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39 Minimally Processed Foods 753coagulation properties of milk in terms of curd firmness (Yu et al.
2010). The result implied that treating milk with PEF and mild
temperature impact less changes in terms of milk coagulation
properties compared to conventional heat pasteurization. This
result is consistent with the finding of Dunn (1996), who stud-
ied the PEF-treated raw milk and concluded that no significant
physicochemical changes were observed.
Although the interest is increasing rapidly, there is much
less information on application of PEF on solid food materi-
als. Bazhal et al. (2003) applied 300μs pulses of 1 kV/cm at the
rate of 1 Hz on apple tissues and studied the resulting structural
and morphological changes. The authors reported an increase of
porosity from 63% to 69.4% after PEF electroplasmolysis. The
sizes of the induced pores were smaller compared to the pores of
untreated samples and were comparable with the tissue cell-wall
thickness. When PEF-treated apple slices were compressed, it
was observed that there was a relationship between mechanical
failure stress and degree of electroplasmolysis as shown in Fig-
ure 39.1. Influences of PEF on physical and textural properties of
food materials have been reported (Rastogi et al. 1999, Angers-
bach et al. 2000, Gadmundsson and Hafsteinsson 2001, Taiwo
et al. 2001, Gachovska et al. 2009, Grimi et al. 2009). Several
other authors have exploited this phenomenon in using PEF to
enhance extraction of juice from different food materials (Bazhal
and Vorobiev 2000, Eshtiaghi and Knorr 2002, Gachovska et al.
2006, Gachovska et al. 2008, Gachovska et al. 2010, Hou et al.
2010, Loginova et al. 2010). The various studies indicate im-
proved pressing efficiency, faster extraction processes, and im-
provement in drying processes as advantages of PEF-enhanced(^00)
0.5
Stress (MPa)
1
1.5
2
0.25
Strain
Number of pulses
0
1
2
60
0.5 0.75
Figure 39.1.Typical stress–strain curves from the compression test
on apple samples treated by electric field pulses (field strength,E=
1 kV/cm; treatment time= 300 μs; frequency=1 Hz). Control
sample has no treatment (n=0) (Bazhal et al. 2004).
processes. Despite the enormous progress that has been made
on PEF processing of food, some of the challenges of the tech-
nology include difficulty in comparing different systems and
process parameters. Different equipment and treatment cham-
bers may present different treatment conditions. There is need
to standardize various parameters and systems in order to facili-
tate effective evaluation of various processes reported by various
authors.
Low-Dose Irradiation
Ionizing radiations have been used to control spoilage microor-
ganisms and extend food shelf life. As it involves no heating,
the food retains most of its organoleptic features. The technique
involves application of precisely controlled ionizing radiation
to either bulk or packaged product for a preestablished dura-
tion in order to destroy microorganisms, insects, or other pests;
minimize postharvest losses; decrease or inhibit sprouting; or
other specific purposes. The form of ionizing radiations used in
food processing includes gamma rays from radioisotopic sources
such as Cobalt-60 or Cesium-137, high-energy electrons from
electron beam devices, and X-rays from electron beam acceler-
ators (Patterson and Loaharanu 2000). The use of gamma rays
or X-rays of energies no greater than 5 MeV is generally recom-
mended for food processing, whereas for accelerated electrons,
energy levels below 10 MeV are suitable (Wilkinson and Gould
1998). The mechanism of microbial inactivation by ionizing
radiations has been ascribed to direct interaction of the radia-
tion with cell components and food molecules or indirect action
from radiolytic products such as water radicals H+,OH−, and
eaq(Wilkinson and Gould 1998, Morris et al. 2007). It primar-
ily targets the cell’s chromosomal DNA and exerts a secondary
effect on the cytoplasmic membrane, either of which can result
in microbial inhibition or inactivation. Energy from the radia-
tion sources may be sufficient to dislodge electrons from food
molecules, converting them to electrically charged particles or
ions. However, the ionizations are too low to induce radioactivity
in food products. To irradiate the food, it is important to use the
radiations that can reach the core of the irradiated food in order
to achieve uniform treatment. Gamma rays have high penetrat-
ing power and thus can be used to treat foods in large packages.
The energy decreases exponentially with depth of the absorb-
ing product. However, accelerated electron beams continuously
lose energy in a series of interactions with orbital electrons in
the absorbing medium. Their penetration depth is low such that
10 MeV electrons may only penetrate about 4 cm (Wilkinson
and Gould 1998).
Microorganisms, enzymes, insects, and vegetable sprouting
have different degrees of sensitivity to radiation. Required ir-
radiation doses for a specific goal depend on the rays (type,
quantity, and radiation time) and the irradiated environment
(absorption capacity, physical, chemical and biological modi-
fications, and secondary reactions). Irradiation dose is measured
in grays (Gy), defined as the absorption of 1 J of ionizing ra-
diation by 1 kg of matter (i.e., 1 Gy=1 J/kg). High-radiation
dose in the range of 10–74 kGy is usually applied for microbial
sterilization (Morris et al. 2007). Milder doses less than 10 kGy