BS 4285-3.11: 1985 pdf download.Microbiological examination for dairy purposes.
1 Scope This Section of BS 4285 describes a method for the detection and enumeration of faecal streptococci in milk and dairy products. NOTE The titles of the publications referred to in this standard are listed on the inside back cover.
2 Definition For the purposes of this Section of BS 4285, the following definition applies: faecal streptococci those bacteria that form characteristic colonies when grown on the selective medium under the conditions specified
3 Principle A test portion or series of decimal dilutions is mixed with a selective culture medium in Petri dishes and incubated. The numbers of characteristic colonies on selected plates are counted and the number of faecal streptococci per millilitre or per gram of the original sample are calculated.
4 Diluents and culture medium 4.1 Diluents Use diluents described in BS 4285-1.2.
4.2.2 Preparation. Add the ingredients to the water and heat to boiling to dissolve. Adjust the pH so that it will be 7.2 after sterilizing. Dispense in flasks in convenient volumes and sterilize for 10 min at 121 ± 1 °C. Cool to about 60 °C and add 1 mL of a filter-sterilized 1 % aqueous solution of triphenyltetrazolium chloride per 100 mL of sterile medium. Mix to obtain uniform distribution. Cool to 45 °C and prepare the plates. NOTE It is recommended that a dehydrated complete medium (also known as KF streptococcus agar) be used following the manufacturer’s instructions. WARNING. Sodium azide is toxic and great care is required in the preparation of the medium. It also forms explosive compounds with metals and before disposal should be rendered safe by addition of excess sodium nitrite.
5 Apparatus
NOTE For details of apparatus including its preparation and sterilization, see BS 4285-1.2.
5.1 Usual microbiological laboratory apparatus.
5.2 Incubator, at 37 ± 1 °C.
5.3 Waterbath, at 45 ± 1 °C.
5.4 Petri dishes.
5.5 Total delivery pipettes, of 1 mL nominal capacity.
5.6 Colony counting equipment.
6 Sampling
Take the laboratory sample in accordance with BS 4285-1.1.
7 Preparation of the test sample
Prepare the test sample from the laboratory sample in accordance with BS 4285-1.1.
8 Procedure
8.1 Test portion and dilutions
Measure the test portion and prepare serial decimal dilutions from the test sample (see BS 4285-1.1), making as many dilutions as are necessary to produce acceptable counts at two successive
dilutions.
8.2 Inoculation of Petri dishes Inoculate the Petri dishes as described in BS 4285-2.1.
8.3 Addition of medium to Petri dishes Add 10 mL to 12 mL of the medium (4.2.2) at 45 ± 1 °C to each inoculated dish, mix and allow to set as described in BS 4285-2.1.
BS 4285-3.11 pdf download.