136 Produce Degradation: Reaction Pathways and their Prevention
5.2.2.5.2.1 Enzymatic Action
Enzymatic peroxidation of unsaturated fatty acids (namely, linoleic and linolenic
acids) is the most dramatic example of the biochemical alteration of the natural
flavor of either fresh or fresh-cut vegetables. This peroxidation by oxygen is cata-
lyzed by lipoxidase and leads to the formation of volatile aldehydes and ketones
[118]. It has been demonstrated that the concentrations of n-hexanal, a byproduct
of hydroperoxide degradation, is well correlated with postharvest development of
off-flavor in peas [119]. Bruising of peas and snap beans has been shown to be an
important factor in the development of delayed off-flavor. Hand-shelled peas do not
deteriorate in flavor as rapidly as vine-shelled peas. The hydroperoxides are very
unstable and may be cytotoxic, affecting particularly proteins and membranes [120].
The positive and negative effects of lipoxygenase activity on aroma and flavor
compounds of fruits and vegetables were recently reviewed by Lamikanra [121].
5.2.2.5.2.2 Switch to Anaerobic Catabolism
Anaerobic catabolism results not only in the production of ethanol and CO 2 but also
in a drastic modification of the aromatic profile of the stored commodity. Ke et al.
[122] demonstrated that exposure of strawberries to hypoxic conditions (O 2 < 0.25%)
resulted in the production of ethanol, ethyl acetate, and acetaldehyde during storage.
Guichard et al. [123] studied the effect of controlled atmosphere on the volatile
compounds of strawberries stored for 7 d at 10°C. They found that some aromatic
molecules such as methyl-ethyl acetate, ethyl propionate, and ethyl hydroxy-3-
butanoate were not present originally but appeared during storage (i.e., ethyl hex-
anoate, ethyl octanoate, butyl acetate, and most volatile compounds responsible for
off-flavor) proportionally to CO 2 concentration. Conversely, some compounds,
including volatile acids such as butanoic, hexanoic, and octanoic acids decreased
with increasing CO 2. Furaneol or dimethyl-2,5-hydroxy-4-(2H)-furanose is consid-
ered to be a key compound in strawberry aroma [124] (Figure 5.12), but its concen-
tration in fruit, even at the highest CO 2 concentration, remained constant during
storage. It seems that CO 2 significantly alters strawberry flavor only at concentrations
higher than 20% [122,123].
Hyperoxygenation is also detrimental to strawberry flavor. Perez and Sanz [114]
stored strawberries at 8°C in four different atmospheres containing from 5 to 90%
O 2 and 10 to 20% CO 2. After 1 week of storage they found high contents of off-
flavor–related compounds in the samples stored in high CO 2 –high O 2 atmospheres.
They concluded that hyperoxygenation was responsible for the alteration of ester
biosynthesis and suggested that stress induced by high CO 2 and stress induced by
high O 2 had an additive effect on strawberry flavor alteration.
The switch to anaerobic catabolism changes the taste of all plant tissues. As
shown in Figure 5.13, both high CO 2 and low O 2 can promote ethanol production
in litchis, which can be stored for 4 weeks under 15% CO 2 (with 5% O 2 ) at 1°C
without developing a noticeable off-flavor. Keeping stone fruits under carboxic
anaerobiosis for 2 to 5 d modifies their aroma with a relative increase in their ester
and terpeneol contents and a decrease in their C6 aldehydes and ketones. A moderate
exposure to CO 2 would improve the flavor of apricots [124a]. As a general rule it
is advisable to prevent any catabolic deviation of intact and fresh-cut fruit because
this physiological disorder provokes tissue necrosis and favors yeast growth [125].