Lactose isn’t fermented by normal beer yeast and is added to make it sweeter without increasing alcohol content or carbonation. But some yeast and bacteria can use lactose, and some of the species that can (*) are known to be common in kombucha, so I sweetened a micro-batch of kombucha with it (in a 1:1 replacement for my typical white sugar) to see what would happen.
- 12 days: pH 4.0. A scoby is forming, but it’s thin (maybe 1mm) and sank when I took a sample with a straw. It didn’t taste very sweet, or at all sour, and mostly tasted like tea. However, as long as the pH was dropping I guessed it was working. I wasn’t sure how lactose tasted, and guessed it may not have tasted sweet at the beginning. Regardless, any activity was happening slowly.
- 19 days: pH still 4.0. It didn’t appear that anything had changed in the week between samples and decided to give it another week. A quick test showed that lactose on the tongue wasn’t sweet at all. 1/4 cup of lactose dissolved in 1 cup hot water had a very mild sweetness that was strangely soft and “powdery”. A quick search online shows that lactose is only 20% as sweet as the same amount of sucrose.
- 26 days: pH now appears to be over 4.0, apparently due to the acetobacter metabolizing the acid because it’s the most available substance.
My conclusion is that while my kombucha culture can work to the tea well enough to lower the pH to 4.0, it can’t use lactose, stunting the growth. I may experiment with using lactose as a sweetener at bottling time, but it’s really resistant to dissolving. I’m not sure it’s worth the effort.
(* Some strains of brettanomyces bruxellensis can under the right conditions, but we don’t know what strains are commonly found in kombucha, what species/strains are in my kombucha, or to what degree my normal procedures provide the right conditions for lactose metabolism.)