What is ultrasound?
Before digging into ultrasound, let’s begin with the basics, shall we? The sound is an oscillation of particles in an elastic medium. These oscillations can transmit their movements to neighboring particles which, in turn, transfer their movements to others. The latter phenomenon causes a local pressure variation. The difference between sound and ultrasound is that ultrasound consists of mechanical waves that propagate in a medium through the transfer of energy and not particles. By definition, ultrasound is a sound wave at a frequency that exceeds the hearing limit of the human ear (20 kHz). Additional ultrasound classifications refer to their frequency range. These are known as 1) the diagnostic ultrasound (1-10 MHz), 2) high-frequency ultrasound (100 kHz – l MHz), and 3) power ultrasound (20–100 kHz).
During the past decade, the unique properties of ultrasound have attracted the attention of the food industry as it has excellent potential. Food companies have used ultrasound in various processes, including filtration, extraction, inactivation of microorganisms and enzymes, dehydration, homogenization, and fermentation. However, the main focus of ultrasound treatment is on the removal of contaminants from foods. The safety of different foods is heavily affected by chemical contaminants such as mycotoxins, heavy metals, pesticides, and allergens. Exposure to these contaminants can range from poisoning to more challenging situations like anaphylactic shock in a severe allergic reaction. Currently, there are available conventional processing technologies for the removal of contaminants such as thermal processing, washing with chemical agents, cooking, peeling, and chemical oxidants, but it’s not possible to always obtain a satisfactory result using these techniques. These methods cannot always remove chemical contaminants from foods altogether. Additionally, some of these methods can affect both the sensory quality of foods and their nutritional value as they cause changes in taste, texture, color and result in loss of dietary components.
For that reason, food scientists have looked for novel food processing technologies to remove chemical contaminants from foods. The application of ultrasound treatment is currently at its initial evaluation steps, though the initial results indicate a very promising, effective, and high-potential technology. There are still some challenges regarding this technology, but technological advancements will improve its efficiency in the future. Let’s examine below how ultrasound can reduce chemical contaminants from foods.
Which food contaminants can ultrasound eliminate?
- Pesticides. Fruits and vegetables are the food sources containing the highest amount of pesticides compared to other food categories. There are available traditional methods for removing pesticides from foods, such as cooking, salting, washing with chemical agents, and peeling, but these methods have certain limitations. For example, thermal heating for removing pesticides may result in the loss of nutrients like vitamins. On the other hand, reducing the pesticide content with water washing may not be very effective, while chemical agents may result in the production of toxic by-products. Ultrasound can reduce insecticides and pesticides contents in strawberries, with a removal rate of 45.6% to 91.2%. The latter rate is higher than the one obtained with tap water washing (i.e., 19.8% to 68.1%). Additionally, ultrasound washing reported better results for removing fungicides from apples compared to boiling at 100 ◦C and tap water washing. However, the efficiency of ultrasound for removing pesticides depends on several factors such as pH, temperature, solubility, frequency, intensity, and volatility. Additionally, ultrasound can degrade pesticides in some liquid foods as the production of free radicals in the liquid medium can oxidize and hydrolyze pesticides.
- Allergens. Allergens are, in most cases, a harmless substance -usually a protein- capable of triggering an immune response. Ultrasound can disrupt the tertiary and quaternary structures of casein and impair the function of whey protein. Other studies reported that ultrasound effectively reduced peanut, soy, and shrimp allergens by disrupting the structures of allergenic proteins. Its activity also is further enhanced when used in combination with heat treatment. Nevertheless, there are some adverse effects when using ultrasound for the removal of allergens from foods. In particular, ultrasound is not adequate for removing milk allergens as parameters such as treatment duration, intensity, and frequency need to be further optimized.
- Heavy metals. Heavy metal accumulation in foods is a widespread food safety issue. Heavy metals can impose severe health threats on human health and life, and decontamination from foods is necessary. The effects of ultrasound on heavy metal decontamination from foods need further investigation, but several studies have reported positive results. Ultrasound might reduce the levels of arsenic present in certain mushroom species and increase the rate of cadmium release from crabs. Furthermore, ultrasound can reduce the size of proteins and fat globules in milk, which can help decontaminate milk from heavy metals bound to them.
- Mycotoxins. Mycotoxins are toxins produced by certain fungal species capable of causing disease and death in humans and other animals. Mycotoxin-producing fungal species can grow on various crops and foodstuffs like cereals, spices, nuts, dried fruits, and coffee beans. Their adverse health effects can range from acute poisoning to immune deficiency and cancer, and for that reason, they represent a significant food safety issue. Ultrasound can result in the degradation of aflatoxin by causing molecular changes in the molecular structure of the toxin. Different studies indicated that the effect of ultrasound and thermal processing has about the same reduction rate. Nevertheless, the advantage of ultrasound is that it does not generate harmful thermal processing products like acrylamide. Ultrasound can generate⋅H, ⋅OH, and H2O2, which lead to aflatoxin hydrolysis and oxidation. The results are very promising. However, more studies are needed to assess and analyze the elimination mechanism of mycotoxins using ultrasound.
Impact on food quality
Ultrasound treatment has many potentials besides food product decontamination from allergens, mycotoxins, heavy metals, and pesticides. As with all food processing methods, ultrasound has an impact on food quality which in some cases it can be positive, while in other cases, it can negatively impact the quality of foods. Parameters such as texture, color, flavor, and overall nutritional quality need to be carefully evaluated. Researchers tested the impact of this technique on various foodstuffs and gathered information on its effects on food quality.
- Flavor. Flavor is a crucial parameter in food quality as it affects the overall acceptance of consumers. It includes taste (volatile compounds) and aroma (non-volatile compounds), which can be affected by many factors like high temperature, for example. As ultrasound is a non-thermal processing method, it can prevent the loss of volatile compounds in foods under appropriate conditions. It can successfully maintain the aromatic compounds of bayberry juice, and the research indicated that it has a higher retention rate of aroma components in juices than thermal pasteurization. Ultrasound can also increase the rate of protein and fat oxidation in meat products resulting in the improvement of meat product flavor.
- Texture. Texture is a critical sensory feature of foods and an essential parameter of food quality. Ultrasonic treatment can affect many properties of food texture, such as viscosity, transparency, structure, and thermal properties. This treatment can reduce meat product chewing force and shear force and significantly affect tenderness since it impacts meat myofibrillar protein structures and fragments collagen macromolecules. However, high intensity and long-time ultrasound treatment can hurt meat texture and generate a tougher meat texture.
- Color. Food color can be affected by ultrasound treatment. It can cause both enzymatic browning when used continuously. Free radicals during treatment may degrade the pigments in fruit juice. The color change cause by ultrasound can be either positive or negative. For instance, a color change in avocado is a negative outcome, whereas the color enhancement of blueberry and wine adds value to these products. There were cases also like in mango juice where it was not observed any color change. Therefore, ultrasound is not suitable for foods for which a color change would decrease their quality.
- Bioactive compounds. Bioactive compounds can be affected by ultrasound as ultrasonic treatment can disrupt the cell membrane and lead to the release of bioactive molecules; hence the availability of these substances increases. Nonetheless, as this treatment can increase the temperature locally and produce free radicals, it can increase the oxidation rate of vitamins, phenols, and carotenoids, which leads to their degradation. However, some studies revealed that ultrasound could increase the retention rate of bioactive compounds during drying. Consequently, it may degrade some bioactive compounds in some cases, but in other cases, it can effectively retain bioactive substances in food.
Ultrasound technology can provide impressive results for decontaminating foods from various food contaminants. Despite the promising results, there are still some issues food scientists need to solve before its commercial application. One of the most critical issues is that currently, there is no appropriate equipment available for large industrial applications. Sophisticated equipment is needed to meet industrial demands and requirements. Moreover, as ultrasound can serve as an alternative to thermal processing technologies, food engineers need to design a system that will effectively eliminate food contaminants carefully. They will also need to optimize parameters such as frequency, power output, temperature, pH, properties of contaminants, and treatment time. Another critical issue is that this technique may cause adverse effects on certain food products. Research studies are currently investigating how to minimize vitamin and phenolic compound degradation and the loss of anthocyanin and color changes. A thorough assessment of ultrasound-treated food products will help understand whether contaminant removal produces other toxic by-products. Toxicological assessments and evaluations on a vast range of ultrasound-treated food products will help understand the latter issue.
Ultrasound is an up-and-coming technology with high potential as a new food processing method. It is an environmentally friendly method since it consumes less energy than conventional food processing techniques (e.g., thermal processing). It can improve efficiency and reduce the cost for the industry. Tests on a variety of food products indicate a high efficiency in removing food contaminants. Another positive effect of this technique is that it can synergize with other food processing techniques and improve their results. Future studies will reveal how to adapt ultrasound processing systems depending on the characteristics of specific food products. Solving this last issue will contribute to the success of ultrasound food processing in food decontamination.