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Whey protein isolate (WPI) solutions, with different levels of aggregated protein, were prepared by heating (5% protein, pH 7, 90 °C for 30 min) WPI solutions with either 20 mM added NaCl (WPI + NaCl), 5 mM N-ethylmaleimide (WPI + NEM) or 20 mM added NaCl and 5 mM NEM (WPI + NaCl + NEM). Gel electrophoresis demonstrated that the heated WPI and WPI + NaCl solutions had higher levels of aggregated protein, due to more covalent interactions between proteins, than the heated WPI + NEM and WPI + NaCl + NEM solutions. There were marked differences in the levels of amino groups between all heated WPI solutions when measured by the OPA and TNBS methods, with lower levels being measured by the TNBS method than by the OPA method. These results demonstrate that the measurement of available amino groups by the OPA method is less impacted than by the TNBS method after heat-induced structural changes, arising from disulfide or sulfhydryl-disulfide bond-mediated aggregation of whey protein molecules.

The effect of conjugating konjac glucomannan with fluorescein isothyocyanate (FITC) via covalent labelling on selected physicochemical properties of FITC and konjac was investigated using spectrophotometry, rheometry, and size exclusion chromatography (SEC). The binding of the lectin concanavalin A (ConA) to sugar residues of konjac was investigated using SEC. Covalent conjugation of konjac with FITC led to a shift in the absorbance spectrum peak of FITC to a lower wavelength and a decrease in the average molecular weight distribution of konjac. Furthermore, covalently labelled konjac showed reduced apparent viscosity compared to unlabelled konjac. ConA bound to the sugar residues in konjac. The potential of konjac–FITC covalent labelled conjugate or konjac–lectin labelled ConA complexes as fluorescent markers for localisation of konjac in a phase separated micellar casein–konjac mixture was investigated using confocal laser scanning microscopy. Results indicated that covalently labelled konjac has microscopic phase behaviour similar to that of un-labelled konjac and are therefore suitable for localising konjac glucomannan in a phase-separated micellar casein–konjac system.

Hydrolysates of sodium caseinate (NaCN)–maltodextrin (Md40 or Md100) conjugates were prepared with a limited ( 10%) degree of hydrolysis. When assessed in the pH range 2.0–8.0, each conjugate hydrolysate had improved solubility compared to NaCN and their respective native unhydrolysed conjugate. Oil-in-water emulsions containing NaCN (1%, w/w, protein) and various combinations of conjugate hydrolysates (0.2%, w/w) and/or glycerol monostearate (0.07–0.3%, w/w, GMS) were prepared; emulsion storage stability (at 45 °C for up to 20 days) and heat stability (at 140 °C for up to 20 min) was determined by measuring changes in the mean size of fat globules in emulsions. NaCN plus conjugate hydrolysate-stabilised emulsions had improved storage stability compared to a NaCN stabilised emulsion. In general, NaCN plus conjugate hydrolysate-stabilised emulsions were less heat-stable than NaCN or NaCN plus GMS stabilised emulsions; however, emulsions stabilised by NaCN plus one of the conjugate hydrolysates (CH102) had improved heat stability in comparison to the NaCN stabilised emulsion. The results show that hydrolysates of NaCN–Md conjugates have potential for use as emulsification aids in emulsion-based food products.

A sodium caseinate (NaCN)–maltodextrin (Md100) conjugate was prepared by a Maillard-type reaction by dry heat treatment of a NaCN–Md100 mixture at 60 °C and 79% relative humidity for 4 days. Conjugation resulted in a 35.7% loss of available amino groups in the NaCN and a 25.9% loss of available reducing groups in the Md100. The crude conjugate was purified by batch anion exchange chromatography to remove non-conjugated Md100. Purification reduced the available reducing groups in the conjugate from 74.1% to 23.7% and increased the protein content from 45.6% to 83.9%. The emulsifying properties of the conjugates were assessed in oil-in-water (o/w) emulsions; crude and purified conjugate stabilised emulsions had improved storage stability and freeze–thaw stability when compared to NaCN stabilised emulsions. Purified conjugate stabilised emulsions had better thermal stability than NaCN, NaCN–Md mixture and non-purified conjugate stabilised emulsions. These results indicate a potential for these NaCN–Md conjugates as speciality functional food ingredients.

Sodium caseinate (NaCN)–maltodextrin (Md40 or Md100) conjugates were prepared by a Maillard-type reaction by dry heat treatment of mixtures of NaCN and Md at 60 °C and 79% relative humidity for 4 days. Minimal levels of coloured reaction products were formed during conjugate preparation. Conjugation resulted in a 35.6% and a 36.2% loss of available amino groups in the NaCN, and a 17.8% and a 25.7% loss of available reducing groups in Md40 and Md100, respectively. SDS–PAGE and gel permeation chromatography confirmed conjugation. When assessed in the pH range 2.0–8.0 at 20 °C and 50 °C, conjugates had improved solubility compared to NaCN, particularly around the isoelectric point of the protein. The emulsifying properties of NaCN–Md conjugates were assessed in oil-in-water (o/w) emulsions and in model cream liqueurs. The conjugate stabilised o/w emulsions and liqueurs showed improved stability when compared to NaCN stabilised o/w emulsions and liqueurs. These results indicate a potential for these NaCN–Md conjugates as speciality functional food ingredients.

The surface-active properties of myosin, actomyosin, F- and G-actin at the air-solvent interface, at initial bulk phase concentrations in the range of 10 − 4 % to 10 − 2 % w/v, were determined by the drop volume method. Overall myosin was the most effective surface tension depressor, followed by actomyosin, F-actin and G-actin. The surface pressures attained after 40 min at an initial bulk phase concentration of 10 − 2 % w/v were 21·02, 19·27, 17·25 and 14·91 mNm − 1 , respectively. Furthermore, myosin effected the most rapid change in surface pressure during the initial 5-min period.

Casein is very soluble at pH values Sorption isotherms of acidic caseins and sodium caseinate were similar up to Aw values of 0·52. Between Aw values of 0·52 and 0·87 sodium caseinate showed a greater increase in water uptake than acidic caseins, while at Aw values of 0·87–0·97 water uptake of acidic caseins increased more sharply than that of sodium caseinate, uptake being in the order acidic casein pH 2·0 > acidic casein pH 2·5 > acidic casein pH 3·0 > sodium caseinate pH 7.0.

The effects of pH and thermal denaturation on the surface active properties of muscle proteins at the air-solvent interface were determined by the drop volume method. The surface pressures attained after 40 min, π 40 , by myosin and actomyosin solutions heated at 45°C for 30 min were similar to those attained by the unheated protein solutions. However, the π 40 values were increased when the proteins were heated at 60°C prior to determining surface activity. The rate of surface tension decay was much faster following heating of myosin at 45°C compared with the unheated sample. Myosin heated at 60°C showed an initial induction period, which was followed by a very rapid rate of surface tension decay. The rate of surface tension decay for actomyosin heated at 45°C was similar to that of the unheated protein solution: however, heating at 60°C increased the rate compared to that of the control. The surface pressures attained after 40 min ( π 40 ) by myosin and Factin solutions were not significantly affected by the pH of the solutions. However, π 40 values for both G-actin and actomyosin were greater at pH 11·0 than at pH 5·6 or 7·0. For all the proteins studied, pH greatly affected the rates of surface tension decay, the rates being slowest at pH 5·6 and fastest at 11·0.

The emulsifying capacity of casein was dependent on pH, decreasing from 325 to 264 g oil/g protein as pH increased from 1·5 to 3·5 and increasing from 251 to 268 g oil/g protein between pH 5·5 and 7·0; above pH 7·0 emulsifying capacity increased sharply to ∼ 700 g oil/g protein at pH 8·5. The emulsifying capacity of sodium caseinate increased on addition of NaCl, up to 1 m , or CaCl2, up to 20 m m , while the emulsifying capacity of acidic casein (pH 2·5) increased on addition of NaCl, up to 5 m m , or CaCl2, up to 5 m m and thereafter decreased on further addition of either. The creaming stability of emulsions prepared in acidic casein increased as the pH increased from 2·0 to 3·0. Foaming capacities of caseins were in the order, acidic casein pH 2·0 > acidic casein pH 2·5 > sodium caseinate pH 7·0 > acidic casein 3·0, while foam stabilities were in the opposite order. Addition of NaCl, up to 20 m m , or CaCl2, up to 10 m m , decreased foam capacity of acidic casein (pH 2·0) but increased foam stability.

The rheological properties of plasma protein gels were markedly affected by protein concentration, pH, heating temperature and time and measuring temperature. Gel strength increased with increasing protein concentration (6–18%), pH (4·0–9·0), heating temperature (60–90°C) and time (15–60 min), but decreased with increasing measuring temperature (10–60°C). The ability of gels to recover following cyclic loading and unloading was low in the pH region 4·0–6·0, but increased at higher pH. Recovery increased with increasing measuring temperature, but decreased at heating temperatures above 75°C and with increased duration of heating (15–60 min).

Added salts influenced the rheological properties of heat-induced plasma protein gels. Gel strength increased in the presence of 0·01 m added CaCl 2 and 0·02 m added MgCl 2 . However, gel strength was reduced at higher concentrations of these salts. The ability of plasma protein gels to recover following deformation was lower than for the control (no added salt) in the presence of 0·02–0·1 m added CaCl 2 and MgCl 2 , but gel cohesiveness was markedly higher. The strength of plasma protein gels formed in the presence of added potassium salts was in the order F − >SO 2− 4 >Cl − >I − >SCN − . These salts had similar effects on gel cohesiveness and gumminess. KF and K 2 SO 4 decreased gel recoverability following deformation; KI and KSCN increased recoverability, while KCl had little effect. The study suggests that hydrophobic interactions and electrostatic forces play an important role in the formation and stabilisation of plasma protein gels.