The impact of high power ultrasound for controlling spoilage by Alicyclobacillus acidoterrestris: A population and a single spore assessment

Consumer demand for healthier, microbiologically safe and stable, higher quality and minimally processed foods has increased in the last decades, promoting the application of non-thermal process technologies. Ultrasound processing is gaining attention because of its potential for improving quality and safety while retaining product flavor, texture, color and nutrient composition. Recently, Alicyclobacillus acidoterrestris has been recognized by manufacturers and processors in the fruit industry as a new type of thermoacidophilic, endospore-forming spoilage bacterium for commercialized fruit juices that can withstand the high temperatures applied in pasteurization process and spoil them producing taint compounds. Based on the above, the present study was undertaken to investigate the separate and combined effect of ultrasound treatment operating at 26 kHz, 90 μm, 200 W for 5, 10, 15 min and heat stress (at 80 °C for 10 min) on the recovery of A. acidoterrestris spores in K broth adjusted to pH = 4.5 with 25% (w/v) citric acid at temperatures between 35 and 45 °C at population and individual spore level. At the population level, statistically significant differences in average lag time of A. acidoterrestris spores from different treatments at 35 and 45 °C have been found. After the applied ultrasound (at 100% Amplitude for 10 min) and heat shock (at 80 °C for 10 min), the average lag time of A. acidoterrestris spores at 35 °C (7.24 ± 0.34 h) and 45 °C (6.38 ± 0.18 h) has been shown to be longer compared to the control at 35 °C (5.68 ± 0.36 h) and at 45 °C (2.87 ± 0.19 h), while for this combination, the maximum specific growth rate was not significantly different from the control samples. Additionally, the findings of this study have shown that λ among individual spores was greatly variable when they were treated by ultrasound (at 100% Amplitude for 10 min) and/or thermal treatment (at 80 °C for 10 min). The application of ultrasound and thermal treatment resulted in a more pronounced effect on median lag phase duration (25.74 h) than the heat shock (11.63 h) and affected both the position and the spread of the λ distributions of A. acidoterrestris spores. This work would help to better assess and optimize the proposed combined treatments, since ultrasound and thermal treatments could work in a synergistic way on delaying the lag time of the tested bacterial spores.