Organic Indigo Vat pH

I have started organic indigo vats; one with aloe vera and one with fructose. The pH is quite high. Too high for wool. Will lowering the ph affect the workings of the vat? What should I use to lower it? Tartaric, lemon juice, something else?

The best solution is to add more sugar and wait for about one day: the the sugar is converted in lactic acid: the pH is decreased while the reduction is very strong
Michel