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SD Parameters - Blog Extract [07.03.22]

mwilson's picture

SD Parameters - Blog Extract [07.03.22]



The concept of redox or ORP (oxidation reduction potential), a parameter of fermentations not given much attention in the application or analysis of sourdough.

The influence of oxygen is an important consideration with living organisms and redox reactions occur as part of their normal cellular machinery processes. Some enzymes may also carry out redox reactions that occur extracellularly and are classed according to their reductive or oxidative transformation of substrates.

Viewed wholly as a system the influence of oxygen and its transference into chemical energy invokes a change in the environment affecting cellular responses that can shift metabolic reactions. The redox potential of this system can be quantified with an ORP meter that measures the electrical potential or voltage in millivolts often recorded as Eh. Values can be positive or negative and denote an oxidative or reductive environment respectively.

As a parameter ORP can provide insights into the stage of fermentation development and suggest how doughs might perform rheologically. A starter maintained in a very stiff consistency using high inoculation, and short time leavening cycles as with LM is likely to be more oxidative than most other starters. Helpfully changes in the colour and consistency of the dough can give the baker visual and tactile clues as to the ORP state.

Fermentation and Redox Potential | IntechOpen

ORP measurements as a monitoring tool in sourdough fermentations | Baking + Biscuit [PDF]



In contrast to a stiff consistency, a starter culture kept loose with a high water content and high hydration shows an overall greater population of cell numbers of which the yeasts see the most significant gains. This being the case, contrary to oft repeated pointers, low hydration does not independently give yeast an advantage over LAB, quite the opposite in fact. While it is well established that yeasts generally tolerate lower degrees of water activity compared to that of bacteria the effect is likely to be minimal since even very stiff starters made with just flour and water have water activity values high enough to not put LAB at any significant disadvantage. The addition of solutes such as salt and or sugar can markedly reduce water activity, however.

Low water content reduces the rate of diffusion within the liquid phase, a natural occurrence that describes the dispersion of substances throughout the medium. With higher water content the increased rate of diffusion means substrates and metabolites can respectively enter and leave cells more easily thus the rate of fermentation is more rapid.

The following papers independently demonstrate the above:

Liquid and firm sourdough fermentation: microbial robustness andinteractions during consecutive backsloppings

Diversity of the lactic acid bacterium and yeast microbiota in the switch from firm- to liquid-sourdough fermentation

SueVT's picture

Interesting papers! It looks like firm starters favoring the yeast Candida milleri GY215 suppressed all but one of the lactic bacteria, especially over time. After seven out of ten cycles, the culture was pretty stable. That would suggest 5 to 7 days of feedings in preparation for the baking day refreshes? People talk about 2 or 3 days prep.

I'm also wondering about that one LAB which predominates, is the most competitive; what is its function, and is it inhibited by competition from the other LABs? 

Benito's picture

Interesting papers Michael, thank you for sharing them with us.

I have been using stiff sweet levains a lot lately to reduce the sourness of bread baked using those levains.  I will now have to experiment with liquid sweet levains to see what difference that might bring.  It sounds like this will produce a levain with higher concentration of yeast and lower concentration of LAB compared with a similar stiff sweet levain if the sugar concentration is maintained.