So, you want to coagulate thermally biological tissues? Here are some useful facts about it.

Here are some important aspects related to the protein composition of muscle tissues and their thermal coagulation. There are two structural forms of protein in meat: globular (i.e., myoglobin) and fibrous (i.e., actin, myosin, collagen). Globular proteins unfold and expand on heating, while fibrious proteins contract on heating. The average muscle consists of 75% water, 20% protein, 3% fat and 2% soluble non-protein substances. The proteins can be divided into three major groups: myofibrillar (50-55% of the total protein content), sarcoplasmic (30-34%) and connective tissue proteins (10-15%). The myofibrillar proteins include the myofilamentous fibrous proteins myosin and actin (building up the myofibrillar structure), the regulatory proteins (the tropomyosin-troponin complex, a- and b-actinin, M-protein and C-protein) and the scaffold proteins (titin, nebulin, desmin, vimentin and synemin) supporting the whole myofibrillar structure. The sarcoplasmic proteins are the soluble proteins of the sarcoplasma (which includes myoglobin). Connective tissue proteins build up structures that cover the whole muscle, individual muscle fibers and fiber bundles. The connective tissue proteins include collagen, reticulin and elastin which are fibrous proteins. The muscle fibres occupy 75–92% of the total muscle volume. They accommodate thread-like structures, the myofibrils, wherein the sarcomere, the smallest contractile unit, is lined up. The diameter of the myofibrils is about 1 mm and the length of the sarcomere is about 2.2 mm in a resting muscle. The sarcomere is built from two ‘building blocks’ that consist of a thick filament (myosin) and a thin filament (actin). Most of the water in the living muscle is held within the myofibrils (~80%), in the spaces between the thick and thin filaments.

Heating induces conformational changes in proteins called denaturation that triggers a number of changes in the structure of meat: destruction of cell membranes, mitochondrial disfunction, inhibition of DNA replication (due to enzymes denaturation), impairment of Golgi apparatus, protein-protein interaction leading to aggregation, shrinkage of meat fibers, coagulation and gel formation, shrinkage and solubilisation of the connective tissue. With increasing internal temperature, the denaturation of myosin and actin causes structural changes and expels the sarcoplasmic fluid from the muscle fibres, resulting in water losses from meat tissue. Myosin denaturation likely shrinks the myofilamental lattice spacing thereby reducing the waterholding capacity. Transverse shrinkage to the fibre axis occurs mainly at 40–60 deg.C while at 60–70 deg.C the connective tissue network and the muscle fibres cooperatively shrink longitudinally.

There are several typical zones in thermal coagulation in meat: the first one with a maximum between 54 and 58 deg.C is attributed to myosin, the second one between 65 and 67 deg.C is assigned to collagen and sarcoplasmic proteins, the third one between 80 and 83 deg.C is assigned to actin with denaturation of titin happening at ~75 deg.C. Gelatination of collagen usually occurs at 80 deg.C.

Static light scattering originates from a spatial variation of the optical refractive index. The refractive index depends on the concentration and type of tissue constituencies. A size of refractive index heterogeneities also plays an important role with structures that are between the size scale of nanometers and microns mostly responsible for optical scattering. Out of several basic tissue types (epithelium, connective tissue, blood, nervous tissue, muscle), connective tissue produces the highest scattering due to the presence of collagen fibers that have high mass density. Since hydration affects tissue density, scattering also changes with water content. During thermal denaturation, protein macromolecules lose their characteristic quaternary, tertiary and secondary molecular configurations and are reduced to granules of collapsed and ruptured primary chains. Accumulation of such highly scattering granules is responsible for the opaque white or light tan appearance of coagulated tissues.

It is well known that cooking in the presence of water causes the hydrolysis of collagen resulting in gel formation and hence, softer tissues. Dry-heat or broiling methods, on the other hand make collagen hard and impermeable since these methods are known to induce less hydrolysis.

However, the difference in the coagulation procedures may induce more subtle changes. The absence of a direct contact with water would arrest diffusion to/from water thus preventing ionic exchange that can affect ionic strength and pH of the tissue.

Ionic strength (as well as pH) affects electrostatic interaction between proteins (both myofibrillar and sarcoplasmic).  Light scattering from meat has been associated with protein denaturation and changes that may involve the packaging of the myofibers. Low pH decreases the lateral negative electrostatic repulsion between myofilaments so that they move closer together, shrinking myofibrillar diameters and releasing fluid. Ionic strength (mostly NaCl) and pH also determine if the myosin exists in monomeric form or as filaments. Alterations in pH and/or ionic strengths may cause conformational changes that allow an increase in the hydrophobicity and aggregation of the enzyme.

It was noted that shrinkage of meat fibres, the aggregation and gel formation of sarcoplasmic proteins and the shrinkage and the solubilisation of the connective tissue may proceed differently in different heating protocols.