Heating temperature and water activity of alfalfa seeds affect thermal inactivation of Salmonella and maintaining seed viability

Abstract

Sprouts have been involved in many outbreaks of salmonellosis where seeds were identified as the likely source of contamination. This study aimed to develop an effective heat treatment that could achieve a >5-log reduction of Salmonella inoculated on alfalfa seeds while maintaining seed viability and vigor. Effects of seeds' water activity (aw) and heat treatment temperature on Salmonella inactivation and seed viability were determined. Alfalfa seeds were dip-inoculated with a four-strain Salmonella cocktail and dried to aw of 0.05–0.20. The inoculated seeds were then placed in sealed glass tubes and heated at 65.9, 71.0, and 76.6 °C for up to 180 h. Increasing aw of seeds greatly improved thermal inactivation of Salmonella. For example, to achieve a >5-log reduction of Salmonella on seeds, treatment times of 140 and 60 h at 71.0 °C were required for aw of 0.1 and 0.2, respectively. Treatment temperature also greatly affected inactivation of Salmonella on alfalfa seeds. For example, to achieve a >5-log reduction of Salmonella on seeds with aw of 0.2, treatment times of 180 and 60 h were required for temperatures of 65.9 and 71.0 °C, respectively. Seeds' aw was critical for preserving seed viability. When seeds were treated at 71.0 °C for 60 h, increasing aw from 0.1 to 0.2 decreased the sprout yield ratio from 103.9 % to 73.7 %. Treatment of seeds with aw of 0.1 at 71.0 °C was found to be optimum for achieving a desirable Salmonella inactivation level while maintaining seed viability, resulting in 4.2 and 6.0 log reductions of Salmonella and yield ratios of 100.7 % and 96.1 % after 100- and 140-h treatments, respectively. This optimum heat treatment was compared with the traditional 20,000-ppm chlorine wash in terms of Salmonella inactivation and preservation of seed viability and found to be a far superior disinfection method. The chlorine treatment achieved 1.8 and 2.0 log reductions of Salmonella and yield ratios of 70.9 % and 65.1 % after 15- and 20-min treatments, respectively.