Throughout the methods for food sampling it is suggested that initial 1: 10 dilution of food samples be prepared from 10 g amount of the food, as frequently this is the largest sample that can conveniently be handled in many laboratories. Nevertheless, it is recommended that, if possible, 1:10 dilutions are prepared by suspending or homogenizing 50 g, as this gives much more reliable and representative results, whilst remaining a quantity reasonably easy to handle.
It is preferable to prepare the initial 1: 10 dilution by using a sterile bottle marked at 100 ml (for a 10 g sample) or at 500 ml (for a 50 g sample), the homogenate being made up to volume with diluents. Subsequent dilution can then be prepared volumetrically by pipette. Calculations of counts can correctly be expressed per gram of sample. The more usual method of preparing a 10-1 dilution by adding 90 ml of diluents to 10 g of food introduces an uncertainty into the calculation as the extent of solubility of the food and the food volume are unknown.
It should be noted that the use of refrigerated diluents when the sample is not at refrigerator temperature may cause a drop in count as the result of cold-shock. Most workers have been in agreement that it is usually Gram-negative bacteria that show cold-shock, that exponential-phase cells are most susceptible (stationary-phase cells being most resistant) and that the effect is more marked in quarter-strength Ringer’s solution than in sucrose solutions or complex diluents. However, there has been at least one report of a Pseudomonas that became more susceptible to cold-shock as the cells aged.