Preserved food products are divided into two types depending on whether their pH value is higher than 4.6 or lower/equal to 4.6. Different thermal treatments are given to the two groups so differentiated; in particular, microbiological stabilization or sterilization treatments are applied depending on whether the pH values are, respectively, lower/equal to 4.6 or higher than 4.6. This is due to the different growth ability, depending on the pH value, of the pathogenic bacterium Clostridium botulinum, responsible for lethal intoxications due to neurotoxins. In fact, scientific data show that the minimum pH limiting the C. botulinum spore germination is equal to 4.6. This bacterium is therefore not capable of multiplying, in the form of spores, at lower values; moreover, the vegetative cells of C. botulinum, as well as the cells of all the other microorganisms present in food, are certainly inactivated by the microbiological stabilization heat treatment because cell resistance to heat is very low. In preserved foods, which have been given a microbiological stabilization heat treatment, with a pH lower than/equal to 4.6, the growth ability of C. botulinum and, therefore, the production of lethal neurotoxins is inhibited. The low pH value (and the high acidity) of these preserved foods is, by itself, sufficient to inhibit spore outgrowth and the growth of the afore-mentioned bacterium.
For naturally non-acid vegetable products, therefore, correct acidification with acidity correctors such as citric acid, acetic acid, lactic acid, etc. is of fundamental importance. This technological operation that also includes the continuous control of the pH value before packaging in order to ensure homogeneity. The control of preserved foods with pH values ≤ 4.6 is performed by the test method called “Microbiological stability”. According to the method, the sample is analyzed after incubation at 30°C for 14 days in order to verify the presence of spoilage microorganisms, possibly present in the product already heat treated and cooled, even at concentration of a single cell per container, regardless of the capacity of the latter. Therefore, the analysis of “microbiological stability” is aimed at verifying the presence of microorganisms capable of growing at the aforementioned pH values (depending on the product: lactic bacteria, fungi, enterobacteria, B. coagulans); it is carried out in qualitative mode, using exclusively selective media for these microorganisms. Therefore, qualitative or quantitative analyses are not carried out on general culture media such as PCA, TSA, SA, TSC, OPSP etc., since in these occurs growth of bacterial spores of the genus Bacillus and/or Clostridium, generally present in the raw materials, not inactivated by the heat treatments given to preserved acid foods in order to obtain their microbiological stability. The heat treatments imparted to preserved foods with a pH lower than/equal to 4.6, in fact, are not intended to inactivate all the spores present in the raw materials but only those capable of germinating (turning back into metabolically active cells) even at those values of pH and spoil the product. The spores not inactivated by heat treatments do not germinate and, therefore, a bacterial flora capable of spoiling the food does not grow, due to its acidity and pH values. These microbial structures remain, therefore, in such preserved foods in silent or dormant form but grow only when they come into contact with general culture media whose pH values are significantly higher than those of preserved acid foods. This analytical finding has no significance for the purpose of microbiological stability of preserved foods having pH ≤ 4.6.