Minimizing Salmonella Contamination in Sprouts by Controlling the Germination Temperature

Introduction: Since 1996, contaminated sprouts have been linked to at least 30 outbreaks and over 2,000 cases of illness in the U.S. Seed is often the source of contamination, but sprouts pose a particular concern as the conditions that promote germination of the seed also promote the growth of pathogens.  FDA has recommended that seeds destined for sprout production be disinfected with one or more treatments such as 20,000 ppm of calcium hypochlorite, Ca(OCl)₂. But this treatment is unable to completely eliminate pathogens on seeds and surviving ones can grow to significant numbers during sprouting. Thus, controlling the germination conditions such that proliferation of pathogens, if present, can be prevented or minimized is a crucial step in the overall approach to reduce microbial food safety hazards in sprouts. 

Purpose: This study examined how the proliferation of Salmonella is affected by the temperature used for sprouting, and how this temperature effect is influenced by the pathogen load, seed lot used, and seed disinfection with 20,000 ppm Ca(OCl)₂.

Methods: Alfalfa seeds artificially inoculated with ~3 log CFU/g of Salmonella were used to prepare the contaminated seeds for sprouting. 200 g of seeds spiked with different levels (0.1, 1, or 10 % by weight) of inoculated seeds were allowed to germinate in glass jars for 3 days at 10, 20, and 30 °C.  The same growth studies were performed on seeds treated with 20,000 ppm Ca(OCl)₂ for 15 min. Sprout samples were taken from each jar daily and analyzed for the level of Salmonella, either by plating on XLD plates or by the three-tube most probable number method as described in the FDA BAM.

Results: The level of Salmonella increased during sprouting at all three temperatures and reached the highest level at 48 h. Sprouting at a lower temperature resulted in a smaller increase in Salmonella counts. At a 1% spiking level, the level of Salmonella increased by approximately 1.5, 4, or 6 log CFU/g in sprouts grown at 10, 20, or 30 °C, respectively. Similar increases were observed during germination of seeds inoculated at a 0.1% level. Using a seedlot with a higher background microflora (~ 2 log higher), the level of Salmonella reached at 48 h in sprouts grown at 20 and 30 °C was slightly lower than those observed in sprouts grown from seeds with a lower level of background microflora. Treatment with Ca(OCl)₂ caused an initial reduction in Salmonella counts on seeds and a delay in the proliferation of Salmonella where the highest level was observed at 72 h of sprouting.

Significance: Re-growth of Salmonella occurred during germination of seeds that have been treated with 20,000 ppm of Ca(OCl)₂.  Lowering the sprouting temperature could reduce the extent of pathogen proliferation.