April 30, 2015 - 1:09pm
Buffering capacity of whole grain flours?
Having read a lot of claims about whole grain flour extending the LAB growth phase of starter refreshment and dough fermentation (and believing it in general) I have not been able to find a reference that quantifies it.
Does anybody have real data (or a link to a paper containing real data) on the buffering capacity of whole grain flours? Before I go to the trouble of doing this for myself, I thought I would just ask.
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Both acetic and lactic acid are weak acids and the pH of any sour dough will be limited on the down side. certainly one would not approach a point of 100% acetic acid. Whole grain flour will have additional nutrients that are stripped away in the processing that results in white AP or bread flour. They also will give different flavor profiles depending on the relative amounts of the whole grain to the normal flour in the bread. There are obviously other parameters such as water absorption and protein amount that will play a role as well. IMHO controlling the temperature via retardation has a bigger impact on LAB growth providing all other variables are controlled.
While the pH of sourdough is limited on the downside to around 3.6-3.8, it is the total acid concentration (lactic for taste and acetic for smell) and not the pH that determines how sour the bread is. Thus acid production during fermentation is of interest.
In any sourdough mixture, the LAB will outgrow the yeast until the pH gets down to around 3.8 at which point the LAB stops replicating even though it continues to produce acid. If you can buffer the mixture so that the pH stays above 3.8 for one more doubling time of the LAB then you have twice as much LAB making acid for the remainder of the cycle. The claim is made (and easily verified) that adding whole grain flour holds the pH up for a longer time than using only white flour. The question is how much and how long. Thus the search for data.
Flour contains a bunch of pretty neutral compounds and it's hard to see what might be contributing to buffering capacity even with whole grain flour. What you note in your post are two different issues: capacity to produce acid which are the end metabolites and the capability for cell division. Cell division requires a lot of energy and this will be at the expense of metabolite production. pH could be maintained at a higher level if there is a lot of cell growth as acid production will be limited. Whole grain flour with the extra nutrients is likely to contribute in this manner rather than as an actual buffer.
I'll have to go back and see what I can find out about flour composition to see if what I said above is correct (aside: my graduate degree is in biochemistry and I spent a fair amount of research time studying several pathogenic microorganisms but alas no food producing ones).
Alan
It seems to be generally accepted that the ash content of a flour is a good proxy for its buffering capacity, so I would go looking for the components of the bran as a place to start.
...from
Modeling of Growth of Lactobacillus sanfranciscensis and Candida milleri in Response to Process Parameters of Sourdough Fermentation
by Gänze et al.
Ref: Applied and Environmental Microbiology, July 1998, p. 2616-2623
The glucose concentration in rye flours and whole-wheat flours remains high enough to support yeast growth throughout the fermentation (19, 23). Fermentations that employ white wheat flours as the raw materials are characterized by low concentrations of glucose, and small amounts of lactic acid are produced because of the low buffering capacity. In these doughs, depletion of glucose and fructose may occur and limit the growth of yeasts.
It's not much, I'm afraid. But there are a couple of other references in the paper that may have additional information, especially the last one. I don't have these papers.
Martinez Anaya, M. A., and O. Rouzaud. 1997. Influence of wheat flour and Lactobacillus strains on the dynamics of by-products from amylolytic activities. Food Sci. Technol. Int. 3:123–136
Röcken, W., and P. A. Voysey. 1992. Sour-dough fermentation in breadmaking. J. Appl. Bacteriol. 79:38S–48S
Saunders, R. M., H. Ng, and L. Kline. 1972. The sugars of flour and their involvement in the San Francisco sour dough French bread process. Cereal Chem. 49:86–91
-Brad
Thanks. Here is a link to the last paper you reference (a significant piece of work from which I have learned a lot though it does not address the issue raised here):
http://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=1&ved=0CB4QFjAA&url=http%3A%2F%2Fwww.aaccnet.org%2Fpublications%2Fcc%2Fbackissues%2F1972%2FDocuments%2FChem49_86.pdf&ei=cYBDVdn7AsuqggSdnIGgAw&usg=AFQjCNEfGJnzM7kKVyiIVEMrDMjgx9rJPg&sig2=GeTRGWdOwynysYCGJYNzXA&bvm=bv.92189499,d.eXY
As noted in the paper the yeast in the sourdough starter does not contribute to acid production which is what the LAB does. In the case of this particular strain of LAB, metabolism of maltose appears to be favored. The key is always going to be to control cell division and divert consumption of sugar to metabolic products. This is why retarding the dough at a lowish temperature leads to enhanced flavor. Metabolism is favored over cell division and LAB production will be favored over yeast. I'll have to do some more reading on this but it would appear that grains that result in higher maltose amounts will be beneficial.
In sourdough starters, there is little competition between the LAB and the yeast (as shown by Gänze) so LAB production is not really "favored" over yeast. The LAB consume maltose and deposit glucose back into the mix along with acid. The yeast preferentially consumes glucose but will use maltose and is not significantly inhibited by the presence of either undissociated acid or declining pH.
The paper sited above demonstrates that (presumably) amylase enzymes in bread flour create a large amount of maltose from damaged starch soon after water is added such that maltose then makes up about 5.5% of the total (dry) flour weight, up from 0.12% in the flour prior to wetting.
used as an indicator to a flour's ability to buffer acid being formed in the dough, would it be too simple to think that the insoluble fibre that makes the ash (what is left after burning the flour) is absorbing or binding acids as they are being produced?
I don't think buffering means extending the fermentation time of the dough, it only temporarily blocks or delays a gradual process of deterioration on the same time line. I do think buffering extends the LAB growth phase, that when perhaps fibre has absorbed/reacted with all the acid that it can, measurable acid levels appear to suddenly increase dramatically in the dough making up for a delay. Rapid deterioration of the dough soon follows.
Flours with more fibre and protein from the outer layers of grain, or whole flours, ferment faster as a general rule than their sifted counterparts and I think that buffering, only refers to the flour's ability once wet, to appear low in measurable acid then suddenly show an increase in acid as the flour ferments. I also think that finely sifted flours tend to buffer acid better than their whole counterparts even though whole flours contain more fibre.
So perhaps what you are looking for is in the grain endosperm and since ryes are said to buffer acids well (although for a shorter time) something that also increases in sifted rye over sifted wheat in the endosperm. How about a study that compares rye to wheat endosperm? If you find out anything about whole Einkorn flour while searching around, let me know, that flour seems to have a great buffering ability, more so than any flour I've ever worked with. ... interesting that rye endosperm contains a high amount of fibre.
Mini,
The specific mechanism by which whole grain flour provides a buffering effect is obscure. The observation is that the TTA of sourdough made with whole grain flour is higher than the TTA of dough made with white flour only. The hypothesis is that the whole grain flour acts to hold the pH up (above 3.8) longer than white flour which allows the LAB to replicate to higher numerical density which contributes to production of acid at a higher rate so that more total acid accumulates before the LAB shut down completely (whether because of declining pH or because of a lack of consumable sugars does not matter).
I would like a simple model that allows me to predict the impact of using a particular whole grain flour. It would be useful to know how much additional acid is needed to have a “very sour” loaf vs a “mildly sour” loaf. With numbers for “mild” and “sour” and a way to predict the increase in acid production as a result of using a particular flour (at a particular hydration, at a particular level of salt, at a particular temperature, for a particular time), it might be possible to consistently navigate a course to various styles of sourdough.
Obviously we have to concurrently answer to the needs of the yeast, but the relative immunity of the yeast to pH makes that less of a concern.
Your point about gluten deterioration under high acid conditions is a good one. A few years ago it was explained to me (perhaps by you) that naturally occurring proteases in the flour are activated at low pH and begin to break down the gluten. This process is apparently slowed down at low temperatures which may be one motivation for a long cool retard (in addition to the relative growth rate advantage of LAB over yeast which increases as the temperature declines below about 20°C). I am assuming that LAB acid production is proportional to LAB growth rate which may not be true.
With so few data points for TTA vs flour type vs other parameters I may have to do the experiment. This raises the question of how best to set it up to collect the needed data so I am open to suggestions.
as I discuss a number of the issues that you raise from a biochemical view point.
You post: "Your point about gluten deterioration under high acid conditions is a good one. A few years ago it was explained to me (perhaps by you) that naturally occurring proteases in the flour are activated at low pH and begin to break down the gluten. This process is apparently slowed down at low temperatures which may be one motivation for a long cool retard (in addition to the relative growth rate advantage of LAB over yeast which increases as the temperature declines below about 20°C). I am assuming that LAB acid production is proportional to LAB growth rate which may not be true".
The pH dependency of any protease will depend on the organism that is derived from. Certainly acid adapted microorganisms might have proteases that are more active at lower pH but this may not be true for organisms that do better under neutral conditions. When retarding dough you get enhanced acid production but very little gluten degradation. I've done lengthy bulk retardations at 40dF and the gluten appears unaffected. The statement about LAB acid production and cell growth I don't think is true as an organism can either make metabolites or reproduce. If energy goes into cell reproduction it's not going be spent on producing extra end metabolites (either acetic or lactic acid).
There are several charts on TFL showing relative growth rates of LAB/yeast and LAB outgrow yeast at lower temperatures and higher temperatures. The bigger thing is at what point does LAB shift from cell reproduction to metabolite production. I'm still looking for some papers here. However, empirical we know that refrigerated sourdough starter gets more sour over time. It's been my experience that cold retardation either at the bulk stage (easiest to do) or the final proof stage can lead to enhanced tang in the bread.
AlanG,
see: http://www.ncbi.nlm.nih.gov/pubmed/8795211/
I was able to download the complete paper. As I thought the optimal activity of the enzymes discussed take place at neutral pH where cell growth is maximal. These three enzymes degrade proteins so that the LAB can grow (needs amino acids for growth). We don't know what happens with these enzymes as the pH drops during fermentation and production of acids. They don't present data but do note that the activity drops with increasing acid production. Whether this has an impact on cell growth relative to metabolite production (lactic and acetic acids) is not clear from the paper. Certainly it makes sense that retardation away from the optimal temperature of 30dC makes sense as the proteinases will be less active at lower temperatures.
Lots of factors to consider in sourdough baking which makes it so intriguing. I need to get back into the laboratory!!!! Too bad the National Institutes of Health (one of my former employers) doesn't do food science research. They are right down the street.
I forgot to add that two of the proteinases from Lactobacillus sanfranciscensis require metals for activity. These two enzymes further breakdown small peptides to amino acids necessary for cell growth. Ash content is a measure of residual metals in the flour thus higher metal contents found in whole grain flour will enhance growth. Thus, I don't think it's buffering capacity but rather the necessary metals (Magnesium, calcium and cobalt) that is key here.
info that rye flour has a lower starting pH and einkorn as well. Could it be that when these flours are mixed with water that their acid levels are already low enough to curb some acid production in bacteria? Thus making it "buffer" acids? I think we need to define "buffer." Are we talking strictly in terms of acid evidence or are we talking perceived acid taste in the dough and/or finished bread?
As some of us know, a sour tasting dough doesn't always end up as a sour tasting bread.
I'll try to address both the points that Mini and Doc made in their posts above. Microorganisms are complicated because of how metabolism is controlled in terms of cell growth versus diverting towards metabolic products which in our case are acetic and lactic acids. We also know that the ratio of acetic/lactic acid can be altered as well which will give a different flavor profile. According to the papers that I've read, maltose is not metabolized by the principal yeast in sourdough but it is used by LAB. I suspect that the addition of malt extract which can manipulate maltose will lead to a more 'sour' bread because of this. It is incorrect to say that glucose will be deposited back into the bread as maltose is a simple dissacharide of two glucose molecules and when it is broken down by the LAB both glucose molecules will be metabolized by the bacteria. IIRC, yeast do not possess the needed enzyme to breakdown maltose.
It's been a lot of years since I took analytical chemistry but I do remember that we did crucible burning of material to reduce samples to ash for analysis. The combustion process eliminates the organic matter but not the residual metals which I think is what the ash measurement is gettng at. This makes sense since rmilling flour to a refined stage does away with a lot of the important structure of the kernel. One is left with largely starch and protein as the bran and some other structural stuff is gone.
All the buffering agents that we used in the laboratory are either weak acids or bases and one would select the compound that buffered in a particular pH range. From a biochemist's perspective most of the time the range of interest was a neutral range at pH 7 which is where most all the organisms and enzymes I used to work on were most active. I believe it is a misnomer to categorize residual metals as buffers since they don't have inherent buffering capacity. However, metals are required for bacterial reproduction. A long long time ago I was studying the whooping cough bacterium and we started cultures on blood agar plates before transfering to larger flasks as the enhanced iron content of the agar spurred growth. I'll plead ignorance about the various factors needed for LAB growth but would suspect that metals might be important since whole grains seem to markedly assist in starter initiation (rye and whole wheat flour have high ash contents and we know how they impact starter growth).
Mini notes that a sour smelling dough doesn't necessarily transfer to a sour taste. Absolutely the case as these smaller molecules are volatile and some amount will be lost during the baking process (which is why your kitchen smells so good while bread is baking). I routinely bake sourdough at 460dF and would imagine that if the temperature were dropped down to 380dF the resultant bread might have more tang (but of course probably less oven spring and a crust that was not optimal).
These are just my thoughts this Saturday AM. The making and baking of sourdough bread is a complicated process!!!