1a. Objectives (from AD-416):
ARS is interested in developing effective intervention technologies during post-harvest handling and fresh-cut processing to inactivate pathogens, and/or reduce their survival and growth while maintaining produce quality and shelf-life. A team has been assembled, consisting of researchers from University of Illinois, North Carolina State University, University of Arizona, and University of California, each contributing their unique expertise to achieve the goals of the research. The proposed research is a part of an awarded NIFA grant entitled “Innovative Technologies and Process Optimization for Food Safety Risk Reduction Associated with Fresh and Fresh-cut Leafy Green Vegetables”.
1b. Approach (from AD-416):
The Cooperator has expertise in developing ultrasonic processes to improve pathogen inactivation while maintaining produce quality. Specifically, in year one: ultrasound treatment will be optimized at the bench scale, and reductions in microbial populations in wash water and on produce and impact on produce quality will be documented. In year two: ultrasound treatment with sanitizers and surfactants will be optimized at bench scale, and reductions in microbial populations in wash water and on produce and impact on produce quality will be documented. In year three: a pilot-scale unit for testing of optimized ultrasonic treatment will be completed and the efficacy of on the unit in disinfecting whole-head lettuce will be documented.
3. Progress Report:
Traditionally, processors have cut iceberg lettuce heads prior to washing. However, bacteria preferentially attach to cut edges of lettuce and are difficult to remove once attached. Beltsville scientists earlier demonstrated that washing whole Iceberg lettuce heads before cutting improved the sanitization efficacy, compared to the traditional approach of cutting the lettuce before washing. Collaborators at the University of Illinois further tested this wash sequence on pathogen reduction using ultrasound. Attenuated E. coli O157:H7 cells were spot-inoculated on the outer layer of Iceberg lettuce heads and dried in a laminar-flow fume hood for 2 hours at 22 °C. Lettuce heads were washed for two minutes with a chemical sanitizer, Tsunami (80 ppm of peroxyacetic acid), chlorine (20 ppm free chlorine), or water, in an ultrasound-equipped continuous-flow washing channel. Lettuce heads were cut into 1×1 inch pieces either before or after washing. In sanitizer-only wash tests, the E. coli count reduction for lettuce treated by washing-before-cutting was higher by 0.79 and 0.80 logs per g in chlorine and peroxyacetic wash, respectively, compared to the traditional cutting-before-washing process. When ultrasound was used in combination with chemical sanitizer, a further reduction in the E. coli population of 0.68 and 0.37 logs per g (again, for chlorine and peroxyacetic acid, respectively) was achieved by the washing-before-cutting treatment. Using both treatment, the total reduction was, 2.43 and 2.24 log per g for the chlorine and peroxyacetic washes, respectively. Studies were also conducted to evaluate the effect of surface area on carrot washing efficacy. Carrots were cut with different surface areas, i.e. whole baby carrots, carrot sticks, and shredded carrots. The decay of free chlorine and peroxyacetic acid in the three carrot surface area treatments was monitored. Chlorine and peroxyacetic acid concentrations decreased linearly with increase in carrot surface area from whole carrots to carrot sticks, to shredded carrots. After 1 min contact with shredded carrots, a nearly 60 % reduction in chlorine and 20 % reduction in peroxyacetic acid concentrations was recorded. When inoculated shredded and whole baby carrots were washed with chlorine under the same washing conditions, the E. coli population reduction on shredded carrots (0.73 log per cm2) was significantly lower than that on whole baby carrots (1.45 log per cm2), correlating well with the chlorine concentration reduction in shredded carrots.