Ultrasound pretreatment of cassava chip slurry to enhance sugar release for subsequent ethanol production

Abstract

The use of ultrasound pretreatment to enhance liquefaction and saccharification of cassava chips was investigated. Cassava chip slurry samples were subjected to sonication for 10-40 s at three power levels of low (2 W/mL), medium (5 W/mL), and high (8 W/mL). The samples were simultaneously exposed to enzymes to convert starch into glucose. The cassava particle size declined nearly 40-fold following ultrasonic pretreatment at high power input. Scanning electron micrographs of both unsonicated (control) and sonicated samples showed disruption of fibrous material in cassava chips but did not affect the granular structure of starch. Reducing sugar release improved in direct proportion to the power input and sonication time. The reducing sugar increase was as much as 180% with respect to the control groups. The slurry samples with enzyme addition during sonication resulted in better reducing sugar release than the samples with enzyme addition after sonication. The heat generated during sonication below starch gelatinization temperature apparently had no effect on the reducing sugar release. The reducing sugar yield and energy efficiency of ultrasound pretreated samples increased with total solids (TS) contents. The highest reducing sugar yield of 22 g/100 g of sample and efficiency of 323% were obtained for cassava slurry with 25% TS at high power. The reducing sugar yield at the completion of reaction (R(infinity)) were over twofold higher compared to the control groups. The integration of ultrasound into a cassava-based ethanol plant may significantly improve the overall ethanol yield.

Ultrasound irradiation in the production of ethanol from biomass

“…The ultrasonic treatment enhances disintegration of starch granules, leading to an acceleration of starch hydrolysis due to exposing a much larger surface area to enzymes, increasing the release of fermentable sugars and thereby increasing ethanol productivity [62,77]. Nearly complete disintegration of corn starch and cassava cells was observed with large numbers of fragmented cell materials by high-power ultrasonic treatment for 40 s [11,62,78].…”

“…The reduction in particle size and opening up of starch fibrous structure can decrease the dose of enzymes needed, shorten the conversion time of starch to ethanol, improve the efficiency of the starch hydrolysis, increase the overall sugar yield, and even eliminate some of the unit processes, which ultimately results in an overall improvement in ethanol yield and reduction in cost [78]. Reduction in starch particle size can also enhance mass transfer and lead to an increase in enzyme activity [62].…”

“…Several studies have investigated the effects of ultrasound on the particle sizes of starches [62,77,78,83,85]. The hydrodynamic shear force introduced by ultrasound irradiation causes the disintegration of starch particles into a slurry into finer particles, significantly enhancing the surface area for enzyme activity [85a].…”

“…Particle size reduction of rice grains resulted from ultrasonic treatment caused in shorter cooking times and faster gelatinization [86]. Ultrasonic treatment reduced cassava particle size nearly 40-fold at an output power as high as 8 W/mL [78]. The ultrasonic treatment not only contributes to the particle size reduction of starches in solutions, but also heat generated assists starch gelatinization, which is required in enzymatic starch hydrolysis [85a].…”

“…The destruction of starch structures might result in release of more individual starch granules to the aqueous phase, which enhances the enzymatic hydrolysis [78]. Structural changes of starch granules pretreated after the liquefaction is started were more than that before liquefaction.…”