Sanitizing Pecan Nutmeats

July 3013

There is concern among public health professionals that foods with low aw are being implicated with increased frequency as vehicles of foodborne pathogens. Included in this food category are tree nuts. Consumption of almonds, pine nuts, and hazelnuts has been associated with outbreaks of infections. Pecans, pistachios, and walnuts have been recalled due to possible contamination with foodborne pathogens.

We have observed reductions of Salmonella of up to 3.4 log CFU/g of in-shell pecans treated with chlorine and organic acid sanitizers. While these treatments show promise, concern still exists that pecan nutmeats may become contaminated during cracking, shelling, and subsequent handling. A potential point of contamination is the vacuum flotation step in which pieces of nutmeats other than halves are immersed in chlorinated water under vacuum to separate them from inedible in-shell nut components. Infiltration of pathogens into subsurface nutmeat tissue may occur during this process.

We did a study to evaluate the efficacy of chlorine and organic acids, with and without a detergent, for effectiveness in killing Salmonella on immersion- and surface-inoculated pecan nutmeats. The effect of intermittent vacuum and atmospheric pressure on reduction of the pathogen was investigated.

The efficacy of chlorine (200 - 1,000 µg/ml), lactic acid (0.5 - 2%), levulinic acid (0.5 - 2%), sodium dodecyl sulfate (SDS, 0.05%), lactic acid plus SDS, levulinic acid plus SDS, and a mixed peroxyacid sanitizer (Tsunami 200, 40 and 80 µg/ml) was evaluated. The addition of SDS to treatment solutions containing lactic acid or levulinic acid resulted in generally higher reductions of Salmonella but differences in these reductions were not always significant. Lactic and levulinic acids (2%) containing SDS (0.05%) were equivalent in killing Salmonella on immersion-inoculated nutmeats. Tsunami 200 (40 µg/ml) was less lethal or equivalent to 1 or 2% lactic and levulinic acids, with or without 0.05% SDS. Reductions did not exceed 1.1 log CFU/g of immersion-inoculated pieces and halves, regardless of sanitizer concentration or treatment time (up to 20 min).

Reductions on surface-inoculated pieces and halves were 0.7 to 2.6 log CFU/g and 1.2 to 3.0 log CFU/g, respectively. Treatment with 2% lactic acid plus SDS (0.05%) and Tsunami (80 µg/ml) was most effective in killing Salmonella on surface-inoculated pieces; treatment of halves with chlorine (1,000 µg/ml) or lactic acid (1 or 2%), with or without SDS, was most efficacious.

Exposure of immersion-inoculated pecan pieces to chlorine (200 µg/ml), lactic acid (2%) and levulinic acid (2%), with or without SDS, and Tsunami (80 µg/ml) during intermittent vacuum (18 ± 2mbar) and ambient atmospheric pressure treatments for up to 20 min reduced Salmonella by only 0.1 - 1.0 log CFU/g.

Previous studies have shown that hot water treatment of in-shell nuts during the conditioning step preceding the cracking and shelling operation can reduce Salmonella by 5 log CFU/g. Hot air treatment of nutmeats that may become cross contaminated during cracking and separation from inedible components, however, cannot be relied upon to achieve a 5-log reduction without adversely affecting sensorial qualities.

Observations on the general ineffectiveness of sanitizers tested in this study to reduce Salmonella on nutmeats further emphasize the importance of preventing contamination, particularly during cracking and shelling operations. Once nuts are contaminated, the lethality of sanitizers tested in these studies is minimal.