Contribution of collagen and connective tissue to cooked meat toughness; some paradigms reviewed

Variations in the quantity and thermal stability of collagen in intramuscular connective tissue (IMCT) play a role in variations in cooked meat tenderness. This review is focussed on sources of variability, especially in the perimysial IMCT, and challenges some of the accepted ideas about its denaturation behaviour, its contribution to cooking shrinkage at high temperatures and the concept of IMCT as an immutable "background toughness". IMCT dominates the shear strength of raw and lightly cooked muscle, but at cooking temperatures of 70-80° its contribution is smaller than the myofibrillar component. The thermal denaturation temperature of IMCT collagen given by differential scanning calorimetry at fast heating rates is usually in the range of 62-67 °C, but collagen denaturation is a multistep, non-equilibrium process that is highly heating-rate dependent and can occur at 55-60 °C in slow heating regimes, such as slow roasting or sous-vide cooking. Although it is commonly assumed that collagen shrinkage drives shrinkage of meat and greater cooking losses above 65 °C, an examination of currently available information suggested that there is no evidence to support this idea. The amount and thermal stability of IMCT collagen can be varied by factors manipulating growth rate, and a recent model of these effects is discussed. The characteristics of the thermally-stable fraction of IMCT collagen as the most important component influencing the connective tissue contribution to cooked meat toughness and the need for future work investigating mechanisms to degrade or turnover this specific fraction is suggested.