Milk quality and udder health: Test methods and standards

There are a number of measurements which may be used to evaluate the quality of milk, whether in terms of the physiological state of the producing animal, the suitability of the milk for processing, or its hygienic quality. These may be divided broadly into methods that enumerate the somatic cells in milk and those that enumerate the bacteria in milk. In both cases, measurements may count the total cells only (bacterial or somatic) or differentiate into different types (or species, in the case of bacteria) of cell to provide additional diagnostic or discriminatory information. In many countries, based on cutoff levels, which vary from country to country the results of total counts of somatic cells or bacteria may be used in accept or rejecting milk samples for processing or consumption. For all measurements of this type, a range of methods with different advantages and disadvantages have been reported. In addition, alternative surrogate indices that are based on the measurement not of cells directly but of correlated biochemical parameters (e.g., enzyme activity, acute-phase proteins, bacterial DNA) have been reported. In this article, the principal methods for this type of assessment of milk quality are described and compared.