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Pulsed electric field (PEF) treatment at 49 kV/cm for 19.36 μs (up to 70 °C) was performed on reconstituted skim milk (10 wt%). The milk condition was varied either by pH adjustment (pH 6.7–8.0) or by the addition of ethylenediaminetetraacetic acid (EDTA) (up to 30 mM). Except for milks at pH ⩾ 7.5, PEF treatment did not affect size and ξ -potential of the milk particles, the amounts of protein in the serum or the state of the whey proteins. For milks at pH ⩾ 7.5, a decrease in the size of the milk particles and an increase in the amount of proteins in the serum were measured. Sodium dodecyl sulphate – polyacrylamide gel electrophoresis (SDS–PAGE) measurements showed that the amount of caseins in the serum increased, while the whey proteins were not affected. This indicates that the decrease of the size of the milk proteins is due to the dissociation of the casein micelles. Further, this study demonstrates that the PEF effects on milks at pH ⩾ 7.5 is due to ohmic heating during the PEF treatment. [ABSTRACT FROM AUTHOR]

Abstract: This study investigated the effect of storage temperature (20–50°C) and time (0–60days) 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. [Copyright &y& Elsevier]

Abstract: The adsorption of sodium caseinate (NaCAS) and whey protein isolate (WPI) to both normal and waxy rice starch granules was investigated using SDS-PAGE. This showed that the proteins present in NaCAS and WPI do adsorb to normal and waxy rice starch granules and that in the case of NaCAS, for both normal and waxy rice starch, αs-casein adsorbed preferentially and in higher amounts than β-casein. In the case of WPI, although the amount of adsorbed β-LG is higher than that of α-LAC, no preferential adsorption is observed. Confocal laser scanning microscopy, with specific dyes, was also used in an attempt to locate the milk proteins adsorbed to the starch granules. The adsorption isotherms of these two milk protein ingredients to rice starch granules, were modelled using the BET equation which is usually used to model multilayer adsorption, and a simple equation is proposed to take into account possible absorption of the proteins into the core of the starch granules. Good agreement is found between these models and the experimental results. Possible mechanisms involved in the interaction between milk proteins and starch granules are discussed. [ABSTRACT FROM AUTHOR]

Abstract: The dynamic rheological behaviour of skim milk gels containing 2% normal rice starch granules pasted to different temperatures was investigated. The extent of swelling of the starch granules was also measured. It was found that the complex modulus G* was maximal when the starch granules were pasted to the temperature of maximum swelling and not to the temperature of maximum viscosity. It is suggested that, in these systems, the starch granules behave as inactive fillers and that their main effect is to increase the milk protein concentration during swelling by absorbing water from the continuous phase. A simple empirical model that agrees qualitatively well with the experimental results is proposed. [Copyright &y& Elsevier]

The objective of this study was to examine the effects of added xanthan gum on the formation and properties of emulsions (4 wt% corn oil) formed with a highly hydrolysed commercial whey protein (WPH) product under a range of conditions. Stable oil-in-water emulsions could be formed using 4 wt% WPH and hydroxylated lecithin as emulsifiers. Addition of xanthan to these emulsions above a certain critical concentration caused flocculation of the oil droplets, by depletion mechanisms, which resulted in a marked increase in the creaming rate. Interestingly, this depletion flocculation also promoted the coalescence of droplets during storage of the emulsions for up to 1 week at room temperature. The rate of coalescence was enhanced considerably with increasing concentration of xanthan in the emulsion up to 0.12%. At higher levels of xanthan addition, the rate of coalescence decreased gradually, apparently because of the high viscosity of the aqueous phase. [Copyright &y& Elsevier]

The influence of the addition of κ-carrageenan to 2% whey protein isolate (WPI) at pH 7 and heated at 75 °C for different times, has been investigated by size-exclusion chromatography with multi-angle laser light scattering and gel electrophoresis. Under the conditions used, the addition of κ-carrageenan to WPI did not influence the loss of native β-lactoglobulin or α-lactalbumin. However, κ-carrageenan addition affected the molecular weight (Mw) of the aggregates formed by the heat treatment. The Mw of the whey protein aggregates formed in the early stages of heating in the presence of κ-carrageenan were lower than the aggregates formed in WPI solutions heated in the absence of κ-carrageenan. At longer heating times, the Mw of the aggregates in the WPI/κ-carrageenan mixtures were similar to the Mw of the aggregates in the WPI solutions heated alone. [Copyright &y& Elsevier]

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. [Copyright &y& Elsevier]

Samples of cheese whey or acid casein whey obtained from commercial plants during the different stages of whey protein concentrate (WPC) manufacture were analyzed for protein composition and the extent of protein aggregation. Protein composition was determined by size exclusion chromatography (SEC) and electrophoresis, while the degree of protein aggregation was determined through the changes in molecular weight distributions monitored by SEC combined with multi-angle laser light scattering. The results show that there were some differences between the compositions of protein between the two types of wheys with the acid casein whey also containing lower proportions of “soluble” aggregates than the cheese whey. Ultrafiltration/diafiltration processes caused a removal of the non-protein, low molecular weight components, but had no significant effect on protein composition and aggregation. Evaporation and subsequent spray drying of the ultrafiltered retentates caused no significant changes in any of the whey protein components. Differences in functionality of WPC products are probably more related to modifications of proteins and other components caused by the processes that are used in cheese or casein manufacture rather than to the WPC manufacturing process. [Copyright &y& Elsevier]

Abstract: This study investigated the high-pressure-induced gelatinization of different starches, namely normal rice, waxy rice, normal corn, waxy corn, tapioca and potato starches. The high-pressure-treated starch solutions were characterized by pasting behaviour, degree of swelling and changes in birefringence. Potato starch was found to be less affected by pressure treatment than the other starches, as it retained birefringence after a pressure treatment of 600MPa for 30min. Waxy and tapioca starches showed complete gelatinization after the same treatment, whereas normal starches were only partially gelatinized. The pasting curves of the normal starches showed an increase in the initial viscosity after pressure treatment, whereas the initial viscosities of the waxy starches after the 600MPa for 30min treatment were already equal to their respective peak viscosities. [Copyright &y& Elsevier]

Dextran and guar solutions and their mixtures were investigated by dynamic light scattering. The measurements have shown that dextran solutions present a major fast decay mode and another slower decay mode. Solutions of guar alone and dextran/guar mixtures have shown up to four different modes. The large number of modes may be caused by the aggregation of guar molecules. Although guar/dextran form very complicated mixtures, partly because of the aggregation of guar, an attempt is made to interpret these measurements within the framework of the theories of ternary polymer solutions. [Copyright &y& Elsevier]

Aqueous solutions of sodium caseinate and xanthan at pH 7 and containing 0.1 M NaCl, and their mixtures were investigated using dynamic light scattering. Sodium caseinate solutions showed a bimodal distribution of relaxation rates; with the aggregate peak distribution predominating. Xanthan solutions showed a single distribution at low concentrations (≤0.06 wt.%) and a bimodal distribution at higher concentrations. The sodium caseinate–xanthan mixture modes were independent of the total biopolymer concentration, and behaved as a superposition of sodium caseinate solution alone and xanthan solution alone. This indicates that there is no interaction between xanthan and sodium caseinate in the range of concentrations considered in this study. [Copyright &y& Elsevier]

Reconstituted skim milk was gelled with a crude protease extract from tamarillo (Cyphomandra betacea) fruit and compared with gels prepared with calf rennet. The effects of temperature and pH on the gelation of skim milk were investigated by small deformation oscillatory rheology. The tamarillo extract-induced gels had a faster rate of increase in the elastic modulus (G′) at the early stage of gelation than rennet-induced milk gels. This was probably due to the broader proteolytic activity of tamarillo protease extracts as shown by sodium dodecyl sulfate–PAGE analysis. Confocal microscopy also showed that the milk gels resulting from the addition of tamarillo extracts had larger voids than rennet-induced milk gels. The proteolytic activity of tamarillo extracts was found to be optimal at pH 11. For both rennet and tamarillo extracts, the aggregation time was similar between pH 6.7 and 6.5, but the aggregation time of rennet-induced milk gels was lower than that of milk gels obtained by the addition of tamarillo extracts at pH lower than 6.5. An increase in temperature was found to have a significant effect on aggregation time, particularly at 20°C, where rennet did not coagulate milk in 3 h but the tamarillo extracts coagulated milk within 2 h. The results of this study suggest that extracts from tamarillo fruit could be used for milk gelation, particularly under lower temperature or high pH conditions. [ABSTRACT FROM AUTHOR]

Corn starch pastes (5 or 10 w%) with different amylose content (∼2.7%, ∼5.1%, ∼29.6%, or ∼52.7%) were treated using high-power ultrasound (frequency 20 kHz, power 13.5 or 29.9 W) for up to 20 min. Changes in the physical properties were determined using viscosity measurements and dynamic light scattering. Results show that both the viscosity ( η ) and hydrodynamic radius ( R H ) decreased markedly with the increase in ultrasound treatment time. FT-IR showed that the molecular scission occurred at the C–O–C bond of α-1,6 glycosidic linkage, and that the extent of breakage was inversely correlated with amylose content. Further, high-amylose starch pastes were found to be more resistant to ultrasound treatment due to their aggregation. A master curve for the behaviour of η and R H is proposed, which is also confirmed by a similar study carried out on the ultrasound treatment of rice starch pastes with different amylose contents. The findings of this work can be used to tailor starch solutions with different viscosities using high-power ultrasound. [ABSTRACT FROM AUTHOR]

Abstract: The effect of heat treatment on reconstituted 10 wt% skim milks containing up to 20mM added CaCl2 at different pH values (pH 6.0–7.2), was investigated both in situ and after cooling of the heat treated milks. Measurements of pH in situ showed that pH decreased at high temperature, that the decrease in pH increases with the increase in the initial pH and that the magnitude of the decrease in pH was greater for milks with added CaCl2. Marked increases in the viscosity at 25°C of heated milks indicated that milks without added CaCl2 with initial pH ≤6.2 and milks with 10mM added CaCl2 with initial pH ≤6.4, heated at 90°C were not heat stable. At a given heating temperature, it was possible to superimpose the measured viscosity of samples with or without added CaCl2 on the same curve when these were plotted as a function of the pH at that temperature instead of the initial milk temperature. To further demonstrate this finding a DWS experimental set-up was built and in situ measurements were performed on the milk samples heated at 75°C for 10min. The DWS measurements showed that ηa, the product of the viscosity with particle size, can also be superimposed for all the measured samples, when plotted as a function of the pH at the heating temperatures. Both the viscosity and DWS data demonstrate the importance of the pH at the heating-temperatures in influencing heat-induced changes in milk. [Copyright &y& Elsevier]

Abstract: The rheological properties of acid skim milk gels, prepared from milk with added potato starch and pH adjusted (pH 6.5–7.1) prior to heat treatment and acidification, were investigated. The storage modulus, G′, of the final acid gels was increased by heating the milk at higher pH and further increased by adding starch. The effect of pH at heating and addition of starch appeared to be additive and independent of each other up to a starch addition level of 1%. Above this starch level, the pH at heating had a lesser effect. This may have been due to the increased viscosity of the aqueous phase as a result of starch gelatinization or to direct contributions of the starch to the gel network structure. Confocal microscopy showed that milk proteins developed fewer but broader protein clusters at higher pH than at lower pH. Starch addition resulted in an increased density of the protein network. [Copyright &y& Elsevier]

Abstract: Potato starch was added to skim milk at levels of 0–1.5%. The milks were heated and then acidified to form acid milk gels. The properties of the milks during acidification and the final properties of the acid gels were studied. The addition of starch resulted in a higher storage modulus, G′, in the final acid gels, and increasing the level of starch caused a linear increase in the final G′. Compared with acid gels prepared with no starch, the gelation time was reduced and the gelation pH was increased. However, the temperature and frequency dependences of the acid gels were not affected by the addition of starch. Furthermore, the breaking strain of the acid gels was not markedly affected by the addition of starch, whereas the breaking stress was dependent on the level of starch added. Confocal microscopy showed that the acid gels contained swollen starch granules embedded in a protein network, and that the protein network increased in density as the level of starch added increased. [Copyright &y& Elsevier]

Abstract: 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. [Copyright &y& Elsevier]

Abstract: Diffusing-wave-spectroscopy-based micro-rheology was used to measure the rheological behaviour of dextran solutions of concentration up to 40%. Three dextrans of different molecular weights, namely 2×105, 5×105 and 2×106 Daltons, were studied. It was found that these solutions were mainly viscous, with the loss modulus G″ higher than the elastic modulus G′. However, at high frequencies and for high dextran concentrations, a crossover between G′ and G″ was observed. A plot of the specific viscosity η sp as a function of the dextran concentration showed two critical concentrations c* and c**. For cc**, η sp ∼c 3.8. These results were in very good agreement with those found in the literature. [Copyright &y& Elsevier]