Effect of high pressure CO2 and mild heat processing on natural microorganisms in apple juice

Apple juice was pasteurized by high pressure carbon dioxide (HPCD) at 20 MPa with CO2 concentration of 4.5–5.3% and mild heat (MH) at atmospheric pressure. Microbial inactivation and stability of natural microorganisms in apple juice were investigated. The temperatures were 37, 42, 47, 52, 57 and 62 °C, treatment time was 30 min, and storage temperatures of pasteurized apple juice were 2 and 28 °C. The aerobic bacteria (AB) treated by MH at 62 °C and by HPCD at ≥ 52 °C were almost totally inactivated, the microbial counts were < 10 CFU/mL. The yeasts and moulds (Y&M) treated by MH at ≥ 57 °C and by HPCD at ≥ 42 °C were totally inactivated. HPCD increased the susceptibility of these natural microorganisms to temperature and enhanced their microbial inactivation. The AB in apple juice treated by HPCD at ≥ 52 °C and the Y&M treated by HPCD at ≥ 57 °C, the AB and the Y&M treated by MH at 62 °C showed a better stability during storage at 2 and 28 °C, but apple juice treated by HPCD at ≤ 47 °C was characterized with high microbial counts of the AB ≥ 2.75 × 103 CFU/ml. A viable but non-culturable (VBNC) state of the Y&M treated by MH at 57 °C and by HPCD at 42, 47 and 52 °C was observed during storage at 28 °C. Apparently the proper temperature of HPCD treatment of apple juice at 20 MPa and stored at 2 and 28 °C was greater than or equal to 52 °C, while for MH treatment the proper temperature increased to 62 °C in this study.