Changes of Casein Complex during Storage of Sterilized Skim Milk

Two types of sterilized skim milk were prepared; one was HTS–1 milk which was heated at 130°C flashly and the other was HTS–2 milk which was heated at 130~135°C for 75 sec. The changes of casein complex during storage of HTS–1 and HTS–2 milks were examined and compared with those of AUT milk which was heated at 120°C for 15 min. The results obtained are summarized as follows.

(1) Visible sediment was formed in HTS–1 and HTS–2 milks after 8 and 14 months of storage, respectively, while no sediment was observed in AUT miik throughout 15 months of storage. (2) The amount of calcium in the ultracentrifugal wheys of HTS–1 and HTS–2 milks decreased gradually with prolonged storage, while that in the ultracentrifugal whey of AUT milk was kept constant after 1 month of storage. (3) Almost no differences among the three samples were observed in the increments of Ca/N ratio of ultracentrifuged casein complex during storage. (4) The amount of soluble casein increased in AUT milk during storage, but decreased in HTS–1 and HTS–2 milks.

On the basis of the above results, the destabilization of casein complex during storage was discussed.     

Acute versus chronic effects of whey proteins on calcium absorption in growing rats

The acute and chronic effects of whey proteins on calcium metabolism and bone were evaluated. In acute studies, 8-week-old male rats were gavaged with 50 mg whey protein concentrate (WPC) and 25 mg calcium. 45Ca was administered intravenously or orally. Kinetic studies were performed, and femurs were harvested. Four of seven WPCs significantly increased femur uptake of 45Ca compared with controls. One WPC at 50 mg enhanced calcium absorption over a range of calcium intakes from 35.1 +/- 9.4% to 42.4 +/- 14.0% (P < 0.01). Three of the most effective WPCs were tested further in a chronic feeding study. One hundred 3-week-old rats were randomly divided into four adequate dietary calcium (ADC; 0.4% Ca) groups (control of 20% casein and three WPC groups with 1% substitution of casein with each of three WPCs) and two low calcium (LC; 0.2% Ca) groups (control of 20% casein and one WPC group with 1% substitution of casein with one WPC). After 8 weeks, there was no effect of WPCs on femur uptake of 45Ca among ADC groups and there was no effect of WPCs on calcium retention, femur breaking force, femur bone mineral density, or total femur calcium at either dietary calcium intake. However, whole body bone mineral content (BMC) was significantly higher (P < 0.05) in the three whey protein concentrate ADC groups compared with the ADC control group. Total BMC at the proximal tibia in whey protein ADC groups was increased, as shown by peripheral quantitative computed tomography. Our results indicate that the acute calcium absorption-enhancing effect of whey proteins did not persist through long-term feeding in rats. However, the initial enhancement of calcium absorption by whey protein was sufficient to increase BMC.     

Bioavailability of zinc and its binding to casein in milks and formulas

Differences in zinc bioavailability among milk and formulas may be attributed to binding of zinc to various ligands. We determined the distribution of zinc and protein at different pHs and zinc and calcium concentrations. We used radiolabelled cow's milk, human milk, whey-predominant (WPF) and casein-predominant (CPF) infant formula. Lowering the pH changed zinc and protein distribution: zinc shifted from pellet (casein) to whey in cow's milk, from fat to whey in human milk and from fat and pellet to whey in formulas. Protein shifted from whey to pellet in human milk and from whey and pellet to fat in formulas. Increasing zinc and calcium concentrations shifted protein and zinc from pellet to whey for cow's milk and from whey and pellet to fat for the formulas. Protein distribution was not affected by calcium or zinc addition in human milk or CPF, while zinc shifted from whey to fat in human milk and from fat and pellet to whey in CPF. Zinc and calcium binding to isolated bovine or human casein increased with pH. At 500 mg/L of zinc, bovine casein bound 32.0 +/- 1.8 and human casein 10.0 +/- 0.9 mg zinc/g protein. At 500 mg/L of calcium, calcium was preferentially bound over zinc. Adding calcium and zinc resulted in 32.0 +/- 1.8 mg zinc/g bound to bovine casein and 17.0 +/- 0.8 mg zinc/g to human casein, while calcium binding was low. Suckling rat pups dosed with 65Zn labelled infant diets were killed and individual tissues were gamma counted. Lower zinc bioavailability was found for bovine milk at pH = 4.0 (%65Zn in liver = 18.7+1.4) when compared to WPF (22.8 +/- 1.6) or human milk (26.9 +/- 0.8). Lowering the pH further decreased zinc bioavailability from human milk, but not from cow's milk or WPF. Knowledge of the compounds binding minerals and trace elements in infant formulas is essential for optimizing zinc bioavailability.     

Inhibition of the formation of protein storage granules by glutaurine in the larval fat body cells of Mamestra brassicae (Insecta, Lepidoptera)

Correlative changes in the protein contents of haemolymph and fat body and the accumulation of protein storage granules in the fat body cells of Mamestra brassicae were investigated during the last larval stage in normally developing larvae and following administration of glutaurine (1 X 10(-4) mg/g body weight). The protein content of the haemolymph of untreated larvae increased up to the 4th day of the stage, declined during days 5 and 6, and increased again before pupation. In the glutaurine-treated larvae the amount of proteins in the haemolymph was as high as in the controls during the first four days but continued to rise up to the end of the stage. The protein content of the fat body started to increase from the 3rd day and heavy accumulation of protein storage granules in the cells of fat body was observed on the 5th and following days. The protein content of the fat body of glutaurine-treated larvae remained at a low level and the protein storage granules were absent in the cells. The inhibition of the selective uptake of haemolymphatic storage proteins by fat body following glutaurine treatment is suggested.     

Dietary whey protein modulates liver glycogen level and glycoregulatory enzyme activities in exercise-trained rats

This study compared the effects of dietary whey protein with dietary casein or soy protein on glycogen storage and glycoregulatory enzyme activities in the liver of sedentary and exercise-trained rats. Male Sprague-Dawley rats (ca. 130 g) were divided into one sedentary and three exercise-trained groups, with eight animals in each group. Casein was provided as the source of dietary protein in the sedentary group while the exercise-trained groups were fed casein, whey, or soy protein. Rats in the exercise-trained groups ran for 30 mins/day, 4 days/week on a motor-driven treadmill. In the exercise-trained rats, animals fed whey protein had higher liver glycogen content than animals in the other two diet groups. Glucokinase activity was significantly higher in rats fed whey protein compared to that in rats fed soy protein, while glucose 6-phosphatase activity was significantly decreased in animals on the whey protein diet compared with those the other two diets. Although 6-phospho-fructokinase activity was significantly lower in the whey protein group than in the soy protein group, we found that fructose 1,6-bisphosphatase activity was significantly higher in the whey group compared with either the casein or soy groups. Pyruvate kinase activity in rats fed the casein diet was significantly higher than in rats fed either the whey or soy protein diets. In addition, hepatic alanine aminotransferase activity and serum alanine level were also increased in the whey protein group compared with the casein or soy protein groups. Taken together, these results demonstrate that the whey protein diet in exercise-trained rats results in significantly higher levels of liver glycogen, because of the combined effects of regulation of rate limiting glycolytic and gluconeogenic enzyme activities and activation of glycogenesis from alanine via alanine amino-transferase.     

Improvement of the Surface Functional Properties of β-Lactoglobulin and α-Lactalbumin by Heating in a Dry State

Controlled heating in a dry state greatly improved the surface functional properties of whey proteins (β-lactoglobulin and α-lactalbumin). Although whey proteins were completely insolubilized by heating at 80°C in an aqueous solution, their solubility was kept even after heating at 80°C in a dry state (7.5% moisture content) for 5 days. The surface hydrophobicity of α-lactalbumin was increased during the dry-heating, while that of β-lactoglobulin was decreased. In addition, the fluorescence spectra excited at 280 nm of dry-heated whey proteins suggested the significant conformational changes. High-performance gel chromatography showed that a considerable amount of soluble aggregates was formed in the dry-heated β-lactoglobulin, while a small amount of soluble aggregate was observed in the dry-heated α-lactalbumin. The foaming properties of dry-heated whey proteins were increased to about 3 times that of untreated proteins. The emulsifying properties of dry-heated whey proteins were also increased, compared to untreated proteins, although a slight decrease in the emulsion stability was observed in dry-heated β-lactoglobulin. The improvement of the surface properties seemed to come from the partial unfolding suitable for the formation of foam film and the entrapment of oil droplets.     

果实膨大期喷布磷酸二氢钾对三月红荔枝果实贮藏性的影响

在三月红荔枝(Litchi chinensis cv.Sanyuehong)果实膨大期对树冠喷施0.2%磷酸二氢钾(KP)水溶液,以探讨磷酸二氢钾对荔枝果实贮藏性的影响。结果表明:(1) KP处理的果实在贮藏期前17d,失重率及果肉可溶性固形物、酸、VitC、花色素苷等指标的变化趋势与对照基本相似;(2) KP处理的果肉可溶性蛋白质含量变化与对照有明显差异,而果皮的可溶性蛋白质含量在贮藏期前10d变化动态与对照一致,此后呈相反的变化趋势;(3)果皮POD活性显著高于果肉,KP处理和对照的果肉POD活性在贮藏第3d后、果皮POD活性在第17d前分别具有相似变化趋势;(4)果肉中CAT活性在贮藏第3~17d期间都显著或极显著高于果皮,KP处理和对照果肉、果皮CAT活性动态变化均为单峰曲线。     

Influence of Cross-linked Waxy Maize Starch on the Aggregation Behavior of Casein Micelles During Acid-induced Gelation

The influence of cross-linked waxy maize starch on the aggregation behavior of casein micelles was investigated using a combination of physico-chemical techniques. Milk was homogenized at two different temperatures (55 and 65 °C) and then heated at 95 °C for 5 min in a pilot scale system. The possible interactions between modified starch and milk proteins during lactic acid fermentation were evaluated. While 1% starch did not show differences in the whey protein complexes formed during heating compared to milk with no starch (as measured by size exclusion chromatography), a higher (2.5%) concentration of starch clearly showed an increased amount of heat-induced whey protein aggregates. The gelation pH also increased significantly with 2.5% starch compared to that of the control samples. The storage modulus (G′) increased with increasing levels of starch, and confocal microscopy confirmed that the microstructure of the casein gels was altered by the presence of modified starch. Milk-starch mixtures preheated and homogenized at 55 or 65 °C exhibited similar physico-chemical behavior during acidification. The results suggested a lack of interaction between starch granules and casein micelles during acidification, and scanning electron microscopy images collected with a self-assembled monolayer technique also confirmed that starch granules were not attached to milk caseins but only embedded in the protein gel matrix.     

欧洲榛子贮藏及萌发生理特性研究

对欧洲榛子(Corylus avellanaL.)在沙藏、萌发、幼苗生长阶段的种仁养分和保护酶活性动态变化特点进行了研究,结果表明:种子萌发后107 d营养耗尽;沙藏过程中种仁的干重、粗脂肪含量略下降,萌发后下降迅速且与时间呈线性负相关;可溶性糖在沙藏至萌发后12 d内下降而后上升,萌发24 d后上升迅速,72 d后进入高峰期;淀粉含量在沙藏过程中下降,萌发后上升并在萌发48 d取得11.12%的峰值后下降;游离氨基酸总量在沙藏期间下降,萌发前12 d开始升高,在萌发36~48 d达155.31 mg/100 g的高峰后降低;可溶性蛋白50.87 mg/g的峰值出现在萌发前12 d;SOD在萌发前12 d至萌发后12 d、CAT由萌发起24 d内、POD在萌发36 d后分别出现活性高峰,对种仁营养按测定时间进行的聚类分析结果与依据幼苗干重划分的幼苗生长阶段相吻合。     

Whey Protein Reduces Early Life Weight Gain in Mice Fed a High-Fat Diet

An increasing number of studies indicate that dairy products, including whey protein, alleviate several disorders of the metabolic syndrome. Here, we investigated the effects of whey protein isolate (whey) in mice fed a high-fat diet hypothesising that the metabolic effects of whey would be associated with changes in the gut microbiota composition. Five-week-old male C57BL/6 mice were fed a high-fat diet ad libitum for 14 weeks with the protein source being either whey or casein. Faeces were collected at week 0, 7, and 13 and the fecal microbiota was analysed by denaturing gradient gel electrophoresis analyses of PCR-derived 16S rRNA gene (V3-region) amplicons. At the end of the study, plasma samples were collected and assayed for glucose, insulin and lipids. Whey significantly reduced body weight gain during the first four weeks of the study compared with casein (P<0.001–0.05). Hereafter weight gain was similar resulting in a 15% lower final body weight in the whey group relative to casein (34.0±1.0 g vs. 40.2±1.3 g, P<0.001). Food intake was unaffected by protein source throughout the study period. Fasting insulin was lower in the whey group (P<0.01) and glucose clearance was improved after an oral glucose challenge (P<0.05). Plasma cholesterol was lowered by whey compared to casein (P<0.001). The composition of the fecal microbiota differed between high- and low-fat groups at 13 weeks (P<0.05) whereas no difference was seen between whey and casein. In conclusion, whey initially reduced weight gain in young C57BL/6 mice fed a high-fat diet compared to casein. Although the effect on weight gain ceased, whey alleviated glucose intolerance, improved insulin sensitivity and reduced plasma cholesterol. These findings could not be explained by changes in food intake or gut microbiota composition. Further studies are needed to clarify the mechanisms behind the metabolic effects of whey.     

Developmental changes in rat hepatic casein kinases 1 and 2

Cytosolic histone kinase and casein kinase activities varied considerably in the late fetal and postnatal periods of liver development. Both activities showed a maximum at day 21 of gestation and decreased at birth to values close to those of adult rats. The changes in total casein kinase activity were due to variations of casein kinase 1 and casein kinase 2. Similarly the activities of both the cyclic-AMP-dependent protein (histone) kinase and the cyclic-AMP-independent histone kinase varied during development. Besides the changes in total activity, the affinity of casein kinases 1 and 2 for casein also varied in fetal and postnatal development. The Km values of casein kinase 2 increased from day 18, reached a maximum at day 20 of gestation and then started to decrease until one day after birth. In contrast the Km values of casein kinase 1 decreased from day 18, reached its lowest value at day 21 of gestation and attained values similar to those in the adult at the day of birth. Changes in this parameter were also observed when insulin (3 IU/kg) was administered by intraperitoneal injection to one-day-old rats. The Km values of casein kinase 1 decreased while those of casein kinase 2 increased after administration of this hormone. On the other hand, the Km values for ATP of casein kinases 1 and 2 as well as their apparent molecular masses and sensitivity to heparin and GTP did not significantly change during ontogeny of rat liver.     

Whey and casein labeled with L-[1-13C]leucine and muscle protein synthesis: effect of resistance exercise and protein ingestion

Muscle protein turnover following resistance exercise and amino acid availability are relatively well described. By contrast, the beneficial effects of different sources of intact proteins in relation to exercise need further investigation. Our objective was to compare muscle anabolic responses to a single bolus intake of whey or casein after performance of heavy resistance exercise. Young male individuals were randomly assigned to participate in two protein trials (n = 9) or one control trial (n = 8). Infusion of l-[1-(13)C]leucine was carried out, and either whey, casein (0.3 g/kg lean body mass), or a noncaloric control drink was ingested immediately after exercise. l-[1-(13)C]leucine-labeled whey and casein were used while muscle protein synthesis (MPS) was assessed. Blood and muscle tissue samples were collected to measure systemic hormone and amino acid concentrations, tracer enrichments, and myofibrillar protein synthesis. Western blots were used to investigate the Akt signaling pathway. Plasma insulin and branched-chain amino acid concentrations increased to a greater extent after ingestion of whey compared with casein. Myofibrillar protein synthesis was equally increased 1-6 h postexercise after whey and casein intake, both of which were higher compared with control (P < 0.05). Phosphorylation of Akt and p70(S6K) was increased after exercise and protein intake (P < 0.05), but no differences were observed between the types of protein except for total 4E-BP1, which was higher after whey intake than after casein intake (P < 0.05). In conclusion, whey and casein intake immediately after resistance exercise results in an overall equal MPS response despite temporal differences in insulin and amino acid concentrations and 4E-BP1.     

Characterization of proteins and fatty acid composition in Galapagos fur seal milk. Occurrence of whey and casein protein polymorphisms

1. Milk proteins of the Galapagos fur seal (Arctocephalus galapagoensis) were separated adequately into whey and casein fractions using bovine milk analysis methods. 2. In samples from days 5-30 of lactation 40% of the total proteins were whey and 60% caseins; in mid-lactation, day 150, 25% were whey and 75% casein proteins. 3. Electrophoretic and isoelectric focusing patterns of fur seal whey protein differed widely from bovine patterns, whereas those of caseins were similar. 4. Polymorphisms of fur seal whey and casein proteins were noted and did not seem related to different stages of lactation. 5. C-16 and C-18 fatty acids contributed about 70% of fatty acids; 63% of the total acids in milk fat were unsaturated.     

Regulation of intracellular casein degradation by secreted milk proteins

Intracellular degradation of newly synthesised casein was measured by a pulse-chase method in freshly prepared goat mammary explants. After incubation in medium containing L-[5-3H]proline, explants were washed and cultured again in unlabelled medium containing 5 mM proline; at intervals up to 24 h the amount of radiolabel incorporated in casein was measured. Tissue was obtained in week 33 of lactation after goats had been milked incompletely in one gland (the test gland) for 24 weeks; the contra-lateral (control) gland was milked normally. In explants from the control gland, casein was not degraded during or after secretion: L-[5-3H]proline incorporated in casein increased to a maximum value which was maintained through the chase period. For four out of five goats, explants from the test gland showed a decrease in total [3H]casein radiolabel at 0-4 h of the chase, indicating that a proportion of casein was degraded during secretion. Intracellular casein degradation was also observed when control gland explants were cultured in chase medium containing a goat whey fraction known to inhibit casein production and milk secretion (Wilde, C.J. et al., (1987) Biochem. J. 242, 285-288). This suggests that the greater volume of residual milk left by incomplete milking reduced secretory efficiency, rendering casein susceptible to intracellular degradation, and that this occurred through the action of a secreted milk constituent, which acts as a chemical feedback inhibitor of milk secretion.     

Adsorption of Staphylococcal Bacteriophage by Milk Proteins

Propagation of homologous bacteriophage in a culture of Staphylococcus aureus (1:1 ratio of phage to bacteria) in Trypticase Soy Broth (TSB) and in skim milk indicated more activity of phage in TSB. Early lysis of bacteria in skim milk followed by a pronounced rise in bacterial population suggested that staphylococcal phages were being inactivated by milk. Titration of phages from skim milk, whey, and TSB indicated about 90% adsorption of phages by acid- and heat-precipitable proteins of skim milk, whereas numbers recovered from whey were quite comparable to those recovered from TSB. Reducing the pH from 6.5 to 4.0 increased the percentage of phages recoverable from skim milk from 10 to 56%. Apparently, the changes in electrical charges on the casein micelles at this low pH were responsible for release of many phages from their complex with casein.     

Synthesis of α-Adenosine 5′-Monophosphate

The distributions of protein, calcium and inorganic phosphate among casein micelles of skimmilk before and after frozen storage were investigated by gel filtration through Sephadex G-200 with 6.6 m urea solution as eluant.

The results showed that the native casein micelles were fractionated into two fractions. Fast eluting fraction contained large amount of calcium and inorganic phosphorus. Slow eluting fraction contained calcium but was essentially free of inorganic phosphorus. Frozen storage caused the increase of proportion of the fast eluting fraction accompanying the increase of calcium and inorganic phosphorus contents in it. It is suggested that the salt linkage newly formed during frozen storage causes the increase of proportion of the fast eluting fraction.     

The Chronic Effects of Whey Proteins on Blood Pressure,Vascular Function,and Inflammatory Markers in Overweight Individuals

Limited evidence suggests that dairy whey protein may be the major dairy component that is responsible for health benefits currently associated with increased dairy consumption. Whey proteins may reduce blood pressure and improve cardiovascular health. This study evaluated the effects of whey protein supplementation on blood pressure, vascular function and inflammatory markers compared to casein and glucose (control) supplementation in overweight/obese individuals. The subjects were randomized to either whey protein, casein or glucose supplementation for 12 weeks according to a parallel design. In all, 70 men and women with a mean (±s.e.m.) BMI (kg/m²) of 31.3 ± 0.8 completed the study. Systolic blood pressure (SBP) decreased significantly at week 6 compared to baseline in the whey and casein groups, (P = 0.028 and P = 0.020, respectively) and at week 12 (P = 0.020, and P = 0.017, respectively). Diastolic blood pressure (DBP) decreased significantly compared to baseline in the whey and casein groups (P = 0.038 and P = 0.042, respectively) at week 12. DBP decreased significantly in the whey and casein groups (P = 0.025, P = 0.038, respectively) at week 12 compared to the control group. Augmentation index (AI) was significantly lower from baseline at 12 weeks (P = 0.021) in the whey group. AI decreased significantly in the whey group at 12 weeks compared to control (P = 0.006) and casein (P = 0.006). There were no significant changes in inflammatory markers within or between groups. This study demonstrated that supplementation with whey protein improves blood pressure and vascular function in overweight and obese individuals.     

Change of Salt Distribution in Milk during Frozen Storage and Its Partial Reversion after Thawing

The change of salt distribution of skimmilk during frozen storage at ?7°C and its reversion after thawing were investigated. Losses of ultrafiltrable calcium, inorganic phosphate and citrate were indicated. A part of insolubilized calcium and citrate reversed to a soluble form after thawing, but insolubilized phosphate hardly reversed when the storage period was prolonged. Comparison on the changes in skimmilk and in its dialyzate suggested that the insolubilization of salt constituents in milk due to frozen storage involved the interaction of these constituents with calcium caseinate phosphate complex.     

采后钙处理对梨果实钙的形态和果胶及相关代谢酶类影响的研究

研究了黄花梨经浸钙处理后,果实钙形态转变及果胶含量、多聚半乳糖醛酸酶（PG）和果胶甲酯酶（PME）活力的变化,以及果实硬度的变化。结果表明：浸钙处理的果实总钙含量显著提高,其硬度明显高于对照,且有利于细胞膜透性的保持;梨果实中的NaCl溶性钙最多,其次是水溶性钙,醋酸溶性钙和HCl溶性钙含量较少。在果实贮藏21d时,水溶性钙含量有一个上升的过程,而NaCl溶性钙则有一个下降的过程。浸钙处理后,除醋酸溶性钙外,果实中的水、NaCl和HCl溶性钙含量均有显著的提高。浸钙处理明显抑制了果胶的降解进程与PG的活力,但对PME抑制作用不明显。浸钙处理能提高果实硬度可能与浸钙处理抑制了PG活力有关。     

Enrichment of n-Alkane Assimilation-Deficient Mutants of Candida Yeast by Synergistic Effect of Nystatin and Pyrrolnitrin

In order to clarify further the relationship between the heat stability of casein micelles and the formation of soluble casein upon heating concentrated milk, the effect of formaldehyde was examined. The addition of formaldehyde up to 20 mM markedly increased the heat stability of both concentrated skim milk and concentrated whey protein-free (WPF) milk. The stabilizing effect of formaldehyde was greater for concentrated skim milk than for concentrated WPF milk. The addition of formaldehyde depressed the formation of soluble casein upon heating concentrated milk. No soluble casein was formed on the addition of 20 mM formaldehyde. It was confirmed by Sephadex G-200 gel filtration in the presence of 6.6 M urea that cross-links among the casein components were formed in heated concentrated WPF milk containing formaldehyde. These facts suggest that formaldehyde may introduce cross-links among the casein components and prevent the formation of soluble casein accompanying the release of K-casein from micelles, thus stabilizing the casein micelles.