78 Meat refrigeration ight on colour stability is clearly seen At.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-5C and -27C, it was found that they were highest at -1l to-12C and lowest below.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 -18C 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 -20C. 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 ing(Jul, 1969). The gravy acts as a barrier to oxygen ano vy prior to freez- rotects 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 xception of meat which has been very slowly frozen Jakobsson and engtsson(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 werelight 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