Efficacy of weak acid preservatives on spoilage fungi of bakery products

Organic acids and their salts are usually the first choice in the bread industry to restrict fungal spoilage, but their efficacy is pH-dependent and spoilage by fungi remains as a common threat. Therefore, this study aimed to evaluate the susceptibility of spoilage fungi of bakery products to acetic, sorbic, and propionic acids at different pH. Penicillium roqueforti, Penicilium paneum, Aspergillus pseudoglaucus, Aspergillus montevidensis and Hyphopichia burtonii strains isolated from spoiled products had their minimum inhibitory concentration (MIC) defined by macrodilution. The concentrations tested were: (i) sorbic acid up to 32 mM; (ii) propionic acid up to 1024 mM and (iii) acetic acid up to 800 mM with pH adjusted in 4.5, 5.0, 5.0 and 6.0 after setting the agent concentration. The lowest MICs for all agents were obtained at pH 4.5, usually doubling with every 0.5 pH increase. P. roqueforti strains isolated from spoiled products were the most resistant to all tested preservatives; while strains of the related species P. paneum, showed similar tolerance to acetic and propionic acids but was double more susceptible to sorbic acid. Strains of A. pseudoglaucus and A. montevidensis were indistinctly susceptible to the preservatives and were the most susceptible species to propionic and acetic acids. H. burtonii strains demonstrated the most variable behaviour in comparison to the other strains being the most susceptible to sorbic acid, were like Aspergillus strains regarding propionic acid, but tolerate well acetic acid. Propionic acid concentrations usually allowed in baked goods are lower than the concentrations required to inhibit the most tolerant isolates tested in this study. The same is true for sorbic acid at higher pH levels. Spoilage species of bakery ware presents a distinct susceptibility profile to the preservatives commonly used in this sector, but the high tolerance observed is a cause of concern.