Example:   Sourdough Bread:

can an old, “Dead”, starter be recovered?



Typically, we chemists, avoid using the equations describing ionic equilibria which we learned in school; relying instead on the general principles and intuition gained from our introduction to those equilibrium equations of state.

My desire is to show that detailed knowledge can be quickly obtained - without the direct use of equations. This knowledge can then be used, in conjunction other existing data, with the intuition and general principles to inform us of a better solution path which will lead to a better result.

The majority of problems encountered in the pursuit of science and engineering are related to problems associated with existing technology.  This is especially true for science which ushers in new technology - so called ‘Earth shattering science’.  The new technology inevitably overcomes some fundamental limitation of the previous ‘standard’ technology.

Science marches forward by utilizing standardized procedures, which permits  direct comparison of results from different workers and different studies. Procedures and process in science, engineering and manufacturing also rely materials which conform on standard specifications (which are also based on standard procedures) in order limit variability.

The bread making process is typical of most processes performed in the advance of science and in the manufacturing world.  For a given application, there is a ‘standard’ process and a ‘standard’ bill of materials. The materials need to conform to quality assurance specifications. The combination of process and materials for bread making is called a recipe.

In science, as in manufacturing, most processes are performed by semi-skilled laborers (technicians) who typically have a general understanding of the principles involved in the making the process successful.  Frequently these laborers lack detailed knowledge of the physics and chemistry at work. 

When a process fails, or produces a less than desirable product, the fault is usually ascribed to the operator or the materials used. Rarely is the procedure itself questioned. This is especially true for processes which have been established many years. In the case of bread making - the basic process has been established for thousands of years.

It is interesting to note that the original bread making process was centered around the sourdough process. A cult of superstition developed around baker’s secrets and the heritage of a given baker’s starter. This lore continues to this day.

The great variability in product quality and consistency was the inspiration to apply science to the art of bread making, with the goal of achieving more uniform quality. Nonetheless, sourdough breads, which exhibit great  batch to batch uniformity, are produced on a commercial scale .

The sourdough process involves multiple stages of dough development, each requiring many hours or days of fermentation.  This process converts the nutrients in the flour to nutrients in a large yeast colony. Much of the flavor and texture of a sourdough bread comes from this yeast colony. Some estimates suggest that this yeast contributes as much as 30% of the mass of the resulting bread .

Most recipes call for large quantities of sourdough starter. A  portion of the dough produced in each batch is set aside to be used in subsequent batches. This portion is the starter.

Basic Problem:

Frequently, doughs made from sourdough starters produce heavy, dense bread with a grainy texture and poor crumb.

These poor results often are attributed to a starter which has gone bad or to poor technique.  Most recipes for starters recommend discarding a starter that has been left to stand, unused, for 7 days.   Nonetheless, some bakers ascribe their success to starters which have been in use for over a hundred years!

The ‘sour’ taste of the dough is largely attributed to the presence of acetic acid. The fermentation of the flour by the yeast produces carbon dioxide and ethanol.  The carbon dioxide is trapped by fibers of gluten creating a closed-cell foam. This is what causes the dough to expand prior to baking.

Acetic acid is not produced by the yeast, but by a colony of acetobacter bacteria which feeds on the ethanol.

A good sourdough starter contains both yeast and acetobacter. The longer a starter is used, the more acid resistance it evolves.

It is pretty clear that the acid level (pH) affects the dough.  The conditions surrounding gluten formation, yeast growth and acetobacter growth have all been studied. The internet is a good source for this information. 

Let’s begin by getting an idea of the pH in the dough resulting from the acetic acid.  My quick internet search provides an upper limit for wine of 6% by mass, which is a reasonable starting point as the acid production self arrests and the fermentation and acid production processes for wine are very similar to those for bread.  This estimate will be based on a dilute aqueous soluiton, so it will truly be a lower pH limit.

Six percent by mass acetic acid corresponds to roughly 1 (mole/Liter), or 1 (M).

6 (g) acetic acid / (0.1 (L) * 60 (g acetic acid / mole) )= 1 (mole /L)

Figure 1 show the pH of a pure acetic acid solutions in deionized water ( DI water), with concentration of about 1 (M)

Based on these graphs, we see that it is possible that the gluten may be damaged if the starter is left to produce acetic acid until the acterbacter self arrest. The pH of the 1(M) could be as low as 2.4, which is below the lower limit for gluten stability: pH = 3. This pH is well below the limit for yeast fermentation: pH =4;  A poor product might result either from the inability of the dough to capture CO2 due to loss of gluten or it may not rise due to lack of CO2 production.

This may explain why staters left to stand, unused, for too long produce poor quality bread. An obvious idea is to dilute the starter so that the initial pH will be lower.

NOTE: A typical sourdough recipe uses a large amount of starter, which comprises about 1 half of the total water used to make the dough. This would correspond , roughly , to a 2 fold dilution of the acetic acid in the starter.  A typical recipe calls for about 2 cups , or about ~ 500 (mL), of water

What is missing is an understanding of the effect of the ‘standard’ bread making processes on dough pH. One cannot get a reliable pH measurement from a typical bread dough, however we can estimate bounds for the pH of the water in the dough using the equiligraph.

Figure 1. Equiligraph of 1.0 (M) acetic acid in pure water. The pH of the pure acid is 2.4.

Figure 2. Equiligraph of 0.01(M) acetic acid in pure water, which is 100 times more dilute than the solution shown in figure 1. The pH of the pure acid is 3.4.

Figure 3. Equiligraph of 0.0001 (M) acetic acid in pure water, which is 10,000 times more dilute than the solution shown in figure 1. The pH of the pure acid is 4.5.

Figures 2 and 3 show similar graphs for 0.01 (M) and 0.0001 (M) acetic acid, which represent the maximum acetic acid content of the starter, a 100 fold dilution and a 10,000 fold dilutions.   A 100 fold dilution could be made by adding only a teaspoon or about 5 (mL) of starter to 2 cups of water.

A 100 fold dilution could result in a pH of 3.4. This is above the lower limit for gluten stability, but still below the limit for yeast fermentation. 

The excessively large 10,000 fold dilution is required to put the initial pH of the dough above the lower limit for yeast fermentation.

Unlike home bakers, commercial bread production, even in small local bakeries, can mix quanitites larger enough for dilution to help.

This small change in pH is due to the square root dependence of pH on total acid concentration, because acetic acid is a weak acid. The equiligraph accounts for this, and you - the analyst - are reminded of this relationship by the graph. You did not need to remember this square root relationship prior to the beginning the analysis.

Note that these pHs are merely the starting points.  A starter which has sat long enough to develop this much acetic acid must also have a large amount of ethanol, which is the food for the exisiting acetobacter colony. Based on these data alone, it seems unlikely that a dough made from even a small amount of old starter will produce a quality bread.

One wonders that this process could work at all, yet thousands of years of bread making show that it can work exceptionally well.  It is easy to see why modern bread production relies so heavily on using fresh yeast of a pure strain.

Here is the central paradox: 

Recipes call for very long fermentation times - on the order of days - and successful bread requires the dough to develop  lots of ethanol and with it, develop a healthy acetobacter colony. On the other hand, lots of ethanol and a healthy acetobacter colony will foul the starter.  How is it possible to maintain a viable starter?


Most stories about truly successful sourdough bakeries include anecdotes about “Spores in the bakery” or a special wooden bowel that has never been washed.  What remains unsaid is that bread is made everyday!

Sourdough recipes in cookbooks almost all begin with a fresh starter, which more often than not uses the reliable modern yeast. The starter can be made by soaking fresh grapes too.  Almost all published recipes suggest that you can keep the starter alive, but warn that it should be discarded if left more than a week.

It seems that the dough must be allowed to ferment without the accumulation of ethanol for as long as possible, such that the dough pH remains high for a long time.  Most published sourdough recipes call for periodic addition of water and flour, which are food for the yeast. 

At this point, it is a good idea to remind ourselves that we have been considering solutions of pure acetic acid in water. we have been neglecting the major component of the dough - the flour.  Studies have shown that the flour itself has a large buffer capacity with respect to pH changes and that the yeast fermentation is relatively insensitive to dough pH, at least until the dough pH drops to about 4.

This gives the home baker hope that an old starter can successfully be used by altering the recipe.  Using, for example, a teaspoon of starter  in a cup of water, rather than a cup of starter.

What other process modifications are possible?

Figure 4 shows an equiligraph of 0.01 (M) sodium acetate, which exhibits a pH of about 8.4.  This solution is well below the pH 11 upper limit for gluten stability.

If we assume that the starter is old, and has reached the maximum acteic acid content of 6%, then 500 (mL) of a 100 fold dilution would contain about 0.005 (mole) of acetic acid.  This could be neutralized by adding aobut 0.005 (mole) of either sodium bicarbonate ( NaHCO3) or cream of tartar ( potassium bitartarate, KC4H5O6). Figure 5 shows an equiligraph for tartaric acid similar to figure 4.

Figure 4. Equiligraph showing pH of 0.01 (M) sodium acetate solution, as end-point of titration.  The pH is about 8.4. An alkalinity of 0.01 (M) is required to transform pure acid to this point.

Figure 5 Equiligraph showing pH of 0.01 (M) potassium tartarate solution, as end-point of titration.  The pH is about 6.2. An alkalinity of 0.01 (M) is required to transform pure acid to this point.

It seems reasonable to dilute the starter ( 100 fold) and neutralize the acid added from the starter.  While this may not change the dough pH substantially, it will certainly return the buffer capacity of the dough back to a state similar to what it would be if fresh yeast were used.

The cream of tartar seems like a better option simply because the upper pH  bound (6.2) is lower than the pH for  baking soda (8.4). In practice, the difference may be insignificant due to the large buffer capacity of the flour.

The quantity of sodium bicarbonate and potassium bitartarate needed is:

0.005 (mole) * 84 (g NaHCO3 / mole) =  0.42 (g) NaHCO3

about 1/2 (mL), or 1/8 (teaspoon), in 500 (mL).

0.005 (mole) * 188(g KC4H5O6 / mole) = 0.94 (g) KC4H5O6

about 1 (mL), or 1/4 (teaspoon), in 500 (mL).

The dough will still contain ethanol from the starter, but it’s level will also be reduced by about 100 fold. Assuming the ethanol content was originally 11%(v/v), the resulting ethanol concentration will be less than 0.1%(v/v).

Both of these proposed process changes should encourage yeast fermentation and retard acetobacter growth. 

It remains to test these process changes!


We have looked at a common, persistent, chemistry problem - poor quality sourdough bread - using the equiligraph to gain perspective that we previously did not have. We obtained numerical results without explicitly using the equations for solution equilibria.  Those results guided us to improve our understanding of a very complex biochemical system. We used some numerical data, which was readily available from internet searches as inputs and as limits.

I hope that you, the reader, now have a better understanding of how equiligraphs can be used to solve problems you encounter daily.

Comparison of bread made by traditional method (left) and starter recovery method proposed here (right). Both were made using the same flour, proportions of water , flour and oil. The difference was the amount of one week old starter used, and a small amount of baking soda. Both were kneaded for 15 minutes and baked as free standing loaves.

The bread on the left side  has a poor crumb, with  a dry, dense texture and the dough never rose more than double the original volume.  This is a typical example of failed sourdough. The loaf fell when put into the oven.

The bread on the right has a good crumb  with a moist  and light texture. It quadrupled its volume and retained its volume when put into the oven.

The simplicity of the equiligraph is attractive, however many people find themselves unable to identify a problem where it would be useful, or are unsure how to extract information from it’s use.

We will use bread making as an example of how to use the equiligraph to trouble shoot an complex industrial or research problem - a problem with more variables than can be identified or controlled.

Problem: Sourdough starters left unused and unfed tend to make poor bread. Most cookbooks recommend discarding the starter if it has sat, unused and unfed, for a week or more.

Question: Why does a starter go bad?

Question: Can good quality bread be made from an old starter?

Question: Can the starter be recovered?

Putting it to the Acid Test

Figure 9.  Sourdough starter which has been in the refrigerator for two weeks. It has not been fed or used. The liquid on the surface has a black cast. It has been sitting at room temperature ( 24 C) for 1.5 hours (since midnight  0000). There are two identical glass bowls, each has 1/2 cup water and 1 tablespoon olive oil.

Figure 6.  Measured and theoretical pH values for household vinegar. the measure pH is quite low, and is comparable to theoretical values for acetic acid concentrations between 1.1 and 1.2 (M).  This agrees nicely with our previous estimate of 1 (M)

Figure 23.  The Recovery method loaf at 1800. the surface remains smooth, unlike the loaf from the traditional method.

Figure 16.  Amount of starter used to make the sponge: 1/2 cup for traditional and 1/2 teaspoon for the recovery method. Not shown is the 1/4 teaspoon of baking soda added to the recovery recipe.

Being good scientists, we have postulated:

* The pH of old starter is too low for yeast to ferment well.

  1. *A 1:100 Dilution of the starter will permit us to 

  > neutralize the acid

  > Reduce the ethanol, which will slow acetobacter growth

We will now proceed to the kitchen with a few lab tools and test our hypotheses.

I have a sourdough starter that I have left in te refrigerator for 2 weeks - just for this test!

Our first hypothesis seems to be confirmed by the data in figure 10, where the pH of the old, unfed and unused starter is 4.1, which is very near the pH = 4.0 lower limit for yeast fermentation. 

We originally assumed that the worst case concentration of acetic acid would be on the order of 1 (M).  Our quick and dirty titration brackets the concentration between 0.1 (M) and 1(M). The equiligraph helps us refine the estimate - to between 0.4 and 0.7 (M). It seems as this assumption is valid!

Let’s see what happens to the pH when we add water and new flour ( e.g. feed it) to make what bakers call a ‘sponge’.

Beacuse we now know that a 1 : 10 dilution does not significantly later the pH, we can take small amounts of dough and dilute it to get an estimate of the bread pH. This will be useful for comparison purposes.

Figure 7.  Numeric results from equiligraph for a 1.1 (M) solution of acetic acid in the pure acid form. The equiligraph method used is described on the Equiligraph Basics page.

Figure 10.  measured pH of liquid standing over two week old starter, shown in figure 9. The dilution was made using distilled water. Note that the pH did not change upon diution, indicating that the acid in the solution is partially neutralized, which creates a buffer.

Figure 8. Recipe used for this test. Traditional referes to the proportions of starter commonly given in cookbooks. Recovery refers to the method suggested by this analysis, where the goal is to recover a starter that most cookbooks state should be discarded. This recipe is equivalent to a 1 : 1/4 ratio of water to flour, which is a bit wetter than most recipes suggest. This test is conducted in Arizona, where the local humidity is low, so the flour will have a low initial moisture content. This recipe should  slump due to being wet. No salt is added to retard fermentation.

Let’s start with testing the accuracy of the equiligraph prediction of the pH of diluted Acetic acid, using household vinegar. Note that the pH of NaHCO3 solution is about 8.4, so it is not a strong enough base to reach the true equivalence point.

Figure 11.  Equiligraph showing the composition of an acetic acid solution at pH = 4.1. The relative proportion of acid to conjugate base is fixed by the pH and to a good first order approximation is independent of concentration. See Figure 12.

Figure 13.  Numeric data for a pH 4.1 buffer of acetic acid and sodium acetate. Note that the acid fraction (α0 = 0.82) and base fraction (α1 = 0.18) are highlighted in yellow.

We can deduce from the data in figures 11, 12 and 13 that the flour neutralizes about 20% of the acid in the starter. This gives us a good idea of the buffer capacity and alkalinity of the flour.   We need to find out the concentration of the acid in the starter.  In order to do this ,we perform a very simply titration using baking soda. All we really need is to bracket the order of magnitude of the acid concentration and get a one significant digit estimate. 

We need to make a 1 (M) sodium bicarbonate solution (NaHCO3).  We will then make serial diluitons to prepare 1x10-1, 1x10-2, 1x10-3 (M) solutions.  At room temperature, a saturated soluiton of NaHCO3 is about 1.25 (M), so make a saturated solution and dilute 80 (mL) to 100 (mL) final volume to make a 1 (M) NaHCO3 solution.

This is an interesting use of the equiligraph - to find the concentration of a solution simply my measuring the pH. To bracket the concentation, one need only perform a quick trial and error search. Once the range has been established one simply gets results spanning the range using appropriate intervals.  Quite easy to do - and very quick!

Here’s the very basic whole wheat sourdough recipe that we will use to test our hyupothesis and check the validity of the very crude assumptions we used to make our estimates of pH, bacteria growth and yeast fermentation.

Figure 8 shows the recipe.  We are using whole wheat, as it is widely know to be a challenge for sourdough.

Figure 15.  Equiligraph showing the relative fraction of the acid form at pH 6.9  is  at least 2  orders of magnitude smaller than the base form.  At pH=6.3, there is about 1.5 orders of magnitude difference. The first end point of a 0.7 (M) acetic acid solution is pH= 9.3.  The presence of the flour is buffering the pH change, and masking the acetic acid endpoint.

Figure 14.  Results of a quick and dirty titration, used to bracket the concentration of acetic acid in the starter. The solution tested was 2 (mL) of the liquid standing over the starter diluted to 20 (mL) . the pH probe shown in figure XXX was used. A small volume of a “standard” was added. the concentration was increased ten fold if the pH change was small.   The most probable concentration is shown in red. The equiligraph was used to identify the reaction with acetic acid as nearly complete - see figure 15.

The acetic acid titration is nearly complete by pH = 6.3, as shown in figure 15.  The equivalence point for a 0.7 (M) acetic acid solution occurs at pH=9.3. The amount of base required to go from pH 6.3 to pH 9.3 would be quite small if the flour were not present.

In Figure 14, the computed acetic acid concentration, [HAc],  accounts for the18% which is present in the base form at the beginning of the titration.

It seem reasonable to bracket the acid concentration between 0.4 and 0.7 (M).

What happens when we dilute the starter with water and fresh  flour?  We postulated that the traditional recipe would not alter the pH much and that the proposed recovery recipe would raise the pH to near 7.

We now have enough data to estimate the alkalinity of the flour. Assume 1/2 cup of 0.7 (M) acid, of which 18% is neutralized by 1/2 cup of whole wheat flour.  1/2 cup is about 125 (mL) and flour has a mass of 600 (g/L).  The alkalinity, in equivalents per gram of flour is about 2 (Equivalents /g).

The pH change for the traditional method sponge is consistent with the added alkalinity of the flour:

75 (g) * 2E-4 (Eq/g) / 0.125 (L) = 0.12 (Eq/L). 

Starting with 0.125 (L) of 0.7 (M) = 0.088 (Eq) Acid present. Reduce by 82% to get 0.072 (Eq) acid present and 0.016 (Eq) of  acetate ion.

We then  subract the flour alkalinity: 0.125 (L) * 0.12 (Eq/L) =  0.015 (Eq) .  The ratio of acid to base is now:

α0 = (0.072 -0.015) / 0.088  = 0.64   


α1 = (0.016 + 0.015)/0.88) = 0.36

The equiligraph gives a pH = 4.5 for α0 = 0.64 and CTotal = 0.7 (M). This agrees well with the measured pH = 4.6.

The pH change of the recovery sponge is also consistent, however the yet unknown acid capacity of the flour complicates the issue.

The acid fraction, α0 = 0.003 at pH = 7.3,  and the base fraction, α1 = 0.997.

Total acetate species in recovery sponge is:

0.7 (M) * 0.0025 (L) = 1.8e-3 mole, with 82% present, or 0.015 (mole), as the acid.

We added about 1.6 (g) of NaHCO3, or 1.9e-2 (mole).  This is a large excess.  The equiligraph for carbonic acid tells us that α0 = 0.09, α0 = 0.91 at pH = 7.3 and CTotal = 0.018 (M).  The acid added from the starter is about 10% of the bicarbonate added. 

The equiligraph helps us understand the measurement and gives us confidence that we are beginning to understand the system.

Now that we have a better understanding of the alkalinity of the flour, we realize that we probably do not need to add the baking soda!

Figure 17.  The initial pH of the traditional sponge measures between 4 and 5 using pH paper and pH test strips.

Figure 21. Graph of pH vs time for the two recipes. See Figure 21.

Figure 18.  The initial pH of the traditional sponge measures between 4.6 using the glass electrode pH meter.

The amount of starter shown in figure 16 is added to 1/2 cup water, 1/2 cup flour and 1/tablespoon oil. Each batch was stirred with 200 strokes to build gluten.

The pH of the dough will be measured by taking 1/4 teaspoon (~1 mL) of dough and adding 20 (mL0 of distilled water. This is sufficient quantity to test with pH paper, pH test strips and a glass electrode pH meter.

Figures 17 and 18 show the initial pH of the traditional sponge Figures 19 and 20 show the initial pH of the recovery sponge.

Figure 19. The initial pH of the recovery sponge measures about 7 using both pH paper and pH test strips.

Figure 20.  The initial pH of the recovery sponge measures 7.3 using the glass electrode.

Our earlier analysis of the effect of small dilutions on dough pH seems to be valid, as is our crude analysis of the effect of a 1 : 100 dilution and addition of a small amount of base ( NaHCO3) on the dough pH.

At this point, it appears that the equiligraph has helped us understand a complex system. The equiligraph did not solve the problem, but it certainly gave us guidance.  It is interesting that we did not directly solve any equations describing ionic equilibrium, yet we obtained results which are rigorously derived directly from the solution of those equations.

Figure 21 shows the measured pH and other observations at various stages of the process. Figure 22 is a graph of dough pH vs time. The traditional method dough has a pH that remains low over time, while the recovery method dough initially rises, then begins to drop. The rate of decrease of the two methods appears to be similar during the later stages.

Another difference between the two doughs was observed when kneading. The Traditional loaf had a very sticky dough and required an addtional half cup of flour .

Figure 24 shows how much the traditional dough stuck to the board even after the additon of extra flour. The recovery method dough had better cohesion and did not stick to any surfaces.

Figure 21. The pH changes of the traditional dough compared to the starter recovery dough. It is interesting that the pH of the recovery dough increases while the traditional dough  pH remains fairly constant during the initial stages.

Figure 25. Loaves in oven.  The Traditional loaf fell when moved into oven. This is typical behavior for a loaf made from old starter.

Figure 26.  Side view of finished bread

Figure 24. The dough made from the traditional method  was very sticky and required more flour in order to knead it. The recovery method loaf did not stick to the board.

Figure 27. Comparison of the baked bread. The traditional loaf is lon the left and the recovery loaf is on the right.  The texture of the traditional loaf suggests that some of the foam cells merged. t This loaf fell when put into the oven. The more unifor texture of the recovery loaf suggests more stable foam structure.

Figure 22.  The traditional loaf started showing bubbles preaking the surface around 1800. THis indeicates that the dough is forming an open cell foam rather than the needed closed cell foam.

The traditional method loaf fell as it was put into the oven. This is a common occurance with brad made from old starter. Figure 22 shows gas bubbles breaking the surface of the traditional loaf about 2 hours before it was ready to bake. This suggests that the dough cannot maintain a closed cell foam structure, which is consistent with the loaf falling.

Figure 23 shows that the Recovery loaf does not exhibit bubbles breaking  the surface, suggesting that this dough can maintain a closed cell foam structure.

It is interesting to note that both loaves continued to rise and that the volumes of the dough remained similar through out the process. The Recovery loaf volume lagged a bit after kneading. This suggests that the low pH of the starter is NOT affecting the yeast fermentation.

Two possibilite

Figure 25 shows the both loaves when first put into the oven. The positions are consistent with other photos, with the traditional on the left side and the recovery on the right side.

The consistency difference between the two doughs is clear. the surface textures are different, as is the shape. The traditional loaf appears to slump more than the recovery loaf.

Figure 25 shows the both loaves when first put into the oven. The positions are consistent with other photos, with the traditional on the left side and the recovery on the right side.

The consistency difference between the two doughs is clear. the surface textures are different, as is the shape. The traditional loaf appears to slump more than the recovery loaf.

Figure 26 shows the baked loaves. Figure 27 is a side view of the cut bread, with the images cropped and superimposed to provide a better comparison of the inside textures.

The traditional loaf  has a darker appearance and has a stiff texture. The crumb has non-uniform bubbles, which is consistent with an open cell foam structure.

The recovery loaf  has a lighter color and has a soft and spongey texture. The crumb is more uniform.  The appearance and texture are consistent with closed cell foam structure.

The Traditional method loaf has a very sour taste, almost too sour to eat. The texture and taste suggest that it would be best used as toast, croutons or made into zweiback.

The recovery method loaf has a mild sour taste and a good texture.  I would serve this loaf, as is, proudly to guests.

In summary:

I think we have shown that good quality bread can be made using an old sourdough starter, one which most cookbooks recommend discarding.

In short, we have been successful in modifying a very old, well established industrial process. 

We identified a problem and used the Equiligraph to help us evaluate the problem and identify system variables which could be important. The Equiligraph allowed us  to make some simplifying assumptions and put reasonable numerical boundaries on process variables. 

We then used this information to make assumptions guide the development of a process change.

We tested the proposed process change and evaluated the results, again using the equiligraph to interpret the results.

As it turns out, our initial hypothesis for the cause of the problem may not be valid, but we did validate several assumptions we made about the process: specificaly the low pH of the dough, the acid concentration of the dough and the effect of small dilution on dopugh pH.

Nonetheless, we now understand the chemistry of the process better than we did and have numerical data and the beginnings of physics based model! The equiligraph helped us use the physics of solution chemistry and the equations of state for ionic equilibrium without requiring us to directly solve any equations more detailed than mole conversions.

In retrospect, it does not seem that the original hypothesis is valid: that the high acidity of the old starter inhibits yeast fermentation.

Instead, based on the results, it appears that the use of large volumes of old starter affects the texture of the dough and ultimately causes the dough to form an open cell foam rather than a closed cell foam.

It is possible that acidity could still play a role. This seems less likely now, as the pH of the final dough was similar for both methods. Other research suggests that the gluten properties are unaffected by dough pH.

It is also possible that the ethanol content of the original starter could affect the properties of the dough. Ethanol could certainly alter the surface tension and cohesion of the dough forming the cell walls. The ethanol concentration is also significantly reduced by the 1:100 dilution.

“I often say that when you can measure what you are speaking about, and express it in numbers, you know something about it;

but when you cannot measure it, when you cannot express it in numbers, your knowledge is of a meagre and unsatisfactory kind;

it may be the beginning of knowledge, but you have scarcely, in your thoughts, advanced to the stage of science, whatever the matter may be.”

Lord Kelvin

Keywords: chemistry, sourdough, acid, base, equilibrium, equiligraph, Buffer Capacity,  alpha, End point, Equivalence point, pH, pKa, pKb, pK1,Composition, graphic solution.

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