Pecans are one of the most desired tree nuts in the United States. The demand for the nut and its production has been increasing every year. In comparison to 2016, pecan production in the U.S. increased by 8.5 percent in 2017, which resulted in approximately 300 million pounds of pecans. In the past few years, various tree nuts have been frequently associated with several outbreaks and/or recalls due to contamination with bacteria of public health concern (Zhang et al., 2017). However, pecans have a clean safety record. There have been no outbreaks associated with the nut and its products as of yet, but there have been several recalls due to potential salmonella contamination.
Pecan orchard environmental conditions, harvesting practices, and processing steps are potential sources of contamination. Shucks, which enclose the pecans when on the tree, are high in nutrients that are suitable for bacterial growth. Studies have shown that pathogenic bacteria such as salmonella can survive on the shucks for several weeks. During rainfall, pathogens present on shucks can cross-contaminate nearby enclosed pecans.
Another point of possible contamination, when pecans are ready for harvest, the trees are mechanically or manually shaken off or the nuts naturally fall to the ground, where they rest on the orchard floor for several days until collected. This process allows the nut to absorb moisture from the soil that can be potentially contaminated with bacteria from wild and domestic animal feces, inadequately composted manure, or irrigation and run-off water from land grazed by livestock. Cattle grazing is still prevalent in some parts of the U.S. and is one of the most common forms of ground cover management in pecan orchards. It provides a second source of income to pecan growers from the same parcel of land (i.e. pecans and beef as meat source) and a significant reduction in orchard mowing costs. However, cattle manure has been found to be the main source of health hazardous bacteria that can survive up to several months and, thereby, contaminate the pecans. In addition, worker health and hygiene on orchards and processing areas play an important role in maintaining the safety of pecans.
Pecan growers have to meet the federal and market-driven food safety requirements. Currently, pecan growers are exempted from the Food Safety Modernization Act (FSMA) Produce Safety Rule. When the FSMA Produce Safety Rule was first proposed, there was a provision where producers using raw manure as a soil amendment had to wait nine months between the last day of application to harvest. That means a waiting time of nine months from the last day of grazing cattle in the orchards to the first day of harvest.
Because this condition was hard to implement by many pecan growers, the Louisiana State University Agricultural Center, or the LSU AgCenter, in collaboration with the Louisiana Farm Bureau Federation hosted a meeting between pecan stakeholders and Food and Drug Administration (FDA) produce safety experts, and it was highlighted that any pecan process with a “kill step” will be exempted from the Produce Safety Rule. Currently, pecan is exempted from the rule, but the nut could still be covered by the rule in the future, depending on the risk to the health of consumers.
As per FDA guidance, any treatment process that gives a 5 log units reduction of health hazardous bacteria can be regarded as a kill-step. Any food product processed with a kill-step ensures the safety of the final product.
Pecan conditioning is one of the important post-harvest processing steps. Prior to shelling, pecans are moistened with water or steam, which increases the kernel moisture content and reduces kernel breakage during cracking. This process increases the shelling efficiency. Additionally, this step can also make the nut free of health hazardous bacteria. Various conditioning methods applied in the pecan processing areas are hot water, cold water, sanitizing solution (such as chlorine water), steam treatment, or moisture equilibration in a humidity-controlled room. Studies have shown that most of the conditioning processes remove bacteria from the pecan by only a maximum of 3 log units. An increased reduction of bacteria can be achieved only when two or more conditioning methods are paired together (Beuchat & Pegg, 2013).
Our research project carried out at the LSU AgCenter found that hot water treatment alone on inshell pecans was effective in removing the bacteria by 5 log CFU/g or greater. The time-temperature combinations of hot water that growers and processors could use to maintain the safety of pecans are 70 degrees Celsius for 8.6 minutes, 80 degrees Celsius for 6.6 minutes or 90 degrees Celsius for 4.6 minutes. These time-temperature combinations were found to be effective against potential health hazardous bacteria in pecans, such as Salmonella enterica, E. coli O157:H7 and Listeria monocytogenes.
As part of this study, pecans treated at these time-temperature conditions and raw pecans were roasted and presented to 112 consumers at the Louisiana State University Sensory Science Lab. Consumers liked the color/appearance and aroma of the hot-water-treated pecans more than the raw pecans. Interestingly, the consumers did not find any effect of hot water treatment on the texture and flavor of the pecans.
Later in the study, consumers were made aware of the role of hot water treatment in maintaining the safety of pecans. This further helped in increasing the consumers’ overall liking for the hot-water-treated nut. The study also found that for scientific validation of equipment or hot-water-treatment processes, Enterococcus faecium could be used in the pecan processing area to evaluate the efficacy of the system (Kharel, Yemmireddy, Graham, Prinyawiwatkul, & Adhikari, 2018). This study concluded that hot water conditioning is an effective processing technique to maintain the safety and quality of the pecan.
A major byproduct of pecan farming is the shells. Pecan shells account for nearly 50 percent of the harvested pecan crop (Worley et. al. 1994). Unlike the nutmeat, the shell is not desired for its delicious taste. However, research has shown that pecan shells are a rich source of bioactive compounds, specifically a group known as the phenols (Christina et. al. 2014; de la Rosa et. al. 2014). These compounds are known for their strong antioxidant and antimicrobial properties, which can be used for health and wellness purposes as well as food quality and safety applications.
Our research team isolated bioactive compounds from shells of twenty-two different pecan cultivars and evaluated their ability to prevent food spoilage by oxidation and kill or inhibit the growth of most problematic foodborne bacteria, such as E. coli O157:H7, salmonella, and Listeria monocytogenes. We found that the antioxidant potential of the shell extracts was highly dependent on the pecan cultivar. Bioactive extracts from ‘Curtis’ and ‘Pointe Coupee 2’ cultivars were found to have the greatest potential to be used as a natural antioxidant in food application. The tested pecan shell extracts were found to inhibit the growth of these bacteria at varying degrees when tested at 1.25 to 5 mg/mL concentrations. E.coli O157:H7, Salmonella, and Listeria monocytogenes were found to have the highest to lowest resistance to pecan shell extracts in that order.
However, the real challenge lies in how these extracts function when applied to food products. For example, we tested the shell extracts of ‘Nacono’ and ‘Caddo’ pecan varieties in inhibiting the growth of Listeria monocytogenes on catfish fillets over a five-day storage period at 4 degrees Celsius. The fillets not treated with pecan shell extracts showed an overall increase of about 2 to 3 log units in the growth of bacteria from zero to five days. Fillets treated with pecan shell extracts showed a significant inhibition (3 to 4 log units) of growth after one day of storage and about 1 log unit lesser growth on the third day when compared to the fillets without antimicrobial treatment. Similarly, testing these extracts on fresh-cut cantaloupes reduced the growth of Listeria about 0.75 to 1.78 log units after a five-day storage period at 4 degrees Celsius when compared to control.
Our results indicate that pecan shells have the potential to be used as natural antioxidants and antimicrobial agents in food, thus increasing the total crop value. Pecan safety validation and byproduct utilization will increase the economic competitiveness of pecan growers and processors while enhancing consumer food safety.