I have been fermenting milk for the last 10 years and used various commercial lactic acid bacteria cultures and starters. I am well aware that there are thermophilic and mesophilic cultures and that it's important to maintain a certain temperature during fermentation.
Recently, I decided to ferment a lactose water solution. I tried several times with a couple of different LAB starters and was very surprised to see that my lactose water solution ferments very little and much much less than actual milk.
For example, I used Biena YO-CULT (Yogurt Type I) yogourt starter and other starters that ferment milk very well.
For preparing lactose solution, I used distilled water. I tried to get lactose powder from different sources: stores selling beer and wine making supplies as well as health food store brand "Now Real Food" . I tried to dissolve as little as 50g of lactose powder in 1 liter of water to as much as 200g of lactose powder in 1 liter.
The result was always the same: lactose water solution ferments very little and much much less than actual milk.
For the sake of completeness, a few years ago, i did ferment milk prepared from dried milk powder and it fermented very well as it should. So preparing the solution from a dried source is not a factor by itself.
So, commercially available lactose powder is not the same as the lactose in milk?