Human milk protein content

RP-HPLC was applied to separate and characterize 3-casein and a-lactalbumin fractions of human milk and to quantify milk protein contents.The determinations were performed in the linear range of 0.09-1.5 and 0.45-3.6 g/L for 3-casein and a-lactalbumin, respectively. Analysis of variance (ANOVA) showed significant differences (p < 0.05) between protein content in milk from different mothers at the same lactation time (l /2 months). A significant decrease (p < 0.05) was observed for the casein and a-lactalbumin concentrations during the first 6 months of lactation. 

It is well known that the protein content of milk from different species varies and is inversely related to the period of development of the young. The content of the B vitamins is likewise much higher in the milks of small, rapidly maturing animals63 than in human or cow s milk. There can be no serious doubt that the ability to produce milk at all, and also the composition of milk, is controlled to a large extent by genetic factors. On this basis one would expect that milk would vary in composition from individual to individual. 

In contrast, the whey proteins are relatively small globular proteins. a-Lactalbumin represents about 20 % of the protein content of bovine whey (3.5 % of total bovine milk protein), and it is the principal protein in human milk (Brew and Grobler, 1992). Nanotube assembly has been discovered in some solutions containing a hydrolysed derivative of this protein. And it appears that the a-lactalbumin nanotube is unique in the sense that it is the only artificial nanotube that has so far been made from a food protein (Graveland-Bikker et al., 2004 Graveland-Bikker and de Kruif, 2006). As for p-lactoglobulin, it has the capacity under certain specific conditions to form nano-fibres in aqueous media (as can various other globular food proteins, such as ovalbumin, soy proteins, and bovine serum albumin) (van der Linden, 2006 Nicolai, 2007). 

The high casein content of cow s milk is responsible for the formation of a large, firm curd which may be difficult for some infants to digest compared with the finer, soft curd formed from human milk. Consequently, cow s milk often is modified to conform more closely to the nutrient and physical requirements of infants (Fomon 1974). When cow s milk is heated, homogenized, or acidified to produce softer curd formation, the protein is used by infants as efficiently as that of human milk, which contains less casein than cow s milk (Fomon 1974). 

As it can be seen in Table 13.2, human milk composition is quite different from that of cow s milk. Casein and mineral contents are lower in human milk than in cow s milk, whilst the lactose content is higher in the former. With regards to fatty matter, both types of milk present similar contents, but the total protein is over three-fold higher in cow s milk. 

Proteins The total amount of proteins in milk is much higher during the colostrum period than in that of mature milk (once milk composition is already stable). The protein content is 12 and 35 g l-1 in human and cow s milk, respectively. This higher protein content is directly related to the higher growth rate of calves. The chemical contents of the principal proteins present in human and cow s milk are summarized in Table 13.4. 

It is interesting to note that /3-lactoglobulin is the major protein in cow s milk whey, but it is absent from human milk whey. However, the content of a-lactoalbumin and immunoglobulins (Ig) is higher in human than in cow s milk whey. Serum albumin and lactoferrins are found in similar concentrations in human and cow s milk whey, and the amount of lactoferrin is higher in colostrum than in mature milk. In any case, lactoferrin is present in milk throughout the lactation period. 

Q12 Human milk is a bluish-white fluid with approximately 88% water, 6-8% carbohydrate (lactose), 3-5% fat and 1 -2% protein. The composition of milk varies from day to day and changes during a single feed the milk is watery at the start of a feed to satisfy thirst, but the fat content of the milk increases towards the end of the feeding period. 

According to Lonnerdahl and Glazier (1985), only 1% of the calcium content of human milk and 0.15% of the calcium content of cow milk are bound to a-lactalbumin. Hence, this protein is quantitadvely unimportant for calcium nutridon of the infant. They point out, rather, that the primary role of calcium may be to regulate lactose synthesis and possibly to aid in the secredon of a-lactalbumin. On the other hand, Rao and Brew (1989) have found that Ca(II) is essential for the formation of correct disulfide bonds and the development of nadve conformadon. They suggest that Ca(II) may funcdon to guide the folding of the nascent protein. 

Typically, soymilk contains 8-12% total solids, depending on the water bean ratio used during processing. Protein content is about 3.6% fat, 2.0—3.2% carbohydrates, 2.9—3.9% and ash, -0.5%. The composition of soymilk compares favorably with those of cow s milk and human milk (Chen, 1989). In addition, soymilk is lactose-free and contains higher levels of protein, iron, unsaturated fatty acids, and niacin however, it contains lower amounts of fat, carbohydrates, calcium, riboflavin, thiamine, methionine, and lysine (Kosikowski, 1971). Therefore, many commercial soymilks are fortified with vitamins, minerals, and in some cases, amino acids. The most widely used nutrients for fortifying soymilk are vitamin calcium, and methionine. 

The nutritive value of a protein depends on its capacity to provide nitrogen and amino acids in adequate amounts to meet the requirements of an organism. Thus, in theory the most logical approach for evaluating protein quality is to compare amino acid content (taking bioavailability into account) of a food with human amino acid requirements. A number of comparisons have been made using reference patterns such as those derived from egg and milk protein. The first major change in procedure was substitution of a provisional pattern of amino acid requirements for the egg protein standard. 

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