Does the ph level change over time i.e. if I have a sauce that I haven't fermented yet but it's already at 3.5 will it ferment and go lower or stay the same?
Rocketman or Salsalady will be your best bet on this.
I'll offer what I can..., you say "if I have a sauce that I haven't fermented yet but it's already at 3.5 will it ferment and go lower or stay the same?" the answer to this is it depends.
It can ferment if there are available sugars for the lacto to eat. If the benificial bacteria shows up then in theory it would reduce the ph further. I really cant remeber how low ph can go before it become inhospitable for the lacto??
Will something after fermenting down to 3.5 ever go to 4.0 or higher over time? To my knowledge no, unless tampered with.