Minimizing Salmonella Contamination in Sprouts by Controlling the Irrigation Conditions during Germination

Introduction: Sprouts pose a special food safety concern as low levels of pathogens present on seeds can multiply and reach high numbers during germination. The FDA has recommended that seeds be disinfected with one or more treatments such as 20,000 ppm of calcium hypochlorite, Ca(OCl)₂, before sprouting. 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 to prevent or minimize the proliferation of pathogens, if present, is an important part of the overall strategy to reduce microbial hazards in sprouts. 

Purpose: This study examined how the proliferation of Salmonella is affected by varying frequencies of irrigation during germination of seeds that have been either treated or not treated with 20,000 ppm of Ca(OCl)₂ prior to sprouting.

Methods: Two hundred grams of alfalfa seeds spiked with 2 g (or 1 %) of inoculated seeds (~1 log CFU/g of Salmonella) were allowed to germinate in glass jars or an automatic sprouter (EasyGreen®) for 5 days at room temperature. The sprouts were irrigated with either sterile tap water or chlorinated water (containing 100 - 500 ppm of calcium hypochlorite, Ca(OCl)₂) at varying frequencies (once every 1, 2, 4 or 24 h). The same growth studies were performed on seeds treated with 20,000 ppm Ca(OCl)₂ for 15 min. Sprout samples were taken daily and analyzed for the level of Salmonella using the three-tube most probable number method as described in the FDA BAM.

Results: Using untreated seeds, the level of Salmonella in sprouts changed from an increase of > 5 log MPN/g to a decrease of ~ 1 log during 3 days of sprouting when the frequency of irrigation with sterile tap water increased from once every 24 h to once every hour for 15 min. Seed treatment with Ca(OCl)₂ lowered the level of pathogen to an undetectable level (< -2.5 log MPN/g) at the start of sprouting. Salmonella re-grew from treated seeds by > 1 log when sprouts were irrigated with water once every 24 h but remained undetected when sprouts were irrigated once every hour for 15 min. Irrigation with chlorinated water further inhibited Salmonella re-growth but resulted in a lower yield of sprouts. Overall, seed treatment combined with frequent irrigation with tap water or chlorinated water (e.g., once every hour) can maintain the level of Salmonella at an undetectable level during sprouting.

Significance: Controlling irrigation conditions provides an additional means to reduce the proliferation of Salmonella during sprouting, thus minimizing the microbial hazards in sprouts.