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.

    64/192 (page 56)
    5. incidence of high udder cell content due to sub-clinical mastitis or end of lactation. The different applications of the resazurin test which have been used in England and Wales are described below. It is recommended that whenever disc readings are taken a suitable North Daylight Cabinet should be used. Ten-Minute Platform Rejection Test 1. Application Since 1942 this test has been applied to samples taken from cans of milk which are set aside by the platform examiner at creameries because of appearance, smell, or which from previous records are suspected to be of doubtful bacteriological quality. The object is to detect the relatively small quantities of milk of really poor keeping quality, which, if added to a much larger bulk of average milk at a country creamery, would be likely to cause the loss of the bulked consignment through souring or tainting. This test continues to be used at all creameries. 2. Technique Cans of producer’s milk set aside by the platform examiner are sampled by the creamery laboratory staff and the test commenced within half an hour of arrival of the milk on the creamery platform. One ml of standard resazurin solution is added to 10 ml of the milk sample in a 6 x # in. test-tube, closed with a rubber stopper. After inversion, the tube of milk is placed in a water bath at 37° and after 10 min incubation the stage of resazurin reduction (disc reading) is determined, using a comparator. Full details are given in Technique No. 32. 3. Guide Standards Cans of milk giving disc readings of 34 or less are rejected and returned to the producer, whereas milk giving a disc reading of 4-6 is accepted, though milk with pronounced taint or defects (blood, etc.) likely to affect the bulk is rejected even if the results of this test are satisfactory. 4. Interpretation . It can be safely assumed, from the farm advisory point of view, that milk rejected on the ten-minute platform resazurin test has either been produced under unhygienic conditions, handled in heavily contaminated equipment, insufficiently cooled, or held overnight at high atmospheric temperatures. High cell counts (more than 750,000/ml) in bulk milk rarely affect the reduction of resazurin to such an extent as to give a disc reading of 34 or less within 10 min. Most milk samples giving disc readings of $ or o on this test have colony counts of more than 108/ml and are usually tainted or sour. . One-Hour Test 5. Application This test has sometimes been employed for routine sorting at receiving depots to detect raw milk supplies of doubtful bacteriological quality, and