Hydration related properties of protein isolates prepared from heated milks

The hydration related properties of sodium casemate, conventional casein-whey protein co-precipitate preparedfrom milk heatedat 90°C × 15 min at pH 6.6 and milk protein isolates prepared from milk heated at 90°C × 15 min at pH 7.5 or at 60°C × 3 min at pH 10.0 were determined. Conventional acid-precipitated casein and the acid-precipitated protein isolates preparedfrom milks heated at pH >7.0 had similar solubilities and reconstitution properties which were better than those of a conventional co-precipitate. Water sorption isotherms for all the proteins were similar. Viscosities followed the order: conventional co-precipitate > milk protein isolates > sodium caseinate.     

Proteins recovered from milks heated at alkaline pH values

Approximately 95% of available nitrogen can be precipitated from milk on adjustment to pH 4.6 after heating at 90°C × 15 minutes at its natural pH and pH 7.5, while 89% can be precipitated after heating at pH 10.0 at 60°C × 3 minutes. Non-recovered protein includes some serum albumin, β-lactoglobulin, α-lactalbumin and proteose peptones. Protein isolates precipitated from milk heated at pH >7.0 are more soluble in the pH range 6.0–7.0 than those precipitated from milk heated at its natural pH. Whey proteins complex onto the casein micelles after heating milk at its natural pH, while on heating at pH >7.0 whey proteins appear to interact with k-casein in the serum phase. When N-ethylmaleimide is present in milk during heating the percentage protein recovered on pH 4.6 precipitation is decreased, confirming that disulphide linkage is involved in complex formation. However, addition of β-mercaptoethanol to recovered isolates did not result in dissociation of the casein/whey protein complex, suggesting that forces other than disulphide bonding are also involved in maintaining the complex.     

Proteins recovered from acid whey/skim milk mixtures heated at alkaline pH values

Skim milk and mixtures prepared by combining acid whey with skim milk at volume ratios of 2:1, 1:1, 1:2, 1:3 and 1:4 were adjusted to pH 7.5 and heated at 90°C × 15 min. Protein was isolated from these heated samples by precipitation at pH 4.6 and it was found that 65% of the whey protein was recovered in each case. Non-recovered proteins included the proteose peptones and small quantities of β-lactoglobulin, α-lactalbumin and bovine serum albumin. The solubility of these isolates, which contained from 10–25% whey protein, decreased to > 95% when the whey protein exceeded ˜16%. Further characterization of the isolate, prepared from the 1:1 volume ratio of acid whey and skim milk, showed that ˜50% of the whey protein was insoluble, bound to casein and non-functional while the other ˜50% was complexed with casein and was soluble. The addition of a reducing agent suggests that sulphydryl bonding alone is not responsible for complex formation.     

Gastric digestion of milk protein ingredients: Study using an in vitro dynamic model

The coagulation behavior and the kinetics of protein hydrolysis of skim milk powder, milk protein concentrate (MPC), calcium-depleted MPC, sodium caseinate, whey protein isolate (WPI), and heated (90°C, 20 min) WPI under gastric conditions were examined using an advanced dynamic digestion model (i.e., a human gastric simulator). During gastric digestion, these protein ingredients exhibited various pH profiles as a function of the digestion time. Skim milk powder and MPC, which contained casein micelles, formed cohesive, ball-like curds with a dense structure after 10 min of digestion; these curds did not disintegrate over 220 min of digestion. Partly calcium-depleted MPC and sodium caseinate, which lacked an intact casein micellar structure, formed curds at approximately 40 min, and a loose, fragmented curd structure was observed after 220 min of digestion. In contrast, no curds were formed in either WPI or heated WPI after 220 min of digestion. In addition, the hydrolysis rates and the compositions of the digesta released from the human gastric simulator were different for the various protein ingredients, as detected by sodium dodecyl sulfate-PAGE. Skim milk powder and MPC exhibited slower hydrolysis rates than calcium-depleted MPC and sodium caseinate. The most rapid hydrolysis occurred in the WPI (with and without heating). This was attributed to the formation of different structured curds under gastric conditions. The results offer novel insights about the coagulation kinetics of proteins from different milk protein ingredients, highlighting the critical role of the food matrix in affecting the course of protein digestion.     

大豆蛋白-瓜尔豆胶水解物Maillard反应共聚物的乳化特性研究

研究了不同水解时间的瓜尔豆胶对蛋白质-多糖Maillard反应共聚物的乳化特性的影响及不同水相条件下共聚物与酪朊酸钠乳化特性的差异。研究表明,瓜尔豆胶的酸水解时间对大豆分离蛋白-多糖共聚物的乳化活性和乳化稳定性都有明显的影响。水解40min的瓜尔豆胶与大豆分离蛋白反应10 d的共聚物具有优良的乳化性能;在0.3mol/L NaCl和pH 4.0的酸性条件下,共聚物的乳化活性和乳化稳定性都明显高于商品乳化剂酪朊酸钠;在90℃热处理60 min后其乳化活性和乳化稳定性仍接近未经热处理时的酪朊酸钠的乳化活性和稳定性。该共聚物作为安全高效的天然高分子食品乳化剂具有广阔的应用前景。     

Improvement of functional properties of acid-precipitated soy protein by the attachment of dextran through Maillard reaction

The functional acid-precipitated soy protein (SAPP)–dextran conjugate was prepared by dry-heated storage at 60 °C under 79% relative humidity (RH) for 5 days through Maillard reaction between the ε-amino of lysine in soy proteins and the reducing-end carbonyl residue in the dextran. The covalent attachment of dextran to SAPP was confirmed by SDS-polyacrylamide gel electrophoresis and gel filtration chromatography. Functional properties of soy protein depend on the structural and aggregation characteristics of their major components (storage globulins 7S and 11S). The conjugate seemed to be predominantly formed by 7S, and the acidic subunits of 11S in soy protein. The emulsifying properties of the SAPP–dextran conjugate were about four times higher than those of SAPP. The solubility of the protein was not enhanced as a result of preheating, but rather it was not decreased when the conjugated protein was heated at 90 °C for 20 min due to the presence of the polysaccharide. The excellent emulsifying properties of SAPP–dextran conjugate were maintained even at pH 3.0 and were further improved at pH 10.0. The object of Maillard reaction is to guarantee the suitable reaction degree, and the resulting soluble conjugate can have excellent emulsifying properties.     

Maillard Reaction Products as Encapsulants for Fish Oil Powders

The use of Maillard reaction products for encapsulation of fish oil was investigated. Fish oil was emulsified with heated aqueous mixtures comprising a protein source (Na caseinate, whey protein isolate, soy protein isolate, or skim milk powder) and carbohydrates (glucose, dried glucose syrup, oligosaccharide) and spray‐dried for the production of 50% oil powders. The extent of the Maillard reaction was monitored using L*, a*, b* values and absorbance at 465 nm. Encapsulation efficiency was gauged by measurement of solvent‐extractable fat and the oxidative stability of the fish oil powder, which was determined by assessment of headspace propanal after storage of powders at 35 °C for 4 wk. Increasing the heat treatment (60 °C to 100 °C for 30 to 90 min) of sodium caseinate‐glucose‐glucose syrup mixtures increased Maillard browning but did not change their encapsulation efficiency. The encapsulation efficiency of all heated sodium caseinate‐glucose‐glucose syrup mixtures was high, as indicated by the low solvent‐extractable fat in powder (<2% powder, w/w). However, increasing the severity of the heat treatment of the sodium caseinate‐glucose‐glucose syrup mixtures reduced the susceptibility of the fish oil powder to oxidation. The increased protection afforded to fish oil in powders by increasing the temperature‐time treatment of protein‐carbohydrate mixtures before emulsification and drying was observed irrespective of the protein (sodium caseinate, whey protein isolate, soy protein isolate, or skim milk powder) and carbohydrate (glucose, glucose/dried glucose syrup, or oligosaccharide/dried glucose syrup) sources used in the formulation. Maillard reaction products produced by heat treatment of aqueous protein‐carbohydrate mixtures were effective for protecting microencapsulated fish oil and other oils (evening primrose oil, milk fat) from oxidation.     

Effect of Trichoderma reesei tyrosinase on rheology and microstructure of acidified milk gels

The aim of this work was to study the potential of tyrosinase enzymes in structural engineering of acid-induced milk protein gels. Fat free raw milk, heated milk or a sodium caseinate solution were treated with tyrosinases from Trichoderma reesei (TrTyr) and Agaricus bisporus (AbTyr) and the reference enzyme transglutaminase (TG) prior to acid-induced gelation. TrTyr treatment increased the firmness of raw milk and sodium caseinate gels, but not that of heated milk gels, even though protein cross-linking was detected in heated milk. AbTyr did not cross-link proteins in any of the studied milk protein systems. TG was superior to TrTyr in gels prepared of heated milk. In acidified heated milk and sodium caseinate, TrTyr and TG treatment resulted in a decrease of the pore size. Scanning electron microscopy revealed more extensive particle interactions in the heated milk gels with TG than with TrTyr.     

Covalent Crosslinking in Heated Protein Systems

Changes in the solubility (in water or in 1% sodium dodecyl sulfate plus 1%β-mercaptoethanol), isoelectric point, and degree of browning were followed for a range of food proteins when they were heated to 105 to 145°C at 3 relative humidities (RH). In general, there is a decrease in solubility with increasing RH and temperature of heating, but most proteins also showed an increase in solubility on extensive heating. The order for the proteins, based on the temperatures at which an increase in solubility occurred was: gelatin < gluten < soya isolate, sodium caseinate < egg albumin, bovine serum albumin < whey isolate, milk powder, β-lactoglobulin. The bonds that could be formed and broken during this thermal treatment are considered.     

Comparison of the Emulsifying Properties of Fish Gelatin and Commercial Milk Proteins

ABSTRACT: We have compared the flocculation, coalescence, and creaming properties of oil-in-water emulsions prepared with fish gelatin as sole emulsifying agent with those of emulsions prepared with sodium caseinate and whey protein. Two milk protein samples were selected from 9 commercial protein samples screened in a preliminary study. Emulsions of 20 vol% n -tetradecane or triglyceride oil were made at pH 6.8 and at different protein/oil ratios. Changes in droplet-size distribution were determined after storage and centrifugation and after treatment with excess surfactant. We have demonstrated the superior emulsifying properties of sodium caseinate, the susceptibility of whey protein emulsions to increasing flocculation on storage, and the coalescence of gelatin emulsions following centrifugation.     

Thermal Aggregation of Whey Proteins in Model Solutions as Affected by Casein/Whey Protein Ratios

Model solutions (32.5 g protein/L) prepared from milk, ultrafiltration permeate, and whey protein isolate were adjusted at pH 6.7 to casein:whey protein (C:W) ratios of 80:20, 60:40, 40:60,20:80, and 0:100. Heating was performed in test tubes at 95 °C for 5 min. Observations of the heated suspensions revealed the occurrence of heterogeneous particulates from the existing casein micelles complexed with denatured whey proteins and from aggregates essentially consisting of denatured whey proteins. The proportion of whey protein aggregates increased as C:W was changed from 80:20 to 20:80. The results from this study confirmed that heat-induced aggregates were formed not only from casein micelles but also from heat-denatured whey proteins.     

A comparison of the interfacial behaviour of three food proteins adsorbed at air—water and oil—water interfaces

The adsorption behaviour of three food proteins—a soya protein isolate, a sodium caseinate and a whey protein concentrate—at a soya bean oil-water interface has been studied by the drop volume method. The interfacial behaviour has been compared with that at an air—water interface. The kinetics of surface tension decay were evaluated in terms of different rate-determining steps at different ionic strengths and concentrations of the proteins. The ranking order with respect to the surface activity of the proteins adsorbed at an air—water interface was the same as that at a soya bean oil—water interface. In the high concentration range the surface activity of the proteins was higher at an air-water interface than at a soya bean oil-water interface, whereas thereverse was found in the low concentration range. In general, the adsorption of the proteins was more diffusion controlled at an air-water interface than at a soya bean oil-water interface; this suggested that proteins were less folded at the soya bean oil-water interface. A comparison of the rates of the diffusion controlled steps for the proteins at air-water and soya bean oil-water interfaces indicated that the solvation energy gained when caseinates adsorb at the soya bean oil interface was enhanced compared with the other two proteins. This indicated an enhanced loop formation of the caseinate molecules in the oil phase when adsorbing at this interface, as compared with the air-water interface.     

Soluble protein fractions from pH and heat treated sodium caseinate: physicochemical and functional properties

The physicochemical (solubility and hydrophobicity), and functional (emulsifying activity index and emulsifying capacity) properties of soluble sodium caseinate fractions were studied as a function of pH (3–8) and temperature (50–100°C). Solubility was determined by measuring protein with the Bradford and 280 nm absorbency methods. Hydrophobicity was determined fluorometrically with 1-anilino-8-naphtalenesulfonate (ANS), and cis-parinaric acid (CPA). Sodium caseinate solubility was minimal at pH 3.75–4 but the ANS and CPA-hydrophobicities and the functional properties of the soluble proteins increased in this pH range. Circular dichroic and 280 nm absorptivity measurements detected conformational changes. SDS-PAGE and reversed phase HPLC revealed substantial losses of αs₁ and β caseins following pH and heat treatment (pH 3.75 and 92.5°C) and the concomitant appearance of modified compounds. Under these same conditions, the o-phtaldialdehyde values increased suggesting partial hydrolysis of sodium caseinate. The soluble protein fractions from sodium caseinate heat treated near the pI of the caseins were shown to have enhanced emulsifying activity and capacity.     

Protein co-precipitates from milk and blood plasma: Preparation and investigation of some functional properties

Various processing methods were investigated for the production of milk and porcine blood plasma co-precipitates. Factors considered included pH and temperature treatments as well as the ratio of milk to plasma proteins in mixtures of the raw materials. The recovery of protein in precipitates was measured and the following method was selected accordingly for further studies: pH adjustment of skim milk and blood plasma mixtures to pH 9.5, heating to 70° C, readjustment of pH to 9.5, holding for 3 min, cooling to 68°C, pH adjustment to 3–5, holding for 5 min, cooling to ambient temperature and final pH adjustment to 4.7. Two co-precipitates (70/30 M/P and 30/70 M/P) were prepared from a 70:30 and a 30:70 mixture of skim milk (M) and blood plasma (P). Some of the functional properties of these preparations were measured and compared with those of total milk protein (TMP) and blood plasma precipitate (P) prepared by the same procedure as well as acid-precipitated casein. The protein contents of preparations freeze-dried at pH 7.0 varied between 91.5 and 92.3% and those freeze-dried at pH 4.7 varied between 93.1 and 96.0%. The solubility profile and emulsifying capacity of the 70/30 M/P compared favourably with those of caseinate and TMP. The solubilities of 30/70 M/P and 100% P were, however, poor. The viscosity of solutions of 70/30 M/P was considerably higher than those of caseinate and TMP solutions. Water adsorption isotherms of protein preparations were constructed and are presented in graphical form. Precipitates freeze-dried at pH 7.0 adsorbed more moisture than the same preparations freeze-dried at pH 4.7. These differences were especially evident a water activities >0.8.     

Emulsifying Effects of Food Macromolecules In Presence of Ethanol

The influence on droplet size of ethanol present during homogenization was investigated for emulsions stabilized by macromolecular emulsifiers: sodium caseinate, whey protein isolate, gelatin and gum arabic. Emulsions produced with polysaccharide gum arabic had increasing droplet size as ethanol concentration increased, in contrast to the protein-stabilized emulsions which had decreasing droplet size (up to 20 % ethanol for gelatin and 30 % ethanol for the milk proteins), followed by increasing droplet size with increasing ethanol concentration. Interfacial tension measurements indicated that the emulsifying property of the macromolecules depended on adsorption at the oil-water/alcohol interface during emulsification.     

Effect of thermal treatments on the properties of coconut milk emulsions prepared with surface-active stabilizers

Previously we have demonstrated improved stability of coconut milk emulsions homogenized with various surface-active stabilizers, i.e., 1 wt% sodium caseinate, whey protein isolate (WPI), sodium dodecyl sulfate (SDS), or polyoxyethylene sorbitan monolaurate (Tween 20) [Tangsuphoom, N., & Coupland, J. N. (2008). Effect of surface-active stabilizers on the microstructure and stability of coconut milk emulsions. Food Hydrocolloids, 22(7), 1233–1242]. This study examines the changes in bulk and microstructural properties of those emulsions following thermal treatments normally used to preserve coconut milk products (i.e., −20 °C, −10 °C, 5 °C, 70 °C, 90 °C, and 120 °C). Calorimetric methods were used to determine the destabilization of emulsions and the denaturation of coconut and surface-active proteins. Homogenized coconut milk prepared without additives was destabilized by freeze–thaw, (−20 °C and −10 °C) but not by chilling (5 °C). Samples homogenized with proteins were not affected by low temperature treatments while those prepared with surfactants were stable to chilling but partially or fully coalesced following freeze–thaw. Homogenized coconut milk prepared without additives coalesced and flocculated after being heated at 90 °C or 120 °C for 1 h in due to the denaturation and subsequent aggregation of coconut proteins. Samples emulsified with caseinate were not affected by heat treatments while those prepared with WPI showed extensive coalescence and phase separation after being treated at 90 °C or 120 °C. Samples prepared with SDS were stable to heating but those prepared with Tween 20 completely destabilized by heating at 120 °C.     

pH Induced Aggregation and Weak Gel Formation of Whey Protein Polymers

Whey protein polymers were formed by heating (80 °C) a 4% (w/v) whey protein (WP) isolate dispersion at pH 8.0 for 15, 25, 35, 45, or 53 min. Dispersions were adjusted to pH 6.0, 6.5, 7.0, 7.5, or 8.0 after heating and the rheological properties were determined. Viscosity increased with increased heating time and decreased pH. At pH 7.0 and 7.5, high-viscosity dispersions with pseudoplastic and thixotropic flow behavior were formed, while weak gels were formed at pH 6.0 and 6.5. The storage (elastic) and loss (viscous) moduli of pH-induced gels increased when temperature was increased from 7 °C to 25 °C, suggesting that hydrophobic forces are responsible for gelation. Key Words: weak-gels, whey proteins, polymers, gelation, functionality     

Functional properties of hydrolysates from Phaseolus lunatus seeds

Use of low degree of hydrolysis (DH < 10%) with enzymatic treatment can produce protein hydrolysates with functional properties superior to the raw material. Suspensions of Phaseolus lunatus protein isolate (PPI) were treated with one of two commercial enzymes (Alcalase or Flavourzyme) at 50 °C and pH 8.0. DH with Alcalase was greater than Flavourzyme at 5 or 15 min of reaction. Alcalase-prepared hydrolysates had more peptides than those prepared with Flavourzyme. All the hydrolysates had higher solubility than the PPI, the highest being for the Alcalase-prepared hydrolysate at 15 min reaction time. Overall, the Alcalase-prepared hydrolysates had better solubility characteristics, whereas the Flavourzyme-prepared hydrolysates had better film properties (maximum emulsifying capacity and the highest foam formation values). This is probably because of the greater ease of movement toward the interface as shown by their high surface hydrophobicity values. The Alcalase-prepared hydrolysates had generally low or nonexistent film properties.     

Small and large deformation rheology and microstructure of κ-carrageenan gels containing commercial milk protein products

The rheological behaviour of commercial milk protein/κ-carrageenan mixtures in aqueous solutions was studied at neutral pH. Four milk protein ingredients; skim milk powder, milk protein concentrate, sodium caseinate, and whey protein isolate were considered. As seen by confocal laser microscopy, mixtures of κ-carrageenan with skim milk powder, milk protein concentrate, and sodium caseinate showed phase separation, but no phase separation was observed in mixtures containing whey protein isolate. For κ-carrageenan concentrations up to 0.5 wt%, the viscosity of the mixtures at low shear rates increased markedly in the case of skim milk powder and milk protein concentrate addition, but did not change by the addition of sodium caseinate or whey protein isolate. For κ-carrageenan concentrations from 1 to 2.5 wt%, small and large deformation rheological measurements, performed on the milk protein/κ-carrageenan gels, showed that skim milk powder, milk protein concentrate or sodium caseinate markedly improved the strength of the resulting gels, but whey protein isolate had no effect on the gel stength.     

The choice of homogenisation equipment affects lipid oxidation in emulsions

Milk proteins are often used by the food industry because of their good emulsifying properties. In addition, they can also provide oxidative stability to foods. However, different milk proteins or protein components have been shown to differ in their antioxidative properties, and their localisation in emulsions has been shown to be affected by the emulsification conditions. The objective of this study was to investigate the influence of homogenisation equipment (microfluidizer vs. two-stage valve homogeniser) on lipid oxidation in 10% fish oil-in-water emulsions prepared with two different milk proteins. Emulsions were prepared at pH 7 with similar droplet sizes. Results showed that the oxidative stability of emulsions prepared with sodium caseinate was not influenced by the type of homogeniser used. In contrast, the type of homogenisation equipment significantly influenced lipid oxidation when whey protein was used as emulsifier, with the microfluidizer resulting in lower levels of oxidation.