276 Produce Degradation: Reaction Pathways and their Prevention
9.5.2 RELATIVE HUMIDITY
Humidity is measured using dry and wet bulb thermometers and established from
a psychrometer and a psychrometric chart (Gaffney, 1978). Grierson and Wardowski
(1975) and Kays (1997) discussed percentage relative humidity, absolute humidity,
vapor pressure, and dew point as the different ways of expressing water vapor in
the environment. Relative humidity refers to the ratio of the quantity of water vapor
present and the maximum amount possible at that temperature and pressure. Because
relative humidity is affected by temperature and pressure, absolute humidity is a
more precise measure of humidity. It refers to the weight of water in a given weight
of dry air. Water vapor pressure difference between locations is a useful criterion
for measuring potential of fresh produce to lose water to the environment. The
movement of water within the produce and between the produce and its surroundings
occurs in response to a gradient. Gaffney (1978) reported that moisture loss from
fruits and vegetables is directly proportional to the difference between the water
vapor pressure at the surface of the produce and that of the air surrounding the
produce. Therefore, the water vapor deficit or the difference in water vapor pressure
between fresh produce and the immediate surroundings gives an indication of the
potential for the loss or gain of water by the produce (Burton, 1982; Kays, 1997;
Wills et al., 1998; Thompson, 2002).
Dew point is another key parameter related to the postharvest life of fresh
vegetables. It refers to the temperature at which the air is saturated with water vapor
(i.e., 100% relative humidity). Water condensation occurs when the temperature is
lowered below the dew point since the air can no longer hold as much water. By
using a psychrometric chart, the dew point can be determined from the air temper-
ature (dry bulb) and the relative humidity.
9.5.3 PRESSURE, AIR MOVEMENT, AND LIGHT
Although elevated pressure increases the free energy of water molecules, resulting
in increased water molecule movement, this increased free energy has minimal
impact on fresh produce. However, reducing pressure within the surrounding envi-
ronment has an impact on water exchange in fresh produce. Reduced pressure
decreases the free energy of the water molecules, resulting in an increased concen-
tration gradient between liquid-phase molecules of water within the tissue and
gaseous water molecules in the surrounding environment. The net effect of the
concentration gradient is the movement of water out of the tissues to the surrounding
atmosphere exterior to the product. Products stored under partial vacuum also lose
water to the exterior until an equilibrium is established (Thompson, 2002).
The presence of an air boundary layer on the surface of produce reduces water
loss to the surrounding environment (Ben-Yehoshua, 1987). Air movement over the
produce’s surface decreases the thickness of the air boundary layer. This decrease
of the air boundary decreases the boundary’s resistance to the exchange of water
molecules between the produce and its immediate surroundings. In succulent fresh
produce such as lettuce, this results in increased water loss. Air movement in closed
storage systems, as is the case in refrigerated storage, can influence vapor pressure