Dextrose Tryptone Agar with Starch

Flat sour spoilage is produced by two species of Bacillus, Bacillus coagulans and Bacillus stearothermophilus. The spoilage occurs in low-acid foods. These organisms produce acid without gas, causing the pH to drop to between 0.3 and 0.5.

Flat sour spores are not directly pathogenic microorganisms. If the product has not been cooled properly, these organisms have the potential to grow and decrease the shelf life of the canned foods. A food spoiled by flat sour organisms will produce a sour smell and taste.

Tryptone provides nitrogen, vitamins, minerals and amino acids essential for growth. Dextrose is the fermentable carbohydrate providing carbon and energy. Starch in the medium acts as a growth factor, functioning like a protective colloid, and neutralizes toxic products that form during the development of the organisms. Bromocresol purple is the pH indicator. When dextrose is fermented, acid is produced and bromocresol purple turns from purple to yellow. Bacteriological agar is the solidifying agent.

Typical formula in g/L, adjusted and/or supplemented as required to meet performance criteria.

Preparation

Suspend 31 grams of the medium in one liter of distilled water. Mix well and dissolve by heating with frequent agitation. Boil for one minute until complete dissolution. Dispense into appropriate containers and sterilize at 121 °C for 15 minutes. Cool to 45-50 °C and distribute into Petri dishes.

Instructions for use

1. Inoculate the prepared sample and incubate the plates at a temperature of 55 ± 2 °C for 40-48 hours.
2. Colour change of the medium from purple to yellow indicates dextrose fermentation.

• Downes and Ito (ed.). 2001. Compendium of methods for the microbiological examination of foods, 4th ed. American Public Health Association, Washington, D.C.