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