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.

    18/192 (page 10)
    free distilled water to make up the volume to about 95 ml. Filter, mix well and adjust the pH to 5-2 by adding N/20 NaOH solution (about 1°5 ml). Make up the volume to Ioo ml with carbon dioxide-free distilled water. Note A pH meter should be used in the pH adjustment to ensure addition of the correct amount of NaOH solution. If a meter is not available 1-5 ml of N/20 NaOH solution may be added and the pH adjusted to §-2 with a suitable buffer solution. When correctly prepared a 10 ml sample of the undiluted solution should require the addition of not less than 0:4 ml and not more than 0:45 ml of N/50 NaOH solution to give a colour similar to that of a 10 ml sample of N/1 potassium dichromate. (c) Acid Solution of Methyl Red. This solution should be freshly prepared for each day’s test. Transfer about 750 ml of carbon dioxide-free distilled water to a clean 1,000 ml measuring flask (B.S. 1792). Using a 10 ml grad- uated pipette (B.S. 700), add 8-3 ml of N/50 hydrochloric acid. Add 20 ml of strong solution of methyl red by means of a bulb pipette (B.S. 1583). Make up to 1,000 ml with carbon dioxide-free distilled water and mix well. (d) Comparison Solution of Methyl Red. Add 0-1 ml of N/20 NaOH solution to 10 ml of the acid solution of methyl red. (e) Other Solutions and Chemicals N/1 Potassium dichromate (49°04 g K,Cr,O, in 1,000 ml) N/20 Sodium hydroxide (2-00 g NaOH in 1,000 ml) N/s50 Sodium hydroxide (0-80 g NaOH in 1,000 ml) N/50 Hydrochloric acid (09-7293 g HCl in 1,000 ml) Methyl red (4—dimethylaminoazobenzene—2— carbolic acid, indicator grade) Ethanol (95 per cent alcohol). Procedure To represent the batch of tubes under investigation select at random at least six tubes from among all the tubes of one size made to one order, and test each one as described below. For a batch to comply with the re- quirements for ‘Special Grade,’ each of the selected tubes shall pass the test. As the tubes may have been stored for short, unequal periods, rinse each one first with tap water, then with 5 per cent v/v aqueous solution of glacial acetic acid, followed by three thorough rinsings inside and out with tap water. Drain for I min, then rinse inside and out with distilled water three times, draining for I min between rinsings. Immediately after the rinsing procedure, fill the tubes to nine-tenths of their overflow capacity with the acid solution of methyl red and close each with a free-fitting cap of inert metal foil (e.g., tin or platinum). Place the capped tubes in a rack or holder of chemically inert material so that they are held upright, and place this in an autoclave with the tubes com- pletely above water-level. Close the autoclave, leaving the valve open, and heat for 3 min with steam issuing freely from the valve to ensure that no air remains inside. Then close the valve and allow the temperature to rise to to 121°. Maintain this temperature for half an hour, then discontinue heating and open the valve slightly to allow the temperature and pressure to fall until it is possible to remove the tubes from the autoclave.