Did you know that pecans are the only tree nuts native to the U.S.? As the world’s largest supplier of pecans, the U.S. produces, on average, 325 to 350 million pounds annually, according to the U.S. Pecan Growers Council, Tifton, Ga.; some 51 percent of global production in 2017/2018, as stated by the International Nut & Dried Fruit Council.
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Fortunately, there have not been any foodborne disease outbreaks traced to pecans, but pecan orchard conditions before and after harvest indicate there are potential food safety risks, says Achyut Adhikari, PhD, an assistant professor and extension food safety specialist at the Louisiana State University (LSU) School of Nutrition and Food Science, Baton Rouge.
Contamination of in-shell pecans and nutmeats can occur preharvest, at harvest, and during postharvest handling and processing, Dr. Adhikari points out.
For starters, during rainfall, the shucks surrounding pecans get wet along with the nut. “The nutrient-rich shucks provide a suitable environment for microbial growth,” Dr. Adhikari explains. Salmonella can survive for several weeks, making it one of the favored routes for contamination.”
During harvesting, Dr. Adhikari continues, pecan tree nuts are shaken off, or the nuts are naturally allowed to drop on the ground. “They can remain there for several days until collected,” he relates. “The nut absorbs moisture from soil that can be potentially contaminated with bacteria from wild and domestic animal feces, inadequately composted manure, irrigation, or runoff water from land grazed by livestock. Additionally, worker health and hygiene in orchards and processing areas play an important role in maintaining the safety of pecans.
“Conditioning prior to cracking is an essential step in pecan processing to not only reduce kernel breakage and improve shelling efficiency, but to also eliminate microorganisms that may be on the shell,” Dr. Adhikari says.
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Pecan Conditioning Methods
According to Dr. Adhikari, some of the pecan conditioning methods currently used commercially are:
- Soaking in hot water that is at least 177.8 degrees Fahrenheit for one to eight minutes or steam processing for six to eight minutes;
- Immersing in cold, usually chlorinated, water for eight hours and then draining for 16 to 24 hours, or soaking in chlorinated water with a minimum free chlorine concentration of 200 parts per million at 59 to 86 degrees Fahrenheit for two minutes; and
- Moisture equilibration in a humidity-controlled storage room.
“FDA recommends that a treatment process must achieve a 5-log reduction of bacteria to be regarded as a kill step,” Dr. Adhikari points out. “Food products processed with a kill step will ensure food safety in the final product, minimizing public health concerns. But most of the conditioning methods currently used remove a maximum of only 3-log colony forming units per gram (CFU/g) bacteria from the pecan shell. So, to increase the efficiency of pathogen control, those conditioning methods must be paired with other techniques.”
Examples of other techniques include spraying with chemicals such as peracetic acid, chlorine dioxide, or hydrogen peroxide; thermal treatment such as hot air and dry roasting; high pressure processing; and treatment with irradiation.
Published in November 2018, recent landmark research by Dr. Adhikari and several LSU colleagues demonstrates hot water treatment alone on in-shell pecans is effective in removing bacteria by more than 5-log CFU/g. “Based on our findings, growers and processors could use hot water either at 158 degrees Fahrenheit for 8.6 minutes, 176 degrees for 6.6 minutes, or 194 degrees for 4.6 minutes,” Dr. Adhikari relates. “These time-temperature combinations were found to be effective against E. coli O157:H7, Salmonella enterica, and Listeria monocytogenes. Also, for scientific validation of pecan processing equipment or a hot water treatment process, Enterococcus faecium can be reliably used to evaluate the efficacy of the system.”