Subject: 27. How do lactic bacteria affect sourdough bread?
13 Feb 1997 10:49:32 +0100
Dear Daniel Wing!
Your letter to Prof. Hammes has reached Hohenheim, and Prof. Hammes has asked me to take care of the communication. I am a Ph.D. candidate in Hammes' lab working on the physiology of sour dough lactobacilli.
Please feel welcome to address questions to us concerning sour dough microbiology and technology! I will mail two recent publications or our lab concerning the physiology of sour dough lactic acid bacteria by mail, but as they may take a week or longer to reach you, I will give a few comments on the questions in your letter:
- yeasts do not produce appreciable amounts of either lactic or acetic acids, their main metabolites are ethanol and CO2. If acidification of the dough is desired or required (e.g. if rye flour is used), lactic acid bacteria or organic acids (most commonly lactic or citric acids) are added.
- homefermentative lactic acid bacteria do produce solely lactic acid from maltose or glucose under anaerobic conditions (as they are prevailing in sour dough fermentations). Thus, doughs acidified with homofermentative lactic acid bacteria (LAB) contain but little acetic acid. As homofermentative lactic acid bacteria do not produce CO2, yeast must be added to ensure leavening of the dough.
- In sour doughs with a tradition of continuous propagation (such as the San Francisco French Bread Sour Dough process, German rye sour doughs or sour dough employed in Pannettone production in Italy), heterofermentative lactobacilli, especially L. sanfrancisco, are dominating the fermentation. Heterofermentative lactobacilli produce lactate, ethanol, and CO2 from hexoses (most strains do not ferment pentoses), HOWEVER, if additional substrates are present that serve as electron acceptor to balance, acetate is produced instead of ethanol. I do not know whether or not you are familiar with the concept of the "redox balance": Degradation of hexoses via the pentose-phosphate pathway as employed by heterofermentative LAB results in phosphorylation of ADP to ATP, and in the reduction of NAD to NADH. As there is no use for NADH, it must be oxidized to NAD again. In the absence of other substrates, acetyl-Phosphate is reduced to ethanol, with two NADH becoming oxidized to HAD in the process. If either fructose, oxygen, citrate or malate are present, these become reduced to mannitol, H2O, lactic and acetic acid, and succinate, respectively, and acetyl-P is dephosphorylated to acetate. (This explanation may not be very straightforward, I hope we did a better job in the publications I`m about to send you; these also include a diagram showing the metabolic pathways of L. sanfrancisco). The consequence for the molar ration of lactate:acetate (fermentation quotient, FQ) in sour dough fermentations is, that acetate in produced only if one or more of the above mentioned co-substrated is present. Oxygen is present only in the beginning of the fermentation, and the amounts of oxygen are too low to result in significant amounts of acetic acid, though, in principle, it is possible to increase the acetate content by aeration of dough. Fructose is present in sucrose and other glucofructans with higher molecular weights. Fructose is released from these compounds by cereal or dough enzymes (many strains of L. sanfrancisco don`t even cleave sucrose) and consequently reduced to mannitol by L. sanfrancisco. The ration of mannitol : acetate in sour dough fermentation is approximately 2:1, suggesting that fructose is the most important electron acceptor. Furthermore, citrate and malate are present in the dough in amounts less than 10 mmol/kg, these are utilized also.
Thus, the effect of substrates and oxygen on the FQ is nicely explained by the metabolic characteristics of the dominating fermentation organisms. Dough yield (=kg dough per 100 kg flour) and temperature also influence the FQ. Spicher reports that softer doughs lead to an increased FQ; an increase in temperature results in higher amounts of lactic acid, while the amount of acetic acid remains more or less the same, thus, the FQ is increased again. I do not have a straightforward explanation for these phenomena, but changes in dough yield and temperature will result in changes in buffering capacities of the dough, modified activities of cereal and microbial enzymes, as well as a changed ration of yeasts : lactobacilli counts, all of which are likely to influence the FQ.
Dear Michael Gaenzele
Thank you for sending one of the most gracious letters I have ever received in response to any kind of an inquiry. Since I wrote to Prof. Hammes I have been able to copy a number of articles from English language publications by Drs. Brummer, Spicher, Vogel, and so forth. Unfortunately, some of them have been in non-technical journals and were thus short on details, and even the less technical ones were not as clearly and idiomatically written as your letter. I DID have a hard time understanding what was meant by Dough Yield, for instance, although I had figured it out before I got your letter. I am still not sure I understand some of the statements those authors made about the acid content of doughs (such as the units of measurement), but I have been piecing things together by looking at all the articles cumulatively. Your letter has clarified a great deal. I will put stars next to my current questions to make THIS letter easier to answer. Like this *.
One problem for me was that I did not realize how predominant rye flours were in German sourdough baking. I know that typical rye pentose is about 8% and that pentose viscosity is important in gas-trapping in rye doughs (He and Hoseney, 1991) but I still don't know how an acidified rye dough behaves differently from a more neutral one. *Does it affect viscosity somehow? He and Hoseney studied neutral doughs only.
I also do not understand why Brummer says "Anstellgut" is a non-translatable term. *What do you think it translates as? *I take it that this a very ripe starter, very acid, maintained at room temperature at some infrequent rate of refreshment? *Is it always rye based? *Always a high-ash flour? *How is it different from the type of French and American wheat starters that are refreshed 1:1 every eight hours, or 1:4 every 12 hours? *What is its consistency, pH, Total Titratable Acid? *My assumption is that my lack of understanding comes from the German use of sourdough as primarily acidification, whereas here we look for a little acidification, a good flavor, and good leavening power.*Do German bakers ever make wheat breads leavened with higher starter percentages than those Brummer cites, for example 20% or 30% starter? *Or do they acidify with very ripe starters and leaven with commercial yeast?
I am curious about the flavor/sensory aspects of the FQ: *When a bread is fairly sour (SF Sourdough, some rye breads) is the perceived sourness mostly lactate, mostly acetate, or due to the pH or TTA of the bread? Calvel brings this subject up, but does not resolve it to my understanding.
As for your answers to my previous questions, thank you -- I will look this material over again, and let you know if I have questions. *Do you mind if I put the text of your letter (with attribution) on the internet as a posting to the newsgroup Rec.Food.Sourdough? I will NOT put your address or email address in the posting, unless you want me to. Please let me know, as I think it might become part of the FAQ file there (Frequently Asked Questions). I will forward your entire letter to a very few people in academia here who have been helping me, so you might hear from one of them.
14 Feb 1997 15:50:30 +0100
Dear Dan Wing!
I do not mind if the answer is posted to the rec.food.sourdough: I've also been browsing in that newsgroup.
To answer a few of your questions:
I) There is no rye bread without acidification of the dough. Rye flour does not contain gluten (or a different type of gluten that does not have the gas-retaining properties), so that the structure of rye bread relies mainly on gelatinized starch. Rye flour does have a higher amylase activity than wheat flour, furthermore, the gelatinization temperature is a few degrees lower than that of wheat starch. Thus, with the temperature optimum of rye amylase being about 50 - 52C (with substantial activity up to temperatures of 70C) and starch gelatinization starting at 55C, starch is degraded during the baking process UNLESS the amylases are inactivated by lowering the pH below 4.5. The situation is exacerbated if there was wet weather during the harvest, as germinating rye has higher amylase activities and the starch granules are damaged, thus facilitating hydrolysis.
II) "Anstellgut" is more or less the same as the continuously propagated wheat starters of the SF sour dough bread, so no harm is done if it is translated as "starter sponge" or something like. German sourdoughs usually are rye based for two reasons: 1) Due to the climatic conditions in Germany, especially in the northern and eastern parts that make it difficult to grow wheat, rye flour is just as important for bread production as wheat flour. 2) As these is no necessity to acidify wheat flour (though it enhances the flavor), most bakers do not use sour dough to produce wheat bread. Starter sponges are not necessarily propagated separately. If the dough is taken care of according to traditional methods, it is re-inoculated three times to produce bread dough (reading Bruemmer and Spicher, you probably have already encountered the "three stage sour dough method." A part of the bread dough is used to prepare the sour dough for the next day. This makes 3 - 4 inoculations a day, the ratio of sour dough to fresh dough being approximately 1:3. One has to make a point of it: there is no typical sourdough without continuous propagation! The microflora of these rye starters is actually the same as for wheat starter in SF or Italy: Lactobacillus sanfrancisco and Candida milleri or Saccharomyces exiguus. The pH of a ripe sour dough will be between 3.6 and 4.0 (L. sanfrancisco does not grow below pH 3.6). The total titrable acidity (TTA) depends on the flour employed: as the lactobacilli acidify to pH 3.6, flours with high buffering capacity (amount of acid required to lower the pH), e.g. whole flours, have a higher TTA than white flours with a low buffering capacity. Furthermore, if "hard" water with high concentrations of Me2+ CO3- is used, the TTA will be higher.
3) Acidification vs. leavening: As mentioned above, rye flour or mixtures of rye and wheat flours containing more than 20% rye must be acidified in order to get bread. As the propagation of sour dough is very time consuming if the full leavening capacity of the organism is to be obtained, quite a few processes have been developed in Germany that ensure that the dough is acidified (or that the sour dough added to the bread dough contains enough acid to bring the pH of the bread dough below ca. 4.5), but no leavened by the sour dough microflora. Leavening is achieved by bakers yeast. Basically, there are three possibilities: 1) Dried sourdough with a high TTA (>20) is added to the bread dough, there are no lactobacilli involved in the fermentation (sometimes they are present in the dried sour dough preparation anyway, as in Germany, something called sour dough must contain viable lactic acid bacteria. The dried dough is sold much more readily if it can be called sourdough). 2) A sour dough is kept at room temperature for up to one week. The TTY of that dough is high enough to use it for baking, but as the organisms are rather stressed in such an environment, they will not contribute to the leavening of the dough. Such doughs do not contain lactobacillus sanfrancisco, but other lactobacilli that are more acid tolerant (the ph of such a dough reaches 3.4 - 3.6 after one day, and stays there for the four or five more days that the dough is kept). 3) One stage or two stage processes with starter sponges. One or two stage processes usually do not ensure that the lactobacilli in the dough are fully metabolically active if the bread dough is prepared, thus, the leavening capacity is rather poor, but enough acid has been produced. As far as I know (I never made a survey, though), only few bakers make bread with traditional processes without bakers yeast added to leaven the dough. Acidification of the bread dough with sour dough is rather common, and the sensory quality of such bread is quite close to that of bread made without bakers yeast. Straight processes with bakers yeast and chemical acidification (citric, lactic, and acetic acid, or mixtures thereof) are also quite common to produce rye bread.
4) Lactic acid and acetic acid will change taste and flavor of bread beyond the decrease of pH: the taste buds (sour, bitter, sweet, salty) are on the tongue, any other aroma is perceived with the nose; therefore, the aroma compounds must be volatile. Acetic acid is more volatile than lactic acid, thus, it's impact on the flavor is more pronounced than that of lactic acid. Spicher says that a ratio of 20 acetate to 80 lactate is optimal. It must also be taken into account, that the lowering of the pH influences the formation of other aroma compounds during the baking process. The acetic acid is furthermore important as growth of spoilage organisms such as molds or rope causing bacilli (Bacillus subtilis) is inhibited by high acetic acid concentrations.
I hope that I could answer your questions
With kind regards
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