some microorganisms may fail to grow and form visible colonies on the agar medium as a result of unfavourable conditions of temperature, oxygen or nutrition, or because the cells are weak.
Hence the colony count may nor reflect the actual number of
viable bacteria in the food.
Apparatus an a) b) c) d) e)
C d Glassware Petri dishes 90-100mm, glass or plastic Pipettes 1,5 and 10ml, graduated (total-flow) Water bath, 45+10C Colony counter Incubator,35+ 1o Culture media and diluent
Buffered peptone water (BPW)
Plate count agar(PCA)
Procedure Preparation of food homogenate
Weigh 25 g of the mixed sample aseptically into a sterile blender jar or into a stomacher bag and add 225ml of BPW.
Blend the food at a speed of 15000- 20000rpm for not more that 2.5 min or mix in the stomacher for 20 sec.