Keeping the area you choose for fermentation warm is important because kombucha likes warmth. The closer to 80 degrees, the better. The faster fermentation takes place, the less chance for mold to develop.
If you want to be scientific about your Kombucha brewing, you can test for its pH. The correct pH will keep your Kombucha culture healthy and viable. Keep the pH of the culture below 4.5, which is quite acidic. Most molds can grow only in a higher pH solution.
Some people keep the pH low by adding what is called 'starter tea' to every new batch of tea. You can do this by using a good amount of fermented Kombucha' from your last batch to lower the pH of the new batch. Testing your new batch of tea using pH test strips with a narrow range of 0-6 pH will tell you acid content of the tea. Just keep adding starter tea until the pH is lowered to a range between 2.5 and 4.6.
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