Keywords

1 Introduction

Bioprocess employs complete living cells (microbes) or their metabolites (enzymes) for getting the desired end product. Industrial biotechnology deals with the application of biotechnological tools in different industrial processes, including bioprocessing and production of value-added products from renewable feedstocks. Manipulation of microbes and their physiology through the application of genetic engineering tools facilitates the development of new and cost-effective, cleaner, and environmentally friendly industrial manufacturing processes. Substantial amount of biomass generated from different agro-processing industries is not being commercially exploited, because of lack of infrastructure for collection, handling, and management of such a biomass. Despite being rich in nutrients, this valuable biomass is burnt leading to environmental pollution problems and loss of important rich resource. Thus, this enormous biomass considered as “waste” possesses great potential to be converted into a great variety of value-added products, such as biofuels, animal feeds, and human nutrients, which besides mitigating the greenhouse effect and atmospheric pollution problems is likely to help in better biomass management (Pothiraj et al. 2006).

According to a recent survey, global consumption of oil rose from 1.1 in 2014 to 1.9 million barrels per day (b/d) in 2016 (BP Statistical Review of World Energy 2016). In addition to the rising global oil demand, there are growing concerns about diminishing known petroleum reserves and the adverse effects of atmospheric greenhouse gases (GHG). This has resulted in search for alternative sources of energy and renewed interest in the production of fuels from plants or organic wastes, termed as “biofuels.”

Biofuels mainly derived from food crops, such as corn, sugarcane, soybean, vegetable oil, etc., are regarded as “first-generation biofuels.” In addition to their potential of being used after blending with petroleum-based fuels, they face many challenges including food-versus-fuel debate resulting in hike in food prices, production of food crops leading to change in land use pattern, and potential increase in GHG emissions. Moreover, the economic feasibility of the processes implied for first-generation biofuel production is based on the type of feedstock and the area of cultivation of that particular feedstock (Naqvi and Yan 2015). Therefore, second-generation biofuels largely produced from lignocellulosic biomass are cheap and abundantly available as nonfood materials which offer a potential alternative to meet the growing global fuel demands. In addition, second-generation biofuels also provide several benefits to the society, such that they (i) are renewable and sustainable, (ii) help to mitigate the greenhouse gases (GHG) emission in the atmosphere, (iii) facilitate development of local economy through creation of job opportunities, (iv) reduce air pollution caused by burning or rotting of biomass in fields, and (v) ensure energy security for countries dependent on oil being imported from the other countries (Greenwell et al. 2012; Lee and Lavoie 2013). Comparison on advantages and disadvantages of the first-generation biofuels, petroleum fuels, and second-generation biofuels is presented in Table 3.1 which corroborates the importance of the second-generation biofuels.

Table 3.1 Comparison of merits and demerits of first- and second-generation biofuel and petroleum fuels
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There are two routes for converting biomass to biofuels: “thermochemical” route, commonly known as biomass-to-liquid (BTL) conversion process, and the “biochemical” route. In the case of BTL, the biomass is subjected to pyrolysis or gasification to generate syngas (composed of carbon monoxide and hydrogen) which is subsequently converted to fuels using either a catalytic process, such as the Fischer-Tropsch reactions, or by a biological conversion method (Balan 2014). In “biochemical” route, holocellulose content (cellulose and hemicellulose) available in the biomass is converted to monomeric sugars, which can be further utilized for the production of valuable compounds, like fuels, organic acids, etc. It is estimated that in economic terms, second-generation biofuel production processes are two to three times more expensive than the petroleum fuels on an energy-equivalent basis and over five times that of similar capacity first-generation bioethanol plants (Wright and Brown 2007; Carriquiry et al. 2011). Several challenges in the areas of bioprocessing of feedstocks, such as feedstock production and logistics, development of energy-efficient technologies for pretreatment and enzymatic hydrolysis of biomass, upstream and downstream processing cost, biofuel distribution, its acceptance in the society, and environmental impacts, need to be addressed to alleviate the fears and to lower down the production costs of the second-generation biofuels (Luo et al. 2010; Menon and Rao 2012).

A consortium of hydrolytic enzymes known as molecular scissors is required to cleave the complex network of cellulose, hemicellulose, and lignin in the lignocellulosic biomass to produce specific monomeric sugars (Zhang et al. 2012). Lower enzyme quantity and enzyme efficiency and higher enzyme costs required to hydrolyze lignocellulosic biomass are the limiting factors in bioprocessing applications. In the recent years, companies manufacturing enzymes commercially have made significant progress in overcoming these problems using different biotechnological and process engineering approaches (Alvira et al. 2013). Development of an efficient and potent process for production of ethanol as a liquid fuel after conversion of cellulose in the lignocellulosic biomass depends on many factors, such as lignin content in biomass, pretreatment effectiveness, cellulose crystallinity, substrate concentration, and productive hydrolysis of cellulose into monomeric units (Jeoh et al. 2007; Hall et al. 2010; Zeng et al. 2014). Moreover, high enzyme concentrations are needed to achieve efficient hydrolysis of biomass, thereby increasing the processing costs. The major difficulties surface up during the recycling of enzymes adsorbed on the residual lignocellulosic material. Thus, different ways to enhance the enzyme efficacy are required to reduce their consumption during hydrolysis, maintaining higher sugar productivity.

Surfactants are amphiphilic surface-active agents having both hydrophilic as well as hydrophobic entities that lessen surface tension between two immiscible fluids after accumulating at their interface (Fig. 3.1). The term “Surfactants” was created and registered as a trademark for the first time by the General Aniline and Film Corp for their surface-active products (GAF 1950; Schramm et al. 2003) and was later released in the public domain (Stevens 1969).

Fig. 3.1
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Structure of several surfactant molecules forming micelles

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Surfactants can either be of synthetic origin or biological origin (biosurfactants). Biosurfactants present an edge over chemical surfactants due to their numerous advantages, such as biodegradable and environment-friendly nature, production at lower temperatures, effectiveness at low concentrations, low toxicity, high selectivity because of the presence of specific functional groups, and efficiency to work at extreme environmental conditions of temperatures, pH, and salinity, which renders them suitable for different industrial applications (Kapadia and Yagnik 2013; Santos et al. 2013). However, their large-scale commercial application is hindered due to their high production costs, ineffective methods of bioprocessing, less efficient microbial strains, and the exorbitant downstream processing costs (Marchant and Banat 2012; Campos et al. 2013; Banat et al. 2014). Although, there is a restricted commercial production of biosurfactants due to huge production cost and low yields, in the recent times, new microbial strains and process interventions have helped in improving their productivity by 10–20-fold. Some of the companies manufacturing different types of biosurfactants are listed in Table 3.2.

Table 3.2 List of global biosurfactant manufacturers
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Due to their important characteristics, surfactants are in high demand for bioprocessing applications (Saharan et al. 2011). This is largely because of the increasing interest in surfactant-aided hydrolysis of cellulose (Hseih et al. 2015; Min et al. 2015; Li et al. 2016). Different mechanisms, such as surfactant adsorption on air-liquid interface preventing enzyme denaturation, increase in available cellulose surface, and/or removal of inhibitory lignin, have been suggested for efficient enzymatic hydrolysis of cellulose (Min et al. 2015; Li et al. 2016). Based on kinetic analysis, surfactants can also increase the availability of reaction sites, leading to enhanced hydrolysis rate (Samiey et al. 2014).

Therefore, the aim of this chapter is to provide an insight into various possible mechanisms for action of biosurfactants and their application in bioprocessing along with their futuristic potential. Information presented in the chapter also provides an insight into the development of an efficient bioprocess for conversion of lignocellulose into biofuel and sets a platform for research on biosurfactant-aided pretreatment and hydrolysis for production of second-generation bioethanol from lignocellulosic biomass.

2 Surfactants and Their Classification

Surfactants are characteristic organic compounds, possessing both water-insoluble hydrophobic groups (tails) and water-soluble hydrophilic groups (heads) that decrease the surface tension at the interface of two liquids or liquid and solid (Saharan et al. 2011). Surfactants have diverse industrial applications, such as in food processing industries, agrochemical and pharmaceutical products, personal care and laundry products, petroleum, fuel additives, lubricants, paints, coatings and adhesives, photographic films, biological systems and various medical practices, soil remediation techniques, and also other environment-friendly methods (Schramm et al. 2003; Mishra et al. 2009).

It is estimated that during 2015–2020, world consumption of surfactants will continue to expand at an average annual rate of 1–5% (IHS 2016). Global surfactant market is calculated to grow by value at a compound annual growth rate (CAGR) of 5.5% and is predicted to reach a volume of 24,037.3 KT corresponding to $42,120.4 million in monetary terms by 2020. In terms of volumes, the anionic surfactants ruled the global market, with 7686.1 KT, followed by the nonionic surfactants at 6345.7 KT in 2014. Additionally, share of the amphoteric surfactants that represent 7.2% of the global surfactant market in 2014 is predicted to rise at the highest CAGR in the duration of 2015–2020 (www.marketsandmarkets.com/Market-Reports/biosurfactants-market-493.html).

On the basis of the ionic properties of the polar head group, surfactants are categorized into different groups, such as ionic (anionic and cationic), nonionic, and zwitterionic. Cationic surfactants carry a positive charge; on the other hand, a negative charge is present on the polar head groups of anionic surfactants. Zwitterionic or amphoteric surfactants possess both positive and negative charges depending on the environment in which they are present, whereas nonionic surfactants do not carry any charge on their head groups (Fig. 3.2). Different classes of surfactants with examples and their structures are presented in Table 3.3.

Fig. 3.2
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Types of surfactants on the basis of polarity of head group

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Table 3.3 Classification of surfactants on the basis of the nature of the polar head group
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3 Ionic (Anionic and Cationic) Surfactants

Anionic surfactants get dissociated in water as an amphiphilic anion, which acts as surface-active portion of the molecule and a cation, which is generally an alkaline metal (Na+, K+) or a quaternary ammonium ion (Salager 2002). The commonly used anionic surfactants in various industrial applications are (a) carboxylates (alkyl carboxylates-fatty acid salts), (b) sulfates (alkyl sulfates, alkyl ether sulfates), (c) sulfonates: docusates (dioctyl sodium sulfosuccinate, alkyl benzene sulfonates), and (d) phosphate esters (alkyl-aryl ether phosphates; alkyl ether phosphates). These are also employed in pharmaceutical and cosmetic industries. For example, sodium lauryl sulfate BP containing sodium dodecyl sulfate with bacteriostatic action against gram-positive bacteria is applied as a skin cleaner and also a component in the medicated shampoos (Mishra et al. 2009; Sekhon 2013; Azarmi and Ashjaran 2015).

Cationic surfactants segregate in aqueous solution into a surface-active amphiphilic cation and an anion (mostly halogen type). Cationic surfactants majorly constitute a huge fraction of nitrogenous compounds, like amine and quaternary ammonium salts with one or many long alkyl chains originating from natural fatty acids (Salager 2002). Due to their positive charge, they adsorb strongly on negatively charged solid surfaces imparting special characteristics, like anti-caking, corrosion inhibition, dispersion, germicidal action, etc., to the substrates. These also have property to destroy a wide range of gram-positive and some gram-negative microbes. Such surfactants find application in cleansing wounds or burns on the skin (Mishra et al. 2009; Azarmi and Ashjaran 2015). The most widely used surfactants in this category are “Esterquats” with ester bonds which are generally more expensive than anionics and nonionics, because their synthesis involves high-pressure hydrogenation reactions. However, these surfactants show poor detergency (Sekhon 2013).

4 Nonionic Surfactants

Nonionic surfactants do not dissociate in water due to the non-dissociable nature of their hydrophilic group, such as alcohol, phenol, ether, ester, or amide, and are compatible with all other types of surfactants (Salager 2002). These can be described as polyolesters, polyoxyethylene esters, poloxamers polyolesters including glycol, glycerol esters, and sorbitan derivatives. Fatty acid esters of sorbitan (commonly known as Spans, e.g., Span 40, Span 60, Span 80, etc.) and their ethoxylated derivatives (frequently referred to as Tweens, e.g., Tween 20, Tween 40, Tween 80) are one of the most commonly used nonionic surfactants. Polyoxyethylenated mercaptans have slight unpleasant odor. The most repeatedly used surfactants in this group are Polysorbate 20, Polysorbate 80, and Poloxamer 188 which are generally applied in a concentration ranging between 0.001 and 0.1% and mainly find their application as emulsifying agents, dispersants, and solubilizers in pharmaceutical industry (Abraham 2003). Nonionic surfactants are mostly used as gelling and foaming agents and also in the fabrication of several drugs and nano-carriers for drug delivery systems (Mishra et al. 2009; Azarmi and Ashjaran 2015; Carter and Puig-Sellart 2016). In the USA, the neutral agent nonoxynol-9 is commonly used as vaginal spermicide. However, octoxynol has also been approved by the Food and Drug Administration (FDA) to be used in contraceptives and other vaginal drug products (Sekhon 2013).

5 Zwitterionic (Amphoteric) Surfactants

Zwitterionic or amphoteric surfactants exhibit both positive and negative charges on the surface-active portion, for example, betaines or sulfobetaines and natural substances, like amino acids, phosphatidylcholine (lecithin), and phospholipids. These surfactants are very mild, compatible with all the other types of surfactants, and less irritating to the skin and eyes than the other types revealing excellent dermatological properties. Moreover, due to their high foaming potential and insolubility in most organic solvents, amphoteric surfactants are largely used in shampoos, cosmetic products, and hand dishwashing liquids (Mishra et al. 2009; Azarmi and Ashjaran 2015).

6 Biosurfactants

Microbially produced surface-active compounds, possessing both hydrophilic and hydrophobic moieties, are commonly referred to as biosurfactants (Kugler et al. 2015). In biosurfactants, the lipophilic moiety is generally a protein or peptide with a high fraction of hydrophobic side chains or a hydrocarbon chain of a fatty acid with 10 to 18 carbon atoms, whereas the hydrophilic moiety is an ester; hydroxyl, phosphate, and carboxylate group; or sugar (Campos et al. 2013). Biosurfactants are structurally diverse group of secondary metabolites secreted in liquid culture media by aerobic microorganisms requiring a carbon source, such as carbohydrates, hydrocarbons, fats, and oils, to perform vital roles for their metabolic processes (Silva et al. 2014). Biological surfactants help in the microbial growth by facilitating the availability of hydrocarbons to the microbes with increase in the area at the aqueous-hydrocarbon interfaces across their cell membranes, thereby enhancing utilization by microorganisms, and also help in protection of microbes from harsh environmental conditions (Aulwar and Awasthi 2016). These compounds have amphipathic molecules which act between solutions of different polarities reducing surface tension, thereby allowing access to the hydrophobic substrates due to enhanced contact area between insoluble compounds (such as hydrocarbons) resulting in their enhanced mobility, availability to living forms, and, thus, biodegradation of such compounds (Arparna et al. 2011). Biosurfactants find diverse applications in a wide range of industries as mentioned previously (Banat et al. 2010; Marchant and Banat 2012; Campos et al. 2013; Lawniczak et al. 2013; Reis et al. 2013). A general outlay of various industrial applications of biosurfactants is presented in Fig. 3.3.

Fig. 3.3
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Diverse industrial applications of biosurfactants

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Due to the capabilities of biosurfactants for being used in diverse industrial processes, many patents have been granted in recent years involving biosurfactant production by different microbes, specifically Pseudomonas spp., Acinetobacter spp., Bacillus spp., Candida spp., and Torulopsis spp. (Sachdev and Cameotra 2013). Patents on the biosurfactant production by a consortium of microbes involving Corynebacterium spp., Alcaligenes spp., and Methylomonas spp. have also been filed (Shete et al. 2006; Rahman and Gakpe 2008).

7 Classification and Properties of Biosurfactants

Biosurfactants are generally divided into two classes, where class I contains low-molecular-mass molecules, such as glycolipids, lipopeptides, and phospholipids, which efficiently reduce surface and interfacial tensions, and class II comprises high-molecular-mass polymer agents including polymeric and particulate surfactants with effective emulsion-stabilizing properties (Kapadia and Yagnik 2013). Biosurfactants allow two immiscible phases to interact more readily by decreasing interfacial tension between the two dissimilar phases (Chavez and Maier 2011). Efficiency of biosurfactants is based on their concentration required to obtain the critical micelle concentration (CMC). It is noteworthy to mention here that biosurfactant molecules aggregate at concentrations above the CMC to form micelles which further enable them to lessen the surface and interfacial tension resulting in enhanced solubility and bioavailability of otherwise reluctant hydrophobic organic compounds (Fig. 3.4). Therefore, critical micelle concentration is inversely related to the efficiency of biosurfactants, which implies that less biosurfactant concentration is needed to reduce the surface tension (Desai and Banat 1997). In standard terms, an effective biosurfactant can lower down the surface tension of water from 72 to 30 mN per meter and the interfacial tension between water and n-hexadecane from 40 to 1 mN per meter.

Fig. 3.4
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Micelle formation in biosurfactants above critical micelle concentration (CMC)

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Biosurfactants are representative amphiphilic biomolecules that contain both hydrophilic and lipophilic groups. The hydrophile-lipophile balance (HLB) number is used to calculate the ratio of these groups, and its value between 0 and 60 defines the affinity of a surfactant for water or oil. Emulsifiers with low HLB are lipophilic and stabilize water-in-oil emulsification, while the emulsifiers with high HLB are hydrophilic (Desai and Banat 1997; Christofi and Ivshina 2002). HLB numbers calculated for nonionic surfactants are in the range from 0 to 20 wherein surfactants with HLB numbers > 10 possess affinity for water (hydrophilic) and HLB numbers < 10 show affinity toward oil (lipophilic). Recently, ionic surfactants have also been assigned HLB values extending above the value of 60 (Sajjadi et al. 2003). Thus, biosurfactant effectiveness can be estimated by its capability to minimize surface and interfacial tensions, by stabilization of emulsions, and also by measuring its hydrophilic-lipophilic balance (HLB).

8 Microbes Used for Production of Biosurfactants

A variety of microorganisms including bacteria, fungi, and yeasts can produce biosurfactants as mentioned in Table 3.4. Bacteria mainly belonging to the genus Pseudomonas and Bacillus are major bacteria known to produce significant amount of rhamnolipids and lipopeptide biosurfactants, while yeasts belonging to the genus Candida are efficient producers of rhamnolipids and lipopeptide biosurfactants (Arparna et al. 2011; Al-Bhary et al. 2013; Campos et al. 2013).

Table 3.4 Major types of biosurfactants produced by different microbial groups
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Because of the important diverse applications, the economics of biosurfactant production draws increasing attention worldwide. Moreover, due to emerging focus on sustainability and environmental impacts, industrial focus is shifting from the chemical surfactants to biosurfactants, but as mentioned previously, their high production cost is a major deterrent. During the past decade, substantial increase in the global biosurfactant production has been observed which is evident from the fact that in 2013, biosurfactant production was about 344,068 tons, but at a current compound annual growth rate (CAGR) of 4.3%, it is speculated to reach up to 476,512 tons by 2018 (Transparency Market Research 2014). Thus, there is an urgent need to develop new strategies for commercial production of biosurfactants through improved biotechnological processes. Two basic approaches are adopted globally for cost-efficient production of biosurfactants: (1) utilization of abundant, inexpensive, and waste biomass as substrate for the production media resulting in low initial raw material costs required for the process and (2) development and optimization of bioprocesses for maximizing biosurfactant production and recovery, leading to reduced operating costs (Saharan et al. 2011; Peirera et al. 2016). Therefore, it appears to be economically viable to produce biosurfactants using economical, renewable resources, such as agricultural wastes, fruit processing industries waste, molasses, vegetable oils, distillery waste, and dairy wastes as substrates (Makkar and Cameotra 2002; Krieger et al. 2010).

9 Application of Surfactants in Bioprocessing

Disruption of Cell Biomass

In order to address current burning issues of energy security, food-versus-fuel debate and environmental-related issues due to the use of liquid fossil fuels, renewable biofuels are drawing a considerable attention worldwide as substitutes to petroleum-based transportation fuels (Lee 2011). Among various types of renewable fuels, microalgal biodiesel known as “third-generation biofuels” has been considered as one of the best alternatives because of the high photosynthetic efficiency, high growth rate, and high levels of extracted oil (Ahmad et al. 2011). Algal strains, like Scenedesmus, Chlorella, Nannochloropsis, and Chlamydomonas, are known to possess a large amount of high-density lipid inclusions (30–60% lipids as dry weight) which can serve as good feedstock for biofuel production (Xin et al. 2011; Bondioli et al. 2012; Seo et al. 2016). However, extraction of lipids from algal cells requires efficient cell disruption methods to allow penetration of solvents into the intracellular inclusions. Different approaches have been developed for microalgal biomass harvesting and cell disruption including centrifugation, flocculation, filtration, and flotation for harvesting and microwave heating, ultrasonic cavitation, bead milling, enzymatic, pulsed electric fields, and osmotic shock for cell disruption which are either energy or chemical extensive (Sheng et al. 2011; Liang et al. 2012; Halim et al. 2012).

Because of their proven ability to disrupt membranes, surfactants can play a crucial role in harvesting and disruption of microalgal cells in a cost-effective and energy-efficient manner. The hydrophobic components of surfactants have potential to insert themselves into outer membranes, thereby facilitating the lysis of the cells (Nasirpour et al. 2014). Existing literature suggests that the cationic surfactants could easily bind with a negatively charged microalgal membranes, resulting in effective cell disruption (Huang and Kim 2013; Lai et al. 2016; Salam et al. 2016). A recent study by Seo et al. (2016) described utilization of cationic surfactant-decorated Fe3O4 nanoparticles (CS-OTES-MNP) in microalgal cell harvesting, detachment, and cell disruption, schematic illustration of which is presented in Fig. 3.5. Despite the huge potential of surfactant-assisted lipid extraction, there are also certain limitations associated with it. The extent of surfactant binding does not just count on the charge but can also be associated with the hydrophilic-lipophilic interactions between microalgae and surfactant leading to the final cell disruption (Ulloa et al. 2012). Microalgal cell membrane composition not only varies with species but also with the physiological state of a single strain (Gerken et al. 2013; Lai et al. 2016). Thus, any deviation in the cell wall structure can impact the efficiency of surfactant-aided cell disruption method. Therefore, it is imperative to understand correlation between microbial growth cycle and surfactant-aided disruption process.

Fig. 3.5
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Steps in the utilization of cationic surfactant-decorated magnetite nanoparticles (MNP) in microalgae harvesting, detachment, and cell disruption

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10 Hydrocarbon Bioremediation

Hydrocarbons are the hydrophobic organic chemicals which show restricted water solubility and are toxic and of persistent nature and have an adverse effect on the living forms. Moreover, excessive use of hydrocarbons nowadays has created numerous environmental contamination problems. Additionally, moderate to poor recovery of hydrocarbon contaminants by physicochemical treatments and limited accessibility to microbes and to oxidative and reductive chemicals during the in situ and/or ex situ applications make their removal difficult (Plociniczak et al. 2011). Therefore, biosurfactants hold a great potential in the biological remediation technologies.

Biosurfactant-aided hydrocarbon bioremediation is enhanced due to increase in substrate availability to microorganisms which involves enhancement of the hydrophobicity of the cell surface due to its interactions with the biosurfactant, leading to more effective and easy associations with the microbial cells (Mulligan and Gibbs 2004). Biosurfactants, therefore, consequently can help in enhancing biodegradation and removal of hydrocarbons by the bacteria capable of growing on hydrocarbon contaminants present in polluted soil (Urum and Pekdemir 2004; Nievas et al. 2008; Chaprao et al. 2015; Liu et al. 2015; Adrion et al. 2016a; Adrion et al. 2016b; de la Cueva et al. 2016; Sawadogo et al. 2016).

One of the most important applications of biosurfactants is the “microbial enhanced oil recovery” (MEOR) which is generally implied to the recovery of a notable fraction of the residual oil remaining in reservoirs that otherwise is difficult to obtain even after exhausting all the physicomechanical recovery procedures (Banat et al. 2000; Sen 2008). In this process, microbes or their primary or secondary metabolites, such as biosurfactants, biopolymers, acids, solvents, and enzymes, are applied to enhance oil recovery from depleted reservoirs, where biosurfactants stick tightly to the oil/water interface and decrease interfacial tension between oil/water and oil/rock, leading to the formation of an emulsion which stabilizes the desorbed oil in water. This further aids in the oil removal along with the injection water while decreasing the capillary forces that prevent residual oil from escaping through rock pores (Suthar et al. 2008) as illustrated in Fig. 3.6. MEOR methods can be classified into two main groups: (a) ex situ production of the MEOR metabolites wherein microorganisms are grown using industrial fermenters or mobile plants and then injected into the oil formation as aqueous solutions and (b) in situ production of the MEOR metabolites in which the formation of microbial metabolites takes place directly in the reservoir (Yernazarova et al. 2016). Recently, many authors have also reported applications of biosurfactants in MEOR (Amin 2010; El-Sheshtawy et al. 2015; Golabi 2016).

Fig. 3.6
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Illustration of role of biosurfactants in microbial enhanced oil recovery (MEOR)

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11 Heavy Metal Bioremediation

Due to toxic nature of heavy metals, their contamination in soil ecosystems has serious consequences as even low concentration of heavy metals is very hazardous to the living organisms (Plociniczak et al. 2011). Anionic biosurfactants form complexes with metal with the help of ionic bonds, stronger than the existing metal-soil bonds, and due to the reduced interfacial tension, metal-biosurfactant complexes are detached from the soil surfaces to the soil solution. The cationic biosurfactants replace the same charged metal ions with the help of ion exchange competing for negatively charged surfaces (Mulligan and Gibbs 2004; Juwarkar et al. 2007; Asci et al. 2008). In addition to this, metal ions can also be separated from the soil matrices by the biosurfactant micellar inclusions, wherein the polar head groups of micelles chelate metals, mobilizing them in water as shown in Fig. 3.7 (Mulligan 2005). Many authors have previously reported the utility of biosurfactants in bioremediation of soils contaminated with heavy metals (Singh and Cameotra 2004; Juwarkar et al. 2008; Das et al. 2009; Peng et al. 2009; Fu and Wang 2011).

Fig. 3.7
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Schematic presentation of biosurfactant-aided heavy metal removal from the soil

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12 Bioconversion of Lignocellulosic Biomass

Enormous amount of lignocellulosic biomass in nature has the potential for bioconversion into a variety of high value-added products like biofuels, fine chemicals, and cheap energy sources (Anwar and Gulfraz 2014; Kumar et al. 2016). Lignocellulosic biomass mainly comprises cellulose (30–60%), hemicellulose (20–40%), and lignin (10–25%) which are interconnected in a hetero-matrix constituting roughly 90% of the dry matter, while the rest is composed of ash and other extractives (Rosta Estela and Luis 2013; Nanda et al. 2014). Cellulose and hemicellulose that account for more than 50% of total mass can be potentially converted to sugars for their subsequent conversion to ethanol through a series of processes (Oberoi et al. 2010). Enzymatic hydrolysis and fermentation processes have been combined into several process configurations, such as separate hydrolysis and fermentation (SHF), simultaneous saccharification and fermentation (SSF), simultaneous saccharification and co-fermentation (SSCF), and consolidated bioprocessing (CBP) that includes enzyme production, enzymatic saccharification, and fermentation in a single step (Kumar et al. 2016).

Lignocellulosic biomass-derived ethanol is often termed as “second generation” or “2G” offering several advantages, such as greenhouse gas mitigation, near carbon neutrality, lesser dependence on fossil fuels, and improvement in nation’s energy security (Naik et al. 2010). The US Department of Energy (DOE) report of 2011 suggests that in the USA alone, more than a billion ton of lignocellulosic biomass is potentially available at ~$60/ton for conversion into > 20 billion gallons of cellulosic biofuels (Perlack and Stokes 2011). Previous reports suggest that the total crop residue available is more than one billion ton in the USA alone and more than nine billion ton worldwide (Lal 2005). Several authors have reported the bioethanol potential of various abundant major agro-wastes generated globally (Kim and Dale 2004; Perlack et al. 2005; Naik et al. 2010; Sarkar et al. 2012).

An immense amount of food waste, such as raw, cooked, edible, and nonedible portions of food crops, is generated during their production, storage, distribution, transportation, processing, and consumption of food stuffs from the household, commercial, and industrial sources. A recent study on global food loss and waste reported a food loss in the range of 27–32% for all the food produced in the world. According to a recent survey in the world, cereal losses at 19–32%, root and tuber losses at 33–60%, and fruit and vegetable losses at 37–55% have been estimated (Global Food Policy Report 2016). Reduction in the food loss and waste can lead to increased global food availability. This waste is a rich source of important biomolecules such as lipids, carbohydrates, amino acids, and phosphates which can be utilized as a substrate for the development of cost-effective biofuels (Pleissner et al. 2013). Carbohydrate-rich food hydrolysate produced after enzymatic hydrolysis of food waste can be transformed into bioethanol, whereas the lipid fraction from hydrolysate can be converted to biodiesel as shown in Fig. 3.8 (Karmee and Lin 2014). Across the globe, a lot of work has been carried out on the tremendous potential of food waste for the generation of biofuels, by different research groups (Kim et al. 2011; Yan et al. 2011; Yan et al. 2013; Yang et al. 2014; Matsakas et al. 2014; Pleissner et al. 2014).

Fig. 3.8
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Recycling of food waste into value-added products such as biofuels

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Commercial production of biofuels from lignocellulosic biomass is still hindered by many factors such as (1) biomass recalcitrance requiring effective pretreatment process, (2) high enzyme concentrations to achieve high rate of cellulose hydrolysis, and (3) enzyme adsorption to the lignocellulosic material making enzyme recycling difficult (Gregg and Saddler 1996). The possible ways of inhibition of cellulases by lignin during hydrolysis include (1) nonproductive adsorption of cellulase onto lignin, (2) physical blockage of cellulase on lignocellulose chain structure, and (3) enzyme inhibition due to soluble lignin-derived compounds like ferulic acid, syringaldehyde, vanillin, etc. (Saini et al. 2016). Thus, nonproductive binding of cellulases with lignin results in decreased efficiency of lignocellulosic hydrolysis. Therefore, development of economically feasible cellulose hydrolysis process for ethanol production along with identification of methods to increase enzyme effectiveness is mandated.

Addition of surfactants, such as nonionic detergents and protein, has been reported to significantly increase the enzymatic conversion of cellulose into soluble sugars (Kaar and Holtzapple 1998; Erikssson et al. 2002; Kristensen et al. 2007; Bardant et al. 2013; Hseih et al. 2015; Min et al. 2015; Li et al. 2016). On the basis of earlier studies, possible different explanations of surfactant effect on cellulose hydrolysis have been illustrated in Fig. 3.9.

Fig. 3.9
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Possible theories explaining enhanced cellulose hydrolysis in the presence of surfactant: (a) surfactant “protects” cellulase and increases its stability, (b) and (c) surfactant increases enzyme-substrate interactions and also causes structural changes in substrate making it more accessible for effective cellulose conversion, (d) hydrophobic part of the surfactant binds through hydrophobic interactions to lignin and prevents unproductive binding of cellulases to lignin increasing their efficiency, and (e) conversion of cellulose into monomeric units which can be further used for development of value-added products

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The exact mechanism explaining how surfactants improve enzymatic hydrolysis is still unknown, but several possible explanations have been proposed to describe enhanced surfactant-aided enzymatic hydrolysis of lignocellulosic biomass, and these are:

  1. 1.

    Surfactants enhance removal of lignin by forming emulsions, thereby increasing the access of substrate’s reaction sites to the cellulases.

  2. 2.

    Surfactants decrease irreversible, nonproductive adsorption of cellulase to nonproductive sites of biomass allowing its availability in solution to have higher activity, thus resulting in higher yields and better recycling of enzymes (Karr and Holtzapple 1998; Park et al. 1992; Eriksson et al. 2002).

  3. 3.

    Enhanced enzyme activity due to improved electrostatic interaction between surfactant and enzyme occurs either by activation of a certain amino acid in the enzyme active site or by reforming enzyme secondary structure, especially the α-helixes (Eckard et al. 2013a).

  4. 4.

    Surfactants provide protection to enzymes from thermal deactivation and denaturation by reducing the surface tension and viscosity of hydrolysate even after extended incubation period (Yoon and Robyt 2005; Kim et al. 2006; Eckard et al. 2013b). Addition of proteins such as bovine serum albumin (BSA) has also been reported to prevent unproductive binding of cellulases to lignin after blocking its reactive sites (Wang et al. 2013).

Currently, various authors have reported the positive effect of surfactant addition on the enzymatic hydrolysis of lignocellulosics (Qing et al. 2010; Sipos et al. 2011; Parnthong and Kungsanant 2014; Hseih et al. 2015; Min et al. 2015; Li et al. 2016; Mesquita et al. 2016). Liang et al. (2016) measured the effect of molecular structure of lignosulfonate-based polyoxyethylene ether (LS-PEG) on the enzymatic hydrolysis of Avicel and corn stover. They showed that the glucose yield of corn stover increased from 16.7 to 51.9% with a respective increase in PEG content and molecular weight of LS-PEG during hydrolysis. In an unpublished study, we had observed that the addition of 0.2% Tween 20 increased the sugar yield by about 15–20% during hydrolysis of alkali-pretreated sweet sorghum bagasse with crude enzymes produced by Aspergillus terreus using optimized parameters, as compared to control, where no surfactant was used. In addition to the sugar concentration, higher productivity of sugars was observed as surfactant-aided hydrolysis was carried out at 60 °C, instead of 50 °C, generally used for industrial applications. Previous studies on enzymatic hydrolysis of pretreated lignocellulosic biomass have reported the use of thermostable crude cellulases and hemicellulases, which helped in achieving a higher sugar yield and productivity (Soni et al. 2010; Srivastava et al. 2014; Rawat et al. 2014; Sharma et al. 2015). It is a well-established fact that the rate of reaction increases with increase in temperature. Therefore, maintaining a temperature of 70–80 °C during hydrolysis with the use of thermostable enzymes and surfactants is likely to improve the sugar productivity manyfold, highly desired by the biofuel industry.

13 Future Perspectives

Surfactants represent a major class of industrial chemicals widely used in different industries. Petroleum-derived commercially available chemical surfactants have diverse industrial applications, such as applications in pharmaceuticals, including their role as enhancers for percutaneous absorption, in respiration distress therapy, in suspension aerosols, as emulsifying agents, and in influencing drug absorption (Mishra et al. 2009), in biological systems (Ikegami et al. 2000), in synthesis of nanostructured or mesostructured materials with diverse uses, in health and personal care products (Mainkar and Jolly 2001), in food industries (Kralova and Sjöblom 2009), and also in crop protection (Green and Beestman 2007). Notwithstanding these advantages, the toxicity and persistence of surfactants in different environmental situations have also been highlighted by several authors (Ying 2006; Ivankovic and Hrenovic 2010). Thus, there is an imminent requirement to realize the effects of surfactants during their normal applications as well as accidental spills in the environment.

Biosurfactants due to their diverse, less toxic, and biodegradable nature have gained importance in recent times as viable alternatives to the chemical surfactants. There is a wide range of potential commercial applications of biosurfactants in diverse industries, such as pulp and paper industry, food industry, textiles, pharmaceutical industry as therapeutic agents, paint industry, remediation processes, cell-biomass disruption, bioprocessing, and even uranium ore processing (Banat et al. 2010; Silva et al. 2014; Peireira et al. 2016). One of the emerging areas of research using biosurfactants is the conversion of agricultural biomass into sugars for their subsequent conversion to liquid fuels. There is very scanty information available on biosurfactant-aided enzymatic hydrolysis of pretreated biomass. The use of biosurfactants during pretreatment might not only yield good results for lignin deconstruction/removal but may also reduce the higher energy required to achieve the same level of efficiency. This coupled with high-temperature biosurfactant-aided enzymatic hydrolysis as discussed elsewhere is likely to revolutionize the research in second-generation bioethanol production. It is well recognized that pretreatment and hydrolysis are the most cost- and energy-intensive operations in conversion of lignocellulosic biomass to liquid fuels (Parnthong and Kungsanant 2014; Mesquita et al. 2016).

Similarly, the use of biosurfactants in developing valuable compounds from fruit and vegetable processing waste holds promise for the future. Since the fruit and vegetable processing waste is high in moisture, biosurfactants in combination with enzymes or other extraction methods may help in efficient extraction of pigments, carotenoids, and other nutritionally important compounds for their use in the development of functional foods. Because of the presence of lower lignin and high carbohydrates, fruit and vegetable processing wastes offer good opportunities for production of bioethanol and other value-added products. Use of surfactants/biosurfactants in improving ethanol productivity from the fruit and vegetable processing residues is likely to draw attention of many researchers involved in biofuel research.

Commercial production and development of biosurfactants is dictated by their safety issues and high production cost. Biosurfactants, currently under investigation, such as sophorolipids and rhamnolipids have not been reported to pose any safety or health issue (Marchant and Banat 2012). However, in certain isolated cases, rhamnolipids can act as immune modulators or as virulence factors in P. aeruginosa infections known as an opportunistic pathogen (McClure and Schiller 1996; Zulianello et al. 2006). Commercial production of rhamnolipid has been initiated by a company, Jeneil Biotech, Milwaukee, USA (www.jenielbiotech.com), which has reported no health issues associated with its use. Furthermore, large-scale production of sophorolipids, generally produced by yeasts, is also already underway in Asia with no reported health/safety issues (Marchant and Banat 2012).

One of the ways to reduce the overall costs is through improving the stability of biosurfactants. Another way to reduce the high production costs is to use a wide variety of microorganisms, such as Bacillus, Candida, Pseudomonas, Thiobacillus, etc., for producing biosurfactants using various renewable substrates, such as sugars, oils, alkanes, and agro-industrial wastes including molasses, potato processing wastes, olive oil mill effluent, plant oil extracts and waste, distillery and whey wastes, by-products of vegetable industries, dairy and sugar industry wastes, and cassava wastewater (Makkar and Cameotra 1999; Youssef et al. 2004; Makkar et al. 2011). However, these microbes cannot be used for commercial production of biosurfactants on a large scale due to their low productivity. Thus, the microbial strain producing biosurfactants should be carefully selected and engineered for enhanced productivity. Moreover, the production process should also be engineered in such a way so as to minimize overall capital, operating, maintenance, and product recovery costs (Reis et al. 2013).

Keeping in mind the demand and need for green technology, biosurfactants can also have a potential application in the synthesis of nanoparticles, which is an emerging promising environment-compatible method. Use of biosurfactants reduces the formation of aggregates facilitating homogeneous and uniform morphology of the nanoparticles during synthesis (Kiran et al. 2010; Mujumdar et al. 2016). Nano-emulsions are advantageous because of the Brownian motion of very small droplets causing a significant deduction in the gravity force. This further inhibits their coalescence, since these droplets are non-deformable, and hence, surface fluctuations are avoided. Consequently, low surface tension of the whole system results in high penetration of active and the small-sized droplets allowing their enhanced uniform rapid deposition on substrates (Morsy 2014). Reddy et al. (2009) reported stabilization of silver nanoparticles for 2 months using surfactin, known as a biodegradable, less toxic stabilizing agent. These surfactant-aided nano-emulsions have diverse applications in agrochemicals, as lubricants and cutting oils and corrosion inhibitors, in remediation technology, in pharmaceutics, in cosmetics, in foods, and in personal care products (Oliveira et al. 2014; Morsy 2014; Jaiswal et al. 2015; Mujumdar et al. 2016; Santos et al. 2016). Therefore, there is an imperative need to focus research efforts on the biosurfactant-mediated stabilization of the nanoparticles for diverse industrial applications.

14 Conclusions

  • Biosurfactants are potential replacements for synthetic surfactants in several industrial processes due to their biodegradability and lower toxicity.

  • Presently, despite of their many advantages, high production costs and shortage of detailed information on toxicity testing of biosurfactants make them economically incompetent in comparison to chemically produced surfactants available in the market.

  • Measures selected to target simplification and optimization of the kinds of the products for specific applications, such as use of sterilized or pasteurized fermentation broth without the requirement for extraction, concentration, or purification of the biosurfactant, may remarkably decrease the process production cost.

  • Optimization of upstream and downstream approaches during the production processes may also have a significant influence on the overall cost reduction. Success of biosurfactants in bioremediation will be in need of specific targeting and complete information of the physicochemical nature of the pollutant-affected areas.

  • Role of biosurfactants in MEOR has numerous utilities with respect to environment, but extensive research is still required for ex situ production and commercial application.

  • With increased efforts on the development of improved and cost-efficient application technologies, genetic engineering, and strain improvement techniques and production processes, biosurfactants are predicted to be one of the most multifaceted and valued compounds for use in various processes in the coming time.