This allows you to calculate the actual level of protein in the 'dry matter' of the food, which excludes the moisture content.
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E.g. Given that Crude Protein is 9%, Moisture is 80%, actual dry level Protein can be calculated as follows:
Protein in dry matter = [9 / (100-80)] * 100% = 45%
This is especially useful in calculating the actual protein level in various brands of canned food, as canned food is mostly moisture. Hope this helps!