Milk handling and cold storage

A cheese factory operating to capacity may have to store milk cold for a period of 1-3 days prior to manufacture. Prior to arrival at the creamery, the milk may have been cold stored on the farm for up to 3 days and then transported substantial distances from farm to creamery. During storage and transport the temperature will be below 60C, and the milk will be subjected to shear through pumping and agitation. Cold storage and shearing encourage a number of physicochemical changes in the milk which potentially impact on cheese yield. These include the solubilisation of micellar caseins, particularly ^-casein, and of colloidal calcium phosphate [4], leading to an increase in serum casein and soluble Ca. There is an increased susceptibility of serum casein to hydrolysis by plasmin and proteinases from psychrotrophic bacteria [7] or somatic cells [8] and plasmin. The milk fat globule membrane may be damaged by shearing, and free fat may be hydrolysed by lipases from psychrotrophic bacteria or the indigenous lipoprotein lipase in milk, resulting in a decrease in the level of fat. Excessive cold storage of milk can impair its rennet coagulation properties [30], leading to reduced recovery of protein and fat and a reduction in cheese yield.

It is generally agreed that a level of <106 cfu psychrotroph ml-1 will have little effect on the cheesemaking properties of milk, and that pasteurisation effectively reverses the physicochemical effects of cold storage. Provided milk is not excessively cold stored, the psychrotroph counts do not exceed 106cfuml—1 and coagulum cutting times are suitably adjusted, the effects of cold storage of milk for several days will be minimal.

However, to ensure psychrotroph levels do not become excessive, milk may be deep cooled (<20C), or thermised [13] at a subpasteurisation temperature (e.g. 57-68 0C) for 10 to 15 s, prior to cold storage. These treatments reduce the psychrotroph load during storage so that casein and fat degradation are minimised.

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