Optimisation of microwave-assisted production of acid condensate from palm kernel shell and its biological activities

Abstract

This study reports on the microwave-assisted heating optimisation of acid condensate (AC) from palm kernel shell (PKS), using the central composite design (CCD) approach focusing on its total phenolic content (TPC) as response and its antimicrobial activity. Thermogravimetric-derivative thermogravimetric (TG-DTG) analysis clearly depicted the devolatilisation of lignocellulosic content of PKS. The highest TPC in concentrated AC extract (CACE), 451.51 ± 2.37 µg GAE/mg (R2 0.9870), was obtained at microwave power of 580 W, nitrogen flow rate of 2.4 L/min and final temperature of 480 ºC. Nitrogen flow rate had the highest effect on TPC with an F value of 63.65. Relative to ascorbic acid, CACE showed a higher Trolox equivalent antioxidant capacity (TEAC) but almost similar 2,2-diphenyl-1-picrylhydrazyl (DPPH) radical scavenging capabilities which can be attributed to the presence of 1,2-benzendiol, i.e. catechol (27.82%) and 1,3-dimethoxy-2-hydroxybenzene, i.e. syringol (22.76%). CACE also displayed good potential for antimicrobial application with high growth inhibition of Escherichia coli, Pseudomonas aeruginosa, Staphylococcus aureus, Enterococcus faecalis, Aspergillus niger and Fusarium oxysporum. In conclusion, PKS has a great potential to be used as raw material to produce AC (acid condensate) using microwave-assisted heating process.