Microbial Inactivation by Ultrasound in the Food Industry

Microbial Inactivation by Ultrasound in the Food Industry

Eda Adal (Gaziantep University, Turkey)
DOI: 10.4018/978-1-7998-1924-0.ch005

Abstract

Pasteurization is the most common processing method for microbial and enzyme inactivation to preserve foods. With this method, foods are exposed to high temperatures and there are disadvantages for many products: thermal treatments cause modifications of sensory attributes (for instance: flavour, colour, nutritional qualities). Now, another method can replace pasteurization: microbial inactivation by ultrasounds. It is a new alternative technology of food processing also called sonication, and it can be used coupled with pressure and/or heat. These techniques inactivate microorganisms in foods. They are effective and energy efficient to kill them, making the techniques promising for the food industry. In this chapter, the method of microbial inactivation by ultrasounds was explained, after that the applications in food industry for instance in milk, orange juice, wastewater, and whole liquid eggs were well-defined, and finally, the advantages, disadvantages, and the limitations of this method were examined.
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Explanation Of The Method

How Ultrasound Can Inactivate Microorganisms?

Two types of ultrasound are used in the industry. One of them is low power with high frequency and the other one is high power with low frequency. In the food industry, the high power with a low-frequency application is most commonly used to inactivate microorganisms. This type of ultrasound is called a power ultrasound. The power ultrasounds work with 20 to 100 kHz frequency (Povey, 1989; McClements, 1995).

The inactivation of microorganisms with ultrasounds is achieved by transient cavitation (formation of bubbles in liquid medium) in food processing. The killing of microorganisms is mostly dealing with the thinning of cell membranes, localized heating, and free radical production during cavitation (Leighton et al., 1998). The longitudinal waves occur while ultrasonic waves go through the liquid medium which results in compression and expansion cycles periodically during cavitation. This phenomenon is shown in Figure 1.

Figure 1.

Schematic demonstration of a layer of a material where longitudinal sonic energy is propagated (Suslick et al., 1988)

978-1-7998-1924-0.ch005.f01

As a result of these cycles, gas bubbles are formed in the medium. The surface area of these bubbles is great which causes great diffusion of gas during expansion, in the medium. That is why the size of gas bubbles increases at each cycle. At this point, created ultrasonic energy cannot remain the vapor phase and vapor rapidly passed into the liquid phase (condensation). The bubbles begin to collide with great force which results in the formation of shock waves. The localized high pressure (104-105 kPa) and high temperature (5500 ºC) are created by these shock waves. The temperature and pressure indicated are generated during very short periods of time at the point was cavitation occurs with an order of temperature variation of 109˚C (Manas and Pagan, 2003). This is the mechanical effect of collapsing bubbles in the liquid medium. Also, these bubbles can create the formation of OH- and H+ (free radicals) and hydrogen peroxide in the medium known as chemical effects. The bactericidal properties of ultrasound are shown with these effects (Suslick et al., 1988).

There are some important factors that affect cavitation in the ultrasound process. These are hydrostatic pressure, temperature, frequency and amplitude of ultrasonic waves and tensile strength of the liquid. Such as The vapor pressure increase and tensile strength decreases with increasing temperature. As a result of that, the gas bubbles are formed and grown rapidly. However, the intensity of implosion decreases due to the increase of the vapor pressure of liquid and increases with increasing surface tension at the bubble interface (Alliger, 1975). The cavitation increases with greater amplitude because the number of bubbles undergoing cavitation increases with time which supplies the cavitation (Suslick et al., 1988). Also, the hydrostatic pressure effects on the inactivation of microorganism as mentioned above because the intensity of bubbles implosion increases with increasing hydrostatic pressure, the number of bubbles empoisoning decreases with time (Whillock and Harvey, 1997). Also, the type of microorganisms is an important factor for the effectiveness of ultrasound because some microorganisms can resist these effects.

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Materials And Methods Used In This Process

Ultrasound equipment consists of 3 parts (Figure 2):

  • 1)

    Generator

  • 2)

    Transducer

  • 3)

    Coupler

During ultrasound processing, the transducer converts the electric energy to mechanical energy. This energy transmitted through the liquid or solid material and the result is the creation of sonic energy waves. The displacement of the particles causes compressive and expansion, respectively which mentioned above (Povey, 1989; McClements, 1995; Sala et al., 1995).

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