Figure 3 shows the surface formation of metmyoglobin.
Results demonstrated their negative association with red color (a* and H* values) of meat shown above.
Until d 8 of storage, all the patties presented a very similar behavior.
Metmyoglobin percentage in the control and the ascorbic acid-treated beef patties was above 80%
from d 12 of storage.
The effect of oregano was dependent on the concentration; 0.02 and 0.1% oregano delayed metmyoglobin formation (p < 0.01) by 4 and 8 d, respectively.
Borage meal exerted a highly significant effect (p < 0.01), delaying myoglobin oxidation by
12 d with independence of the concentration used.
Regarding rosemary,its antioxidant effect was similar to that of borage until d 20
of storage; however, in combination with vitamin C, it exerted a dramatic inhibition of metmyoglobin formation (p < 0.01), whose percentage was kept below 20% and 40% at d 16 and 20 of storage, respectively.
These results were in agreement with those found by Sánchez-Escalante and others (2001), who reported that rosemary combined with ascorbic acid was very useful for preventing myoglobin
oxidation in ground beef patties, and with those of Djenane and others (2001) who found that the same combination extended the shelf life of fresh beef steaks by about 10 d.
The synergistic effect of ascorbic acid when used in combination with other antioxidants was already proposed by Elliott (1999).
Several authors have reported visually detectable percentages of metmyoglobin or consumer rejection levels of metmyoglobin.
Van der Oord and Wesdorp (1971) reported detection at 50% metmyoglobin.
Harrison (1977) found that metmyoglobin was visually seen when it represented 30 to 40% of total heme pigment.
Greene and others (1971) indicated that consumer rejection occurred at
40% metmyoglobin.
According to them, even though the procedures used by those authors to determine metmyoglobin percentage were different from ours, the oxidized pigment would be detectable early after 8 d of storage in control patties, within the range 12 to 16 d in oregano-, borage-, and rosemary-treated patties, and at d 24 of storage in those treated with rosemary and vitamin C.