What is the basis of the Kjeldahl procedure for total protein?

The Kjeldahl procedure for determining total protein is fundamentally based on the principle that proteins contain nitrogen. Specifically, the procedure quantifies the nitrogen content in a sample, which can then be used to estimate protein concentration.

The basis for this method lies in the fact that the nitrogen content in proteins is relatively consistent, as proteins are formed from amino acids, which commonly contain amino groups (NH2). While it's true that proteins average about 16% nitrogen by weight, the more critical aspect of the Kjeldahl method is that it relies on measuring nitrogen because all proteins will contain this element in a definable and appropriate proportion. As such, if the nitrogen content can be accurately measured, it can be translated into a total protein measurement.

Option A focuses on the consistent presence of nitrogen in proteins, which is integral to the validity of the Kjeldahl method. The measurement of nitrogen allows for the calculation of protein content, making this option the foundation of the procedure. While the percentage of nitrogen content is an important consideration, the first option encapsulates the essential principle driving the Kjeldahl methodology.