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4 Browning Reactions 69reported that antioxidative activity of CPs from different sugars
(fructose, glucose, ribose, and xylose) increased with increas-
ing pH levels (7.0–12.0), heating temperature (80–180◦C), and
heating time (0–180 minutes). The CPs from fructose exhib-
ited the highest antioxidant activity as evidenced by the greatest
scavenging effect and reducing power. In addition, this increase
in antioxidative activity was coincidental with the browning de-
velopment and intermediate formation. This suggested that both
colorless intermediates, such as reductones and dehydroreduc-
tones, produced in the earlier stages of the caramelization, (Rhee
and Kim 1975) and HMW and colored pigments, produced in the
advanced stages, might play an important role in the antioxidant
activity of CPs (Kirigaya et al. 1968)
In addition, the effect of caramelized sugars on enzymatic
browning has been studied by several authors. Thus, Pitotti et al.
(1995) reported that the anti-browning effect of some CPs is in
part related to their reducing power. Lee and Lee (1997) ob-
tained CPs by heating a sucrose solution at 200◦C under various
conditions to study the inhibitory activity of these products on
enzymatic browning. The reducing power of CPs and their in-
hibitory effect on enzymatic browning increased with prolonged
heating and with increased amounts of CPs. Caramelization was
investigated in solutions of fructose, glucose, and sucrose heated
at temperatures up to 200◦C for 15–180 minutes. Browning in-
tensity increased with heating time and temperature. The effect
of the caramelized products on PPO was evaluated, and the great-
est PPO inhibitory effect was demonstrated by sucrose solution
heated to 200◦C for 60 minutes (Lee and Han 2001). More re-
cently, Billaud et al. (2003) found CPS from hexoses with mild
inhibitory effects on PPO, particularly after prolonged heating
at 90◦C.
Antioxidative activity of CPs from higher molecular weight
carbohydrates has also been reported. Thus, Mesa et al. (2008), in
a study on antioxidant properties of hydrolyzates of soy protein-
fructooligosaccharide glycation systems, found that the most
neoantioxidants products are able to prevent LDL oxidation
and to scavenge peroxyl-alkyl radicals derived from the ther-
mal degradation (95◦C for 1 hour) of fructooligosaccharides
rather than from the Maillard reaction.Ascorbic Acid BrowningAscorbic acid (vitamin C) plays an important role in human
nutrition as well as in food processing (Chauhan et al. 1998).
Its key effect as an inhibitor of enzymatic browning has been
previously discussed in this chapter.
Browning of ascorbic acid can be briefly defined as the thermal
decomposition of ascorbic acid under both aerobic and anaerobic
conditions, by oxidative or non-oxidative mechanisms, either in
the presence or absence of amino compounds (Wedzicha 1984).
Nonenzymatic browning is one of the main reasons for the
loss of commercial value in citrus products (Manso et al. 2001).
These damages, degradation of ascorbic acid followed by brown-
ing, also concern noncitrus foods such as asparagus, broccoli,
cauliflower, peas, potatoes, spinach, apples, green beans, apri-
cots, melons, strawberries, corn, and dehydrated fruits (Belitz
and Grosch 1997).Pathway of Ascorbic Acid BrowningThe exact route of ascorbic acid degradation is highly vari-
able and dependent upon the particular system. Factors that can
influence the nature of the degradation mechanism include tem-
perature, salt and sugar concentration, pH, oxygen, enzymes,
metal catalysts, amino acids, oxidants or reductants, initial con-
centration of ascorbic acid, and the ratio of ascorbic acid to
dehydroascorbic acid (DHAA; Fennema 1976).
Figure 4.10 shows a simplified scheme of ascorbic acid degra-
dation. When oxygen is present in the system, ascorbic acid is
degraded primarily to DHAA. DHAA is not stable and spon-
taneously converts to 2,3-diketo-l-gulonic acid (Lee and Nagy
1996). Under anaerobic conditions, ascorbic acid undergoes the
generation of diketogulonic acid via its keto tautomer, followed
byβelimination at C-4 from this compound and decarboxyla-
tion to give rise to 3-deoxypentosone, which is further degraded
to furfural. Under aerobic conditions, xylosone is produced by
simple decarboxylation of diketogulonic acid and that is later
converted to reductones. In the presence of amino acids, ascorbic
acid, DHAA, and their oxidation products furfural, reductones,
and 3-deoxypentosone may contribute to the browning of foods
by means of a Maillard-type reaction (Fennema 1976, Belitz and
Grosch 1997). Formation of Maillard-type products has been de-
tected both in model systems and foods containing ascorbic acid
(Kacem et al. 1987, Ziderman et al. 1989, Loschner et al. 1990,
1991, M ̈olnar-Perl and Friedman 1990, Yin and Brunk 1991,
Davies and Wedzicha 1992, 1994, Pischetsrieder et al. 1995,
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
the spontaneous oxidation. Anaerobic degradation, which occurs
slower 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 ox-
idation of ascorbic acid is sometimes catalyzed by enzymes.
Ascorbic acid oxidase is a copper-containing enzyme that cat-
alyzes oxidation of vitamin C. The reaction is catalyzed by
copper ions. The enzymatic oxidation of ascorbic acid is impor-
tant in the citrus industry. Reaction takes place mainly during
extraction of juices. Therefore, it becomes important to inhibit
the ascorbic oxidase by holding juices for only short times and
at low temperatures during the blending stage, by de-aerating
the juice to remove oxygen, and finally by pasteurizing the juice
to inactivate the oxidizing enzymes.
Enzymatic oxidation has also been proposed as a mechanism
for the destruction of ascorbic acid in orange peels during the
preparation of marmalade. Boiling the grated peel in water sub-
stantially reduces the loss of ascorbic acid (Fennema 1976).
Tyrosinase (PPO) may also possess ascorbic oxidase activity.
A possible role of the ascorbic acid-PPO system in the browning
of pears has been proposed (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 commercial juice, filled into TetraBrik car-
tons, it has been demonstrated that nonenzymatic browning was
mainly due to carbonyl compounds formed froml-ascorbic acid