Rapid yeast growth combined with acidification will begin to inhibit any possible bacterial contamination.ĭuring the cell growth phase wort sugars are brought into the cell and broken down into glucose. This stage also marks the beginning of the acidification of the wort. Yeast cells multiply by “budding,” which is a form of cell division, and they require strong cell walls to facilitate budding. Though the yeast is technically fermenting and giving off small amounts of CO2 and ethanol, the majority of its efforts in the presence of oxygen produce not ethanol but sterols and fatty acids that help build cell walls. Instead, yeast cell growth in the presence of air is a highly complex stage wherein yeast use up all of the oxygen in the wort for energy buildup and reproduction. Since all wort will have a greater glucose concentration than 0.4 percent, respiration cannot occur in brewing. However, a law called the Crabtree Effect dictates that respiration does not occur with brewing yeast in wort that has a glucose content of greater than 0.4 percent w/v (weight/volume). This stage of yeast cell growth is often mistakenly referred to as “respiration.” Respiration (cell growth in the presence of air, or aerobic, which yields CO2 and water) and fermentation (cell growth in the absence of air, or anaerobic, which yields CO2 and ethanol) are the two ways in which yeast cells can use sugars to produce energy. When the cell walls have been prepared, nitrogen and sugars can be transported into the cell and rapid growth begins.

With healthy yeast cells and proper pitching rates, temperature, and aeration, the initial stage should be very short, often just a few hours. If the pitching yeast has been starved, therefore, the glycogen reserves will have been depleted and a long lag time will ensue. The cells need an initial store of energy in the form of glycogen and adequate dissolved oxygen to ensure the permeability of the cell walls. The initial stage is characterized first by yeast cell wall preparation and then uptake of oxygen, nitrogen, and sugar.Ĭell wall preparation consists of the secretion of enzymes needed to make the cells permeable to the sugars and other compounds that will eventually feed it. Because the yeast now finds itself in a new environment, it must adjust and prepare itself for the work of fermentation. The initial stage begins when yeast is pitched into the wort. Minimizing lag time, therefore, is of the utmost importance to a clean, healthy fermentation. The lag phase is the period during which the wort is most susceptible to contamination and spoilage by bacteria.

The lag phase technically describes the time immediately after pitching when the yeast adapts to the new conditions of its environment and no activity is seen. The initial stage is often referred to as the lag phase, though the lag phase also overlaps the cell-growth stage. And it is where many off-flavors are created that ruin beer.įermentation actually consists of three distinct but often overlapping stages: the initial stage, yeast cell- growth stage, and fermentation. Fermentation is where the alcohol and CO2 that make beer are produced, but it is also where the chemical reactions occur that produce all the other subtle flavors and aromas that give beer its complexity. Without it we’d have sweet barley tea to share with friends or something to put on our pancakes.

Peek a brew