CARC Researcher Studies the Growth of Salmonella in Pistachios

By Precious Effiong

February 11 – Dr. Mahta Moussavi is one of the Cooperative Agricultural Research Center’s Post-doctoral Researchers who is making a big difference on the campus of Prairie View A&M University. While serving on a team of researchers from across the country, Moussavi carried out a study to examine the nature of Salmonella presence in pistachios and the team recently published their findings in the article “Growth of Salmonella on Inoculated Inhull Pistachios during Postharvest Handling.” Moussavi’s research contains some key information on how salmonella growth can be reduced in inhull pistachios.


Salmonella has been isolated from dried pistachios in both postharvest and retail surveys. The source of Salmonella in pistachios is unknown, but introduction is possible at points during production, harvest, and postharvest activities. To examine the behavior of Salmonella on pistachios during simulated postharvest conditions, early-, mid-, and late-season inhull pistachios were collected from two commercial processors over five different harvests. Pistachios were inoculated with cocktails of nalidixic acidor rifampin-resistant Salmonella at 0.64 to 1.59 log CFU/g (low) or 2.73 to 3.27 or 4.29 to 4.31 log CFU/g (high) and were incubated for up to 30 h under commercially relevant conditions (23, 35, or 378C and 50 or 90% relative humidity [RH]). Populations of Salmonella were measured by plating onto tryptic soy agar and CHROMagar Salmonella with added nalidixic acid or rifampin. Individual growth curves at the same temperature and RH differed significantly among different lots of pistachios. Except for a single late-season lot in which no significant growth was observed, Salmonella multiplied under all storage conditions. In the first 3 h after inoculation, insignificant (most cases) to small (0.41 to 0.67 log CFU/g) but significant (P < 0.05) mean increases in Salmonella populations were measured; the mean predicted time to achieve maximum populations (5 to 8 log CFU/g) was 16 6 4 h. In paired samples, longer lag phases, lower growth rates, and lower maximum increases were observed with inoculated inhull pistachios incubated at 238C and 50% RH compared with 35 or 378C and 90% RH. Similar growth curves were observed at the low and high inoculum levels; throughout the 30 h of incubation, Salmonella populations were consistently ~1 to 2 log CFU/g lower on pistachios inoculated at the low inoculum level. Managing the time between harvesting and hulling will reduce the potential for growth of Salmonella on pistachios during postharvest handling.

To read the entire article on Dr. Moussavi ’s research, please visit here.

This research was supported, in part, by the California Pistachio Research Board, the Center for Produce Safety, and the Specialty Crop Block Grant Program at the U.S. Department of Agriculture (USDA).


Mahta Moussavi


Dr. Mahta Moussavi
Post-doctoral Researcher
(936) 261-5060