Value Addition of Waste Biomass
Transformation of waste biomass to various biotechnological products and bioenergy is carried out through different routes. The major routes comprise biological, chemical, and thermal processes and are depicted in Fig. 1.5. The conversion of biomass either can result in final products or may provide building blocks for further processing.
Biotransformation of Biomass
Biological transformation involves the utilization of living organisms or enzymes (biocatalysts) to catalyze the conversion of biomass into specialty and commodity chemicals. Generally, it is considered to be the most flexible mode for conversion of biomass into various industrial products (Dale 2003). Compared to chemical transformations, where high temperatures and pressures are involved, operating conditions for biological transformations are relatively mild. Fermentation is the primogenital and the most fundamental and mature area of biotechnology for biological transformation. For centuries, fermentation was used for preserving and processing food and beverages. Only in the last several decades due to current advancements in biotechnology, it has been used to bring to market a wide variety of fermentation-based products, including platform chemicals, renewable fuels, biopolymers, antibiotics, amino acids, organic acids, and pharmaceuticals using various agro-industrial feedstocks. Some commercial bulk chemicals, such as ethanol, lactic acid, citric acid, acetone, and butanol, have been produced via yeast,
Fig. 1.5 Conversion routes of biomass to bioenergy and other biotechnological products
fungal and bacterial fermentation processes (Atsushi et al. 1996; Huang et al. 2005; Ezeji et al. 2007; Dhillon et al. 2011c).
Recently, there has been increasing interest in the utilization of biocatalysts to transform renewable resources into biochemicals, owing to high yield and selectivity, and fewer by-products as compared to chemical synthesis. Table 1.3 shows the biotransformation of different wastes to high-value biochemicals through different processes. However, due to the metabolic restriction in microorganisms, only a few bulk products currently are produced via fermentation (Danner and Braun 1999). Therefore, development of new technologies to broaden the product range is necessary. Advances in genetic engineering have been viewed as a powerful tool for genetic manipulation of multistep catalytic systems involved in cell metabolism (Zha et al. 2004). Recombinant DNA technology has been used to clone and manipulate gene encoding enzymes in organisms. Recombinant microorganisms, with altered sugar metabolism, are able to ferment sugar to few specialty biochemicals, which cannot be produced by the corresponding wild strain (Danner and Braun 1999). For instance, catechol and adipic acid were produced from glucose using genetically modified Escherichia coli. Both glucose and xylose, in cellulosic biomass, have been converted into ethanol by recombinant Saccharomyces strains (Anastas and Kirchhoff 2002). Hence, it is imperative that the recombinant strains can be used for the efficient utilization of pentose and hexose sugars from the abundant lignocellulosic biomass. Moreover, immobilized enzyme systems and whole cells have been used to produce various biochemicals from biomass.
Table 1.3 Biotransformation of different wastes to high-value biochemicals through different processes
Table 1.3 (continued)
Currently, research efforts are ongoing to isolate, identify, characterize, and even tailor microorganisms and enzymes in order to better utilize renewable resources to produce structurally diverse and complex chemicals. Biotransformation of biomass to higher-value chemicals provides advantages of high yield and selectivity, as well as minimum waste streams. However, there are still problems with current biological transformation technologies including both upstream and downstream processes. The capital costs related to energy requirements, such as pretreatment, sterilization, production, agitation, aeration, temperature control, and finally recovery of target products from aqueous systems with low product concentration, result in high-cost processes (Danner and Braun 1999). Further, considerable investment is required to make processes highly efficient and continuous (Dodds and Gross 2007). Therefore, there are research opportunities in the development of new economic biological transformation technologies which could effectively transform biomass into high-value biochemicals.
Biological conversion or biotransformation is a well-established process and comprises of fermentation and anaerobic digestion. Sugar and starchy crops provide the main feedstocks for the process of fermentation in which a microorganism converts the sugars into bioethanol. As an economic alternative to costly sugars, lignocellulosic biomass can be used as feedstock after pretreatment which helps to break it down into simple sugars. The pretreatment can be carried out by enzymes or acids. Although acid hydrolysis offers the more mature conversion platform, enzymatic hydrolysis appears to offer the best long-term option in terms of technical efficiency. Besides its recalcitrant structure, the efficient hydrolysis of lignocellulosic wastes and subsequent conversion of sugar syrups to various value-added products also depends upon various other factors, such as crystalline structure of cellulose, amount and nature of lignin present, and production of various inhibitory compounds during acid hydrolysis (Fig. 1.6). Lignocellulosic conversion would greatly increase the supply of raw materials available for production of various high-value products. The lignin residues could be used as fuel for the energy required and even providing surplus energy, resulting in significantly improved energy balances and resulting potential reductions in GHG emissions. The following sections discusses the potential of two underutilized wastes (marine processing and biotechnological process wastes) for the production of high-value biochemicals.
126.96.36.199 Biotransformation of Marine Processing Wastes
Large quantities of marine processing by-products are accumulated as aquaculture waste and shells of crustaceans and shellfish. Generally, the fishery by-products find applications for production of low-economic-value products, such as fish oil, fishmeal, fertilizer, pet food, and fish silage (Choudhury and Gogoi 1995). Currently, studies have identified a number of high-value bioactive compounds from fish wastes, such as fish muscle-derived peptides, collagen and gelatin, fish oil (source of omega-3 fatty acids), fish bone (consists of 60–70 % of inorganic substances, mainly composed of calcium phosphate and hydroxyapatite), and other
Fig. 1.6 Schematic diagram showing different aspects of lignocellulosic hydrolysis and its value addition
visceral organs (rich in a range of proteolytic enzymes including pepsin, trypsin, chymotrypsin, and collagenases) (Kim et al. 2001; Je et al. 2005). Lipid-based compounds that can be recovered from fish waste include fish oil, omega-3 fatty acids, phospholipids, squalene, vitamins, and cholesterol. Recovery of oil or lipids from fish industry waste offers not only the revenue generation but makes it suitable for other applications, such as spreading on land as a fertilizer or feedstuffs in swine diets to meet the protein requirements and as a substitute for common protein sources (i.e., soybean meal and commercial fishmeal) (Esteban et al. 2006).
Similarly, the other important class of by-products from marine bioprocessing plants includes crustacean shells and shellfish wastes mainly in the form of head and body carapace. These body parts comprise 48–56 % depending on the species. The efficient utilization of shellfish and crustacean shell by-products also becomes an environmental priority due to increased quantity of accumulation from processing plants as well as slow natural degradation of these materials. Shellfish and crustacean shells are a potential source of high-value biochemicals, such as biopolymers (chitin, chitosan), pigments (a carotenoid, astaxanthin), minerals, and proteins (Kaur and Dhillon 2013a, b) (Fig. 1.7). Most crustaceans, such as shrimp, lobsters, and crabs, are important reservoirs of natural carotenoids, such as astaxanthin and its esters (Sachindra et al. 2005).
However, the recovery of shell waste products, such as pigments and proteins, through chemical methods is complicated and the biological value of chemically extracted compounds is low. Additionally, these methods generate large quantities of hazardous chemical wastes. This has led to amplified interest in biotechnology research regarding the identification and extraction of high-grade, low-volume bioactive compounds produced from crustacean shell wastes. Recently, fermentation
Fig. 1.7 Schematic diagram for preparation of proteins, pigments, chitin, chitosan, and their oligomers from marine wastes
has also been reported as a suitable and economic method to extract carotenoid pigments from crustacean shell wastes. These bioactive compounds can be extracted and purified with technologies varying from simple to complex. Furthermore, some of these bioactive compounds have been identified to possess nutraceutical potentials that are beneficial in human health promotion. Therefore, development of new technologies in exploration of new bioactive compounds from marine processing wastes will alleviate costs associated with its safe disposal. The bioactive compounds from marine processing wastes will add high value to marine waste and represent unique challenges and opportunities for the seafood industry.
The commercial applications of marine fish processing by-products are expanded every year. However, their applicability as bioactive compounds and their nutraceutical properties are not well described. High-value profit can be achieved by identifying bioactive compounds and exploring their nutraceutical properties and pharmaceutical and personal care applications. Identification of nutraceutical potential of natural compounds is a growing field and marine processing by-products represent potential feedstocks for this purpose. To date, only a limited number of bioactivities have been identified from isolated compounds and mandate future research developments to apply them for the human health promotion.
188.8.131.52 Biotransformation of Fermentation/Biotechnological Process Wastes
The advancements in bioprocess technology led to commercialization of various biotechnological/fermentation processes for the production of various bioproducts, such as food and beverages, organic acids, antibodies, pharmaceutical products, and renewable fuels among others. These microorganism-mediated processes result in thousands of tons of waste biomass, such as of yeast, bacteria, fungi, and algae. These waste are rich in various kinds of bioactive compounds, such as biopolymers, proteins, lipids, and pigments, among others.
Chitin and chitosan occur naturally in some fungi (Mucoraceae). Fungal cell walls are composed of polysaccharides and glycoproteins. Polysaccharides, such as chitin and glucan, are the structural components, whereas the glycoproteins, namely, mannoproteins, galactoproteins, xylomannoproteins, and glucuronoproteins, form the interstitial components of fungal cell walls (Bowman and Free 2006; Dhillon et al. 2012a). Commercially, chitin and chitosan are mainly derived from the marine processing wastes, such as shrimp, crabs, squids, and lobsters shell by chemical deacetylation, using a hot concentrated base solution (30–50 % w/v) at high temperatures (<100 °C) for a prolonged time (Dhillon et al. 2012a). However, the chitosan obtained by such treatments suffers some inconsistencies, such as protein contamination, inconsistent levels of deacetylation, and high molecular weight (MW), which results in variable physicochemical characteristics (No et al. 2000). There are some additional problems, such as environmental issues, due to the large amount of waste (concentrated alkaline solution), seasonal limitation of seafood shell supply, and high cost (Wu et al. 2005). In this context, production and purification of chitin and chitosan from the cell walls of waste fungal mycelium (Fig. 1.8) offers the advantage of being environmentally friendly and provides greater potential for a consistent product (Dhillon et al. 2012a; Kaur and Dhillon 2013a).
Additionally, β-glucan can also be isolated from the mycelia chitosan–glucan complex and has important applications in biomedicine (Pomeroy et al. 2001).
Edible mushrooms are produced and consumed on a large scale. The amount of waste remaining after removing the edible part mainly consists of stalks and mushrooms with irregular dimensions and shapes and accounts for 5–20 % of the total production volume. In the USA alone, mushroom production results in nearly 50,000 metric tons of mushroom waste material per year with no suitable commercial application (Wu et al. 2005). The huge amount of wastes of edible mushrooms, such as Agaricus bisporus, Lentinus edodes, Pleurotus species, and Volvariella volvacea, among others, can be potentially used for the extraction of the high-value-added product chitosan, which nowadays finds promising applications in various fields (Dhillon et al. 2012a; Kaur and Dhillon 2013a; Dhillon et al. 2013).
Aspergillus niger strains are extensively used for the bioproduction of citric acid (CA) (Dhillon et al. 2011a, b, c, 2012b, c). The annual worldwide production of CA is estimated to be 1.7 million tons, which results in 0.34 million tons of A. niger mycelium waste per year, and furthermore, the industry continues to expand with an annual growth rate of 5 % (Wu et al. 2005; Dhillon et al. 2011c). A. niger strains contain approximately 15 % chitin, which can be separated and transformed into chitosan (Dhillon et al. 2012a).
Fig. 1.8 Extraction of chitosan and other products from waste fungal mycelium (SmF submerged fermentation, SSF solid-state fermentation)
Penicillium chrysogenum is widely used for the large-scale production of antibiotics. As a by-product of the antibiotic industry, a large amount of P. chrysogenum mycelia waste is managed by incineration. Only a small percentage is used as an additive for cattle feed and in agriculture as fertilizers. Similarly, another important microbial strain, Rhizopus oryzae, is widely used in the food industry. Some yeast strains, such as Saccharomyces cerevisiae, find commercial applications in the brewery and bioethanol production. These yeasts strains are rich source of proteins and biopolymers. The development of bio-based economy mandates need to develop some integrative technology to utilize the unlimited waste mycelium resulting from fermentation industries which has not only a commercial advantage but also an ecological benefit.