Subject: 39. What factors affect microbial growth in sourdough?
We've been doing quite some work to figure out which factors affect microbial growth in sourdough. I've done some work in vitro (which is about to be published: Ganzle et al., Modeling of growth of Lactobacillus sanfranciscensis and Candida milleri in response to process parameters of the sourdough fermentation, Applied and Environmental Microbiology, July 1998); and a colleague of mine, Markus Brandt, has tried to figure out how my "model predictions" work out during the actual dough fermentation. Taken together, one can state the following: For sourdough lactobacilli:
A) 32¡C - 33¡C (89.6F - 91.4F) -- optimum growth
B) 37¡C & 20¡C (98.6F & 68F) -- double generation time
C) 39¡C & 15¡C (102.2F & 59F) -- fourfold generation time
D) 41¡C & 4¡C (105.8F & 39.2F) -- no growth
For the yeasts, the figures are as follows:
A) 28¡C (82.4F) -- optimum growth
B) 32¡C & 20¡C (89.6F & 68F) -- double generation time
C) 34¡C & 14¡C (93.2F & 57.2F) -- fourfold generation time
D) 35¡C & 8¡C (95F & 46.4F) -- no growth.
So: if several refreshments are done above 32 C, the yeasts will drop out eventually. The optimum pH for lactobacilli is 5.0 - 5.5 (which is the initial pH of a sourdough with 5 - 20% inoculum), the minimum pH for growth is 3.8 (they usually produce acid until pH 3.6 is reached).
Lactic or acetic concentrations don't affect growth of lactobacilli very much: this is the reason why the buffering capacity of the flour is so important for the organism (a high buffering capacity in high ash flours means that the lactobacilli produce much acid until the critical pH is reached). It also means, that in doughs that are continuously operated with a high inoculum (more than about 30%), you'll find more yeasts and fewer lactobacilli. Eventually, the lactobacilli flora may change, with more acid tolerant lactobacilli (e.g. L. pontis) prevailing. Such a sourdough is found in the Vollmar and Meuser continuous sourdough fermentation machines (there are 6 operating in Germany, and a diploma candidate in our department characterized the microflora of several of these: as the machine is operated with a 50% inoculum, the pH is never above 4.1 - 4.3, and no L. sanfranciscensis is found in those doughs).
Yeasts are different: they don't mind the pH at all, but are strongly inhibited by acetic acid, and to a much lesser extent by lactic acid. Increasing salt concentrations inhibit growth of lactobacilli, but yeasts tolerate more salt. No salt is added to the sourdough until the final bread dough, but the dough yield affects the salt concentration: with a low dough yield (little water), the salt (ash) is dissolved in a smaller water volume, and the salt concentration goes up: resulting in a slower fermentation.
So much for the "in vitro" theory. Surprisingly, Markus has found most of the predictions to come true when he was looking at the cell counts at different temperature, size of inoculum, salt concentration, and pH in rye dough. The variation of the inoculum size was interesting: If he reduced the inoculum size by 2, he had to wait almost exactly one generation time (one doubling time of the lactobacilli) longer until the dough has reached the same cell counts, pH, titrable acidity, and so on as the dough with the higher inoculum. This was true for inoculum sizes between 1% and 20%: at 50% inoculum, the pH is so low that the lactobacilli don't really grow well, and at an inoculum size of 0.1%, the pH and/or the oxygen pressure in the dough are so high that the cells have a lag-time (see above) of an hour. Thus, a scanty inoculum means one generation time longer fermentation.
The generation time of L. sanfranciscensis in rye dough at 28 C is a little less than an hour (figures may vary with different strains in different flours, but it's not much more or less than that), so if the inoculation size is reduced from 20 to 2.5%, it'll take about three hours more until the dough is ripe.
The question is, whether these findings are true for all flours and for all organisms. The strain isolated by Kline and Sugihara does not differ very much from the two strains I've been looking at. All the literature available tells me that - as long as we're looking at sourdoughs with a tradition of continuous propagation - the system behaves the same way. Differences may be between rye flour and white wheat flour: in white wheat flour, the enzyme activities are so low that the organisms may run out of food before the critical pH (lactobacilli) or the critical acetic acid concentration (yeasts) is reached.
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