In sourdough starters you have yeast and lactic acid bacteria. There are three types of lactic acid bacteria, based on the kind of acids that they produce. The LAB ( Lactic acid Bacteria) ferment carbohydrates and produce carbon dioxide and acids.
These produce organic acids that flavour the bread and facilitate the biochemical processes that contributes to the enhanced nutritional profiles of sourdough bread.
Heterofermentative means ‘different’ or mixed end products. They produce both lactic acid, acetic acid and carbon dioxide (CO2) as by-product . They predominantly produce acetic acid, which is the ones that we recognise as vinegary flavour.
See also Homofermentative lactobacilli & Facultative Heterofermentative lactic acid bacteria.