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.

    35/192 (page 27)
    touch the tip against the neck of the sample container. Transfer the charged pipette to the first dilution tube and, with the tip touching the side of the tube at a point 4 to 1 in. above the level of the diluent, blow out the contents: allow 3 sec to elapse and blow out again. With a fresh pipette, proceed as above and mix the first dilution by sucking up and down 10 times: measure I ml and transfer to the second dilution tube: discard the pipette. Prepare further tenfold dilutions in the same way. 2. Inoculation of Petri Dishes Take a fresh 1 ml pipette, and using the mixing technique described in para. I, inoculate 1 ml from the last dilution to a Petri dish (B.S.611: 1956). Hold the tip of the charged pipette about 4 in. above the bottom of the dish, and blow out the contents of the pipette. Allow 3 sec to elapse: touch the tip of the pipette against the dish at a point away from the liquid already delivered, and blow out the last drop. Inoculate Petri dishes from the next dilutions in the same way, using the same pipette or a fresh one. For general work, one Petri dish may be prepared from each dilution examined, but for special work it is preferable to prepare three plates from each dilution. | Alternatively, the Petri dish may be inoculated when the dilutions are made, i.e., before transfer of the 1 ml quantity to the next tube of diluent; the same pipette may then be used to inoculate the appropriate Petri dish. 3. Pouring Petri Dishes To each dish add 10 ml of melted agar medium (see Technique No. 6) at a temperature of 45-47°. The medium should be poured to one side of the inoculum. If tubes of media are used, dry and flame before pouring. Immediately mix the medium and inoculum by five to-an-fro movements, followed by five circular clockwise movements, followed by five to-and-fro movements at right angles to the first set, followed by five circular anti- clockwise movements. Allow the dishes to stand until the medium has set, invert and then transfer to the incubator. The time elapsing between the preparation of the dilutions and the pouring of Petri dishes should not exceed 15 min. The operations detailed in this para. and in paras. I and 2 should not be carried out in direct sunlight. If the poured plates are not placed in the incubator immediately after setting, they should be covered with a clean, dry cloth. 4. Incubation Incubate dishes bottom upwards and do not stack more than six deep. For times and temperatures of incubation the techniques for individual products should be consulted, but for normal colony counts on milk and milk products, swabs and rinses of dairy equipment, cans and bottles, incubation at 30+ 1:0°, for 72-2 hr is recommended. Water-jacketed incubators are to be preferred. Other time and temperature combinations for special purposes may be employed at the discretion of the bacteriologist. 5. Counting of Colonies Colonies should be counted within 4 hr of the expiry of the incubation period. A suitable counting chamber is desirable. To facilitate counting, a