618 Produce Degradation: Reaction Pathways and their Prevention
with hypochlorous acid from sodium hypochlorite or gaseous chlorine) to minimize
levels of spoilage organisms.
Since product contact with cooling water is critical, packaging for produce to
be hydrocooled must tolerate water contact and allow water to flow through.^23 Plastic
or wood containers work well in hydrocoolers.
20.3.2.3 Package Icing
Package icing involves packing the product in crushed or flaked ice. Ice not only
removes heat rapidly when first applied to produce but, unlike other cooling methods,
it continues to absorb heat as it melts.23,25 This cooling method has the advantage
of maintaining a high humidity around the product, thereby reducing moisture loss.
Disadvantages of package icing include its high capital and operating costs, added
package weight due to the ice, the necessity that the product package be capable of
withstanding constant water contact, and melting ice that may damage or contami-
nate other products in a shipment of mixed commodities.
20.3.2.4 Vacuum Cooling
Vacuum cooling is achieved by causing water to evaporate from the product, thereby
using the latent heat of vaporization as the cooling method.^23 The product is placed
in a steel vessel and vacuum pumps reduce pressure in the vessel from an atmospheric
pressure of 760 mmHg to 4.6 mmHg. Since water boils at a pressure of 20 to
30 mmHg, depending on its temperature, reduction in the pressure surrounding the
product causes moisture evaporation and cooling. At the end of the cooling cycle
when the pressure is equal to 4.6 mmHg, water boils at 0°C. If the product is held
at this pressure long enough, its temperature will be reduced to 0°C.
Water loss of about 1% causes a 6°C reduction in product temperature.^26 Gen-
erally, products lose 2 to 4% of their weight during vacuum cooling, depending on
initial temperature. Spraying produce with water, either before starting the cooling
process or towards the end of the vacuum operation, minimizes the moisture loss
from the product (Figure 20.8).
The rate of cooling by this technique is largely dependent on the surface-to-
volume ratio of the produce and the ease with which the product loses water.^2 Leafy
vegetables are ideally suited to vacuum cooling. Other vegetables such as asparagus,
broccoli, brussels sprouts, mushrooms, and celery also cool well in this system.
Fruits and vegetables that have a low surface-to-volume ratio or a surface barrier to
water loss, such as a waxy cuticle, lose water slowly and are not good candidates
for vacuum cooling.
20.4 RESPONSES TO TEMPERATURE EXTREMES
Produce is frequently exposed to temperature extremes. Some, such as low-temperature
storage to extend shelf life and heat treatment to eliminate pests, are intentional. Expos-
ing crops to low temperatures postharvest can be used to kill insect pests.^20 The U.S.
Department of Agriculture recommendations for controlling the Mediterranean fruit fly