When food spoils/goes bad, what happens to the pH?

Question

Because I know people often ferment foods to lower the pH so that it is preserved... so what happens when food just goes bad, does pH stay the same, or go up?

Answer 1

Simple: spoilt food is acted on by bacteria and fungi. Both are usually anaerobic and degrade sugars to lactic acid. As a result, the pH reduces (because the food becomes more acidic). This is what's responsible for the acrid taste.