Purpose: This study aimed to compare the thermal inactivation kinetics of Listeria monocytogenes and Salmonella enterica in buffer and spinach.
Methods: Five-strain cocktails of each microorganism in phosphate buffered saline in 2 ml vials or blended spinach (1:1 (w/v) with water) vacuum-sealed in polyethylene-nylon bags were exposed to 56, 58 and 60°C in a circulating water bath with temperatures monitored by thermocouples. Following heating for appropriate times, samples were removed, placed in an ice bath, then serially diluted and spread-plated on tryptic soy yeast extract agar plates. Colony forming units (CFU) were recorded after 24h at 37°C (Salmonella) and 48h at 32°C (Listeria). Each experiment was replicated thrice. D- and z-values were calculated using a first-order model.
Results: D-values for Listeria in PBS were 4.42 ± 0.16, 1.45 ± 0.22 and 0.58 ± 0.04 min at 56°C, 58°C and 60°C, respectively while for Salmonella they were 2.04 ± 0.24, 0.55 ± 0.03, 0.21 ± 0.01 min, respectively. D-values were higher in spinach: 11.77 ± 2.18, 4.48 ± 0.92 and 1.22 ± 0.12 min at 56°C, 58°C and 60°C, respectively, for Listeria and 3.51 ± 0.06, 1.19 ± 0.02 and 0.47 ± 0.06 min, respectively, for Salmonella. z-values were 4.52°C and 4.06°C in PBS and 4.06°C and 4.59°C in spinach for Listeria and Salmonella, respectively.
Significance: These results indicate that heating medium affects thermal inactivation kinetics. These data will potentially aid the food industry in developing appropriate mild heating processes, such as blanching or microwave processes, to eliminate foodborne pathogens.