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A Closer Look - Project Assesses Food Safety Impacts of Delays between Pistachio Harvest and Drying

February 25, 2016

Even with the best-laid plans, delays can happen during pistachio harvest and post-harvest handling. Not only can the slowdowns reduce overall nut processing efficiency and quality, but they also may have food safety implications.

A two-year research project led by Dr. Linda Harris, Cooperative Extension specialist in microbial food safety, Department of Food Science and Technology, University of California, Davis, examined the potential for those delays to increase food safety risks.

The data generated are being used to develop a model that could be used by the U.S. pistachio industry to assess delays during harvest. A separate objective has generated thermal processing data that will assist the pistachio industry in meeting the new preventative controls rules under the Food Safety Modernization Act, Harris said. Her work was guided by input from several of California’s pistachio processors.

“Before each harvest began, we met with our collaborators, reviewed data from previous years and solicited input in the research plan for the upcoming harvest,” Harris said. “This type of dialogue has been very productive in our research and has directly led to modifications in our approach to the research or in interpreting the results.”

The collaborators also worked with her to collect harvest samples, which occurred on roughly weekly intervals over the busy five-week harvest. “Harvest can be unpredictable, and we have appreciated the willingness of our collaborators to help us collect the samples we needed,” she said.

Bob Klein, manager of the California Pistachio Research Board, said these types of partnerships are important to help the industry better address food safety risks.

“The industry wants to ensure we always have a safe and wholesome product for the consumer,” he said. “And we need outside scientists who can independently determine the risks, assess the risks and provide results so we can act upon them.”

Klein said being able to provide input into the project also helps ensure the outcome will be “real world” and applicable to the industry.

Harris’s project, titled “Assessing postharvest food safety risks and identifying mitigation strategies for foodborne pathogens in pistachios,” examined two points where pistachio movement can be delayed.

The first may occur between tree shaking at harvest and removing the hull from the nut. Like many tree nuts, pistachios are covered with a fleshy exterior hull — sometimes called a husk — that has to be removed to expose the in-shell kernel.

The second delay may occur after the pistachios are hulled and sorted into two main streams — floaters and sinkers. The hulled pistachios are separated in a tank of water, with the typically smaller or damaged nuts rising to the top. The larger, heavier ones sink. The floaters, which constitute about 15 percent of the harvest, are removed and handled separately.

The nuts then move to dryers, where the 30 to 40 percent moisture content at harvest is reduced to less than 15 percent before they are conveyed to silos for additional drying to 5 to 7 percent moisture for storage. But delays can occur before drying, especially with the floater stream where volumes are low at the beginning and end of the harvest.

As part of laboratory trials, Harris and her group inoculated samples of in-hull pistachios as well as hulled floaters and sinkers with Salmonella to identify points during postharvest handling where and under what conditions this organism could multiply.

The results provide a better understanding of how Salmonella behaves under simulated pre-hulling holding, post-hulling holding and drying.

In addition, the research examined the thermal tolerance of various strains of Salmonella, Escherichia coli O157:H7, Listeria monocytogenes and the surrogate organism, Enterococcus faecium, under dry and moist heat conditions. These data will provide a scientific foundation for thermal process validation in the pistachio industry.


Pistachio industry collaborated, providing important input on the project and assisting in collecting timely samples.

Research examined the impact of delays after harvesting pistachios and before drying.

Data were generated to provide the scientific foundation for thermal process validation


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