Amino nitrogen content, measurement

Determination of total nitrogen content of the gastric juice, introduced by Wolff and Junghans (W22) and used later by others (D4), is inadequate for quantitation of gastric juice mucosubstances because proteins, peptides, and amino acids contribute to this measurement. Other authors precipitated gastric juice with methyl alcohol or acetone and determined the amount of alkali bound by the precipitate (M4-M6). These methods determined only the buffer capacity of the precipitate,... 

In field tests, sugar content was determined by refractometry and the total amount of amino acid nitrogen was measured by Kjel-dahl method and the quantity of organic acids was determined by titration. 

The biuret method The bimet method is a colorimetric technique specific for proteins and peptides. Copper salts in alkaline solution form a pruple complex with substances containing two or more peptide bonds. The absorbance produced is proportional to the number of peptide bonds that are reacting and therefore to the number of protein molecules present in the reaction system. Thus, the biuret reaction with proteins is suitable for the determination of total protein by spectrophotometry (at 540-560 nm). The method is used extensively in clinical laboratories, particularly in automated analyzers in which protein concentration can be measured down to 0.1-0.15gl. The use of bovine or human serum albumin to standardize the biuret method is well established. ITigh-purity albumin contains only amino acids its nitrogen content is a constant fraction... 

Reductions in dietary protein level may decrease milk yield and, almost invariably, non-protein nitrogen content. Milk protein is httle affected until intake of protein falls below 60 per cent of requirement. This is probably due to an insufficiency of essential amino acids, primarily methionine, followed by threonine and tryptophan. When duodenal amino acid supply has been measured and deficient amino acids infused directly into the bloodstream, there have been large increases (approximately 5-8 g/kg) in milk protein content. The prediction of the amino acid profile at the small intestine in dairy cows is fraught with difficulties, and feeding studies that have attempted to rectify amino acid imbalances through supplementation have produced variable results. 

The nitrogenous components of grapes and must that are metabolically available to yeast are present as ammonium salts (NH4 ) and amino acids primary, collectively known as yeast assimilable nitrogen (YAN). Therefore, a complete evaluation of the nutritional status of juice or must requires measurement of both fractions. The total nitrogen content of juice not only contains YAN but peptides and proteins as well. However, the latter two fractions are not thought to play a significant role in the nutritional needs of Saccharomyces during fermentation. 

Table 26-2 shows the percent DNA retention for the complete set ofsol/gel coatings, and a reference sample (standard amino-silanized DNAmicroarray). The percent retention values presented are an average of measurements from six spots on three different slides with the same processing history. The nitrogen contents (in at%) for the different coatings, determined by XPS, are also listed in Table 26-2. 

Biomass growth can also be determined by measuring the cell or mycelia component that is not present in solid substrate. Protein content of the biomass can be measured by determining the nitrogen content by the Kjeldahl method. Accurate measurement of biomass can be obtained using an amino acid analyzer. This method of biomass measurement is reliable if the solid substrate has no protein content. 

Walter et al. (38) measured the protein efficiency ratio (PER) of flour prepared from sweet potatoes which were cooked in a drying oven. Because the PER is determined on the basis of a diet containing 10% protein, the Jewel and Centennial sweet potatoes used in this study were stored until sufficient starch had metabolized to increase crude protein content to 11.25% (dry basis). When the flour was fed to Sprague-Dawley strain rats, the corrected PER values were 2.22 and 2.00 for Centennial and Jewel cultivars, respectively, compared to 2.50 for casein. Centennial had the highest PER value of the two cultivars because its NPN content was lower. The net effect of increased NPN content is to lower the amount of essential amino acids as a percentage of the total nitrogen and thus decrease the PER value. 

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