IDF Standard Method

This method is completely described in the International IDF Standard 87 (IDF, 1979). It is specifically designed to determine the dispersibility of instant dried milk and is also a rapid routine method to determine wettability (i.e., wetting time). A certain amount of dried milk powder (26 g for instant dried skim milk and 34 g for instant dried whole milk), held in a glass tube with a glass plate on the top of a beaker, is scattered on the water surface by withdrawing the glass plate underneath the glass tubing. The water in the beaker weighs about 250 g and its temperature is adjusted to around 25◦C. After a defined procedure for stirring, sampling, and solid content measuring, the dispersibility

D is calculated as (IDF, 1979):

D= 962CS

100− (W + CS)

, for instant dried skim milk (3.12)

D= 735CS

100− (W + CS)

, for instant dried whole milk (3.13) where CS is the total solid content of the liquid in % (m/m), and W is the water content of the

pre-treated test sample in % (m/m). According to this IDF standard, a dried milk powder is regarded as failing to possess sufficient instant characteristics if its D value is less than 85% for dried whole milk or less than 90% for dried skim milk.

The apparatus and procedure to measure the wettability of instant dried milk is almost the same as that for the dispersibility measurement, except that the following stirring, sampling and solid content measuring steps are omitted. After the test sample is evenly spread on the water surface, the time required for all the sample particles to sink below the water surface or remain on the surface but have a typical “wet” appearance is recorded. The wettabilityθwetis calculated as:

θwet= θ − 60 (3.14)

whereθ is the recorded time in seconds.

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