Fig. 1 shows microbiological results and pH measurements of base product (BP), UHPH and pasteurized soymilk during 28 days of storage at 4 °C. Initial counts of mesophilic bacteria and spores of BP soymilk were respectively 2.13 log cfu/mL and 1.54 log cfu/mL. In accordance with our previous results , in the present study Enterobacterial counts were not detected in BP and treated soymilks, using detection limit b0.5 cfu/mL. UHPH treatments
at 200 MPa were more effective in the microbial inactivation than pasteurization treatment, being 75 °C of inlet temperature the most efficient UHPH condition. Pasteurized soymilk showed an increase in mesophilic bacterial counts of 4.2 log units from day 1 to day 28, whereas soymilk treated at 200 MPa of pressure and 55 and 75 °C of inlet temperature of just 3.2 and 0.3 log units respectively (Fig. 1A). During storage, mesophilic spore counts increased 2.0 and 0.8 log units respectively for pasteurized and 200 MPa, 55 °C soymilks (Fig. 1B). In general terms, pasteurized soymilk presented
higher bacterial counts compared to UHPH soymilks. From day 1 to day 14, mesophilic bacteria increased progressively in pasteurized and 200 MPa, 55 °C soymilks, but beyond this point an accelerated increase occurred mainly in pasteurized soymilk. In parallel with bacterial growth, pH values of pasteurized sample decreased considerably after 14 days of storage (Fig. 1C). On the last day of measurements, pH reached values about 6.49, indicating a slight acidification produced by this microbial growth. UHPH-treated soymilks revealed similar pH evolution during the period of storage. A decrease in the pH values was observed after 21 days of storage, with values on the last day of analysis of 6.73 and 6.77 for 200 MPa, 55 and 75 °C of inlet temperature, respectively.