The main microbiological aim of UHT processing is to inactivate spore-forming bacteria which could grow during storage and cause spoilage. The main targets are Bacillus species, particularly heat-resistant ones such as B. licheniformis and B. subtilus. Geobacillus stearothermophilus is an extremely heat-resistant sporeformer found in milk but because it only grows at temperatures above about 50°C, it does not cause problems in UHT milk unless the milk is severely temperature abused during storage. In comparatively recent times, another extremely heat-resistant sporeformer, B. sporothermodurans, has caused problems in UHT milk; unfortunately, unlike G. stearothermophilus, this organism is mesophilic, that is, it can grow at room temperature5.