As established in the previous chapter, potato is not a rich source of energy (approx. 335 kJ (80 kcal)/(100 g), but it supplies high-quality protein. This is of considerable importance in developing countries where energy supplies tend to be more readily available than protein. The nitrogenous constituents of the potato tuber have a high nutritional value compared with many other vegetable crops and there is a wealth of literature devoted to the subject.

Part 1 of this chapter addresses the factors affecting the composition and quality of tuber N and hence its contribution to the diet; Part 2 assesses ways of measuring the nutritional value. The last part discusses the possibilities for reclamation of valuable protein from waste processing. This may be of use to developing countries in planning potato processing operations.

Part 1: Composition of tuber nitrogen

Factors affecting total tuber nitrogen

The average contents of total protein in potato are approximately 2% (FWB) and 10% (DWB). Total protein is Kjeldahl N x 6.25, according to van Gelder (1981), although conversion factors of 5.7 and 7.5 have been suggested (Vigue & Li, 1975; Desborough & Weiser, 1974). Wide ranges of crude protein contents have been reported, e.g. 11.6% to 16.1% (DWB) between different species of Solarium and 9.5% to 14% (DWB) between different varieties of S. tuberosum (Hoff et al., 1978; see also Espinola, 1979; Snyder & Desborough, 1980; International Potato Center, unpublished data). As the potato absorbs little water on boiling or steaming, the total protein content of boiled, unpeeled potato is virtually identical with that of the raw, uncooked tuber.