such hidden defects established preharvest or during storage with ap-
pearance in the kitchen need to be improved.
For most vegetables, optimum temperatures for growth and develop-
ment are far above those most suitable for storage and an extended shelf
life. Are such low temperatures beneficial to quality retention and im-
provement besides freshness? Lowering the tissue temperature will in-
fluence the activation energy of enzyme-catalyzed reactions, with certain
reactions being more affected than others leading to imbalances in cell
metabolism. Low-temperature sweetening in potatoes is one well-known
example caused by the temperature sensitivity of some of the glycolytic
enzymes (ap Rees et al., 1988). Transfer of potatoes to a higher tem-
perature before processing may reestablish normal cell metabolism and
avoid undesired browning due to Maillard reactions in fried potato prod-
ucts. Since freshness and turgor are essential for quality of vegetables,
few if any investigations have been made until present about the con-
ceivable effects of low-temperature storage on the taste, texture and fla-
vor of the produce.
An increasing concern among consumers about chemical treatments
applied pre- or postharvest to fruit and vegetables to control insects, dis-
eases, physiological disorders and regrowth has called upon less harm-
ful alternatives. Postharvest heat treatments aimed at insect disinfestation
and disease control can also alter the senescence of fruit by influencing
the rate of protein synthesis, softening, chlorophyl loss, respiration and
ethylene synthesis (Paull, 1990; Klein and Lurie, 1991). Heat treatments
at 38–46°C for 12 hr to 4 days or short-term heat treatment for up to 60
min at 45–55°C by exposure to hot air, vapor heat or immersion in hot
water (Paull, 1990) have been used for slowing the ripening of climac-
teric fruits resulting in a longer shelf life. Hot water treatments of broc-
coli florets at a range of temperatures (43–55°C) with a duration from
10 sec to 30 min have been tested in order to delay yellowing and ex-
tend shelf life (Forney, 1995; Tian et al., 1996, 1997). Heat treatment
inhibited wound-induced ethylene production with delayed degreening
compared with the control. Ethylene production plays an important role
in broccoli yellowing probably due to the climacteric behavior of this
inflorescence tissue (Tian et al., 1994). The effect of heat treatment on
yellowing could be mediated via a transient inhibition of ethylene syn-
thesis similar to results reported for heat treated tomatoes (Biggs et al.,
1988). The potential benefit of heat treatment to delay senescence in
non-fruit and non-flower vegetables should be a fertile field for future
research.
greg delong
(Greg DeLong)
#1