Nitrogen in milk

Reinart, A. and Nesbitt, J. M. 1956A. The distribution of nitrogen in milk in Manitoba. 14th Int. Dairy Cong, 1, 925-933. 

I KinxjTiON OF Nitrogen in Milk (Per Cent Total Nitrogen). 

J. Junsomboon, J. Jakmunee, Flow injection conductometric system with gas diffusion separation for the determination of Kjeldahl nitrogen in milk and chicken meat. Anal. Chim. Acta 627 (2008) 232—238. 

Figure 2. The effect of metabolizable energy intake on the proportion of nitrogen in milk and that excreted in faeces and urine. The responses have been adjustedfor study effect.

Initially, it was believed that milk contained only one type of protein but about 100 years ago it was shown that the proteins in milk could be fractionated into two well-defined groups. On acidification to pH 4.6 (the isoelectric pH) at around 30°C, about 80% of the total protein in bovine milk precipitates out of solution this fraction is now called casein. The protein which remains soluble under these conditions is referred to as whey or serum protein or non-casein nitrogen. The pioneering work in this area was done by the German scientist, Hammarsten, and consequently isoelectric (acid) casein is sometimes referred to as casein nach Hammarsten. 

Antisera to cloxacillin/oxacillin/dicloxacillin and cefuroxime were also produced by similar procedures and successfully utilized in methods for the detection of these antibiotics in milk (34). Unfortunately, a number of other -lactams including aminopenicillins and some cephalosporins were not amenable to this mixed anhydride procedure. Thus, a carrier protein derivatization procedure was used to allow cross-linking of cephalosporins, such as cephataxime that has an acetoxy side chain, to ovalbumin. Because acetoxy groups react readily with the heterocyclic nitrogen atoms, the latter were introduced into ovalbumin through the carbodiimide-mediated derivatization of protein carboxyl groups with amino-methylpyridine (34). 

Gas chromatographic methods have been successfully used for the determination of penicillin molecules bearing neutral side-chains in milk and tissues (95, 97), but cannot be used for amphoteric -lactams. Gas chromatography of penicillin residues is further complicated by the necessity for derivatization with diazomethane. This derivatization step is particularly important because it not only leads to formation of the volatile penicillin methyl esters but also improves their chromatographic properties (thermal stability and decreased polarity). Using a fused-silica capillary column in connection with a thermionic nitrogen-selective detector, excellent separation and sensitivity figures were obtained. 

A large number of N-containing compounds of low molecular weight are not precipitated with proteins by 12% trichloroacetic acid. Some small peptides are included in this group. These nonprotein nitrogen (NPN) constituents aggregate about 1 g/liter and account for about 6% of the total N (i.e., 250-350 mg of N per liter). The principal NPN components are listed in Table 1.6 (Wolfschoon-Pombo and Kloster-meyer 1981). The wide variations in concentrations that have been reported for these constituents probably arise from the fact that many of them are metabolites of amino acids and nucleic acids and from the fact that their concentrations in milk depend on the amounts of those substances consumed by the cow. 

Table 1.7 lists a number of nitrogenous compounds that have been detected in milk, in addition to those listed in Table 1.6. 

The proteolytic systems of psychrotrophic bacteria selectively attack /3- and as-caseins (Cousin and Marth 1977A), whereas whey proteins are relatively unaffected. Growth of psychrotrophic bacteria in milk results in decreased stability of casein, as measured by rennet coagulation time and heat stability (Cousin and Marth 1977B). Growth of psychrotrophs in milk also causes an increased rate of acid production by starter cultures as a result of increased quantities of readily available nitrogen compounds (Cousin and Marth 1977C.D). 

Musty or potato-like flavor and aroma have been observed as a defect in milk (Hammer and Babel 1957) and Gruyere de Comte cheese (Dumont et al. 1975). This off-flavor results from the production of nitrogenous cyclic compounds by Pseudomonas taetrolens and P. perolens (Morgan 1976). Musty-flavored compounds produced by these organisms include 2,5-dimethylpyrazine and 2-methoxy-3-isopropyl-pyrazine. The Gruyere de Comte with potato off-flavor contained 3-methoxy-2-propyl pyridine, as well as alkyl pyrazine compounds (Dumont et al. 1975). Murray and Whitfield (1975) postulated that alkyl pyrazines are formed in vegetables by condensation of amino acids such as valine, isoleucine, and leucine with a 2-carbon compound. Details of the synthetic mechanism in pseudomonads are unknown. 

This is formed of the nitrogenous substances (casein, albumin) and fats contained in milk, separated by coagulation (by rennet or by acidification). As a result of special fermentations which occur during the maturation of the cheese, these give rise to soluble albuminoid substances (albumoses, peptones, etc.), amino-adds (phenylaminopropionic add, tyrosine, leucine, etc.), ammoniacal products, fatty adds (lactic, propionic, caproie), etc. Cheese also contains water and mineral salts, including added sodium chloride. 

The drying technology used exerts a major influence on triacylglycerol oxidation and on the levels of OS in milk and other powder products, both immediately after drying and upon storage. Nitrogen oxides (NOx), which... 

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