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All laboratories use modifications of the Kjeldahl method. These modifications differ with respect to choice of catalyst procedures for digestion, distillation and determination of the ammonia.

It is not only important to know which methods can be used to measure nitrogen or protein values, but it is also important to have insight in the reliability and precision of these data. To obtain that insight you will study the results of an interlaboratory trial in which protein content s of several products is determined by various experienced food labs. Study the results and answer the following questions.

Not right. If you want to determine protein values it is correct to account for the NPN fraction, because the protein values should be based only on the protein-nitrogen fraction. So the true protein values will be more similar to the values of laboratory 19, i.e. the laboratory that did account for the NPN fraction.

Right.

a

Only lab 19 made a correction for NPN (non-protein nitrogen). This correction appears to have the greatest effect on values for French beans (see graph) and milk powder (not shown). What do you expect with respect to the 'true' protein content in French Beans?

b

It will be close to 15.3 g/100g (i.e. Average protein contents taken all laboratories together)

It will be close to 11.7 g/100g (i.e. Average protein contents of laboratory no. 19)