High Power Ultrasound

Introduction

  • High energy ultrasound (20-500kHz) is referred to as High power ultrasound (HPU). The physical, mechanical or chemical effects of high power ultrasound are capable of altering material properties.
  • These effects are promising in food processing, preservation and safety. HPU has much to offer to the food industry such as drying, extraction, crystallization, de-foaming, emulsification, inactivation of microorganism and enzymes, filtration, separation of valuable compounds from vegetables and food products [1].
  • Power ultrasound is an emerging and promising technology for food processing industry. Pressing demands from consumers for quality food products with natural flavor and taste, free from additives and preservatives, has triggered need for the development of non-thermal process methods which offer maximum quality and safety of food product [2]
  • Ultrasound is considered as one such non thermal processing alternative, which can be used in many food processing operations. It travels through a medium like any sound wave, resulting in a series of compression and rarefaction. Due to their important features at ambient or lower temperatures, the non-thermal technologies are regarded as potential and powerful tools in food processing.

High Power Ultra Sound Equipment Design

There are three main components in a typical ultrasonic processing system:

  1. The electrical power generator
  2. The transducer
  3. The emitter(s)
  • The electrical power generator provides the energy for the system, which in most cases is an electrical current. An exception is the “liquid whistle,” which uses purely mechanical energy to generate ultrasound[1]
  • The second component, the transducer, is the central element in any ultrasonic system. The transducer converts electrical energy (or mechanical energy) into sound energy through mechanical vibrations at ultrasonic frequencies.
  • The third component, the emitter, is used to radiate (and in some cases amplify) the ultrasonic waves from the transducer into the medium. Emitters can come in the form of baths, horns, or sonotrodes.
  • Several companies manufacture and supply ultrasonic processing equipment for the food industry. A few of the leaders are Branson, Danbury, Conn.; Australia-based Cavitus; Dukane, St. Charles, Ill; Hielscher, Germany; Innovative Ultrasonics, Australia; Sonimat, France; and Telsonic, Switzerland.

Applications Of High Power Ultrasounds In Food Processing

Drying:

  • Removal of moisture by drying is one of the oldest method of food preservation. The use of ultrasound in combination with or prior to hot air drying was shown to have potential in increasing the drying rate without significantly affected the quality of the product.
  • Ultrasound enhanced the mass transfer during drying of carrot. The product was dehydrated at low temperature therefore, the product quality was found to improve[3].

Osmotic Dehydration:

  • Osmotic dehydration is widely used for partial removal of water from food materials by immersion in a hypertonic solution.
  • One of the main problem encountered while applying this technique is the usually slow kinetics of the process.
  • Osmotic dehydration combined with ultrasonic energy reduced total processing time and increased effective water diffusivity in strawberries compared to osmotic dehydration, which alone increased processing time.
  • Combined effects of micro-channel formation by high power ultrasound treatment and osmotic pressure differential were largely responsible for reducing drying time.

Freezing:

  • Freezing is an important preservation technique that is used in the food industry to preserve the quality of food product and maximize the shelf–life.
  • Ultrasound is known for assisting and/or accelerating various freezing operations. Several studies have indicated the potential of using high power ultrasound in accelerating the freezing rate and improving the quality of frozen food plants such as potatoes (Li and Sun, 2002a; Sun and Li, 2003) and apples.
  • High power ultrasound treated frozen potatoes exhibited a better cellular structure as less extracellular void and cell disruption/breakage appeared than those without acoustic treatment (Sun and Li, 2003).

Enzymatic Inactivation:

  • Inactivation of enzyme is an important process for enhancing the stability, shelf -life and quality of many food products.
  • Use of combination of ultrasound with low pressure and heat (manothermosonication or MCT) was reported to increase the inactivation rate of food quality related enzymes such as tomato pectic enzyme (Lopez, et al., 1998; Vercet, et al., 2002), soybean lipoxygenase (Lopez and Burgos, 1995a), horseradish peroxidase (Lopez and Burgos, 1995b) and orange PME (Vercet, et al., 1999).

Microbial Inactivation:

  • For the inactivation of microorganisms in food products thermal pasteurization and sterilization are the most commonly used techniques.
  • However, long time exposure to high treatment temperatures leads to loss of organoleptic characteristics (e.g. off flavor) and nutritional value of food products.
  • To improve the microbial inactivation in liquid foods, ultrasound is combined with other treatments such as pressure (manosonic), heat (thermosonic), both pressure and heat (manothermosonic) and antimicrobials.
  • The inactivation of Saccharomyces cerevisiae was enhanced by incubating with low molecular weight chitosan prior to ultrasound (Guerrero et al., 2005). Scouten and Beuchat (2002) indicated the decontamination of alfalfa seeds inoculated with Salmonella or E.coli O157 by combined treatments of ultrasound and Ca(OH)2, which could be an alternative to chlorine treatments to avoid contamination.

Extraction:

  • The extraction of organic compounds from plants/seeds has been based on the judicious combination of solvent, heat or agitation.
  • High power ultrasound has been shown to be a promising and innovative technique for facilitating the extraction process of a variety of food compounds (e.g. herbal, oil, protein, polysaccharides) as well as bioactive ingredients (e.g. antioxidants) from plant and animal resources (Vilkhu et al., 2008).
  • Ultrasound treatment to corn in the conventional wet milling process enhanced starch separation and increased final starch yield in addition to higher paste viscosities and whiteness (Zhang et al., 2005).
  • Application of high intensity ultrasound was shown to improve the extraction of edible oil from soyabean ( Haizhou et al., 2004) and flax seed (Zhang et al., 2008) which may reduce the overall cost of production.

DE Foaming:

  • Foams are frequently produced as unwanted side effects in many food processing operating. Foaming problems can result in product losses and reduced efficiencies.
  • High intensity ultrasound (20 kHz) in pulsed operation (1 s/1 s) has been described as an effective procedure to remove foam and dissolved oxygen (80% of foam reduction) with very low energy consumption (40 kJ/l) in super-saturated milk (Villamiel et al., 2000).

Crystallation:

  • Crystallization is an important process in the production of many food products such as chocolate, butter, margarine, whipped cream and ice cream.
  • To obtain good quality food products with specific sensory attributes (e.g., texture, hardness, smoothness, mouth feel), fat crystallization must be controlled by temperature, cooling rate and application of shear or ultrasound.
  • High powered ultrasound can assist the crystallization process in several ways: influence the initiation of crystal nucleation, control the rate of crystal growth, ensure the formation of small and even-sized crystals and prevent fouling of surfaces by the newly formed crystals (Luque de Castro et al., 2007; Virone et al., 2006).

Conclusion

  • Ultrasound is considered to be an emerging technology in the food industry. It has advantages of minimizing flavor loss, increasing homogeneity, saving energy, high productivity, enhanced quality, reduced chemical and physical hazards, and is environmentally friendly [4]
  • The considerable interest in high-powered ultrasound is due to its promising effects in food processing and preservation, such as higher product yields, shorter processing times, reduced operating and maintenance costs, improved taste, texture, flavour and colour, and the reduction of pathogens at lower temperatures.
  • The application of high power (low frequency) ultrasound, on the other hand, modifies the food properties by inducing mechanical, physical and chemical/ biochemical changes through cavitations, which reduces reaction time and increases reaction yield under mild conditions compared to conventional route.
  • By maximizing production while saving energy, power ultrasound is considered a green technology with many promising applications in food processing, preservation and safety.
  • This implies that the ultrasound is a good alternative method for the food preservation and processing and also no adverse effect on human health has been proven.