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 conditionsin 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 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