Non-thermal technologies for Alicyclobacillus acidoterrestris inactivation in apple juice

Non-thermal processes have been applied to food products with the purpose of eliminating spoilage and microorganisms. Ultraviolet-C (UV-C) radiation and ultrasound (US), combined or not with less severe temperature conditions, are giving promising results. In fruit juices, namely apple juices, Alicyclobacillus acidoterrestris is a concern, because it is responsible for quality degradation while being a spore-forming bacterium, able to grow at low pH (2.5–6.0) and high temperature (25–70 °C) conditions. The main objective of this work was to study the influence of UV-C radiation, ultrasounds and combinations of both treatments on A. acidoterrestris inactivation in apple juices. Ultrasounds were also tested at 60 °C (thermosonication). Commercially available juices were artificially inoculated with the bacterium, with initial loads around 107 CFU/mL. The juices were then exposed to the following treatments, for different times: (i) UV-C radiation with 13.44 W/m2 of intensity; (ii) ultrasonication (US) with frequency of 35 kHz and power level of 120-480 W; (iii) thermosonication at 60 °C and (iv) combination of UV-C and US (testing both orders of treatments). The inactivation rate was estimated by fitting a linear model to log-survival data obtained from each treatment. Results showed that ultrasounds had no significant effect on A. acidoterrestris inactivation. When UV-C radiation was applied, the number of spores decreased drastically (around 5-log reduction, which attains US Food and Drug Administration requirements) after 8 min of treatment. The combined treatment US followed by UV-C resulted in higher inactivation (around 6-log reduction) after 25 min of exposure. Inverting the order of treatments, i.e. UV-C followed by US, inactivated approximately 4-log of initial microbial load, after 30 minutes of treatment. Overall it can be concluded that UV-C radiation and combination of US with UV-C are promising treatments with a drastic impact on the loads of A. acidoterrestris in apple juices.