One of the most overlooked aspects of homebrew wine making is the temperature of the fermentation of the wine. Many a batch of homemade homebrew wine has been ruined by fermentation at high temperature - a simple bit of care will improve your results no end.
Many people starting out in wine making are tempted to put their fermenting wine in a position that enjoys high ambient temperature. This has the attraction of making the fermentation complete very quickly - however, it can be a disaster for the wine.
If you try fermenting at a very high temperature, you will simple kill the yeast. This varies depending on the strain of the yeast. Anything about about 35C will kill the yeast, because yeast cannot stand very high fermentation temperature.
If you try and ferment at a very low temperature, much less than 10C, the yeast will fall dormant. Many strains of yeast, especially those for making English table wines, are designed to ferment at low temperatures - even as low as 6C or 8C.
Between this range, it is likely that the yeast will ferment the wine out to dryness. However, if the fermentation temperature is too high, especially during the initial phases of fermentation (i.e. until the wine reaches a gravity of about 1010) various off flavours and nasty enzymes can be produced by the yeast. Volatile flavour compounds can also be driven off by the vigorous fermentation cause by the high temperature. Compounds which we might like to leave in the bottle!
Much of my homebrew wine making takes place during the English summer, when daytime temperatures of between 18C and 25C are not uncommon. Especially with the seasonal Elder flower, strawberry and peach. When making a 5 gallon bucket of wine, the temperature of the fermentation can easily go over 25C, because the fermentation process creates heat on its own!
Please make sure you've got a decent thermometer, and avoid cheap ones because they can be very inaccurate.
I prefer to break the process down into a number of steps.
If we focus on the key main fermentation phase, how do we control the temperature of the fermentation? If it is winter, and the temperature is too low, then simply use a heater, or bring the wine into a warmer place. However, if it is too hot, it is more difficult.
First put the wine in the coolest place possible. I normally leave the wine outside in the shade, wrapped in a wet towel. The water in the towel evaporates in the breeze, cooling the wine. Add water to the towel several times a day.
If the temperature of the wine is too high you will need to take more drastic action. I usually take a 4 pint glass mixing bowl, and fill it with fermenting must. I then put this in the freezer for 12 hours so it is frozen solid. Placing the bowel in a sink of cool water will free up the ice block and you simply float this wine iceberg in the fermenting wine - this will bring the fermenting temperature of a 5 gallon bucket down by about 3C or 4C . Repeat this twice a day during the first week of rapid fermentation.
Control of fermentation temperature is key to producing quality wines - and a little bit of attention in the first week of fermentation will reap rewards 6 months later when you drink the wine!
Note some high alcohol yeasts need a very specific temperature as specified by the supplier. If you deviate by even a couple of degrees, the yeast will not work to its full potential.
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