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      The Chemical Composition and Nitrogen Distribution of Chinese Yak (Maiwa) Milk

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          Abstract

          The paper surveyed the chemical composition and nitrogen distribution of Maiwa yak milk, and compared the results with reference composition of cow milk. Compared to cow milk, yak milk was richer in protein (especially whey protein), essential amino acids, fat, lactose and minerals (except phosphorus). The contents of some nutrients (total protein, lactose, essential amino acids and casein) were higher in the warm season than in the cold season. Higher ratios of total essential amino acids/total amino acids (TEAA/TAA) and total essential amino acids/total non essential amino acids (TEAA/TNEAA) were found in the yak milk from the warm season. However its annual average ratio of EAA/TAA and that of EAA/NEAA were similar to those of cow milk. Yak milk was rich in calcium and iron (p < 0.05), and thus may serve as a nutritional ingredient with a potential application in industrial processing.

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          Composition, physiochemical properties, nitrogen fraction distribution, and amino acid profile of donkey milk.

          This study investigated the changes in chemical composition, nitrogen fraction distribution, and AA profile of milk samples obtained during lactation from the Jiangyue breed of donkey in Northwest China. Results showed that donkey milk contained 9.53% total solids, 1.57% protein, 1.16% fat, 6.33% lactose, and 0.4% ash on average, which is more similar to mare and human milk than to the milk of other mammals. Throughout the lactation investigated, pH and density were constant, protein and ash content showed an apparent negative trend (an increase in lactose content during 120 d postpartum, followed by a decrease), fat content exhibited wide variability, and variations in the content and percentage of whey protein, casein, and AA were small. The casein to whey protein ratio of 52:37 was between the lower value of human milk and the higher value of cow milk. Sodium dodecyl sulfate-PAGE results demonstrated that donkey milk is rich in beta-lactoglobulin and lysozyme. The percentages of 8 essential AA in protein of donkey milk were 38.2%, higher than those of mare and cow milk; donkey milk also had higher levels of serine (6.2%), glutamic acid (22.8%), arginine (4.6%), and valine (6.5%) and a lower level of cystine (0.4%).
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            Nonprotein nitrogen and protein distribution in the milk of cows.

            The NPN content of milk represents only 5 to 6% of the total N in milk. However, the significance of this milk N fraction to energy and N metabolism in the dairy cow has not been well characterized. The single largest contributor to the NPN fraction of milk NPN is urea. Urea equilibrates in body water, and blood urea is the primary source of milk urea. The urea in milk can be derived from at least two sources: the end product of digestion and amino acid catabolism. Blood urea N was positively associated with intakes of ruminally degradable and undegradable protein and negatively associated with intake of net energy. Consequently, it might be possible to develop a system to evaluate the dietary protein and energy status of the lactating dairy cow employing milk urea in conjunction with milk true protein.
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              Composition of milks of dairy cattle. I. Protein, lactose, and fat contents and distribution of protein fraction.

              Milks from commercial dairy herds in Southeastern Pennsylvania were analyzed for total protein, casein, whey protein, beta-lactoglobulin, nonprotein nitrogen, and lactose contents. Data for fat contents and milk yields were from Dairy Herd Improvement Association records for the same lactation. Milk samples were from a single milking of healthy cows (151) in midlactation. Since the remainder of the milk was returned to the bulk milk of the farm, the data represent market milk composition. The data were grouped and analyzed by breed and beta-lactoglobulin phenotype; there were 18 to 33 cows per breed. In true protein percentage, the breeds ranked: Jersey 4.07 plus or minus .49, Brown Swiss 3.84 plus or minus .47, Guernsey 3.56 plus or minus .53, Ayrshire 3.30 plus or minus .52, Milking Shorthorn 3.17 plus or minus .47, Holstein 3.07 plus or minus .43. Breeds differed in all other components and in milk yield. Brown Swiss ranked highest in yield of protein. Only whey protein and beta-lactoglobulin contents were influenced by the beta-lactoglobulin genotype with beta-lactoglobulin A greater than AB greater than B in whey protein content.
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                Author and article information

                Journal
                Int J Mol Sci
                ijms
                International Journal of Molecular Sciences
                Molecular Diversity Preservation International (MDPI)
                1422-0067
                2 August 2011
                2011
                : 12
                : 8
                : 4885-4895
                Affiliations
                [1 ] School of Food Science and Engineering, Harbin Institute of Technology, Harbin 150090, China; E-Mails: haimeili@ 123456ht.edu.cn (H.L.); chengjinju1987@ 123456163.com (J.C.)
                [2 ] Research and Development Central of New Hope Dairy Holding Company, Chengdu 610041, China; E-Mails: qm258@ 123456126.com (Q.L.); wjq722006@ 123456126.com (J.W.)
                [3 ] Guelph Food Research Center, Agriculture and Agri-Food Canada, 1341 Baseline Road, Ottawa, Ontario, K1A 0C5, Canada; E-Mails: Jun.Xue@ 123456agr.gc.ca (J.X.); John.Shi@ 123456agr.gc.ca (J.S.)
                Author notes
                [* ]Author to whom correspondence should be addressed; E-Mail: maying@ 123456hit.edu.cn ; Tel.: +86-451-86282903; Fax: +86-451-86282903.
                Article
                ijms-12-04885
                10.3390/ijms12084885
                3179139
                21954332
                91772919-8bc5-45a4-b3aa-ee13046c1db3
                © 2011 by the authors; licensee MDPI, Basel, Switzerland.

                This article is an open-access article distributed under the terms and conditions of the Creative Commons Attribution license ( http://creativecommons.org/licenses/by/3.0/).

                History
                : 10 June 2011
                : 13 July 2011
                : 22 July 2011
                Categories
                Communication

                Molecular biology
                chemical composition,nitrogen distribution,seasonal changes,yak milk
                Molecular biology
                chemical composition, nitrogen distribution, seasonal changes, yak milk

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