The Salt, pH and Thermotolerance of A Novel Nonstarter Lactic Acid Bacterium That Might Be Associated With Slit Defect In Ripened Cheddar Cheese
An obligate heterofermentative lactic acid bacteria, Lactobacillus wasatchii WDC04 (WDC04), isolated from an aged Cheddar cheese, was studied for salt, pH and thermotolerance. We investigated the pH and salt tolerance of WDC04 in MRS+1.5% Ribose (MRS-R) under conditions that mimic Cheddar cheese ripening. In addition, the thermotolerance of WDC04 was tested in 2% milk to estimate its pasteurization tolerance. WDC04 was inoculated in MRS-R at two different pH levels (5.2 or 6.5), each containing 0, 1, 2, 3, 4, or 5% salt-in-moisture (S/M) levels (w/w). Growth was monitored by OD600 measurements every 8 h under anaerobic conditions at 23°C for 60 h. After 48 h, an OD600 of 2.0 was reached in all media except for 5% S/M at pH 5.2 which had an OD600 of 1.75. At pH 6.5, WDC04 growth rates were similar at S/M levels of 0, 1, 2, and 3% after 24 hr. Growth rates for 0, 1, and 2% S/M levels at pH 5.2 after 24 h were also similar, while growth rates at S/M levels of 4, and 5% at either pH were slower. Two different thermotolerance tests were performed. Milks containing ~7x106 CFU/ml WDC04, was heated at 72°C for 15 s followed by a cooling by placing in a water bath at 31°C (set temperature for cheesemaking) for 2 h. In the second heat treatment, WDC04 inoculated milks were heated at 63°C for 30 min with samples collected at 0, 15 and 30 min intervals followed by incubation at 31°C for 2 h. Samples were plated on MRS-R agar in triplicate. There was a 4-log reduction of WDC04 after the 72°C for 15 s heat treatment with 9.2x101 CFU/ml after cooling to 31°C. However, there were no detectable colonies (<102CFU/ml) when heated for 30 min at 63°C. This suggests that WDC04 is maybe sufficiently thermoduric for some cells to survive HTST pasteurization but perhaps not LTLT pasteurization. It also appears that WDC04 is able to grow in the same environment that occurs during Cheddar cheese ripening, high S/M (2.5 to 4.5%) and low pH. The ability to survive pasteurization and grow under cheese ripening conditions allow WDC04 to be considered a NSLAB, placing it in a position to be involved with late gas production and slit defect in ripened Cheddar cheese.
Keywords: Nonstarter lactic acid bacteria, salt, pH, and thermotolerance