The status of fluoride added to bovine milk

The use of bovine milk as a carrier for fluoride has been advocated as a prophylactic measure. The effect of the addition of sodium fluoride to cold milk, up to 1000 ppm, was examined by particle micro electrophoresis, centrifugation, and dialysis.
The fluoride had little effect on the electrokinetic potential, and no sedimentation calcium fluoride occurred. Even with centrifugation at 30 000 g , the bulk of the fluoride remained in solution. Dialysis showed that the fluoride could easily be removed, and that the eluent contained fluoride in the ionic (free) form.
The addition of fluoride to calcium phosphate free (CPF, milk showed that the bulk of the fluoride remained with the protein, even after addition of excess calcium chloride. Thus the fluoride does not affect the stability of milk and the element forms a reversible ionic complex. The evidence suggests that it is held in solution by the smaller milk proteins.     

蛋白质组学在人乳及牛乳蛋白研究中的应用现状

乳蛋白是生命最初阶段中最重要的营养物质,利用蛋白质组学技术能够直观、整体地研究乳蛋白质的组成与调控生命的活动规律,可以更全面、深入地阐明乳蛋白质的表达信息。通过阐述蛋白质组学在人乳与牛乳蛋白研究中的应用进展,以及乳酪蛋白质组学、乳清蛋白质组学、乳脂肪球膜蛋白质组学、乳铁蛋白质组学的研究,为寻找疾病的临床诊断和治疗、乳品加工保存条件、开发新型婴幼儿食品提供重要的理论依据。     

Dried milk powder containing fluoride

A milk powder was prepared from a skim milk containing 10 p.p.m. fluoride. The powder showed that the size of the powdered milk particles produced were not affected by the presence of the fluoride. The fluoride in the reconstituted milk existed in at least two reversible ionic equilibria and all of it could be recovered as 'free' fluoride after precipitation of the protein. The fluoride was evenly distributed throughout the range of milk particles, and thus mechanical handling and particle segregation would not produce unequal concentration of fluoride in a stored milk powder. The fluoride did not change its status even after 2 years storage.     

Oxidative stability of bovine milk determined by individual variability in herd irrespective of selenium status

The effect of supplementing dairy cows with organic selenium (25 mg day^?1) on oxidative stability of milk when exposed to Cu(II) ions or to fluorescent light was evaluated using solid-phase micro-extraction gas chromatography with flame ionisation detection. Neither formation of volatile lipid (hexanal) and protein (dimethyl disulfide) oxidation products nor radical scavenging activity was affected by selenium supplementation. We found that other factors must be determinant for the oxidative stability of milk, and that these factors vary considerably between individual cows. Milk from two cows that were either sensitive or resistant to oxidation was compared in an attempt to identify the molecular basis for these differences. It was found that oxidation of milk could not solely be explained on the basis of fatty acid composition, or content of low molecular weight antioxidants, such as uric acid, ascorbic acid, and tocopherol.     

Zinc binding in bovine milk

About 90% of the Zn in bovine skim milk was sedimented by ultracentrifugation at 100,000 g for 1 h. About half of the non-sedimentable Zn was non-dialysable, indicating that it was associated with protein, probably non-sedimented casein micelles. Casein micelles incorporated considerable amounts of Zn added to skim milk as ZnCl2, and at Zn concentrations greater than or equal to 16 mM coagulation of casein micelles occurred. Ca was displaced from casein micelles by increasing ZnCl2 concentration and approximately 40% of micellar Ca was displaced by 16 mM-ZnCl2. Micellar Zn, Ca and Pi were gradually rendered soluble as the pH of milk was lowered and at pH 4.6 greater than 95% of the Zn, Ca and Pi were non-sedimentable. These changes were largely reversible by readjustment of the pH to 6.7. About 40% of the total Zn in skim milk was non-sedimentable at 0.2 mM-EDTA and most of the remainder was gradually rendered soluble by EDTA over the concentration range 1-50 mM. This indicates that there are two distinct micellar Zn fractions. No micellar Ca or Pi was solubilized at EDTA concentrations up to 1.0 mM, indicating that both colloidal calcium phosphate (CCP) and casein micelles remained intact under conditions where the more loosely bound micellar Zn fraction dissolved. Depletion of casein micelles of colloidal Ca and Pi by acidification and equilibrium dialysis resulted in removal of Zn, and in colloidal Pi-free milk non-dialysable Zn was reduced to 1.2 mg/l (approximately 32% of the original Zn). Thus, approximately 32% of the Zn in skim milk is directly bound to caseins, while approximately 63% is associated with CCP. Over 80% of the Zn in colloidal Pi-free milk was rendered soluble by 0.2 mM-EDTA, indicating that the casein-bound Zn is the loosely bound Zn fraction in casein micelles. A considerable fraction of the Zn in acid whey (pH 4.6) co-precipitated with Ca and Pi on raising the pH to 6.7 and heating for 2 h at 40 degrees C, indicating that insoluble Zn phosphate complexes form readily under these conditions. Studies on dialysis of milk against water, or dilution of milk or casein micelles with water, showed that CCP and its associated Zn is very stable and dissolves only very slowly at pH 6.6. The nature of Zn binding in casein micelles may help to explain the lower nutritional bioavailability of Zn in bovine milk and infant formulae compared with human milk.     

Toxigenic status of Staphylococcus aureus isolated from bovine raw milk and Minas frescal cheese in Brazil

A group of 291 Staphylococcus aureus isolates from mastitic cow's milk (n = 125), bulk tank milk (n = 96), and Minas frescal cheese (n = 70) were screened for staphylococcal enterotoxin (SE) genes (sea, seb, sec, sed, see, seg, seh, sei, selj, and sell) and for the tst-1 gene encoding staphylococcal toxic shock syndrome toxin 1 by PCR assay. A total of 109 (37.5%) of the isolates were positive for at least one of these 11 genes, and 23 distinct genotypes of toxin genes were observed. Of the S. aureus isolates bearing SE genes, 17 (13.6%) were from mastitic cow's milk, 41 (41.7%) were from bulk tank milk, and 51 (72.9%) were from Minas frescal cheese. The occurrence of exclusively more recently described SE genes (seg through sell) was considerably higher (87 of 109 PCR-positive strains) than that of classical SE genes (sea through see, 15 strains). The SE genes most commonly detected were seg and sei; they were found alone or in different combinations with other toxin genes, but in 60.8% of the cases they were codetected. No strain possessed see. The tst-1 gene was found in eight isolates but none from mastitic cow's milk. Macrorestriction analysis of chromosomal DNA from 89 S. aureus isolates positive for SE gene(s) was conducted with the enzyme SmaI. Fifty-five distinct pulsed-field gel electrophoresis patterns were found, demonstrating a lack of predominance of any specific clone. A second enzyme, Apa I, used for some isolates was less discriminating than Sma I. The high genotype diversity of potential toxigenic S. aureus strains found in this study, especially from Minas frescal cheese, suggests various sources of contamination. Efforts from the entire production chain are required to improve consumer safety.     

The analysis of milk components and pathogenic bacteria isolated from bovine raw milk in Korea

Bovine mastitis can be diagnosed by abnormalities in milk components and somatic cell count (SCC), as well as by clinical signs. We examined raw milk in Korea by analyzing SCC, milk urea nitrogen (MUN), and the percentages of milk components (milk fat, protein, and lactose). The associations between SCC or MUN and other milk components were investigated, as well as the relationships between the bacterial species isolated from milk. Somatic cell counts, MUN, and the percentages of milk fat, protein, and lactose were analyzed in 30,019 raw milk samples collected from 2003 to 2006. The regression coefficients of natural logarithmic-transformed SCC (SCCt) on milk fat (−0.0149), lactose (−0.8910), and MUN (−0.0096), and those of MUN on milk fat (−0.3125), protein (−0.8012), and SCCt (−0.0671) were negative, whereas the regression coefficient of SCCt on protein was positive (0.3023). When the data were categorized by the presence or absence of bacterial infection in raw milk, SCCt was negatively associated with milk fat (−0.0172), protein (−0.2693), and lactose (−0.4108). The SCCt values were significantly affected by bacterial species. In particular, 104 milk samples infected with Staphylococcus aureus had the highest SCCt (1.67) compared with milk containing other mastitis-causing bacteria: coagulase-negative staphylococci (n = 755, 1.50), coagulase-positive staphylococci (except Staphylococcus aureus; n = 77, 1.59), Streptococcus spp. (Streptococcus dysgalactiae, n = 37; Streptococcus uberis, n = 12, 0.83), Enterococcus spp. (n = 46, 1.04), Escherichia coli (n = 705, 1.56), Pseudomonas spp. (n = 456, 1.59), and yeast (n = 189, 1.52). These results show that high SCC and MUN negatively affect milk components and that a statistical approach associating SCC, MUN, and milk components by bacterial infection can explain the patterns among them. Bacterial species present in raw milk are an important influence on SCC in Korea.     

Cysteine protease activity in bovine milk

Proteolytic activity of native cysteine proteases was studied in bovine milk. Five fractions (fI–fV) with cysteine protease activity were separated from acid whey prepared from raw bovine milk by ion-exchange chromatography on Q-Sepharose. The hydrolytic action of the most active fractions (fIII and fV), after further purification using gel permeation chromatography on Superdex S75, was studied against individual caseins. The two fractions contained different cysteine protease activities capable of hydrolyzing both α_s1- and β-casein. Studies of the effects of different reagents on the activity of partially purified fIII showed that the activity in this fraction was unaffected by aprotinin, slightly inhibited by p-chloromercuribenzoate and o-phenanthroline and completely inhibited by -trans-epoxysuccinyl- -leucylamido (4-guanidino) butane, consistent with identification as a cysteine protease. Phenylmethylsulphonyl fluoride and pepstatin A reduced activity of fIII by 40% and 50%, respectively. The partially purified fIII retained 20% of its cysteine protease activity after heating at 55°C, 60°C, 65°C and 72°C for 40 min, 20 min, 10 min and 30 s, respectively. Immunoblotting of fIII with antibodies to the bovine lysosomal cysteine protease, cathepsin B, clearly indicated the presence of immunoreactive cathepsin B in this fraction. This study presents strong evidence for the presence of a heterogeneous group of cysteine proteases in bovine milk, with one of these enzymes probably being cathepsin B.     

Hormones in bovine milk and milk products: A survey

A survey of the published data on the occurrence of hormones in milk and milk products is presented. Bovine milk and colostrum contain a large number of hormones from either steroidic or peptidic origin. The main categories to which these molecules belong are gonadal (estrogens, progesterone, androgens), adrenal (glucocorticoids), pituitary (prolactin, growth hormone) and hypothalamic hormones (GRH, LH-RH, TRH). Other molecules, such as proteins related to the parathyroid hormone, insulin, somatostatin, calcitonin, bombesin, erythropoeitin and melatonin, are also of interest. The exact role of hormones in the development of the newborn is still not known, but it is believed that they may contribute to the growth of the newborn and to the development and maturation of its gastrointestinal and immune systems.     

Enzymatic-fluorometric quantification of cholesterol in bovine milk

The present paper describes an enzymatic-fluorometric method for the determination of cholesterol in milk and other opaque matrices. The initial step of the method is to liberate chemically and physically bound cholesterol from the milk fat globule membrane by enzymatic action. The method is able to discriminate between esterified and free cholesterol in milk. The analysis is cost effective and is developed to work directly on whole, fresh milk thereby eliminating time consuming and tedious pre-treatment procedures of the sample. More than 1000 milk samples were analysed on the day of sampling. The total concentration of milk cholesterol ranged from 80 to 756μM (n=1068; mean 351μM). Milk cholesterol was significantly correlated to milk fat concentration as analysed by mid-infra red spectrometry (r=0.630; n=853) and by an enzymatic-fluorometric method (triacylglycerol) (r=0.611; n=842).     

Metabolites of nucleic acids in bovine milk

To investigate metabolites of nucleic acids in milk as by-products of protein biosynthesis, a method for determination of pyrimidine and purine compounds by reversed-phase high-pressure liquid chromatography was developed. Reproducibility of the measured compounds was 2%. Recovery of the main constituents averaged about 99%. In addition to orotic acid, allantoin, and uric acid, the free bases hypoxanthine, xanthine, and guanine, the ribonucleosides uridine, cytidine, and pseudouridine, and the ribonucleotides, guanosine 5'-monophosphate and cytidine 5'-monophosphate, were quantified in milk samples of Holstein-Friesian cows. Milk production from days 45 to 65 of lactation influenced concentrations of individual metabolites differently. Concentrations of orotic acid, allantoin, and uric acid decreased with increasing milk production, whereas concentrations of uridine, cytidine, pseudouridine, hypoxanthine, xanthine, and guanine remained unchanged. The results allowed conclusions on rate limiting steps of catabolic pathways of pyrimidines and purines and indicated relationships with protein biosynthesis. A possible role of metabolites of nucleic acids as taste factors and as substrates for milk microbes is suggested.     

牛羊乳蛋白组分比较研究

羊乳被国际营养学界誉为"奶中之王",逐渐被人们列为日常生活的营养保健佳品。该文对羊乳和牛乳的蛋白组分(主要是酪蛋白和乳清蛋白)含量、氨基酸组成及变异体等方面的差异进行了综述,并且对两者酪蛋白胶束的差异进行了比较,为羊乳检验和加工提供理论依据。     

Astringency of bovine milk whey protein

Whey protein solutions at pH 3.5 elicited an astringent taste sensation. The astringency of whey protein isolate (WPI), the process whey protein (PWP) that was prepared by heating WPI at pH 7.0, and the process whey protein prepared at pH 3.5 (aPWP) were adjusted to pH 3.5 and evaluated by 2 sensory analyses (the threshold method and the scalar scoring method) and an instrumental analysis (taste sensor method). The taste-stimulating effects of bovine and porcine gelatin were also evaluated. The threshold value of astringency of WPI, PWP, and aPWP was 1.5, 1.0, and 0.7 mg/mL, respectively, whereas the gelatins did not give definite astringency. It was confirmed by the scalar scoring method that the astringency of these proteins increased with the increase in protein concentration, and these proteins elicited strong astringency at 10 mg/mL under acidic conditions. On the other hand, the astringency was not elicited at pH 3.5 by 2 types of gelatin. A taste sensor gave specific values for whey proteins at pH 3.5, which corresponded well to those obtained by the sensory analysis. Elicitation of astringency induced by whey protein under acidic conditions would be caused by aggregation and precipitation of protein molecules in the mouth.     

Antioxidant nature of bovine milk beta-lactoglobulin

Heating is necessary for processing milk in the dairy industry, which evidently produces a conformational change in β-lactoglobulin (β-LG). β-Lactoglobulin, a major protein that accounts for approximately 10 to 15% of total milk proteins, is a globular protein consisting of 162 AA with a relative molecular mass of 18.4 kDa. The purpose of the present study was to determine the antioxidant role of β-LG in milk and the possible mechanism involved. We showed that β-LG is a mild antioxidant whose potency is less than that of vitamin E and probucol (the latter being an antioxidant used for clinical therapy). The conversion of the β-LG monomer to dimer was responsible, in part, for the mode of action in protecting low-density lipoproteins against copper-induced oxidation. Cross-linking the free thiol groups of β-LG by heating (100°C for 2 min), or chemically modifying the β-LG by carboxymethylation to block the thiol groups resulted in a substantial loss of antioxidant activity. The data suggest that Cys-121 plays an essential role in the antioxidant nature of β-LG. By using an anti-LG antibody affinity column to deplete the β-LG from milk, we observed from the lost antioxidant activity that β-LG contributes approximately 50% of the total activity. Because β-LG is extremely sensitive to thermal denaturation, to maintain its antioxidant nature, dairy products consumed daily should not be overheated in order to maintain its antioxidant nature.     

Cholesterol content in European bovine milk fats

Data about the cholesterol content in edible fats like bovine milk fat are important for balancing the cholesterol intake with food. A comparison of 3 different cholesterol determination methods showed that with the direct analysis by a 25 m long TAP steel capillary column the same results could be obtained as with a time-consuming saponification standard method including thin-layer chromatographic cleaning and subsequent silylation. On the other hand with a rapid direct method using a short packed column 21% unsaponifiables as e.g. minor sterols or hydrocarbons could be found in the "cholesterol peak". The analysis of 1142 German milk fats led to a mean cholesterol content of 265.6 +/- 20.0 mg/100 g fat (range: 204.4 to 382.5). For 165 milk fats from other 12 EU-countries, a similar mean cholesterol content of 258.5 +/- 19.9 mg/100 g fat (range: 215.0 to 331.6) was detected. Compared with sufficiently fed cows, underfed cows demonstrated an approx. 10.1% lower mean cholesterol content (238.7 +/- 9.7 mg/100 g fat). On the other hand, during the first 7 days post partum, the colostrum showed a significantly higher mean cholesterol content of 327.2 +/- 99.0 mg/100 g fat (n = 15; range: 213.1 to 583.9). Further, with special conditions as feeding of rape-seed the cholesterol content can be significantly lowered by 8-13%. An extraordinary lowering up to 50% can be reached by dry fractionation of milk fat (stearin "hard" fraction).     

Bioactive lipids naturally occurring in bovine milk

Bioactive properties of food components increasingly gain in importance in the modern diet. Bovine milk fat (BMF) exhibits bioactive substances mainly in the class of fatty acids. Currently, most interest is addressed to trans fatty acids (TFA) and particularly conjugated linoleic acids (CLA) with BMF being the main source of CLA in food. Whereas saturated fatty acids (C12–C16) and TFA are reported to be positively correlated (negatively for oleic acid) with atherosclerosis and coronary heart disease, CLA are regarded as potent anticarcinogens. Also butyric acid (C4) as well as some phospholipids and ether lipids present in BMF are thought to have anticarcinogenic properties. Furthermore, BMF contains the essential fatty acids C18 : 2 n-6 and C18 : 3 n-3 that have many and diverse functions in human metabolism and, thus, control a variety of biochemical and physiological processes. Altogether, BMF contains approximately 75 wt% of bioactive substances. However, the overall impact on human health can hardly be assessed.     

牛乳体细胞数与稳定性关系的研究

对某牧场30头荷斯坦奶牛的200个奶样进行体细胞数、酒精稳定性和热稳定性的测定。结果表明,所采样本的平均体细胞数为87×104cfu/mL,标准差为147×104cfu/mL,不服从正态分布;随着牛乳体细胞数升高,酒精稳定性随之降低;不同的体细胞组间,牛乳热稳定性随着SCC的升高而降低,组间有极显著差异(P<0.001)。     

Distribution and fatty acid content of phospholipids from bovine milk and bovine milk fat globule membranes

The phospholipid (PL) content was determined comparatively in the milk fat globule membrane (MFGM) and whole milk including their fatty acid profiles. The possible role of milk PLs in defence against pathogens was also addressed. The MFGM and whole milk showed a similar distribution of PL species; however, the fatty acid contents of the PL species were different. Total PL from MFGM showed a decrease in C18:0 content in parallel with an increase in C18:1 and C18:2 and very long-chain fatty acid (more than C20) content. No significant differences in the fatty acid content of phosphatidylcholine and sphingomyelin from either source were found. However, the phosphatidylethanolamine from MFGM had more C18:1 and C18:2 and less C14:0 and C16:0 than that from whole milk. A similar but less pronounced result was found for phosphatidylserine/phosphatidylinositol. Enterotoxigenic Escherichia coli strains failed to bind to PL, which had been previously separated by high-performance thin-layer chromatography.