DEXTROSE TRYPTONE AGAR WITH STARCH
CAT Nº: 1146
For the cultivation of ‘flat-sour’ thermophilic and mesophilic microorganisms
in canned foods
FORMULA IN g/l
Tryptone
10.00
Bromocresol Purple
0.04
Dextrose
5.00
Bacteriological Agar
14.00
Soluble Starch
2.00
Final pH 6.9 ± 0.2 at 25ºC
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. Cool to 45-50 ºC and distribute into Petri dishes. The prepared medium
should be stored at 8-15°C. The color of the prepared medium is purple, slightly opalescent.
The dehydrated medium should be homogeneous, free-flowing and light greenish beige in color. If there are any
physical changes, discard the medium.
USES
DEXTROSE TRYPTONE AGAR WITH STARCH is used for the cultivation of ‘flat sour’ thermophilic and mesophilic
microorganisms in canned food. Flat sour spoilage should be controlled by food processors that want to ensure a long
shelf life of a product and to prevent early spoilage.
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, this 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 provide 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 colloid
protector 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.
Inoculate and incubate at 55 ± 2°C for 40-48 hours.
MICROBIOLOGICAL TEST
The following results were obtained from type cultures in the performance of the medium after incubation at a
temperature of 55± 2°C and observed after 40-48 hours.
Growth
Dextrose Fermentation
Bacillus coagulans ATCC 1050
Good
+ (yellow)
Bacillus stearothermophilus ATCC 7953
Good
+ (yellow)
Microorganisms
1
LABORATORIOS CONDA, S.A.
www.condalab.com
BIBLIOGRAPHY
Downes and Ito (ed.). 2001. Compendium of methods for the microbiological examination of foods, 4th ed. American Public Health
Association, Washington, D.C.
STORAGE
25ºC
Once opened keep powdered medium closed to avoid hydration.
2ºC
2
LABORATORIOS CONDA, S.A.
www.condalab.com