Raw milk composition for payment purposes is reported in units of Kg of component per hL of milk at 4C. This is referred to as weight over volume (w/v) measurement. Measurement in units of w/v is dependent on milk density which in turn is affected by both composition and temperature. Weight over weight (w/w) measurement (eg., Kg component per 100 Kg of milk) results in a significantly smaller value because the density of milk is more than 1 Kg/L. Measurement by w/w has the advantages that: (1) most wet chemical reference analyses used to calibrate milk analysers report composition in units of w/w; (2) w/w values are independent of milk temperature. However, milk composition for payment purposes is reported in units of w/v because the volume of milk is easily measured with dip sticks or volumetric meters. Weight measurement would require installation of farm bulk tanks on expensive load cells. Volume rather than weight measurement of milk and other liquids is also more convenient in the plant.
In any case, the important point with respect to accurate standardization is to ensure that all measurements and calculations use the correct units. When component estimates are given as percentages, the basis of measurement must be stated as w/w percent (eg., Kg fat per 100 Kg milk) or w/v percent (eg., Kg fat/hL of milk). In this manual composition values given in percent always mean w/w. Cheese composition will always be stated in percent w/w (eg., 30% fat in Cheddar cheese means 300 g fat per Kg cheese). Similarly, 3.3 % fat in milk means 3.3 Kg fat per 100 Kg milk. If weight over volume units are used I will always state the specific units, eg., 3.3 Kg/hL. Because composition of producer milk is reported to processors in units of Kg/hL and because milk metering systems are volumetric, I will usually report milk composition in units of Kg/hL.
It is important to ensure that milk analysers are calibrated in the appropriate units and the correct units are subsequently used for milk standardization calculations and calibration of automated standardizing systems. Wet chemical analysis is normally done by weight, so reference results for milks used to calibrate milk analysers are normally reported in units of percent by weight and it is convenient to calibrate milk analysers in percent by weight (eg., Kg/100 Kg). If required, w/v values can be estimated using the following equation.
w/v=w/w x pT where pT is density at temperature T
Note, that the density must be known at the given temperature. For example, if the milk composition was given in units of w/w and you are metering milk into your cheese vat at 32C you need to know the density of the milk at 32C. For milk of average composition (4.0 % fat), the density can be estimated according to the following equation(1).
pT = 1.0366 - .00035T where pT is density at temperature T
Density values for milk of average composition (4% fat) at some temperatures relevant to cheese manufacture are: