Effects of dilute acid and alkaline pretreatments on enzymatic saccharification of palm tree trunk waste for bioethanol production

Abstract

Increasing energy demands, rising crude oil prices, and security issues regarding oil supply are key motivators for research into the development of alternative energy sources. The sugar palm tree Arenga pinnata is a nonfood plant that is abundant in Indonesia and produces a lignocellulosic biomass with high cellulose contents that can be used to produce bioethanol. However, because lignins in A. Pinnata biomass inhibit saccharification enzymes, pretreatments that degrade lignins are required to increase quantities of cellulose that are available for conversion to ethanol. We investigated the effects of pretreatments using nitric acid (HNO3) or ammonium hydroxide (NH4OH) at 2%, 4%, 8%, and 10% (v/v) on reducing sugar and ethanol contents and compared with the effects of steam pretreatment. Pretreated samples were hydrolyzed using cellulase enzymes at pH 5.0 with a substrate concentration of 10% (w/v) for 24, 48, and 72 h at 50 °C. Subsequent assessments of enzymatic saccharification following pretreatment with 10% (v/v) HNO3 showed maximum reducing sugar and total sugar contents in palm tree trunk waste of 5.320% and 5.834%, respectively, after 72 h of saccharification. Following pretreatment with 10% (v/v) of NH4OH, maximum reducing sugar and total sugar contents of palm tree trunk waste were 2.892% and 3.556%, respectively, after 72 h of saccharification. In comparison, steam pretreatments gave maximum reducing sugar and total sugar contents of 1.140% and 1.315% under the same conditions. These data show that HNO3 pretreatment produces greater yields than NH4OH pretreatments. Simultaneous saccharification and fermentation (SSF) was conducted at 37 °C (pH 4.8) and 100 rpm for 120 h using 10% (v/v) Saccharomyces cerevisiae and cellulase enzyme with a substrate concentration of 10% (w/v). Our results show that the highest ethanol content can be produced with a high concentration of acid or alkaline solutions. The highest ethanol content was 2.648% and was achieved using the 10% (v/v) HNO3. Use of 10% (v/v) NH4OH led to a yield of only 0.869% ethanol and steam pretreatment produced 0.102% ethanol. This demonstrates palm tree trunk waste is the potential feedstock for ethanol production.