The two keys to a great kombucha are quality starting ingredients and the right brewing environment. Apart from keeping everything clean and free from contaminants, temperature is probably the most important environmental factor for brewing kombucha.
Kombucha will brew easily between 20-30°C. At the colder end the brew may take a bit longer and taste a bit weaker with a yeasty smell. At the hot end the brew will go quickly and things will taste and smell more vinegary. If you a brewing in hotter temperatures you will need to regularly do taste tests so it does not overbrew and end up as vinegar.
The best temperature for kombucha is about 25°C – and in a perfect world you will keep the temperature as even as possible.
For those living in the southern half of Australia, getting the right temperature for brewing through winter time can be quite tricky. Use a thermometer to measure what temperature you get in your brewing location during the day and also at night before going to bed. Take measurements over a few days and calculate the average daytime and nighttime temperatures.
Once you have a baseline temperature you know what needs to be done to keep your brew within the ideal temperature range. This may be as easy as moving the brew to a warmer (or cooler) part of the house, but in most cases you are going to need some sort of heating element.
There are two styles of heating that will work for homemade kombucha – a heating pad and a heat wrap. The pad is the cheapest option and works best for smaller jars. You can probably fit between four and eight kombucha brews on the one pad. The downside of this is that the temperature will not be evenly spread – the base of the jar will be hot and the surface of the liquid is likely to be still fairly cool. In some cases this temperature imbalance can encourage certain strains of yeast in your brew to become overactive and dominate the brew. If this happens you will know about it! There will be a lot more yeast strings than normal and the flavour will be noticeably ‘beery’. Keep a close eye on things and regularly do taste tests to see how your brew is progressing with the heat mat to help things along.
The other option is to use a heat wrap. This is the best option in terms of brew quality, however it means you will need to buy a heat wrap for each brewing jar – and you are also going to need several powerpoints available in your brewing location. If you are finding the heat wrap is making things a bit too warm, wrap a thin towel around the jar and then put the heat wrap over the top of that. If you need to warm things up a bit more – do the opposite, put the towel around the heat wrap for extra insulation.
A basic heat mat is not the perfect solution – however it is much, much better than having your brew too cold. When your kombucha is cold, the rate of fermentation and the formation of organic acids will be slower than normal. The slowed reaction means the pH of the tea will remain higher for longer, leaving a greater window of time where mould and other bacteria may spoil your batch.
Seeing your SCOBY turn out like this can be heartbreaking – make sure you keep your brew warm to prevent this from happening!