Surdough fermentation (I part)
The study highlights the action of sourdough both in hydrolyzing (breaking) the proteins rich in proline (gliadin) involved in activating the human immune system both in the hydrolyser gluten (favoring digestibility) and ,above all, the high glutenins molecular weight.
Sourdough and its potential for degradation of gluten
Sourdough is produced using a culture of lactobacillus, frequently in combination with yeast. Sourdough is the oldest method for leavening bread and is still used for some applications. For example, in making bread from rye, perhaps because the dough made from rye flour needs a low pH to be appropriate for baking (Arendt et al., 2007). In comparison with yeast-treated doughs for wheat- or rye-based breads, sourdough produces a distinctively tangy or sour taste, mainly as a result of lactic acid produced by the lactobacilli. Moreover, during sourdough fermentation, proteolysis provides compounds that are precursors for the aroma volatiles and amino acids which are converted by microbes to compounds which are precursors of flavours (Gänzle et al., 2008). Traditionally sourdough is added as an ingredient to unmodified flour of wheat or rye for breadmaking. However, some authors (Rizzello et al., 2007) have proposed sourdough as the major ingredient and the only source of proteins for making gluten-free bread.
Mechanisms of degradation of gluten in sourdough
Lactobacilli produce a complex system of peptidases, including proline-specific peptidases (Gerez et al., 2008). However, no single strain produces the whole spectrum of peptidases required for hydrolysis of the proline-rich proteins involved in CD (Gobbetti et al., 2007). Therefore, it is necessary to consider the capacity to hydrolyse wheat prolamins when selecting lactobacilli for sourdough (Di Cagno et al., 2004; Rizzello et al., 2007). Thus, a pool of Lactobacillus sanfranciscensis, L alimentarius, L. brevis and L. hilgardis, was added as a starter for sourdough of wheat flour. The ethanolic extract from such sourdough reduced the activation of peripheral blood mononuclear cells from CD patients, compared to the activation that occurred when cells were challenged by native prolamins (Rizzello et al., 2007). Fig. 1B summarises the effect of high proteolysis due to sourdough.
Proteolysis by sourdough is not only due to bacterial peptidases but also to the grain enzymes, with the activities of endogenous acidic enzymes, such as aspartic peptidases and carboxypeptidases in wheat and rye flours, being activated under the fermentation conditions (Gänzle et al., 2008; Wieser et al., 2008). Another important effect of sourdough fermentation is to disrupt the gluten protein network. The highest molecular weight proteins in gluten are glutenins which are polymers stabilised by disulphide bonds. When glutenins are partially hydrolysed, the depolymerisation and solubilisation of the polymers occurs (Thiele et al., 2004). In addition, glutathione is an endogenous reducing agent in dough that can cleave disulphide bonds particularly when the pH is slightly acidic as during the first hours of sourdough fermentation (Grosch and Wieaser, 1999; Wieser et al., 2008). Furthermore, the activity of glutathione reductase is increased due to the effect of the lactobacilli on the redox potential (Jänsch et al., 2007). Finally, proline-rich polypeptides released by disruption of the gluten network, are exposed to the action of proline-specific peptidases from lactobacilli. (Trends in wheat technology and modification of gluten proteins for dietary treatment of coeliac disease patients . F Cabrera-Chávez, AM Calderón de la Barca* Coordinación de Nutrición. Centro de Investigación en Alimentación y Desarrollo, A. C. Carretera a la Victoria Km 0.6 P. O. Box 1735. Hermosillo 83000, Mexico)