V. B. Tolstoguzov

Functional properties of food proteins and role of protein-polysaccharide interaction

Abstract The review paper discusses the findings on the functional properties of food proteins in multicomponent systems as well as some working hypotheses advanced during recent years at the laboratory of new food forms at the A.N.Nesmeyanov Institute of Organoelement Compounds of the USSR Academy of Sciences. Consideration is given to the following trends of research work: (i) complexing of proteins and polysaccharides, including oligomeric proteins; (ii) nature of biopolymer incompatibility in solution; (iii) effect of thermodynamic incompatibility and complexing of proteins and polysaccharides on the functional properties of food proteins, including solubility and behaviour of proteins at the water-oil interface during emulsion stabilization, as well as on protein gelation; (iv) incompatibility of proteins and polysaccharides in their water-plasticized melt mixtures and the effect of this phenomenon on the formation of structure and properties of proteins textured by thermoplastic extrusion. The importance of investigations into protein-polysaccharide interaction for understanding the mechanisms of structure formation processes in food systems is shown.

Thermodynamic incompatibility of proteins and polysaccharides in solutions

Abstract The thermodynamic incompatibility of proteins and polysaccharides is of a general nature. This phenomenon is observed under conditions which inhibit complexing between proteins and polysaccharides and promote association between macromolecules of the same type, i.e. provide self-association of biopolymers. Conditions for incompatibility are dependent on the structure and composition of a given biopolymer pair. Incompatibility decreases in the order: carboxyl-containing polysaccharides > neutral polysaccharides > sulphate-containing polysaccharides. Normally, linear polysaccharides are more incompatible with proteins than branched polysaccharides. The difference in hydrophilicity between proteins and polysaccharides, the so-called Δχ-effect, is of great importance for phase equilibria in protein—polysaccharide—water systems and significantly affects this phenomenon. This is reflected in the marked tie slope asymmetry of the phase diagrams of mixed pro tein—polysaccharide solutions.

Thermodynamic incompatibility of proteins

Abstract Unlike mixtures of synthetic polymers, mixtures of polysaccharides and protein—polysaccharide mixtures, the thermodynamic incompatibility of proteins in solution is notable for the following general features. This phenomenon is typical of mixtures of proteins belonging to different classes according to the Osborne classification. A relatively high phase separation threshold is the other characteristic feature of mixed protein solutions. Factors such as pH, ionic strength and temperature only slightly affect protein incompatibility. This is probably due to the compactness and rigidity of protein molecules. Accordingly, incompatibility is strongly dependent upon the conformational state of the proteins and is enhanced by protein denaturation.

Thermodynamic compatibility of gelatin with some D-glucans in aqueous media

Abstract The thermodynamic compatibility of gelatin with a number of D -glucans (amylopectin, glycogen, dextran) in aqueous media has been studied. Particular attention was given to the effect of the nature of electrostatic interaction of gelatin macro-ions on the compatibility with D -glucans. It has been established that all the systems investigated, namely water-gelatin- D -glucan, are thermodynamically unstable under isoionic conditions at sufficiently high concentrations of polymers. On centrifugation (≈ 1000 g ), they separate into two liquid phases. The phase separation is accompanied by almost complete separation of the gelatin and the corresponding D -glucan. However, when the pH was shifted towards the acid or alkaline region in relation to the pI of gelatin, as well as when the ionic strength was sufficiently increased, these systems undergo a reversible phase transition from a two-phase to a single-phase state. This suggests that the thermodynamic incompatibility of gelatin with D D-glucans in isoionic conditions is determined by self-association of gelatin macro-ions due to interaction of charge fluctuations.

Application of phase-volume-ratio method for determining the phase diagram of water-casein-soybean globulins system

SummaryThe possibility of using the phase-volume method for determining phase diagrams of polymer mixtures in a common solvent has been ascertained. The above method was used to determine a phase diagram of water-casein-soybean globulins system.

Structural studies of the solutions of anionic polysaccharides. IV. Study of pectin solutions by light-scattering☆

Abstract Aqueous solutions of pectins, with a degree of esterification (DE) varying from 0 to 95%, have been studied by light-scattering. A set of samples of similar molecular sizes were prepared by methylation of sodium pectate with diazomethane. The solutions were subjected to ultracentrifugation to ensure the removal of small amounts of gel fraction. The second virial coefficient of pectin was positive and constant for DE values varying between 43 and 95%, but increased by a factor of three when the DE was reduced to zero. This increase is at least partly due to coulombic electrostatic interactions. All of the pectins investigated behaved as semi-rigid-chain polymers. The chain flexibility was a maximum in the DE range 43 to 58%. In pectin solutions with a DE higher than 58% attractive forces exist between pectin macromolecules due to the presence of ester groups. It is expected that the interactions between ester groups contribute to both the chain rigidity and the gel-forming ability of pectins.

Structural characterisation of thermoreversible anionic polysaccharide gels by their elastoviscous properties

Abstract Previously reported results obtained for the elastoviscous properties of some thermoreversible gels formed from anionic polysaccharides (high methoxyl pectin, furcellaran and κ-carrageenan) and also gelatin and maltodextrin are discussed and some conclusions about the structure of the gels are presented. The rate at which the relaxation processes take place in the gel is independent of the polymer concentration suggesting that the gels are structurally inhomogeneous. If the helical conformation of the individual macromolecule is stable the standard enthalpy change on crosslink breakdown is less than 45 kJ mol −1 . A relatively small decrease in standard enthalpy is sufficient for network stability because of the low standard entropy loss on gelation which is typical of semi-rigid chain polymers. If, however, the helical conformation is unstable the gelation process is cooperative and the standard enthalpy change on crosslink breakdown exceeds 200 kJ mol −1 .

Phase equilibria in water-protein-polysaccharide systems: II. Water-casein-neutral polysaccharide systems

SummaryPhase separation and thermodynamic stability of water (W) — casein (C) — neutral polysaccharide (NPS) systems have been studied. Dextrans of different molecular weights D40, D150, D500 and D2000 (10−3Mw=40, 150, 500, 2000), ficoll (10−3Mw=400) and amylopectin (10-−6Mw= 38) have been used as neutral polysaccharides. Phase diagrams of W-C-NPS systems, as well as their effect on the thermodynamic stability of the molecular weight and structural features of polysaccharides, low-molecular salts, pH and temperature have been considered. There has been shown the similarity of conditions under which the stability of W-C-NPS systems is disturbed, resulting in their separation, as well as conditions favorable to self-association of casein. A decrease in the pH value, an increase in the ionic strength and a rise in temperature are favorable both to self-association of casein and separation of W-C-NPS systems. Proceeding from this fact and from the results obtained earlier for W-C-acidic polysaccharide and W-albumin-D-glucan systems, a conclusion is drawn as to the common nature of the relationship between self-association of polymers and their compatibility. At constant temperature and pH, the stability limit of W-C-NPS systems is determined by the concentrations of polymers (w2 andw3) and salt (C4). The totality of cloud points C4* (pH) at givenw2 andw3 being considered as the stability limit. It is also shown that the relationship C4* (pH) is an increasing function within the range of pH values from 6.5 to 11.5 and in all cases where lim C4* (pH)=0. The sequence of values 947-1PH→PH IEP C4* is determined by the nature of the low-molecular salt (Na2SO4 ≊ NaCl ≪ KSCN) and the specific nature of the polysaccharides (D2000≊ amylopectin < D 150 < ficoll < D 40).W-C-NPS-NaCl have been found to possess a lower critical point in a system of coordinates: temperature-composition. W-C-NPS-urea (6M) systems feature, in a system of coordinates: pH-polymer concentration, two areas of separation, overlapping at sufficiently high total concentrations. This fact is indicative of the specific nature of interaction of casein macromolecules capable of association near the isoelectric point due to interaction of charge fluctuations and, in the acid region, due to interaction of non-ionized carboxyl groups. The obtained results are discussed with the stability limit being expressed in terms of second virial coefficients.

The factors affecting the compatibility of serum albumin and pectinate in aqueous medium

Abstract Compatibility is observed in aqueous solutions of serum albumin and pectin (degree of esterification 57%) at pH levels above the isoelectric point of protein. Both the variation in the pH values from 5 to 8 and the increase of ionic strength from 0·1 to 1·0 do not result in phase separation. These facts enable us to conclude that the affinity in this system is of a nonelectrostatic nature. The interaction of serum albumin and pectinate fractions with different degrees of esterification was studied by light scattering. The negative sign of A 24 (the second virial coefficient component of the mixture associated with the interaction between different polymer molecules) means that for any degree of esterification there is affinity between pectin and serum albumin. Information concerning excess thermodynamic functions was obtained from the temperature dependence of light scattering. Mixing microcalorimetry was used for precise measurement of enthalpy. The experimental results indicate that thermodynamic compatibility of serum albumin and pectin is controlled by increase of mixing entropy, which mainly stems from dehydration of biopolymer macromolecules during contact formation.

Studies of the gel formation of κ-carrageenan above the coil-helix transition temperature range

Abstract The temperatures of gel formation and coil-helix conformational transition of κ-carrageenan have been compared. For a 1·5% solution of the potassium salt of κ-carrageenan the gelation temperature is higher than the coil-helix transition temperature range when the KCl concentration exceeds 3−4 × 10 −2 mole litre −1 . The gelation temperature, T g , and the midpoint temperature of the coil-helix conformational transition, T 1 2 c−h , are linearly increasing functions of the logarithm of the concentration of potassium ions, and δT g δ( lg [ K + ]) > δ(T c− h 1 2 δ( lg [ K + ]) Creep compliance studies on a 1·5% gel of κ-carrageenan in 0·11 m KCl have shown that the standard breakdown enthalpy of gel network crosslinks at temperatures above the coil-helix transition range is about 12 kJ mole −1 . Upon helix formation this value increases 20 times. The relaxation behaviour of the κ-carrageenan gel in the high-temperature region is typical of thermoreversible gels. The network of such gels is generally made up of associates of macromolecules.