Inactivating salmonella on alfalfa sprouting seeds and increasing seed viability using dry thermal treatments
Date
2020
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Publisher
University of Delaware
Abstract
Alfalfa sprouts, a popular food item containing many researched and proposed health benefits, are often eaten raw or lightly cooked. Though associated with healthy living, these seedlings have been implicated in over 48 outbreaks all over the world between 1996 to 2016. The seed is thought to be the primary source of contamination, so most antimicrobial treatments are done before the sprouting process. Salmonellosis is the most common result of contaminated sprouts due to the pathogen’s strong attachment to the seeds and ability to survive in low moisture conditions only to proliferate during the germination period. Physical, chemical and biological treatments have been explored with sprouts, and the Food and Drug Administration (FDA) no longer recommends a 20,000 ppm sodium hypochlorite wash as it has proven ineffective, only reducing initial populations by 2- to 3-log CFU/g. Thermal treatments are capable of inactivating pathogens inside the seed coat as opposed to only the surface, and have been shown to even increase germination when done properly. The aims of this study were to find the time and temperature combinations that would remove all pathogens (~5-log CFU/g) from the seed without negatively affecting germination, with the additional goal of increasing germination yields with the same treatment by influencing the initial water activity of the seed prior to treatment. If a 5- log CFU/g kill was unobtainable without harming germination, then the goal was to find a comparable treatment to the previously recommended chlorine wash. In this study, the chlorine wash achieved an average 2.67-log reduction in Salmonella populations. Dry heat at 65°C achieved an average 5.56-log reduction in Salmonella populations when seeds were treated for seven days with an initial water activity (aw) between 0.20-0.25. This treatment increased germination to the point where it was significantly different from the control at a 95% confidence level (α =0.05). Heat treatment at 72.5°C reached a 4.02-log reduction on seeds treated for 18 hours with an initial aw of 0.20 to 0.25. At the same temperature, seeds treated for 24 hours with a aw between 0.19 and 0.25 reached a 4.93-log reduction. Germination for seeds treated at 72.5°C for 18 or 24h were not established as significantly different from the control. However, the average mean germination of seeds treated for 18h was higher than the control, whereas seeds treated for 24h showed a reduced mean germination to about 93%. Seed incubation at 85°C resulted in a 3.94-log reduction when incubated for 4 hours, and a 4.37-log reduction when incubated for six hours with an aw around 0.23. Germination was not significantly affected for the six-hour treatment, but seeds grown after this treatment visibly did not look appealing, as a large percentage of seeds did not make it past the water uptake phase and by the end of the germination process these seeds were oversaturated and soggy. Seeds incubated at 85°C for 4 hours showed no difference in germination from the control. Adding 0.1 mL of water into the seeds one hour prior to treatment increased the Salmonella kill by an average 0.60- log CFU/g, which was more effective for the less aggressive treatments at each temperature, but these seeds also generally had significantly lower germination as a result. ☐ Looking at these results together, germination only increased for seeds treated at 65 or 72.5°C, though 85°C could achieve a kill comparable to chlorine without affecting germination and in a shorter amount of time. Seeds treated at 85°C did show the largest amount of variation in germination results based on initial water activity range, so at the expense of treatment duration, the lower temperatures would be recommended with respect to Salmonella inactivation and germination results.
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Keywords
Alfalfa, Dry heat, Germination, Seeds, Sprouts, Water activity