Hydroxycinnamic acids such as p-coumaric (4-hydroxycinnamic acid) and ferulic acid (4-hydroxy-3- methoxycinnamic acid) are the most abundant of phenolic acids found present in most agricultural residues (Figure 2.6). These hydroxycinnamic acids form lignin-phenolic-carbohydrate complexes within the cell wall of the residues. They are attached to both the lignin and carbohydrate by ether and ester bonds. Buranov and Mazza (2008) found that wheat straw contains 0.38% and 0.17% esterified and etherified p-coumaric acid, respectively, as well as, 0.10% and 0.22% esterified and etherified ferulic acid, respectively. The ether bonds are usually linked to lignin whereas the ester bonds to the carbohydrate. A cereal crop such as wheat has more ferulic acid present in the bran than in the straw, with more p-Coumaric acid present in the straw than in the bran (Boz, 2015; Ou et al., 2012).
Figure 2.6 Structure of ferulic acid (right) and p-Coumaric acid (left) (Saad et al., 2019)
The hydroxycinnamic acids are usually extracted from natural sources by enzymatic and chemical treatments. Enzymes such as ferulic acid esterase used in the hydrolysis of ferulic acid are usually produced in very low quantities by microorganisms, also the hydrolysis of the hydroxycinnamic acid from these agricultural residues takes a very long time and results in low yields due to the its inability to release bound acids (Ou et al., 2007). Due to these reasons, the option of using chemical hydrolysis to release these hydroxycinnamic acids has become the most favourable method. However, hot acid hydrolysis has been shown to cause degradation and result in lesser yield of hydroxycinnamic acids such as p-coumaric and ferulic acid in comparison to hot alkaline hydrolysis (Kim et al., 2006). The
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ester linkages are hydrolysed using an alkali such as NaOH. These compounds found within the cell wall of biomass can also serve to induce swelling of the cell wall, thereby increasing solubility of the individual cell wall components such as lignin and hemicelluloses (Sun et al., 1995). Although, this is an advantage provided by the extraction of these hydroxycinnamic acids, it also results in an issue in terms of the purity of these acids. Due to this a number of purification methods have been investigated including the use of activated carbon, macro-porous resins and the use organic solvents such as ethanol (Buranov and Mazza, 2009; Couteau and Mathaly, 1997; Ou et al., 2007).
It is however interesting to note that, although these hydroxycinnamic acids make up a small percentage of the biomass composition, they have much commercial value. For example, 1 g ferulic acid is sold at ZAR 668 whereas 1 g p-Coumaric acid is sold at ZAR 710 by Sigma Aldrich. Furthermore, ferulic acid and p-Coumaric acid have been of interest due to their antioxidant properties. They have been applied as a high-value additive in both the pharmaceutical, food industry and cosmetic industries, among others. In the pharmaceutical industry ferulic acid is used as an additive to help reduce liver cholesterol and coronary diseases (Ou et al., 2007). Whereas, p-Coumaric acid is used as an additive to reduce the risk of stomach cancer (Max et al., 2009). In addition, ferulic acid has been found to be a precursor in the production of a value-added product such as vanillin (Tapin et al., 2006). Meanwhile, p-Coumaric acid has also been found to be a precursor in the production of p-hydroxybenzoic acid (K. Jiang et al., 2016).
Fractionation of Wheat Straw and Wheat Bran into Hemicellulose,
Lignin, Cellulose and Hydroxycinnamic Acids
Due to the difference in their compositions (Table 2.1), treatments such as destarching, which can be conducted on wheat bran will not have any significant effect on wheat straw. Comparing the two biomass, wheat straw has received more research attention for fractionating processes, as well as the nanocellulose extraction (Barman et al. 2012; Curreli et al. 1997; Heikkinen et al. 2014; Kamel, 2007; Kaushik and Singh, 2011; Koti et al. 2012; Montane et al. 1998; Sun et al. 2004; Zimmermann et al. 2010). The focus of research utilising wheat bran has been more on the fractionation methods to produce arabinoxylan and ethanol because of its high starch and hemicelluloses contents (Brillouet and Mercier, 1981; Chotěborská et al. 2004; Favaro et al. 2012; Liu and Ng, 2016; Merali et al. 2013; Palmarola- Adrados et al. 2005; Sánchez-Bastardo et al. 2013; Tobergte and Curtis, 2013). To date, there has been limited research on the extraction of nanocellulose utilising wheat bran as raw material due to its high starch content and low crystallinity among others (Nilsson, 2017).
Therefore, components other than cellulose must first be extracted from the raw biomass material. Extraction of other cell wall components such as hemicellulose and lignin results in obtaining a cellulose-rich pulp that can be used for nanocellulose production. The minimum requirements stated in most literature for the cellulose-rich pulp for nanocellulose production are cellulose content ≥ 62%, crystallinity index ≥ 48%, hemicelluloses content ≤ 23% and lignin ≤10% (Espinosa et al., 2017; Kallel
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et al., 2016; Teixeira et al., 2010). It is therefore necessary to reduce the hemicellulose and lignin contents while increasing the cellulose content and crystallinity before nanocellulose production from wheat straw and wheat bran. However, the process of reducing the hemicelluloses and lignin provides an opportunity to obtain additional value-added products in addition to nanocellulose. In addition removal of the two products would increase the purity of the nanocelluloses to be produced It is therefore necessary to develop a method to extract lignin, hemicellulose, ferulic acid and p-Coumaric acid from the biomass while improving on its cellulose content and crystallinity to meet the minimum criteria for nanocellulose production.