Did you know that pecans are the only tree nut 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 according to the International Nut & Dried Fruit Council.
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.
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.”
With these encouraging data in hand, the next step was to get some consumer feedback on kill step-validated, hot water-conditioned pecans. To that end, Dr. Adhikari and his team evaluated consumer acceptance and purchase intent of dehulled and roasted pecans that had been preconditioned in hot water according to the aforementioned protocol. Results were published in January 2019.
“We presented 112 consumers with roasted raw pecans as a control and roasted pecans pre-treated with hot water at three respective time-temperature combinations,” Dr. Adhikari says. “The volunteer panel included LSU faculty, staff, and students, roughly equal numbers of males and females. Consumer acceptance was higher for hot water-treated pecans, with higher ratings on color/appearance and aroma. No effect of hot water pretreatment was observed by consumers on other sensory properties, such as texture and flavor.”
Based on all of these results, Dr. Adhikari believes hot water conditioning holds greater promise than ever before for the commercial pecan industry. “Hot water conditioning has the potential to be regarded as a kill step to ensure the safety of pecans,” he emphasizes. “The treatment will also enhance the color and aroma of the pecan without affecting its texture and flavor. Since hot water conditioning is already in practice by most U.S. pecan shellers, no additional cost is required for setting up the system.”
During the Louisiana Pecan Growers fall field day in September 2018, Dr. Adhikari surveyed the producers and processors in attendance after he presented the results of his hot water conditioning research. “Some 80 percent of the respondents agreed that they will evaluate the microbial food safety risks associated with their pecan production and processing practices,” he reports. “About 75 percent of the growers said they would be willing to adopt this technology within the next two to three years.”
Dr. Adhikari says the hot water treatment process may hold promise for application to other nut species. “Hot water treatment has already been extensively used for tree nuts, especially almonds,” he relates. “Recent research indicates that pine nuts, black walnuts, and chestnuts could benefit from hot water treatment by reducing food safety risks and increasing shelf life. However, thermal processes validated for one nut type cannot be generalized to all tree nuts. The efficacy of hot water treatment may be affected by the shape, size, surface area, or other characteristics associated with each specific nut species. Therefore, validation of hot water treatment for each type of nut must be performed before commercial use.”