324 Produce Degradation: Reaction Pathways and their Prevention
Efforts to optimize thermoinactivation efficiency have continued for many years.
New methods of heating have been developed and used. Ohlsson^ (2002) reviewed
the recent developments in thermal processes in minimal treatment of fruit and
vegetables and food. One of the principles is the application of HTST (high tem-
perature, short time). Inactivation of microorganisms is more dependent on temper-
ature. With increasing temperature the rate of killing increases faster than the rate
of thermodestruction of food components (see z values in Table 10.10). When
produce is processed in the package, to achieve the HSTS condition the package
size and the degree of convection inside the package could be optimized. More
recent developments in the heat treatment of packaged food are aseptic or semi-
aseptic filling. Heating equipment used includes direct steam injection and steam
infusion or indirect systems that involve the use of plate, tubular, or scraped surface
heat exchangers. The trend in thermoinactivation technologies will probably continue
in the area of sophisticated systems such as Twintherm, in which particulate food
is heated by direct steam injection in a pressurized, horizontal, cylindrical vessel
that rotates slowly. Once the particles have achieved the required inactivation effect
(F value), they are cooled under lower pressure (Ohlsson, 1992). Similar systems
for pasteurization of vegetables with vacuum steam heating followed by vacuum
cooling or a vacuum/steam/vacuum surface intervention process to reduce bacteria
on the surface of fruits and vegetables (Kozempel et al., 2002) have been developed.
Fruits and, especially, vegetables are also processed using the sous-vide pro-
cessing process. Raw material is vacuum-packaged in multilayer plastic film and
cooked at temperatures below 100°C, followed by cooling to 3°C. As is in the case
of sous-vide processing of meat, vegetables maintain nutrients compared to tradi-
tional cooking and the sous-vide produce is juicier (Schellekens, 1996).
Thermoinactivation also involves direct methods of heating, in which heat is
generated inside the products such as in ohmic heating and dielectric heating using
radiofrequency electric field and microwaves. In ohmic heating processes foods are
made a part of an electric circuit by flowing alternating current and the resulting
heat generated due to the electrical resistance of the foods through them. Ohmic
heating allows a fast increase of temperature (1°C/s) with an absence of temperature
gradients. The technology is used for processing liquid foods or liquid foods with
small particles (Ruan et al., 2001).
When radiofrequency electric field or microwaves are used, dipolar molecules
in water and other ionic components attempt to orient themselves to the field. The
rapid oscillation creates frictional heat. Brody (1992) reviewed the microwave pas-
teurization process. Usually, microwaves can be used in blanching, thawing, and
tempering of frozen products and in dehydration processes (Fellows, 2000c).^ Indirect
heating methods have no effect on fruit and vegetable components (other than those
caused by heating). Compared to traditional heating processes, this could be shorter
and therefore more gentle with reduced loss of nutrients.
10.2.3.2.2 Other Physical Methods of Killing Microorganisms
Nonthermal preservation methods of foods include traditional technologies such as
irradiation, UV radiation, ultrasound treatment, and also more modern emerging
technologies such as high-pressure treatment and pulsed electric field treatment.