Fix - Principles of brewing science: a study of serious brewing issues

Fermentation. As long as beer flavor is the final arbiter of what is desirable, the fermentation of beer will be the area in brewing of greatest practical importance. Moreover, this is as true for conventional beers brewed with conventional techniques as it is for specialty beers.

A mastery of the various metabolic pathways is highly recommended for even beginning brewers. The major pathway, often called the Embden-Meyerhof-Parras pathway, consists of Glycolysis, where each molecule of glucose is transformed to two molecules of ethanol and two molecules of carbon dioxide. A strong case can be made that the first part of this pathway is the most crucial. It is certainly the part of the fermentation where we as brewers can exert the greatest control. In this regard, consider the following four points.

First, the yeast used must be in excellent physiological condition. The yeast membrane controls what enters and leaves the yeast cell, and its viability is a necessary condition for a satisfactory start of the fermentation. Also important is for the yeast to have an adequate food supply within their cells.

The second and closely related issue is to add an adequate number of yeast cells. At the start of the fermentation more yeast is definitely better than less, however, too much is not good either. Indeed, one of the major themes that permeate this book is that successful brewing is one where the correct balance is struck in all issues. Fortunately, there are elementary procedures for measuring viability food reserves and cell counts.

The third point associated with the initial period is the indispensable role that oxygenation of chilled wort plays as a yeast nutrient. The most important aspect of exogenous oxygen is associated with yeast synthesis of sterols and unsaturated fatty acids, a crucial step in the maintenance of yeast viability. Yet there is more, and the inevitable conclusion one reaches is that wort oxygenation can never be omitted by quality conscious brewers.

The fourth and final point about the start of the fermentation relates to wort composition, and the vital role played by key wort constituents such as nitrogen from amino acids.

Other parts of beer fermentation that fall into the primary relevance category are the various minor metabolic pathways. These produce such flavor active products as esters, fusel alcohols, diacetyl, and phenols, to cite a few examples. The term minor is used for these pathways because only a miniscule amount of products are produced in them, typically less than 1 mg per liter. This is in striking contrast to the main pathway where over 40,000 mg per liter of ethanol is produced in beers of normal strength. Yet, what I find most fascinating about brewing is that minor products can influence beers flavors in ways that far exceed their concentrations. Put differently, the minor pathways can become major once the quality of beer flavor is put on the table.