Selecting the right blood glucose monitor for the determination of glucose during the enzymatic hydrolysis of corncob pretreated with different methods
Highlights
► Ionic liquid, alkaline and dilute acid pretreatments were applied to corncob. ► Blood glucose monitors were used to determine glucose in the enzymatic hydrolyzates. ► Optium Xceed was the most accurate BGM with % error values between 3.9% and 10.5%. ► Accu-Chek Active was the least accurate BGM yielding similar results to DNS method. ► Ionic liquid and alkaline pretreatments gave similar glucose yields.
Introduction
The pretreatment of lignocellulosic biomass, which is among the most expensive and crucial stages for the conversion of lignocellulosic feedstocks to value added products including ethanol, improves the glucose production yield and rate by changing biomass structure, making cellulose found in the lignocellulosic biomass more prone to enzymatic hydrolysis (Mosier et al., 2005). There are several biomass pretreatment methods having different mode of actions with different effects not only on the sugar yield but also on the subsequent ethanol production conducted via fermentation (Hendriks and Zeeman, 2009). Glucose, xylose and total sugar yields obtained via enzymatic hydrolysis following various pretreatment methods conducted on the same lignocellulosic biomass source can be used to compare the performance of different pretreatment methods (Wyman et al., 2005), together with the effect of each pretreatment on the ethanol production (Wyman et al., 2009). Ionic liquid (IL) pretreatment, which is mainly based on the dissolution of cellulose (Remsing et al., 2006, Swatloski et al., 2002), is a recent pretreatment technique that generally involves the complete dissolution of lignocellulosic biomass in the ionic liquid followed by the regeneration of a portion of the dissolved material upon precipitation via the addition of an anti-solvent, usually water (Mora-Pale et al., 2011). At the end of IL pretreatment, both the crystallinity and the degree of polymerization of cellulose decreases while the surface area of cellulose increases causing the enzymatic hydrolysis of pretreated cellulose to proceed more rapidly and efficiently as reflected by the improvements in glucose production rate and yield (Dadi et al., 2006, Kuo and Lee, 2009, Zhao et al., 2009).
Comparison of IL pretreatment with other pretreatment methods including dilute acid and crude glycerol pretreatment indicate that IL pretreatment is more efficient in terms of the glucose and reducing sugar production upon enzymatic hydrolysis (Guragain et al., 2011, Li et al., 2010). 1-ethyl-3-methyl imidazolium acetate (EMIMAc) was shown to be the most efficient IL via a high through put (HTP) experimental setup among 21 ILs for the dissolution of cellulose, which was also shown to achieve complete dissolution of various lignocellulosic biomass (Zavrel et al., 2009). EMIMAc is also capable of partial extraction of lignin from lignocellulosic biomass (Sun et al., 2009), improving the sugar yields obtained upon subsequent enzymatic hydrolysis (Arora et al., 2010, Doherty et al., 2010, Fu et al., 2010, Lee et al., 2009, Li et al., 2010, Singh et al., 2009).
Although there are many glucose determination methods available, such as chromatographic, chemical and enzymatic methods, these are either expensive by means of the equipment or the reagents required or they might be time consuming (Choy et al., 2007, Decker et al., 2009). Another important drawback of many of these methods is that they might be problematic to integrate to a HTP experimental setup, since they are either slow or may require high temperatures, which may cause the partial evaporation of the limited amount of liquid phase in the enzymatic hydrolyzates causing inaccurate measurements (Decker et al., 2009). Therefore a rapid, low-cost and practical method, which does not require high temperatures or intensive sample preparation stages, for the determination of glucose is an important aspect within the framework of lignocellulosic biomass research. By means of these considerations, utilization of blood glucose monitors (BGMs), which are easily accessible, low cost and simple to operate devices capable of conducting measurements within 5–8 s with a sample volume of approximately 0.6–1 μl, for the determination of glucose concentration in the enzymatic hydrolyzates of pretreated lignocellulosic biomass appears promising although these devices are originally designed and produced for measuring the glucose concentration in human blood. Reviews that describe the working principles of BGMs in detail are available in the literature (Heller and Feldman, 2008, Wang, 2008).
The utilization of two BGMs with different measurement mechanisms for the determination of glucose concentration in the enzymatic hydrolyzates of untreated lignocellulosic biomass, namely corn stover, has been previously reported by FitzGerald and Vermerris (2005). A similar approach for the determination of glucose via a BGM was reported by Sopade and Gidley (2009), where a single BGM was used for the determination of glucose during the in vitro digestion of starch and found to give very close results to a glucose oxidase based spectrophotometric method. BGMs were also used for the determination of glucose concentration during fermentation where Choy et al. (2007) tested two BGMs and obtained similar results with reference methods. Other than these works, which describes the utilization of BGMs in detail, there are also certain examples in the literature in which different BGMs were used for the determination of glucose upon the enzymatic hydrolysis of dilute acid pretreated bagasse (Alriksson et al., 2009), waste fiber sludge (Cavka et al., 2011) and native or ionic liquid pretreated micro crystalline cellulose (Jones and Vasudevan, 2010).
The present work investigates the applicability of four different BGMs for the determination of glucose in the enzymatic hydrolyzates of corncob pretreated with dilute acid, alkaline and ionic liquid pretreatment methods by first measuring the response of each BGM to glucose as well as cellobiose. Each of the four BGMs used in the present work uses a different enzymatic mechanism for the determination of glucose. The results obtained via BGMs are compared with those obtained via HPLC and DNS methods, in order to assess the accuracy of BGMs during the measurement of glucose in the enzymatic hydrolyzates of pretreated lignocellulosic biomass. Finally the efficiency of ionic liquid pretreatment by means of the glucose obtained upon enzymatic hydrolysis is compared with dilute acid and alkaline pretreatment methods.
Section snippets
Materials
Corncobs obtained from Turkey were milled to pass a 10 mesh (2 mm) screen and dried at 60 °C for 16 h followed by drying at 105 °C for 2 h prior to pretreatments. Sulfuric acid (H2SO4), sodium hydroxide (NaOH), tri-sodium citrate dihydrate, citric acid monohydrate, sodium azide and d-glucose were purchased from Merck (Darmstadt, Germany). Potassium sodium tartarate, 3–5 dinitro salicylic acid, phenol, sodium sulfate, cellobiose and 1-ethyl-3-methylimidazolium acetate (BASF, ⩾90% purity) were
Response of BGMs to glucose and cellobiose
Prior to their utilization during the measurement of glucose concentration in the pretreated lignocellulosic hydrolyzates, BGMs were first characterized by means of their response to standard solutions of glucose and cellobiose in 0.05 M sodium citrate buffer at pH 4.8, since this buffer is used both during the enzymatic hydrolysis of lignocellulosic biomass and as the diluent of samples withdrawn from the enzymatic hydrolysis systems throughout the study. As shown in Fig. 1, Accu-Chek Active
Conclusions
BGMs may provide a rapid and practical alternative to conventional glucose measurement techniques since they can conduct the measurements within approximately 5–8 s with simple sample preparation. The pretreatment method applied to lignocellulosic biomass has significant impact on the accuracy of BGMs. Among the tested BGMs, Optium Xceed was the most accurate BGM for the determination of glucose in the enzymatic hydrolyzates. Among the different pretreatment methods applied to corncob, IL and
Acknowledgements
This research was partially funded by the Middle East Technical University research fund via projects BAP-03-04-2010-05 and BAP-07-02-2011-101. Erinc Bahcegul is provided with a scholarship from The Scientific and Technological Research Council of Turkey (TUBITAK).
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