Nitrogen
fractions
The
total nitrogen of potato tuber comprises soluble protein, insoluble protein and
soluble non-protein nitrogenous compounds (NPN). The NPN compounds are free
amino acids, the amides asparagines and glutamine and small amounts of nitrate
nitrogen and basic nitrogen compounds including nucleic acids and alkaloids.
Though reducing sugars are the major determinants of processing quality,
anomalies in the relationship of reducing sugars to fry colour do occur and free
amino acids may also act as determinants of processing quality. Protein
degradation has been associated with the end of tuber dormancy and the
mobilization of nitrogen reserves for sprout formation and the breakdown of
proteins is consistent with an increase of proteolytic enzyme activity. Protein
synthesis may also occur during potato storage, although low temperature
conditions reduce the rate of this process while enhancing protease activity.
The free amino acid content changes with storage duration. Glycoalkaloids are
another important nitrogen fraction and about 95% of the total glycoalkaloids
present in potato tubers are accounted for by two compounds viz. alpha solanine
and alpha chaconine. It has been suggested that at low concentrations, the
presence of glycoalkaloids may enhance the flavour of potatoes, but at levels
exceeding 15 mg/100g fresh weight, a bitter taste may be perceived. Potatoes
containing more than 20 mg of glycoalkaloids per 100g fresh weight of tuber are
not safe for human consumption. The rate of glycoalkaloid accumulation was
independent of the levels found at harvest and significant interactions are
observed between the cultivars and temperature.
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