310 Produce Degradation: Reaction Pathways and their Prevention
higher susceptibility to the enzymatic browning reaction due to the higher concen-
tration of substrates and enzymes; therefore, these tissues must be completely
removed during peeling. The mass losses after peeling or other types of technological
processing also depend on the type of plant tissue. Agar et al.^ (1999) observed higher
mass losses in peeled and sliced kiwi fruits than in unpeeled, sliced fruits, and these
changes did not correspond with the effect of wounding on ethylene and CO 2
production rates in tissues. The wounding stress expressed as the rate of ethylene
and CO 2 was much higher in the unpeeled, sliced kiwi fruits.
10.2.3 PREVENTION OF ENZYMATIC REACTIONS
10.2.3.1 Application of Additives
Browning reactions occur principally as a result of cell damage during peeling,
cutting, slicing, and other treatments that produce new surfaces from one or more
layers of injured cells. The prevention of undesirable enzymatic and chemical
changes consists usually of surface treatments. This may involve rinsing out enzymes
and substrates released from the damaged cells, as well as reducing access to
exogenous substrates such as oxygen. Various chemical additives that are capable
of diffusion through the damaged cells may also be used. Surface treatments are the
most commonly used during processing of fresh produce. These are applied by
dipping, spraying, or rinsing with browning inhibitors, firming agents, growing
inhibitors or stimulators, disinfectants, or antimicrobial agents. The groups of anti-
browning additives are summarized in Table 10.6 and a comparison of the inhibitory
effects of selected additives is shown in Figure 10.1. Dipping times normally range
from 1 to 5 min (Soliva and Martin-Belloso, 2003).^ Increased temperature of a
dipping solution can accelerate the diffusion process. Lowering the pH value of the
dipping solution is recommended to minimize growth of microorganisms, but in some
cases the pH value may need to be modified to a value that is close to neutral pH. For
example, antibrowning treatments that involve cystein require relatively higher pH
values when used to minimize the risk of formation of pinkish-red compounds in foods
(Sapers and Miller, 1998). Drying of wet surfaces to avoid microbial decay should
follow a dipping treatment. Some of the postdip drying methods involve draining, gentle
spinning, or drying with cheesecloth^ (Bett et al., 2001; Gorny et al., 1999).
Intensity of browning is influenced by the activity of enzymes and by the amount
of phenolic substrate. In climacteric fruits, partial ripeness corresponds to a lower
tendency for browning. Sapers and Miller^ (1998) found minimal color changes in
slightly underripe Anjou and Barlett fresh-cut pears preserved with a combination
antibrowning treatment and MAP. The enzymatic browning caused by polyphenolox-
idase depends also on the access to oxygen and can be reduced by MAP, vacuum
packaging, and/or waxing. In some fruits and vegetables, enzymatic color changes
are also caused by peroxidase enzymes (POD). Peroxidase reduces hydrogen per-
oxides in fruit or vegetable tissues and oxidizes any hydrogen donor. The reaction
does not necessarily require oxygen (Robinson, 1991).
POD color-induced changes are probably marginal relative to PPO browning
reactions. The POD browning reactions usually proceed within the complex of