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Everyday science

Everyday science

Sound waves deliver a faster pint

01 Jan 2001

Good things come to those who wait, according to one beer commercial. However, brewers - and pharmaceutical companies - are now turning to ultrasound to speed up their production processes, as Valerie Jamieson discovers.

Cool crystals

With millions of litres of beer and lager sold every day, brewing is big business. One of the main aims of the industry is to increase production by reducing the time needed to turn the barley, hops, yeast and water into a refreshing pint. Now a team of physicists and engineers at EA Technology in Chester and UMIST in Manchester, both in the UK, has teamed up with a major brewer to investigate how ultrasound could slash days off the “de-gassing” process – the most time-consuming part of the brewing cycle.

Currently, it takes anything from five to seven days to produce the perfect pint from scratch. For most of that time, the beer is stored in large vessels while much of the carbon dioxide produced during fermentation is removed. To do this, brewers typically pass tiny bubbles of nitrogen gas through the liquid, which displaces the heavier carbon dioxide – a process that can take several days. Now Chris Ellwood at EA Technology together with Alexandra Clark and Peter Payne of UMIST are developing an ultrasonic “whistle” that will help beer manufacturers to de-gas their ales and lagers almost instantly.

The stainless steel whistle is 30 cm long and has a nozzle through which carbonated liquid is forced. The whistle also has a reed that vibrates to produce sound waves at ultrasonic frequencies. As the sound wave compresses and rarefies, gas that is dissolved in the water diffuses into the micron-sized carbon-dioxide bubbles, causing them to expand. These large bubbles then simply float to the top of the liquid.

Wet your whistle

It sounds simple, but the device must be carefully designed so that the vortices that are shed by the fluid as it flows through the nozzle excite the reed at ultrasonic frequencies. Moreover, for the gas to diffuse into the bubbles, the intensity of the resulting sound wave must be above a certain threshold that depends on the bubble size and the frequency. If the intensity is too high, then a process called transient cavitation – best known for causing damage to the propellers of ships – can occur. In this case, the sudden growth and collapse of bubbles when they reach regions of high pressure leads to extreme pressures that could corrode the metal surfaces of the whistle and the vessel holding the liquid.

In preliminary tests with carbonated water, Clark has shown that the whistle can remove up to 40% of the carbon dioxide as 110 litres of liquid rushes through the whistle every minute. Further improvements are expected when the design of the nozzle and reed is optimized. The results were presented at the Institute of Physics Physical Acoustics Group conference at the end of October last year.

The whistle developed by EA Technology is also attracting the attention of soft-drinks manufacturers. Currently, the amount of carbon dioxide in fizzy drinks can be controlled by cooling the pipes and other components in the processing stream. Ellwood predicts that the ultrasonic whistle could eliminate the need for costly refrigeration equipment that is also expensive to run.

Crystal growth

A more sober application of ultrasound is in the pharmaceutical industry, where sound waves are improving the quality of crystals produced from solution. Crystallization is a complex affair with few ways of controlling the process on an industrial scale. However, Peter Cains and co-workers at AEA Technology in Harwell, UK, have developed an ultrasonic technique that can speed up crystallization and allow manufacturers to tailor the size of their crystals (see www.aeat.co.uk/sono/index.html).

The size and shape of crystals affects the rate at which they dissolve – an important factor in the pharmaceutical industry. In general, crystals are made by dissolving increasing amounts of a solid compound in a liquid until the solution is saturated. The solution is then cooled slowly until the first few atoms arrange themselves in an orderly crystalline pattern. The crystals grow as more atoms gather at these so-called nucleation sites. With a quick and powerful blast of ultrasound, however, the first crystals can form at higher temperatures, thereby speeding up the process.

Alternatively, manufacturers can forgo speed and initiate crystal growth in much less concentrated solutions using ultrasound. In this case, Cains and co-workers have demonstrated that smaller numbers of larger and purer crystals can be produced (see figure). They have also found that additional ultrasound pulses generate secondary nucleation, which leads to smaller, more uniform crystals that are ideal for certain pharmaceuticals that are later dissolved in water.

The ultrasound causes bubbles a few tens of microns wide to form in the liquid, which then expand and collapse violently. These collapsing bubbles act as sites where new crystals can form and grow. But no one really understands what makes such cavitation initiate crystal growth. It could be the pressure changes inside the liquid or the extremely high temperatures that are reached inside the bubble when it is compressed (see Physics World May 1998 pp38-42).

Cains and co-workers are now investigating compounds that are difficult to crystallize by conventional methods, such as sugars and proteins, and the food industry starting to take an interest in the work. Chocolate manufacturers, for example, have previously used sound waves to control the size and type of crystals formed as the molten confection solidifies.

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