Muscle denaturation

Sugar alcohols have also found appHcation in foods containing sugars. Sorbitol is an effective cryoprotectant in surimi, preventing denaturation of the muscle protein during fro2en storage. 

Meat products have to be stabilised in some cases, as meat lipids contain no natural antioxidants or only traces of tocopherols. Most muscle foods contain, however, an efficient multi-component antioxidant defence system based on enzymes, but the balance changes adversely on storage. The denaturation of muscle proteins is the main cause of the inbalance as iron may be released from its complexes, catalysing the lipid oxidation. Salting contributes to the negative effects of storage, as it enhances oxidation. Using encapsulated salt eliminates the deleterious effect of sodium chloride. 

By 1950, Hogeboom and Schneider had prepared relatively pure mitochondria (Chapter 9) which could be used as the oxidizing system rather than crude particulate preparations like the heart muscle system of Keilin and Hartree. The problem still remained for the extraction and identification of the components from the (inner) mitochondrial membrane without denaturation. 

Traditionally pork bundle is homemade. In this study, muscle of Rectus femoris was sliced into 20 x 10 x 10 cm3 cubes and boiled in water (100°C) for 1.5 hours to denature the muscle protein. When... 

The exact form in which non-crosslinked elastin is secreted from smooth muscle cells is yet to be clearly defined. Foster et al. (36) have suggested that a non-cross linked elastin (pro-elastin) is secreted from smooth muscle cells in a form that is approximately 120,000 to 140,000 daltons. They have suggested that proelastin is cleaved to smaller molecular weight forms of non-crosslinked elastin. It should be noted, however, that this view is not entirely supported by data from other laboratories. There are two reports on the use of isolated mRNA from chick aorta suggesting only a 70,000 dalton non-cross linked elastin is the major product of translation (37,38). There is also a recent report suggesting that aortic mRMA translates a 200,000 dalton putative elastin product (39). We have recently isolated a non-crosslinked elastin from the aortas of copper deficient chicks that appears to be 100,000 daltons (27). Its amino acid composition is similar to that for tropoelastin (Table III). A major problem in resolving these points is that the trypsin-like proteinase associated with elastin is not easily denatured or separated from the non-crosslinked forms of elastin. The proteinase is also not readily inhibited by commonly used inhibitors for trypsin-like proteinases (26). 

Most of the studies indicate that denaturation of muscle proteins plays the dominant role in the quality changes of the frozen stored meats. The muscle proteins of fish and other aquatic animals have been found to be much less stable than those of beef animals, pigs and poultry (1 ). The present paper will be limited primarily to fish muscle as one representative of vertebrate muscle and it will also deal primarily with the behavior of fish proteins at sub-zero temperatures. In order to do a thorough analysis within the space limit permitted, focus will be on the changes of the proteins per se leaving peripheral problems to other reviews (2-18). 

Actomyosin. Frequently, the change in amount of soluble actomyo-sin is regarded as the primary criterion of freeze denaturation. It must be remembered that solubility data do not tell precisely how much protein is denatured and how much is native rather, it provides a relative measure of denaturation. Solubility decreases have been found in frozen storage experiments with either intact muscle, protein solutions or with suspensions of isolated actomyosin. 

On the other hand, King (69) and Anderson and coworkers (70,71), based on detailed analyses of ultracentrifugal patterns of extracts of frozen stored cod muscle and experiments on the effect of lipids on protein denaturation, have proposed that denaturation of F-actomyosin occurs by two parallel pathways which lead to insolubilization (Figure 2). As indicated by Connell (61), the occurrence of G-actomyosin at an intermediary stage needs experimental verification. Possibility of an alternate pathway involving lipids will be discussed later. 

Jarenback and Liljemark (75,76) found similar changes in cod actomyosin solution and cod muscle during frozen storage. The denatured myosin was not extracted with salt solution. 

Connell has proposed that insolubilization of actomyosin during frozen storage of cod muscle is attributable to the denaturation of myosin rather than actin (89). During 40 weeks storage at -14°C, extractability of actomyosin and myosin decreased in parallel, while that of actin appeared to remain constant. The decrease in extractability of myosin was biphasic, while that of actomyosin followed an exponential curve. 

Some enzymes and enzyme systems are still active at the temperature of frozen storage (123-132). Such enzymatic activity, especially of proteases, may cause loss of biological activity of actomyosin and other muscle proteins. Products of such enzymatic activity, e.g. free fatty acids and formaldehyde, may effect a secondary denaturation of muscle proteins. 

Cause of denaturation. Many hypotheses have been proposed to explain the denaturation of muscle proteins (9-17). These hypotheses include 1) the effects of inorganic salts concentrated into the liquid phase of the frozen system 2) water-activity relations 3) reactions with lipids 4) reaction with formaldehyde derived from trimethyl amine (in fish) 5) auto-oxidation ... 

Among the above hypotheses, effects of lipids (4-17,59-62, 69-71,155-159), formaldehyde (160-166), and gas-solid interface TMJ appear to be very important in Gadoid fishes. Denaturation of myofibrillar proteins caused by free fatty acids and/or lipid peroxides must occur during frozen storage. To prove this, Jarenback and Liljemark have shown by electron microscopy that, in muscle stored frozen with added linoleic and linolenic hydroperoxides, myosin became resistant to extraction with salt solution (168). 

The concentrated salt solution may denature the proteins (9-17, 169-177). Whereas experiments with isolated muscle protein preparations cannot exclude the effects of salts such as NaCl or KC1 (since they are required to solubilize the proteins), denaturation during frozen storage has been decreased or prevented completely when an efficient cryoprotectant such as sodium glutamate or glucose was added (66,67,82,93,145-150). Hence, the effect of salts may not be of primary importance, though they may contribute. 

The water-activity relations, effects of displacements of water or effects of changes in the state of water must be the most important factors to trigger and to promote the denaturation of muscle proteins during frozen storage. 

As described by Fennema (9J, several refined hypotheses such as "physical barrier and structured water hypothesis" (134,178, 179), "ice-moderator hypothesis" (180-183), and "minimum cell volume hypothesis" (184) have been proposed. However, the author will take a more naive approach in interpreting the results on denaturation of muscle proteins during frozen storage at the same time taking advantage of the basic ideas of the above hypotheses. 

---