Adaptive responses of Alicyclobacillus acidoterrestris in acidic broth and fruit juices: Focus on the influences of organic acids and temperature conditions

Acid adaptive response (AAR) is a survival mechanism that allows bacteria to develop enhanced stress tolerance. Our previous research identified AAR in Alicyclobacillus acidoterrestris, a thermo-acidophilic bacterium responsible for fruit juice spoilage. However, the roles of specific acidulants, adaptive temperatures, and acidic juice matrices in triggering AAR remain elusive. In this work, acid adaptation of A. acidoterrestris in broth acidified with various organic acids and in fruit juices was investigated, while also considering the ambient temperature. Results revealed that acid adaptation (at pH values of 3.0, 3.2, and 3.5, adjusted with malic, tartaric, or citric acids, and at pH 3.5 adjusted with lactic, succinic, or ascorbic acids, for 1 h) enhanced acid resistance (pH = 2.2, 1 h) of A. acidoterrestris, across all tested temperatures (45 °C, 35 °C, 25 °C, and 10 °C). Moreover, heat tolerance (65 °C, 5 min) was improved, except when using tartaric acid. Among acidulants used during adaptation (pH 3.5, 45 °C), succinic acid induced the highest level of acid resistance, followed by lactic, citric, malic, ascorbic, and tartaric acids, in descending order. For heat resistance, the ranking was succinic, citric, tartaric, lactic, ascorbic, and malic acids. Furthermore, acid adaptation in apple or orange juices enhanced heat resistance (65 °C) of A. acidoterrestris, and the induced resistance increased with extension of adaptation period. Adaptive temperatures of 25 °C and 35 °C were more effective in promoting resistance than 10 °C. These findings highlight the importance of considering adaptive responses of A. acidoterrestris to different preservation stresses and acidic juice environments during juice processing.