Alicyclobacillus spoilage in fruit juices

The low pH of juices and pasteurisation has always been regarded as an effective combination to control spoilage organisms such as lactic acid bacteria, yeast and mould.?There are however bacteria, such as Alicyclobacillus, that is heat resistant due to its ability to form endospores. Instead of eliminating the organism, pasteurisation will activate the spores which will germinate into vegetative cells and proliferate to high levels if conditions are favourable.?Juices provide a favourable environment for these bacteria as Alicyclobacillus thrive at low pH levels. Signs that juices are contaminated with Alicyclobacillus are not that easy to identify as it causes no gas.?The juice will appear normal and might only have slight sedimentation. Often the spoilage will only be detected once it has been opened, and a distinct off-flavour is noticeable.

The off-flavour from Alicyclobacillus has been described as smoky, medicinal and antiseptic. This is due to the production of guaiacol, which can be detected by humans at a low concentration of 2 ppb. Components such as 2,6- dibromophenol and 2,6-dichlorophenol has also been associated with Alicyclobacillus spoilage.?The level of Alicyclobacillus cannot be correlated to the level of guaiacol production as research studies have found contradicting results. It has however been found that the bigger the headspace in bottles, the quicker the taint will be produced.

It has been reported that Alicyclobacillus is able to survive pasteurisation temperatures of 23 min at 90°C and 2.4 - 2.8 min at 95°C.?Once germinated, Alicyclobacillus strictly grows aerobically between pH 2 – 6.5, with an optimum pH range between pH 3 – 5.5. As these bacteria are thermophilic, it can grow between temperatures ranging from 17° - 70°C, with the optimum temperature for growth ranging between 40° - 60°C. Alicyclobacillus cannot grow in products, such as fruit concentrates, that has a °Brix of 18 or higher. Therefore, if present in products that has a very high brix value, it will only be present in its spore form and will not lead to germination until the product has been diluted and exposed to heat.

As Alicyclobacillus has frequently been isolated from environmental sources such as soil, compost, fruit and manure it is understandable that one of the major contamination sources is the unwashed or poorly washed raw fruit. Spoilage is most prominent during warmer summer months, when volumes are large and natural cooling occurs slower.?Early detection and control of the organism during manufacturing and in the processing, environment is therefore of great importance.?

Numerous species have been identified in the genus, but only three have been implicated to cause spoilage in juice products namely, A. acidoterrestris, A. acidocaldarius and A. pomorum.?Various low pH media and methods have been evaluated to enumerate the different species in fruit juices. The general incubation temperature of 45°C will favour the growth of all species present in a sample, however, to distinguish between, A. acidoterrestris and A. acidocaldarius, the cultures need to be incubated at different temperatures. For A. acidocaldarius plates are incubated between 50° - 55°C and at 30°C for A. acidoterrestris.

"The juice will appear normal and might only have slight sedimentation. Often the spoilage will only be detected once it has been opened."        

?Alicyclobacillus spp. shows that pasteurisation and low pH combination cannot be considered the only way to ensure juices will be spoilage free. The organism is a real threat to juice industry that can cause severe financial damages and it is therefore recommended that juice manufacturers include Alicyclobacillus to their general micro testing criteria.??

?Author: Anneri Carinus (MSc Food Science - Stellenbosch University)

Photo credit: jmexclusives & silviarita