Abstract

In the United States and most everywhere in the world, the leading cause of health threats, including hospitalizations and deaths related to foodborne illnesses, are caused mostly by nontyphoidal Salmonella spp., Escherichia coli and Listeria monocytogenes. In the nut industry, Salmonella entiritidis has been associated with outbreaks on almonds. Due to these outbreaks, regulatory standards have been established requiring that almonds delivered in North America for human consumption must have sufficient sanitation processing to achieve a minimum of 4 log10 CFU/g reductions of Salmonella. Today, and with existing technologies, this requirement has not been achieved efficiently, reliably, and without quality deterioration. However, this study identified and studied a potential solution based upon the use of an additive combination of radiofrequency (RF) heating (a thermal process) with the rapid and transient application of alternative surface disinfectants (SD). It was also assumed that this new approach would be capable of achieving at least >4 log10 CFU/g reduction levels. This combination of thermal RF and chemical disinfection mechanisms is a unique approach available for further evaluation as RF heating has recently emerged as a commercial process. The application of the surface disinfectants can be accomplished by using standard spray technology, and recent research has shown that can be easily adapted to the industry. In order to evaluate this approach and with the objective of achieving high log10 reduction, a heat-spray process was developed and tested at laboratory scale by studying critical operating parameters of the combined process including mode of applications and concentrations of selected surface disinfectants (SD) such as ethyl and isopropyl alcohols, hydrogen peroxide, peracetic acid, and acetic acid. These studies included the use of radiofrequency as the heat technology, SD contact times, temperature-SD interactions from 40 to 116°C surface almonds temperatures, atmospheric and low-pressure spraying, single and multiple sprays, interactions of bacteria and nut types, and several postharvest (quality and shelf life) parameters. These factors and levels were studied using conventional one, two, three, block and fractional factorial experimental designs. For the log10 reduction experiments commercial-quality samples of nuts (mostly almonds) were infected using wet inoculation at levels of 107-109 CFU/g of either Enterococcus faecium NRRL-B-2354 (a surrogate organism for Salmonella) and live Salmonella enteritidis ATCC 1045. The combinations of high log10 reduction and postharvest maintenance of the nut quality lead to the use of ethanol 70%, isopropyl 70%, hydrogen peroxide 3%, acetic acid 5%, and peracetic acid 500 ppm. Among these SD, ethanol and isopropanol 70% showed the best attributes to be used in a short-timed heat-spray process achieving log10 reductions in the range of 2-3 CFU/g1 with more than 70% evaporation from nut surfaces in less than 30 min at 25±2°C without affecting sensory and nutritional qualities. The spray process achieved high log10 reduction under the following parameters: (1) Use of 70% ethanol or isopropyl (p<0.05), (2) a sprayed amount of 4.5% v/v (p<0.05), (3) a contact time of at least 30 min, (4) alcohols applied on almond surface temperature lower than 75°C, and (5) use of two consecutive spray applications (p<0.05). Spraying under low pressure technology did not significantly improve the disinfection levels. Using conventional heating as simulation of RF, the thermal treatments alone achieved ~3 log10 CFU/g reductions. A heat-single spray process has an additive response (p<0.05) on log10 reduction achieving an average of 4.5 log10 CFU/g at a surface almond temperature of 115°C. Under equal conditions a heat-double spray process has additive synergism (interaction) achieving a mean of ~6.6 log10 CFU/g reduction. Differences on the response of bacteria type (Salmonella or Enterococcus) to SD were not found (p>0.05) for single spray process, but were found for the single and double heat-spray process (p<0.05). Both single and double heat-spray process at low surface almond temperature of ~105°C achieved at least 4 log10 CFU/g reductions. The use of radiofrequency in the heat-double spray process also achieved high log10 reduction of 6.7, 4.7, 3.8, and 3.5 log10 CFU/g of Enterococcus faecium for almonds, pecans, pistachios, and walnuts, respectively. Furthermore, the use of radiofrequency heating and ethanol 70% in the single heat-spray process achieved 5.8 log10 CFU/g reductions. In this experimental mode, heat and spraying alone achieved 2.9 and 3.0 log10 CFU/g reduction, which corresponds to a contribution of 50% each to the efficacy of the combined heat-spray process using radiofrequency heating. These results corroborate previous findings using conventional heating as a simulation of radiofrequency; the single heat-spray process has additive model, while the double heat-spray process has shown interaction in addition to additive characteristics. Therefore the use of spray process after radiofrequency heating further enhances the log10 reduction of a heat phase alone. The implications of these findings is that both the heat-spray and heat-double spray process may become alternatives to current almond disinfection processes to achieve high log10 reduction with similar postharvest characteristics. This has been demonstrated during this research and by current nut industry practices that have already adopted radiofrequency heating at an industrial scale. In addition, some current research has shown that installation of spray systems in the nut industry can be affordable, effective, and practical.