Shapton Medium

Principle And Interpretation

Bacillus stearothermophillus is a thermophile that is widely distributed in soil, hot springs, ocean sediment, and is a cause of spoilage in food products. It is commonly used as a challenge organism for steam sterilization validation studies. Flat sour spoilage occurs chiefly in low acid foods, such as peas, corn and lima beans but can also occur in medium acid foods such as spinach, green beans etc. Shapton and Hindes (1) formulated the medium for the enumeration of spores of Bacillus stearothermophilus, which cause flat sour spoilage in canned foods with pH more than 4.5.

Casein enzymic hydrolysate, peptic digest of animal tissue, beef extract and yeast extract in the medium provide carbon, nitrogen, vitamins and minerals required for bacterial metabolism. Dextrose is the fermentable carbohydrate. Bromocresol purple is the pH indicator, indicating dextrose fermentation visualized as a colour change from purple to yellow.

The sample under test is suspended in Ringers salt solution and then added to sterile molten Shapton Medium and is held at 100°C for 20 minutes. Then the temperature is slightly raised to 108.4°C and maintained for 10 minutes after which this is cooled to 50°C and plates are poured. The plates are incubated at 55°C for 48 hours.