Bacteriological techniques for dairy purposes.

  • Great Britain. Ministry of Agriculture, Fisheries and Food.
Date:
1968
Catalogue details

Licence: Public Domain Mark

Credit: Bacteriological techniques for dairy purposes. Source: Wellcome Collection.

    41/192 (page 33)
    selected colonies is an essential preliminary, and the inoculum for all tests should be a 24 hr culture in peptone water. 18. COLI-AEROGENES COLONY COUNT ON VIOLET RED BILE AGAR The determination of the coli-aerogenes content of milk, dairy products and rinses and swabs of dairy equipment by means of the colony count on violet red bile (V.R.B.) agar has been shown to be a convenient method, which may have certain advantages over the presumptive coli-aerogenes test using MacConkey’s broth. It has also been found suitable for investigating the incidence of different types of coli-aerogenes organisms in milk products and on dairy equipment. British and American brands of dehydrated V.R.B. agar are available and give comparable colony counts. It is recommended that either should be used in preference to V.R.B. agar made up in the laboratory from ingredients. 1. Preparation of the Medium Rehydrate the medium by suspending the recommended quantity in one litre of cold distilled water and steaming till all granules have completely dissolved. Dispense the hot medium into sterile containers. The medium should not on any account be overheated. If the heated medium is stored at 3—5° it may be used up to 3 weeks after preparation. 2. Preparation of Dilutions It is recommended that the following amounts of different materials should be plated. (a) Raw milk: 1 ml-1o-? ml. (b) Pasteurized milk: duplicate quantities of 5 ml with an additional plating of 1 ml in duplicate. (c) Rinses and swabs: 1 ml and 107! ml of rinse or swab solution. (d) Butter and cheese: duplicate 5 ml quantities of a 1071 dilution with an additional plating of 1 ml of a 107} dilution. 3. Inoculation of Petri Dishes The method prescribed in Technique No. 16 should be used. 4. Pouring Petri Dishes To each dish add 15 ml of melted V.R.B. agar at a temperature of 45-47°. The tubes or bottles of medium should be dried and the mouths flamed before pouring. Immediately mix the inoculum. As soon as the medium has set, an additional 4-5 ml of V.R.B. agar should be poured over the surface of the inoculated agar and allowed to set. 5. Incubation Incubate the Petri plates bottom upwards in stacks of not more than six deep. For coli-aerogenes colony counts on milk and milk products, rinses and swabs of dairy equipment, and milk cans and bottles, incubation at