Digestibility of modified milk proteins: nutritional implications

Chemical changes in the structure of several milk proteins were quantitated and the effects on digestibility evaluated using a number of in vitro techniques developed in our laboratory. An assay for digestibility was developed and standardized incorporating immobilized gastric, pancreatic, and intestinal mucosal proteinases and peptidases and thereby simulating in vivo digestion. The method displayed a direct relationship to digestibilities determined in human feeding studies. Various degrees of racemization were induced in a number of proteins and quantitated with the aid of a bioreactor containing coimmobilized D-amino acid oxidase and catalase. Digestibility values were directly related to the degree of racemization and to the amount of lysinoalanine. In a separate study of these proteins, various degrees of Maillard reaction were induced, and the available lysine was quantitated using the fluorometric o-phthalaldehyde method. Digestibility values determined for these modified proteins were also directly related to the amount of available lysine. A bioreactor containing various immobilized proteinases and peptidases catalyzed complete hydrolysis of a number of proteins including beta-lactoglobulin. Because this method does not degrade residues such as tryptophan and methionine nor hydrolyze aldosyl lysine to give unmodified lysine, analysis of these hydrolysates yields more nutritionally meaningful chemical scores. In summary, these techniques appear to be useful for quantitating and monitoring changes in the structures of proteins resulting from processing and storage, which affect their nutritional quality.     

Chemistry of milk proteins in food processing.

This paper analyzes the current knowledge of mild protein chemistry to explain the reactions and their control for the major processes utilized by the modern milk processing industry. The compositon and chemical properties of casein micelles and whey proteins are summarized. The effect of processing upon denaturation, aggregration, and destabilization of milk proteins is updated. The role of milk proteins in the gelation of sterile milk concentrates, destabilization of frozen milk, rennet-clotting of milk, and stabilization of the fat emulsion in milk is also described.     

山羊奶与牛奶和人奶营养成分的比较

对山羊奶、牛奶和人奶中蛋白质、脂肪、维生素、矿物质等主要营养成分进行了比较,分析了3种奶中主要营养成分的差别。通过比较发现,山羊奶在总体营养成分方面优于牛奶,更接近人奶。但山羊奶存在铁、叶酸和维生素B12含量较低以及羊奶膻味的问题,应在营养上对这方面加以重视。     

Seaweed proteins: biochemical,nutritional aspects and potential uses

Seaweeds are traditionally used in human and animal nutrition. Their protein contents differ according to the species and seasonal conditions. Little information is available on the nutritional value of algal proteins and, especially, on the compounds that decrease their digestibility. This paper is a short review of the biochemical and nutritional aspects associated with seaweed proteins. Some perspectives on the potential uses of algal proteins for the development of new foods or additives for human or animal consumption are also discussed.     

Structure,function and nutritional potential of milk osteopontin

Osteopontin (OPN) is a multifunctional protein present in most tissues and body fluids, with the highest concentrations found in milk. Processes for isolation of OPN from bovine milk for use in infant formula have been developed and studies have investigated the effects of oral administration of milk OPN. At the same time, plasma OPN levels have been shown to be elevated in some types of cancer, and OPN has been suggested as a potential diagnostic marker for cancer. OPN exists in several different isoforms in vivo, of which presumably only a minority is directly or indirectly implicated in cancer related events. In this article, we review the differences between milk-derived OPN and OPN derived from transformed cells and compare the structure of OPN from human and bovine milk. Furthermore, current knowledge about the function of OPN in milk and recent findings about the effect of orally presented OPN is discussed.     

Amino acid enrichment and compositional changes among mammalian milk proteins and the resulting nutritional consequences

Milk is a hallmark of mammalian evolution: a unique food that has evolved with mammals. Despite the importance of this food, it is not known if variation in AA composition between different species is important to milk proteins or how it might affect the nutritional value of milk. As milk is the only food source for newborn mammals, it has long been speculated that milk proteins should be enriched in essential AA. However, no systematic analysis supports this assumption. Although many factors influence the overall nutritional value of milk, including total protein concentration, we focused here on the AA composition of milk proteins and investigated the possibility that selection drives compositional changes. We identified 9 major milk proteins present in 13 mammalian species and compared them with a large group of nonmilk proteins. Our results indicate heterogeneity in the AA composition of milk proteins, showing significant enrichment and depletion of certain AA in milk-specific proteins. Although high levels of particular AA appear to be consistently maintained, orthologous milk proteins display significant differences in AA composition across species, most notably among the caseins. Interspecies variation of milk composition is thought to be indicative of nutritional optimization to the requirements of the species. In accordance with this, our observations indicate that milk proteins may have adapted to the species-specific nutritional needs of the neonate.     

Water sorption by proteins: milk and whey proteins

The content and physical state of water in foods influence their physical, chemical, quality, safety, and functional behavior. Information concerning the sorption behavior of dairy proteins, in the water activity (Aw) range 0 to 0.9, is collated in this paper. The sorption behavior of proteins in general, the kinetics of absorption, factors affecting water binding, the phenomenon of desorption hysteresis, and the chemical and physical nature of water/protein interactions are reviewed in general terms. This is followed by a discussion of thermodynamic aspects of sorption phenomena and the adequacy of the various equations for describing sorption isotherms of proteins. After a discussion of the methods available for measuring sorption by milk proteins, the sorption behavior of various milk protein preparations, i.e., nonfat dry milk, whey proteins, caseins, and milk powders is summarized. Finally, the water activity of cheese and its relationship to solute mobility and solvent water are discussed. Some of the unique features of protein behavior, i.e., conformational changes, swelling, and solubilization are cited as possible sources of disparities between various reports.     

Compositional,technological and nutritional aspects of dromedary camel milk

A comprehensive review on Dromedary camel milk composition in comparison with bovine milk, the factors effecting camel milk composition, and an overview of production, properties, nutrition value, dairy products and functionality is provided. The mean values of camel milk composition reported from 1980 to 2009 are as follows: protein 3.1%; fat 3.5%; lactose 4.4%; ash 0.79% and total solids 11.9%. Differences between camel and bovine milk proteins lead to some difficulties in cheese manufacturing. Problems associated with cheese produced from camel milk, and factors reported to improve camel milk coagulation, are highlighted. Fresh and fermented camel milk were found to provide various potential health benefits including angiotension I-converting enzyme-inhibitory activity, hypocholesterolaemic effect, hypoglycaemic effect, antimicrobial and hypoallergenicity effects. The proposed mechanisms behind each health benefit are discussed.     

Comparison of heat and pressure treatments of skim milk, fortified with whey protein concentrate, for set yogurt preparation: effects on milk proteins and gel structure

Heat (85 degrees C for 20 min) and pressure (600 MPa for 15 min) treatments were applied to skim milk fortified by addition of whey protein concentrate. Both treatments caused > 90 % denaturation of beta-lactoglobulin. During heat treatment this denaturation took place in the presence of intact casein micelles; during pressure treatment it occurred while the micelles were in a highly dissociated state. As a result micelle structure and the distribution of beta-lactoglobulin were different in the two milks. Electron microscopy and immunolabelling techniques were used to examine the milks after processing and during their transition to yogurt gels. The disruption of micelles by high pressure caused a significant change in the appearance of the milk which was quantified by measurement of the colour values L*, a* and b*. Heat treatment also affected these characteristics. Casein micelles are dynamic structures, influenced by changes to their environment. This was clearly demonstrated by the transition from the clusters of small irregularly shaped micelle fragments present in cold pressure-treated milk to round, separate and compact micelles formed on warming the milk to 43 degrees C. The effect of this transition was observed as significant changes in the colour indicators. During yogurt gel formation, further changes in micelle structure, occurring in both pressure and heat-treated samples, resulted in a convergence of colour values. However, the microstructure of the gels and their rheological properties were very different. Pressure-treated milk yogurt had a much higher storage modulus but yielded more readily to large deformation than the heated milk yogurt. These changes in micelle structure during processing and yogurt preparation are discussed in terms of a recently published micelle model.     

High-pressure-induced interactions between milk fat globule membrane proteins and skim milk proteins in whole milk

The association of beta-lactoglobulin (beta-LG) and alpha-lactalbumin (alpha-LA) with milk fat globule membrane (MFGM), when whole milk was treated by high pressure in the range 100 to 800 MPa, was investigated using sodium dodecyl sulfate (SDS)-PAGE under reducing and nonreducing conditions. In SDS-PAGE under reducing conditions, beta-LG was observed in the MFGM material isolated from milk treated at 100 to 800 MPa for 30 min, and small amounts of alpha-LA and kappa-casein were also observed at pressures >600 MPa for 30 min. However, these proteins were not observed in SDS-PAGE under nonreducing conditions. These results indicate that beta-LG and alpha-LA associated with MFGM proteins via disulfide bonds during the high-pressure treatment of whole milk. The amount of beta-LG associated with the MFGM increased with an increase in pressure up to 800 MPa and with increasing time of pressure treatment. The maximum value for beta-LG association with the MFGM was approximately 0.75 mg/g of fat. Of the major original MFGM proteins, no change in butyrophilin was observed during the high-pressure treatment of whole milk, whereas xanthine oxidase was reduced to some extent beyond 400 MPa. In contrast to the behavior during heat treatment, PAS 6 and PAS 7 were stable during high-pressure treatment, and they remained associated with the MFGM.     

The nutritional ingredients and antioxidant activity of donkey milk and donkey milk powder

The aim of this work was to shed light on the nutritional ingredients and antioxidant activity of donkey milk and powder. It indicated that basic components of donkey milk were closed to human milk, especially lactose. The fat content of donkey milk was lower than that of cow milk and human milk. The essential amino acids, fatty acids and taurine in donkey milk and powder were richer. The whey protein content of donkey milk was 49.08 g/100 g. The ratio of casein to whey protein in donkey milk and powder was 70.3:100. The cholesterol of donkey milk was 8.6 mg/100 g. Therefore, donkey milk and powder are typical low-fat and low-cholesterol foods. The precious values of donkey milk and powder are suitable calcium and phosphorus proportion, and strongly resistant to oxidation. In conclusion, donkey milk and powder are good supplementary of cow milk and powder, would be better for infant dietary.     

蛋白质氧化及其对乳蛋白结构与功能性的影响

乳清蛋白氧化及其对蛋白质结构与功能性的影响已引起广泛关注。从氨基酸残基侧链的改变、羰基衍生物的生成以及蛋白质聚合物形成等三方面讨论了乳蛋白氧化的可能机理。今后拟系统研究乳品加工条件下乳蛋白氧化对制品功能特性的影响。     

毛细管电泳法对乳及乳制品中乳源蛋白的研究

采用毛细管电泳方法对原料乳、市售鲜奶、不同厂家的巴氏灭菌乳、不同厂家和产地超高温灭菌乳(UHT)、调味乳、乳酸饮料、复原乳、酸奶、奶粉中蛋白成分进行检测。选择聚乙烯醇涂层毛细管,采用柠檬酸缓冲体系,在紫外检测214nm、分离电压20kV条件下对乳及乳制品中的α一乳白蛋白(α-La)、β一乳球蛋白(β-Lg)、α-酪蛋白(α-CN)、β-酪蛋白(β-CN)和k-酪蛋白(k-CN)进行分离测定。结果表明:五种蛋白的含量在原料乳(巴氏灭菌乳、市售鲜奶)、UHT乳、酸奶、调味乳、乳酸饮料、复原乳中依次降低,而UHT乳含量随保质期的增加而减少,奶粉中蛋白质含量因其适应人群而有差异。乳及乳制品中蛋白质的含量与其存在形式、产地及加工工艺相关。     

Changes in proteins,physical stability and structure in directly heated UHT milk during storage at different temperatures

Changes occurring in directly heated UHT milk were studied during storage at 5, 22, 30 and 40 °C. Industrially produced UHT milk samples were analysed for changes in enzymatic activity, protein modification, destabilisation of casein micelles and relocation of milk proteins in relation to sedimentation and gel formation. Sedimentation occurred at all temperatures, and the protein composition of the sediments reflected the composition of its liquid phase; however, there was no α-lactalbumin, β-lactoglobulin or κ-casein present in sediments. Tendrils composed of β-lactoglobulin and κ-casein were seen on casein micelles after UHT treatment and grew in length prior to gelation. High degrees of lactosylation of proteins and peptides were clearly correlated with the absence of gelation and long tendrils. Gelled samples showed complete hydrolysis of intact β-casein, and limited lactosylation of β-lactoglobulin and κ-casein.     

Ultrasound-assisted extraction and modification of plant-based proteins: Impact on physicochemical,functional, and nutritional properties

Ultrasonication is a green technology that has recently received an enormous research attention for extraction of plant-based proteins and tailoring the functionalities of these ingredients. Ultrasonication is generally used as a pretreatment method in the conventional protein solubilization protocols because it can break the cell matrix to improve the extractability. The rate of protein extraction and increase in the extraction yields depend on operating conditions such as sonic energy density, time of sonication, the substrate to slurry ratio, agitation, and so on. Ultrasonication is also applied to modify the physical, structural, and functional properties of protein-based ingredients, besides simultaneous extraction and modifications. Significant changes that occur in protein physical properties due to sonication include size reduction, rheology, electrical conductivity, and zeta (ζ) potential. These changes are due to cavitation-induced shear leading to changes in secondary and tertiary structures, including protein aggregation and cross-linking due to oxidation. Physical and structural changes affect the resulting ingredient functionality and nutritional quality of protein. Changes in the functional properties, especially hydrophobicity, solubility, emulsion, and foaming, depend on the extent of ultrasound energy applied to the protein. This study aims to review major ultrasound process parameters and conditions for extraction and modification of plant proteins and their impact on protein structural changes and resulting physicochemical, functional, and nutritional properties.     

Protein and fat composition of mare's milk: some nutritional remarks with reference to human and cow's milk

Milk composition of mammallian species varies widely with reference to genetic, physiological and nutritional factors and environmental conditions. In this survey, the composition of mare's milk is reviewed and compared to human and cow's milk, considering principal protein fractions and fatty acid content. Protein content in mare's milk is higher than in human milk and lower than in cow's milk; casein concentration in mare's milk is intermediate between the other two milks. Fat content is lower in mare's milk compared to human and cow's milk. Distribution of di- and tri-glycerides in mare's and women’ milk is similar. The proportion of polyunsaturated fatty acids in mare's and human milk is remarkably higher than in cow's milk. Mare's milk shows some structural and functional peculiarities that make it more suitable for human nourishment than cow's milk.