The Fresh Loaf

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Common acids

mwilson's picture
mwilson

Common acids

 

Acid

     
H+ ionsNameBase FormulapKa1pKa2pKa3pH of 1mMMolar mass (g/mol)
mono-LacticC3H6O33.86  3.5190
mono-AceticC2H4O24.76  3.9160
di-TartaricC4H6O62.994.40 3.18150
di-MalicC4H6O53.405.20 3.33134
di-SuccinicC4H6O44.215.64 3.65118
tri-CitricC6H8O73.134.766.343.24192

 

Lactic and acetic acids - by products of heterolactic fermentation e.g., sourdough cultures, pickles, cheese.

Lactic acid - Dairy fermentations e.g., cheese, sour milk, yoghurt, silage.

Acetic acid - vinegar, pickles, kombucha.

Malic, citric and tartaric - common fruit acids. Malic acid and citric are the most common major fruit acids. Tartaric is the principal acid of grapes and raisins. Citric is the major acid in citrus fruits.

Succinic acid - byproduct of yeast fermentation.

 

What effects do lactic and acetic acids have on dough? - both acids show a tightening effect on dough, or put another way, both acids can be shown to decrease extensibility of doughs. However, while lactic acid increases elasticity, acetic acid tends to increase resistance and make the gluten short and hard. The effect of the latter can cause gluten to rupture during dough handling or even as it rises.

HeiHei29er's picture
HeiHei29er

Is there a point where pH begins to effect gluten structure regardless of enzyme activity?  Do any of these get us there?  I’m specifically looking at tartaric and it being dominant in raisins.  A few times I’ve had doughs get really slack when I use a lot of RYW.  I’m wondering if a different fruit source might help alleviate that.  Maybe green apples or blueberries?

mwilson's picture
mwilson

Hi Troy.

Yes, and yes would be the short answer with regard to the effect on gluten structure. All proteins are affected by pH and more broadly ionic interactions this being because proteins have dual charged sites (carboxylic and amino groups at their terminals) and so, as the environment changes, so do they (Tip: search for isoelectric point to learn more).

Gluten is however quite a complex in its makeup, structure and interaction with other flour components.

Gluten as you probably know is formed via two main proteins, glutenin and gliadin. Each of these proteins are defined differently according to their type. Glutenin is polymeric and is bracketed into HMW (high molecular weight) and LMW (low molecular weight) molecules. While gliadin is monomeric and has been defined into alpha, beta, gamma and omega molecular structures. And there are further layers of classes, such as whether they are sulfhydryl (thiol) containing...

And furthermore the structure of gluten is complexed by covalent, ionic and hydrogen chemical bonding interactions.

 

Gluten in an environment with pH lower than its native incurs swelling and this is maximised at pH 4.2 thus its takes on the most amount of water at this pH.

 

Let's look at the effect of pH on rheology.

SampleStabilityDev TimeAbsorption
Control 13.52.557.4
pH 6.1 12.02.056.6
pH 5.3 8.52.555.8
pH 5.0 7.02.054.5
pH 4.4 7.06.052.3
pH 3.9 6.52.546.8

 

 

With the farinograph, a key point of measure of interest is the stability. This figure roughly tells you how strong the flour is but also its tolerance to mixing and therefore fermentation capacity.

 

As the pH lowers so does the stability value (measured in minutes).

 

A lower pH than the native essentially means poorer performance. But that is one viewpoint. It is clear that there are further interactions of acidity such as coagulation and redox potential, and the types of acid involved.

 

I'm not a YW baker, so the practicalities I can't advise on but certainly I can relate having dabbled with it in the past and yes I too noticed considerable slackening (probably enzymatic proteolysis) and much ethyl acetate odour (often a result of stressed yeast), don't know if you experienced that too...

 

Of those acids listed tartaric is the strongest, meaning smaller doses have a greater influence on lowering the pH.

 

Regards,

Michael

 

(Apologies if my response is a bit scrappy, I am suffering vision problems and finding it hard to focus, so I might not be as succinct as I usually like to be)