How starter hydration slows bacterial fermentation

 Since school there was one nugget of sourdough wisdom that had always been drilled into me, “firm starters produce acetic acid and liquid starters produce lactic acid”, the lesson continues that breads made from the appropriate starter will also produce breads of following acidity, with lactic acid being the weaker of the two. Yet, recent discussions of the topic point to the opposite, which does make some sense, with less water to move around in, bacteria would be less able to access appropriate nutrients, therefore, energy, with which to grow and reproduce. So the by-product of their existence, acid, would be lower in quantity, regardless of the relative strength of their specific acid. 

 Two of Hamelman's sourdough formulas, the “Vermont Sourdough” and his, “Pain au Levain” are virtually identical but for two procedural discrepancies, the Vermont Sourdough uses a 125% hydration starter and is retarded for the final proof, whereas the latter utilizes a 60% hydration starter and has absolutely no retarded fermentation whatsoever. This being the perfect experimental set up for proving the truth of this question to myself, I decided to mix these two formulas, and conclusions come what may.

The main question here could be summarized as, “What differing flavor profiles are a result of extending the bacterial fermentation compared to relatively limiting their growth?” A secondary question was, “What is the optimal final proof time of a sourdough loaf?”

Vermont Sourdough

90% Bread Flour

10% Rye Flour

67% Water

1.9% Salt

15% Pre-fermented Flour

Pain au Levain

95% Bread Flour

5% Rye Flour

67% Water

1.8% Salt

15.5% Pre-fermented Flour

Both doughs were mixed to an improved window and medium soft consistency. They were also both bulk fermented for approximately 3 hours with a single fold at 90 minutes. After shaping, the procedure diverges when the Vermont Sourdough is retarded overnight and the Pain au Levain is not. The loadings of both doughs were staggered, with the first half of the batch being loaded when I felt it was appropriate, the second set, after that.

Results:

Interestingly enough, neither dough was particularly acidic. The Vermont Sourdough had some acidity on the back end with particular emphasis on the after taste, but not unpleasantly so. Comparatively the Pain au Levain had no discernible acidity. Despite the rumor that naturally leavened breads are sour, this was an exception to the rule. In both cases the oven spring was greater then desired. This was true in both the first and second loadings of both doughs. Even at a three hour final proof, there was excessive oven spring. There may have been complications with the shaping of the loaves exacerbating any underproofing here; but the second loadings had less spring then the first sets.

Next:

Dough fermented with a liquid starter and retarded overnight produced a more acidic flavor then a dough fermented with a firm starter and no retarded fermentation. This much is clear, which of these two variables contributed more to the acidity is still uncertain. This with a longer final proof will have to be explored next time!