If for Rye the factor 6.25 was used instead of the Jones’ factor (5.83) this

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Therefore Jones et al (1941) proposed food specific conversion factors to account for differences in amino acid composition in foods (see table).

Besides, foods contain also (a small) non protein nitrogen (NPN) fraction: substances such as nucleic acids, urea, ammonia, nitrates, certain phospholipids etc. contain also nitrogen. The following products are examples of products that contain a high amount of NPN:

• Cow’s milk (5-6% of the total nitrogen)

• Seafood (many shellfish and crustaceans)

• Human breast milk (15 % of the total nitrogen)

The Jones’ factors are based on protein nitrogen only and do not include NPN.

Proteins differ with respect to nitrogen contents because of differences in amino acids composition. Consequently, it is reasonable to use food specific conversion factors instead of a factor of 6.25 for all foods.

Right!

Not right. Remember that you should multiply nitrogen values by the conversion factor to get protein values. If you use a conversion factor that is too low you will underestimate the protein values.

a
b
c

leads to over estimation of protein values.

leads to under estimation of protein values.

does not affect protein values.