The Fresh Loaf

A Community of Amateur Bakers and Artisan Bread Enthusiasts.

Sourdough yeast vs bacteria and temperature

venkitac's picture
venkitac

Sourdough yeast vs bacteria and temperature

In the recent "Very liquid sd" post, there was a lot of information on yeast vs. bacteria. One thing that I found confusing is this: I have heard from other sources (including at a class at SFBI, assuming my notes are correct) that w.r.t temperature, lower temperatures restrict yeast activity *more* than bacteria activity. This also seems to match with my (naive?) experience: if I leave a starter in the fridge, it takes forever to double, but the sourness is there in 12-24 hours easily. But, but, in the earlier thread Debra Wink says:

"Dan gave us a good overview of how dough is affected by hydration (some cereal chemistry as well as metabolic effects), but now let's take a look at how the culture is affected---the population dynamics---because that will determine the magnitude of the metabolic effects. Lowering hydration will slow all the microorganisms, yes, but yeasts are not quite as sensitive to it as the lactobacilli. In other words, the growth rate of the bacteria declines more sharply than that of the yeasts. Sourdough LAB thrive in warmth at high hydrations; low hydration and cool temperatures really slow them down. Yeast benefit from this, because they have less competition from the bacteria, so they have more space, and the resources to expand. They aren't quite as hindered by low hydration, low temperature, low pH, salinity, etc., as lactobacilli are, so even if they do slow some, they gain an edge because the bacteria are slowed more."

i.,e, the exact opposite of what I heard at SFBI, read elsewhere, and my experience, w.r.t the effect of temperature on yeast vs. bacteria. Can someone (Debra?:)) clarify the effect of temperature on acidity and bacteria, please? Thanks.

Debra Wink's picture
Debra Wink

I think the misunderstanding comes from a faulty assumption that more acid production means more bacterial reproduction, which isn't necessarily the case. The lower temperatures favor a metabolic pathway leading to acetic acid production (in lieu of the alternate end-product---ethanol---which doesn't contribute any sourness).

On the Very Liquid sd thread, what I'm talking about is maintaining a starter cool, and how that affects reproduction over time and the evolution of the starter, which is very different from retarding. Maybe this will help pull it together:

http://www.thefreshloaf.com/node/17719/front-page-wall-street-journal#comment-116730

Best,
dw

Ford's picture
Ford
venkitac's picture
venkitac

Thanks! I will assume that I took wrong notes at SFBI as well, which would explain everything:)

Debra Wink's picture
Debra Wink

On the contrary, I wouldn't assume there was anything wrong with your notetaking :-)  That is what is generally taught to bakers. But scientifically speaking, it isn't quite accurate, and so is the source of much confusion when results don't match expectations.