Biomedical Engineering Reference
with conventional heat transfer equipment is slow and uneven, and temperature gradients
can easily result in overheating and product decomposition. Pressurized microwave reactors
have been designed for laboratory use and numerous publications describe the use of
microwave heating to prepare starch derivatives, such as esters and ethers, reaction products
of starch with urea, thiourea and thiosemicarbazide, starch phosphates, and starch graft
copolymers. It is predicted that future years will see an increase in the use of microwave
heating to prepare new starch derivatives.
2.5 PROCESSES USING ULTRASOUND
The effect of ultrasonic irradiation on liquids, and on liquid dispersions of suspended
solids, has been reviewed by Suslick and co-workers (Suslick, 1990; Suslick and Flanagan,
2009). Review articles have also been published on the effects of ultrasound on polymers
in solution (Basedow and Ebert, 1977) and on carbohydrates (Kardos and Luche, 2001).
The effects of passing ultrasound through a liquid are the result of acoustic cavitation,
which is the formation of micrometer-sized cavities or bubbles in the liquid followed by the
growth of these cavities, and their subsequent collapse by implosion. When the cavities
collapse, small areas of intense local heating and high pressure are generated, followed by
very high rates of cooling. Cavities are formed in the liquid because the passage of
ultrasound through the liquid medium exerts areas of negative pressure in the liquid; when
this negative pressure exceeds the tensile strength of the liquid, molecular separation
occurs, creating cavities or bubbles. Cavity formation often results from nucleation in areas
of the liquid having reduced tensile strength, for example, in gas-filled crevices of
suspended particles, or in micro-bubbles of gas already present in the liquid medium.
When the cavities reach a certain size, implosion of the cavities occurs. It is the implosion
process that generates the small areas of intense thermal energy and pressure in the
otherwise cold liquid.
Cavitation occurs in a different manner near a solid-liquid interface. Whereas the cavities
formed in pure liquids remain spherical during collapse, cavity collapse occurs asymmetrically
near a solid surface and high-speed jets of liquid are generated during the implosion process
that can damage and degrade the surface of the solid. The solid must be several times larger
than the collapsing cavity for this process to occur; this phenomenon is therefore not observed
in the presence of very small particles. When small particles are present, however, the shock
waves generated by cavity collapse can cause interparticle collisions, resulting in damage of
the suspended particles. High-intensity probes (50-500 W/cm 2 ), as well as lower-intensity
units, such as ultrasonic cleaning baths, are commercially available for laboratory use. Earlier
literature on the modification of starch by ultrasonic irradiation has been reviewed by Tomasik
and Zaranyika ( 1995 ).
The effect of ultrasound at frequencies of 280 and 960 kHz on potato starch granules was
examined under a wide range of treatment conditions by Gallant and co-workers (1972) and
Degrois and co-workers (1974). Potato starch was dispersed in water, and treatments were
carried out under vacuum and in atmospheres of air, oxygen, hydrogen and carbon dioxide.
Light microscopy and SEM were used to determine the effects of these ultrasonic treatments
on starch granule structure. Under vacuum or under an atmosphere of carbon dioxide,
ultrasonic irradiation produced little or no effect on the morphology of the potato starch
granules. However, under an atmosphere of hydrogen, deep pits in the granules were
observed. In oxygen or air, the pits were not as deep and more surface damage was observed.