nuts

Nut meats are derived from processed nuts harvested from trees, shrubs, or plants. Nut meat processing is normally a dry process so bacterial and yeast growth is not important but molds may grow on damp nuts. Nut meats are frequently contaminated with molds especially the storage molds Penicillium, Aspergillus, and Fusarium. The mold composition changes from "field fungi" to "storage fungi" from harvest through processing to storage. Aflatoxin is a concern because of mold growth and mycotoxin formation in the field or during storage if the nuts contain more than about 9% moisture. The U.S. Food and Drug Administration (FDA) guidelines permit 20 ppb of total aflatoxin in nut meats.

The microbial counts on nut meats are often several thousand per g or less. Coliforms are not uncommon, but E. coli is present on 4-6% or less of the samples. The U.S. Food and Drug Administration (FDA) considers nuts to be adulterated when moldy, rancid, infested with insects, tainted with aflatoxins or positive for E. coli. Nut meats rarely may contain Salmonella that may survive the thermal treatment.

Peanut butter has a low water activity and microbial growth is prevented. Peanut butter has been recalled because of the presence of Salmonella. The presence of such a pathogen is usually a result of mishandling after dry roasting. The level of aflatoxin is regulated.

Recommended tests

Aerobic plate count.
Total coliform count. 
E. coli.
Yeast and mold count.

Additional tests

Salmonella test.

Aflatoxin test.

Adapted from

King, A. D. and Jones, T. 2001. Nut Meats, p. 561. In F. P. Downes and K. Ito (eds.), Compendium of Methods for the Microbiological Examination of Foods. American Public Health Association, Washington, DC.