Wine fermentation: ideal temperature and how to fix problems

For the vast majority of artisanal winemakers, alcoholic fermentation is the most stressful moment in making their homemade wine: fermentation that doesn’t start, lacks vigour, gives off weird smells… Many problems can happen during these crucial weeks that determine the quality of the wine. Yet online resources are scattered across vineyard websites, wine shops and forums on the subject. What’s more, they very often deal with only one problem at a time and stay at the surface of it. For all these reasons, a complete, practical and exhaustive article on the subject is needed.

What is the right alcoholic fermentation temperature?

First of all, you should know that managing fermentation temperature is the key to success. I also recommend using a suitable thermometer. Here are the main temperature ranges to keep in mind:

• To start fermentation: between 22 and 25°C
• During white wine fermentation: between 12 and 18°C
• During red wine fermentation: between 22 and 28°C

These ranges assume access to refrigeration to cool the vat below a house’s ambient temperature (about 20°C). Without access to a fridge, a cellar or a large enough cool room, aiming for 12 to 18°C for a white wine can prove difficult in practice. The coolest room in the house, or an unheated basement, is then the best option.

Why won’t my fermentation start?

It’s the most worrying situation and one that frequently happens to amateur winemakers who don’t use commercial yeasts. Since there can be many causes, they are covered from the most likely to the least likely.

Cause 1: the alcoholic fermentation temperature isn’t optimal

It’s very common for the temperature of the vat to be too cold for fermentation to start, especially if your home is air-conditioned or the vat sits on a concrete floor like in a garage. As mentioned above, natural yeasts are finicky and need a precise temperature range to start quickly, unlike those of commercial origin. It’s not an absolute rule, though: fermentations can start in all sorts of conditions outside that range!

How can you easily raise the temperature? Take the vat out on the balcony during a nice fall afternoon so the sun naturally warms the grape juice. Be careful not to overheat the vat, for example beyond 27°C, since that could let other unwanted bacteria become active. A temperature above 30°C could even kill the natural yeasts.

Fermentation could also start below those temperatures but, depending on the case, it might prove too slow and so allow other unwanted yeasts or bacteria to proliferate (see the section “Why does my fermentation give off a strange smell or look odd?”). A telling example is the Brettanomyces yeast, also called “Brett,” which produces the typical “wet leather” and “horse” smell characteristic of certain natural wines. Fermentation speed is the artisanal winemaker’s best weapon against Brett here, since once the natural yeasts are fermenting optimally, they take up all the available sugar.

Cause 2: the natural yeasts were eliminated by mistake

Natural yeasts are finicky and can have been eliminated by mistake in several ways:

• Over-rinsing the grapes: Yeasts live on the skin of the grapes, so rinsing will inevitably remove them. It’s better to remove contaminated berries (insect, fungus, etc.) from a bunch than to try to rinse it.
• An unsuitable temperature: The vat’s temperature may have been too high (above 30°C) or kept too cold for too long, for example during a maceration done in a refrigerator.
• Poorly controlled sulphiting: Since sulphites are readily available and inexpensive, it’s easy to add them to your juice for all sorts of reasons. But an error in the concentration calculation and the yeasts will inevitably be affected.
• A late phytosanitary treatment: These treatments aim to protect the grape from various diseases and are generally stopped at least 2 weeks before harvest. But an unusual event a few days before harvest, such as hail, will require a treatment that will affect the natural yeasts’ ability to ferment properly.

If one of these scenarios seems likely, or the vat has already been taken out on a nice sunny afternoon as suggested in the previous paragraph and nothing is happening, the yeasts are probably dead. Don’t panic! Any winemaking or beer brewing shop sells commercial yeasts. As for which variety to use, I recommend Lalvin 71B, which is suited to young wines, like a good share of homemade wines are.

Finally, here is a procedure for activating commercial dry yeasts every time:

1. Rinse all the equipment with boiling water and dry it with new paper towels to sterilize it.
2. Measure 0.2 grams of yeast per litre of juice. A 5 gram packet is therefore enough for 25 litres.
3. Heat to between 38 and 40°C a volume of bottled spring water (the chlorine in tap water could harm the yeasts) equal to 10 times the weight of the yeasts. For example, 5 grams require 50 mL of water.
4. Sprinkle the yeasts into the water and wait 15 minutes without stirring.
5. Gently mix after 15 minutes with the sterilized spoon until you get a brown liquid.
6. Add an equal amount of grape juice to the brown liquid, then wait 15 more minutes. Going back to the example, that would be 50 mL of juice to add.
7. Pour the now-activated yeasts into the vat containing the grape juice and stir vigorously so the yeasts are well distributed.

Within 24 hours, fermentation will have begun and will be recognizable by a foam forming on the surface of the juice as well as a fruity smell and the release of carbon dioxide (CO₂).

Cause 3: the grape juice is too sweet

It’s rather rare (and even more so in Québec given our climate!), but it happens that the grape’s Brix level, abbreviated Brix, that is, the number of grams of sugar per 100 grams of liquid, is so high that it gets in the way of fermentation. Watch for this when a grape above 26°Brix is picked. A refractometer lets you measure the Brix of a grape right in the field!

In that case, there are several options to consider:

• Add bottled spring water to the juice: This will bring it back to around 23°Brix. Water dilutes sugar, after all!

• Add a sugar-tolerant commercial yeast: As with many winemaking problems, commercial yeasts have a solution to offer, and here it’s called Lalvin EC-1118, which tolerates an impressive 18% alcohol by volume before becoming ineffective.

• Welcome your next sweet wine with open arms: If not all the sugar is converted, it’s no big deal: it will be delicious just the same. The resulting wine will be what’s called a “sweet” wine because it contains a good amount of residual sugar. You will, however, need to prevent what’s called a “refermentation,” which could start in the bottle in the months to come. There are two ways to do that:

  • Natural method: Keep the wine in a cellar or a refrigerator; and
  • Conventional method: Sulphite to 50 mg/L.

If the Brix isn’t measured in the field, you’ll need to measure the specific gravity of the juice once pressed, with a hydrometer. It’s also a very useful tool for determining whether fermentation is finished. These 2 measurements also determine the potential alcohol: if all the sugar in a wine at 26°Brix or 1.1103 specific gravity is fermented, a wine at 15.9% alcohol by volume will be obtained. For other conversions, I’ve added a table below to make your life easier.

Cause 4: the grape juice is too acidic

It’s extremely rare for this to happen and, truth be told, I’ve never met anyone it happened to. As proof, my friend Étienne manages every year, without intervention, to ferment Vitis riparia, that is, North America’s wild grapevine, which is very acidic given its hardiness. Acidity can however be a stress factor for natural yeasts which, when combined with other causes such as a temperature that’s too low or a high Brix, can get in the way of fermentation. There’s not much to do besides measuring your juice’s pH with pH test strips and acting on the other factors: a normal pH is between 3.2 and 3.6. Careful! These strips don’t work with red wine.

Why is my fermentation slow or lacking vigour?

Fermentation has started but it’s very slow or even stuck. The symptom is easy to measure: a specific gravity, which normally goes down day after day in an optimal fermentation, stays almost the same after several days. To fix that, a few solutions:

• Raise the juice’s temperature: As mentioned earlier, don’t hesitate to take the vat out on the balcony on a nice sunny afternoon, because a house’s normal temperature, around 20°C, can be the cause of the slowdown or standstill. If that fixes the problem, bring the vat back inside and leave it in a room heated to between 22 and 25°C.
• Stir the juice (be careful to use a sterile tool!): This introduces a little oxygen (recommended when the specific gravity is still high) while stirring up the lees, which are in fact yeasts that have gone inactive. Stirring also evens out the juice’s temperature.
• Check the specific gravity continuously: Ideally every day to make sure fermentation is really happening. Normally, each measurement should be different. Once the specific gravity is below 1.000, ideally between 0.9950 and 0.9980, fermentation is finished.

Why does my fermentation give off a strange smell or look odd?

It’s unlucky, but sometimes a juice is contaminated by unwanted bacteria and yeasts that the natural yeasts can’t manage to overcome. To that end, I’ve gathered the signs to look for and the various solutions in the following table, ranked by the seriousness of the micro-organism involved.

In conclusion: best practices

A few practices are worth following to prevent all these ills and so ensure a trouble-free fermentation. Here they are:

• Monitor fermentation continuously: Every day you should measure the gravity, check the room temperature and, ideally, taste the juice while it’s transforming to catch any problem before it gets worse.
• Lightly sulphite the pressed grape juice, if needed: If the harvest is perfectly healthy, there’s no need to sulphite. That said, even low-intervention winemakers today will add up to 10 milligrams of sulphites per litre before fermentation begins. That quantity is so small that it’s even undetectable in a lab! Despite that, it’s enough to “calm” the unwanted micro-organisms and let the natural yeasts quickly take over. If the sanitary condition of the grapes isn’t ideal, higher doses will be needed, but careful: an excess of sulphites harms natural yeasts. To learn how to add the right amount of sulphites, see my article Potassium metabisulphite in wine: how much, when and how.
• Maintain impeccable hygiene: Unwanted micro-organisms are everywhere and are just waiting to be carried into the juice by oversight or lack of rigour. So you need to make wine in a clean place with sterile tools.

That’s it, everything’s been said. Now all that’s left is to wait for it to ferment!

I also want to express my gratitude to Véronique Lemieux, who gave me an enormous helping hand by reviewing the winemaking concepts presented here. For those interested in her wines, you’ll find them on Instagram (@les_cuvees_de_veronique).