarily, these are the amylolytic enzymes — alpha- and beta-amylase. The alpha-amylase liquefies native starch and reduces amylose and amylopectin to a stew of carbohydrate fractions. By randomly separating 1-4 linked glucose molecules within the length of polysaccharide chains, it liberates glucose, maltose, maltotriose, and dextrins, leaving “a-limit” dextrins wherever it is stopped by 1-6 link branching points in amylopectin. It reduces complex starch to a-limit dextrins very rapidly and completely, so that its solution gives only a faint-red reaction with iodine. Yet it further generates a predominance of maltose only very slowly and ineffectively. It is present in the unmalted barley.
Beta Amylase
Alpha Amylase
Beta-amylase, on the other hand, does not appear until malting. It has no effect on the native starch. In solution it detaches glucose molecules from the nonreducing ends of amylose and amylopectin chains, rejoining them with a water molecule to produce maltose. Alone, it breaks down amylose very slowly and amylopectin very incompletely, because it proceeds in a linear fashion and only from one chain end. It is ineffective within two or three glucose molecules of amylopectin’s outermost branching points, leaving a very large “ß-limit” dextrin that gives a deep mahogany color reaction with iodine. Where alpha-amylase activity splits soluble starch into smaller fractions, beta-amylase operates more efficiently, capitalizing upon the increased number of exposed chain ends.
Both amylases are made more effective by the activity of debranching enzymes. A-glucosidase (maltase), limit dextrinase, and pullulanase reduce amylopectin and limit dextrins to amylose by cleaving the linkages at their branching points. The debranching enzymes are most active during malting, and very few survive kilning, even with low-color Pilsen malt. At low mash temperatures they may dismantle some amylopectin, but not at hotter saccharification temperatures in the mash.
During fermentation, the zymase enzymes and a phosphoric coenzyme convert glucose to alcohol and carbonic gas; other enzymes are formed during fermentation that split and invert the more complex sugars present in the ferment. Intracellular maltase and glucase reduce maltose to two molecules of glucose; extracellular invertase splits sucrose into glucose and fructose. Finally, proteolytic enzymes within the yeast cell, triggered by a decline in the yeast’s metabolism, autolyze the cell contents to other enzymes, minerals, and vitamins that are slowly released into solution.
It is the enzymatic composition of the yeast cell that determines the nature and vigor of fermentation. Various yeast strains have widely varying enzymatic capability. When the yeast cells do not contain the specific enzymes to reduce the sugars in a wort, they synthesize them. Fermentation lag times, however, are dangerously extended.