4 Browning Reactions 871995, 1997; Rogacheva et al. 1995; Koseki et al.
2001).
The presence of metals, especially Cu^2 and
Fe^3 , causes great losses of Vitamin C. Catalyzed
oxidation goes faster than spontaneous oxidation.
Anaerobic degradation, which occurs more slowly
than uncatalyzed oxidation, is maximum at pH 4 and
minimum at pH 2 (Belitz and Grosch 1997).
Ascorbic acid oxidation is nonenzymatic in
nature, but oxidation of ascorbic acid is sometimes
catalyzed by enzymes. Ascorbic acid oxidase is a
copper-containing enzyme that catalyzes oxidation
of vitamin C. The reaction is catalyzed by copper
ions. The enzymatic oxidation of ascorbic acid is
important in the citrus industry. The reaction takes
place mainly during extraction of juices. Therefore,
it becomes important to inhibit ascorbic oxidase by
holding juices for only short times and at low tem-
peratures during the blending stage, by deaerating
the juice to remove oxygen, and finally by pasteuriz-
ing the juice to inactivate the oxidizing enzymes.
Enzymatic oxidation also has been proposed as a
mechanism for the destruction of ascorbic acid in
orange peels during preparation of marmalade. Boil-
ing the grated peel in water substantially reduces the
loss of ascorbic acid (Fennema 1976).
Tyrosinase (PPO) may also possess ascorbic oxi-
dase activity. A possible role of the ascorbic acid–
PPO system in the browning of pears has been pro-
posed (Espin et al. 2000).
In citrus juices, nonenzymatic browning is from
reactions of sugars, amino acids, and ascorbic acid
(Manso et al. 2001). In freshly produced commer-
cial juice, filled into Tetra Brik cartons, it has been
demonstrated that nonenzymatic browning was
mainly due to carbonyl compounds formed from L-
ascorbic acid degradation. Contribution from sugar-
amine reactions is negligible, as is evident from the
constant total sugar content of degraded samples.
The presence of amino acids and possibly other
amino compounds enhance browning (Roig et al.
1999).
Both oxidative and nonoxidative degradation
pathways are operative during storage of citrus
juices. Since large quantities of DHAA are present
in citrus juices, it can be speculated that the oxida-
tive pathway must be dominant (Lee and Nagy
1996, Rojas and Gerschenson 1997a). A significant
relationship between DHAA and browning of citrus
juice has been found (Kurata et al. 1973; Sawamura
et al. 1991, 1994). The rate of nonoxidative loss
of ascorbic acid is often one-tenth or up to one-
thousandth the rate of loss under aerobic conditions
(Lee and Nagy 1996). In aseptically packed orange
juice, the aerobic reaction dominates first and is fair-
ly rapid, while the anaerobic reaction dominates lat-
er and is quite slow (Nagy and Smoot 1977,
Tannenbaum 1976). A good prediction of ascorbic
acid degradation and the evolution of the browning
index of orange juice stored under anaerobic condi-
tions at 20–45°C may be performed employing the
Weibull model (Manso et al. 2001).
Furfural, which is formed during anaerobic degra-
dation of ascorbic acid, has a significant relationship
to browning (Lee and Nagy 1988); its formation has
been suggested as an adequate index for predicting
storage temperature abuse in orange juice concen-
trates and as a quality deterioration indicator in
single-strength juice (Lee and Nagy 1996). How-
ever, furfural is a very reactive aldehyde that forms
and decomposes simultaneously; therefore, it would
be difficult to use as an index for predicting quality
changes in citrus products (Fennema 1976). In gen-
eral, ascorbic acid would be a better early indicator
of quality.Control of Ascorbic Acid BrowningSulfites (Wedzicha and Mcweeny 1974, Wedzicha
and Imeson 1977), thiol compounds (Naim et al.
1997), maltilol (Koseki et al. 2001), sugars, and sor-
bitol (Rojas and Gerschenson 1997b) may be effec-
tive in suppressing ascorbic acid browning. Doses to
apply these compounds greatly depend on factors
such as concentration of inhibitors and tempera-
tures. L-cysteine and sodium sulfite may suppress or
accelerate ascorbic acid browning as a function of
their concentration (Sawamura et al. 2000). Glu-
cose, sucrose, and sorbitol protect L-ascorbic acid
from destruction at low temperatures (23, 33, and
45°C), while at higher temperatures (70, 80, and
90°C) compounds with active carbonyls promoted
ascorbic acid destruction. Sodium bisulfite was only
significant in producing inhibition at lower tempera-
ture ranges (23, 33, and 45°C) (Rojas and Ger-
schenson 1997).
Although the stability of ascorbic acid generally
increases as the temperature of the food is lowered,
certain investigations have indicated that there may
be an accelerated loss on freezing or frozen storage.