616 Produce Degradation: Reaction Pathways and their Prevention
of a container, heat is often generated by natural respiration more rapidly than it
can be removed, causing the temperature to rise. Some types of produce, such as
strawberries, must be cooled as quickly as possible after harvesting to preserve fresh
quality. Even a delay of several hours may be enough to reduce quality considerably.
In such cases, room cooling is not fast enough to prevent serious damage.
The rate of cooling is significantly increased if the air is forced through the
packages and around each piece of produce, thereby greatly increasing the heat
transfer surface area.^2 Forced-air cooling is accomplished by exposing packages of
produce in a cooling room to higher air pressure on one side than on the other. This
pressure difference forces the cool air through the packages and past the individual
units of produce, where it picks up heat, greatly increasing the rate of heat transfer.
Depending on the temperature, airflow rate, and type of produce being cooled,
forced-air cooling can be from 4 to 10 times faster than room cooling (Figure 20.6).
With both types of cold air methods, cooling time is decreased as airflow rate
is increased. 3,23,25 As shown in Figure 20.7, the 7/8 cooling time in still air is more
than 7 compared to just over 1 for products cooled with an airflow of 1 cfm/pound
of produce.
Causing the air to move faster around the product may shorten cooling time;
however, the movement of air over the product during cold air cooling also causes
some moisture loss.^23 The loss may be virtually nonexistent for products like citrus
fruits that have a low transpiration coefficient, or it may equal several percent of the
initial product weight for products with a high transpiration coefficient. If the relative
humidity of the cooling air is above 80%, moisture loss during cooling is negligible.
An additional advantage of high humidity during cooling is that it will add moisture
to the storage boxes and, therefore, decrease the amount of moisture removed from
the product during subsequent handling.^3
Wrapping the product in plastic or packaging it in bags can reduce the loss of
moisture to cooling air. However, the use of packaging material can hinder airflow
around the product and slow cooling time.^23 The amount of airflow restriction caused
by the packaging material should be a consideration in selecting materials.
FIGURE 20.6Forced-air cooling facility. The photo on the left shows fans mounted along
the wall of the cold room. In the photo on the right, produce is placed in front of the fans so
that cold air from the room is pulled across the produce. (Photo courtesy of Western Precooling
Systems, Fremont, CA.)