Physiological response ofPectinatus frisingensis,a beer spoilage bacterium,to mild heat treatments

Abstract

Genus Pectinatus is strictly anaerobic bacteria described as a new beer spoilage flora. The physiological response of Pectinatus frisingensis to increasing heat treatments has been studied. Cell death occurred at temperatures higher than 50°C and increased with time. During heat treatment at 50°C, a potassium efflux of more than 50% of the internal potassium was measured at pH 6.2 in starving bacteria, whereas a small transient potassium efflux was measured with a similar 50°C treatment in energized cell suspensions. At beer pH values (pH 4.0), potassium content of P. frisingensis cells was not changed by a moderate heat treatment. Internal pH values of cells were only slightly (0.1 pH unit) decreased upon heat treatments. In contrast, membrane potential value was lowered by a heat treatment at pH 6.2 in deenergized cells, while only a transient decrease of delta was measured with glucose in the medium. A moderate heat treatment at 50°C had no effect on the membrane potential value at pH 4.0, even after 1 h of treatment. In addition, compared with a high level of adenylate energy charge (AEC) measured in energized cell suspensions, an AEC of 0.7 was routinely measured in starving cell suspensions. Moderate heat treatments at pH 4.0 lowered the AEC of cells to 0.6. The physiological response of P. frisingensis to mild heat treatments demonstrated a significant ability of the cell to maintain internal homeostasis at pH conditions encountered in beer.Key words: Pectinatus, thermal death, beer spoilage, homeostasis.