Hereâ€™s just something magical about yeast. I remember, as a child, how my mother would make bread and I would watch with fascination as she assembled all the ingredients on our kitchen counter. Out would come the big glass bowls, the measuring cups and spoons, the bread dough, and the smaller containers of sugar, spices, and herbs.
The last item to emerge was always my favorite: the packet of freeze-dried yeast neatly tethered to two other pouches in a threesome of what was, to me, a sunny yellow symbol of the magic that the yeast would soon be performing.
Now that Iâ€™m older and own a bread machine, itâ€™s easy for me to lose my appreciation for yeast cells that somehow, among the whirrings and stirrings of paddles, gears, and motors, manage to turn a few cupfuls of flour into a hearty food.
Even when making wine using Saccharomyces cerevisiae, which is a lot more hands-on than my bread-making by proxy, I still find myself regarding a packet of Red Star Montrachet or prise de mousse as just another part of the equipment, much like my carboys, fermentation locks, and hydrometers.
Yeast cells, however, far from being just another ingredient to be tossed into the pot, are living, breathing organisms that need certain things to function properly.
Aside from sugar, their main food source, yeast cells also require nitrogen and vitamins to live, reproduce, and conduct a healthy and complete fermentation. Smart winemakers make sure that their yeast has an abundance of all of these things before fermentation begins, and their yeast cells respond by happily and healthily working their magic.
Feed Your Yeast:
The phrase â€świne makes itselfâ€ť is familiar to many home wine-
makers. Because grapes have indigenous yeast populations, we know that all we really have to do to make wine is to crush those grapes and let the yeast cells have at it.
Similarly, grape juice is a pretty good medium for yeast cells to grow in because it usually has enough sugar and nutrients to
keep a large population of yeast happy. However, due to differing viticultural practices, climatic variations, and soil profiles, to name a few, sometimes the juices of grapes and other fruits do not carry enough of these compounds for adequate cell growth.
If nitrogen and vitamins are lacking, your wine might suffer from stuck fermentations,
production of acetic acid (vinegar), and formation of hydrogen sulfide gas (smells like rotten eggs), among other reactions. If you are making wine from a fruit other than grapes, you have to pay special attention to yeast nutrition.
The fruits and vegetables that home winemakers choose to ferment often have a nutrient profile that is completely different than that of grapes. In this case you must make doubly sure that your yeast has enough nitrogen sources and vitamins to keep it going.
Yeast needs nitrogen to survive. Nitrogen is used in many places in the biosynthetic pathway of yeast. Cells that donâ€™t get enough of it canâ€™t jumpstart their exponential growth phase at the beginning of the fermentation process, among other things. If the fermentation is thus slowed, the wine will likely develop off-flavors and odors. Stuck or sluggish fermentations can also result.
Nitrogen is also a key element in regulating hydrogen sulfide prod-uction. A juice with low nitrogen levels is more likely to produce offensive amounts of hydrogen sulfide. Wine yeast can get nitrogen from two sources in the juice, ammonia and amino acids. Both sources are often described together with the name free alpha amino nitrogen (FAN), a measurement of the amount of
readily assimilable nitrogen in juice.
Studies have shown that for yeast cells to carry out a healthy fermentation, they need a minimum of 400 to 500 milligrams per liter total FAN. Most grapes already have close to this amount, but without sophisticated scientific equipment at their disposal, home winemakers have no way of knowing the level in their grapes. Fortunately, there are a couple of ways to preventively supplement yeastâ€™s available nitrogen levels and make sure yeast cells have enough to get their jobs done.
Nitrogen, in the form of ammonia, is naturally present in grape juice, and it can be added easily when necessary. Nitrogen is often added to musts in the form of diammonium phosphate (DAP) powder before fermentation begins. DAP powder is available at most home winemaking supply stores and is added directly to juices before the yeast is pitched. This gives the liquid time to absorb the powder and the solution time to become homogenous in composition.
A study conducted by wine scientists found that, on average, grape juices that had their FAN levels measured before fermentation needed about 100 mg/L of DAP extra to achieve the minimum recommended level. In five-gallon carboy terms, thatâ€™s 1,895 mg of powder per
carboy. You just might want to round it off to an even 2,000 mg per five gallons to reach that minimum. Bump it up to 2,500 mg if you know the vineyard you get your grapes from is often nitrogen-deficient or if youâ€™re making wine with fruit other than grapes.
Since every commercial preparation of yeast nutrients can be different, itâ€™s still a good idea to read labels and follow the manufacturerâ€™s directions to give your juice and musts the right concentration.
Besides ammonia, the other source of FAN in grape and other juices is the variety of amino acids that exist naturally in the fruit. Amino acids are the building blocks of proteins, and yeast cells need a lot of them to grow, reproduce, and carry out a strong fermentation. Low levels of amino acids, like low levels of ammonia, can cause both stuck and sluggish fermentations. They also can cause high levels of hydrogen sulfide to be produced during the fermentation.
Levels of amino acids can become especially depleted if the fruit youâ€™re using has any mold on it. These molds degrade amino acids very quickly and if, for example, youâ€™re making a botrytisized white dessert wine (that uses the mold Botrytis cinera to concentrate the flavor and sugar of the grape), be sure that you make up for the lost nitrogen in the amino acids.
Fortunately, a protective dose of amino acids, in the form of lysed (dead) yeast cells, can be added to juices and musts before fermentation begins. These dead yeast cells, sometimes also called â€śyeast hullsâ€ť or â€śyeast ghosts,â€ť are essentially that: the leftovers of what were once
living yeast cells. That may sound a little morbid, but there are a lot of amino acids, nitrogen, vitamins, and other useful goodies left over when yeast cells die en masse as they do after theyâ€™re done with fermentation.
Most grape-juice fermentations could use an addition of a teaspoon or two of yeast hulls per five-gallon carboy. Itâ€™s probably advisable to double that if youâ€™re making a non-grape wine or if youâ€™re using grapes that have been attacked by molds.
People need certain vitamins to be at optimal health. The same goes for yeast. Yeast cells are particularly dependent on biotin, pantothenate, and thiamin for their biological functioning. Vitamins have less to do with preventing stuck fermentations than do nitrogen sources, but a
lack of appropriate vitamins â€” in particular pantothenate â€” can lead to the production of stinky hydrogen sulfide.
Musts and juices with vitamin deficiencies also have been shown to produce higher levels of acetic acid (vinegar) and succinic acid.
A thiamine deficiency has been shown to enable the formation of pyruvic acid, a compound that will preferentially bind with added
sulfur dioxide and make the sulfur dioxide unavailable to kill microbes and prevent oxidation.
Yeast hulls, again, are a great source of these vitamins. Since yeast cells only need these vitamins in minute amounts, itâ€™s almost
certain that the amount of hulls you add to ensure youâ€™re getting enough nitrogen will be sufficient to ensure an adequate supply of
vitamins. Molds also can seriously deplete the levels of vitamins in musts and juices, so if your grapes are moldy, use a little extra.
Keep an eye out for commercial yeast hull/DAP combinations (â€śSuperfoodâ€ť is one brand name that comes to mind) that offer
the best of both worlds in one convenient package. You know that your juice or must is getting the nitrogen it needs from DAP
as well as from the amino acids and critical vitamins supplied by yeast hulls.
Yeast cells are complex living organisms, and they need to be treated as such. Even during an operation as simple as making bread in a bread machine, yeast cells will adamantly refuse to work if theyâ€™re unhappy. Donâ€™t let them go on strike when your barrels of Chardonnay are at stake.
Alison Crowe works at Byington Winery & Vineyards in Los Gatos, Calif. She has a degree in wine-making from the University of California, Davis.