Lactobacillus bulgaricus ATCC 11842 as a Potential Surrogate for Inactivation Studies at ≥ 70°C

Introduction: Pasteurization is an effective method of inactivating bacterial pathogens in a variety of products. However, it has been shown that hepatitis A virus (HAV) has greater thermal resistance than bacterial pathogens with D_72°C of ca. 0.9 min in buffer. Thus, the most common surrogate used for pasteurization studies, Listeria innocua, is not resistant enough for validation studies.

Purpose: The purpose of this study was to determine the thermal inactivation kinetics (D- and z-values) of Lactobacillus bulgaricus in milk as a potential surrogate for HAV. Additionally, the effect of sublethal heat on increasing thermotolerance of L. bulgaricus was also investigated.

Methods: Thermal inactivation of L. bulgaricus ATCC 11842 was performed in 2% fat ultrahigh temperature treated milk at 65, 67, and 70°C for 0 - 60 min. Milk was inoculated by with 1.5 x 10⁷ CFU/ml of L. bulgaricus and placed in 2 ml vials. For the effect of sublethal heating, cells were treated at 45 or 50°C for 30 min prior to inoculation of milk. Following heating for appropriate times, vials were placed in an ice bath, serially diluted and spread-plated onto MRS agar plates. Plates were incubated anaerobically for 48 h at 37°C and colonies enumerated. Each treatment was replicated thrice and D- and z-values were determined using a first-order model.

Results: D-values for L. bulgaricus in UHT milk were 9.98 ± 1.0, 2.68 ± 0.16 and 0.45 ± 0.08 min, at 65, 67, and 70°C, respectively, with a z-value of 3.7°C. After sublethal heating, D_70˚Cfor the 45 or 50°C treatment were 0.34 ± 0.05 or 0.48 ± 0.1 min, respectively.

Significance: Thus, while the heat resistance of L. bulgaricus is lower than HAV, it has potential value in validation studies for inactivation of HAV at high cell concentrations. Sublethal heating did not cause the D-value to increase significantly.