Physiology and Biochemistry of Sourdough Yeasts

The cereal dough is a dynamic system which is characterised by continuous changes in nutrients availability and physic-chemical condition changes. Depending on the type of flour and bread making technology, starvation conditions can be also envisaged. The imbalance between yeast consumption and starch hydrolysis might lead to the rapid depletion of soluble carbohydrates. Overall, microbial starvation induces a quiescent state whose length is conditioned by the presence of the limiting factors. Yeast responds to a changing environment not with a small adjustment in a key control points but with the coherent transcriptional relation as a large set of genes. During fermentation of sourdough yeasts cells may encounter different environmental states. Co-fermentation with lactic acid bacteria and yeasts determines environmental fluctuations not only of availability of nutrients, but also of organic acids concentration, decrease of pH and changes on the texture profile. Maintaining optimal functionality in the presence of such external variability is a central evolutionary constraint. The exposure of microbial cells to stressful and fluctuating conditions during fermentation involves a broad transcriptional response with many induced or repressed genes. The selective pressure exerted by the environmental conditions encountered by yeast cells during sourdough fermentation, accounts for the consolidated dominance of selected yeast species, such as in particular C. milleri and S. cerevisiae. The nutrient availability or limitations are likely the factors that modulate the microbial ecology of sourdough. However, within the sourdough ecosystem there are numerous mechanisms whereby one species may influence the growth of another species. The chapter describe the stress response, of those species, to nutrient availability (starvation), DY, pH (acid stress), presence of sugars, salts and polysaccharides (osmotic stress), oxygen (oxidative stress), temperature fluctuations (heat shock and cold stress) and interactions between lactic acid bacteria and yeasts (e.g., S. cerevisiae, C. milleri and L. sanfranciscensis), and between yeasts (e.g., S. cerevisiae and C. milleri). Moreover, both environmental process parameters and the interaction with lactic acid bacteria affect the metabolism of yeast in terms of fusel alcohols and namely branched chain amino acids metabolites, carbonyl compounds and unsaturated fatty acids oxidation products and induce also the inter-species signalling molecules production. An overview of baker’s yeast in bread making industry is also included. Traditional cultivation methods in combination with phenotypic and genotypic identification adopted to characterize the yeasts of ripe dough revealed the presence of more then 2, 3 species belonging especially to the genera Saccharomyces and Candida