Reference Number: 218
Previous workers from this laboratory observed considerable variation in the proportions of acetic and lactic acids produced in pure broth culture as compared to consistently high proportions of acetic acid produced in the sourdough and flour suspension systems. In the latter the proportion of acetic acid was always in the range of 20 to 35% of the total, whereas in pure broth culture frequently less than 5% acetic acid was produced. In the natural environment, the sourdough bacteria, tentatively identified as lactobacilli, coexist with a yeast, Saccharomyces exiguus, and this study was undertaken to determine whether this yeast or flour ingredients including glucose or other factors were involved in this variable production of acetic acid. The proportion of acetic acid produced in broth culture on maltose, the preferred carbohydrate source, was found to depend almost entirely on the degree of aeration. Essentially anaerobic conditions, as obtained by thorough evacuation and flushing with CO2 or N2, resulted in very low (5% or less) proportions of acetic acid. Aerobic conditions, achieved by continuous shaking in cotton-plugged flasks, yielded high levels (23 to 39% of the total) of acetic acid. Similar effects of aeration were observed with glucose as the substrate, although growth was considerably slower, or in nonsterile flour suspension systems. It is theorized that, under aerobic conditions, the reduced pyridine nucleotides generated in the dissimilation of carbohydrate are oxidized directly by molecular oxygen, thereby becoming unavailable for the reduction of the acetyl phosphate intermediate to ethyl alcohol, the usual product of anaerobic dissimilation of glucose by heterofermentative lactic acid bacteria. Comparative studies with known strains of homo– and heterofermentative lactobacilli showed similar effects of aeration only on the heterofermentative strains, lending additional support to the tentative grouping by previous workers from this laboratory of the sourdough bacteria with the heterofermentative lactobacilli.
What does this mean for bakers?
There are two main acids involved in sourdough culture – lactic and acetic acid. It’s the acetic acid that gives sourdough its distinctive tang. This results of this study suggest that a bacterial culture can be manipulated to produce a greater proportion of acetic acid by providing aerobic conditions. Bakers can use this information and incorporate more air into their dough during proving as a way to increase the sourness of their loaves.