Metals and food oxidation - Oxidation in Foods and Beverages and Antioxidant Applications - page 50

0.9mg/g and 17.6mg/g dry matter, respectively. The stability of the carotenoids in
paprika is very much affected by the water activity being the best at 0.64 a w . When
the a w in powdered paprika exceeded 0.32, carotenoid bleaching proceeded in a
sigmoidal three stages. After an initial bleaching, there was a prolonged period of
stability after which oxidation is resumed.
The intermediate period of stability was absent in the dry products. Results
obtained with the oleoresin-cellulose model showed that the three-stage
carotenoid bleaching at high a w value was characteristic of the combined
action of all the catalyzers only after addition of ascorbic acid. It is concluded
that ascorbic acid, becoming soluble only at high water activities, only then
plays its important antioxidant part in the stabilization of the carotenoids in
powdered paprika or in a model system. Under conditions of high a w , the pro-
oxidant effect of the peroxidase-like protein (hemeprotein) and the metal ions
are overcome by the marked antioxidant action of ascorbic acid present at very
high concentration in this product. In order to increase the stability of paprika
powder by activation of ascorbic acid it is very much recommended to keep it at
a w of 0.64 or ~10% moisture, at 4 ëC. Metal affected oxidation in many other
dehydrated food systems (Delgado-Vargas et al., 2000).
2.7 Metal catalyzed oxidation in muscle foods
Oxidative degradation is a major cause of quality deterioration in muscle foods,
following slaughtering, processing and storage (Kanner, 1994). A critical
question in the control of oxidation in muscle foods concerns the source of the
primordial active species or free radicals that initiate the oxidative deterioration.
When cells are injured, such as in muscle foods after slaughtering, oxidation and
peroxidation are favored and traces of O 2 ·ÿ , H 2 O 2 and hydroperoxides (LOOH)
are formed. The stability of a muscle product will depend on the `tone' of these
peroxides and especially from the involvement of metal ions in the process. The
cytosol contains not only pro-oxidants, but also antioxidants and the tone of both
affects the overall oxidation. Lipid peroxidation is one of the primary
mechanisms of quality deterioration in meat products. The change in quality is
mainly manifested by deterioration in color, flavor and nutritive value, including
production of cytotoxic compounds (Kanner, 1994; Gorelik et al., 2005).
2.7.1 Muscle food color
Oxymyoglobin is the main pigment in fresh muscle food, responsible for the
bright red color. The redox state of the heme-iron and the presence or nature of
the ligand bound to iron account for the meat color. Oxidation of heme-iron
from ferrous state in oxymyoglobin or deoxymyoglobin to the ferric state,
produces the brownish color of fresh meat, which consumers find undesirable
(Lin and Hultin, 1977; Faustman and Cassens, 1990; Kanner, 1994). Heme-
proteins such as oxymyoglobin and oxy-hemoglobin are particularly prone to