Between October 2008 and March 2009, a UK group of scientists studied E. coli and Salmonella contamination on ready to eat shelled nuts. The samples were from retail stores. The whole study can be read here: shellednuts.pdf.
A total of 2,886 nut kernel samples of different varieties were examined for Salmonella spp. and Escherichia coli. Any ready to eat shelled nuts could be sampled as part of the study provided they had not been cooked, covered with chocolate, yoghurt, or other coatings, or flavoured with sauces, spices or salt.
Overall Salmonella spp. and E. coli were detected from 0.1% and 0.8% of edible nut kernels, respectively.
Of the nut kernel varieties examined, Salmonella Senftenberg and Salmonella. Tennessee were detected from two pre-packed samples of Brazil nuts (0.4%). Salmonella Anatum was isolated from a pre-packed mixed nuts sample (0.9%; mix: almonds, brazils, cashews, peanuts, walnuts). The levels of Salmonella from the three contaminated nut samples in this study were low, and ranged from <0.01–0.23/g. Nonetheless, the presence of Salmonella is unacceptable in edible kernels and the Food Standards Agency was immediately informed and full investigations undertaken.
E. coli at unsatisfactory levels (150/g) was present in another pre-packed Brazil nuts sample (0.2%). E. coli was additionally found at lower levels (range: 3.6 – 43/g) in Brazils (1.9%), macadamia (1.5%), pistachios (1.1%), walnuts (0.7%), peanuts (0.7%), hazels (0.5%), cashews (0.4%), and almonds (0.3%). Levels of E. coli did not correlate with the presence of Salmonella.
Prevention of microbial contamination in these products lies in the application of good agricultural, manufacturing and storage practices together with the hazard analysis and critical control points (HACCP) system that encompass all stages of production, processing and distribution.