Yeast Viability and Vitality
Before any batch of pitching yeast is used it is necessary to test for viability and vitality. Yeast viability refers to the ability of cells to grow, reproduce and interact with their environment. Thus, viability is essentially a measure of living cells. In many instances accurately assessing how many cells within a population are alive is sufficient in determining yeast viability. However, it should be noted that viable cells can possess different degrees of vitality which affect the viability tests. Yeast vitality refers to yeast activity or the capacity to recover from physiological stress yeast—how metabolically active it is.
There are many manual and automatic methods available for assessing yeast cell viability that can be used to quickly and accurately determine the relative fitness of the culture for fermentation. The evaluation of yeast viability includes methods based on cell replication, methods using viability stains, and methods based on measurement of cellular products resulting from metabolic activity, i.e., capacitance.
Standard Plate Count. A viable cell is defined as a cell which is able to divide and form a population (or colony). A viable cell count is usually done by diluting the original sample, plating aliquots of the dilutions onto an appropriate culture medium (agar), then incubating the plates under proper conditions so that colonies are formed.
Slide Count Technique. The slide count technique involves the layering of a thin film of agar on a microscope slide and then overlaying a dilute suspension of yeast cells.
Viability stains in general do not depend on the yeast cell’s ability to divide but instead rely primarily on the permeability of the cell membrane as well as the activity of intra-cellular enzymes. The staining techniques are often based on the membrane’s ability to exclude the penetration of the dye and/or the dyes ability to penetrate into the cytoplasm of a dead or dying cell.
Another indicator of viability is capacitance. The principle of this method is that the application of a radio frequency to a viable cell result in a charge buildup within the membrane, and a capacitance is generated. Non-viable cells are unable to generate this capacitance. The capacitance is directly proportional to the number of live cells in the sample chamber. The output is linear over a wide concentration range.
The methods for assessing cell viability presented above provide information only on alive and dead cells in the whole population. However, in many cases, toxic effects of chemical or physical factors do not lead directly to cell death. Such factors may cause a number of morphological, intracellular, or metabolic alterations that will result in the inability of a cell to divide, yet the cell itself may still be alive.
Factors Influencing Viability and Vitality
One of the key stages in yeast handling that has a significant effect on yeast viability and vitality is the collection and subsequent storage conditions of the cropped yeast. Optimally, yeast should be collected and stored at temperatures between 0 and 2 degrees C (32 and 36°F) with a minimum storage time.
Ethanol and Osmotic Stress
Ethanol, a primary metabolic product of yeast fermentation, may reach a concentration where it causes chemical stress to yeast cells. The effect of ethanol and/or ethanol tolerance is strain dependent. When yeasts are exposed to wort or beer they are subjected to an osmotic pressure.
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