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.

    25/192 (page 17)
    Dissolve the Yeastrel and peptone in the distilled water in the steamer and adjust the reaction at room temperature to pH 7:4, using phenol red as the indicator (see Note 1) or using a pH meter. Add the agar (see Note 2) and the milk to the broth and autoclave at 121° for 25 min. In order to ensure thorough mixing and that heat treatment of the bulk at this stage is equivalent to the final sterilization of the tubed media, it is recommended that quantities of not more than 14 or 2 litres are autoclaved in 3-litre conical flasks. The hot medium is then filtered through paper pulp in a Buchner funnel. The pulp is prepared by mashing up small pieces of filter paper in water and boiling. The funnel should be inserted into an Erlenmayer flask fitted with a side piece. A single layer of filter paper should be laid on the top of the Buchner funnel to prevent the pulp being sucked through. The hot pulp is poured on to the filter paper and a filter pump is applied to suck through the excess water, which is then poured away. The pulp should be firmly packed down just before the last of the water is sucked through. At this stage a layer of filter paper is laid on the filter bed, so that the hot agar can subsequently be poured on to it without disturbing pulp. The filter when ready for use should have a total depth of about 1-5 mm. (A pulp layer of suitable and approximately the same depth for any size of funnel may be obtained by pulping an area of filter paper equal to four times the square of the diameter of the funnel. With ordinary grade filter paper 1 g of the dry paper is required for every 20 sq. cm of filtering area). The flask and funnel should be thoroughly hot before filtering commences, and these and the agar should be kept hot during filtering. The agar is taken direct from the autoclave, the hot agar poured on to the pulp where the filter paper was laid and the vacuum pump connected. The reaction of the filtrate is tested at 50° and adjusted if necessary to pH 7:0. Adjustment at this stage should not normally be necessary, but if it is needed at all frequently, the method of preparation should be checked. The medium is distributed in 10 ml quantities in 6x 32 in. test-tubes (B.S.625: 1959) and autoclaved at 121° for 15 min. Alternatively, for storage, I00-250 ml quantities may be filled into suitable bottles. The final reaction of the medium at room temperature should be pH 7:2. Notes 1. A brightness screen must be used with the Lovibond phenol red disc 2/IJ. 2. If shredded agar is used, it should be wrapped in muslin and washed in running water for I5 min, the excess water being squeezed out before the agar is added to the broth. 7. MEDIA FOR COLI-AEROGENES ORGANISMS 1. MacConkey’s Broth Commercial sodium taurocholate, sodium tauroglycholate or other satis- factory bile salt : : : ; 5g Peptone : : ' ; : 20g Sodium chloride. 52g Lactose ; 5 : ; 10g Distilled water , ‘ ; 1,000 ml