TEXTURE-STRUCTURE RELATIONSHIPS IN HEAT-INDUCED SOY PROTEIN GELS

Structural factors responsible for textural properties of heat-induced soy protein gels were investigated using microscopic and mechanical testing techniques. The gels were prepared by heating 20% soy protein pastes for 30 min at temperatures ranging from 25 to 130°C . Gel hardness increased linearly with the heating temperature up to 80°C , and decreased when the gels were heated at over 90°C , especially over 120°C . The equilibrium modulus estimated by tensile stress relaxation experiments was of the order of 10⁴ - 10⁵ dyne/cm², suggesting the presence of crosslinks, and there was a good correlation between the equilibrium modulus and the hardness of the gels. Solubility in phosphate buffer containing 2 -mercaptoethanol and/or urea suggested that the gel network was formed through crosslinking of the disulfide-, hydrogen- and hydrophobic-bonding types, and that the textural properties were governed by the degree of the network formation controlled by the heating temperature. SEM images of the 80°C -induced hard gels revealed a porous structure having membranous walls of thin compact film. With the 40°C -induced soft gels, the formation of the porous structure was not yet adequate, while a partial collapse of this structure was observed in the 120°C -induced fragile gels.     

EFFECT OF ADDED SALT ON TEXTURAL PROPERTIES OF HEAT-INDUCED GELS MADE FROM GUM–PROTEIN MIXTURES

Large deformation rheological tests were employed to determine the textural differences in heat‐induced gel systems. Three different large deformation rheological methods (viscosity index and apparent elasticity, texture profile analysis (TPA) and torsional fracture) were employed to study the dependence of the ion type on the textural properties of heat‐induced mixed protein–gum gels. Protein–gum mixed solutions were prepared with bovine serum albumin (BSA) or egg white albumin (EWA) (20% w/v) with κ‐carrageenan (KCG), gellan (GLN) or xanthan gums (XNT) (0.5% w/v) at 0.1 M sodium chloride (NaCl), potassium chloride (KCl) or no added salt. Despite inherent differences in protein type, the main effect on the textural properties evaluated was for the kind of salt added, since potassium ions, with a strong influence on KCG and GLN gelation, affected the parameters related to the structure or hardness of the samples. There was no significant effect on parameters associated with sample ductility or elasticity. GLN formed stronger gels than KCG, whereas XNT did not perform as well in gel formation since it does not contribute to protein matrix formation. The results indicated that potassium may be substituted for sodium ions at low ionic strengths in foods where the incorporation of KCG or GLN helps to improve texture and related features.     

SUITABILITY OF THE WARNER-BRATZLER SHEAR FOR TEXTURAL MEASUREMENTS OF FISH PROTEIN GELS

Protein gels were prepared without added NaCl, from the minced, washed, dewatered flesh of red hake and flounder. Similar gels were prepared from red hake with added NaCl. Gel strength was measured by means of the Warner-Bratzler shear, Fold Test, and Instron UTM. Flounder gels had the highest moduli of deformability values while red hake (with NaCl) had the lowest. Instron percent recovery values were inversely related to these results. The red hake (with NaCl) gel was the most elastic (greatest percent recovery) and the flounder gel was the least elastic. The highest Warner-Bratzler shear force values were recorded for flounder gels while the lowest were recorded for red hake (with NaCl) gels. Fold Test scores were 4.6 – 4.7) for both red hake gels, and low (2.7) for flounder gels. The Warner-Bratzler shear was very highly correlated with Instron deformability measurements and highly correlated with percent recovery. It was not significantly correlated with Fold Test scores. Fold Test scores correlated very highly with both Instron values.     

MECHANICAL AND WATER-HOLDING PROPERTIES OF HEAT-INDUCED SOY PROTEIN GELS AS RELATED TO THEIR STRUCTURAL ASPECTS

The dependence of mechanical and water-holding properties of soy protein gels upon their structures was examined. Gels varying in extent of network formation were prepared by heating 20% paste of the isolated soy protein (ISP) containing L-cysteine hydrochloride (CySH) which has an ability to cleave intermolecular disulfide bonds. A rapid decrease in gel hardness and cohesiveness was observed with the increase in added amount of CySH up to 2.5 times 10^-5 mole/g ISP, and then a gradual decrease to 5.0 times 10^-5 mole/g ISP. The changes in these mechanical parameters appeared to depend on the degree of the network formation interpreted from the solubility changes of gels in the phosphate buffer containing 6 M urea. The relaxation time, estimated by compression stress relaxation, was also dependent on the degree of the network formation, while the modulus of elasticity was hardly affected. On the other hand, there was an inverse correlation between the NMR line width of the water proton and expressible water by the pressure method as an index of water-holding properties of the gel; the broader the former, the lesser the latter. The change in the water-holding capacity estimated from expressible water was also associated with that in the degree of the network formation. This confirms that both mechanical and water-holding properties of the gel are qualitatively governed by the extent of the structure formation controlled by the intermolecular disulfide bonds.     

A COMPARISON OF THE VISCOELASTIC PROPERTIES OF CONVENTIONAL AND MAILLARD PROTEIN GELS

The properties of gels prepared by heating solutions of bovine serum albumin (BSA) for 30 min at 121C in the presence and absence of glucono-delta-lactone (GDL gels) and xylose (Maillard gels) were compared. During formation the pH of the Maillard and GDL gels decreased to 4.9 whereas the pH of the gels formed in the absence of GDL or xylose remained near neutral. Maillard gels show much less syneresis compared with the GDL gels and contained nondisulphide covalent crosslinks as evidenced by very low protein solubilities in mixtures of sodium dodecyl sulphate and β-mercaptoethanol. Both the GDL and Maillard gels could be formed at much lower protein concentrations than the neutral conventional gels. The stress relaxation of the gels in compression was measured and the response analyzed using Peleg's equation. The parameters in this equation were not strongly dependent on protein concentration or degree of deformation. The neutral pH gels were far more elastic than the low pH gels, but despite the difference in crosslinking mechanisms the viscoelastic behaviour of the Maillard and GDL gels was similar. However, the break strength and asymptotic residual modulus of the Maillard gels were higher. It is suggested that the stress relaxation occurs in weaker, noncovalently linked regions of the gel, whereas the nondisulphide covalent crosslinks in the Maillard gels reinforce strong regions already containing disulphide linkages.     

鸡肉盐溶蛋白质热诱导凝胶质构的研究

采用L9（3^4）正交试验法研究磷酸盐和氯化钙复合作用对鸡胸和鸡腿肉热诱导凝胶的硬度和弹性的影响。结果表明，鸡胸肉和鸡腿肉热诱导凝胶的硬度有极显著差异（p〈0．01），二者凝胶的弹性无显著差异（p〉0．05）。就鸡胸内而言。氯化钙、三聚磷酸盐和焦磷酸盐对其硬度均有极显著影响（p〈0．01），三聚磷酸盐和焦磷酸盐对其凝胶的弹性有显著影响（p〈0．05）；而对于腿内来讲。焦磷酸盐对其硬度有极显著影响（p〈0．01），氯化钙和三种磷酸盐对凝胶的弹性均有显著影响（p〈0．05）。     

A NEW INSTRUMENTAL METHOD FOR MEASURING THE CARBON DIOXIDE CONTENT OF CARBONATED BEVERAGES

Tests of an instrument designed primarily for the estimation of carbon dioxide in blood have demonstrated its suitability for carbon dioxide in carbonated beverages. In respect of speed, simplicity of operation, safety and reproducibility, the instrument's performance compares favourably with the Institute's recommended method.     

REFINEMENTS OF THE CASEIN TURBIDITY METHOD FOR MEASURING PROTEASES IN BEER

The casein turbidity method for determining papain has been modified to allow it to be used for both normal and strong beers. Problems of instant casein coagulation which occur when the original reagents are mixed with strong beers were due to inadequate control of pH, and not to high alcohol levels. Methods are given for preparing convenient substrate and activator solutions that are stable for at least 8 days at 4°C. By carrying out estimations at 60°, rather than 50° as originally proposed, incubation times are roughly halved. It is shown that weak activators, like potassium metabisulphite, can reduce enzyme activities when added to a reaction mixture which also contains cysteine. The mixing sequence of beer, activator solution and substrate solution, as well as buffer strengths, influence the observed activities of papain. Further changes which might improve the method further have been proposed.     

EFFECT OF PROTEIN HYDROLYSATE ON THE INSTRUMENTAL TEXTURE PROFILE OF GELATIN GELS

The texture of gels containing 2 to 8% of gelatin and 1 to 10% of alcalase protein hydrolysate obtained from poultry meat was examined using the instrumental texture profile analysis (TPA) method in compression and penetration mode. Response surface analysis modelled the effects of the two components of the gels on textural properties. In both tests the addition of hydrolysate decreased hardness and chewiness, and the effect was more pronounced for higher gelatin concentrations. The senses of changes in cohesiveness and springiness as measured in both tests employed here were not in agreement with each other owing to different consequences produced on the sample. In the case of compression, both parameters decreased with increased hydrolysate addition, whereas for penetration an increase in cohesiveness was observed as well as small changes in springiness. Hydrolysate produced a statistically significant negative linear and positive quadratic effect with regard to the majority of the studied texture parameters. This component interacted significantly with gelatin with regard to hardness, chewiness, and fracturability which led to a decrease in the first two and an increase in the last texture parameter. The weakening effect of the protein hydrolysate upon texture may be of great importance for the quality of gelatin gels.     

APPLICATION OF SURFACE FRICTION MEASUREMENTS FOR SURFACE CHARACTERIZATION OF HEAT-SET WHEY PROTEIN GELS

The surface properties of heat‐set whey protein gels (14 wt %) was studied by measuring the friction at the gel's surface. A simple device was constructed that can be conveniently attached to a Texture Analyzer. Surface friction forces of gels with and without addition of salt were measured as a function of sliding speed and surface load. Surface friction strongly depended on the sliding speed for all three gel systems over the speed range 0.01 mm/s to 10 mm/s. The gel without salt addition showed the highest speed dependency, while the gel containing 200 mM NaCl had the lowest speed dependency. Surface load tests showed nearly linear relationships for both protein gels (with and without salt addition). Unlike solid materials, both protein gels exhibited a surface friction even as the surface load approached zero. Possible contributions of surface attraction and viscous flow to the measured forces are discussed. Results from surface friction tests were further confirmed by optical observation of the surface using a confocal laser scanning microscope (CLSM), where a very smooth surface was observed for the whey protein gel without salt addition, but a much rougher surface was observed for the gel containing 200 mM NaCl.     

A COMPARISON OF PROTEIN ASSAYS FOR THE DETERMINATION OF THE PROTEIN CONCENTRATION OF BEER

Seven protein assays have been compared for determination of the protein content of beer. Dialysis and ultrafiltration have been used to investigate possible interference and the protein content of the samples has been simultaneously monitored by sodium dodecyl sulphate-polyacrylamide gel electro-phoresis. The Fisons NA2000 Nitrogen/Protein Analyzer gave identical results to the standard Kjeldahl method but both measure nitrogen-containing compounds rather than protein. Colorimetric methods based upon the biuret reaction (including the Lowry and bicinchoninic acid protein assays) are particularly prone to interference from reducing substances and gave protein concentration values dependent upon the sample volume. The protein dye-binding assays (Coomassie Brilliant Blue and pyrogallol red-molybdate methods) were optimal for beer protein analysis displaying minimal inter ference and giving reproducible protein concentration values consistent with the protein content of the beers as indicated by electrophoresis.     

EFFECT OF EMULSION DROPLETS ON THE RHEOLOGY OF WHEY PROTEIN ISOLATE GELS

The effects of droplet size and emulsifier type on the rheology of whey protein isolate (WPI) gels containing emulsion droplets was studied. Gels were prepared by dispersing droplets of corn oil (20 wt%, d₃₂= 0.7 – 4 μm) in a 10 wt% WPI solution (pH 7.0, 50 mM NaCl), and heating at 90C for 15 min. Gel strength was determined by measuring the stress of gels at 20% compression using an Instron Universal Testing Machine. Droplets stabilized by WPI increased the gel strength, those stabilized by non-ionic surfactants (Tween 20 and Triton X-100) decreased it slightly, and those stabilized by SDS decreased it drastically. Gel strength increased as the droplet size decreased for droplets stabilized by WPI, but was relatively insensitive to the size of droplets stabilized by the small molecule surfactants. These observations may be explained in terms of the interactions between the emulsifiers and the protein network. Droplets coated with emulsifiers which can be incorporated into the protein network reinforce the structure and so increase gel strength, whereas droplets coated with emulsifiers which cannot be incorporated into the protein network disrupt the three dimensional structure of the gel and decrease its strength.     

纺织品中纱线密度的测量方法

在简单介绍常规的纱线密度人工测量方法(包括拆纱点数法、放大镜观察计数法以及密度板法)的基础上，综述了计数型和计算型两种自动测量密度的方法，同时对自动密度测量的改进与展望提出了个人的看法。     

RHEOLOGICAL PROPERTIES OF WHEY PROTEIN CONCENTRATE GELS

The rheological properties of heat whey protein concentrate gels were studied by dynamic oscillation rheometry. A whey protein concentrate of 75% protein was used to make solutions of 10.3, 12.5 and 14.5% protein (w/w), which were heated to 90C for gel formation. Specific attention was focused on the temperature dependence of the mechanical properties of the gels during cooling and reheating. In all cases the magnitude of the complex modulus |G*| was found to increase with decreasing temperatures from 90 to 30C. The tan δ, which is related to the relative viscoelasticity of the gels, increased with decreasing temperatures from 90 to 60C. At temperatures between 60 and 30° C, tan δ remained constant. The dependence of |G*| and tan δ on temperature was found to remain constant during heating and cooling between 30 and 70C, indicating that rheological changes were reversible within this temperature range.     

RHEOLOGICAL PROPERTIES OF PROTEIN/STARCH MIXED GELS

The influence on the viscoelastic behaviour of the heat transition temperatures of starch and protein in gelatinous systems prepared from the two components was studied. Four different starches were combined with Bovine Serum Albumin (BSA) in several protein-to-starch ratios. Both the transition temperature and the rates of gelation of the components were critical for the behaviour of the complex systems. In those cases where the starch gel was formed before the protein gel, the storage and loss moduli of the complex system could be predicted by a simple addition of the moduli of the components at corresponding concentrations. When the gelation occurred in the reverse order, the gels were considerably stronger than predicted by the additivity model. When the starches were combined with gelatin, the complex gels were all weaker than predicted. The gelation of gelatin was very slow compared to that of the starches and the BSA, and it is likely that the weakness of these gels was due to the aggregation of the amylose molecules or some other part of the starch retrogradation process.