- Hazelnuts are commonly consumed raw and are valued for their numerous health benefits and antioxidant properties. Increased foodborne illness outbreaks associated with Salmonella and Escherichia coli O157:H7 contamination of tree nuts and peanuts generate a need for improving agricultural sanitation procedures. Food-safe chemical sanitizers have shown promise for reducing pathogenic organisms on fresh produce, but minimal research has been conducted for in-shell nuts. The purpose of this study was to determine the effects of water and three food-safe sanitizers on a) the natural microbial load of postharvest in-shell hazelnuts and b) populations of pathogenic Salmonella (S. enterica subsp. enterica ser. Panama) inoculated and dried onto the surfaces of in-shell hazelnuts.
The first phase of the study investigated the effectiveness of water, sodium hypochlorite (NaOCl; 25 ppm, 50 ppm), peroxyacetic acid (PAA; 80 ppm, 120 ppm), and acidified sodium chlorite (ASC; 990 ppm) as sanitizers for use on postharvest in-shell hazelnuts.
Treatments were applied to two groups of freshly harvested hazelnut samples to examine their effects on total aerobic microorganism populations during different times of harvest (Group 1 = early season, dry weather; Group 2 = late season, rainy weather). Treatments within each group included hazelnuts that underwent a tap water rinse, a tap water rinse followed by a water spray, and a tap water rinse followed by a chemical spray. Due to excess soil attached to shell surface, hazelnuts harvested later in the season (Group 2) had an initial population mean 2.24 log CFU/hazelnut greater than hazelnuts harvested earlier in the season (Group 1). All treatments, including water, resulted in significant population reductions compared to untreated controls (P≤0.05). Rinsing with tap water produced reductions of 0.38 log units in both groups, and additional water spraying resulted in reductions of 0.83 and 0.73 log units in Group 1 and Group 2, respectively. None of the chemical treatments were significantly more effective than the water spray treatment in Group 1; however, several chemical treatments in Group 2 were significantly more effective than water spraying. Tight adherence to shell surfaces during dry weather may have increased the chemical resistance of microorganisms on hazelnuts. Treatment with ASC produced the greatest reduction in Group 1 and Group 2 compared to the control (1.22 and 2.08 log units, respectively) and water spray treatments (0.39 and 1.39 log units, respectively), but the efficacies varied between treatment groups. Wide variation between Group 1 and Group 2 treatment results made determination of chemical efficacy difficult.
The second phase of the study analyzed the effectiveness of water, sodium hypochlorite (NaOCl; 25 ppm, 50 ppm), peroxyacetic acid (PAA; 80 ppm, 120 ppm), and acidified sodium chlorite (ASC; 450 ppm, 830 ppm, 1013 ppm) as sanitizers for reducing Salmonella on in-shell hazelnuts. Hazelnut samples were soaked in pure cultures of S. Panama for 24 h, air dried for 66 h, and then sprayed with water and chemical treatments. Surviving S. Panama populations were evaluated using a non-selective medium (tryptic soy agar), followed by a selective overlay (xylose lysine deoxycholate agar) after a 3 h incubation period. Tight adhesion prevented significant population decreases from
physical removal by water, which allowed for clear demonstration of chemical effectiveness. All of the chemical treatments significantly reduced the S. Panama population (P≤0.05) compare to untreated and water-sprayed samples. The most effective concentrations of ASC, PAA, and NaOCl treatments resulted in mean microbial population reductions of 2.65, 1.46, and 0.66 log units, respectively.
Overall, physical removal of excess dirt appeared to have the greatest effect on the microbial population reductions of postharvest in-shell hazelnuts, and adherence to shells during dry weather appeared to increase the chemical resistance of microorganisms. Future sanitation experiments should consider weather and levels of excess soil on hazelnuts as factors in the apparent efficacy of chemical sanitizers. Testing chemical sanitizers against tightly-adhered Salmonella cells provided consistent results with clear demonstration of chemical efficacies. Acidified sodium chlorite at 1013 ppm was significantly more effective at reducing Salmonella populations than other treatments and shows the greatest potential for use as a postharvest sanitation treatment. Thorough rinsing of hazelnuts in clean tap water, followed by spraying with high concentrations of acidified sodium chlorite could help increase the efficacy of current hazelnut processing.