Estimation of phenylalanine (Phe) contents in dehydrated soups: significance of non-protein nitrogen

Abstract

The direct analysis of phenylalanine (Phe) in food with low protein content is difficult, time consuming and expensive. In general, more convenient indirect methods based on raw protein contents have been used, considering that natural proteins contain about 4% of Phe. In this paper the Phe contents of 22 commercial dehydrated soups were estimated taking into account the contents of total N, protein N and non-protein nitrogen as well as the contents of monosodium glutamate (GMS) which were analyzed independently. The amounts of crude protein (total N multiplied by an adequate conversion factor) ranged between 6.1 and 21.5 g/100 g, showing similarity with protein values on nutritional labels. The actual protein contents were significantly lower and Phe contents from these data varied between 51 and 651 mg/100 g sample. The GMS contents showed a significant contribution representing 2.5 to 42.7% of total N. In conclusion, due to the high contents of this flavor enhancer in some formulated soups, Phe should be estimated only after a previous extraction of non-protein nitrogen in order to avoid an overestimation of protein, and as a consequence, of Phe contents. Some dehydrated soups with low Phe contents had been found, creating the possibility to include them as an additional food choice for phenylketonurics.