An Update on Ultrasonics

When last we discussed ultrasonics in food processing (Food Technology, March 2007), cutting cheesecake and laboratory cell disruptions were typical applications. Some recent developments point toward a wider range of uses that include accelerating chemical reactions, improving extractions from fruits, and helping to clean equipment and vessels.

Darren Bates is involved with two Australian firms—Innovative Ultrasonics (phone +61 7 54570026, www.innovativeultrasonics.com) and Cavitus (phone +61 8 84259602, www.cavitus.com) —that offer proprietary technology, off-the-shelf ultrasonic generators, and process development. He describes how ultrasonic energy uses the phenomenon of cavitation to create high temperatures and pressures in a liquid medium.

Cavitation occurs when the passage of sound waves creates very small bubbles, as the low pressure "valley" of the sinusoidal wave is temporarily below the vapor pressure of the fluid. Then, as the sound wave pressure increases, the bubbles collapse. In addition to high temperature and pressure (5,000 K and 1,000 atmospheres), the creation and collapse of these bubbles causes high local shear, as well. The combination of temperature, pressure, and shear can accelerate chemical reactions, mass transfer, and heat transfer and change the properties of fluids by affecting viscosity.

Ultrasound is typically divided into three regions of frequency, all above the threshold of hearing for humans. Power ultrasound is 16–100 kiloHertz (1 Hertz is 1 cycle/sec). High-frequency ultrasound is 100 kHz– 1 MHz. Diagnostic ultrasound is 1–10 MHz.

Ultrasound is generated with either piezoelectric or magnetostrictive transducers, which are tunable devices that create high-energy vibrations. (Piezoelectric devices convert electrical signals into physical vibrations of a certain type of crystal. Magnetostrictive devices convert magnetic fields into physical vibrations of a different type of material.) These vibrations are amplified and transferred to a sonotrode or probe, often fabricated from titanium, which is in direct contact with the fluid. Sound waves are attenuated quickly in air or gas, compared with their passage in fluids. Ultrasound directed through air at foam can cause the foam to deflate, improving liquid filling operations of foaming fluids such as beer and soft drinks, but most other applications occur in fluids.

Ultrasound is characterized by energy and intensity. The energy is expressed as kwh/L (kilowatt hours per liter). Typically, the more energy, the greater the effect. Intensity is expressed as power per surface area of sonotrode, W/cm² (watts per square centimeter).

According to Bates, some representative return on investment payback figures for scale-up systems using either single or multiple 16kW or 10kW units are between 1 and 3 years. The equipment includes generator, transducer, flow cell, sonotrode, booster, and a cabinet.

Increasing pressure can increase the intensity of a process, while changing temperature has a more-complex effect, such that there usually is an optimum temperature for maximum effect—cleaning, reaction-rate increase, or emulsification. Increasing temperature reduces viscosity, allowing for more-violent collapse of bubbles, but increased temperature also increases vapor pressure, somewhat cushioning the impact.

Ultrasound Applications
According to Bates, some known applications of high-power ultrasound in food processing include the following: extraction (release of plant material), emulsification/ homogenization, crystallization (formation of smaller ice crystals in freezing), filtration/screening (increased flux due to disturbance of boundary layer), separation (agglomeration of small particles in presence of standing waves), viscosity alteration (can increase or decrease viscosity of purees), defoaming, and extrusion (vibration of die to increase flow rate).

Other manufacturers have claimed increased chemical reaction rates using ultrasound in such processes as transesterification of triglycerides to make biodiesel.

While chemical reactions are not common in food processing, there surely must be occasions where ultrasonics could be helpful. Some unproven, but promising applications include enzyme and microbial inactivation, fermentation, heat transfer, and cleaning.

Ultrasound inactivates enzymes and bacteria, especially at somewhat elevated temperatures, by breaking cell membranes due to the violence of cavitation. On the other hand, low-intensity ultrasound improved the fermentation rate of beverages, probably by improving mass transfer and by promoting the removal of carbon dioxide, which can inhibit fermentation. In heat transfer, the vibrations of ultrasound break up the insulating boundary layer and thus reduce the resistance to heat transfer at surfaces.

Bates points out that ultrasound is relatively inexpensive to apply because the equipment is energy-efficient, has no moving parts (and hence is low-maintenance), and lends itself to retrofitting an existing process. The sonotrode is a wear part and lasts 3–24 months, depending on the conditions and the intensity of the energy applied. Titanium is used because it has low mass and high rigidity, so less energy is lost deforming the tool and instead is transmitted to the fluid. Financial benefits can be high compared to the costs of the investment, leading to apparent payback times of months.

Some of the previously mentioned benefits of ultrasound apply to winemaking. These include the observed improvement of fermentation, the improved extraction of color and flavor from grapes, and the removal of excess gas. In addition, Bates and his colleagues have identified some relatively unique applications.

Significant spoilage microbes for wine are Brettanmyces/Dekkera yeasts, which can produce compounds with potent off-flavors and odors. These spoilage microbes grow from small numbers during maturation of wine in oak barrels. Oak barrels, by their nature, are difficult to clean and sanitize.

After the grapes themselves, barrels are the highest-cost element in winemaking. There are more than 7 million barrels in wineries around the world. Barrels used for aging red wine can be re-used at least once, but—for a variety of reasons—are usually discarded after about 4 years.

Used barrels become contaminated with spoilage microbes, such as Lactobacillus and Brettanmyces/Dekkera, which are capable of infiltrating the wood to a depth of 8–10 mm, the same depth to which the wine can penetrate. This makes the common practice of scraping the surface to expose fresh wood ineffective at disinfection.

Further complicating the challenge is the precipitation of tartrates as wine matures. The tartrates are difficult to re-dissolve and also block the pores of the wood. During aging, it is essential that a small amount of oxygen diffuse through the barrel into the wine while some alcohol evaporates and diffuses out from the wine. (The loss of alcohol is greater from more-potent spirits, such as brandy and whiskey.)

Some common approaches to barrel cleaning and sanitation include the following: low/high-pressure cold and hot water, chemicals (caustic, citric acid, sulfur dioxide, and ozone), shaving the wood, dry ice blasting, and microwaving.

None of the common practices are completely effective. Steam can disinfect barrels, but only for about 2 mm into the wood. Many chemicals require 24–48 hours of contact time to be effective. Sulfur dioxide is often added to a closed barrel, if it is not to be filled immediately, and this does reduce contaminating yeasts, but those microbes that are protected in the wood are not affected. (Sulfur dioxide was found to have another beneficial effect, however. It reacts with compounds in the wood and wood char—barrels are normally lightly charred before use—to form desirable flavor compounds that are extracted into the wine as it ages.)

Ozone appears to be effective, but requires good cleaning, and some winemakers are concerned that reaction products of ozone with wood may contribute off-flavors.

Shaving and re-charring can only be done a few times for 3–5 mm and does not completely sterilize the barrel. Dry ice blasting can remove surface contamination but does not affect embedded yeasts. A French company uses high-pressure water, alkaline cleaners, acidified solutions, and microwaving to clean and disinfect barrels for the secondary market.

Bates and his colleagues studied the effect of ultrasound on staves from used barrels and were able to demonstrate that a few minutes of sonication in water with 400 W or 1 kW removed tartrate deposits. In separate studies, they demonstrated that sonication of suspensions of Brettanomyces/Dekkera killed up to 99.9% of the cells.

At this point, Cavitus is launching its patented ultrasonic barrel cleaning and disinfection system into the wine industry following 12 months of in-house development and independent pilot trials. The Cavitus system is a fully automated cleaning system designed to retrofit existing wine cleaning operations. Used wine barrels would be filled with water at a predetermined optimum temperature, a sonotrode inserted through a bung hole, sonic energy applied for a few minutes, the barrel emptied, maybe filled with sulfur dioxide, and sealed until needed. With this approach, tartrates are removed, and Brett organisms are eliminated on the surface and in the pores of the oak barrel.

Cavitus is also launching its proprietary ultrasonic defoaming system into the food and beverage industry for controlling or eliminating foam on bottling and packaging lines for carbonated and noncarbonated products, filler bowls, fermentation tanks, or general food processing vessels where foam is a problem. The system allows for improvements in bottling/packaging line speed, reduced waste, reduced mold and microbial contamination at the cap, reduced anti-foaming chemical costs, and greater capacity/production volume in vessels.

by J. Peter Clark,
Contributing Editor,
Consultant to the Process Industries, Oak Park, Ill.
[email protected]