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Nuts - about BAX

The DuPont Qualicon BAX® System lets you quickly, easily and reliably test nuts and nut products for Salmonella.

A recent outbreak of foodborne illness in the United States was traced to Salmonella typhimurium in peanut meal, which is used to make peanut butter and peanut paste. Shortly thereafter, an FDA investigation into the recall of 2 million pounds of pistachios found samples that tested positive for several serotypes of Salmonella, including Montevideo.

If your company needs to test nut products for Salmonella, you should know that the BAX® System is certified by the AOAC Research Institute as a Performance Tested Method for detecting Salmonella in many foods, including peanut butter. The AOAC Emergency Response Validation program has also determined that the BAX® System is ERV-certified as a rapid, reproducible and effective method for detecting Salmonella in peanut butter.

Outsourced testing

If you outsource your testing, please check this list for a service lab near you that uses the BAX® system for fast, reliable pathogen testing.

In-house testing

If your testing is carried out in-house, you should know that, although Salmonella detection with the BAX® System is straightforward, sample preparation for this food type is key to optimal performance.

Tips on sample prep for testing peanut butter

To prepare nut meats for Salmonella testing, the FDA Bacteriological Analytical Manual recommends blending your samples 1:10 in lactose broth. After allowing the blend to stand for 60 minutes, adjust pH to 6.8 and incubate 24 hours at 35°C. The BAX® system protocol then calls for a 1:50 dilution in BHI, incubated for 3 hours at 35°C before beginning the test.

Peanut meal is finely ground and should disperse well in solution, but peanut butter is highly lipid and hydrophobic, tending to form globules. These characteristics could prevent Salmonella cells from being exposed to the enrichment media and lead to overgrowth by competing flora. For best results, use a blender--or Mason jar and standard blender base--to thoroughly disperse any Salmonella cells in the enrichment broth. Blended enrichments can then be incubated directly in the Mason jars.

Another important technique is to bring the food sample to room temperature (25°C), which will lower the viscosity for easier blending. You should also pre-heat the enrichment broth to 35°C before blending to ensure low concentrations or injured cells have a full 24 hours at the correct temperature to grow to detectable levels.

Tips on sample prep for testing whole nuts

Internal studies performed on spiked whole pistachios, with and without shells and salt, spiked trail mix and unspiked whole almonds suggest the same enrichment protocol, minus the blending, is sufficient for accurate detection of Salmonella with the BAX® system. Place whole nuts at room temperature in 1:10 dilution of pre-warmed (35°C) lactose broth. Allow enrichment to stand for 60 minutes, then adjust the pH to 6.8 and incubate for 24 hours at 35°C. Dilute this enrichment 1:50 in BHI and incubate 3 more hours at 35°C


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