Colour changes in chilling, freezing and storage of meat The appearance of meat at its point of sale is the most important quality attribute governing its purchase. The ratio of fat to lean and the amount of marbled fat are important appearance factors and another is the colour of the meat. The changes in colour of the muscle and blood pigments (myoglobin and haemoglobin, respectively) determine the attractiveness of fresh red meat, which in turn influences the consumers acceptance of meat products(Pearson, 1994). Consumers prefer bright-red fresh meats, brown or grey-coloured cooked meats and pink cured meats(Cornforth, 1994) Reviews of the affect of chilling and freezing on the colour of meat were carried out by MacDougall in 1972 and 1974, respectively. The principle factors governing colour changes from those reviews have been included ir 4.1 Meat colour Objects appear coloured when some wavelengths of light are selectively absorbed. meat looks red because it absorbs all other colours other tha red, which is reflected. When meat is examined in reflected light its colour will depend on (1) the nature of the illuminating light, and(2)changes taking place during reflection Light sources contain a varying spectrum of intensities and wavelengths and meat viewed in tungsten light, for example, will appear redder because of the abundance of red light produced by the source. The physical structure of the meat and the chemical changes to the pigment govern the changes taking place during reflection
4 Colour changes in chilling, freezing and storage of meat The appearance of meat at its point of sale is the most important quality attribute governing its purchase. The ratio of fat to lean and the amount of marbled fat are important appearance factors and another is the colour of the meat. The changes in colour of the muscle and blood pigments (myoglobin and haemoglobin, respectively) determine the attractiveness of fresh red meat, which in turn influences the consumers acceptance of meat products (Pearson, 1994). Consumers prefer bright-red fresh meats, brown or grey-coloured cooked meats and pink cured meats (Cornforth, 1994). Reviews of the affect of chilling and freezing on the colour of meat were carried out by MacDougall in 1972 and 1974, respectively. The principle factors governing colour changes from those reviews have been included in this chapter. 4.1 Meat colour Objects appear coloured when some wavelengths of light are selectively absorbed. Meat looks red because it absorbs all other colours other than red, which is reflected. When meat is examined in reflected light its colour will depend on (1) the nature of the illuminating light, and (2) changes taking place during reflection. Light sources contain a varying spectrum of intensities and wavelengths and meat viewed in tungsten light, for example, will appear redder because of the abundance of red light produced by the source. The physical structure of the meat and the chemical changes to the pigment govern the changes taking place during reflection
72 Meat refrigeration Table 4.1 Main form of pigment found in uncured meat Colour Reduced myoglobin Bright red Metmyoglobin Denatured globin haemachrome Brown Source: MacDougall, 1972. Instrumental measurements of meat colour are usually expressed in terms of"lightness, hue'and'saturation' 'Hue'is the psychological appre- ciation of colour describing purple to red to orange to yellow, and so on andsaturation' is the lack of greyness or increase in purity(MacDougall, 1972). The principles of colour measurement for food are described by MacDougall (1993) and instrumental measurement of meat colour is reviewed by Warriss(1996) Myoglobin is the primary meat pigment and exists as bright-red oxymyo- globin(MbO2), purple-red deoxymyoglobin(Mb), or brown metmyoglobin (MetMb). Haemoglobin which is responsible for the colour of blood plays only a small role in the colour of red meat, although it may be more sig- nificant in paler meat(Bendall, 1974). For example, in back bacon, about 40% of the colour intensity is attributable to haemoglobin and 60% to myo- globin. In most beef muscles myoglobin is by far the more dominant pigment, whereas in the calf 20-40% of the total is haemoglobin. The main forms of the pigments found in uncured meat are given in Table 4.1 The purple colour of freshly cut meat is due to the deoxymyoglobin. On xposure to air, it is converted to the bright red pigment oxymyoglobin, which gives fresh meat its normal desirable appearance. The brown colour of cooked meat is due to denatured globin hemichrome. In extreme con ditions the pigment can decompose and green choleglobin and colourl bile pigments are formed MbO2 Metmb Myoglobin Metmyoglobin oxygenated oxidised (bright-red) (purplish-red) The depth of the oxymyoglobin layer is controlled by several factors, the more important of which are the duration of exposure(Brooks, 1929), the emperature(Urbin and Wilson, 1958)and the oxygen tension( Brooks, 1929; Landrock and Wallace, 1955; Rikert et al., 1957).Other important factors are the diffusion of the oxygen through the tissue(Brooks, 1929), and its utilis
Instrumental measurements of meat colour are usually expressed in terms of ‘lightness’, ‘hue’ and ‘saturation’. ‘Hue’ is the psychological appreciation of colour describing purple to red to orange to yellow, and so on and ‘saturation’ is the lack of greyness or increase in purity (MacDougall, 1972). The principles of colour measurement for food are described by MacDougall (1993) and instrumental measurement of meat colour is reviewed by Warriss (1996). Myoglobin is the primary meat pigment and exists as bright-red oxymyoglobin (MbO2), purple-red deoxymyoglobin (Mb), or brown metmyoglobin (MetMb). Haemoglobin which is responsible for the colour of blood plays only a small role in the colour of red meat, although it may be more significant in paler meat (Bendall, 1974). For example, in back bacon, about 40% of the colour intensity is attributable to haemoglobin and 60% to myoglobin. In most beef muscles myoglobin is by far the more dominant pigment, whereas in the calf 20–40% of the total is haemoglobin. The main forms of the pigments found in uncured meat are given in Table 4.1. The purple colour of freshly cut meat is due to the deoxymyoglobin. On exposure to air, it is converted to the bright red pigment oxymyoglobin, which gives fresh meat its normal desirable appearance. The brown colour of cooked meat is due to denatured globin haemichrome. In extreme conditions the pigment can decompose and green choleglobin and colourless bile pigments are formed. 72 Meat refrigeration MbO2 Mb MetMb Oxymyoglobin Myoglobin Metmyoglobin Fe2+ Fe2+ Fe3+ oxygenated oxidised (bright-red) (purplish-red) (brown-red) Table 4.1 Main form of pigment found in uncured meat Pigment Colour Reduced myoglobin Purple Oxymyoglobin Bright red Metmyoglobin Brown Denatured globin haemichrome Brown Source: MacDougall, 1972. The depth of the oxymyoglobin layer is controlled by several factors, the more important of which are the duration of exposure (Brooks, 1929), the temperature (Urbin and Wilson,1958) and the oxygen tension (Brooks,1929; Landrock and Wallace, 1955; Rikert et al., 1957). Other important factors are the diffusion of the oxygen through the tissue (Brooks, 1929), and its utilisa-
Colour changes in chilling, freezing and storage of meat 73 tion in the tissue(Watts et aL., 1966). At low partial pressures of oxygen, oxidation to brown metmyoglobin occurs( George and Stratmann, 1952)and the desirable red colour is lost. Such conditions occur, for example, at the limit of oxygen penetration in meat at the oxymyoglobin myoglobin bound ry Metmyoglobin can be converted back to myoglobin(Stewart et aL., 1965; Watts et al., 1966) by the products of enzymic activity, if present(Saleh and Watts, 1968).The pigment status, therefore, depends on the balance between enzymic activity, oxygen tension and oxidation 4.2 Factors affecting the colour of meat 4.2.1 Live animal The pigment concentration in meat is affected by many factors affecting the live animal. These include: 1 Species- beef for example contains substantially more myoglobin than 2 Breed 3 Age-pigment concentration increases with age 4 Sex-meat from male animals usually contains more pigment than that from female animals 5 Muscle- muscles that do more work contain more myoglobin There are also two specific meat defects, dark, firm, dry(dFD)and pale, soft, exudative(PSE)associated with the live animal that result in poor meat colour DFD meat has a high ultimate pH and oxygen penetration is low. Consequently, the oxymyoglobin layer is thin, the purple myoglobin layer shows through and the meat appears dark In PSE meat the pH falls while the muscle is still warm and partial denaturation of the proteins occur. An increased amount of light is scattered and part of the pigment oxidised so that the meat appears pale 4.2.2 Chilling Red colour is more stable at lower temperatures because the rate of oxi- dation of the pigment decreases. At low temperatures, the solubility of oxygen is greater and oxygen-consuming reactions are slowed down. There is a greater penetration of oxygen into the meat and the meat is redder than at high temperature Changes in colour have been reported resulting from chilling treat ment. Taylor et al. (1995) found that electrical stimulation of pork pro. duced higher lightness(L), i.e. paler, values than those measured in non- stimulated sides. Spray chilling of pork has some effect on its colour during the initial chilling period(Feldhusen et al., 1995a). After 4h of chilling, the musculature of sprayed ham becomes lighter and red and yellow values
tion in the tissue (Watts et al., 1966). At low partial pressures of oxygen, oxidation to brown metmyoglobin occurs (George and Stratmann,1952) and the desirable red colour is lost. Such conditions occur, for example, at the limit of oxygen penetration in meat at the oxymyoglobin myoglobin boundary. Metmyoglobin can be converted back to myoglobin (Stewart et al., 1965; Watts et al., 1966) by the products of enzymic activity, if present (Saleh and Watts, 1968).The pigment status, therefore, depends on the balance between enzymic activity, oxygen tension and oxidation. 4.2 Factors affecting the colour of meat 4.2.1 Live animal The pigment concentration in meat is affected by many factors affecting the live animal. These include: 1 Species – beef for example contains substantially more myoglobin than pork. 2 Breed. 3 Age – pigment concentration increases with age. 4 Sex – meat from male animals usually contains more pigment than that from female animals. 5 Muscle – muscles that do more work contain more myoglobin. There are also two specific meat defects, dark, firm, dry (DFD) and pale, soft, exudative (PSE) associated with the live animal that result in poor meat colour. DFD meat has a high ultimate pH and oxygen penetration is low. Consequently, the oxymyoglobin layer is thin, the purple myoglobin layer shows through and the meat appears dark. In PSE meat the pH falls while the muscle is still warm and partial denaturation of the proteins occur. An increased amount of light is scattered and part of the pigment oxidised so that the meat appears pale. 4.2.2 Chilling Red colour is more stable at lower temperatures because the rate of oxidation of the pigment decreases. At low temperatures, the solubility of oxygen is greater and oxygen-consuming reactions are slowed down. There is a greater penetration of oxygen into the meat and the meat is redder than at high temperatures. Changes in colour have been reported resulting from chilling treatment. Taylor et al. (1995) found that electrical stimulation of pork produced higher lightness (L), i.e. paler, values than those measured in nonstimulated sides. Spray chilling of pork has some effect on its colour during the initial chilling period (Feldhusen et al., 1995a). After 4 h of chilling, the musculature of sprayed ham becomes lighter and red and yellow values Colour changes in chilling, freezing and storage of meat 73
74 Meat refrigeration decrease. However, after 20h there is no significant difference in the colour values. The surface of the skin becomes lighter after spray chilling 4.2.3 Conditioning Newly cut conditioned meat is known to show a brighter surface after a short exposure to air than unconditioned meat(Doty and Pierce, 1961 Tuma et aL., 1962: 1963). MacDougall (1972)studied the effects of condi- ioning on colour and on subsequent storage in packages of high oxygen permeability typical of those used for display and in vacuum packages of low oxygen permeability. The average colour values for the selection of muscles are given in Table 4.2 along with the statistical significance of the olour differences. Meat, when cut and exposed to air, changed from dull purple red to a bright cherry red, which is measured as an increase inlight ness, a hue change towards red and an increase insaturation. The mag nitude of the change on blooming for conditioning meat as compared with unaged was the same size for 'lightness but was two-fold greater for hue and three-fold greater for'saturation'. Conditioned meat, when freshly cut, was lighter but more purple than the unconditioned. After I h exposure air, conditioned meat had a redder hue which was considerably more satu rated and intense than the unconditioned samples. These changes in light ness, hue and saturation produced by conditioning result in a brighter, more attractive appearance. The overall colour improvement was of a similar magnitude to that which occurred on blooming Conditioned meat is superior to unconditioned because of its eating uality and bloomed colour. However, this improved colour is not main tained on subsequent packaging for retail display. Both the improvements in the colour of conditioned meat when freshly cut and the faster accumu- lation of metmyoglobin can be accounted for by the diminution of the meats enzymic activity which occurs during the conditioning process: First, a thick layer of oxymyoglobin forms in conditioned meat because of the lowering of the rate of oxygen consumption and oxygen therefore penetrates faster and further into the tissue. Second, metmyoglobin formed Table 4.2 Effect of conditioning for up to 22 days on meat colour when cut and after l h exposure to air at 2C Colour when fresl Lightness Hue Saturation 28.1 15.6 om Ioi 28.824.2173 Conditioned 30.3214 214 Source: Macdougall, 1972
decrease. However, after 20 h there is no significant difference in the colour values. The surface of the skin becomes lighter after spray chilling. 4.2.3 Conditioning Newly cut conditioned meat is known to show a brighter surface after a short exposure to air than unconditioned meat (Doty and Pierce, 1961; Tuma et al., 1962; 1963). MacDougall (1972) studied the effects of conditioning on colour and on subsequent storage in packages of high oxygen permeability typical of those used for display and in vacuum packages of low oxygen permeability. The average colour values for the selection of muscles are given in Table 4.2 along with the statistical significance of the colour differences. Meat, when cut and exposed to air, changed from dull purple red to a bright cherry red, which is measured as an increase in ‘lightness’, a ‘hue’ change towards red and an increase in ‘saturation’. The magnitude of the change on blooming for conditioning meat as compared with unaged was the same size for ‘lightness’ but was two-fold greater for ‘hue’ and three-fold greater for ‘saturation’. Conditioned meat, when freshly cut, was lighter but more purple than the unconditioned. After 1 h exposure to air, conditioned meat had a redder ‘hue’ which was considerably more saturated and intense than the unconditioned samples. These changes in lightness, hue and saturation produced by conditioning result in a brighter, more attractive appearance. The overall colour improvement was of a similar magnitude to that which occurred on blooming. Conditioned meat is superior to unconditioned because of its eating quality and bloomed colour. However, this improved colour is not maintained on subsequent packaging for retail display. Both the improvements in the colour of conditioned meat when freshly cut and the faster accumulation of metmyoglobin can be accounted for by the diminution of the meat’s enzymic activity which occurs during the conditioning process: First, a thick layer of oxymyoglobin forms in conditioned meat because of the lowering of the rate of oxygen consumption and oxygen therefore penetrates faster and further into the tissue. Second, metmyoglobin formed 74 Meat refrigeration Table 4.2 Effect of conditioning for up to 22 days on meat colour when cut and after 1 h exposure to air at 2 °C Colour when freshly cut Lightness Hue Saturation Unaged 28.1 22.9 15.6 Conditioned 29.7 18.0 16.0 Colour after bloom for 1 h Unaged 28.8 24.2 17.3 Conditioned 30.3 21.4 21.4 Source: MacDougall, 1972
Colour changes in chilling, freezing and storage of meat 75 in the region of low oxygen tension is no longer converted back to myo- globin. The band of metmyoglobin below the surface is established soone and any advantage in appearance conditioned meat could have over uncon- ditioned is soon lost as the brown band displaces and dilutes the surface oxymyoglobin layer The difference in the mechanism of brown discolouration in packages of high and low oxygen permeability, is that in the former metmyoglobin is formed several millimetres below the surface, while in the latter it appears on the surface Boakye and Mittal(1996) observed that the lightness of longissimus dorsi muscle increases with the length of conditioning. The change was greatest over the first 2 days and then became almost linear with time at a decreased rate. Similar effects were observed in total colour difference brightness difference and hue difference. Yellowness decreased between 2 and 4 days of conditioning and then increased 4.2.4 Chilled storage The muscle surface of fresh meat undergoes extensive oxygen penetration and oxygenation of myoglobin after short periods of exposure to air. The length of time meat is kept in chilled storage has an effect on the rate of colour change during retail display. Feldhusen et al (1995b)showed that there were clear colour changes after exposure in beef longissimus dorsi muscle stored for up to 5 days at 5'C. The degree of lightness(L),per centage of red(a)and percentage of yellow(b) all increased by 3-4 units The colour of meat stored for longer periods showed less intense colour anges during 5h of exposure Bacterial activity is another factor in pigment changes in fresh meat ( Faustman et al., 1990). The primary role of bacteria in meat discolouration the reduction of oxygen tension in the surface tissue(Walker, 1980) Initial oxygen concentrations in packaging over approximately 0.15% will seriously compromise the colour stability of both beef and lamb(Penney and Bell, 1993). Pork appears able to tolerate oxygen concentrations above 1% without obvious detrimental effect during short-term storage at chilled temperatures. Gill and McGinnis(1995)have shown clearly that control of both storage emperature and oxygen content are required to stop colour deterioration in controlled atmosphere storage of beef Samples were packaged in either N2 or COz containing oxygen at concentrations between 100 and 1000 ppm. The colour of samples of longissimus dorsi, which has a high colour stability, had deteriorated after 4h at either 5 or 1C. Samples stored it -15C with oxygen concentrations <400 ppm had not deteriorated after 48h. At 0C samples deteriorated after 24 h at >200ppm and 48h at 100 ppm O2 Beef muscles with low colour stability discoloured under all conditions
in the region of low oxygen tension is no longer converted back to myoglobin. The band of metmyoglobin below the surface is established sooner and any advantage in appearance conditioned meat could have over unconditioned is soon lost as the brown band displaces and dilutes the surface oxymyoglobin layer. The difference in the mechanism of brown discolouration in packages of high and low oxygen permeability, is that in the former metmyoglobin is formed several millimetres below the surface, while in the latter it appears on the surface. Boakye and Mittal (1996) observed that the lightness of longissimus dorsi muscle increases with the length of conditioning. The change was greatest over the first 2 days and then became almost linear with time at a decreased rate. Similar effects were observed in total colour difference, brightness difference and hue difference. Yellowness decreased between 2 and 4 days of conditioning and then increased. 4.2.4 Chilled storage The muscle surface of fresh meat undergoes extensive oxygen penetration and oxygenation of myoglobin after short periods of exposure to air. The length of time meat is kept in chilled storage has an effect on the rate of colour change during retail display. Feldhusen et al. (1995b) showed that there were clear colour changes after exposure in beef longissimus dorsi muscle stored for up to 5 days at 5 °C. The degree of lightness (L), percentage of red (a) and percentage of yellow (b) all increased by 3–4 units. The colour of meat stored for longer periods showed less intense colour changes during 5 h of exposure. Bacterial activity is another factor in pigment changes in fresh meat (Faustman et al., 1990). The primary role of bacteria in meat discolouration is the reduction of oxygen tension in the surface tissue (Walker, 1980). Initial oxygen concentrations in packaging over approximately 0.15% will seriously compromise the colour stability of both beef and lamb (Penney and Bell, 1993). Pork appears able to tolerate oxygen concentrations above 1% without obvious detrimental effect during short-term storage at chilled temperatures. Gill and McGinnis (1995) have shown clearly that control of both storage temperature and oxygen content are required to stop colour deterioration in controlled atmosphere storage of beef. Samples were packaged in either N2 or CO2 containing oxygen at concentrations between 100 and 1000 ppm. The colour of samples of longissimus dorsi, which has a high colour stability, had deteriorated after 4h at either 5 or 1°C. Samples stored at -1.5 °C with oxygen concentrations £400 ppm had not deteriorated after 48 h. At 0 °C samples deteriorated after 24 h at >200 ppm and 48 h at 100 ppm O2. Beef muscles with low colour stability discoloured under all conditions. Colour changes in chilling, freezing and storage of meat 75