In the modern world, the consumption of packaged foods by the people has been increased day by day. However, the most significant hurdle of the food industry is the limited shelf life of packaged food products due to contamination by food spoiling pathogens which results in a global public health issue, trade, and the economy. Given access to improper food preservation, bacteria and fungi rapidly colonize, increase in population that leads food spoilage (Hammond et al., 2015). The World Health Organization reports that unsafe food results in the illnesses of at least 2 billion people worldwide annually and can be deadly (Sharif et al., 2017). Addition of synthetic antimicrobial agents effectively controls the growth of food contaminants and extends the shelf life of foods (Irkin and Esmer, 2015). Though, recent toxicological studies indicate that specific concentrations of synthetic preservatives and their continuous use may be potentially mutagenic and genotoxic. For example, Sales et al. (2018) recently proved that artificial synthetic additives induced the formation of micronuclei in the bone marrow erythrocytes and believed as cytotoxic and genotoxic in the animal study. This apprehension rising consumers demand additional of natural preservatives that must be non-toxic as well as excellent defensive from microbial attack (Gyawali and Ibrahim, 2014; Brandelli and T.M. Taylor, 2015; Piran et al., 2017). Therefore, search for new alternatives to preserve foods is of great interest in the food industry. Natural antimicrobials attract considerable attention in the food industry because these substances would not cause any toxic or undesirable effect on the consumers (Brandelli and T.M. Taylor, 2015; Wang et al., 2018).
Natural bioactive compounds from many plants, bacteria and animal sources for food application have been extensively studied by various researchers and reported the possibility of commercial use. However, each source has own disadvantages and thus that bottleneck 100% usage for commercial purpose. Most of the essential oils from plants show instability (Moghimipour et al, 2012) and very less effective against gram-negative bacteria (Tiwari et al., 2009; Naik et al., 2010; Nazzaro et al., 2013). Moreover, overuse of bacteriocins can lead to resistant pathogens (Cavera et al., 2015), loss of their activity by proteolytic enzymes (Bradshaw, 2003; Fahim et al., 2016). Some of the animal source antimicrobial peptides like lysozyme and pleurocidin are not showing strong effects on gram-negative bacteria (Aloui and Khwaldia, 2016) and was inhibited by magnesium and calcium in the foods which may limit the use (Tiwari et al., 2009). There is an urgent need of new and alternate to above mentioned antimicrobial agents. In the recent years, seaweeds have been recognized as one of the wealthiest and most unexplored new source of antimicrobial compounds and nanofibers for therapeutic and food preservation. This review focus on various antimicrobial compounds extracted from marine algae and their biochemical compositions, antimicrobial activities against food pathogens. Also, this article pivots the potential use of marine algae for nanofibers synthesis used for incorporating antimicrobial agents for greater delivery and their stability during food preservation.
2. Natural antimicrobial compounds from marine algae
Over the last few years marine algae has attracted many researchers worldwide to isolate high value bioactive compounds for food and pharmaceutical industries due to its broad spectrum of various biological activities such as antioxidant, antibacterial, antifungal, anticancer, anti-inflammatory and antidiabetic (Thomas and Kim, 2011; Eom et al., 2012; Hamed et al., 2015; El Shafay et al., 2016; Sathya et al., 2017; Davoodbasha et al., 2018). Algae are the fastest growing plant in the world and generally divided into macroalgae and microalgae based on morphology. The macroalgae or “seaweeds,” are more abundant, multicellular plants growing up to 60 meters in length growing in an ocean. Microalgae are microscopic, mostly existing as small cells of about 2–200 µm and habitat in fresh, sea and even wastewater systems (Sirajunnisa and Surendhiran, 2016). Generally, marine algae are categorized into four main groups namely Rhodophyceae (red algae), Chlorophyceae (green algae), Phaeophyceae (brown algae) for the attribution of different pigments like phycobilins, chlorophyll and fucoxanthin respectively (Kadam et al., 2013; Barbosa et al., 2014). Asian countries like China, Japan, and Korea used seaweeds for medicinal and food purposes since prehistoric times (Thomas and Kim, 2011).
Many research reports revealed that marine algae could act as a potential alternative source of antimicrobial agents because of their functional groups with excellent antibacterial activity include phlorotannins, fatty acids, polysaccharides, peptides, terpenes, polyacetylenes, sterols, indole alkaloids, aromatic organic acids, shikimic acid, polyketides, hydroquinones, alcohols, aldehydes, ketones, and halogenated furanones, alkanes, and alkenes (Barbosa et al., 2014; Shannon and Abu-Ghannam, 2016; Sathya et al., 2017; Pina-Pérez et al., 2017; Zouaoui and Ghalem, 2017). Screening for bioactive compounds from marine algae with antimicrobial properties under exploited to be employed in food applications. Hence, the research has moved towards to find natural antimicrobial compounds against food pathogens and to replace synthetic compounds. Various primary bioactive compounds from marine algae are shown in Fig.1. Antimicrobial agents from terrestrial plants such as spices and herbs and their antimicrobial activity against food pathogens have already been well documented in many kinds of literature. There are more than 100,000 species of algae existing on earth (Sirajunnisa and Surendhiran, 2016). However, information about their potential activity against food pathogens is sparse since it is a recent field of research worldwide. Recently, some research reports have been published on antimicrobial potential of bioactive compounds extracted from marine algae against food pathogens and obtained notable positive results by various researchers globally. For instance, Rajauria et al. (2012) reported that methanol extract of polyphenolic compounds from the Irish brown seaweed Himanthalia elongates showed potent bactericidal activity against Gram-positive Listeria monocytogenes and Enterococcus faecalis and Gram-negative Pseudomonas aeruginosa and Salmonella abony at a concentration of 60 mg/mL. Dussault et al. (2016) reported that low concentrated algal extracts (?500 µg/ml) from Padina and Ulva sp. showed potential antimicrobial activity against Gram-positive foodborne pathogens such as Listeria monocytogenes, Bacillus cereus, and Staphylococcus aureus. A summary of antimicrobial agents from marine algae and their antimicrobial activities against food pathogens is shown in Table 1.
Marine algae contain many different kinds of polysaccharides as their storage compounds and show good antibacterial, antiviral and antioxidant property. Many of them are soluble dietary fibers (Chojnacka et al., 2012) and could be converted into nontoxic bioactive oligosaccharides by simple hydrolysis (Pina-Pérez et al. 2017). For example, sulphated polysaccharides from seaweed, Chaetomorpha aerea containing alginates, fucoidans and laminaran showed potent antimicrobial activity against food pathogens E. coli and Staphylococcus aureus at the MIC concentration of 50 mg/mL of extract (De Jesus Raposo et al., 2015). Kadam et al. (2015) recorded the remarkable effect of ultrasound assisted extraction of laminarin from two Irish brown seaweeds Ascophyllum nodosum and Laminaria hyperborean against essential food pathogens such as Staphylococcus aureus, Listeria monocytogenes, Escherichia coli, and Salmonella typhimurium. Another report published by Pierre et al. (2011) that carrageenans and the sulphated exopolysaccharide from the red microalga Porphyridium cruentum are effectively inhibiting one of the most important foodborne pathogen, Salmonella enteritidis. Treating Helicobacter pylori, is one of the most dangerous foodborne pathogen, is big challenge till now which affecting 50–80% of the worldwide population and responsible for gastric ulcer (Pina-Pérez et al., 2017). Chua et al. (2015) successfully inhibited the growth of H.pylori using sulphated polysaccharide fucoidan isolated from edible brown algae Fucus vesiculosus at the concentration of 100 µg /mL. Moreover, Araya et al. (2011) demonstrated that fucoidan shows no toxic effects on human trials with the daily intake of 6 grams which reveals the possibilities of applying into food industries.
2.3. Phenolic compounds
Phenolic compounds also known as Polyphenols are a group of tannin compounds that contain hydroxyl (?OH) substituents on an aromatic hydrocarbon moiety. Polyphenols in marine algae include phenolic acids, flavonoids, isoflavones, cinnamic acid, benzoic acid, quercetin, lignans, catechins, anthraquinones, phlorotannins (Chojnacka et al., 2012; Kadam et al., 2013; Pina-Pérez et al., 2017). Among various polyphenols, phlorotannins show excellent, potent free radical scavenging properties than polyphenols derived from terrestrial plants due to eight interconnected phenol rings (Sathya et al., 2017). Phlorotannins are found in many brown seaweeds such as Ecklonia cava, E. kurome, E. stolonifera, Eisenia aborea, Eisenia bicyclis, Ishige okamurae, Pelvetia siliquosa have medicinal and pharmaceutical benefits and have shown strong anti-oxidant, antiinflammatory, antiviral, anti-tumor, anti-diabetes and anti-cancer properties (Eom et al., 2012). Phlorotannins are polymers of phloroglucinol units (1,3,5-trihydroxybenzene) with molecular weights of 126 Da to 650 kDa (Kadam et al., 2013). In recent studies depicted that phlorotannin has excellent antimicrobial activity against food pathogens and gives a roadmap to replace synthetic chemicals for food preservation.
A research group led by Kim et al. (2017) investigated the antimicrobial activity of phlorotannin extracted from edible brown seaweed, Eisenia bicyclis and act against Listeria monocytogenes, is one of the essential food contaminants in the meat processing industry. They evidenced that phlorotannins have excellent anti-listerial activity in the ranges of 16–256 µg/ml. Choi et al. (2010) recorded the potent antimicrobial activity of eckol rich phlorotannin from E.cava against food pathogens methicillin-resistant S. aureus (MRSA) and Salmonella sp. in the range of 125–250 µg/mL. Moreover, some research results concluded that phlorotannin showed no cytotoxic effects on animal models with oral administration (Nagayama et al., 2002; Eom et al., 2012) which is highly suitable for food applications. A team led by Al-Saif et al. (2014), investigated the effects of flavonoids include rutin, quercetin, and kaempferol extracted from marine alga G.dendroides. They recorded the antibacterial activity of these compounds against some critical food contaminants such as E. coli, S. aureus and E. faecalis at the concentration of 10.5 mg/kg (rutin), 7.5 mg/kg (quercetin) and 15.2 mg/kg (kaempferol). Besides, marine algae can be directly added into human foods such as breads, pizza, cheese, pasta and meat products (Pina-Pérez et al., 2017) and used for edible coatings to preserve food products (Sánchez-Ortega et al., 2014; Pina-Pérez et al., 2017) which would add additional benefits of using marine algae in food industries.
2.4. Proteins and peptides
Many short chains and long chain peptide and proteins have been isolated from marine algae, and their antimicrobial activities have been exposed by many recent publications (Shannon and Abu-Ghannam, 2016). Crude extract from marine algae that contains the high amount of proteins showed higher antimicrobial potency (Al-Saif et al., 2014). Compare with larger peptides; small chain peptides have more potent antimicrobial property due to their simple molecular structure (Pina-Pérez et al., 2017) which aids the effortless invasion of the bacterial cell wall. Beaulieu et al. (2015) reported that antibacterial peptides extracted from Saccharina longicruris, brown seaweeds and showed a noticeable inhibiting activity against Staphylococcus aureus with the concentrations of 0.31 mg/mL to 2.5 mg/mL. Recently, a carbohydrate-recognizing protein called Lectin has been recognized as potential antimicrobial agents isolated from many biological sources including marine algae (Pina-Pérez et al., 2017). The possible antimicrobial activity of lectin isolated from red alga Solieria filiformis and its antimicrobial activity have been evaluated by Holanda et al. (2005). They found that the isolated lectin acted against both Gram-positive and negative bacterial species such as Pseudomonas aeruginosa, Enterobacter aerogenes, Serratia marcescens, Salmonella typhi, Klebsiella pneumonia and Proteus species at the concentration of 1000 µg/mL. Smith et al. (2010) also reported the antimicrobial potential of lectins extracted from red algae, namely Eucheuma serra and Galaxaura marginata and hinder the growth of Vibrio vulnificus and V. pelagicus.
2.5. Fatty acids
Apart from secondary metabolites, some of the fatty acid molecules present in marine algae also have potent antimicrobial properties and have been well documented by many researchers. For example, Cakmak et al. (2014) tested that fatty acids extracted from marine microalga D. salina and found that noticeable growth inhibition against Listeria monocytogenes ATCC 7644 at a concentration of 5 mg/mL and revealed this activity is due to the presence of valuable FA molecules like ?-3 and ?-6. In another study conducted by Desbois et al. (2008) observed that fatty acids from the diatom Phaeodactylum tricornutum with a very potent antibacterial activity against the multidrug-resistant Staphylococcus aureus and have characterized three different polyunsaturated fatty acids involved in the antibacterial activity such as eicosapentaenoic acid (EPA), the monounsaturated fatty acid palmitoleic acid (PA). The fatty acids profile in algae, with a predominance of myristic, palmitic, oleic and eicosapentaenoic acids (EPA), is a specific feature associated with the antimicrobial potential of algal species (Pina-Pérez et al. 2017). Furthermore, palmitic acid has been assumed to be primarily responsible for the antibacterial activity of algae (Al-Saif et al., 2014; Pina-Pérez et al. 2017). However, in our previous study, the results demonstrated that fatty acid methyl esters produced from marine microalgae Nannochloropsis oculata show higher antimicrobial efficacy on gram-negative bacteria than the gram-positive one (Surendhiran et al., 2014). This finding was in agreement with the statement given by Mubarak Ali et al. (2012) that the action of fatty acid methyl esters was not limited to the cell wall variations. Because fatty acids are made up of carboxylic acid (an acid with a -COOH group) with long hydrocarbon side chains that creates hydrophobic in nature that quickly pass through a lipid bilayer of bacterial cell membrane resulted in cell lysis due to leakage of cytoplasm (Burt, 2004; Oussalah, et al., 2006; Dussault et al., 2016).
2.6. Terpenes and lactones
Terpenoids are dominant group of secondary metabolites found in many marine algal species and displayed potential biological activity including antibacterial and antiviral properties (Bajpai, 2016). Abad et al. (2011) elucidated that sesterterpenoids, sesquiterpenoids, and meroterpenoids are the primary compounds of terpenoids which are responsible for antimicrobial properties. The highest antimicrobial potential of algae extracts against S.aureus was reported by Tüney et al. (2006), who observed zone of inhibition > 50 mm by diethyl ether extract (0.5 g/mL) of Enteromorpha linza (0.5 g/mL) and 38 mm inhibition zone in the case of Ulva rigida. It is due to that most effective antimicrobial compounds found in these algal species are terpenes, (e.g., usneoidone E, zosterdiol A, zosterdiol B, zosteronol, and zosteronediol) responsible for the antimicrobial and antiviral activity attributed to them (Pina-Pérez et al., 2017). Xu et al. (2003) conducted experiments on tetracyclic brominated diterpenes isolated from the organic extract of Sphaerococcus coronopifolius collected from the rocky coasts of Corfu Island that showed MIC value of 16 and 128 ?g/mL against MRSA S.aureus.
According to Etahiri et al. (2001), two bromoditerpenes, 12 S-hydroxybromospha-erodiol, and bromosphaerone, isolated from red seaweed and Sphaerococcus coronopifolius showed antibacterial activity against S. aureus with a minimal inhibitory concentration of 0.104 and 0.146 ?mol/L, respectively. Besides, bioactive compounds from marine algae show excellent antimicrobial efficacy against fungal food contaminants as well. It was evidenced by Indira et al. (2013) that seaweed Halimeda tuna extracted using various solvents like ethanol, methanol, and chloroform which hinder the growth of A. niger, A. flavus, A. alternaria, C. albicans and E. floccosum at the concentration ranging between 250 and 500 mg/mL. Bromophycolides (diterpene-benzoate macrolides) extracted from red alga Callophycus serratus and showed significant growth inhibition of methicillin-resistant S.aureus and vancomycin-resistant Enterococcus faecium (Lane et al., 2009). Recently, Rodrigues et al. (2015) isolated sphaerane bromoditerpenes, an uncommon dactylomelane called sphaerodactylomelol, from the red alga Sphaerococcus coronopifolius using dichloromethane solvent and observed efficient growth inhibition against bacteria Escherichia coli, Pseudomonas aeruginosa, Staphylococcus aureus and yeast Candida albicans. Furanones, cyclic esters of lactones extracted from red seaweed Delisea pulchra has been reported on inhibition of growth of Pseudomonas aeruginosa and E.coli by damaging their biofilm formation (Ren et al., 2004; Shannon and Abu-Ghannam, 2016).
3. Mechanism of action of marine algal bioactive compounds on bacteria
Antimicrobial mechanism of action of marine algal compounds remains unclear still, and only limited information has been specified by some researchers. Different concepts have been proposed by some investigators. Most of the studies depicted that cell wall and cell membrane damage are primary targets of marine algal antimicrobial compounds (Wang et al., 2009; Smith et al., 2010; Guedes et al., 2011; Hierholtzer et al., 2012; Wei et al., 2015; Falaise et al., 2016). Mechanism of action of different natural antimicrobial compounds from marine algae on bacteria is shown in Fig.2. Antibacterial mechanism action of the marine algal polysaccharide is due to glycoprotein-receptors present on the cell-surface of polysaccharides which binds with compounds in the bacterial cell wall, cytoplasmic membrane, and DNA. This action resulted in increased permeability of the cytoplasmic membrane, protein leakage and damaging of bacterial DNA which leads to cell death (He et al., 2010; Pierre et al., 2011; Shannon and Abu-Ghannam, 2016). An important mechanism was proposed by Shannon and Abu-Ghannam (2016) that the marine algal fatty acids target essential metabolic pathways in the bacterial cell, i.e., electron transport chain (ECP) and oxidative phosphorylation which take place in cell membranes. These resulted in damages of adenosine triphosphate (ATP) energy transfer and hindered enzymes such as bacterial enoyl-acyl carrier protein reductase, essential for the production of fatty acids within the bacterial cell. Another report claimed by Guedes et al. (2011) that the possible mechanism of action of fatty acids in cell leakage due to morphological damages of the outer cell membrane. It was corroborated by the recent research findings of El Shafay et al. (2016). They elucidated the mechanisms of action of fatty acids extracted from seaweeds, Sargassum vulgare and Sargassum fusiforme on gram-positive bacteria Staphylococcus aureus and gram-negative Klebsiella pneumoniae using Transmission electron microscopy (TEM). The results show that there was a shrink, ruptured and distorted shape of bacteria cells were observed due to puncture of the cell wall.
Not all the marine algal bioactive compounds demonstrate potent effects on all groups of bacteria. It depends on algal species, the efficiency of the extraction method and concentration of the compositions present in the extracts. For example, diethyl ether extract of red seaweed S. fusiforme shows high activity against gram-positive bacteria S.aureus; however, the methanol extract of same marine algae shows higher inhibition activity against gram-negative bacteria P.aeruginosa (El Shafay et al., 2016). Another example, the phycobiliproteins, and exopolysaccharides from the red microalgae Porphyridium aerugineum and Rhodella reticulata respectively were active against the gram-positive bacteria S.aureus and Streptococcus pyogenes but presented no effect against the gram-negative bacteria E.coli and Pseudomonas aeruginosa (Najdenski et al., 2013). Thus, the difference in sensitivities between bacteria may be due to their complex membrane permeability, making the penetration and the bactericidal action of the compound more difficult. Lane et al. (2009) suggested that the mechanism of antibacterial activity of terpene compounds was due to the hydrophobicity and conformational rigidity of the tetrahydropyran structure.
Wang et al. (2009) accounted that mechanisms of action of phlorotannin were also similar to fatty acids documented by Shannon and Abu-Ghannam (2016). Moreover, they added that phloroglucinol units of phlorotannin contain Phenolic aromatic rings and hydroxyl group that makes the hydrogen bond with the amine group of bacterial proteins that cause cell lysis. It was further evidenced by Wei et al. (2015) that phlorotannin extracted from S.thunbergii injured the cell membrane of V.parahaemolyticus, leads to cytoplasm outflow and disassembly of cell inclusions. However, a higher amount of phloroglucinol is required to destroy Gram-negative bacteria than the Gram-positive. This reason is due to the multifaceted structure of gram-negative cell wall than the gram-positive one (Kamei and Isnansetyo, 2003). It was corroborated by the previous research reports of Hierholtzer et al. (2012). In their report, a mechanism of action of phlorotannin was directly evidenced by electron microscope images shows that total cell membrane damages of bacterial cells. Paiva et al. (2010) explained that the antibacterial activity of lectins is due to the binding and damages of cell wall compositions of bacteria such as teichoic acids, peptidoglycans, and lipopolysaccharides and destroy them by leaking cell inclusions. However, more research findings must be required, and various advanced techniques such as molecular techniques, protein, enzyme and DNA analysis and scanning and transmission electron microscope analysis should be carried out to support the direct evidence of the mechanism of action of bioactive compounds isolated from marine algae.
4. Application of nanofibers in food preservation
Nevertheless natural bioactive compounds show ideal antimicrobial properties, to desire to be used in food preservation, there is also a need for an efficient method for their delivery into foods (Balasubramanian et al., 2009). Nowadays nanotechnology has become the call of the century. It has a thriving application in several other sectors, and its application in the food industry has been a recent event (Pradhan et al., 2015). Many nanotechniques include Nanoemulsions, Nanoliposomes, and Nanoencapsulation using nanofibers are employed to incorporate natural antimicrobial agents with different supportive materials used for food preservation. Among various nanodelivery systems, nanofibers have more advantages than other methods due to its rapid, controlled release, high surface to volume ratios shows higher antimicrobial activity and stability. These assets create the mats composed of electrospun fibers outstanding candidate for immobilization of natural antimicrobial compounds for food applications globally.
Many different methods such as phase separation, self-assembly, drawing, and electrospinning can be used to produce nanofibers (Esentürk et al., 2016; Akhgari et al., 2017) and chemical method (Saurabh et al., 2016; Berglund et al., 2016; Martelli-Tosi et al., 2016; Xie et al., 2016). Among the other nanofiber production, electrospinning is the most cost-effective one with simple tooling, and it applies to produce ultrafine fibers with a simple step-up production for the encapsulation of various bioactive compounds (Esentürk et al., 2016; Wen et al., 2017). Recently, the electrospun nanofibers have drawn significant interest to the food industry because of their high surface area-to-volume ratios. This property makes the mats composed of electrospun fibers excellent candidates for various applications, like edible films and additive delivery systems (Padilla et al., 2014).
The working principle of electrospinning is to use electrostatic repulsion of charged polymer jets to generate arbitrarily oriented or united nanofibers on the exterior of the collector. In general, electrospinning set up consist two electrodes; one is attached with polymer mixer and second one associated to a collector. Electrically charged polymers produce a Taylor cone at the end of the needle and are evicted at a positive charge. Nanofibers are generated after the evaporation of solvents while the mixer of polymer solution goes faster towards rotary collector or an auxiliary electrical field (Brandelli and Taylor, 2015). Electrospinning is widely accepted and superior method for production of nanofibers, owing to its ease, less cost, good elasticity, the potential to massive scale production, the capability to produce nanofibers from a most of polymers (Esentürk et al., 2016; Wen et al., 2017). Furthermore, both hydrophobic and hydrophilic compounds such as protein and amino acids could be directly encapsulated on nanofibers by electrospinning technique (Wen et al., 2017). Also, immense stability and high encapsulation efficiency of natural antimicrobial agents could be achieved by this method (Yang et al., 2017). Moreover, heat sensitive bioactive compounds could be efficiently immobilized in nanofibers during electrospinning method since it operates at ambient environment when compared to conventional techniques like spray drying which runs at high temperature (Wen et al., 2017). Basic electrospinning set up is illustrated in Fig.3.
Efficient nanofiber formation and its physicochemical properties such as mechanical strength, structure, release features of drug including burst effect and biocompatibility are influenced by the selection of polymers to electrospun because that would impact the interactions of the mixer of bioactive compounds/polymer/solvent (Esentürk et al., 2016). Nowadays, considerable attention has been given to natural biopolymers due to their remarkable advantages including biocompatibility, biodegradability, renewability, and sustainability as carriers for encapsulation of bioactive compounds in the food industry (Safi et al., 2007; Wen et al., 2017). Many researchers have been successfully synthesized Nanofibers using electrospinning from biopolymers such as cellulose acetate (Dods et al., 2015; Mehrabi et al., 2017; Liakos et al., 2017), chitosan (Tripathi et al., 2009; Liu et al., 2018), gelatin (Agudelo et al., 2018), dextran (Fathi et al.,2017), pullulan (Liu et al., 2016; Wen et al., 2017), pectin (Liu et al., 2016), hyaluronic acid (Zhao et al., 2016; Wen et al., 2017), collagen and silk fibroin (Zhao et al., 2016). The basic concept of immobilization and delivery of natural antimicrobial agents through nanofibers for food preservation is represented in Fig. 4.
6. Marine algae as potential source of nanofibers
Nowadays, there has been much research focused on marine algae as a source of potential biopolymer source for large-scale production of nanofibers due to their ubiquitous and abundant and easy to harvest. Moreover, marine algal biopolymers such as sodium alginate, agar, fucoidan, and carrageenans come under GRAS category recognized by FDA (Tavassoli-Kafrani et al., 2016). Some researchers have already been successfully synthesized nanofibers from different kinds of marine algal biopolymers by electrospun methods such as alginate (Saquing et al., 2013; Hu et al., 2015; Wongkanya et al., 2017), ulvan (Kikionis et al., 2015), agar and agarose (Sadrearhami et al., 2015; Cho et al., 2016), fucoidan (Jang et al., 2015; Zhang et al., 2017) and carrageenan (Basilia et al., 2008; Serdar Tort and Füsun Acartürk, 2016; Goonoo et al., 2017). Though, in aqueous solution whole biopolymers alone cannot be fabricated into electrospun due to their poor mechanical properties and processability. Therefore, some synthetic polymers to be added with biopolymers as blending agents to form strong intermolecular hydrogen bond which helps easy spun of nanofibers (Safi et al., 2007; Saquing et al., 2013; Zhao et al., 2016). Many synthetic, semisynthetic and natural polymers have been applied for production of electrospun nanofibers. Synthetic polymers and copolymers such as poly (lactic acid) (PLA), poly (lactic-co-glycolic acid) (PLGA), poly(?-caprolactone) (PCL), poly(vinyl pyrrolidone) (PVP) and poly(ethyleneoxide) (PEO) have been used to produce Nanofibers (Brandelli and Taylor, 2015; Akhgari et al., 2017; Wen et al., 2017). Various marine algal biopolymers blended with synthetic polymers for nanofibers synthesis is shown in Table 2.
6.1. Sodium alginate
Alginates or sodium alginate (SA) or algins are biopolymer composed of two different linear copolymers such as uronic acids, ?-D-mannuronic acid (M) and ?-L-guluronic acid (G) linked in position 1?4. The salt forms (alginates), with several cations (Na+, K+, Mg2+ and Ca2+), are the significant components of brown seaweed cell walls and also of the intracellular matrix (Fathi et al., 2014; Pérez et al., 2016; Abdul Khalil et al., 2017). It is hydrophilic in nature with the molecular weight of alginate ranges of 500 and 1000 kDa (Pérez et al., 2016). Due to its biocompatibility, biodegradability, nontoxicity and low cost, sodium alginate has been well recognized for nanofibers synthesis for food application. Alginate is distinct from chitosan due to its high solubility in water (Zhao et al., 2016) which is an additional advantage to be used in electrospinning to produce nanofibers mats. In general, sodium alginate is composed of three different types of regions such as G, M and MG distributed in different extent in the polymeric chain which determine the physical properties of alginate. The gelling property of alginate is mainly decided G region which are composed of L-guluronic acids and M regions are entirely composed of D-mannuronic acid. MG regions consist of both M and G which determine the dissolving property of alginate in many solvents (Tavassoli-Kafrani et al., 2016). Sodium alginate makes strong bond with multivalent cations particularly calcium ions by producing hard gel which is the exclusive property of alginate (Tavassoli-Kafrani et al., 2016; Abdul Khalil et al., 2017). Therefore, SA is highly suitable for making nanofibers mat because for most of the cross-linking of biopolymers depends on calcium ions which strengthen the nanofibers.
Sodium alginate is the most studied biopolymer for electrospun among other biopolymers from marine algae. Saquing et al. (2013) produced alginate nanofibers blend with synthetic polymer Polyethylene Oxide (PEO) by electrospinning method. Hajiali et al. (2015) fabricated sodium alginate nanofibers containing lavender oil by the electrospinning method and demonstrated potential growth inhibition of bioactive nanofibrous against S.aureus. Citric acid cross-linked sodium alginate/PVA electrospun nanofibers were prepared by a team led by Stone et al. (2013). They used a homogeneous blend of sodium alginate-polyvinyl alcohol (1:1 weight ratio) containing cross-linking agent citric acid (5 wt%) for electrospun and found that cross-linked nanofibers were more heat stable and water-insoluble even after two days of immersed in water than the non-cross linked electrospun nanofibers which dissolved immediately. Recently Rafiq et al. (2018) successfully immobilized some essential oils (EOs) such as cinnamon, clove, and lavender on electrospun nanofibers synthesized from alginate and polyvinyl alcohol (PVA) blend and evaluated antimicrobial activity against Staphylococcus aureus. The results show that all EOs displayed good antimicrobial activity and the FTIR study was confirm the successful incorporation of essential oils in nanofibers.
Carrageenan is a sulfated water-soluble polysaccharide present in red algae, which consists of a linear sequence of other residues forming (AB)n sequence, where A and B are units of galactose residues. They are linked by alternating ?-(1?3) (unit A) and ?-(1?4) (unit B) glycosidic bonds. Carrageenans are polyanions due to the presence of sulfated groups (Cardoso et al., 2016). Carrageenans are classified into three groups based on degree of sulfation: they can be kappa (?) which contain 4-sulfated galactose and a 4-linked 3,6-anhydrogalactose, iota (?) is like kappa but with addition of sulfate ester group on C-2 of the 3,6-anhydrogalactose residue and lambda (?) contains 2-sulfated, 3-linked galactose unit, and a 2,6-disulfated 4-linked galactose unit (Tavassoli-Kafrani et al., 2016). Generally, carrageenan is extracted from following marine algal species include Kappaphycus alvarezii, Eucheuma denticulatum, Hypnea musciformis, Lamoroux and Solieria filiformis (Tavassoli-Kafrani et al., 2016; Cardoso et al., 2016)
Carrageenan is widely used as a functional ingredient in many food industries for various purposes (Tavassoli-Kafrani et al., 2016; Abdul Khalil et al., 2017). These three different carrageenan exhibits distinct gelation properties to each other. i.e., ? -carrageenan produce rigid and brittle gels, ?-carrageenan produces softer, elastic and cohesive gels and ? -carrageenan doesn’t form gels. This is due to the presence of different sulphate groups and anhydro bridges in carrageenan (Abdul Khalil et al., 2017). Some authors pointed out that carrageenans have biological properties such as anticoagulant, antitumor, immunomodulatory, anti-hyperlipidemic and antioxidant activities. They also have protective action against bacteria, fungi and some viruses (Silva et al., 2010; Zhou et al., 2004; Panlasigui et al., 2003; De Souza et al., 2007).
Basilia et al. (2008) produced polycaprolactone/carrageenan nanofibers by the electrospinning method and studied in vitro and in vivo for tissue engineering applications. Carter (2016) successfully encapsulated two essential oils such as carvacrol and eugenol in nanofibers synthesized from iota-carrageenan and tested against food pathogens L.monocytogenes and L.innocua. His results elucidated that carrageenan nanofiber encapsulated essential oils effectively inhibit the growth of tested food pathogens and potential release characteristic features. Another team led by Goonoo (2017), have reported the possibilities of producing nanofibers from mixed components of biodegradable polyhydroxybutyrate (PHB) or polyhydroxybutyrate valerate (PHBV) with the anionic sulfated polysaccharide ?-carrageenan (?-CG) by electrospinning method. Since, carrageenans are polyanionic, which makes strong bonds with polycations compounds (Eg. chitosan), thus, applying organic solvents and toxic cross-linkers can be avoided during nanofibers synthesis (Cardoso et al., 2016). It is an additional advantage of using carrageenans for nanofibers synthesis by electrospinning technique.
Ulvan is a complex, water-soluble sulfated anionic polysaccharide obtained from cell wall matrix of green algae, members of Ulvales (Chlorophyta) (Toskas et al., 2011). The name ulvan is derived from the original terms ulvin and ulvacin and usually extracted by the process of hydrolysis at around 80-90 °C using divalent cation chelator such as ammonium oxalate (Lahaye and Robic, 2007). Generally, ulvan extracted from following species such as Ulva pertusa, Ulva lactuca, Ulva clathrata, Ulva compressa, Ulva conglobata, and Enteromorpha prolifera (Majee et al., 2018). Ulvan is typically composed of ?- and ?-(1?4)-linked sugar residues, namely of ?-1,4- and ?-1,2,4-linked L-rhamnose 3-sulphate, with branching at O-2 of rhamnose, ?-1,4- and terminally linked D-glucuronic acid and ?-1,4-linked D-xylose, partially sulphated on O-2. The primary structural units found in ulvan include ?-D-glucuronosyluronic acid- (1,4)-L-rhamnose 3-sulphate dimer (?-D-GlcpA-(1?4)-LRhap 3-sulphate) and ?-L-IdopA-(1?4)-?-L-Rhap 3-sulphate, also known as ulvanobiuronic acid A and B, respectively. The main difference between these aldobiuronic acids is the presence of glucuronic acid in A, which is replaced by iduronic acid in B (Lahaye 1998; Quemener et al. 1997; Alves et al., 2013).
One extraordinary feature of ulvan is the occurrence of uncommon sugars within its fibers, i.e., sulphated rhamnose and iduronic acid. Rhamnose is an unusual sugar, typically found in bacteria, plants and accumulates uncommon in animals, and branching of O-2 of 1, the associated ?-L-rhamnose residue was found only on an exopolysaccharide produced by the bacterium Arthrobacter sp. The presence of iduronic acid in the ulvan chain signifies another salient characteristic since it does not recognize in algal polysaccharides (Quemener et al. 1997; Alves et al., 2013). The possibility of producing nanofibers from ulvan using electrospinning technique was first reported by Toskas et al. (2011). They obtained various sizes of nanofibers with different ratios of ulvan and copolymer PVA. For the rate of ulvan/PVA (50:50), they obtained nanofiber in the range of 105±4 nm, for 70:30, 84±4nm and for 85:15 ratios for 60±5nm were formed. Many reports indicated that higher concentration biopolymer than the synthetic copolymer in the mixer leads to the formation of beads in nanofibers. In contrary, in their findings, a higher concentration of ulvan shows the smaller size of nanofibers without any bead formation, and it was evidenced by SEM and TEM analyses. Another research report has published by Kikionis et al. (2015) from Greece, that ulvan could be converted into electrospun nanofiber by blending with two biodegradable synthetic polymers such as polyethylene oxide (PEO) and polycaprolactone (PCL). They investigated the synthesized nanofibers by SEM, FTIR which revealed the strong interaction and good compatibility between ulvan and the two copolymers. Moreover, the stability of ulvan nanofibers was examined and found that it does not lose its balance even after 18 months of storage which conclude that ulvan could represent new promising biomaterials for producing strong nanofibers for various biological applications.
6.4. Agar and agarose
Agar and agarose widely used as a gelling agent in the food industry and microbiological purposes, extracted from red seaweeds (Khalil et al., 2017). In general, agar is composed of two polysaccharides such as agarose and agaropectin with similar structural and functional properties as carrageenans. Agarose is the significant components of agar than the agaropectin, and it consists of high molecular weight polysaccharides composed of repeating units of (1?3)-?-D-galactopyranosyl-(1?4)-3,6-anhydro-?-L-galactopyranose. The structure of agaropectin, with a lower molecular weight than agarose, is mainly made up of alternating (1?3)- ? -D-galactopyranose and of (1?4)-3,6-anhydro- ? -L-galacto-pyranose residues (Pérez et al., 2016). Agar and agarose are associated with several biomedical applications especially as hydrogels for the release of bioactive agents, taking advantage of its ability to jellify, biocompatibility and biodegradability in nature (Cardoso et al., 2016). Only sparse details are available on nanofiber synthesis from agar and agarose due to limited research have been carried out on electrospinning of this marine algal polysaccharides.
Sadrearhami et al. (2015) attempted to produce nanofibers from agar with Polyacrylonitrile as copolymer to immobilize Methotrexate for cancer therapy and succeeded by electrospinning method. To evaluate the effects of polymer ratio and drug concentration on release rate, solutions with different polyacrylonitrile/agar ratios were prepared and subsequently electrospun with varying proportions of the weight of Methotrexate-polymer. The results demonstrated that increasing the drug and agar concentration led to rising in diameter due to increase in the solution blend viscosity which was evidenced by SEM analysis. Moreover, drug release rates increased with increasing agar ratio due to the increased hydrophilicity of the drug delivery systems. They concluded that novel agar nanofibers proved to be a potential candidate for controlled release of drugs or any other bioactive compounds for various biological applications. UV-irradiated agarose/polyacrylamide cross-linked double-network electrospun nanofibers were produced in the range of 187 nm by a group of Korean scientist led by Cho (2016). Strong interaction and compatibility of agarose/polyacrylamide in the nanofibers were proved by SEM and FTIR studies. Moreover, they analyzed the thermal stability of double-network nanofibers using thermogravimetric analysis (TGA) and showed the excellent thermal property at 290–500 °C. From the results, they recommended that agarose/polyacrylamide nanofibers could be used as possible material in biomedical and bioengineering applications.
Fucoidan is a water-soluble sulfate-rich polysaccharide mainly composed of fucose (Puvaneswary et al., 2016). The molecular weight of fucoidan have been recorded in the range of 100 to 1600 kDa, and the differences are due to differing in their composition and chemical structure including the degree of branching, substituents, sulphation and type of linkages (Rioux et al., 2007; Pérez et al., 2016). Mostly fucoidans are extracted from seaweeds like Laminaria spp., Analipus japonicus, Cladosiphon okamuranus, Chorda filum, Ascophyllum nodosum and Fucus sp. The molecular structure of fucoidans comprises of backbone structure of (1?3)-linked ?-L-fucopyranosyl ? (1?3) and ? (1?4)-linked L-fucopyranosyls (Tutor and Meyer, 2013; Pérez et al., 2016). Many names include fucans, fucosans, fucose is used for this group of polysaccharide extracted from other marine species, but fucoidan is the term hold for the algal source by IUPAC naming system (Pérez et al., 2016).
Lee et al. (2012) fabricated nanofiber using fucoidan and polycaprolactone (PCL) by electrospinning technique at various concentrations of fucoidans such as 1, 2, 3, and 10 % weight. The result shows that electrospun nanocomposites of fucoidan/PCL exhibited improved hydrophilicity, tensile and strength properties than the PCL fiber mats. In another study, fucoidan nanofiber was successfully produced with Chitosan and Poly(vinyl alcohol) for vascular tissue engineering (Zhang et al., 2017). After the electrospun, they obtained well defined interconnected nanofibers composed of F/CS/PVA which was evidenced by SEM and FTIR analysis. Moreover, they concluded that XRD pattern confirmed that electrospinning process of F/CS/PVA mixer has lowered the crystallinity of the polymers and have more excellent water uptake ability, sufficient porosity, enhanced drug release. Besides, fucoidan per se has various biological functions like anticoagulant, antiviral, immunomodulatory activity (Lee et al., 2012) and potential antibacterial activity against food pathogens (De Jesus Raposo et al., 2015; Chua et al., 2015). Hence, fucoidan is the promising candidate for making nanofibers to incorporate natural antimicrobial agents to preserve foods.
7. Advantages of marine algae for extraction of antimicrobial agents and nanofibers
The following essential features highlight that the marine algae are an ideal candidate than the other sources for isolation of antimicrobial compounds and to fabricate nanofibers to be used in food preservation. Since, marine algae exist in the complex living environment and are exposed to high salinity, consequently able to synthesize many different kinds of potential bioactive compounds that cannot be found in the terrestrial environment (Hamed et al., 2015).
• They won’t need agricultural land for cultivation as like terrestrial plants and oceans occupy 70% of the earth which makes them offers the unlimited resource for bioactive compounds (Tittensor et al., 2010; Hamed et al., 2015; Bajpai, 2016).
• It doesn’t compete with food chain since they grow in the marine environment (Sirajunnisa and Surendhiran, 2016). Whereas, herbs and spices are used for extraction of bioactive compounds that hikes food price. Because some South Asian countries like India, Pakistan, Bangladesh, etc. using spices are their primary food ingredients (Zaidi, 2016).
• Marine algae are measured as the best potential reservoir for antimicrobial compounds by their abundant throughout the coastal areas of many countries (Eom et al., 2012; Kadam et al., 2013); whereas some plant derivatives are seasonal.
• They don’t need any nutrients and fertilizer for their growth (Sirajunnisa and Surendhiran, 2016).
• Many organisms produce marine natural products that possess unique structural features as compared to terrestrial metabolites (Eom et al., 2012).
• Most of the marine algal bioactive compounds show both antimicrobial and antioxidant activities which are highly suitable for food application than the other sources (Senthilkumar and Sudha, 2012).
Marine algae are abundant in the seas that offer an unlimited renewable resource of natural antimicrobial and other bioactive compounds as well as biopolymers. Since marine bioactive compounds are unexplored in the terrestrial environment, this could actively restrict the growth of resistant food pathogens. Recent studies revealed that marine algal species represent an inspirational model for the development of new alternative antimicrobial agents to be used in food preservation. Moreover, marine algal biopolymers stay an unexploited pool of novel biomaterials for nanofiber synthesis due to their biocompatibility, biodegradability and low cost of the extraction process. Besides, marine algae contain an enormous quantity of nutritionally rich biomolecules that could be directly added into foods as functional ingredients. In conclusion, due to these reasons, marine algae have been widely studied as the raw material for the development of novel antimicrobial agents and carrier device for delivering them in food system without cause any harmful effects on consumers. Furthermore, the extraction and application of marine algal compounds in the food industry is in the nascent state and currently under investigation. Finally, more research finding is needed to achieve the possibilities of commercialization and succeed in near future.