Lag Phase is the period between adding (pitching) yeast into wort and the beginning of fermentation. After its arrival in the wort, yeast requires a certain amount of time to acclimate to its new environment and to shift from dormancy to metabolic activity, that is, to active fermentation. During the lag phase, the yeast turns on genetic pathways that allow it to import sugar and other materials needed for cell replication as well as nutrient absorption (fermentation). The lag phase may last anywhere from 3 to 15 h, depending on such factors as wort type and gravity, temperature, yeast strain, yeast health, pitching rate, and aeration.

During the lag phase, yeast cells rapidly absorb available oxygen. Oxygen is needed for yeast to produce important compounds—most significantly sterols (unsaturated steroid alcohols), which are critical in yeast membrane permeability. Although higher temperatures result in a shorter lag phase, brewers usually keep the lag phase temperature below the temperature at which the yeast will eventually ferment. This is because higher temperatures during the lag phase promote the synthesis of such substances as alpha acetolactate, which is a precursor to diacetyl. See diacetyl. Overall, however, yeast produces few flavor compounds and ethanol or other alcohols during the lag phase than it does in the fermentation phase.

The pitching rate, too, plays a significant role in the effectiveness and length of the lag phase. Overpitching can decrease the lag phase, but, because each cell grows the same number of new cells, the result may be too many old, worn-out cells at the end of fermentation. This can lead to off- flavors and low viability if this yeast is subsequently repitched. See pitching.

It is also important that the lag phase not last too long, because cool, well-aerated wort is an ideal habitat for bacteria and wild yeast. It is essential, therefore, that vigorous fermentation with the desired yeast begins before any other organisms can gain a foothold. Although most worts will remain stable for at least 24 h, it is best to err on the side of caution and aim for active fermentation within 15 h.

See also fermentation.