Agriculture Reference
In-Depth Information
and vegetables (APHIS 1993; Couey 1989). Steam treatment has been applied mainly
for insect control, and hot dry air has been used for both fungal and insect control
(Lurie 1998). Steam and forced hot-air treatment systems are less damaging to com-
modities and more versatile than hot water; however, hot water is preferred for most
applications because it is more effi cient than air in delivering thermal energy to fresh
produce. The cost of a typical commercial hot-water technology system is signifi cantly
less than that of a commercial steam treatment system (Fallik 2004). Hot-water treat-
ment has been used commercially for controlling decay and insects in sweet peppers,
mangos, melons, tomatoes, and many other fruits, most noticeably in Israel and
Central and South America (Geysen and others 2006). In a typical application, hot
water is applied at temperatures between 43 and 53 °C for periods of several minutes
up to 2 h for quarantine treatments (Fallik 2004). In addition to hot-water immersion,
a hot-water rinsing and brushing system has been developed (Fallik 2004). Because
of relatively lower temperatures used in the systems for the purpose of disinfestation,
ripening inhibition, and decay control, the systems may reduce human pathogens only
marginally. However, the systems may be modifi ed to enhance microbial safety of
fresh fruits and vegetables. Other forms of thermal treatments such as microwave and
radio frequency (RF) treatments have also been investigated for controlling insects in
many fruits (Wang and others 2003). The effectiveness of thermal treatments to control
decay, disinfest produce, and reduce physiological disorders has been reviewed previ-
ously (Fallik 2004; Geysen and others 2006; Lurie 1998; Paull and Chen 2000). This
chapter focuses on use of thermal treatment to improve microbial safety and extend
shelf life of fresh and fresh-cut fruits and vegetables.
Thermal Treatment Fundamentals
Heat Transfer
During thermal treatment of any kind of food, heat is transferred by a combination of
conduction, convection, and radiation, depending on the source and types of the
heating medium. The objective of thermal treatment is to increase the temperature of
the product to a point that causes lethal damage to the bacteria of concern without
causing irreversible physical damage to the product. Because most fruits and vegeta-
bles are solid in nature, internal conduction heating is the predominant mode of heat
transfer during thermal treatment. When hot water, steam, or a mixture of steam/air
is used as a heating medium, convection heating is involved. If an infrared heater is
used, the heat would be transferred to the produce surface by radiation. In a typical
industrial operation, hot water, steam, or a mixture of steam/air is used; the process
of heat transfer is unsteady; and the transient heat transfer for any shape of food is
governed by the following:
2
2
2
∂
∂
T
t
k
C
⎛
⎜
∂
∂
T
x
+
∂
∂
T
y
+
∂
∂
T
z
⎞
⎟
Eq. 13.1
=
2
2
2
ρ
p
(
)
=
Txyxt
,,,
0
T
Eq. 13.2
0
∂
∂
T
n
(
)
k
=−
hT
−
T
a
Eq. 13.3
