Inactivation Of Microbes Using Ultrasound: a Review

1. Introduction
Conventional thermal pasteurization and sterilization
are the most common techniques currently used
to inactivate microorganisms in food products; however,
the demand for new methods that have a reduced
impact on the nutritional content and overall food
quality is increasing. New preservation techniques
have been developed that could eliminate microbial
activity whilesignificantly reducing or completely
eliminating the amount of heat required. These processes
are, for the most part, less energy-intensive and
therefore more cost-efficient and environmentally
friendly than conventional thermal processing. Thermal
processing does kill vegetative microorganisms
and some spores; however, its effectiveness is dependent
on the treatment temperature and time.Unfortunately,
the magnitude of treatment, time and process
temperature are also proportional to the amount of
nutrient loss, development of undesirable flavours and
deterioration of functional properties of food products.
Some of the common nonthermal alternatives to
conventional thermal processing of foods include
pulse-electric field inactivation, microfiltration,
pulse-light inactivation,high pressure and ultrasonication.
In recent years, the food industry has discovered
that ultrasonic waves have a wide variety of applications
in the processing and evaluation of products.
From grading beef to sterilization, ultrasound has a
number of applications in an increasing number of
0168-1605/03/$ - see front matter D 2003 Published by Elsevier Science B.V.doi:10.1016/S0168-1605(03)00075-8
$ Contribution No. S121 from the Food Research Program.
* Corresponding author. Tel.: +1-519-829-2400x3147; fax: +1-
519-829-2600.
E-mail address: piyasenap@agr.gc.ca (P. Piyasena).
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International Journal of Food Microbiology 87 (2003) 207– 216
areas in the food industry. In combination with heat, it
can accelerate the rate of sterilization offoods, thus
lessening both the duration and intensity of thermal
treatment and the resultant damage. The advantages of
ultrasound over heat pasteurization include: the minimizing
of flavor loss, especially in sweet juices;
greater homogeneity; and significant energy savings
(Crosby, 1982). This paper provides a comprehensive
summary of literature recently published on food
processing andmicrobial food safety using ultrasound.
1.1. Ultrasound waves
Ultrasound, in its most basic definition, refers to
pressure waves with a frequency of 20 kHz or more
(Brondum et al., 1998; Butz and Tauscher, 2002).
Generally, ultrasound equipment uses frequencies
from 20 kHz to 10 MHz. Higher-power ultrasound
at lower frequencies (20 to 100 kHz), which is
referred to as ‘‘power ultrasound’’, hasthe ability to
cause cavitation, which has uses in food processing to
inactivate microbes.
Types of transducers that can accomplish the generation
of ultrasonic waves, equipment and their
functions are given in details by Povey and Mason
(1998). This review will focus on those aspects
applicable to microbial food safety.
1.2. Microbial inactivation
The elimination of microorganisms is anarea of
increasing concern to consumers. Effective microbial
destruction is imperative in food processing; a single
report of contamination could put the reputation and
future success of a manufacturer at risk. In order to
minimize the bacterial load of a product, the manufacturer
must reduce initial contamination, inactivate
any microorganisms present in the food, and implementprocedures to prevent or retard the subsequent
growth of microbial populations which have not been
inactivated (Sala et al., 1995). Ultrasonic irradiation
has the potential to be used for the inactivation of
bacterial populations.
Conventional methods of bacterial inactivation
usually involve thermal treatment (i.e., pasteurization,
ultra high temperature). These treatments often result
in the...