Bacterial concentration reduction in swine waste amended livestock feed using a single-screw dry-extrusion process

A study was conducted to determine the efficiency of a dry-extrusion process to reduce or eliminate bacterial contamination of swine waste amended livestock feed. Separated swine waste solids were mixed with ground corn and soybean hulls and dry-extruded at temperatures of 110-135 degrees C for no more than 30 s to produce animal feed. Swine waste, pre- and post-extrusion livestock feed, and commercial swine feed samples were collected aseptically and analysed for total coliform, Escherichia coli, fecal coliform, heterotrophic, and non-specific anaerobic/facultative bacteria using standard culturing techniques. Selected pre-extrusion feed samples were inoculated with liquid cultures of Bacillus stearothermophilus to test efficiency of the dry-extrusion process to eliminate heat-resistant spore-forming bacteria. Bacterial concentrations recovered from post-extrusion livestock feed were significantly reduced from other sample types analysed. Based on these data, it is apparent that a single-screw, dry-extrusion process can consistently disinfect animal feed. However, careful monitoring of the extrusion process may be necessary for consistent sterilization. (C) 2000 Elsevier Science Ltd. All rights reserved.