Effects of calcium chelating agents on the solubility of milk protein concentrate

The aim of this study was to determine the effects of calcium chelating agents on the dissolution and functionality of 10% (w/w) milk protein concentrate (MPC) powder. MPC powder dissolution rate and solubility significantly (P > 0.05) increased with addition of sodium phosphate, trisodium citrate (TSC) and sodium hexametaphosphate (SHMP), compared to MPC dispersions alone. Trisodium citrate and SHMP addition increased viscosity as a result of micelle swelling. However, dispersions containing SHMP showed a decrease in viscosity after prolonged time due to micelle dissociation. Overall, MPC powder dissolution was aided by the addition of calcium chelating agents.     

Rehydration process of milk protein concentrate powder monitored by static light scattering

Static light scattering (SLS) was applied to monitor the rehydration process of milk protein concentrate (MPC) powder. The size distribution and volume concentration of the suspended powder particles were measured to quantify the dissolution kinetics of MPC powder. The results obtained showed that the low solubility index reported for MPC85 (85% protein) powder at room temperature was the consequence of slow dissolution kinetics rather than the presence of a large amount of insoluble material in the rehydrated powder. The rehydration process of MPC85 powder occurs in two overlapping steps: the disruption of agglomerated particles into primary powder particles and, simultaneously, the release of material from the powder particles into the surrounding aqueous phase. The latter process appeared to be the rate-limiting step of dissolution of MPC85 and was accelerated by an increase of the solvent temperature.     

Ageing-induced solubility loss in milk protein concentrate powder: effect of protein conformational modifications and interactions with water

BACKGROUND: Protein conformational modifications and water‐protein interactions are two major factors believed to induce instability of protein and eventually affect the solubility of milk protein concentrate (MPC) powder. To test these hypotheses, MPC was stored at different water activities (a_w 0.0–0.85) and temperatures (25 and 45 °C) for up to 12 weeks. Samples were examined periodically to determine solubility, change in protein conformation by Fourier transform infrared (FTIR) spectroscopy and water status (interaction of water with the protein molecule/surface) by measuring the transverse relaxation time (T₂) with proton nuclear magnetic resonance (¹H NMR). RESULTS: The solubility of MPC decreased significantly with ageing and this process was enhanced by increasing water activity (a_w) and temperature. Minor changes in protein secondary structure were observed with FTIR which indicated some degree of unfolding of protein molecules. The NMR T₂ results indicated the presence of three distinct populations of water molecules and the proton signal intensity and T₂ values of proton fractions varied with storage condition (humidity) and ageing. CONCLUSION: Results suggest that protein/protein interactions may be initiated by unfolding of protein molecules that eventually affects solubility. Copyright © 2011 Society of Chemical Industry     

Functionality of milk protein concentrate 80 with emulsifying salts and its applications in analogue cheeses

Milk protein concentrate 80 (MPC80) was prepared with different emulsifying salts (ES). The effects on particle size (D50), solubility, and surface hydrophobicity (H₀) of MPC80 were then observed after production. The molecular weight and secondary structure of MPC80 protein were also investigated through sodium dodecyl sulphate-polyacrylamide gel electrophoresis (SDS-PAGE) and Fourier Transform Infrared (FTIR) spectrometry. Particle size (D50) was reduced from 31.37 to 20.67 μm following the addition of sodium phosphate (SPS). The solubility of MPC80 fortified with sodium citrate salt (SCS), SPS, and sodium pyrophosphate (SPP) was improved by 80.32, 78.23, and 55.80%, respectively. The SDS-PAGE pattern showed no significant difference between the control and single-ES-fortified samples, but major protein bands (α_s-CN, β-CN, κ-CN, BSA, and β-LG) had a stronger intensity than binary-ES-fortified MPC80. The FTIR results showed that the β-sheet content of ES-fortified MPC80 was relatively higher than that in the control. Compared with analogue cheeses made by MPC80 with and without ES, SCS–SPP (92:8)-modified MPC80 presented a better fluid mass and homogeneous colour.     

Physical aging of glassy normal and waxy rice starches: effect of aging time on glass transition and enthalpy relaxation

Physical aging and glass transition characteristics of amorphous normal and waxy rice starches (11 and 15% moisture contents) were investigated under differential scanning calorimetry as function of aging time. Normal rice starch showed higher T_g than waxy rice starch. The T_g and ΔC_p at glass transition gradually increased with aging time, whereas fictive temperature was slightly reduced regardless of moisture content and starch type. The relaxation enthalpy and relaxation peak temperature increased with aging time until structural equilibrium was reached. Enthalpy increase was more significant in the early stage of aging whereas temperature increase was constant during the aging period tested (120 h). Aging kinetic analysis using Cowie and Ferguson model revealed that the amorphous normal and waxy rice starches behaved in different modes for the physical aging. Relaxation distribution parameter (β) of both starches was in a range of 0.3<β<0.7, but higher at a lower moisture content, and for normal starch than for waxy starch. Maximum relaxation enthalpy for normal starch (1.10 and 2.69 J/g, respectively, at 11 and 15% moistures) was higher than those of waxy starch (0.77 and 2.48 J/g). Based on the characteristic time (t_c), normal starch has slower progression toward an equilibrium than waxy starch. Overall results proved that physical aging kinetics were highly dependent on starch structure and composition.     

乳粉中的玻璃化相变及对乳粉品质的影响

在无定形聚合物玻璃化相变理论的基础上 ,介绍了乳粉中乳糖的玻璃化相变及测定方法 ,讨论了玻璃化相变的影响因素以及对乳粉品质的影响     

Correlations between the solubility and surface characteristics of milk protein concentrate powder particles

The solubility of high-protein milk protein concentrate (MPC) may decrease significantly during storage, particularly at relatively high temperatures and humidity. The objective of this study was to seek correlations between the solubility loss of MPC during storage and various surface characteristics determined on the basis of simultaneous nanoscale topographical imaging and nanomechanical mapping of MPC particle surfaces using atomic force microscopy. A control MPC and a calcium-depleted MPC were stored at 45°C and 66% relative humidity for up to 60 d. The solubility of the control MPC was 56% at the beginning of the storage and gradually decreased to 10% at the end of the 60-d storage. The calcium-depleted MPC exhibited more rapid decreases from almost 100% at the beginning of the storage to 18% after storage for 45 d, after which we observed no significant difference in solubility between the control and calcium-depleted MPC. Averaged or root mean squared roughness values calculated using topographical images were found to have no correlation with the solubility. Deformation, Derjaguin-Muller-Toropov modulus, and adhesion images revealed the presence of individual casein micelles and larger clusters of aggregated casein micelles at MPC particle surfaces, whereas we observed no correlation between the solubility and averaged values of these nanomechanical properties. Furthermore, Derjaguin-Muller-Toropov modulus and adhesion images showed that the peripheral edges of individual casein micelles and their clusters had significantly higher values of the corresponding nanomechanical properties than other regions in the images, indicating the occurrence of the fusion of casein micelles. The surface area coverage or the percent area of the fused regions in an image revealed significant negative linear correlations with the solubility for both the control and calcium-depleted MPC. The present results support the hypothesis that the fusion of casein micelles at MPC powder particle surfaces is a causative factor for the solubility loss of MPC during storage and in turn suggest that the solubility loss may be alleviated by inhibiting the formation of a crust or skin on powder particle surfaces.     

Effect of storage time and temperature of milk protein concentrate (MPC85) on the renneting properties of skim milk fortified with MPC85

This study investigated the effect of storage temperature (20–50 °C) and time (0–60 days) on the renneting properties of milk protein concentrate with 85% protein (MPC85). Reconstituted skim milk was fortified with the MPC85 (2.5% w/w) and the renneting properties of the skim milk/MPC85 systems were investigated using rheology. It was found that the final complex modulus (final G∗) and the yield stress of the rennet-induced skim milk/MPC85 gels decreased exponentially with storage time of the MPC85 for storage temperatures greater than 20 °C, with a greater effect at the higher storage temperatures. Changes in the solubility of MPC85 with storage time were correlated with the rheological properties. The primary phase of renneting (cleavage of κ-casein) was not affected by the storage of the MPC85; hence the effect was related to the secondary stage of renneting (aggregation/coagulation of rennet-treated casein micelles). Using a temperature–time superposition method, a master curve was formed from the final G∗, yield stress and solubility results. This suggested that the same physical processes affected the solubility and rennet gelation properties of the milks. It is proposed that the MPC85 protein in rennet-treated skim milk/MPC85 solutions may transform from an interacting material, when solubility is high, to an inert or weakly interacting material, when solubility is low, and that this results in the reduced final G∗ and yield stress of the rennet gels when MPC85 is stored at elevated temperatures for long periods.     

Development of highly soluble and functional buffalo milk protein concentrate 60 by modifying ionic environment and characterisation thereof

The ionic and protein environment of buffalo skim milk was modified by instant pH drop and its restoration for the development of highly soluble milk protein concentrate-60 (BuMRate–60^@), which resulted in depletion of 88.64% calcium, 89.18% magnesium, 91.69% potassium and 93.96% phosphorous. BuMRate–60^@ displayed excellent wetting, rehydration and solubility (97.76%). The Fourier transform infrared spectrometer spectra revealed significant stretching (C-H and O-H), N-H bending and a large extent of H-bonding. Scanning electron microscopy micrographs presented particles with porous nature and dimples, while transmission electron microscopy confirmed a greater release of minerals with altered casein micelles.     

脱钙预处理对提高浓缩乳蛋白溶解性的影响

以脱脂乳为原料,通过超滤、洗滤、离子交换和喷雾干燥制备了脱钙率为0%、11%、19%、27%和37%的浓缩乳蛋白(milk protein concentrate,MPC),并在35℃加速贮藏4个月,采用聚丙烯酰胺凝胶电泳、激光共聚焦、激光粒度仪等方法分析了MPC中可溶性蛋白的含量、组成及其溶解液的微观结构和粒径分布,旨在探讨脱钙程度对MPC溶解性和贮藏稳定性的影响。脱钙MPC的初始溶解度都在95%以上,且随脱钙程度的增加而略有提高。在贮藏过程中,0%脱钙MPC的溶解度显著降低;11%脱钙MPC的溶解度随贮藏时间增加而显著降低;19%~37%脱钙MPC的溶解度在贮藏期内几乎不变。MPC溶解度的降低,主要是由酪蛋白聚集所致。MPC的脱钙率越高,其在水中的分散程度越高,且分散程度随贮藏时间降低的速度越慢。综上所述,当脱钙率达到19%及其以上时,MPC具有良好的溶解性和贮藏稳定性。     

On quantifying the dissolution behaviour of milk protein concentrate

Milk protein concentrate (MPC) is a newly developed dairy powder with wide range of applications as ingredients in the food industry, such as cheese, yogurt, and beverage. MPC has relatively poor solubility as a result of their high protein content (40–90 wt%), with distinct dissolution behaviour in comparison to skim milk or whole milk powders. Here, a focused beam reflectance measurement (FBRM) was used to monitor the dissolution process of an MPC powder, with the data used to develop a kinetic dissolution model based on the Noyes–Whitney equation. The model was used to estimate the dissolution rate constant k and the final particle size in suspension d_∞, describing dynamic dissolution behaviours and final solubility respectively of a particular powder. In this work, the effects of dissolution temperature, storage duration and storage temperature on dissolution properties of an MPC powder were also investigated. A quantitative understanding of relationship between process and storage conditions with powder functionality could be achieved from k and d_∞ profiles. This approach can potentially be applied to predict the dissolution behaviour of specific dairy powders in a more robust manner than conventional solubility tests.     

Comparative study of deteriorative changes in the ageing of milk powder

Skim milk powders produced by spray drying (mostly amorphous) are commonly used in downstream food industries and have to be stored for long times after production before final use. However, their physicochemical qualities and properties decline during long-term storage due to the hygroscopicity of amorphous lactose. Surface modification of particles by crystallisation of the outer layer, giving so called “egg-shell” particles, has shown significant improvements in the physical properties of the powder in their fresh state. In this study, the raw powder and processed powder (with modified surface) were stored at around 33% relative humidity and 25–30 °C for 30 weeks to investigate the effect of ageing on these two types of powders. Agglomeration, large lactose crystal formation on the surface, surface composition changes and protein modifications were studied. The changes between the raw powder and process powder were compared after ageing. The non-hygroscopic crystalline surface layer showed significant benefits in maintaining the physicochemical qualities of the powders over long storage times. The aged raw powder showed a 24% change in the crystallinity, a 3% change in the lactose/protein ratio on the surface and 6% protein denaturation compared with the aged processed powder with a 4% change in the crystallinity, a 1.5% change in the lactose/protein ratio on the surface and 2% protein denaturation, respectively, after 30 weeks storage.     

Effects of standardization of whole milk with dry milk protein concentrate on the yield and ripening of reduced-fat cheddar cheese

Commercial milk protein concentrate (MPC) was used to standardize whole milk for reduced-fat Cheddar cheesemaking. Four replicate cheesemaking trials of three treatments (control, MPC1, and MPC2) were conducted. The control cheese (CC) was made from standardized milk (casein-to-fat ratio, C/F approximately 1.7) obtained by mixing skim milk and whole milk (WM); MPC1 and MPC2 cheeses were made from standardized milk (C/F approximately 1.8) obtained from mixing WM and MPC, except that commercial mesophilic starter was added at the rate of 1% to the CC and MPC1 and 2% to MPC2 vats. The addition of MPC doubled cheese yields and had insignificant effects on fat recoveries (approximately 94% in MPC1 and MPC2 vs. approximately 92% in CC) but increased significantly total solids recoveries (approximately 63% in CC vs. 63% in MPC1 and MPC2). Although minor differences were noted in the gross composition of the cheeses, both MPC1 and MPC2 cheeses had lower lactose contents (0.25 or 0.32%, respectively) than in CC (0.60%) 7 d post manufacture. Cheeses from all three treatments had approximately 10(9) cfu/g initial starter bacteria count. The nonstarter lactic acid bacteria (NSLAB) grew slowly in MPC1 and MPC2 cheeses during ripening compared to CC, and at the end of 6 mo of ripening, numbers of NSLAB in the CC were 1 to 2 log cycles higher than in MPC1 and MPC2 cheeses. Primary proteolysis, as noted by water-soluble N contents, was markedly slower in MPC1 and MPC2 cheeses compared to CC. The concentrations of total free amino acids were in decreasing order CC > MPC2 > MPC1 cheeses, suggesting slower secondary proteolysis in the MPC cheeses than in CC. Sensory analysis showed that MPC cheeses had lower brothy and bitter scores than CC. Increasing the amount of starter bacteria improved maturity in MPC cheese.     

Use of milk protein concentrate to standardize milk composition in Italian citric Mozzarella cheese making

To better exploit manufacturing facilities and standardize cheese quality, milk composition could be standardized by fortifying its protein content with a milk protein concentrate (MPC) addition so avoiding partially skimming the milk. With this aim Mozzarella cheese was obtained adding citric acid into milk standardized at 4% protein and a fat to protein ratio of 1.0. Protein fortification was obtained adding MPC produced by ultrafiltration. Milk, whey, curd, cheese and stretching water were weighed and analysed for total solid, fat and protein content, to measure component recovery and yield. Yield increase (from 13.8% to 16.7%) was due to the higher recovery of the milk total solids and proteins in MPC cheese (48.2 and 78.3%, respectively) and to the slightly higher cheese moisture, obtained with a little modification of the cheese technology when adding MPC. Milk fat in cheese was lower than that reported in literature. Hot water stretching of the curd resulted in very low losses (1%) of protein and considerable losses (14%) of fat for both control and MPC cheeses. The likely reasons of this low recovery are discussed and it can be supposed that a further cheese yield increase is possible by changing the curd stretching procedures.     

Preparation and functional properties of protein coprecipitate from sheep milk

The optimum conditions for the production of coprecipitate from sheep milk were studied. The best percentage of calcium chloride added to milk was 0.2%, which resulted in a recovery of 97.5% of milk proteins. At low pH (4.5–5), the recovery of protein was low, but it increased at higher pH values (5.5–6.5); recovery was greatest at pH 6.5. The optimum heating temperature to obtain coprecipitate from sheep milk was 85–95°C. The functional properties of the sheep milk coprecipitate were studied. At pH values higher than 6, there were no differences between the solubility of sheep milk coprecipitate and sheep milk sodium caseinate, but the solubility of coprecipitate at pH values lower than 5 was relatively higher than those of the caseinate. At pH ≥6, the emulsion activity index (EAI) for emulsions of sheep milk coprecipitate and caseinate increased as pH increased; at all pH values, the EAI of the coprecipitate was higher than that of the caseinate. Sheep milk coprecipitate showed higher foaming ability and stability than sheep milk sodium caseinate.     

Effect of NaCl addition during diafiltration on the solubility, hydrophobicity, and disulfide bonds of 80% milk protein concentrate powder

We investigated the surface hydrophobicity index based on different fluorescence probes [1-anilinonaphthalene-8-sulfonic acid (ANS) and 6-propionyl-2-(N,N-dimethylamino)-naphthalene (PRODAN)], free sulfhydryl and disulfide bond contents, and particle size of 80% milk protein concentrate (MPC80) powders prepared by adding various amounts of NaCl (0, 50, 100, and 150 mM) during the diafiltration process. The solubility of MPC80 powder was not strictly related to surface hydrophobicity. The MPC80 powder obtained by addition of 150 mM NaCl during diafiltration had the highest solubility but also the highest ANS-based surface hydrophobicity, the lowest PRODAN-based surface hydrophobicity, and the least aggregate formation. Intermolecular disulfide bonds caused by sulfhydryl-disulfide interchange reactions and hydrophobic interactions may be responsible for the lower solubility of the control MPC80 powder. The enhanced solubility of MPC80 powder with addition of NaCl during diafiltration may result from the modified surface hydrophobicity, the reduced intermolecular disulfide bonds, and the associated decrease in mean particle size. Addition of NaCl during the diafiltration process can modify the strength of hydrophobic interactions and sulfhydryl-disulfide interchange reactions and thereby affect protein aggregation and the solubility of MPC powders.     

Effect of heat treatment on milk protein functionality at emulsion interfaces. A review

Heat treatment affects the molecular structure of milk proteins at the interfaces of oil-in-water emulsions and in aqueous media. Experimental evidence of the impact of thermal processing on milk protein structure is presented and the contribution of whey proteins and caseins at film formation during emulsification is discussed. Recent advances in understanding the effect of heat treatment in milk protein functionality at emulsion interfaces are reviewed with particular emphasis on the emulsifying ability of whey proteins with or without the presence of the casein fraction. The major findings regarding the destabilizing mechanisms of oil-in-water emulsions brought about by heat-induced denaturation of milk proteins are presented. This paper aims to combine recent knowledge on how thermal processing of milk proteins affects their molecular configurations in bulk and particularly at interfaces, which in turn appear to be important with respect to the physico-chemical properties of milk protein-stabilized emulsions.     

Effect of processing on nutritive values of milk protein

Milk is an essential source of nutritionally excellent quality protein in human, particularly in vegan diet. Before consumption, milk is invariably processed depending upon final product requirement. This processing may alter the nutritive value of protein in a significant manner. The processing operations like thermal treatment, chemical treatment, biochemical processing, physical treatments, nonconventional treatments, etc. may exert positive or negative influence on nutritional quality of milk proteins. On one side, processing enhances the nutritive and therapeutic values of protein while on other side intermediate or end products generated during protein reactions may cause toxicity and/or antigenicity upon consumption at elevated level. The review discusses the changes occurring in nutritive quality of milk proteins under the influence of various processing operations.     

Effects of storage temperature on the solubility of milk protein concentrate (MPC85)

The effect of storage time and temperature on the solubility of milk protein concentrate (MPC85) was investigated using solubility tests, gel electrophoresis and mass spectrometry. It was found that, at a given temperature, the solubility of MPC85 decreased exponentially with time and a master curve was obtained using a temperature–time superposition. Gel electrophoresis indicated that the insoluble proteins were the caseins, whereas the whey proteins remained soluble. Mass spectrometry showed that, with storage time, the casein was lactosylated. In the light of these measurements, it is speculated that the insolubility of the MPC85 could have been due to cross-linking of the proteins at the surface of the MPC85 powder. However, other mechanisms, such as the cross-linking of the proteins by hydrophobic and/or hydrogen bonding, are not ruled out.     

Instant infusion pasteurisation of bovine milk. II. Effects on indigenous milk enzymes activity and whey protein denaturation

Direct heat treatment of two milk types, skimmed and nonstandardised full‐fat, was performed by instant steam infusion and compared with indirect heating. Infusion conditions were temperatures of 72–120°C combined with holding times of 100–700 ms, and indirect heat conditions were 72°C/15 s and 85°C/30 s. The activity of indigenous enzymes such as alkaline phosphatase, lactoperoxidase, xanthine oxidase and γ‐glutamyl transpeptidase was evaluated. Infusion temperature was the main determinant of inactivation. Whey protein denaturation represented by β‐lactoglobulin increased significantly with infusion temperature. The nonstandardised milk had a higher denaturation rate than skimmed milk. The effect of instant infusion on pH and milk fat globule size in relation to whey protein denaturation and association is discussed.