Integral Utilization of Red Seaweed for Bioactive Production: History Edit
Subjects: Plant Sciences

1. Introduction

Seaweeds are widespread and traditionally used in Eastern countries for food and for medicinal purposes. In Western countries, despite being included recently in the diet, direct human consumption is still unusual, mainly being used for the production of hydrocolloids with thickening and gelling properties. Among seaweeds, red algae (Rhodophyta) contain high amounts of polysaccharides (floridean starch and sulfated galactans, such as carrageenans or agarans), proteins and derived peptides (phycobiliproteins, phycolectins and mycosporine-like amino acids), minerals and other valuable compounds, such as polyphenols and lipids [1,2]. The whole algae of red seaweeds have been traditionally used as food, while agars and carrageenans have been extracted for multiple purposes, namely for food, pharmaceutical applications and biotechnological applications.

The reader can find compilations on the chemical and nutritional characteristics of seaweeds as a feed livestock resource [3] and its health-promoting properties [4], including the anticancer [5] and antiviral [6] features. Additionally, there have been some interesting reviews on the use of red seaweeds for carrageenans [7], agar and carrageenan oligosaccharides [8]. The extraction technology, which influences the composition, structure and properties of the target solutes, is conventionally addressed using chemicals, in long term operation, and with high energy consumption [9]. The recent developments of blue biotechnology and novel extraction techniques meeting the requirements of low cost, sustainability, food-compatibility and industrial scale feasibility to obtain seaweed components [10], the systematical selection of operational variables of emerging extraction technologies [11] or the use of alternative solvents, such as ionic liquids [12], or supercritical CO2 for macro and microalgae [13] have been reviewed. Scarcer studies are focused on the extraction of red seaweed components, such as carrageenan and agar fractions [9,14], photoprotective substances [15] and pigments [16]. Particular interest has been on the influence on the depolymerization of saccharidic and protein components, since both the type of extraction process and the operational conditions have to be controlled depending on the future uses [17]; however, further development and additional studies are still needed [18]. The mentioned studies are oriented to the selective extraction of some valuable fractions, but scarce information is found for the simultaneous utilization of the different components in a more rational scheme, following the philosophy of the biorefineries [19]. Biomass refineries, with a production scheme analogous to the petroleum refineries, are aimed at obtaining a wide range of products from renewable raw materials, including value added components for the food, cosmetic and pharmaceutical industries, as well as biofuels. These multistage multiproduct processes are based on the sequential fractionation of the biomass and on their subsequent physical, chemical or biotechnological transformation into the target final products. This sustainable approach adopts an integral utilization of resources, promoting the development of a marine bio-economy.

The present review presents an overview of the properties and potential applications of red seaweed bioactives, the specific technologies for extraction and also for the depolymerization of agar and carrageenan into oligosaccharides, as well as the potential of these techniques for the extraction of other red seaweed components. Both conventional and emerging extraction and depolymerization technologies are discussed with the aim of promoting the sustainability based on i) the development of clean processes and ii) the integral utilization and valorization of resources following the philosophy of biomass biorefineries.

  1. Components: Properties and Extraction

2.1. Polysaccharides

Polysaccharides are the main components in marine algae according to their abundance and their current commercial value based on their technological features [4,20]. More recently, attention has been directed to their health benefits [21,22]. These polysaccharides, generally not digested by humans, are considered to be dietary fibers [23]. The composition, structure and rheological properties are influenced by the algal source, life-stage, growth, environment and by the extraction method [24]. Agars and carrageenans are major cell wall polysaccharides in red macroalgae, also known as galactans, accounting for up to 40–50% of the dry weight. They are highly anionic homopolysaccharides, composed of a backbone built from disaccharide blocks of D-galactose and 3,6-anhydrogalactose (L-AHG in agar and D-AHG in carrageenan) with different sulfation, methylation and pyruvation patterns that vary among species [21,25]. The high electronegative charge density from their sulfated esters favors the electrostatic interactions with specific proteins, determining their biological effects, which are also closely related to the structural features [20,26–28]. Proteins, minerals and lipids also confer red seaweed important structural value [29].

2.1.1. Agar

  • Composition, structure, occurrence and properties

Agar is a linear polysaccharide composed of alternating (1,3) linked D-galactose and (1,4) linked 3,6-anhydro-l-galactose [25] and substituted in some degree by sulfate, methyl or pyruvate groups [30–32]. The molecular structure of agar polysaccharides, particularly the type and location of sulfate esters, appears to be species-specific [33]. Agar has two different constituents: agarose and agaropectin (Figure 1). Agarose is a neutral linear polysaccharide composed of three linked β-d-galactose and four linked 3,6-anhydro-α-l-galactose. Agaropectin is an acid polysaccharide containing sulfate groups, pyruvic acid, and D-glucuronic acid conjugated to agarobiose. Agarose accounts for up to 70% of the mixture and is responsible for gelling, whereas agaropectin is responsible for thickening characteristics. Different derived agarose molecules can be obtained from chemical or enzymatic degradation. Most of the corresponding hydrolysis products such as agarooligosaccharides (AOSs), neoagarooligosaccharides (NAOSs), neoagarobiose (NAB) and 3,6-anhydro-l-galactose (L-AHG) exhibit biological activities [34].

Agar is mainly found in the cell matrix of seaweeds of the order Gelidiales (Gelidium and Pterocladia) and Gracilariales (Gracilaria and Hydropuntia), which have become the major worldwide source. Its abundance and easier exploitation made G. tenuistipitata an economically important raw material for agar production [35]. In comparison with agars from Gelidium and Pterocladia, agars from Gracilaria can have higher degrees of sulfation, methoxylation and pyruvylation [31].

The agar properties are dependent on the species and environmental characteristics of the collection or cultivation area, such as season, life cycle and geographical features [36,37] and the storage, extraction processes and postharvest storage [24,32,38–41]. The quality of agar is determined by the type, pattern and degree of substitution as well as molecular weight, chemical composition (pyruvate, methyoxyl and sulfate) and physical properties (gel strength, gel syneresis, viscosity, gelling and melting temperatures) that determine its market value [30,32,42]. The agar gel strength, in terms of elastic modulus (G’), of systems formulated at 1.5% agar in milli-Q water is around 238 Pa at 25 °C, with gelling temperatures of 48 °C, and those agars with gel strengths greater than 6.9 104 Pa are referred to as high quality agars [24].

 

Figure 1. Scheme of the agar constituents (agarose and agaropectine) and different derived molecules with biological activities, adapted from [43,44].

Agar is a generally recognized as safe (GRAS) food additive in the United States and a food additive approved in Europe (E406). Agar cannot be digested in the gastrointestinal tract because humans lack α/β-agarases, but can be metabolized by intestinal bacteria to D-galactose [45]. Agar is demanded as gelling agent and stabilizing agent, and as cryoprotectants in the pharmaceutical, cosmetics and food industries [39,46–49]. The human food industry demands for 80% production, and biotechnological applications for the remaining 20% [50]. The importance of these products is based on high market demand for agar and the higher price compared to alginates and carrageenans [24,32,51]. It is used as a gelling, thickening and stabilizing agent in food formulations and it has also been used in microbiological media and in chromatographic techniques. Most native agars from Gracilaria are not bacteriological grade agar due to their high content of methoxyls, but they can be food and reactive grade [52].

  • Extraction processes: conventional and emerging technologies

The storage conditions and duration before extraction affects the agar quality from Gracilaria, since seaweeds are susceptible to degradation by agarolytic enzymes and bacteria. Some species from temperate and cold water could be more resistant to hydrolysis during storage. Postharvest treatment with acid, alkali or formaldehyde is necessary to prevent enzymatic and microbial degradation [32,37,40,47,50,53]. Another factor requiring attention after harvesting algae is correct drying under 20% moisture and packing, and avoid wetting during the transporting and storage period, but dewatering pre-treatments have to be defined according to the species and to collection season [54].

Although Gelidium agar has better quality and is easily extracted with boiling water, the gelling ability of agars from Gracilaria sp can be enhanced by an alkali pretreatment to convert α-l-galactose 6 sulfate into 3,6-anhydro-α-l-galactose. This treatment reduces the sulfate content and improves the gelling properties as evidenced by higher gel strength, gelling, melting temperatures and viscosity [42]. Generally, the alkali-treatment was most effective for obtaining more galactose-rich hydrocolloids [24]. However, agar degradation and diffusion towards the aqueous medium could occur, reducing the extraction yield [29,55], although in some cases, no reduction was observed [24]. Alkaline pretreatment variables, such as alkaly type and concentration or heating time and temperature affected the yield and quality of the agar. Regardless the alkaline concentration, NaOH rendered agar with a higher quality than KOH [35,56]. Compilations of conditions are also found in [37], being the optimal in the range 5–7% NaOH, up to 80–100 °C for 0.5–3 h [29,35,46,48,50], but higher alkali concentrations (10%) [55], shorter times [57] and the application of several stages [53] have also been reported. An alternative pretreatment was proposed by Roleda [58], which consisted of soaking the Gelidiella acerosa air dried sample in 0.5% acetic acid for 1 h at 16–20 °C, then 1 h steam pressure at 15–20 psi and boiling at 100 °C. Freile-Pelegrín [59] proposed the cultivation of Gracilaria cornes under dark and salinity treatments (50 and 25% salinity) to replace the alkali treatment. Pigments, such as chlorophyll, carotenoids and phycoerythrobilin, can be leached out during the alkaline pretreatment and an alternative environmentally friendly scalable photobleaching process for Gracilaria asiatica and Gracilaria lemaneiformis with 3–5% NaOH and photobleaching for 5 h was proposed [60]. The pigments and the agar sulfate contents decreased during the photobleaching agar extraction process, and the gel strength increased during the photolysis.