326 Produce Degradation: Reaction Pathways and their Prevention
the treatment. Nevertheless, the technology is promising and especially suitable for
fruits and vegetables because of its ability to preserve the flavor of fresh produce.
When a high-intensity pulsed electric field (HIPEF) is applied to a food in short
pulses (1 to 100 μs), an external electric field induces an electric potential over the
cell membranes. When this potential greatly exceeds a critical value (for vegetative
cells, 15 kV/cm) irreversible pores are formed, membranes are destroyed, and cells
die^ (Fellows, 2000c). The degree of inactivation depends on the intensity of the
electric field and the number and shape of pulses and also on conditions in treated
food such as temperature, pH, ionic strength, and electrical conductivity (Vega-
Mercado et al., 1999). According to studies with inoculated food, HIPEF processing
reduces the number of target microorganisms of up to 6 log cycles. Treatment has
no effect on bacterial spores and only limited effect on enzymes^ (Barbosa-Cánovas
et al., 1998).^ The technology is not commercially used for food preservation, but it
is used for recovery of edible oils and fats^ (Fellows, 2000c)^ and seems to be promising
pretreatment for improving drying of fruit or vegetables (Ade-Omowaye et al., 2001).
Sound waves (18 to 500 MHz) of high intensity (10 to 1,000 W/cm^2 ) produce
in food cycles of compression and expansion; this phenomenon is known as cavi-
tation. The implosion of bubbles generates spots with very high pressures and
temperatures that disrupt cells (Kader, 1986). The lethal effect is relatively low and
is affected by properties of treated product. Clear liquids of low viscosity are better
suited than mashed vegetables with a high content of insoluble solids. Ultrasound
is therefore used mainly for disinfection of product surfaces to increase the effect
of disinfectants (Mason et al., 1996). Recently ultrasound treatment was observed
to increase lethality of heat treatment, probably due to enhancement of the heat
transfer. D values decrease 10- or 20-fold when heating is combined with ultrasound
compared with heating alone (Leadly and Williams, 2001).
Ultraviolet light and high-intensity pulsed white light (25% UV, 45% visible light,
30% infrared) are used for treating produce and equipment surfaces. Produce such as
whole fruits and vegetables without flat surfaces are more difficult to treat with ultra-
violet radiation and the preservation effect is lower (2 to 3 log cycles reduction).
10.2.3.2.3 Chemosterilation
Chemosterilation refers to the addition of chemicals that kill microorganisms. They
usually leave residues in food. Chemosterilation can be the application of peroxy-
acetic acid, hydrogen peroxide, or other disinfectants during the dipping or washing
treatment of fresh-cut produce. Chemosterilation is applied in drinking water treat-
ment (chlorination, ozonization). Dimethyl dicarbonate (DMDC) is commonly used
for the chemical sterilization of soft drinks and wine^ (Ough, 1993; Dunn, 1997).
DMDC is usually added to the drinks before filling. It hydrolyzes on contact with
water. In pH about 2.8 and temperatures in the range of 10 to 30°C it is broken
down to the detection limit within 65 to 260 min to produce methanol and carbon
dioxide. It kills the microorganisms faster than conventional preservatives; after the
addition of DMDC the number of microorganisms is reduced. DMDC acts primarily
against yeasts; its effect against bacteria, and especially against molds, is weaker or
nonexistent (Genth, 1979). The effect of DMDC on the selected microflora of acidic
fruit juices is given in Table 10.9.