Alicyclobacillus acidoterrestris, acido-thermophilic spore forming bacteria, which produce an unpleasant (disinfectant) odour, cause fruit juices to become spoiled. An automated microbial detection system, BacT/ALERT^®, and a LYM medium (pH 3,7) were used to detect those bacteria. The objective of the study was to determine the impact the content of extract (the content of soluble solids) in concentrated apple juice and the inocolum dose had on the detection time (length of lag phase) of A. acidoterrestris in the BacT/ALERT^® system, and to adjust the mathematical multiple regression model. Five Alicyclobacillus acidoterrestris strains isolated from the samples of concentrated apple juices and beverage production emulsions were investigated. As for the juice containing 35.6 % of extract (of the soluble solids), it was found that the growth of two strains was partially inhibited, and the growth of one of the examined strains was totally inhibited. Additionally, as for the juice containing 23.7 % of extract (of the soluble solids), it was found that the growth of three strains was partially inhibited. The detection time (length of lag phase) of all the examined A. acidoterrestris strains became shorter along with the decrease in the extract content in the concentrated apple juice and along with increase in the dose of inoculum. As for juice with 11.8 % of extract, the detection time, when the dose of inoculum was about 10 cfu per 20 cm³ juice sample, ranged between 23.5 h up to 43.7 h depending on the strain. Polynomial equations of second degree, which describe the dependence between the detection time (length of lag phase), the content of extract (of soluble solids), and the inoculum level, was adjusted. The R² determination coefficients calculated ranged from 0.83 to 0.95 depending on the strain. It is possible to apply the BacT/ALERT^®automated microbial detection system to detect Alicyclobacillus sp. in concentrated apple juices when performing routine investigations. The content of extract (of soluble solids) in the juice investigated should be 17.8 % at the most.