Effect of dry heat treatment on inactivation of salmonella and germination of alfalfa, mung bean, radish, and red clover seeds
Date
2022
Authors
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Journal ISSN
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Publisher
University of Delaware
Abstract
Alfalfa, mung bean, radish, and red clover are world’s main sprout vegetables production categories and are consumed by a huge population both in the US and around the world. Due to their good taste and many health benefits, there is a growing trend to consume sprouts. However, sprouts are often associated with food-borne outbreaks. Salmonella is the major pathogenic bacteria associated with sprout outbreaks, due to their strong attachment to seeds and proliferation under drought environment. Seeds could be contaminated during growing, harvesting, transportation or distribution and the initial contamination load is usually low. During seeds’ sprouting process, temperature, humidity, pH, and available nutrients in the sprouting environment provide an ideal warm environment for bacteria to multiply and the final pathogenic bacteria population could reach 6-7 log. Without cooking treatment, consumption of raw sprouts can easily make people sick and cause food safety issues. Therefore, effective decontamination methods are needed to treat seeds before sprouting to prevent pathogen contamination. Currently the Food and Drug Administration (FDA) no longer recommends using 20,000 hypochlorite chemical wash to decontaminate seeds because it was proven to be very inefficient; only achieve 2-3 log reduction on pathogens. The new 2017 FDA sprout production guidelines recommends using physical treatment or combined treatment methods to achieve 5 log reduction of pathogens. Dry heat treatment is considered as a physical treatment method and heat can penetrate the whole seed. Some previous research already showed the high effectiveness of dry heat treatment on seed decontamination but not too many studies on seed germination after treatment. ☐ In this research, different dry heat temperature, treatment time, and seeds’ water activity (aw) were precisely controlled to develop the optimal dry heat treatment conditions for alfalfa, mung bean, radish and red clover seeds. Both microbial log reduction and germination results were monitored during the treatment. The optimal conditions are considered to reach 5 log reduction of Salmonella population as well as maintain seeds germination after the dry heat treatment. Microbial and germination results after 20,000 ppm sodium hypochlorite wash for 15 or 20 minutes were also studied and compared to dry heat treatment results. Our results showed that when alfalfa seeds with aw of 0.1 were dry heat treated at 71.0°C for 140 hours and 120 hours, Salmonella inactivation could reach 6.03 log and 4.53 log, respectively, while germination weight yield ratio could maintain 96.42% and 101.42%, respectively. When mung bean seeds with aw of 0.2 were dry heat treated at 71.0°C for 50 hours, Salmonella inactivation could reach 5.14 log, while germination weight yield ratio could maintain 98.60%. When radish seeds with aw of 0.2 were dry heat treated at 71.0°C for 50 hours, Salmonella inactivation could reach 3.80 log, while germination weight yield ratio could maintain 61.03%. When red clover seeds with aw of 0.2 were dry heat treated at 71.0°C for 50 hours, Salmonella inactivation could reach 3.36 log, while germination weight yield ratio could maintain 66.73%. ☐ Taking together, dry heat treatment has a very promising application on seeds decontamination. Our results showed that lower water activity level provided a better overall trade-off on each seed. Higher treatment temp could decrease seeds’ viability, but could also shorten treatment time. Therefore, a proper treatment temperature is needed for each seed. Microbial results were similar among 4 seeds, while germination results varied per seeds. Germination results behaved well on alfalfa and mung bean, but not so well on radish and red clover. 71.0°C/aw 0.1/140h is considered as the most optimal treatment condition for alfalfa, while 71.0°C/aw 0.2/50h is considered as the most optimal treatment condition for mung bean, radish, red clover. Dry heat treatment showed better results for microbial/germination overall trade-off on alfalfa and mung bean, but not as ideal as for radish and red clover. However, even for radish and red clover, dry heat treatment method is still considered superior to the results from chlorine wash treatment groups. ☐ From industry perspective, required processing equipment for dry heat treatment is very simple and low cost. Dry heat treatment technology developed in this study could be easily scaled up for large scale decontamination of seeds and easily applied to the sprout industry. Despite the cost of equipment investment, this new technology could also cut down the cost for sprouting yield loss and outbreak recall for sprout producers caused by using inefficient seeds decontamination methods. Overall, dry heat treatment is a very promising decontamination technology for sprouting seeds. It is recommended for sprout producers to use dry heat treatment technology for post-harvest treatment before sprout growth.
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Keywords
Alfalfa, Mung bean, Radish, Red clover, Dry heat treatment, Salmonella, Sprout yield, Viability, Water activity