Improving pasteurization validation methods for pistachio processing

Microbial safety of low-moisture foods is a particularly difficult challenge, as reflected in recent outbreaks and/or recalls associated with Salmonella-contaminated nuts and other low-moisture products. Therefore, processing interventions are an emerging imperative to reduce the risk of Salmonella in low-moisture products, including pistachios. The Food Safety Modernization Act (FSMA) proposed Preventive Controls rule will mandate that the low-moisture food industry implement and validate interventions against identified hazards, such as Salmonella. Although a number of pathogen-reduction technologies are available to the pistachio industry (e.g., dry heat, steam, radio-frequency), there are several significant problems: (1) No single technology will be universally applicable, so that product specific/scalable solutions are needed; (2) The cost of stand-alone pasteurization technologies is an impediment to small processors; and (3) Robust validation protocols have not been widely tested or disseminated. Therefore, the overall goal is to improve the methods for validating pathogen-reduction processes for pistachios, with particular attention to improving existing processes and enabling any processor to reliably validate those processes. The work plan includes laboratory- and pilot-scale experiments with Salmonella inoculated pistachios, and a process validation demonstration at the commercial-scale. A key project outcome will be a guidelines document for methods to validate preventive control measures for pistachios.

Microbial safety of low-moisture foods is a particularly difficult challenge, as reflected in recent outbreaks and/or recalls associated with Salmonella-contaminated nuts and other low-moisture products, including 2009 and 2013 recalls of pistachios. Therefore, processing interventions are an emerging imperative to reduce the risk of Salmonella in low moisture products, including pistachios. The Food Safety Modernization Act (FSMA) proposed Preventive Controls rule will mandate that the food industry implement and validate interventions to prevent or control identified hazards, such as Salmonella in low-moisture products; large, small, and very-small facilities will need to comply by 2016, 2017, and 2018, respectively. Therefore, the overall goal of this project is to improve the methods for validating pathogen reduction processes for pistachios, with particular attention to enhancing existing operations and enabling processors of any scale to reliably validate those processes. The specific objectives are: (1) To develop and test process modifications suitable for existing processing equipment (e.g., roasters) that could improve achievement of preventive controls (i.e., log reductions of Salmonella), and (2) To develop, test, and disseminate a “Guidelines for Validation of Pistachio Pathogen Reduction Processes” document that will assist processors to effectively validate various thermal treatments. Laboratory-scale experiments will quantify the effects of key product and process factors (i.e., salt, water activity, process humidity) on Salmonella inactivation. Pilot-scale roasting/pasteurization experiments (with Salmonella-inoculated pistachios) will validate the laboratory-scale data. Results from these tests, and from significant prior and current work by the PI/co-PIs, will be used to develop, test, and distribute a process validation guidelines document. Industry partners will play a key role in that effort by assisting with commercial-scale tests to quantify process variability and by evaluating the practical relevance and completeness of the protocols outlined in the guidelines document. Therefore, this proposed project will provide timely information to help the pistachio industry comply with the requirements of the forthcoming rule. This project will help ensure the microbial safety of processed pistachios, prevent unnecessary capital expenses, and ensure continued operation of existing processes.

1. To develop and test process modifications suitable for existing processing equipment (e.g., flat bed or rotary roasters) that could improve achievement of preventive controls (i.e., log reductions of Salmonella), particularly aimed at achieving lowest-cost solutions for smaller processors. 

2. To develop, test, and disseminate a “Guidelines for Validation of Pistachio Pathogen Reduction Processes” document to help processors effectively validate various treatments, including dry roasting and thermal pasteurization, via either time temperature calculations or the use of a nonpathogenic surrogate (Enterococcus faecium).

This project has generated significant new findings, relative to factors affecting pathogen reduction in pistachio thermal processes. The key findings and recommendations are as follows: 

1. Process humidity significantly enhances pathogen lethality. Pistachio processors should consider monitoring, and perhaps ultimately controlling and enhancing, humidity in the early stages of multi-stage roasting processes, in order to enhance the efficacy of roasting as a pathogen reduction step. 

2. Brining significantly enhances pathogen reduction during roasting. For roasted, unsalted pistachios, a water pretreatment of some type might be considered, in order to enhance process efficacy. Dry/unbrined nuts in dry roasting conditions yielded, in several conditions, nearly negligible pathogen reduction on the time scales relevant to pistachio roasting. 

3. Initial moisture content may not be a significant factor in process lethality. Although the data are not yet sufficient to preemptively exclude pistachio moisture content as a critical factor in process validations, they do suggest that, for certain hot-air treatments, it might be possible to do so (with additional supporting evidence). 

4. The non-pathogenic surrogate (E. faecium) appears to be a very conservative tool for validating pistachio hot-air/roasting processes. Prior work by the project Co-PI has shown a very close correlation between E. faecium and Salmonella thermal inactivation in water and oil; however, this study suggests that there is a significantly conservative difference in the hot-air treatments. 

5. The predictive microbial inactivation model worked similarly well, with conservative predictions of lethality. Although not yet sufficient to be the sole tool for a process validation, the predictive model may be a useful tool for evaluating equipment/process modifications (e.g., to improve monitoring and control of existing roasters to support process validations) and for designing validation studies. 

6. The pilot-scale data on lethality uncertainty informs the quantitative microbial risk assessment (QMRA) being published by the project Co-PI. Using conservative estimates, that study concluded that an overall mean number of cases of pistachio-linked salmonellosis per year in the U.S. would be less than one if a uniform 4 ± 0 log (but not a 3 ± 0 log) reduction were applied to 100% of the domestic product. However, that conclusion increased to >1 case per year when variability in the pathogen-reduction step increased to 4 ± 1 log reduction. Obviously, there is no such thing as ± 0 uncertainty in real-world processes. Therefore, the lethality uncertainty values reported in the present study suggest that a 1-log uncertainty in lethality outcome is not an unreasonable estimate of the actual results associated with real-world process validations. It is critically important that process controls, monitoring, and variability are properly incorporated into validation strategies for any pathogen reduction process, and that the industry continues to seek ways to maximize the reliability of those processes. 

7. Next steps. Although this project is officially complete, and the team has completed the research tasks proposed, the PI and Co-PI will continue to work on pathogen reduction processes for pistachios, in the following ways: a. The Co-PI is meeting in March 2017 with pistachio industry food safety leaders, to solicit feedback on the guidance documents, in terms of content, format, etc. b. The Co-PI and PI intend to carry out on-site testing of the guidance documents with (hopefully) two industry partners – one medium/large and one very small processor. The goal will be to “debug” the guidance documents and prepare them for industrywide/public release. c. The PI currently leads a $4.7M USDA-funded project (2015–2020) entitled “Enhancing low-moisture food safety by improving development and implementation of pasteurization technologies.” The outreach component of that project includes development of decision-support tools to help the low-moisture food industry evaluate alternatives for pathogen reduction operations, and to reliably validate technologies and processes. The intent is to utilize the results from this CPS project (and the subsequent implementation of the research results and guidance documents) as a unique opportunity to assess impact of new knowledge and tools on industry practices (and ultimately food safety outcomes) within a specific industry sector (i.e., the pistachio industry).