The effect of high hydrostatic pressure on the microbiological quality and safety of carrot juice during refrigerated storage.

The microbial quality of untreated and pressure-treated carrot juice was compared during storage at 4, 8 and 12 °C. High pressure treatment at 500 MPa and 600 MPa (1 min/20 °C) reduced the total counts by approximately 4 log CFU ml⁻¹ and there was very little growth of the survivors during storage at 4 °C for up to 22 days. Total counts increased during storage of pressure-treated juice at 8 °C and 12 °C but took significantly longer to reach maximum levels compared to the untreated juice. The microflora in the untreated juice consisted predominantly of Gram-negative bacteria, identified as mostly Pantoea spp., Erwinia spp. and Pseudomonas spp. Initially the pressure-treated juice contained low numbers of spore-forming bacteria (Bacillus spp. and Paenibacillus spp.) and Gram-positive cocci; the spore-formers continued to dominate during storage. When irradiation-sterilised juice was inoculated with a cocktail of Listeria monocytogenes, numbers decreased during storage at 4 °C and 8 °C by 1.50 and 0.56 log CFU ml⁻¹ respectively. When the inoculated carrot juice was pressure treated (500 MPa/1 min/20 °C) no L. monocytogenes were found immediately after pressure treatment or during storage at 4, 8 and 12 °C (>6 log inactivation). In contrast, pressure treatment in TSBYE only resulted in 1.65 log inactivation and survivors grew rapidly. This suggests that the antilisterial effect of carrot juice is enhanced by HPP.