274 Produce Degradation: Reaction Pathways and their Prevention
Nearly 90% of the water lost as water vapor to the environment occurs through
the leaves from the cell walls lining the intercellular spaces adjacent to stomata. In some
produce in which there are few remaining stomata, lenticels replace the function of
stomata; in apples, lenticels account for up to 21% of the transpiration (Maguire et
al., 2001). The outer surfaces of most common leaves are covered with cuticle, a
multilayered, waxy deposit mainly composed of cutin (Figure 9.2). The waxes are
hydrophobic and thus offer high resistance to diffusion by water and water vapor
from underlying cells. The cuticle thus serves to restrict evaporation of water directly
from the outer surfaces of leaf epidermal cells and protects both the epidermal and
underlying mesophyll cells from potentially lethal desiccation. The epidermis of the
leaf contains small pores called stomata, each surrounded by a pair of specialized
cells called guard cells, which are responsible for the opening and closing of the
stomata. The photosynthetic mesophyll cells are loosely arranged and create an
interconnected system of intercellular air spaces that allows escape of gases and
water vapor through the stomata.
9.5 EXTERNAL FACTORS AFFECTING WATER
MIGRATION IN FRESH PRODUCEIn most harvested fresh produce, the key is to minimize the loss of water from the
produce to the environment. Water loss from the produce to the environment can be
replaced to a limited extent using techniques such as misting or submerging the produce
in water, as is the case in the ornamental plant industry. The rate of water loss from the
harvested produce is generally affected by temperature, humidity, pressure, and air
movement around the product. All of these factors influence the rate of water vapor
diffusion between the substomatal air chamber and the ambient atmosphere.
9.5.1 TEMPERATURE
Water loss from fresh produce is greatly increased with an increase in the temperature
of the produce and the surrounding environment. The increase in temperature results
in increased free energy of water molecules, which in turn increases their movement
and potential for exchange. After harvest and in storage, fresh produce continues to
respire and give off heat, which leads to a slight increase in the temperature of the
produce and results in water loss.
Temperature has a profound effect on the postharvest quality of harvested fresh
produce. Perkins-Veazie and Collins (1999) found that transferring blackberry fruit to
20°C for 2 days after low-temperature storage intervals was detrimental to fruit quality,
resulting in increased weight loss, leakage, decay, and softening in ‘Navaho’ and
‘Shawnee’ blueberry cultivars. The loss in water was dependent on the fruit cultivar:
‘Shawnee’ fruit lost 25% of marketable fruit, while ‘Navaho’ had a 10% loss. In general,
freshly harvested produce is held under reduced temperature to minimize respiration
and weight loss. The reduction in temperature should just be above the produce’s
freezing-point temperature (Table 9.2) or just above its chilling threshold temper-
ature in the case of chilling-sensitive produce. Lowering the temperature of fresh
produce lowers its rate of deterioration and thus maintains quality and extends
shelf life. Fresh produce subjected to 0°C or below undergoes freezing injury,