American consumers lost their innocence about peanut butter when manufacturers of such well-known brands as Peter Pan, Great Value, King Nut, and Organic Trader Joe’s Salted Valencia all failed to remove the Salmonella contamination from their once-trusted products. Peanut butter with spoon in jarPeanuts are grown in fields, along with manure and mud and other materials, so the presence of Salmonella is always possible. But consumers who knew anything about it knew enough to assume that those peanut roasters with their temperatures set to reach 300 degrees F killed any bug that was present. Then came ConAgra, Peanut Corporation of America, and Sunland — the manufacturers that somehow let Salmonella find its way back in their peanut butter after the peanuts were roasted and before the products were shipped. Three outbreaks in 10 years have caused plenty of thinking about what needs to be done to kill Salmonella in peanut butter and prevent it from coming back. The latest voices come from Seoul National University scientists Won-Jae Song and Dong-Hyun Kang. In the February 2016 edition of Food Microbiology, Song and Kang report on their experiments to inactivate Salmonella Senftenberg, Salmonella Typhimurium, and Salmonella Tennessee in peanut butter by subjecting it to 915 MHz microwave heating. Highlights of their work include:

  • Peanut butter inoculated with Salmonella was treated with 915 MHz microwave system.
  • Six kW microwave heating effectively reduced Salmonella in peanut butter.
  • No significant change of color, acid and peroxide value was observed.
  • Microwave heating can be used as a control method for peanut butter pasteurization.

“This study evaluated the efficacy of a 915 MHz microwave with 3 different levels to inactivate 3 serovars of Salmonella in peanut butter. Peanut butter inoculated with Salmonella enterica serovar Senftenberg, S. enterica serovar Typhimurium and S. enterica serovar Tennessee were treated with a 915 MHz microwave with 2, 4 and 6 kW and acid and peroxide values and color changes were determined after 5 min of microwave heating. Salmonella populations were reduced with increasing treatment time and treatment power,” according to the researchers’ abstract. “Six kW 915 MHz microwave treatment for 5 min reduced these three Salmonella serovars by 3.24–4.26 log CFU/g. Four and two kW 915 MHz microwave processing for 5 min reduced these Salmonella serovars by 1.14–1.48 and 0.15–0.42 log CFU/g, respectively. Microwave treatment did not affect acid, peroxide, or color values of peanut butter. These results demonstrate that 915 MHz microwave processing can be used as a control method for reducing Salmonella in peanut butter without producing quality deterioration,” they wrote. Since those three Salmonella outbreaks linked to peanut butter occurred, processors have focused on fixing leaky roofs and addressing basic pest control and sanitation issues. Some possible interventions, such as irradiation, cannot claim to leave peanut butter without changes in color or odor. If microwaving leaves no trace, as reported by the Koreans, it’s an intervention that might attract some attention. However, there is no indication the authors are suggesting that consumers microwave their peanut butter at home. The Korean study found in Food Microbiology is available through ScienceDirect on either a pay-per-view basis or by subscription to a participating scientific publication.

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