Agriculture Reference
In-Depth Information
Storage relative humidity
Relative humidity is the percentage that air is saturated with water vapour
at a particular temperature and pressure. At low temperatures such as 0°C,
the amount of water vapour required to saturate the air is fi vefold less than
at 25°C (Gaf rey, 1978). This dif erence in amount of water vapour at lower
temperatures means that condensation is a greater problem on fruit when
small changes in temperature occur. Relative humidity is used as a handy
measure of water vapour pressure (WVP) in air relative to standard pressure.
Psychrometric charts give a graphical representation of the relationship
between temperature, RH and WVP in moist air. The water vapour pressure
defi cit (WVPD) is the dif erence between actual vapour pressure and the
saturated vapour pressure and determines the rate of evaporation from a fresh
commodity at the same temperature (Paull, 1999).
Relative humidity in storage rooms is dependent upon the surface area
of the refrigeration evaporator coil and the temperature dif erence between
the coil and the air, along with air exchange rates, temperature distribution
in the room, commodity and packing material used. Practical dii culties are
encountered in maintaining RH in large storage rooms within a narrow range
at high relative humidity. To illustrate the dii culty, to maintain 95% RH at
0°C, the mean temperature dif erential between the air and the evaporator
must be ~0.5°C. A measurable and controllable temperature dif erence
of ~1°C is available at 90% RH. These small dif erences test the limits of
sensitivity needed to measure temperatures to this degree of accuracy. At high
RH, a small fl uctuation in temperature (<0.5°C) can result in condensation
on cool surfaces. Poor air distribution could mean that air at 0°C, 95%
RH from the coil would be 70% RH in an area at 5°C. Fibreboard and wood
absorb water and may decrease RH in a room. A fi breboard box held at 50%
RH has a moisture content of 7% (dry mass basis); at 90% RH, the moisture
content would be 16%. High RH will not prevent moisture loss if the product
temperature is not near the air temperature. Newer refrigeration controls,
more rugged humidity detectors and humidifi cation technologies have
increased the ability to vary both temperature and RH and reduce energy
consumption.
The nature of the commodity evaporative surface is determined by
commodity type and cultivar, and both have a major infl uence on the rate
of evaporation. Surface area to volume ratios are a signifi cant commodity
factor infl uencing evaporation (Ben-Yehoshua, 1987). The ratio varies
widely for dif erent commodities: individual edible leaves have 50-100 cm
2
/
cm
3
, strawberries 2-5 cm
2
/cm
3
and bananas 0.5-1.5 cm
2
/cm
3
. In order to
compare fresh commodities, it is necessary to incorporate these factors into
loss units such as %/day/mPa WVPD (Table 5.3). This enables the time needed
to reach a permissible water loss as a percentage of the original mass at which
a commodity becomes unmarketable or has to be sold for a lower price to be
