Yeast and Winemaking
Wine Yeast Culturing
Commercially prepared yeasts are either available as dry active yeast or as liquid culture. Dry yeasts are vacuumpacked freeze-dried yeast. Liquid cultures are live, viable yeasts in a juice medium prepared from either a live mother culture or dry yeast. Liquid cultures can be purchased from a reputable laboratory by the liter or in 400-mL vials (slants). Liquid culture is usually more expensive than dry yeast, but it is ideal for small inoculations. When freezedried or slants are cultures are used, the winery needs to be able to build them up into pure starter cultures. This is done by a series of serial transfers in increasing volumes of sterile nutrient medium. The final sterile inoculation is then added to the juice/must.
Dried Yeast Rehydration
In order to resume the fermentative activity, the moisture content of a dried yeast cell must be restored in order for the yeast to recover their original form. Extensive research shows that the yeast cell wall is very fragile during the first few minutes of rehydration and if not rehydrated properly the yeast will lose viability. When a desiccated yeast cell rehydrates, its cell wall is swelling and the membrane is gaining back its elasticity. If rehydration is not properly carried out, the cell can leak important cellular compounds through the membrane, which is extremely permeable at the time of rehydration.
Yeast rehydration is one of the most important steps in a fermentation, and for that reason alone the use of a rehydration nutrients is the simplest and most costeffective step a winemaker can take to ensure a successful fermentation. Ensuring the optimal condition of the yeast is of paramount importance, both in terms of quality and kinetic outcomes. Rehydration is vitally important to replace the water removed by drying, thereby restoring the yeast to its metabolically active form. Rehydration can be done in water only or in a water juice mixture (? juice: ? water). typical of today's harvests. Some grapes from some vineyards have notoriously low YAN levels almost every year. Most musts contain between 80 and 400 mg/L of YAN depending on grape variety, vintage, and microclimate (Reynolds, 2010).
Rather than rehydrate yeasts on a tank-by-tank basis, economics and time may dictate preparation of starter tanks in which larger volumes of yeast are propagated and used over a short period. After the addition of yeast inoculum to the starter tank, 24 to 72 hours may be required before the expanded starter reaches a sufficient cell number to add to the must.
The number of yeast cells added to a substrate is called the inoculation rate. To start a good and healthy fermentation, one has to add enough living yeast cells to the must. To ensure the rapid onset of fermentation, the inoculation rate is generally rates are between 20 and 40 g/hL; some specify 25 g/hL which equates to a cell density of 5x106 cells/mL (5 million cells/mL). Rates can be confusing as they are often expressed in different units. Here is an example conversion: 25 g/hL is equivalent to 25 g in 100 L or 250 g in 1000 L.
Wine Yeast Viability and Vitality
Yeasts require optimal conditions to be in the healthiest state during fermentation. To maintain cells in a viable, healthy state to achieve the best winemaking outcomes, constant monitoring is required to prevent the occurrence of stuck or sluggish ferments. Analytical methods commonly employed in the wine industry tends to focus on measurement of yeast viability that is, the percentage of live and dead cells in a ferment. Viability is measured as a percentage, so a viability of 75 percent would mean that 75 percent of the yeast cells you are measuring are living, while the remaining 25 percent are dead.
Microscope and Hemocytometer
A microscope can be quite informative on the physical condition/state of the yeast. It is relatively simple to use and provides an insight into the number of yeast present and whether they are multiplying, as noted by budding cells. By using a hemocytometer (gridded counting chamber) it is possible to determine the yeast cell numbers and count the number of budding cells. This provides an indication of the state of the yeast population.
Automated Yeast Cell Counting
Using a hemocytometer for measuring yeast viability and concentration is relatively tedious and suffers from human error in preparation and the subjectivity of visual measurements using the human eye. An alternative method is automated yeast cell counting that provides a fast but also accurate and precise measurement of yeast cells.
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