Conversion of Sorrel (Hibiscus Sabdariffa) Calyces To Glucose

The utilization of agrowaste for the production of bioethanol has attracted worldwide attention as a strategy for reducing global warming and improving global energy security. Sorrel drink, popularly known as “zoborodo” in Nigeria is a non-alcoholic drink, which is obtained from the calyces of Hibiscus sabdariffa, by solid-liquid extraction process, leaving the calyces pulp as the raffinate. The calyces obtained after extraction being rich in carbohydrate, is used in this work as raw material for glucose production. The rate of glucose production and its concentration using hydrochloric acid was found to increase with increase in temperature, acid concentration and reaction contact time and reduction in calyces’ particle sizes. The conversion after 120 minutes was found to be 25% at 60C with 6% w/v acid strength. The results suggest that acid strength greater than 6%w/v would be more effective from 60C and above. The finding also shows a shift in the influence of mass transfer control to reaction control mechanism, with increase in temperature. It is established that under favorable condition sorrel calyces could serve as alternative source of energy and raw material for food and pharmaceutical industries. Keyword: zoborodo, acid hydrolysis, Sorrel calyx, cellulose, bioethanol, glucose INTRODUCTION The calyx of Hibiscus sabdariffa is widely used by humans, as food, jams, jellies, juice drinks, wine and as medicinal syrups (Akanya et al, 1997; Daltziel and Burkill, 1985). The solid residue obtained after leaching of the calyces constitutes solid waste. This solid waste could increase soil acidity owing to its characteristic low pH (Olawale and Ajayi, 2008). Additionally, due to its high organic material content and high biogradability (Babalola et al, 2005) it could constitute a serious environmental problem, if not properly dispose, as is the current practice in Nigeria. The composition of sorrel calyces reported by Samy (1987), Duke(1983) and www.hort.purdue.edu (2007), suggest that this waste could serve as source of glucose (a major source of energy), food and/or raw material for the production of many organic chemicals. Conversion of abundant cellulosic biomass to useable product presents an important opportunity to improve energy security, reduce the trade deficit, reduce green house gas emission, and improve price stability (Wyman, 1999), while reducing waste arising from agricultural produce. The cellulose and hemicellulose content in any biomass can be hydrolyzed chemically or enzymatically (Karimi et al, 2006). Cellulose normally has strongly resistance to attack both by enzymes and chemical agents and various workers have attempted to hydrolyse cellulose using enzymatic and/or chemical hydrolysis under mild or harsh conditions. Enzymatic hydrolysis of cellulose is a slow process and is not likely to be applied in industrial practice in the immediate future (Gan et al, 2003; Sun et al, 2009). In the meantime, Chosdu and co-workers (1993) upgraded cellulosic waste material via radiation and chemical pretreatment, while Gregg and Sadder (1996) investigated factors affecting cellulose enzymatic hydrolysis. Imai and others (2004) employed ultrasonic approach to enhance hydrolysis of cellulose. Yet enzymatic hydrolysis with/without steam explosion have also been employed in conversion of ethanol, from cellulosic materials, to glucose (Highina et al, 2006; Grous et al, 1986). Acid hydrolysis of cellulose is a relatively faster process with higher yield and the conversion is usually strongly affected by acid concentration, temperature and reaction time (Liao et al, 2006; Iranmanhboob et al, 2002; Choi and Matthew, 1996). These types of application commonly utilize either concentrated acid at low temperature or dilute acid at high temperature (Liao et al, 2006; Sun and Cheng, 2002). Grahmann et al, (1985) reported that concentrated acid hydrolysis is much more effective than dilute acid hydrolysis, while Herrera et al, (2004) reported on use of hydrochloric acid for sorghum straw hydrolysis. Despite, its effectiveness, concentrated acid hydrolysis major drawback is the associated serious environmental concerns (Sun et al, 2009). Interestingly though, various authors have adopted means of minimizing this negative effect through neutralization (Alves et al, 2002). However, the economics of such process viz-a-viz product yield makes it a less profitable approach. Different lignocellulosic materials exhibit varying hydrolytic behaviors due to differences in composition and other properties. Thus each material needs to be studied to establish its hydrolysis parameter. No related work has been reported on the production of glucose from sorrel calyces, using either acid (dilute or concentrated) or enzymatic hydrolysis route. This paper presents results of investigation carried out on production of glucose from sorrel calyces, using dilute hydrochloric acid, under different operating conditions.