Propionate Resistant Yeast

Mould spoilage is a serious and costly problem for bakeries and use of preservatives is therefore an attractive means to diminish spoilage and insure food safety. Propionic acid inhibits yeast and mould growth and it is used in a variety of bakery products and bread.

Weak organic acids such as propionic, benzoic, and sorbic are used to suppress the growth of microorganisms and to lengthen usable life of bakery products. The antimicrobial activity of these weak acids is mainly dependent upon the undissociated molecules.

 Scientific studies have shown that calcium propionate, is effective to inhibit mold and yeast from bakery products at pH 4.5 when they where applied at a concentration of 0.3%. Weak organic preservatives have a poor efficacy in bakery products of high pH (like spongecakes).

 When calcium propionate is dissolved in water, it dissociates to form calcium and propionate ions. the ratio of propionate to propionic acid is dependent on pH. At lower pH the ratio of propionic acid is higher, and at higher pH it’s lower.  The inhibitory agent is propionic acid, while the propionate form is not inhibitory. Calcium propionate is an effective mold inhibitor but also reduces the fermentation rate of most bakers yeast strains.

Cultures of S. cerevisiae can be induced to adapt to weak acid stress with addition of around propionic acid. Neither of these preservatives can be degraded by S. cerevisiae such that their presence effectively provides continuous weak acid stress. Immediately following this weak acid addition the cells usually exit the cell cycle and enter a short period of stasis. Eventually, they resume growth. They are now propionate adapted, in that they will not display transient growth arrest if introduced into fresh dough containing the same levels of Calcium propionate.  

When the propionate sensitivities of several different yeast species were compared, these were, to a rough approximation, inversely proportional to rates of diffusional entry of propionate into the cells. Reducing

Diffusional entry of the acid into the cells is therefore probably a key mechanism of resistance.

The strategy adopted by S. cerevisiae, whereby resistance is conferred largely through high levels of proton and acid extrusion, Active anion extrusion at the plasma membrane can only export the weak acid as far as the periplasm. From there, undissociated acid can possibly just as readily diffuse back into the cell as out through the cell wall. Ensuring that the initial diffusion of the acid through the cell wall or membrane is much more restricted, so that lower amounts initially reach the periplasm or cytosol, should be a much better strategy for achieving resistance.

Controlled addition of propionic acid towards to the end of the commercial fermentation stage is a well-known practice to improve the resistance of the baker’s yeast to Calcium propionate.