Controlling Fermentation Temperature

Knowing and Making Wine, the classic text by Émile Peynaud, is now sadly out of print. I often turn to it when looking into any technical matter involving wine. On the subject of fermentation temperature control, he puts it very bluntly: “It is above all by acting on the fermentation temperature that the winemaker can intervene effectively in the vinification process.”

Despite such expert advice, many home winemakers seem to believe temperature control is beyond their reach. They crush and destem the grapes (and press them for white or rosé) and let the fermentation take off under whatever ambient conditions prevail. I want to dispel that notion. Fermentation temperature control is important and you can do something about it. In this article, I will cover what happens in a fermentation at various temperatures and why you want control over those conditions.

Reading Temperature

To manage your fermentation temperature you need to know what that temperature is. It can also be important to know if it is changing and, if so, how rapidly. In a white wine fermentation of pressed juice, the natural stirring caused by the evolution of carbon dioxide will probably mix it well enough that a probe thermometer anywhere in the wine will provide a reasonably accurate reading. For thin wall tanks or carboys, a surface-mounted thermometer may suffice. Red wine fermentations are trickier. Typically hottest just below the cap, they will be cooler near the bottom. I like to check the temperature under the cap by pushing a dial-top stainless steel probe thermometer through the cap just prior to punchdown. Punching down mixes and cools the fermenter, so I check the temperature again in the just-mixed container. Write down your readings at least once per day and preferably twice. Monitor the findings for trends.

Hot or Cold?

In winemaking, temperature control goes both ways. You want the fermentation to be neither too cold nor too hot. There is not, however, just one optimum temperature for all fermentations. Rather, the style choices of the winemaker, the grapes that are available, and the yeast strain selected all influence the choice of temperature. The challenge comes about because there are desirable wine characteristics to be had from cool fermentation and others available from warm fermentation. In any fermentation, rapid evolution of carbon dioxide tends to sweep volatile compounds out of the must; that wonderful fermenting-wine smell is an example of desirable volatile aroma compounds leaving the wine! Because white (and rosé) wines rely for much of their appeal on the native aroma compounds that come from the grapes, keeping those compounds in the wine becomes a priority. On the other hand, higher temperatures, especially in the presence of alcohol, help to extract desirable compounds from skins, as in the production of red wine. Indeed, Peynaud notes, “Everything that differentiates red wine from white, to the eye and on the palate, is the result of the phenomena of extraction.”

Wine yeast, mostly Saccharomyces cerevisiae, exhibits optimum growth characteristics in the range of about 77 to 91 °F (25 to 33° C). If you are culturing yeast, use that temperature range. However, for optimum preservation of aromas in white wine, the best fermentation range is usually considered to be about 64 to 68 °F (18 to 20 °C). For desirable extraction of color compounds and tannins in red wine, the range is usually cited as something like 79 to 86 °F (26 to 30 °C). During fermentation, the yeast metabolizes sugar to derive energy for its life processes and growth. There is also waste energy that is released in the form of heat, so if there is insufficient loss of heat from the fermenter, the temperature will rise. In Concepts in Wine Chemistry, Yair Margalit provides the theoretical outcome of this effect. As each 1 °Brix that ferments releases 1.14 Kcal as heat, the complete fermentation of a 22 °Brix must would result in a 40 °F (25 °C) rise over the initial temperature. Starting at a room temperature of 68 °F (20 °C), such a fermentation that was closed off to all loss of heat would rise to 113 °F (45 °C) by the end! While any real-world system will necessarily lose some heat to a cooler ambient environment, this calculation makes clear that some heat management will probably be needed to keep a white wine fermentation cool and may be needed to manage the heat in red wine as well. Above 91 °F (33 °C), wine yeast is stressed, above 113 °F (45 °C) it is severely stressed, and at 122 °F (50 °C), 99% of viable cells will die within five minutes. For red wine fermentation, if your tank is large or ambient temperatures in your winery are high at harvest time, plan on cooling. Small tanks, because of a larger surface-to-volume ratio, lose heat to the surrounding environment faster. In a cool climate and with small fermenters, you may not need further cooling for red wine and you may even need to warm your fermentation for optimum extraction.

In planning to ferment white wine, once you have decided to keep it cool for your style choice, you also need to pay attention to preventing it from getting too cold. The optimum range for white wine fermentation is already below the best temperature for yeast production, and there is risk of a sluggish fermentation as yeast metabolism slows down at low temperatures. If allowed to drop very low, the temperature may cause the yeast to flocculate and drop out, stopping the fermentation entirely. If a white wine fermentation is allowed to stall out due to cold but has only 1 or 2 °Brix left, it may be very difficult to restart.

The sum of all these factors boils down to four thermal conditions that the winemaker needs to guard against: White wine too cold, white wine too hot, red wine too cold, and red wine too hot.

White wine too cold: If cold fall weather sets in early in your climate, or if your cooling system is difficult to manage, your white wine fermentation may get too cold. The primary risk in that case is a stuck or sluggish fermentation when the yeast simply cannot maintain its metabolic activity and stops working. This does not mean, however, that you cannot make wine at temperatures below the “optimum” ranges noted earlier. Some selected yeast strains are capable of fermenting at even cooler temperatures (more on yeast strains later). If your style choice or winery conditions dictate a lower temperature, make that a factor in choosing your yeast. If you do have a white fermentation that goes sluggish while residual sugar is still present, act promptly to restart it. Put the fermenter in a warmer place or take other steps to reverse the temperature drop, but do not do it abruptly. A sudden change in temperature can shock the yeast and retard your efforts to get fermentation going again. Once you have adjusted the temperature back into a range suitable for your yeast, vigorously mix it. The yeast may have settled out and “rousing” it can help get things rolling again. If fermentation does not resume within a day or two, consider reinoculating. For that purpose, you may want to repeat with the same strain you started with, switch to one with higher alcohol tolerance, or select one with a wider and more forgiving temperature range.

White wine too hot: If you are attempting a cool white fermentation and the heat generation gets away from you, it is unlikely that it will get so hot (above 90 °F/32 °C) as to cause harm to the yeast metabolism. It may, however, cause harm to your wine quality objectives at temperatures as low as 75 °F (24 °C). Volatile aroma compounds are released into a wine fermentation through enzymatic actions that separate them from glucose molecules (the enzymes may be natural to the yeast and grapes or may be added by the winemaker). By their nature as volatile compounds, their evaporation rate accelerates with higher temperature. Particularly in white wines where these native aromas are critical to varietal identity, as in Gewürztraminer or Viognier for example, the loss of aroma from warm fermentation may be a serious detriment to the wine. If you detect a rapid temperature rise in your fermentation, take steps to cool it right away. Once again, however, do not shock it with a large, sudden temperature drop. Doing so could cause the yeast to immediately flocculate out and leave you with a stuck fermentation. Cool promptly but gradually back into your desired range. You cannot replace aromas already lost, but you can minimize further losses.

Red wine too cold: Although the same sluggish fermentation problem can occur in a cold red must as in a cold white fermentation, if you are attempting a warm fermentation it is unlikely that you will drop that low. Do note, however, that some yeast strains have a recommended minimum temperature as high as 68 °F (20 °C). If there is risk that your winery temperature will drop below that level, you may wish to choose a different yeast strain. More likely in a too-cool red wine scenario is not a failure of the fermentation but rather a loss in quality of the wine. The anthocyanin polyphenolic compounds that provide grape and wine color extract fairly quickly in a red must while the tannin polyphenolics that provide structure and body extract more slowly. Both classes of compounds become more soluble at higher temperatures. To assure good development of color and adequate tannin balance, most red wine fermentations need to proceed under warm conditions. I usually recommend a steady fermentation temperature over 80 °F (27 °C) and allowing the must to rise to at least 90 °F (32 °C) at least once during the fermentation. A red wine fermented all the way to dryness at a significantly lower temperature, even if completed without stopping, may yield a wine with lighter color and less intense tannin profile.

If you cold soak, or harvest fruit when cool, fermentation temperatures may be low for the first few days and prove difficult to start. Spoilage organisms can grow at lower temperatures than optimum red wine fermentation temperature and you do not want those to get started before your yeast gets going. If more than a day or two passes after yeast inoculation and the temperature is still too cool, warm the must. Again, proceed gradually to avoid shocking the yeast, even if you have not seen the cap rise yet. Also consider warming if the fermentation is not generating enough heat to reach your target maximum.

Red wine too hot: While much more likely in large commercial wine tanks, overheating from fermentation heat is a condition home winemakers should keep in mind as a possibility. While 90 °F (32 °C) at least once may be desirable, a rise above 100 °F (38 °C) could cause significant loss of yeast viability, especially if it occurs quickly. If the weather is hot where you make your wine, watch the temperature closely. If you approach your desired maximum and fermentation is very active, be prepared to quickly apply (gradual) cooling to the must.

Once you have determined your desired temperature of fermentations and made provisions for achieving it, check your yeast choice. Most manufacturers and retailers of wine yeast have charts that recommend optimum temperature ranges for the yeasts they sell. For a cool white fermentation, for instance, you might consider Lalvin ICV D254 with its recommended range of 50 to 85 °F (10 to 30 °C) or Red Star Premier Cuvée at 45 to 95 °F (7 to 35 °C). If you do not have good temperature control capability in your winery, avoid a yeast with a rather narrow range.

Techniques for Temperature Control

If you have the opportunity to design and build your home winery, it is most efficient to include provisions for temperature control right from the beginning. Depending on whether you make white wine or red wine and depending on your climate, you may need heating, cooling, or both. Most home winemakers do not get to build an ideal facility and instead need to come up with ad-hoc temperature management methods as needed. From simplest to most elaborate setups, here are some of the techniques for controlling temperature I have seen:

Cooling

Usually used to maintain the temperature for white wine fermentations, for cooling red must for cold soaking, or for cellaring.

A cool cellar: If you have a naturally cool place like a basement, root cellar, or even crawl space under the house, it may be worth adapting for your white wine fermentations. Ambient cooling is not always adequate to maintain an ideal temperature for fermentation, but it can moderate temperature extremes and, once set up, requires little or no intervention.

Dry ice: Especially for use at harvest when the grapes are warm or you want to initiate a cold soak. Take a hard-side picnic cooler to your local dry ice vendor (often at welding gas supply locations) and fill it up! Don’t touch the dry ice pellets with your bare skin as they can cause burns and be very careful of the production of large amounts of carbon dioxide gas that are released when dry ice is used. Make sure you have adequate ventilation! Shovel the pellets directly into the fermenter. Stir periodically until the desired temperature is reached (usually somewhere below 55 °F or 13 °C for a cold soak).

Water tub: If you are using a small fermenter (like a carboy) for white wine, you can set the whole thing in a large plastic or galvanized steel tub full of water. The water will absorb some heat of fermentation and also serve as a ballast against wide temperature swings from day to night. If just a little more cooling is needed, put an old T-shirt over the carboy with the tail of it in the water. Water will wick up and evaporate off, increasing the cooling effect. Or combine this method with the next technique.

Frozen water bottles: Buy or refill some 1-L water bottles. Freeze them. Sanitize the outside and put one or two directly into a too-warm red must or put them in a tub of water around a white wine fermenter. As they thaw, if more cooling is needed, put those back in the freezer and get some more out to continue the cooling.

Chillers: There are stainless coils sold at home winemaking shops that are intended to cool beer wort for homebrewers. You can hook up one of these to a submersible pond pump in a pail of water (iced, if necessary) and put the coil into your must. There are also stainless-steel coil chillers available that connect to a portable refrigeration unit so all you supply is electricity. Plate chillers are also available (although are a bit more costly). You will, however, need to stir the container frequently to take full advantage of the chiller’s effect.

Cold jacket: Variable capacity stainless steel tanks are readily available in home winemaking sizes from about 50 L (13 gal.) to 1,000 L (260 gal.). One factory variation for these tanks is a cooling (or heating) jacket on the outside. Usually in the form of a stainless steel “belt” welded in place around the middle of the tank, the jacket has attachments for recirculating a cooling fluid. You can use a pump and ice water as described earlier, or purchase a mechanical refrigeration chiller that pumps cold glycol fluid through the jacket. Those systems are thermostat controlled and therefore require less monitoring, but they can also be quite expensive.

Chill the cellar: If you have the chance to build it, you can cool the entire space. A through-the-wall cellar cooling unit like a Breezaire or CellarPro can be sized for any well-insulated room. Purchase extra capacity if you are also going to cool tanks or carboys of white wine in the room so you can remove the heat of fermentation that is released. Air conditioners can help, too. If your air-conditioned space is large enough and the mechanical equipment sized properly, you may have a suitable fermentation space. Also available are dual purpose cooling and heating controllers like the UNI-STAT that can maintain fermentation temperatures within a re-purposed refrigerator for smaller-volume fermenters.

Heating

Heat may be needed when making wine in cooler climates to keep a red wine fermentation proceeding at a favorable temperature. Heat is also used to maintain sufficient warmth when malolactic fermentation is carried out (usually above 65 °F, 18 °C).

Hot water bottles: Fill plastic water bottles with hot tap water. Cap tightly, sanitize the outside, and drop them right in the must.

Space blankets: Since your fermentation is generating heat anyway, just keeping it locked in can help raise the temperature. These thin, lightweight reflective blankets are made by evaporating a very thin layer of pure aluminum onto a Mylar plastic sheet. Their highly reflective coating reflects the heat back around the fermenter. The blankets are inexpensive and readily available at camping and sporting goods stores.

Heating pad: Electric rubber pads that are used to help with garden starts can be purchased at nurseries and home improvement stores. Placing a plastic fermenter on the pad will add some gentle heat and will insulate the tub from a cold concrete floor, reducing heat loss.

Electric blanket: Over fermenters or across barrels, an automatic electric blanket can be set with a thermostat to maintain a desired temperature. Be aware that safety concerns have led to automatic shutoff systems in modern blankets. Most will turn off after 8 to 12 hours and will need to be reset to keep heating.

Space heater: If your fermentation space is small or you can partition it off with something like insulated plywood panels, a small electric space heater may be all you need. The oil-filled ones that look like an old-fashioned radiator provide a gentle source of heat. They are not very energy efficient, but they may be suitable for short-term use during primary or secondary fermentation.

Aquarium heater: Glass-jacketed heating elements are designed for immersion in tropical fish tanks and are equipped with thermostats. For wine, they find their best effect in barrels used for malolactic fermentation.

Heat the winery: Just as with building in systematic cooling, a design decision to include a furnace (or ducting from the household heating system) in the winery can make operation very simple. The capital cost is high, but the operating complexity is low. Make sure a thermostat is provided in the winery room, set the temperature to where you want it, and go!