Inactivation of Sporeforming Spoilage Bacteria in Milk and Juice Using Ultra High Pressure Homogenization

Inactivation of spores is essential for extending the shelf life of fluid milk and other food products. Three studies were conducted to evaluate the effects of ultra-high pressure homogenization (UHPH) on spores from three spore forming bacteria. The first experiment studied UHPH effects on Geobacillus stearothermophilus, Paenibacillus lautus, and Bacillus licheniformis in fluid milk. Homogenization pressures of 100 - 500 MPa were applied to spore-inoculated samples, and spore viability was determined by plating onto agar media. Heat shock treatments (80°C, 10 min) were applied to a portion of each sample prior to UHPH. UHPH treatment significantly reduced spore populations on the heatshocked sample of P. lautus at 100 MPa (P<0.05, but had no significant effect on G. stearothermophilus and B. licheniformis (P≥0.05).

The second experiment was conducted with a similar protocol to the first experiment but with the addition of a heat treatment at 95°C within the homogenizing system. Fluid milk (3.79 L) was inoculated with a10 ml spore P. lautus spore suspension and was treated with pressures of 100 - 500 MPa at 95ºC. A heat shock treatment of 80°C for 10 min was applied to a portion of the UHPH-treated samples. Generally, these treatments had no significant effect on spore reduction (P≥0.05).

The third experiment evaluated the effect of UHPH on P. lautus spores under acidic conditions. Apple juice (3.79 L) was inoculated with 10 ml of a P. lautus spore suspension and was treated at pressures of 100 – 500 MPa. A heat shock treatment was applied to a portion of each UHPH treated juice sample. Significant reductions (P<0.05) in spore concentrations were observed in nonheat shocked and heat shocked samples at all pressures.

This study indicates that UHPH is effective at inactivating P. lautus spores in apple juice and provides some beneficial reductions in spore numbers in milk. These findings have promising applications for the industry to extend shelf life of milk, apple juice, and similar products.