AS 5013.14.2:2009 pdf – Food microbiology Method 14.2: General procedures and techniques-Colony count-Membrane filtration method.
Where used, a filter pad shall be soaked with liquid medium as follows:
(a) Using forceps. place a filter pad in a Petri dish.
(b) Add sufficient liquid niedium to saturate the pad and to result in a small excess. NOTE: A 5 cm (in diameter) filter pad will require 2.0 mL to 2.5 mL of medium.
6.2.2 Solid me’diun, Where a solid medium is used, the plates should be prepared as follows:
(a) Pour sufficient volume of’ molten medium into a Petri dish to give a depth of approximately 5 mm. Allow to set.
(b) As soon as the medium has set, remove excess moisture from the plates by one of the following methods:
(i) Incubate the plates open with the internal surface of the base facing downwards and with the base resting on the lid, for the minimum time necessary to obtain plates free from condensate. e.g. at 37°C for about 2 h or at 45°C for about I h. Do not dry plates at temperatures above 45°C’.
(ii) Incubate at 37°C for 16 Ii in the inverted position with the lids on. If the plates are not then free of condensate. open them and incubate as described in Step (i) above until dry.
(iii) Other suitable time/temperature regimes. For example, on bench over night. NOTE: In humid climates either extend drying time or place a tray of desiccant. such as silica gel, in the base of the drying oven.
7 FILTRATION PROCEDURE
7.1 General
The optimum volume of liquid to he used will depend upon the amount of undissolved solids in the sample and the expected count.
If the expected count is high. suitable dilutions of the sample should be made (see Note). Where the expected count is uncertain, it is recommended that two determinations be made using two different volumes. In this way. the probability thai at least one determination will be within the range 20 to 80 will be increased.
NOTE: The optimum number of colonies on the filter is about 50 and the volume of sample filtered should be such that the number of colonies 10 be counted on the membrane is not greater than so. For example. samples expected to contain less than 80 organisms per 100 ml require the filtration of at least 100 mL of sample for each test.
7.2 Procedure
The procedure shall he as follows:
NOTE: Where a pre-assembled filtration unit is used. Steps (a) to (c) below are not required.
(a) Assemble the funnel base in the filter flask and connect the flask to the source of vacuum. Apply a slight vacuum.
AS 5013.14.2:2009 pdf – Food microbiology Method 14.2: General procedures and techniques-Colony count-Membrane filtration method
