Your first post mentioned tomato paste.
The practices addressed in fermenting 101,while focusing heavily on peppers, can be applied to fermenting vegetables.
Many folks who ferment peppers, also ferment other vegetables, including tomatoes. I do so myself, and after fermentation is complete I puree into a sauce.
Fermentation is a preservation method, where an acid is produced by a string of lacto bacteria that consumes carbohydrates and expels the lactic acid as a bi-product...which lowers the pH and preserves the vegetables against the growth of harmful bacteria that causes spoilage.
The naturally occurring lacto bacteria (LAB)is present,to varying degrees,on all vegetables. And will grow and create lactic acid. Some vegetables(like cabbages) have such an abundance of LAB on the leaves(as do many ground vegetables) that it ferments very easily.
The next time you are in the market, go check out a bag of coleslaw mix in the produce department that is near its expiration date. Take a big whiff.....that sour smell is the LAB doing its thing making lactic acid.
CM