Ultrasound techniques for producing high quality and safe food products
Namita Rani, Anil Kumar Verma, Pritam Chand Sharma, Shivani and Manoj Kumar
Increasing consumer demand for high quality food products with natural flavour, taste and free from preservatives, encourages the need for development of non-thermal innovative approaches for food processing. Pasteurization, sterilization and drying are the traditional methods used in food processing pose problems like loss of nutrients, vitamins, enzymatic browning, prolonged processing time, fruit juice sedimentation and microbial spoilage.
Ultrasound is considered as one of the non-thermal processing technologies having potential to be a suitable alternative to thermal food processing technologies. Ultrasound is a form of energy, generated by sound waves of different frequencies that are too high to be detected by the human ear, i.e. above 16 kHz. It is one of the new technologies which increases and ensures quality and reduces the time of processing and cost of the food products.
It is used to assess the composition of meat, fish, and poultry products and in quality control of vegetables, cheeses, oils, breads, and cereals as well as detecting adulteration of honey and protein analysis. There are also several reports of its application in mass transfer and marination processes, meat tenderization, crystallization, freezing, drying, degasification, filtration, foam production and reduction, and emulsification, as well as homogenization and inactivation of microorganisms. Hence, ultrasound can be one of the viable techniques for quality assurance and food safety.
Thermal treatments like pasteurization and sterilization have been used to produce safe food products such as juice, milk, beer, and wine in which the final product has a shorter storage life that result in less vitamins, taste, colour, and other sensorial characteristics. It often creates the need for additives to improve the products (Piyasenaet al., 2003). Non-thermal food processing methods such as high pressure processing, pulsed electric fields, cold plasma, ozonation and oscillating magnetic fields, offer maximum quality and safety to food products.
Among these, ultrasonication is an important non thermal method that can be combined with heat treatment or can be used as an alternative method to heat. It is a form of energy, generated by sound waves of different frequencies that are too high to be detected by the human ear, i.e. above 16 kHz. It is a significant food processing technology with the capability for large commercial scale-up and good payback on capital investment; and one of the fast, versatile, emerging and promising non-destructive green technologies used in the field of Food science and technology from the last few years. Ultrasound is being applied as an effective preservation tool in many food-processing fields viz. vegetables and fruits, cereal products, honey, gels, proteins, enzymes, microbial inactivation, cereal technology, water treatment, diary technology, etc. Presently, ultrasound technology has gained wider applications in almost all fields including medical scanning ultrasonic therapy, mineral processing, nanotechnology, food and beverage technology, non-destructive testing, industrial welding, surface cleaning, and environmental decontamination applications. Ultrasonication combined with heat can accelerate the rate of sterilization of foods, thus lessening both the duration and intensity of thermal treatment and the resulting damage. The advantages of ultrasound over heat treatment include Minimization of flavor loss, especially in sweet juices; greater homogeneity and significant energy savings. There are many other methods that can be combined with ultrasound other than heat. The use of ultrasound in the food industry involves various novel ideas and methods which are interesting as compared to conventional methods or techniques. This introduces us to the food processing methods which are alternatives to conventional ones. The use of ultrasound is an active subject within the food industry for both of the research and development.
HISTORY OF ULTRASONIC APPLICATION
|1987||Combination of heat and high power UW (20 kHz) was first explored by and the term of thermoultrasonication was used. According to this study the inactivation effect of thermal ultrasonication was greater than UW at room temperature. More recently, the term Manothermosonication (MTS), which is the combination of heat, pressure and ultrasound, has been coined for the combined treatments.||Ordondezet al., 1987|
|1960||Ultrasonic irradiation has the potential to be used for the inactivation of bacterial populations. Investigation of ultrasound as a potential microbial inactivation method began in the 1960s, after it was discovered that the sound waves used in anti-submarine warfare killed fish
|Earnshaw et al., 1995|
|1930||According to another source, the inactivation of microorganisms by Ultrasonic Waves (UW) was reported in the early 1930s but its scant lethal effect prevented its use as a sterilization method. However, improvements in UW generation technology over the last few decades have stimulated the interest of investigators in microbial inactivation by UW.
|Pagan et al., 1999|
|1994||A resistometer was designed and built to apply high- power UW under pressure at nonlethal (manosonication) and lethal (manothermosonication) temperatures. The results indicated that the rate of vegetative cell inactivation by Manosonication (MS) increased when the static pressure was raised. It was also observed that the inactivation rate by MS increased exponentially with the amplitude of UW||Oagan, 1977; Palacios et al., 1991|
Types of sonication
Ultrasound can be used for food preservation in combination with other treatments to increase the efficiency of the technique. There have been many studies combining ultrasound with either pressure, temperature, or pressure and temperature.
It is the application of ultrasound at low temperature. Therefore, it can be used for temperature sensitive products where there is a concern about the loss of nutrients like vitamin-C, denaturation of protein, non-enzymatic browning etc. However, it needs a long period of exposure to kill/ inactivate stable enzymes and/or microorganisms which may cause high energy requirements. During ultrasound application there may be rise in temperature depending on the ultrasonic power and time of application and needs control to optimize the process.
It is a combination of ultrasound and heat. In this method the product is subjected to ultrasound combined with moderate heat. As a result of additional heat, the ultrasound produces a high amount of cavitation which in turn gives a greater effect on inactivation of microorganisms than heat alone. Therefore, the combination of low frequency ultrasound with mild heat will help in reducing the time of processing by 55 % and temperature of processing by 16 % by reducing product sensory quality
It is a combined method in which ultrasound and pressure are applied together. Manosonication helps to inactivate enzymes and/or microorganisms by combining ultrasound with moderate pressures at low temperatures. Its inactivation efficiency is higher than ultrasound alone at the same temperature.
It is a combined method of heat, ultrasound and pressure. In this method the applied temperature and pressure will maximize the cavitation and give greater efficiency for inactivation of enzymes and microorganisms. MTS treatments inactivate several enzymes at lower temperatures and/or in a shorter time than thermal treatments at the same temperatures. Microorganisms that have high thermo tolerance can be inactivated by MTS. Thermoresistant enzymes, such as lipoxygenase, peroxidase and polyphenoloxidase are reported to be inactivated by MTS.
Classification of ultrasound application
Ultrasound applications are classified based on their mode of application and based on the intensity of sound waves.
Classification based on mode of application
Direct application to Coupling with the device Submergence in
the product an ultrasonic bath
Classification based on intensity of sound waves
High power ultrasound Low power ultrasound
- In high power ultrasound, sound waves are having at low frequency (20 Hz to 100 kHz) which is having the ability to cause cavitation with sound intensity of > 1 W/cm2. It gives impact to physical, chemical and biological properties of food in processing, preservation and safety (Feng and Yang 2005).
- In low power ultrasound, sound waves are having higher frequency of > 100 kHz with an intensity of < 1 W/cm2. It is used in non-destructive analytical measurements and monitoring of composition and physico-chemical properties of food for quality control.
Principle of Ultrasound
The basic principle on which ultrasound can work is the cavitation which is the phenomenon of generation, growing and eventual collapse of the bubbles. As ultrasound waves propagate, the bubbles oscillate and collapse which causes the thermal, mechanical, and chemical effects. Mechanical effects include collapse pressure, turbulence, and shear stresses, while the chemical effects include generation of free radicals. The effects in the cavitation zone generate extremely high temperatures (5,000 K) and pressures (1,000 atm)
Application of Ultrasonication in Food Industry
Microbial inactivation mechanisms of ultrasound are simply explained by cavitation phenomena that are caused by the changes in pressure. Extremely rapid creation and collapse of bubbles formed by ultrasonic waves in a medium creates the antimicrobial effect of ultrasound. During the cavitation process, localized changes in pressure and temperature cause break- down of cell walls, disruption and thinning of cell membranes, and DNA damage via free radical production.
Wordon suggested that high frequency of ultrasound was more effective in irradiation of microorganisms. Microbial inactivation using ultrasound has been investigated for application to a range of liquid foodstuffs. Levels of E. coli O157:H7 were reduced by 5 log cfu/mL with ultrasound in apple cider and the inactivation of E. coli K12 was enhanced using ultrasound at ambient temperatures. D’Amico et al. (2006) showed that ultrasound treatment combined with mild heat (57˚C) for 18 min. resulted in a 5-log reduction of L. monocytogenes in milk, a 5-log reduction in total aerobic bacteria in raw milk, and a 6-log reduction in E. coli O157:H7 in pasteurized apple cider. With ultrasonic probes particularly in combination with higher temperature and amplitude. The method thermosonication produces a greater effect on inactivation of microorganisms than heat alone.
Ultrasonication in Fruit and Vegetable Processing
Ultrasonication is used to maintain both pre and post-harvest quality attributes in fresh fruits and vegetables. It is considered a substitute for washing of fruit and vegetables in the food industry. In an attempt to meet the consumers needs of not only maintaining but also improving the nutritional value of fruit juices. It is reported to retain fresh quality, nutritional value, and microbiological safety in guava juice and orange juice. Ultrasound treatment can also be used to recover the nutrient loss occurred during blanching, resulting in achieving the collaborative benefit of both the techniques.
Enhancing Fruit Juice Quality and Shelf Life
Many beverages and concentrated juice are vital food products due to their massive demand in the global market. Using traditional preservation methods like pasteurization and sterilization, it is not possible to disturb the cell structures in juice suspension, but when sonication is applied to fruit juice, it will affect juice’s macrostructures and convert them into micro structures thereby reduces fruit sedimentation and increases and enhances the nutritional compounds in fruit juices.
Ultrasound Assisted Drying
The aim of ultrasound-assisted drying is to overcome some of the limitations of traditional convective drying systems, especially by increasing drying rate without reducing quality attributes in a short period of time. When ultrasounds pass through the product, mass transfer can be effectively achieved by cavitation phenomenon which creates micro streaming channels in the food product, and gives better results compared to traditional methods. The drying which is enhanced sonically can be carried out at lower temperatures due to which the probability of oxidation or degradation reduces in the material. This method of drying is useful in case of heat sensitive material.
This is one of the oldest applications of ultrasound which is involved in the degradation of polymers. The depolymerization mainly involves two possible mechanisms:
- a) by collapsed cavitation bubble mechanical degradation of the polymer.
- b) Chemical degradation. Chemical method involves reaction between the high energy molecules and the polymer. The high energy molecules like hydroxyl radicals are produced from cavitation phenomena.
Ultrasound has important potential for the conversion of raw materials like carbohydrates which are polymeric in nature to useful less weight molecules or its simpler components. In the food industry, the area in which the use of ultrasound is active is to depolymerize starch. So due to its progress in sonochemical engineering it may play a big role in the carbohydrate industry.
Brining, Pickling and Marinating
Pickling and marinating are the techniques which are used with a variety of vegetables and meat products. Most commonly used salt-brining or pickling fermentation has mainly three limitations.
- For brining, a high quantity of sodium chloride is used which may require the desalting process for the use of food products
- The process is difficult to control during fermentation
- By soaking methods the products may soften, bloated or structural damage may occur.
So to eradicate these limitations there is a need for alternative technologies. Ultrasound reduces pickling time of products particularly the foods which have crunchy texture. Also the products contain low sodium content so there is no need for desalting. This process provides a product which is uniformly salted. Brining involves two main mass transfer processes: water migration happens from the meat to the brine and the solute migration happens from the brine to the meat. Ultrasound energy can be used with combination for the methods like brining or marinating raw foodstuffs by submerging it in the brine or marinade.
The ultrasound is effective in the dairy industry for the processes like pasteurization. It is found effective for killing the micro-organisms like E.coli, Pseudomonas etc. and it does not have a detrimental effect on the total protein or casein content of milk.
Used for reducing the size of fat globules and can be applied effectively to homogenize milk.
The most important factor in the application of ultrasound in processes of emulsification and homogenization of milk is to control possible negative effects such as oxidation of fats, inactivation of enzymes, and protein denaturation.
Ultrasound treatment is applied in dairy industry for removal of fat from dairy wastewater using enzyme as a catalyst , improvement in whey ultrafiltration, cutting of cheese blocks, crystallization of ice and lactose, alter the functionality of dairy proteins, cleaning of equipment, pasteurization, and homogenization which involve minimum loss of flavor, and increased homogeneity and considerable savings in energy.
A large number of applications of ultrasonic treatment are reported in meat technology like, reduction of meat toughness due to large proportion of connective tissue, examining the composition of fish, poultry, raw, and fermented meat products by supporting genetic enhancement programs in case of livestock and in the tenderization of meat products.
The traditional method of meat tenderization is mechanical pounding. But this method makes poor quality meat. Power ultrasound is one of the methods which is very useful in this technique. Ultrasound act by using two methods:
- By the breakage of integrity of muscular cells or
- By increasing the rate of enzymatic reactions by using biochemical effects. Ultrasonic tenderization can be achieved with poultry meat, veal and beef. So ultrasound is used for producing processed meats. Meat products are present in the recombined form such as beef rolls. These meat pieces are held together by a protein gel which is formed by the myofibrillar proteins released during processing.
Ultrasonication has been used to influence enzyme activity and to obtain intracellular enzymes from microbial cells. Ultrasound treatment helps in the release of glucose-oxidase from Aspergillus niger, galactosidases from Lactobacillus strains and E. coli, and invertase from A. niger. Despite positive implications on enzymatic activity, high-intensity ultrasonication leads to denaturation and hence making ultrasound treatment enzyme-specific and sonication parameter-specific. Thermosonication a combination treatment of incorporating high static pressure in an ultrasound treatment chamber is used as a means for enzyme inactivation such as lipoxygenase, peroxidase, lipase, and protease, and tomato or orange pectin methylesterase.
In the food industry, to produce solid-free liquid or to isolate solid from its mother liquor, the separation of solids from liquids is an important step. In this case two specific effects is involved:
- Agglomeration of fine particles in the nodes of the acoustic waves
- Generation of sufficient vibrational energy to keep the particles partly suspended and therefore leave more free ‘channels’ for solvent elution.
Acoustic filtration also called as ultrasonically assisted filtration is successfully used to increase the vacuum filtration of difficult mixtures to separate like coal slurry. But the main problem in filtration is deposition of solid materials on the surface of the filtration membrane. Ultrasound is useful in the filtration processes because it can increase the flux by breaking the concentration polarization and cake layer on the surface of the membrane without creating an effect on the permeability of the membrane. This method is mainly applied to extract the fruit juice and drinks from the pulp. From other studies it is clear that high-power acoustic or ultrasound is used to remove the cake which is accumulated in the filtration processes.
In conventional cooking methods either by frying or boiling the exterior of the food may be overcooked as compared to the interior. This may reduce the quality of the product. Ultrasound has the ability to provide improved heat transfer characteristics so there is no problem such as the conventional method. So this technology has been utilized in cooking.
So cooking by ultrasound leads to greater cooking speed. It also provides an energy efficient and rapid method which also improves the textural attributes of food. The post-cooking moisture content is also preserved by using this. The use of high-intensity ultrasound thus has the potential to increase the water-binding properties of meat. So ultrasound is useful in cooking of moist meats and hence is useful in the food industries for food processing. A patent describes a cooking vessel in which ultrasound is applied to a hot oil to provide better and more even overall frying and it is claimed to reduce energy consumption.
Advantages and Limitations of Ultrasonication
- Ultrasound applications offer numerous advantages in the food industry some of which are enlisted as follows:
- Ultrasonication in combination with other non-thermal methods is considered an effective means of microbial inactivation.
- Ultrasound waves are non-toxic, safe, and environmentally friendly..
- Ultrasound has gained huge applications in the food industry such as processing, extraction, emulsification, preservation, homogenization, etc.
- Ultrasonication involves lower running cost, ease of operation, and efficient power output.
- Ultrasonication does not need sophisticated machinery and a wide range of technologies.
- Ultrasonication involves minimum loss in flavor, superior consistency (viscosity, homogenization), and significant savings in energy expenditure.
- Use of ultrasound provides more yield and rate of extraction as compared to other conventional methods of extraction.
- Ultrasound application needs more input of energy which makes industrialists to think over while using this technique on a commercial scale.
- Ultrasonication leads to the formation of radicals as a result of critical temperature and pressure conditions that are responsible for changes in food compounds. The radicals (OH and H) produced in the medium deposit at the surfaceof the cavitation bubble that stimulates the radical chain reactions which involve formation of degradation products and thus lead to considerable quality defects in the product.
- Ultrasound induces physicochemical effects which may be responsible for quality impairment of food products by development of off-flavors, alterations in physical properties, and degradation of components.
- Ultrasound due to shear stress developed by swirls from the shock waves (mechanical effects) cause inactivation of the released products.
- Ultrasonic power is considered to be responsible for change in materials based on characteristics of medium. So, this power needs to be minimized in the food industry in order to achieve maximum results.
- Frequency of ultrasound waves can impose resistance to mass transfer.
Ultrasound being non-toxic and eco-friendly is an emerging technology which is considered as green technology as it saves a lot of energy and maximizes production. Ultrasound technique may become a new path to improve the nutritional quality of different foods, enhance the shelf life and at the same time minimize the effect on the product sensory properties. Ultrasonic efficiency can also be enhanced by combining with other non-thermal and thermal technologies. The technology is more effective in inhibition/killing of harmful enzymes and microorganisms without affecting the beneficial one. The technique may not be able to get 100 % success in replacing the traditional methods of food preservation, but they can certainly best complement or be integrated with the existing ones. A lot of research has been conducted on ultrasound technologies in food technology, but still a great deal of future research is necessary in order to produce industrial-automated ultrasound systems that will help in reduction of labor, cost, energy, and should ensure the maximum production of high value and safe food products.