Comparison of Alternative Wetting Agents for Drag and Bootie Swabs for Agricultural Soil Sampling

Abstract

Drag and bootie swabs have been used in animal (e.g., poultry litter) and produce (e.g., soil) production for food safety purposes in place of grabs. Skim milk, the industry standard wetting agent for drags and booties, is not ideal for produce soil sampling due to its allergenic properties and animal-based origin, and (depending on preparation) low shelf stability. This study evaluated alternative wetting agents – tryptic soy broth, buffered peptone water, phosphate buffered saline, or deionized water – for hydrating drags and booties. Sampling was performed in fields with untreated swine manure and untreated dairy manure, with a total of 220 drags, 220 booties, and 44 grabs collected along 100 m paths. Indicator organisms including aerobic plate counts (APCs), total coliforms, and Escherichia coli were enumerated. Both wetting agents (p < 0.001) and sampling methods (p < 0.001) significantly affected the recovery of indicator organisms. In the field with swine manure, mean recovery differences between wetting agents ranged from 0.1 to 0.2 log(CFU/g) for APCs and 0.1 to 0.6 log(CFU/g) for total coliforms. In the field with dairy manure, mean recovery differences between wetting agents ranged from 0.0 to 0.2 log(CFU/g) for APCs, 0.1 to 0.4 log(CFU/g) for total coliforms, and 0.1 to 0.4 log(CFU/g) for E. coli. Overall, differences between wetting agents were small and suggest one could select wetting agents for future method development and industry use based on which are most practical for use in produce safety, such as most shelf stable and not animal sourced.