Alicyclobacillus is a genus of Gram-variable, rod-shaped, spore-forming bacteria. The bacteria are able to grow in acidic conditions, while the spores are able to survive typical pasteurization procedures.
Alicyclobacilli are strictly aerobic, acidophilic, mesophilic to thermophilic, soil-dwelling organisms, and have been shown to grow at temperatures between 20 and 70 °C (with the optimum temperature range being 42–60 °C) and pH values of 2.0 to 6.0. Alicyclobacilli are of special interest to the fruit juice canning industry because common pasteurization techniques (92 °C for 10 seconds) do not deactivate the spores; Alicyclobacillus species can have a D-value of over 8 minutes (requiring treatment of over 8 minutes at 95 °C to kill 90% of spores). When a product is spoiled by Alicyclobacillus, the juice products develop a disinfectant-like odor and/or flavor (due to guaiacol production), but the bacteria do not cause swelling of the package or discoloration of the product, nor is it pathogenic to humans. Alicyclobacilli have been implicated in spoilages of pear, orange, peach, mango, and white grape juice, fruit juice blends, and tomato products. Not all Alicyclobacilli produce guaiacol, and thus not all species are of spoilage concern.
The first Alicyclobacillus species was isolated in 1967 from hot springs, and was named Bacillus acidocaldarius. However, it was not until 1982 that the organisms were implicated in the spoilage of apple juice. The next outbreak occurred in 1994, and based on 16S rRNA studies, a separate genus was proposed. A. acidoterrestris is considered the most important spoilage species within the Alicyclobacillus genus, but A. acidocaldarius, A. pomorum, and A. herbarius have also been isolated from spoiled product.
The canning industry works under the assumption that bacterial spores will not germinate at pH values below 4.6, and that acid-tolerant organisms are not very heat resistant. In this case, a low heat pasteurization process is applicable. However, the emergence of Alicyclobacillus as a spoilage organism has led some researchers to advocate using A. acidoterrestris as the reference organism to design pasteurization processes for high acid foods, just as the thermal death time of Clostridium botulinum was used to design the sterilization process for low acid canned foods. High-pressure processing has been shown to be effective at inactivating A. acidoterrestris spores in orange juice. One survey of 8556 samples of fruit and vegetable juices found Alicyclobacillus in 13% of samples, while another study found Alicyclobacillus in 6% out of 180 samples, and another found the bacteria in 14% out of 75 samples.
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