74 Part I: Principles
(Marshall et al. 2000). Structures of common pheno-
lic compounds present in foods are shown in Fig-
ure 4.3. Phenolic subtrates of PPO in fruits, vegeta-
bles, and seafood are listed in Table 4.1. The sub-
strate specificity of PPO varies in accordance with
the source of the enzyme. Phenolic compounds and
polyphenol oxidase are, in general, directly respon-
sible for enzymatic browning reactions in damaged
fruits during postharvest handling and processing.
The relationship of the rate of browning to phenolic
content and PPO activity has been reported for vari-
ous fruits. In addition to serving as PPO substrates,
phenolic compounds act as inhibitors of PPOs
(Marshall et al. 2000).
CONTROL OFBROWNING
Enzymatic browning may cause a decrease in the
market value of food products originating from
plants and crustaceans (Perez-Gilabert and García-
Carmona 2000, Kubo et al. 2000, Subaric et al.
2001). Processing such as cutting, peeling, and bruis-
ing is enough to cause enzymatic browning. The rate
of enzymatic browning is governed by the active
polyphenol oxidase content of the tissues; the phe-
nolic content of the tissue and the pH, temperature,
and oxygen availability within the tissue. Conse-
quently, the methods addressed to inhibiting the
undesired browning are focused on either inhibiting
or preventing PPO activity in foods. The methods
are predicated on eliminating from the reaction one
or more essential components (oxygen, enzyme,
copper, or substrates). Numerous techniques have
been applied to preventing enzymatic browning.
Table 4.2 gives a list of procedures and inhibitors
that may be employed for controlling enzymatic
browning in foods. According to Marshall et al.
(2000), there are six categories of PPO inhibitors
applicable to control of enzymatic browning: reduc-
ing agents, acidulants, chelating agents, complexing
agents, enzyme inhibitors, and enzyme treatments.
The inhibition of enzymatic browning generally pro-
ceeds via direct inhibition of the PPO, nonenzymat-
ic reduction of o-quinones, and chemical modifica-
tion or removal of phenolic substrates of polyphenol
oxidase.Figure 4.3.Structure of common phenols present in foods Maillard reaction.