Fermentation Primer Lesson XI
Fermentation Primer - Lesson XI
In last month’s lesson we discussed Torulaspora delbrueckii and its potential to be an alternative to Saccharomyces cerevisiae. In this month’s lesson we take a look at the different stages yeast go through during fermentation.
Stationary Phase
Q: Does yeast start producing alcohol immediately upon introduction into the fermentation medium?
In typical fermentations, yeast follow a predictable sequence of stages or phases. Not all of the yeast undergo the exact same transformations at the same time, but the majority do. These phases are: Lag Phase, Log (or Exponential) Phase, Stationary Phase and Death (or Decline) phase.
Lag Phase
The lag phase is the time during which the yeast become acclimatized to the wort and prepare to reproduce and consume massive amounts of sugar. As with all phases in the yeast life cycle, nothing is clear-cut.
In this phase, oxygen is extremely important. Oxygen is used by yeast for synthesis of sterols and unsaturated fatty acids that are necessary growth factors. Without oxygen, these lipids can’t be biosynthesized and growth will be very limited. The sterols and fatty acids produced are also very important in the structure of the cell membrane and the ability of the yeast to respond to external and internal stresses. Adequate oxygen in the wort when the yeast is pitched is essential to a successful fermentation and long-term health of the culture.
During the lag phase, yeast also take in amino acids, peptides, other proteins, and various nutrients. Internal cellular energy reserves (glycogen) fuel the initial yeast activity until the membrane becomes permeable to the wort sugars.
Log Phase
The log phase is a time of exponential growth of the yeast culture. The preparation the yeast made during the lag phase allows rapid multiplication of cells and consumption of sugar. Yeasts reproduce asexually by budding; the adult cell forms a daughter cell that is an exact genetic copy itself. Nitrogen, amino acids, nutrients and sugar are consumed while the cells are reproducing.
Many of the significant aromatic and flavor compounds are by-products of cell growth and are produced during the log phase. Many large alcohol plants try to limit the amount of yeast growth by pitching larger quantities of yeast and therefore minimize ester synthesis. Keeping fermentations cold also limits ester production by limiting the rate of growth.
Stationary Phase
The stationary phase is the last active stage, where the yeast population reaches maximum density and the remaining sugars are consumed. Triggers that are not yet completely understood cause the yeast to begin increasing cellular energy reserves in the form of glycogen and trehalose. As the available sugar and nitrogen decreases, the yeasts begin to prepare for a period where there is a lack of food.When the yeast have consumed all of the sugar, flocculation (the act of yeast aggregating into “clumps” and settling out) begins. During this time, many alcohol plants will allow the fermentation to “rest” (at different temperatures, depending on the alcohol being produced). The resting time allows the yeast to convert or reduce some less desirable compounds, especially diacetyl, to more acceptable or preferred ones.
Death Phase
When all available food sources have been consumed or when the amount of ethanol (or temperature) surpases the level tolerated by the yeast, the yeast dies, marking the end of the four-phase fermentation cycle.
Batch fermentations must go through all these phases in the order described above. Fermentation tanks are then emptied, cleaned and prepared for the next batch. Over the years, many scientists have wondered if it is possible to improve on this process...
Join us again next month as we wrap up our exploration of the fascinating world of fermentation!
Source: https://wyeastlab.com/yeast-fundamentals