Utilization of sweet sorghum: production of bio-ethanol from nonedible part and evaluation of the juice crystallization potential

Abstract

ABSTRACT The availability of cheaper sources of energy is a key driver of any economic development, more so for a developing country like Kenya. The vision of the Kenya Government Biofuel Policy is to increase access to energy through sustainable biofuel production, and reduce dependence on fossil fuels by 25% in volume by the year 2030. The objective of this research was to characterize sweet sorghum cultivars which could be used to produce crystal sugar from its juice and bio-ethanol from the bagasse. The bio-ethanol could then be blended with gasoline and used in the transport sector. This blending will serve to reduce the import bill of Kenya‟s fossil fuel while at the same time mitigating the environmental change. Furthermore, since the juice can be crystallized to produce crystal sugar, this will meet Kenya‟s annual sugar deficit of 200,000 tons. Sixteen sweet sorghum varieties namely: Madhura, Dale, Wiley, Brandes, Theis, Rema, Ramanda, Rio, CMSXS636, CMSXS633, CMSXS644, SPV1411, IESV91018LT, IESV92008DL, IESV92038/2SH, IESV93042SH, were planted at the JKUAT experimental farm. After 16 weeks, 4 stalks of each variety were cut, seeds, pinnacle and leaves removed. The juice was expressed from the stalks using a sugarcane presser. The Brix was measured for each variety and glucose, fructose and sucrose content were analyzed using HPLC. Rio cultivar with the highest apparent purity (AP) of 83.91 was then chosen for crystal sugar and bio-ethanol production. Rio variety was then harvested and juice extracted from the stalks, treated, concentrated and then crystallized by cooling. The Rio sweet sorghum bagasse was comminuted after drying in the green house and subjected to alkaline peroxide and phosphoric acid pretreatments. This was then subjected to enzymatic hydrolysis using cellulase produced by Trichoderma reesei, after which the hydrolysate was fermented using the baker‟s yeast.The 0 Brix for the juices ranged from 15.1 – 21.5, Sucrose content, 6.05 - 72.77 g/L, Glucose content, 2.65 - 16.42 g/L, Fructose content, 2.66 – 12.53 g/L and AP, 33.89 – 83.91%. The Rio syrup did not crystallize into raw crystal sugar. The yield for enzymatic hydrolysis was 50%, 78% and 88% for the untreated bagasse, H acid pretreated and alkaline peroxide pretreated bagasse, respectively. The bioethanol yield was approximately 60%, 40% and 15%, respectively, for acid pretreated, alkaline pretreated and untreated bagasse. Due to the lower purity(ratio of the %wt. of sucrose to the %wt. of soluble) of the sugar extracted from sweet sorghum (about 75 AP) compared to that of sugar cane or sugar beet (80-85 AP), it may require further technological input to produce white sugar from sweet sorghum. Thus, the more likely markets for sorghum sugar can be syrup for local foodstuffs or as raw material for the food industry. According to the study, the following sweet sorghum cultivars namely; Rio, CMSXS636, IESV91018LT, IESV93042SH and SPV1411 could have the potential to be used in raw sugar production. Bio-ethanol can also be obtained from the Sweet Sorghum bagasse after pretreatment and saccharification. This work therefore provides a complimentary source of sugary products and bio-fuel.