Calculation of nitrogen-to-protein conversion factors for food products

Twenty-three food products, selected to represent the major classes of foods, were analyzed for amino acids (AA's), with a Beckman 119 BL AA analyzer and nonprotein nitrogen (NPN), with a stirred cell ultrafiltration and a Beckman 119 BL AA analyzer. The amide N was determined separately to estimate the Gln and Asn contents. Nitrogen-to-protein conversion factors ( N:P factors ) were calculated from these AA plus amide N data. Variability in AA content within the animal and grain products were not extensive, except for the total basic AA's (BAA's) plus amides, Pro, Cys, and Leu. The fresh food products had greater variation in their AA compositions than those of the former foods. Generally, Glu and Asp concentrations were highest among the food products than the other AA's, while the levels of Sulfur AA's (SAA's) were quite low among the fresh vegetables, fruits and mushroom. Nitrogen-to-protein conversion factors for milk products, meat and fish products, grains and the fresh foods were variable, ranging from low of 4.8 to 6.1. On statistical analysis, the twenty-three foods were combined into eight groups in which the N:P factors are similar. For practical purposes, the N:P factors for the various food products were 6.1 for milk, cheese, casein, fish and tomato, 5.65 for chicken, beef, corn, sorghum, wheat, egg, rice, field peas, carrot and mushroom, 5.2 for potato, beet, lettuce, banana and as low as 4.8 for cabbage. For accurate protein contents of the individual food product, the use of their individual N:P factors is proposed. The NPN contents of the food products were higher in the plant foods than those of the animal foods. While the values for the animal foods ranged from 0.4-35.5 mg/gN, those of the plant origin varied from 12.5 - 238.5 mg/gN. Variability in the total nucleic acid nitrogen (NAN) of the animal products was not great, ranging from 0.6-3.7 mg/gN. While carrot, apple and banana had high values (67.1-95.9 mg/gN), the other foods contained 13.9-32.4 mg/gN. The AA compositions of the dialyzable NPN of these food products were quite variable but Glu and Asp were the main AA components of the NPN common to them. In general, the AAN ranged from 21.1-91.4% of the total diffusible NPN. The remainder plus NAN constituted the nonnutritive N (NNN) and ranged from as low as 1.1 mg/gN for casein to a high value of 24.6 mg/gN for sorghum among the animal and grain products. This NNN was quite considerable in the fresh foods and accounted for 26.4 - 156.1 mg/gN or 2.6-15.6% of their total N contents