4 Browning Reactions 85down of oligo- and polysaccharides to nonvolatile
reaction products. Homoki-Farkas et al. (1997) stud-
ied, through an intermediate compound (methylgly-
oxal), the caramelization of glucose, dextrin 15, and
starch in aqueous solutions at 170°C under different
periods of time. The highest formation of methyl-
glyoxal was in glucose and the lowest in starch sys-
tems. The authors attributed the differences to the
number of reducing end groups. In the case of glu-
cose, when all molecules were degraded, the con-
centration of methylglyoxal reached a maximum
and began to transform, yielding low and high
molecular weight color compounds. Hollhagel and
Kroh (2000, 2002) investigated the degradation of
maltoligosaccharides at 100°C through-dicarbonyl
compounds such as 1,4-dideoxyhexosulose, and
they found that this compound is a reactive interme-
diate and precursor of various heterocyclic volatile
compounds that contribute to caramel flavor and
color.
Perhaps, as mentioned above, the most striking
feature of caramelization is its contribution to the
color and flavor of certain food products under con-
trolled conditions. In addition, it is necessary to con-
sider other positive characteristics of this reaction
such as the antioxidant activity of the caramelization
products. Kirigaya et al. (1968) suggested that high
molecular weight and colored pigments might play
an important role in the antioxidant activity of
caramelization products. However, Rhee and Kim
(1975) reported that caramelization products from
glucose have antioxidant activity that consists main-
ly of colorless intermediates, such as reductones and
dehydroreductones, produced in the earlier stages of
caramelization.
In addition, the effect of caramelized sugars on
enzymatic browning has been studied by several
authors. Pitotti et al. (1995) reported that the anti-
browning effect of some caramelization products is
in part related to their reducing power. Lee and Lee
(1997) obtained caramelization products 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 car-
amelization products and their inhibitory effect on
enzymatic browning increased with prolonged heat-
ing and with increased amounts of caramelization
products. Caramelization was investigated in solu-
tions of fructose, glucose, and sucrose heated at
temperatures up to 200°C for 15–180 minutes.
Browning intensity increased with heating time and
temperature. The effect of the caramelized products
on polyphenol oxidase (PPO) was evaluated, and the
greatest PPO inhibitory effect was demonstrated by
sucrose solution heated to 200°C for 60 minutes
(Lee and Han 2001). More recently, Billaud et al.
(2003) found caramelization products from hexoses
with mild inhibitory effects on PPO, particularly
after prolonged heating at 90°C.ASCORBICACIDBROWNINGAscorbic acid (vitamic 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 dis-
cussed 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 nonoxidative mechanisms, in either the presence
or absence of amino compounds (Wedzicha 1984).
Nonenzymatic browning is one of the main rea-
sons for the loss of commercial value in citrus prod-
ucts (Manso et al. 2001). These damages, degrada-
tion of ascorbic acid followed by browning, also
concern noncitrus foods such asparagus, broccoli,
cauliflower, peas, potatoes, spinach, apples, green
beans, apricots, melons, strawberries, corn, and
dehydrated fruits (Belitz and Grosch 1997).Pathway of Ascorbic Acid BrowningThe exact route of ascorbic acid degradation is high-
ly variable and dependent upon the particular sys-
tem. Factors that can influence the nature of the
degradation mechanism include temperature, salt
and sugar concentration, pH, oxygen, enzymes, met-
al catalysts, amino acids, oxidants or reductants, ini-
tial concentration 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 degradation. When oxygen is present in the sys-
tem, ascorbic acid is degraded primarily to DHAA.
Dehydroascorbic acid is not stable and spontan-
eously converts to 2,3-diketo-L-gulonic acid (Lee
and Nagy 1996). Under anaerobic conditions, DHAA
is not formed and undergoes the generation of dike-
togulonic acid via its keto tautomer, followed by