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. 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 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 oxymyo- globin (MbO 2 ), 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 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 con- ditions the pigment can decompose and green choleglobin and colourless bile pigments are formed. 72 Meat refrigeration MbO 2 Mb MetMb Oxymyoglobin Myoglobin Metmyoglobin Fe 2+ Fe 2+ Fe 3+ 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- 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- ary. 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 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 temperatures. 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 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 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 condi- tioning 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 ‘light- ness’, a ‘hue’ change towards red and an increase in ‘saturation’. 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 1 h exposure to 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 quality 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 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. in the region of low oxygen tension is no longer converted back to myo- globin. The band of metmyoglobin below the surface is established sooner 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 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 N 2 or CO 2 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 O 2 . Beef muscles with low colour stability discoloured under all conditions. Colour changes in chilling, freezing and storage of meat 75 4.2.5 Freezing The colour of frozen meat varies with the rate of freezing. Taylor (1930, 1931) reported that as the speed of freezing diminished, the appearance of the product changes and at very low rates there is a marked development of translucence. Later experiments have demonstrated a direct relationship between freezing rate and muscle lightness; the faster the rate the lighter the product. Guenther and Henrickson (1962) found that 2.5 cm thick steaks frozen at -9 °C were dark. Those frozen at -34 to -40 °C had the most desirable colour and those frozen at -73 to -87 °C tended to be pale. Jakobsson and Bengtsson (1969, 1973) obtained similar results; very rapid freezing in liquid nitrogen spray at a freezing rate of about 13cmh -1 pro- duced meat which was unnaturally pale. Air blast freezing at 2 cmh -1 gave the best frozen appearance while very slow freezing at 0.04 cmh -1 resulted in a darker colour and the formation of ice on the product surface. Zaritzky et al. (1983) reported that the surface of liver frozen at high rates was lighter in colour. These differences in frozen meat lightness result from the depen- dence of ice crystal growth on the freezing rate. Small crystals formed by fast freezing scatter more light than large crystals formed by slow freezing and hence fast frozen meat is opaque and pale and slow frozen meat is translucent and dark. Meat frozen at -55 °C or lower need not be pale (Tuma, 1971). If expo- sure time to liquid nitrogen is based on thickness of the product its appear- ance at an equilibrated temperature of -18 °C can be assured (Bernholdt, 1971). Studies have been carried out using freezing times, from -1 to -7°C, of 1–37 minutes which are believed to represent the range of times found at the surface in normal blast freezing operations (Lanari et al., 1989). No effect of freezing rate on colour within this range was found. These results were similar to those of Jul (1984) who stated that the colour of beef was not affected by variations in freezing rate between 0.2 cm h -1 (still air freez- ing) and 5 cmh -1 (quick frozen). In addition to the rate of freezing, the duration of exposure to air prior to freezing is important. Lentz (1971) found that blast freezing at -30°C produced a frozen product similar in appearance to fresh beef if exposed to air for 30min before packaging and freezing. Freezing before or after the development of optimum ‘bloom’ affected the appearance of the frozen material adversely. Tuma (1971) also bloomed meat for 30 min before pack- aging, but suggested that 5–10 min might be adequate. 4.2.6 Frozen storage ‘Freezer burn’ is the main appearance problem that traditionally affected the appearance of meat in frozen storage. Desiccation from the surface tissues produces a dry, spongy layer that is unattractive and does not re- cover after thawing. This is commonly called ‘freezer-burn’. It occurs in 76 Meat refrigeration unwrapped or poorly wrapped meat. The problem is accentuated in areas exposed to low humidity air at high velocities, and by poor temperature control. Since most meat is now wrapped and temperature control much improved this is less of a problem than it once was commercially. Provided problems of freezer burn can be eliminated, the major appearance prob- lem that affects frozen meat arises from oxidation of oxymyoglobin to metmyoglobin. Both temperature and illumination level affect the rate of discolouration during frozen storage, but light is by far the more serious factor. Townsend and Bratzler (1958) found that frozen beef steaks stored under fluorescent light (60 dekalux) discoloured in 2–3 days at -18 °C. Lane and Bratzler (1962) found that metmyoglobin formation in frozen extracts of meat was similar to the pattern seen in frozen steaks and was increased by exposure to fluorescent illumination.Tuma (1971) showed that the acceptable storage life of M. longissimus dorsi beef steaks was 3–4 weeks at -26°C under fluo- rescent illumination (110 dekalux) although colour changes could be seen within 7 days. M. psoas major steaks were unacceptable under these con- ditions within 7 days. Lentz (1971) reported the progress of discolouration in the light (160–220 dekalux) and in the dark for frozen beef stored at a range of tem- peratures in terms of the Munsell colour notation. The prefreezing colour of most samples in the study was 7.5R 4/8. Based on the notation, this means that the hue was 7.5 Red, which is an intermediate step between Red and Yellow-Red. The lightness was 4 on a scale where 10 is white and 0 is black, and the chroma or saturation was 8 on a scale which ranges from 0 (neutral or grey) to about 12 (intense or strong). During frozen storage the chroma decreased and the hue became more yellow, producing a brown appear- ance. A chroma of 6 at a hue of 7.5–10R is no longer an attractive meat colour but is greyish red to reddish brown. The time from freezing to reach a chroma of 6 is shown in Table 4.3 where the effect of temperature and Colour changes in chilling, freezing and storage of meat 77 Table 4.3 Effect of storage temperature and light on the colour stability of frozen beef Temperature (°C) Time after freezing to reach Munsell Chroma 6 (days) No light 160–220 dekalux -731 -18 60–90 3 -29 60 7 -40 90 7 Munsell Chroma immediately after freezing was 8 to 10. Source: MacDougall, 1974 after Lentz, 1971. light on colour stability is clearly seen. At -18 °C, a temperature typical of good commercial display, the colour remained attractive for 3 months in the dark but only 3 days in the light. The relationship between frozen storage temperature and oxidation rate was studied by Zachariah and Satterlee (1973) for purified bovine, ovine and porcine myoglobins. When the rates were measured between -5°C and -27 °C, it was found that they were highest at -11 to -12 °C and lowest below -18 °C. The autoxidation of porcine myoglobin was faster than ovine or bovine myoglobin. Porcine myoglobin is precipitated by freezing which leads to the conclusion that the more rapid rate for this protein is due to a combination of autoxidation and precipitation. The results indicate that the red colour in frozen beef, pork and lamb can best be preserved if the tem- perature is less than -18 °C. Ledward and MacFarlane (1971) showed that metmyoglobin formation and lipid oxidation both depend upon the treat- ment meat receives prior to and during frozen storage. Meat frozen promptly was most stable while meat that had been subjected to cyclic thawing was least stable. Thus, it is desirable during prolonged aerobic frozen storage to avoid both delay in freezing and any subsequent thawing and refreezing of the surface. Lanari et al. (1994) have shown that dietary vitamin E supplementation improved pigment and lipid stability of frozen beef stored under illumina- tion and in the dark at -20 °C. These results complemented their earlier publication (Lanari et al., 1993) which showed that the colour of control samples of longissimus lumborum deteriorated in 1 day compared with 11 days for treated samples stored in the dark. Under an illumination of 1614 lux the treated samples deteriorated after 38 days. The advantages of using vitamin E supplementation in the extension of chilled and frozen storage life was reviewed by Liu et al. (1995). Vacuum packaging of frozen beef increases colour stability maintaining metmyoglobin levels lower than those found in just frozen samples wrapped in polyethylene (Lanari et al., 1989). The storage life of precooked frozen meat, for example sliced roast beef and pork, can be extended if the slices are covered with gravy prior to freez- ing (Jul, 1969). The gravy acts as a barrier to oxygen and protects against surface changes and oxidation. 4.2.7 Thawing Although the freezing rate has a marked effect on the colour of the frozen product it does not affect the lightness of the meat when thawed, with the exception of meat which has been very slowly frozen. Jakobsson and Bengtsson (1969, 1973) found that slowly frozen beef, which also darkened on freezing, showed considerable loss of redness after thawing. In contrast, meat frozen in liquid nitrogen and then defrosted was a light bright red. Little difference was also found between thawed beef steaks which were 78 Meat refrigeration frozen at 15 cmh -1 in liquid nitrogen spray and those which were blast frozen at 4 cmh -1 (Pap, 1972). In both cases, they were frozen to the same final temperature of -30 °C. However, the bloomed meat before freezing was redder than the same material after thawing. In thawed meat, the rate of pigment oxidation is increased (Cutting, 1970) and therefore the colour will be less stable than when fresh. On pro- longed frozen storage, a dark brown layer of metmyoglobin may form 1–2 mm beneath the surface so that on thawing the surface colour will rapidly deteriorate. Meat which has lost its attractiveness during frozen storage because of oxidation of oxymyoglobin on the surface will remain brown after thawing. Unwrapped meat thawed in high humidity air, water or in steam under vacuum appears very white and milky after thawing. However, if then stored in a chill room for 10–24 h it will be almost indistinguishable from fresh meat. Unwrapped meat thawed in air at high temperatures and low humidities will take on a dark, dry, tired appearance. It will not recover its appearance during chilled storage and will often require extensive trimming before sale. 4.2.8 Retail display 4.2.8.1 Chilled During refrigerated display, oxymyoglobin oxidises to brownish green metmyoglobin (MacDougall, 1993). Twenty per cent dilution of surface oxymyoglobin with metmyoglobin causes the product to be rejected at retail because of its faded colour (Hood and Riordan, 1973). The colour stability of fresh meat is influenced to a very marked degree by the temperature of display. Landrock and Wallace (1955) showed that meat held at 2 °C in packaging films whose oxygen permeability is greater than 5000mlm -2 atm -1 day -1 will remain attractive for 4 days.In practice,com- mercial display cabinets are not controlled to maintain their contents at near freezing temperatures; meat temperatures during display at times may exceed 10 °C (Malton, 1971, 1972). Heiss and Eichner (1969) showed that the rate of discolouration is roughly doubled for a 5°C rise in temperature, while Buck and Peters (1970) demonstrated that the rate of colour deterioration is dependent upon the temperature of the meat, which in turn is dependent upon location within the showcase.Similarly,MacDougall and Malton (1972) also found that the rate of colour change is influenced by position in the showcase and temperature differences of the order of 5 °C have a large effect on the rate of colour change. For example, a change in redness, which takes 72–168h at 0°C, will occur in 24–48h at 5°C. The discolouration rate is also different for different muscles. MacDougall and Malton (1972) found, for example, that the fillet dis- coloured faster than the loin. Similar observations were made by Colour changes in chilling, freezing and storage of meat 79 Hood (1971) who showed that the rate of metmyoglobin formation on the surface of these muscles was different. The increase in the rate of dis- colouration with an increase in temperature is explained by the lowering of the solubility of oxygen in the meat (Urbin and Wilson, 1958) and the faster formation of metmyoglobin (George and Stratmann, 1952) nearer the surface. Changes in appearance are normally the criteria which limit display of unwrapped products, rather than microbiological considerations. Deterio- ration in the appearance of unwrapped meats has been related to the degree of dehydration (Table 4.4) which makes the product unattractive to consumers. 4.2.8.2 Frozen The major problem in retail marketing of frozen meat is its appearance.The freezing process causes changes in the structure and colour of the muscle, and the deterioration in appearance during frozen storage and display ulti- mately leads to rejection of the product by the consumer. Storage tempera- ture, light intensity on the display area and method of packaging all affect the rate of deterioration. The appearance of fresh meat is a primary factor in acceptability at retail level and the same criteria of attractiveness will apply to frozen meat, retailed either frozen or after thawing. The poor colour of the frozen product and the drip associated with it when it thaws, have in the past both contributed to consumer resistance. The problem of light-catalysed pigment oxidation remains the largest single problem in the display of frozen meat. It can be overcome by opaque packaging in cartons, but the trade and consumer have to develop a very high level of mutual trust for it to be accepted. Where it has been tried in the past (Trieb, 1971) sales of meat packed in cartons were less than those in transparent film. Frozen imported carcass meat has been an item of commerce in the United Kingdom for almost a century and its retail marketing is an estab- lished part of the meat trade. Consumer satisfaction is evident by the 80 Meat refrigeration Table 4.4 Relationship between evaporative weight loss and appearance of sliced beef topside after 6 h display Evaporative loss Change in appearance (g cm -2 ) up to 0.01 Red, attractive and still wet; may lose some brightness 0.015–0.025 Surface becoming drier, still attractive but darker 0.025–0.035 Distinct obvious darkening, becoming dry and leathery 0.05 Dry, blackening 0.05–0.10 Black Source: Swain and James, 1986. demand and acceptance of New Zealand lamb although the colour of the product in the frozen state is different from that when fresh. Frozen beef similarly differs from fresh and is often extremely unattractive when dis- played for sale in the frozen state. However, if it is allowed to partly thaw and bloom during display its attractiveness improves. Undoubtedly the price differential between frozen and fresh meat is an important factor in the acceptance of the frozen product by the consumer. The appearance of frozen meat is markedly improved if retail sized por- tions are first packed in film to exclude air between the meat surface and the film and then rapidly frozen. With this product, however, the price dif- ferential between fresh and frozen would necessarily be small and the con- sumer would have to be persuaded by the trade that such frozen meat was in no way inferior to fresh. 4.3 Conclusions 1 Consumers prefer bright-red fresh meats, brown or grey-coloured cooked meats and pink cured meats. 2 Myoglobin is the primary meat pigment and exists as bright-red oxymyoglobin (MbO 2 ), purple-red deoxymyoglobin (Mb), or brown metmyoglobin (MetMb). 3 Red colour (oxymyoglobin) is more stable at lower temperatures because the rate of oxidation of the pigment decreases. At low tempera- tures, the solubility of oxygen is greater and oxygen-consuming reac- tions are slowed down. There is a greater penetration of oxygen into the meat and the meat is redder than at high temperatures. 4 Conditioned meat is a brighter and more attractive red than uncondi- tioned meat but its colour stability becomes progressively poorer the longer it is conditioned. 5 Commercial refrigerated display temperatures require close control if maximum shelf-life is to be obtained. Meat display temperatures of -1 °C would be ideal, but the higher temperatures commonly found limit shelf-life to 2 days or less. 6 Very fast freezing rates can potentially affect the colour of meat. However, the range of rates available commercially are unlikely to have a significant effect on colour. 7 Provided problems of freezer burn can be eliminated, the major appear- ance problem that affects frozen meat arises from oxidation of oxymyo- globin to metmyoglobin. 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