As a general rule: Procedure A is used for samples that are known to contain low levels of free galactose. Procedure B is used for samples that are known to contain high levels of free galactose. If the free galactose content of a sample is unknown it is recommend that the galactose is measured as per the galactose assays in Procedure A.
For “lactose free” samples it is generally recommended that procedure B is used to maximise the absorbance range and enable the sensitive detection of lactose.
“Lactose free” dairy products have usually been processed whereby lactose in the original sample has been hydrolysed to glucose and galactose. These samples will contain high levels of free galactose and should be processed using procedure B. Some samples will be “lactose free” because the original sample never contained lactose so, assuming that the free galactose level is low, these samples can be processed using procedure A.

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