Wine yeast — or Saccharomyces, which is Latin for 'sugar-fungus' — converts sugar into alcohol. While alcoholic fermentation is the most important aspect of winemaking, yeast does more than just bubble out happy-juice. It also produces compounds that influence flavor, aroma, body, structure and the finish of wine.
Wine kit manufacturers are among the heaviest purchasers of winemaking yeast in the world. Commercial wineries buy 500 grams here and there, while others buy pure strains and grow their own in the laboratory. In contrast, because kit manufacturers have to include a fresh packet of dried yeast in every kit, we order our yeast by the ton!
But what about the yeast that came in your wine kit? If you've made a few kits, you may have seen the same strain used more than once, or wondered who decided which strain was right for each kit? It turns out that wine kit manufacturers pay close attention to yeast, using a whole slate of criteria for their choice. First, they (loosely) follow the yeast company's recommendations. Second, they run many yeast trials on every kit they develop, to ensure that the yeast meets the following criteria:
Rapid onset of fermentation — the faster a kit gets going, the less chance there is for an indigenous yeast or spoilage organism to affect the juice.
Speedy fermentation — most kits are designed to be bottled between 4 and 8 weeks after yeast pitching. If the yeast is poky, carboy turnover will be too slow, and many home winemakers are impatient to get their wine in the bottle.
Thorough fermentation — unfinished (sweet) table wines or wines that re-ferment in the bottle are major concerns for beginning winemakers.
Flocculation — the ability of the yeast to settle out after fermentation, again allowing wine to be bottled after a few weeks.
Low foaming — too much foam can overwhelm primary fermenters.
The ability of the yeast to cope with Pasteurized juice and concentrate —
The last one is crucial. All kit wines have to go through Pasteurization — that is, they get heated to around 160 °F (71 °C) for approximately one minute to kill off any live organisms, such as wild yeast or fungi. While careful Pasteurization ensures any impact on flavor and aroma is minimized, the heating causes some sugars to bond to other compounds in the juice. This makes Pasteurized juices more difficult for the yeast to process.
Some yeast, in fact, cannot successfully ferment out Pasteurized juice. Kits fermented with such an underperforming yeast will have a fruity-candied character, kind of like grape Jolly Ranchers™. When someone says, "I can detect a 'kit taste,'" it's probably this flavor that they perceive. That's why sometimes a varietal kit won't come with the most obvious varietal-specific yeast: some of these specialty yeast can't get all of the sugars, negating any potential benefit from varietal expression.
So, once the speed, thoroughness and ability to correctly ferment Pasteurized juice are taken care of, manufacturers do a number of trials to choose which yeast is going to yield the best flavor and aroma characteristics for a kit. From years of experience (including ongoing trials for new strains), the field of appropriate yeast for kit wines has been narrowed to only five or six dozen. While this number represents a lot of potential choices, it's pretty small compared to the many hundreds of different strains out there.
Once the obvious failures are weeded out, the wines go to tasting panels to determine which one best expresses the varietal character of the wine and the terroir of the grape region. These tests are performed both when the wine is very young, less than two months old, and over a longer aging period. Sometimes in the gap between two age tastings, an early favorite drops out: some of the young fermentation characteristics diminish with age as the esters and terpenes that comprise the higher flavor notes recombine and change.
Changing Your Kit Yeast
From the above it's obvious that kits don't react in a straightforward fashion to normal grape winemaking manipulations. A yeast strain that might guarantee a wonderful expression of varietal character in a grape wine may not deliver the same character in a kit. However, you can experiment with your kits if you wish: there's no guarantee that you'll dislike the results of a change in yeast, and in fact you may be pleased. You'll need to research the yeast manufacturer's literature to get a better handle on the character of the yeast. Don't be surprised if it doesn't turn out exactly the way you had intended. Unless the kit manufacturer has run a fermentation trial for that particular yeast, even they don't know.
If you're thinking of dipping your toe in the (yeasty) water, then you might consider running a trial yourself. Split your kit into two 11.5 L (3 US-gallon) batches and ferment one with the original yeast, and the other with the alternate you've chosen. You'll need some extra equipment, and some careful measuring with finings and stabilizers to split the packages that came with the kit. While this does entail the cost of another primary fermenter and a couple of three-gallon carboys, it has some advantages: if your experiment doesn't yield optimum results you can blend the two together and still salvage a drinkable wine. If it fails completely, you've still got half a batch to console yourself.
Liquid Yeast Strains
Liquid wine yeast is starting to become more popular. Originally, liquid yeast strains were championed by home brewers of beer. In beer brewing, something like 70% of the flavor of the finished product is derived from the yeast used. So specialty liquid yeast helps beer express its style precisely and deliciously.
Kit wine fermentations? The results aren't all in yet. The liquid yeast manufacturers are graciously working with the kit companies at this time to see which kits respond to the liquid yeast best, but so far no kit manufacturer has released the results of any trials. (It's a bit early, but enthusiasts can rest assured we're looking very closely at liquid yeast.)
In the meantime, if you want to conduct your own liquid yeast trials, you can split your batch as described above. Keep in mind, though, that you will need to carefully follow the specific yeast manufacturer's instructions regarding the preparation, pitching rates and fermenting temperatures of their yeast. Often it's a bit more complicated than shake and pour, so make sure you understand the steps necessary before embarking on a liquid yeast trial.
Rehydrating Kit Wine Yeast
If you look at the instructions in your wine kit, they will likely instruct you to sprinkle your packet of yeast directly on to the must. Yet if you read the yeast package, and many winemaking textbooks, they recommend rehydrating the yeast. If the objective is to deliver the maximum number of yeast cells to the must, which technique is best?
It turns out that the answer is not as simple as choosing between one or the other. When performed correctly, re-hydrating gives the highest live cell counts, and the quickest, most thorough fermentation. The catch is that it has to be done very precisely. The Lalvin yeast folks for instance, ask you to add the yeast to 10 times its weight in water at 40–43 °C (104–109 ° F).
Breaking it down, first it's important to use the correct amount of water at the correct temperature. It's important to be exact in this measure — 10 times 5 grams of water is 50 grams. (And, since one milliliter (mL) of water weighs one gram, it's also 50 mL.) Using 25 or 100 mL would be bad, as the yeast are only happy in a narrow range of rehydration density. Getting the water to yeast ratio wrong could reduce yeast viability.
Second, the temperature range is very important. The outer integument of a yeast cell is made up of two layers of fatty acids. These layers soften best in warm water. Once it has softened up, it will allow the passage of nutrients and waste products in and out of the cell much more efficiently. If the water isn't warm enough, the cell won't soften. If it's too warm — generally anywhere above 52 °C (126 °F) — the yeast cells will die.
Third, you have to worry about temperature shear. If yeast goes too quickly from a favorable temperature to a less favorable one, weakened cells may die, and others may go dormant. This reduces the numbers of live, viable cells available to ferment the must, and gives spoilage organisms a chance to get a foothold. Thus when re-hydrating yeast, it's important to wait until the yeast cools to within two degrees of the must temperature before adding.
By contrast, while simply dumping the yeast onto the top of the must should result in lower cell counts, empirical evidence shows this isn't the case: the yeast seem to know what they're doing. Generally, a 5-gram packet of yeast will result in a less than six-hour lag phase on an average wine kit. This is perfectly acceptable — and a lot simpler than going through the re-hydrating process!
You can rehydrate your yeast if you absolutely want to, but be sure to do it accurately and precisely.
The biggest difference between kit winemaking and grape winemaking isn't actually in the raw materials, it's in the schedule. Most grape winemakers take a year or more before they go to the bottle, giving the wine plenty of time to finish fermenting, de-gassing and clearing. The instructions from kit wines call for a much shorter pitching-to-bottling schedule (in most cases, under two months).
Cooler temperatures extend fermentation times because the yeast work more slowly in the cold. As a rule of thumb, for every 10 °C (18 °F) you drop the temperature, the fermentation time will double. This throws off fining and stabilizing, increasing the difficulty of clearing the wine for bottling. Even though the yeast works more slowly, the problem isn't usually waiting for the gravity to drop, it's gas saturation.
Think of it this way: carbon dioxide gas (CO2) is soluble in a liquid solution in inverse proportion to the temperature of that solution. Thus colder wine equals fizzier wine. Wine that's fermented too cool is wine that's saturated with CO2 gas. Wine kits have to be de-gassed at fining and stabilizing, both to ensure that the wine clears effectively and to eliminate trapped fermentation odors and "fizz" before going to the bottle. Lowering the temperature of the fermentation could make the kit much more difficult to finish, and would throw off the time schedule mentioned in the instructions.
However, if you're willing to take longer and work without a net, there are some gains to be had when using lower temperatures, with the right strain of yeast. Not every yeast responds well to cooler fermentations — consult the manufacturer's information to be sure.
Two of the dried strains in current kit use that can benefit from cooler temperatures are RC212 and ICV D47. RC 212, originally isolated in Burgundy in France, can enhance fruitiness in red wines, and is usually used in Pinot Noir. ICV D47 is a strain originally isolated from the Rhône Valley in France, and can enhance the mouthfeel, aroma and flavor of white wine, and specifically Chardonnay.
Raising the fermentation temperature is a technique done in some winemaking regions for red grapes. By allowing the must temperature to rise quite high, simple fruity qualities are suppressed, and more complex, "winey" characters are enhanced. In Bordeaux and Burgundy, this is sometimes used to enhance the structure and complexity of the wines. This technique is not likely to yield positive results in a kit wine. The kit has already been through a high-temperature process (Pasteurization) and kit manufacturers usually work at retaining as much fruitiness and varietal character as possible. Also, once you remove the fresh fruity character of some value-priced kits, you may find that it does not have the structure or tannin to age well, or to produce a pleasantly drinkable wine.
Most sweet kit wines are intended to be first fermented completely dry, and then back-sweetened before bottling. Some commercial wineries, on the other hand, halt their fermentation before it is complete, leaving residual sugar in the must for sweetness. Could this be something you can do at home with your kit wine?
While it's tempting to try this, it's actually a highly advanced winemaking technique that could (literally!) blow up in your face. It's much simpler to wait for the fermentation to complete, and then to back sweeten your kit.
A wine kit manufacturer's goal is to guarantee success with every wine you make, from the very first. That's why most manufacturers are somewhat conservative when it comes to customers using experimental techniques on their products.
However, this doesn't mean that you always have to obey the instructions — winemaking is a playground, not a prison. Today's kit winemakers are the most advanced in history, with new yeast strains, technologies, and most importantly, accurate information at their fingertips. With a little research and some careful experimentation, you could come up with an interesting and potentially tasty wine kit by working with your yeast.