Lactic acid and pH as indicators of spoilage for vacuum-packed cooked ring sausages

Lactic acid production and pH changes of 206 vacuum-packed cooked ring sausages stored at 2, 4 and 12°C from 21 different production runs were monitored as a function of time and of microbial growth. The total lactic acid concentrations and pH values were first at a constant level, starting to increase sharply after the lactobacilli count reached about 5 × 10⁷ or 6 × 10⁷ cfu/g, respectively. The lactic acid and pH changes as a function of the lactobacilli count were similar at 4 and 12° C. The sharp increase at high lactobacilli counts was observed in both L-lactic acid and D-lactic acid. The variation was lesser and the increase greater in D-lactic acid formation than in L-lactic acid. Above a level of 3–4 mg lactic acid/g most of the samples were deemed unfit. The pH started to decrease from a level of approx. 6.3; below 5.8–5.9 the samples were deemed unfit. The lowest pH value observed was 4.58. Both a high lactic acid content and a low pH indicated that the sausage was spoiled. These changes, however, took place at later stages of storage, and do not give information about the early phase of spoilage.