Oxidation of sugars and polyalcohols by acetic acid bacteria during surface culture fermentation

Acetic Acid Bacteria (AAB) are well known for their ability to oxidize alcohols, aldehydes, sugars, polyalcohols and others molecules with ketonic or aldehydic functional groups. Species of Acetobacter and Gluconobacter lack a functional Embden-Meyerhof-Parnas pathway and are unable to metabolize hexose sugars by this route. Hexose and pentose sugars are oxidatively metabolized by the hexose monophosphate pathway to acetic and lactic acids. In some cases, hexose sugars may be directly oxidized to gluconate and ketogluconates without further catabolism, leading to an accumulation of these end products in the culture medium. Furthermore, Gluconobacter oxydans has been used to oxidize various sugars and sugar alcohols to substances of industrial significance, such as sorbose, gluconic acid and ketogluconic acids. Dihydroxyacetone, 2,3-butanediol, and acetoin are also significant products of carbohydrate metabolism evolved by AAB Our trials of surface static fermentation was carried out on complex media such as base wine (BW) for Traditional Balsamic Vinegar, where sugars and alcohols are present in different amount. In this conditions AAB sequentially oxidized the several carbon sources, first ethanol, then glucose and glycerol. In particular the oxidation of glucose to gluconate occurred when the ethanol was exhausted, then high amount of gluconate was accumulated in the medium. BW had an initial composition of around 7% (v/v) of ethanol and 25% of sugars with glucose and fructose in the ratio 1/1. At the end of the fermentation the glucose/fructose ratio was in favour of fructose, titratable acidity was very high due to the gluconate formed, while volatile acidity decreased. In summary, when surface fermentation is extended afterward the ethanol run out, other substrates are oxidized and give origin to a vinegar with a completely different composition. The extension of the oxidative step is a tool for increasing vinegar’s sweetness and acidity without the acetic acid sensorial pungency. (empty line)Keywords: gluconic acid, sugar oxidation, surface static acetification.