AS 1766.3.10-1994

1 - AS 1766.3.10-1994 FOOD MICROBIOLOGY - EXAMINATION OF SPECIFIC PRODUCTS - LIQUID MILKS - DIRECT MICROSCOPIC COUNT
1 - PREFACE
1 - METHOD
1 - 1 SCOPE
1 - 2 DEFINITIONS
1 - 2.1 Microbial clump count
1 - 2.2 Individual count
1 - 3 REAGENTS
1 - 3.1 Carbol thionin staining solution
2 - 3.2 Methylene blue staining solution (modification of Newman-Lampert Stain)
2 - 3.3 Methanol
2 - 3.4 Ethanol
2 - 3.5 Xylene
2 - 4 APPARATUS
2 - 4.1 Pipette or micropipettor
2 - 4.2 Microscope
2 - 4.3 Microscope slides
2 - 4.4 Guide plate (template)
2 - 4.5 Suitable platinum ‘rake’†
2 - 5 PREPARATION OF SAMPLE
2 - 6 METHOD A (WITH DEFATTING)
2 - 6.1 Preparation of smear for microscopic examination
3 - 7 METHOD B (WITHOUT DEFATTING)
3 - 8 MICROSCOPIC EXAMINATION OF STAINED SMEARS
3 - 8.1 Mounting the slide
3 - 8.2 Number of fields to be counted
3 - 8.3 Selection of fields
4 - 8.4 Counting
4 - 9 CALCULATIONS
4 - 9.1 Microscope factor (MF)
4 - 9.2 Microscopic count
4 - 10 REPORT

This Standard sets out two methods for counting the number of microbial cells andclumps in liquid milks by microscopic examination of a stained film.

This Standard sets out two methods for counting the number of microbial cells and clumps in liquid milks by microscopic examination of a stained film, viz.Method A- a procedure involving treating the film to remove fat before staining.Method B -a procedure not having a separate defatting step and which may be used on raw milk and similar products with low fat content in order to obtain results more rapidly.NOTE: The degree of accuracy in estimating microbial numbers by these methods depends on the number of microorganisms present in the milk. It is generally recognized that the degree of reproducibility by one operator decreases with counts of less than 500 000 per millilitre of milk, even when the technique is followed exactly.Factors contributing to this poor reproducibility within and between operators are inaccuracies in measuring volumes, area of smear, inadequate fixing, defatting and staining, poor microscopy, irregular distribution of microorganisms, small number of fields actually counted in proportion to fields present in the smear, and eye fatigue.

First published as part of AS 1095.1-1971.
Revised and redesignated in part as AS 1766.3.10-1991.
Second edition 1994.