1  Chemical and Biochemical Aspects of Color in Muscle Foods

José Ángel Pérez-Alvarez and Juana Fernández-López

Abstract: Color is the main aspect that defines a food’s quality and that most influences consumer choice. Sensorial quality, especially color and appearance of meat, can be affected by both internal and external factors.

    Chromoproteins, carotenes, and carotenoproteins are important in meat color. Carotenes are ­responsible for the color of beef fat, poultry meat and skin, fish, and shellfish. Of the hemoglobins present postmortem in the muscle, myoglobin is the one mainly responsible for color. Cytochromes are metalloproteins with a heme group with a role in meat coloration. The principal role of fat is in the brightness of meat products.

    The color of meat may be altered by several factors, including exposure to light, microbial growth, rancidity, and exposure to oxygen.

Keywords: Muscle-based food color, carotenes, hemoproteins, myoglobin, cytochromes, fat color, melanosis, discoloration, premature browning, shelf life

1.1 GENERAL ASPECTS OF MUSCLE-BASED FOOD COLOR

The first impression that a consumer receives concerning a food product is established visually, and among the properties observed are color, form, and surface characteristics. Color is the main aspect that defines a food’s quality, and a product may be rejected simply because of its color, even before other properties, such as aroma, texture, and taste, can be evaluated. This is why the appearance (optical properties, physical form, and presentation) of muscle-based products at the sales point is of such importance (Lanari et al. 2002).

Regarding the specific characteristics that contribute to the physical appearance of meat, color is the quality that most influences consumer choice (Krammer 1994). The relationship between meat color and quality has been the subject of study since the 1950s, indeed, since Urbain (1952) described how consumers had learned through experience that the color of fresh meat is bright red, and any deviation from this color (nonuniform or anomalous coloring) is unacceptable (Diestre 1992). The color of fresh meat and associated adipose tissue is, then, of great importance for its commercial acceptability, especially in the cases of beef and lamb (Cornforth 1994), and in certain countries, for example, the USA and Canada, there have been many studies to identify the factors controlling its stability. Adams and Huffman (1972) affirmed that consumers relate the color of meat to its freshness. In poultry, the consumers of many countries also associate meat color with the way in which the animal was raised (­intensive or extensive) and fed (cereals, animal feed, etc.).

Color as a quality factor for meat can be appreciated in different ways in different countries; for example, in Denmark, pork meat color holds fifth place among qualities that affect consumers’ purchase decisions (Bryhni et al. 2002). The sensorial quality, especially color and appearance (Brewer & Mckeith 1999), of meat can be affected by both internal and external factors. In the case of internal factors, in fish, for example, a particular problem that has been encountered in rearing some Pargus species is the darkening of the body after the capture of wild fish and during farming. During farming and marketing, the skin color (­silver-red) turns dark gray (especially the tail and fins) (Kentouri et al. 1995; Lin et al. 1998). In the case of farmed salmon, too, feeding fish with carotenoid pigments is regarded as the most important management practice for marketing (Moe 1990), because without them, flesh and skin color would be less visually attractive, and therefore would be less valued as a food (Baker 2002).

Food technologists, especially those concerned with the meat industry, have a special interest in the color of food for several reasons—first, because of the need to maintain a uniform color throughout processing; second, to prevent any external or internal agent from acting on the product during its processing, storage, and display; third, to improve or optimize a product’s color and appearance; and, lastly, to attempt to bring the product’s color into line with what the consumer expects. Put simply, the color of meat is determined by the pigments present in it. These can be classified into four types: (1) biological pigments (carotenes and hemopigments), which are accumulated or synthesized in the organism antemortem (Lanari et al. 2002); (2) pigments produced as a result of damage during manipulation or inadequate processing conditions; (3) pigments produced postmortem (through enzymatic or nonenzymatic reactions) (Montero et al. 2001; Klomklao et al. 2006); and (4) pigments resulting from the addition of natural or artificial colorants (Fernández-López et al. 2002).

As a quality parameter, color has been widely studied in fresh meat (MacDougall 1982; Cassens et al. 1995; Faustman et al. 1996) and cooked products (Anderson et al. 1990; Fernández-Ginés et al. 2003; Fernández-López et al. 2003). However, dry-cured meat products have received less attention (Pérez-Alvarez 1996; Pagán-Moreno et al. 1998; Aleson et al. 2003) because in this type of product, color formation takes place during different processing stages (Pérez-Alvarez et al. 1997; Fernández-López et al. 2000); recently, a new heme pigment has been identified in this type of product (Parolari et al. 2003; Wakamatsu et al. 2004a, b). From a practical point of view, color plays a fundamental role in the animal production sector, especially in meat production (beef and poultry, basically) (Zhou et al. 1993; Esteve 1994; Verdoes et al. 1999; Irie 2001), since in many countries of the European Union (e.g., Spain and Holland) paleness receives a wholesale premium.

For fish, skin and flesh discoloration is a very important problem, especially in highly appreciated species. Since the skin and flesh color must be very vivid, many efforts have been directed at improving color, mainly through dietary control (carotene-enriched diets) (Fujita et al. 1983; Mori 1993). Without these pigments, the aquaculture industry would find it hard to undertake the production of some species because fish demand is driven through consumer demand for quality products (Baker 2002). In fish, consumer preference is often influenced by body pigmentation. Fish flesh color is an important quality parameter for most farmed fish, especially with salmonids (salmon, rainbow trout), (Francis 1995; Hyun et al. 1999), in which the pink or red color of fillets is an important feature (Sigurgisladottir et al. 1994; Sigurgisladottir et al. 1997). For example, a uniform red color in rainbow trout is considered to indicate a high-quality product and is a reason for its acceptability, while for the tuna fish industry, it is very important to avoid discoloration in fresh and processed meat and to increase its shelf life (Goodrick et al. 1991; Tze et al. 2001). Fish nutrition has an important impact on several parameters that directly influence the quality of fish, some of which are color and appearance. The color of salmonid flesh is one of the most important quality parameters, because consumers have a preference for red- or pink-colored products in the case of salmonids. This is the reason for using carotenoids in aquaculture.

1.2 CHEMICAL AND BIOCHEMICAL ASPECTS OFMUSCLE-BASEDFOOD COLOR

Of the major components of meat, proteins are the most important since they are only provided by essential amino acids, which are very important for the organism’s correct functioning; proteins also make a technological contribution during processing, and some are responsible for such important attributes as color. These are the so-called chromoproteins, and they are mainly composed of a porphyrinic group conjugated with a transition metal, principally iron metalloporphyrin, which forms conjugation complexes (heme groups) (Whitaker 1972) that are responsible for color. However, carotenes and carotenoproteins (organic compounds with isoprenoid-type conjugated systems) exist alongside chromoproteins and also play an important part in meat color. There are also some enzymatic systems whose coenzymes or prosthetic groups possess chromophoric properties (peroxidases, cytochromes, and flavins) (Faust-man et al. 1996). However, their contribution to meat color is slight. Below, the principal characteristics of the major compounds that impart color to meat are described.

1.3 CAROTENES

Carotenes are responsible for the color of beef fat, poultry meat and skin, fish, and shellfish; in the last two cases, these are of great economic importance. The color of the fat is also important in carcass grading. Furthermore, carotenoids can be used as muscle-based food coloring agents (Verdoes et al. 1999). An important factor to be taken into account with these ­compounds is that they are not synthesized by the live animal but are obtained by assimilation (­Pérez-Alvarez et al. 2000), for instance, in the diet. Salmonids, for example, obtain carotenes in the wild in their preys, but in intensive fish culture, carotenoids must be added to the diet. Farmed fish, especially colored fish (e.g., salmon and rainbow trout), are now a major industry. For example, Norway exports a great part...