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Ultrasound

Hugo Scudino1, Jonas Toledo Guimarães1, Angela Suárez-Jacobo2, Hilda María Hernández-Hernández3, Tatiana Colombo Pimentel4, Socorro Josefina Villanueva Rodríguez5, Vitoria Hagemann Cauduro6, Erick Almeida Esmerino1, Erico Marlon Moraes Flores6, and Adriano Gomes da Cruz7*

1Fluminense Federal University, Faculty of Veterinary, Niteroi, RJ, Brazil

2Industrial Biotechnology Unit, Centro de Investigación y Asistencia en Tecnología y Diseño del Estado de Jalisco A.C., Zapopan, Jalisco, México

3Catedratica CONACYT in Food Technology Unit, Centro de Investigación y Asistencia en Tecnología y Diseño del Estado de Jalisco A.C., Guadalajara, Jalisco, México

4Federal Institute of Paraná, Paranavaí, Paraná, Brazil

5Food Technology Unit, Centro de Investigación y Asistencia en Tecnología y Diseño del Estado de Jalisco A.C., Guadalajara, Jalisco, México

6Federal University of Santa Maria, Department of Chemistry, Santa Maria, Brazil

7Federal Institute of Food Science and Technology (IFRJ), Department of Food, Rio de Janeiro, Brazil

Abstract

Ultrasound is an non-thermal technology which present enormous versatility at the food industry, with positive impact on the intrisic quality parameters of different food matrices. In this chapter, the definitons and the applications of ultrasound are presented as well as the challenges involved in their use.

Keywords: Ultrasound, technology, food matrices

1.1 Introduction

Conventional thermal technologies, such as sterilization, pasteurization, evaporation, and drying, are commonly used by the food industry. They can be used to inactivate enzymes and microorganisms, ensuring a safe product [1]. However, their effectiveness depends on the temperature and time of treatment, which may lead to nutrient loss, undesirable flavor and color development, and impairment of the sensory properties of foods [2-4]. Therefore, there is a considerable demand for new technologies to replace conventional thermal processing, providing microbiologically safe food without nutritional losses that are sensory pleasing to the consumer [5].

Among the various emerging technologies studied, high-intensity ultrasound (HIUS) technology has stood out. HIUS is a non-thermal emerging technology that can be used in food processing because of the ability to cause chemical and physical changes in the medium at mild temperatures. Ultrasound in a liquid medium can cause many changes, and the primary phenomenon that provides these changes is unstable acoustic cavitation [6]. The micro implosions generated during the collapse of the microbubbles result in high pressures and temperatures in the environment. Hence, they may induce chemical, physical, and microbiological changes in food [7].

Ultrasound is gaining prominence for presenting significant advantages compared to conventional thermal processes, such as lower production costs, preservation of the nutritional characteristics of food, improvement of sensory attributes, and improvements of product properties [8, 9]. In addition, it causes a low impact on the environment, and HIUS is known as a green technology [10].

There are currently many applications for HIUS in food processing. Ultrasound can be used individually in the processing or in combination with other methods, such as pressure (manosonication) and temperature (thermosonication), to produce a synergistic effect, increasing the effectiveness [11]. One of the main applications of ultrasound is microbial inactivation, which is widely studied in dairy and vegetable products. The impact on physicochemical properties is also widely explored. HIUS has wide application uses in defoaming, emulsification, texture modification of fatty products, as well as regulation of microstructures, functional properties of food proteins, and sonocrystallization. It can also be applied in various processing such as drying, freezing, concentration, tenderization, and thawing [12-15].

This chapter presents the applications of HIUS in food processing, with a focus on its use in food processing and preservation. The objective is to discuss what makes ultrasound one of the most promising research areas in food science.

1.2 Basic Principles of Ultrasound

Ultrasound research began in the 1920s with the researchers Harvey and Loomis and later in the 1960s. They verified that the sound waves produced by war submarine sonars could inactivate microorganisms [16, 17]. Then, other studies using ultrasound technology were developed using high frequencies and, then, high intensities, in parallel with the development of small equipment that would allow testing in these conditions [18].

Acoustic waves are classified considering the frequency audible to the human ear. Infrasound encompasses frequencies below the human hearing range (0 - 20 Hz), sound refers to the human audible range (20 Hz - 20 kHz), and ultrasound corresponds to the wave range above 20 kHz [19].

The ultrasound applications in food analysis, processing, and quality control can be divided based on the sound wave intensity in low and high-intensity ultrasound (Figure 1.1). Low-intensity ultrasound is related to intensities below 1 W/cm2 and high frequencies (> 1 MHz), resulting in no changes in the treated material. Therefore, it is generally used for analytical applications, such as determining the structures and composition of food [20]. High-intensity ultrasound is related to intensities greater than 1 W/cm2 and low frequencies (20 - 100 kHz). It can result in alterations in the microbiological and physicochemical characteristics of the food. It is considered a non-thermal technology, and it has been used as an alternative to conventional food processing [7]. It is widely studied to replace thermal processes for microbial inactivation to reduce the adverse effects caused by heat [21]. Furthermore, it is used in the development of new products due to its ability to increase the activities of bioactive compounds [22], change viscosity [23], improve food stability by decreasing fat globules [24], alter the crystallization of dairy components, and water [25], among other applications.

Figure 1.1 Ultrasound waves with application in food analysis and processing [26].

1.2.1 Generation of the Ultrasonic Wave

Ultrasound is a form of vibrational energy produced by an ultrasound transducer that converts electrical energy into acoustic energy. A reliable and stable ultrasound is created through the electrical energy produced by a power generator (in general, using a piezoelectric device). This energy is transformed into mechanical energy by ultrasonic vibrations and then applied indirectly or directly to food [27]. When passing through food, the ultrasound will find resistance, and the waves will be partially dissipated. The resistance is dependent on the materials used. Therefore, an increase in the temperature may occur due to the transducer energy dissipation loss in the form of heat [7].

The most used ultrasound systems in food processing are probes and baths (Figure 1.2). The most commonly used is the probe system. An acoustic signal is directed to food in an amplified form and through a metal rod in this system. In this way, a liquid medium between probe and food is not needed. Therefore, energy is applied directly to the medium. In this way, Due to its direct action, there is greater cavitation intensity. The baths are considered an indirect system. In this system, a transducer is attached to the walls or base of the tank. The ultrasonic energy is transmitted to the liquid (generally water) and is then transferred to the food, resulting in greater dissipation of the waves. In this way, the ultrasound intensity is lower than expected [28, 29].

Figure 1.2 Equipment used for ultrasound application. Probe (left) and Bath (right) [26].

1.2.2 Principles of Acoustic Cavitation

Stable cavitation and transient or unstable cavitation are the two forms of acoustic cavitation. Stable cavitation is generally formed at low or moderate acoustic intensities. A micro agitation in the medium is observed because the bubbles oscillate regularly and long cycles around the equilibrium size. However, there is no implosion of the bubbles. The unstable cavitation is generally formed at high-intensity sound fields. The bubbles oscillate in short cycles, and they gradually grow. Contractions and expansions of the bubbles are observed several times, resulting in an unstable size. Then, the bubbles violently collapse, and there is the formation of shock waves and jets [30]. The unstable cavitation is responsible for chemical, physical, and microbiological alterations in food [31] (Figure 1.3).

The bubble size depends on the sound frequency. Other parameters also influence the cavitation, e.g., the viscosity, temperature, surface tension, vapor pressure of the liquid medium. Larger bubbles are produced at lower frequencies (20 - 40 kHz). In a general view, greater ultrasound intensities result in a higher number of bubbles, as the number of bubbles is dependent on the ultrasound amplitude and energy supplied...