Food Spoilage by Bacilli: Combined Effects of pH, aw and Storage Temperature on Spore Germination and Growth in Cultural Broth Added with Solutes and Organic Acids

Aims: The control of Bacillus spores plays an important role in quality assurance and safety of processed foods. Their growth can be avoided by either sterilization or pasteurization in combination with a lowering of pH (≤ 4.6) or water activity (aw < 0.90) or with the combined action of both parameters (spore germination inhibition). In some food products, the reduction of pH to values ​​lower than 4.6 or a thermal treatment of sterilization can lead to nutritional and organoleptic changes, that are not always satisfactory for the consumer. The aim of this work was to produce data for a range of spoilage bacilli at a variety of pH and aw values and to monitor the potential outgrowth of these spores when stored at different temperatures. This study focused on the influence of the type of acid or solute used.

Place and Duration of Study: Quality, Safety and Pre-Industrialization Area, Stazione Sperimentale per l’Industria delle Conserve Alimentari-SSICA, Parma, Italy between March 2018 and November 2019.

Methodology: B. sublitilis, B. licheniformis, B. cereus spore’s growth was studied in broth under the following conditions: aw (0.90 to 0.93), temperature (6, 12 and 30°C) and pH (4.9 to 5.5). Selected water activities were reached by the addition of appropriate amount of sodium chloride, sucrose or glycerol; pH was adjusted to various values with lactic acid, acetic acid or citric acid.

Results: A combination of aw 0.92, with glycerol or sodium chloride and pH 5.1 inhibits the germination of all strains spores. B. subtilis was the species able to grow in the less favorable conditions. Acetic acid proved the strongest antimicrobial action; sodium chloride resulted more effective in inhibiting growth when used in combination with acids tested.

Conclusion: The results achieved can be useful for the food industry, identifying the conditions that could preserve the product by means of a less drastic heat treatment preserving as much as possible the natural organoleptic characteristics.