Mashing is done to create fermentable sugars for use by the yeast during fermentation. The process of mashing involves combining crushed malted barley with water and starchy adjuncts (additives like unmalted barley, corn, oats, potato, rice, rye, sorghum, and wheat). The process is continued over a period of time at different temperatures that activate and denature specific enzymes in order to break down proteins and starches(4). Only two enzymes readily fall under the brewer's control, and these are proteases (proteolytic enzymes), which degrade proteins, and diatase (diastatic enzymes), which degrade starches. We first look at the action of proteolytic enzymes.
The naturally occurring proteolytic enzymes that are used in the mash degrade the proteins in the grains into forms of protein which improve the quality and fermentation characteristic of the beer(4). The yeast used in brewing requires amino acid proteins as nutrients. In a temperature range of 103-122 F (40-50 C), proteolytic enzymes become active that break down nitrogen based proteins into amino acid proteins providing these nutrients for later use by the yeast. These nutrients are very significant in developing the attenuation of the wort, which is the ability of the yeast to ferment and convert fermentable sugars into alcohol and CO2. The temperature of the mash is then raised to between 122-140 F (50-60 C), which activates other proteolytic enzymes that "break down proteins into forms that improve the foam potential of the beer and aid in clarity."(4) This stage in the mashing process is know as the protein rest(3).
Diastatic enzymes in the mash convert starch molecules into fermentable sugars and dextrins. The two enzymes that do all of the work are alpha-amylase and beta-amylase. These two enzymes convert the soluble starch molecules, which are really just long chains of glucose molecules, into fermentable and unfermentable sugars. The glucose chain that is starch is not fermentable because all of the glucose molecules are attached to each other. After the enzymes break the starch up there are four resulting kinds of molecule. Glucose is the single molecule form and is fermentable. Maltose is a short chain of two glucose molecules linked together, and it is highly fermentable. Maltriose is a chain of three glucose molecules and is also fermentable. Dextrins are chains of four or more glucose molecules and are not fermentable, but they give the beer body and fullness(4).
The way alpha-amylase works is by dividing the long starches across the middle of their chains. It keeps dividing the chains until they are one, two, or three glucose molecules long. Until these chains are shorter than four glucose molecules they are called dextrins and are not fermentable by the yeasts used in brewing. This process is called liquifaction or dextrinization(4).
Beta-amylase works somewhat differently. It nibbles at the ends of the starch chains and make them into chains of one, two, or three glucose molecules, all of which are fermentable. This process is called saccharification(4).
The environment that the grains are mashed in is important. Variations in the temperature, the time taken, pH levels and the viscosity of the mash can make or break a batch of beer(3). With that in mind, we now look at the factors involved.
Alpha-amylase works best at temperatures between 149-153 F (65-67 C), and beta-amylase between 126-144 F (52-62 C). At 153 F (67 C), alpha-amylase will become denatured after two hours, and at 149 F (65 C) beta-amylase denatures in 40-60 minutes. The two work well together at temperatures between 145-158 F (63-70 C)(4).
In general, the higher the temperature and shorter the time of the mash, the more dextrins will be produced, resulting in heavy-bodied beers. Since higher temperatures denature the enzymes more quickly, it is not a surprising result. Lower temperatures and longer mash times result in a more fermentable wort which produces beers that are lighter and higher in alcohol content.
The acidity of the mash is measured in pH units. The optimum pH for diastatic enzymes is between 5.2 and 5.8. For proteolytic enzymes the optimum pH is between 4.2 and 5.3. A compromise usually is made at a pH value of 5.2, which fortunately occurs naturally when water is mixed with the grains. The acidity is caused by chemical reactions and other enzymatic processes that are beyond the scope of this paper. These reactions lower the pH even when the water is neutral. Generally a brewer does not have to worry about the pH of the water used unless the water is extremely soft (less than 50 parts per million) or extremely hard (in excess of 110 ppm), in which case the brewer should either use another source of water or chemically adjust the water at hand to suit the mashing process(4).
The thickness of the mash is the ratio of water to extract. In general, a thicker mash will promote the activity of proteolytic enzymes and a thinner mash will promote the activity of diastatic enzymes(3).
I hope that the information in this paper is both relevant and enlightening. Malting and mashing are two interesting biological processes that brewers have performed for many years longer than they have know why the processes worked, and because of this fact I find them to be incredibly fascinating, as I hope you have.
Thanks to Ed Romero and Mike Diehl for getting me interested in home brewing, and thanks to all of the people in the Usenet discussion group rec.crafts.brewing for their answers to my many questions, and for pointing the way to the huge archive of brewing material at ftp.stanford.edu. As the motto of the American Homebrewer's Association suggests: "Relax. Don't worry. Have a homebrew."