A structural approach to understanding the interactions between colour, water-holding capacity and tenderness
Introduction
Considerable variations in colour, tenderness and water-holding occur in pork, lamb and beef, as well as in other meat species. These quality traits are important as they determine visual appeal, and sensory acceptability. These traits are also important for economic reasons as the industry loses money due to undesirable colour (Hughes, Kearney, & Warner, 2014), and due to weight loss of product. The industry can also achieve higher prices for assured tenderness and eating quality (Lyford et al., 2010).
The ultimate pH of meat and the rate of pH fall are influenced by biochemical events pre- and post-mortem which have their action on structural components in muscle cells and their associated connective tissue. In particular, the influence of the rate and extent of pH fall on protein denaturation, the myofibrillar lattice spacing and the shrinkage in muscle cells is fundamental in determining quality of raw and cooked meat. Genetics, on-farm nutrition, pre-slaughter stress and post-slaughter processing are associated with variations in quality traits (Warner et al., 2011, Warner et al., 2010), but the exploration of the fundamental mechanism by which these have their effects is often lacking. We propose that the influence of pH on muscle structure is a fundamental mechanism behind many gene associations with quality.
Post-rigor muscle is about 75% water, with up to 87% of the volume of the muscle cell being myofibrils, which contain the majority of the water. Muscle structure is comprised of soluble (sarcoplasmic, including myoglobin and calpain) and insoluble proteins (myofibrillar, cytoskeletal and connective tissue) and also water. We consider water to be a part of the structure, which not only allows diffusion and interaction of substrate and enzyme, but also determines plasticity, rigidity and gelatinisation of the insoluble proteins. Thus water-holding capacity (WHC) is not only important for visual and sensory acceptability and for economic reasons, but also because of its role in moulding muscle structure and the consequent effects on quality.
Many studies have been conducted on raw meat, such as the changes in WHC, proteases, structure, protein denaturation and degradation, oxidation, etc. There is often a poor correlation between the WHC of raw meat, the water lost during cooking and the juiciness of the final product. As the person determining the acceptability of the meat product is a consumer of a cooked muscle, the changes occurring during cooking are particularly important and are considered in this review.
The variations in WHC, tenderness and colour have been explained from a structural basis (Offer et al., 1989, Tornberg, 1996, Tornberg, 2005), from a biochemical understanding (Ferguson and Gerrard, 2014, Huff-Lonergan and Lonergan, 2005, Huff Lonergan et al., 2010, Kim et al., 2014) and have also been modelled (Dransfield, 1994, Kondjoyan et al., 2014, Offer, 1991, Tornberg, 2013, van der Sman, 2007, van der Sman, 2013). Modelling has identified the areas where data is insufficient, particularly in explaining the complex variations between animals and muscles (Kondjoyan et al., 2014). Gene expression studies (Brunner et al., 2012, Damon et al., 2012, Damon et al., 2013, Ponsuksili et al., 2008, Ponsuksili et al., 2009, Ponsuksili et al., 2013) and proteomics (Di Luca, Elia, Hamill, & Mullen, 2013) demonstrate ever more complex pathways and signalling networks associated with changes in WHC and tenderness. However, the end-effect of gene expression, cellular pathways and biochemical events is to impact on some key structures within the muscle that influence the physical properties of water-holding and tenderness, and to some extent colour. This review will build on previous reviews and offer new insights into these structural determinants of meat quality traits.
Section snippets
Structural determinants of water-holding capacity and water loss during cooking
Water-holding capacity (WHC) is defined as the ability of fresh meat to retain its own water (Pearce, Rosenvold, Andersen, & Hopkins, 2011) during cutting, heating, grinding and pressing and during transport, storage and cooking (Hamm, 1986). The water released can be described as drip, purge, weep, exudate or cook loss (Warner, in press). Poor WHC results in high drip and purge loss, which can represent significant loss of weight from carcasses and cuts and may affect the yield and quality of
Structure, water-holding capacity and cooked meat toughness
Studies show that WHC and tenderness vary from muscle to muscle in the same animal and between different animals of the same species, but variations in these traits are not necessarily correlated. Perry, Thompson, Hwang, Butchers, and E.A. (2001) could not show any significant relationship (P > 0.05) between drip loss in raw meat and cooked meat tenderness, in two experiments on the longissimus muscle from 111 cattle.
However, there are some factors that clearly affect both the moisture content of
Consumer perception of meat colour
When light comes in contact with the surface of meat, it can either be reflected, absorbed or scattered. It is a combination of these three attributes which determine the colour of the meat, but of primary importance is the light that is reflected back to the eye which determines consumer perception and hence acceptability. Thus, the light reflecting from meat to the eye is often compared with quantitative assessment as measured numerically using a spectrophotometer or colorimeter and
Summary and recommendations
The dominant structural features that influence water loss from raw meat are myofibrillar lattice spacing, expulsion of fluid to the extracellular space and development of drip channels. Although this has not been reviewed in detail here, gene expression and proteomic studies have suggested a substantial list of candidate genes, cellular metabolic process and signalling pathways that appear to differ with variations in meat quality, especially WHC (Brunner et al., 2012, Damon et al., 2012,
Conclusions
The high water content of muscle is important to the structural arrangement of the lattice and acts as a plasticiser of proteins. Consequentially, loss of water from the structure influences the properties of the meat, with the majority of research focusing on the raw product. Water loss from the muscle is impacted by a variety of the structural elements of the muscle, the main components being myofibrillar lattice spacing, membrane permeability, extracellular space and drip channel formation.
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