Control of water activity and thermal processing temperature to preserve seed viability and vigor while effectively killing Salmonella

Abstract

In the past three decades, numerous outbreaks linked to seed sprouts have occurred in the United States and many other countries. In most of the sprout-related outbreaks, seeds were identified as the most likely source of contamination. The goal of this study was to develop a heat treatment that could effectively inactivate Salmonella on seeds while preserving their viability and vigor. Mung bean, red clover, and radish seeds were inoculated with a cocktail of 4 Salmonella strains and conditioned to aw of 0.10–0.33. The inoculated seeds were heated at 71.0, 76.6, and 80.6 °C for up to 160 h. Thermal inactivation of Salmonella was greatly improved by increasing seeds' aw. To achieve >5 log reductions of Salmonella on mung bean seeds at 76.6 °C, increasing aw from 0.20 to 0.33 reduced the heat treatment time by 18 h. However, this increase in aw made them less tolerant to thermal treatment; resulting in a 35.1% reduction in sprout yield ratio. Although increasing heating temperature from 71.0 to 80.6 °C improved Salmonella inactivation efficiency, it drastically reduced mung bean seeds’ survival. The aw of 0.2 and heating temperature of 71.0 °C were identified as the optimum processing conditions for mung bean seeds. Heat treatment using these optimum conditions for 40 and 50 h resulted in 4.5 and 5.1 log reductions of Salmonella and sprout yield ratios of 100.8% and 98.4%, respectively. However, those optimum processing conditions did not result in satisfactory sprout yields when they were applied to red clover and radish seeds. A wash treatment of mung bean, red clover, and radish seeds with 20,000 ppm of sodium hypochlorite, as recommended by a former FDA guideline, was much less effective for seeds decontamination than the heat treatment, resulting in Salmonella population reductions ranged from 1.3 to 1.5 logs. Chlorine wash also negatively affected seed viability, reducing the sprout yield ratio by up to 35%. Compared to the chlorine wash, the heat treatment developed in this study achieved much better inactivation of Salmonella and better preservation of seed viability, especially for mung bean seeds.