Meat denaturation

Most chemical reactions, speed up with increases in temperature. However, high temperatures (above about 60°C) destroy, or denature, proteins by breaking up their 3-D structure. For example, the protein in an egg or a piece of meat denatures when the egg or meat is cooked. 

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. 

Denaturation of hemoproteins in cooked meats leads to liberation of the heme and oxidation of the porphyrin ring. Nonheme iron is less available nutritionally than heme iron and affects lipid oxidation more. In methemoglo-bin and metmyoglobin solutions heated for one hour at 78°C and 100°C the degradation of heme was about 22 to 26%, while after two hours at 120°C it increased to about 85 to 95% (Oellingrath, 1988). In meat cookery, however, such severe conditions do not apply. 

WOF is a problem associated with the use of precooked meat products such as roasts and steaks. The term WOF was first used by Tims and Watts (2) to describe the rapid development of oxidized flavors in refiigerated cooked meats. Published evidence indicates that the predominant oxidation catalyst is iron from ntyoglobin and hemoglobin, which becomes available following heat denaturation of the protein moiety of these complexes. The oxidation of the lipids results in the formation of low molecular weight components such as aldehydes, adds, ketones and hydrocarbons which may contribute to undesirable flavor. 

Killday etal. (1988) also provided evidence for internal autoreduction of ferric nitrosyl heme complexes, as previously proposed by Giddings (1977). Heating of chlorohemin( iron-III) dimethyl ester in dimethyl sulfoxide solution with imidazole and NO produced a product with an infrared spectra identical to that of nitrosyl iron(ll) protoporphyrin dimethyl ester prepared by dithionite reduction. Both spectra clearly showed the characteristic nitrosyl stretch at 1663 and 1665 cm. They thus proposed a mechanism for formation of cured meat pigment which includes internal autoreduction of NOMMb via globin imidazole residues. A second mole of nitrite is proposed to bind to the heat-denatured protein, possibly at a charged histidine residue generated in the previous autoreduction step. 

Applications of the Karl Fischer method are numerous food stuffs (butter, margarine, powdered milk, sugar, cheese, processed meats, etc.), solvents, paper, gas, petroleum, etc. Before the determination can be made, solid components that are not soluble must either be ground into powders, extracted with anhydrous solvents, eliminated as azeotropes or heated to eliminate water. Problems are encountered with very acidic or basic media that denature reactants and transform ketones and aldehydes into acetals that interfere with the titration. Special reagents must be used in these instances. 

Condensed phosphates containing three- atoms of phosphorus are tripolyphosphates, the most important of which is sodium tripolyphosphate (.STPP). This compound reacts with the protein in meat, fish, and poultry to prevent denaturing or loss of fluids. This properly is sometimes called "moisture binding. STPP also solubilizes protein, which aids in binding diced cured meat, fish, and poultry. It also emulsifies fat to prevent separation. 

The pink cured meat pigment mononitrosylhemochrome is a complex of nitric oxide (NO), ferrous heme iron, and heat-denatured globin protein (Table F3.2.1). The pink nitrosylheme (NO-heme) moiety may be extracted from the protein in aqueous acetone and quantitated by A540 (see Basic Protocol 1). The percent nitrosylation may be determined from measurement of ppm NO-heme relative to ppm total acid hematin (hemin) extracted in acidified acetone (see Basic Protocol 2), since NO-heme is completely oxidized to hemin in acid solution (i.e., 1 ppm NO-heme = 1 ppm hemin). 

Heating of meat during processing or cooking results in both chemical and physical changes that can be noticed as protein denaturation and solubility loss. Davis and Anderson (98) used SE-HPLC (Spherogel TSK-3000 SW) to evaluate heat-induced changes in the molecular-... 

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