Page 38 - FoodFocusThailand No.244 August 2026

STRONG QC & QA

The use of nitrogen-to-protein conversion factors can content. This approach provides both a more accurate
introduce substantial errors into the reported protein content. estimate of protein content and the amino acid profile
The conventional factor of 6.25 is based on the assumption required for calculating AAS and DIAAS. However, it is more
that proteins contain, on average, 16% nitrogen. In reality, the expensive, more time-consuming, and requires specialized
nitrogen content varies among proteins from different food procedures for certain amino acids. For example, tryptophan
sources. Consequently, Jones factors, which are specific to can be destroyed during acid hydrolysis, whereas cysteine
individual foods, have been developed to improve accuracy. and methionine must be oxidized before analysis
Typical conversion factors include approximately 6.38 for PDCAAS and DIAAS may provide different evaluations
milk, 5.70 for wheat, 5.71 for soybeans, and 5.18 for certain of protein quality. Because PDCAAS values are truncated
legumes and seeds. For example, a sample containing 2.0 g at 1.00, proteins such as whey protein, casein, egg protein,
of nitrogen per 100 g would contain 12.5 g of protein when the and soy protein isolate may receive similar scores despite
conventional conversion factor of 6.25 is used. If the sample differences in their nutritional quality. In contrast, DIAAS better
is wheat, however, using the appropriate factor of 5.70 gives distinguishes between different protein sources, particularly
11.4 g of protein, whereas milk would require a factor of 6.38. between animal- and plant-based proteins. However,
Thus, selecting an appropriate food-specific conversion factor protein quality is influenced by various factors, including raw
is essential for accurate protein quantification. material characteristics, processing methods, heat treatment,
Another limitation is the presence of non-protein nitrogen fermentation, and the food matrix. Moderate heat treatment
(NPN), which includes compounds such as urea in milk; can improve protein digestibility by reducing enzyme inhibitors
creatine and creatinine in meat; free amino acids and small in legumes. However, excessive heat treatment may cause
peptides in fermented foods; nucleotides in yeast or seafood; lysine to participate in the Maillard reaction, thereby reducing
chitin and chitosan; nitrate and nitrite; and nitrogen-rich its nutritional availability.
contaminants such as melamine and urea. These compounds The assessment of dietary protein requires consideration
may result in crude protein values that are higher than the true of both protein quantity and protein quality. The Kjeldahl and
protein content. Dumas methods are commonly used to estimate total protein
For instance, if milk powder contains 5.0 g of nitrogen per content; however, the selection of an appropriate conversion
100 g, of which 0.25 g is non-protein nitrogen, using the milk factor and consideration of non-protein nitrogen (NPN) are
conversion factor (6.38) gives a crude protein content of 31.9 essential for accurate determination. Protein quality can be
g per 100 g. Excluding the NPN before calculation yields (5.0 − evaluated using Amino Acid Score, PDCAAS, and DIAAS,
0.25) × 6.38 = 30.3 g per 100 g, a difference of approximately with DIAAS providing a more accurate assessment of the
5%. Therefore, accurate protein determination should combine digestibility and availability of indispensable amino acids.
food-specific conversion factors with the exclusion of NPN Therefore, the development of high-protein foods, infant
whenever appropriate. foods, senior nutrition, medical foods, and alternative protein
A more direct estimate of true protein is obtained from the products should consider multiple factors to ensure that
sum of amino acid residues. Proteins are hydrolyzed, individual nutritional quality is evaluated accurately and appropriately
amino acids are quantified by high-performance liquid for the intended population.
chromatography (HPLC) or ion-exchange chromatography,
and the amino acid residues are summed to determine protein
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