The structural and mechanical properties of gels formed from biopolymers are discussed both in terms of the techniques used to characterise these systems, and in terms of the systems themselves The techniques included are spectroscopic, chiroptical and scattering methods, optical and electron microscopy, thermodynamic and kinetic methods and rheological characterisation The systems considered are presented in order of increasing complexity of secondary, tertiary and quaternary structure, starting with gels which arise from essentially ‘disordered’ biopolymers via formation of ‘quasicrystalline’ junction zones (eg gelatin, carrageenans, agarose, alginates etc), and extending to networks derived from globular and rod-like species (fibrin, globular proteins, caseins, myosin) by a variety of crosslinking mechanisms Throughout the text, efforts are made to pursue the link (both from experiment and from theory) between the structural methods and mechanical measurements As far as we are aware this is the first major Review of this area since that of J D Ferry in 1948 — The interest shown by polymer physicists in more complex biochemical systems, and the multi-disciplinary approaches now being applied in this area, make the format adopted here, in our opinion, the most logical and appropriate

Structural and mechanical properties of biopolymer gels

Effects of heat on meat proteins – Implications on structure and quality of meat products

Publisher Summary The chapter discusses the protein stability with emphasis on compact globular proteins representing a single cooperative system. All the small compact globular proteins represent cooperative systems; they exhibit an extreme cooperativity that integrates the whole of their structure into a single structural unit. The large proteins, to which fibrillar proteins are also related, do not present single cooperative systems, but are subdivided into definite cooperative subsystems—structural blocks or domains. The advances in studying the stability of complicated proteins are connected with two methodical achievements: (1) the appearance of the precise scanning microcalorimetric technique, which affords reliable information on the heat capacity function of proteins in a broad temperature range; and (2) realization of the fact that the complicated heat effect of disruption of a complex macromolecular structure can be analyzed thermodynamically. The thermodynamic specificity of collagen has been considered. The volume of globular proteins does not increase at denaturation but decreases, as seen from their ability to denature under high pressure. The results of calorimetric studies are discussed, presenting the specific melting enthalpy of various protein structures—globular proteins, double-stranded coiled coils, and triplestranded coiled coils. The practical importance of thermodynamic studies of protein stability—that is, its importance not only for understanding the principles of organization of these molecules, but just for obtaining structural information on the domain level is emphasized.

Stability of proteins. Proteins which do not present a single cooperative system

Summary Protein gelation is important to obtain desirable sensory and textural structures in foods. Gelation phenomenon requires a driving force to unfold the native protein structure, followed by an aggregation retaining a certain degree of order in the matrix formed by association between protein strands. Protein gelation has been traditionally achieved by heating, but some physical and chemical processes form protein gels in an analogous way to heat-induction. A physical means, besides heat, is high pressure. Chemical means are acidification, enzymatic cross-linking, and use of salts and urea, causing modifications in protein–protein and protein–medium interactions. The characteristics of each gel are different and dependent upon factors like protein concentration, degree of denaturation caused by pH, temperature, ionic strength and/or pressure.

A review of physical and chemical protein-gel induction

Functional Properties of the Myofibrillar System and Their Measurements

Myosin molecules are cleaved by chymotrypsin digestion into two fragments: subfragment 1, which originates from the globular heads of myosin, and the myosin rod, which originates from the helical tail of the myosin molecule. The heat-induced gelation of these subfragments was compared to that of intact myosin by measuring rigidity, turbidity, and other physico-chemical characteristics of each system. Two features of the heat-induced gelation of myosin, aggregation and three-dimensional network formation were found to be imparted by the subfragment 1 and the rod, respectively. The former involves disulfide exchange and the latter relates to conformational changes arising from a partially irreversible helix-coil transition during heating. Possible relationships are suggested between these physicochemical changes of the myosin head and tail regions upon heating and the heat-induced gelation of myosin.

Relative Roles of the Head and Tail Portions of the Molecule in Heat‐Induced Gelation of Myosin

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Recent Advances in Meat Science in Japan : Functionality of Muscle Proteins in Gelation Mechanisms of Structured Meat Products

Myosin solutions and suspensions have been monitored during heating at pH 6.0 by using dynamic rheological measurements. The storage modulus (G′), the loss modulus (G) and the phase angle (δ) all showed a marked dependence on ionic strength in the temperature range 25–75°C. The filamentous gels (ionic strength <0.34) displayed a temporary reduction in G′ at temperatures between 50 and 60°C, presumably due to denaturation in parts of the rod portion of the myosin molecule. In the same temperature region the concentration dependence of G′ changed by a power of 2. The loss modulus also showed a marked concentration dependence, while the phase angle varied with concentration primarily at low (<50°C) temperatures. For the final gels, heated to 75°C, only G′ indicated marked differences due to different protein concentrations and ionic strengths; all gels were almost completely elastic (δ⋍1°). Adenosine triphosphate was shown to have a pronounced temporary effect on the filamentous gel formed at low temperatures, i.e. on the gel with the highest concentration dependence, while pyrophosphate had no such effect. However, both adenosine triphosphate (or rather its hydrolysis product: adenosine diphosphate) and pyrophosphate appeared to have a small, lasting effect on the heat-gelling ability of myosin: the former a detrimental effect, the latter an improvement.

Dynamic rheological measurements on heat‐induced myosin gels: Effect of ionic strength, protein concentration and addition of adenosine triphosphate or pyrophosphate

Gelation of myosin in 0.6M KC1 at pH 7.0 and 6.0 during heating at various temperatures between 25–70°C was quantitatively measured by a simple shear modulus tester devised in our laboratory. Shear modulus data indicated that gelation occurs with a stepwise elevation of temperature from 30–60°C. Heat-treatment of the protein solution remarkably influenced the structure and water mobility in the system. Myosin gel formed by heating had a micro-network structure, which was not-present in the original myosin solution. The myosin gel had a slower value of spin-spin relaxation time compared to that of the original untreated sample, suggesting that the water mobility in the gel system was somehow restricted. The heat-induced gelation of myosin may be the result of the development of a three dimensional network structure which holds water in a less mobilized state.

Changes in shear modulus, ultrastructure and spin-spin relaxation times of water associated with heat-induced gelation of myosin

The rabbit muscle contractile proteins, myosin, actin and reconstituted actomyosin were mixed in 0.1–1.0 M KCl, 20 mM buffers, pH 5.0–8.0, and were tested quantitatively for thermally induced gelation properties by measuring the rigidity (shear modulus) of the system at 20–70°. Scanning electronmicroscopy (SEM) was also used to study the structure of the gels formed by gelation of myosin in the presence of F-actin. Under the standard condition, i.e. at 0.6 M KCl, pH 6.0 and 65°, decrease of the myosin/actin mole ratio to about 1.5–2.0 in the reconstituted acto-myosin system resulted in substantial augmentation of the rigidity of the gel formed. Further decreases in the myosin ratio relative to F-actin reduced the rigidity value of the gel to close to the level of myosin alone. Gel-formability of the reconstituted actomyosin was maximal at pH 5.5–6.0 and between 0.5 and 0.8 M KCl and decreased considerably at other pH values and KCl concentrations.



The SEM studies revealed progressive changes in three dimensional ordering as actin concentration in the actomyosin varied. These were in concordance with the results of gel strength.

Kunihiko Samejima

Papers

Relative Roles of the Head and Tail Portions of the Molecule in Heat‐Induced Gelation of Myosin

Recent Advances in Meat Science in Japan : Functionality of Muscle Proteins in Gelation Mechanisms of Structured Meat Products

Dynamic rheological measurements on heat‐induced myosin gels: Effect of ionic strength, protein concentration and addition of adenosine triphosphate or pyrophosphate

Changes in shear modulus, ultrastructure and spin-spin relaxation times of water associated with heat-induced gelation of myosin

Heat‐induced gelation of myosin in the presence of actin