1.1 Introduction
The industry is facing one of the major challenges nowadays in its effort to transition from an economic system based primarily on nonrenewable resources to one based on renewable ones, a sustainable production and conversion of biomass to provide food, health, fiber, industrial products, and energy (Venkata et al., 2016; Yamamoto et al., 2014). The vast amount of available biomass represents an attractive potential feedstock for many energy and chemical processes. From an economic standpoint, some evidence suggests that energy production from renewable carbon sources may be competitive with production from nonrenewable carbon sources. Industry must play a decisive role in the transition, and the development of cost competitive biomass-based processes is required. Agriculture, a significant global activity, is producing massive amounts of lignocellulosic residues that can be used as feedstock in fractionation/hydrolysis processes to produce fermentable sugars and cellulosic fibers. A large variety of feedstocks such as corn stover, rice barn, wheat straw, corn shell, vegetables and fruits peel, seed, food matrices, etc., can be used as a raw material for extracting valuable resources (phenolic compounds, water-soluble sugars, organic acids, etc.) that could be used in pharmaceutical, cosmetics and biofuel processing industries (Deng et al., 2012). These feedstocks are renewable sources of biofuels and bioproducts which are abundant, inexpensive, and do not compete with food. Industrial food processing residues are another source of biomass that can be transformed into biofuels and biochemicals. Biomass from food waste typically consists of 50–75% carbohydrates (cellulose and hemicellulose), 5–25% lignin (Mohan et al., 2015; Silva et al., 2009), and modest amounts of other substances, including phenolic compounds, which may be processed into higher value-added products (Cardenas-Toro et al., 2014; Yang et al., 2015). Therefore, such biomass feedstocks are ideal sources for obtaining simple sugars (monomers), which are used as fermentation substrates for the production of bioethanol (Prado et al., 2014) and phenolic compounds that possess antioxidant properties or other biological activities in pharmaceutical, food, and health applications (Pourali, Asghari, & Yoshida, 2010; de-Oliviera et al., 2016).
Biofuels represent a class of renewable energy with the potential to contribute significantly to the sustainable energy mix required to meet future energy demands (Awaluddin et al., 2016). Microalgae, which is an aquatic biomass, is heavily researched as feedstock for the production of advanced biofuels as a result of its fast growth rate and the capacity to accumulate high concentrations of biochemical compounds such as lipids and carbohydrates (Chen et al., 2013). Microalgae primary metabolites, such as proteins, fatty acids, and carbohydrates, are produced intracellularly and entrapped within the cells; thus, an effective extraction technology is required to release these biochemical products (De Morais et al., 2015). The primary metabolites are a source of bioactive metabolites, such as vitamins and enzymes, which are commercially beneficial due to their antioxidant, anti-inflammatory, antiangiogenic, anti-obesity, anticancer properties (De Morais et al., 2015). Commonly used extraction technologies via chemical and mechanical methods include expellers, liquid-liquid extraction (organic solvent extraction), supercritical fluid extraction (SFE), and ultrasound techniques (Castejon, Luna, & Senorans, 2017) or subcritical water extraction (SWE) (Ibanez et al., 2003). SWE technology is gaining popularity as a method of valuable material recovery in high yields and high quality of extracted products, is inexpensive, has a short residence tim...
