Downstream processing of microalgae for pigments, protein and carbohydrate in industrial application: A review

Highlights

• Microalgal products based on pigments, proteins and carbohydrates.

• Downstream processing of microalgal products.

• Recent patent applications on downstream processing of microalgal products.

• Current research trends and market scenario.

Abstract

Numbers of high valued and low volume products from microalgae are already in the market and more number of products is yet to be launched. Downstream processing is the key steps to maintain the quality of the products. Therefore, this review is focused on the downstream processing of the microalgae and cyanobacterial cells for commercial production of biomolecules such as pigments (chlorophyll and carotenoid), protein, carbohydrate (agar, carageenan, alginate, fucoidan) and biopolymers. It also covers the most recent preferred downstream processes for the industries for accomplishing the desired quality of the product and recent important filed patents on downstream processing of microalgae. Further, this article highlights the recent investments made by the industries towards commercial production of microalgal-based biofuels and bio-products worldwide.

Introduction

Microalgae (microalgae mean both microalgae and cyanobacteria) are considered as potential source of high-value nutrients such as pigments, proteins, carbohydrates and lipid molecules (Ghosh et al., 2016, Pignolet et al., 2013, Ursu et al., 2014). Industrial scale production of microalgae has evolved worldwide due to human consumption of microalgae as nutritional supplements (Spolaore et al., 2006). In the past, successful industrial scale production of microalgae was made using green algal species Chlorella and cyanobacterial species Spirullina (Ciferri and Tiboni, 1985, Liu and Chen, 2014). Large production of microalgal biomass is costly and required a high volume of seed culture to ensure purity of the cultures (Chisti, 2016, Das et al., 2015). However, research is still continuing on the sustainable, economical production of microalgae biomass at large scale (Su et al., 2008). Harvesting processes of microalgal biomass at large scale, are also costly and energy intensive. Harvesting processes include several techniques such as centrifugation, foam fractionation, chemical flocculation, electro-flocculation, membrane filtration, and ultrasonic separation etc. (Zeng et al., 2016). Some other harvesting process like gravity sedimentation required low energy, but economically non-viable in the industry due to very slow rate (Pittman et al., 2010). Selection of harvesting technique for the processing of microalgae is largely depend on the type of strains, culture condition and the cell density in the harvesting medium (Singh et al., 2010). Therefore, finding an efficient harvesting process is still under research for the industrial application for the production of primary and secondary metabolites from the microalgae (Ghosh et al., 2017, Ummalyma et al., 2016, Vandamme et al., 2013, Wan et al., 2015).

Apart from biomass, microalgae produces variety of pigments molecules like chlorophyll, carotenoids and β-carotene that are used as colourants in cosmetic and food industry. Algae strains, Chlorella sp., Dunaliella sp., and Scenedesmus sp., and cyanobacterial strains (Spirulina sp. and Nostoc sp.) are used as sources of fine chemicals and nutrient rich food supplements (Kay, 1991, Sanmartin et al., 2010, Seo et al., 2013). Further, pigments from the microalgae are used in cosmetics industry as anti-ageing cream, refreshing or regenerating care products for healing and repairing of damaged skin with nourishments (Tominaga et al., 2012, Zhu, 2015). For example, the microalgae, Haematococcus pluvialis, is known as the natural source of keto-carotenoid astaxanthin. Red pigment astaxanthin is the precursor molecule of vitamin A and this pigment play important role in embryo development and cell reproduction in poultry and aquaculture firms (Goto et al., 2015, Lorenz and Cysewski, 2000). Moreover, astaxanthin has superior antioxidant properties compared to those of β-carotene, α-carotene, lutein, lycopene, canthaxanthin, and vitamin E and therefore, is becoming popular as a human dietary supplement (Guerin et al., 2003, Johnson and An, 1991). Consequently, a number of industries, such as Cyanotech, Seambiotic, Mera Pharmaceuticals and Fuji Chemical, are producer of microalgae biomass for value added products in cosmetics, nutritious feed and pharmaceuticals (Becker, 2007, Zhu, 2015). Selection of suitable process for pigment extraction from the microalgae depend on the several factors like biochemical features of pigments, choice of solvents for extraction, extraction yield, duration of extraction, reproducibility, denaturation and degradation of molecules, cost and easy operation (Cuellar-Bermudez et al., 2014). Industry often uses classical organic solvent extraction techniques like soaking, percolation, counter-current extraction, pressurised liquid extraction, and soxhlet extraction (Sun et al., 2012). However, these processes have certain disadvantages and limited for industrial application due to large amounts of solvent requirement, environmental pollution, risk of health hazards and denaturation of biomolecules (Peshin et al., 2002). Efficient extraction process of the biomolecules from the microalgae is the growing demand for the industries. A number of processes like ultra high pressure extraction, use of supercritical carbon dioxide for extraction, microwave processing, combination of techniques such as soaking in liquid nitrogen followed by buffer extraction, combination of solid-phase and supercritical fluid extraction are currently being exploited and under research for further development to establish energy efficient, low cost extraction technique for the pigments (Abrahamsson et al., 2012, Biller et al., 2013, Klejdus et al., 2009, Lowrey et al., 2015).

Microalgae also considered as reliable rich source of vegetable protein (Becker, 2007). Nutritional studies on different microalgae have demonstrated that microalgae produced high amount and high quality proteins, which are the source of essential amino acids (Galland-Irmouli et al., 1999, Gutierrez-Salmean et al., 2015, Hosseini et al., 2013). Recently, microalgae shows potential as an alternative expression host for recombinant protein production (Specht et al., 2010). Certain algae species like red algae (Rhodophyta) and cyanobacterial strain produce a group of accessory photosynthetic pigment protein complexes for light harvesting purpose are called phycobiliproteins (Belford et al., 1983, Gantt and Conti, 1966). These proteins have high demand in pharmaceutical industries and specific application in the biological field as fluorophore (Kuddus et al., 2013, Rijgersberg and Amesz, 1980, Yaakob et al., 2015). Conventional methods utilised for cell disruption of microalgae before the extraction of the proteins includes bead milling, high pressure and high speed homogenisation, ultrasonication, microwave treatment, pulsed electric field treatment, pre-heat treatment, enzymatic treatment etc. (Demuez et al., 2015, Gerde et al., 2012, Gunerken et al., 2015, Prabakaran and Ravindran, 2011). Protein extraction from microalgae is done using aqueous, acidic, and alkaline methods, followed by centrifugation, ultrafiltration, precipitation, or chromatography techniques for the recovery of the protein molecules (Sari et al., 2015). Industrial scale extraction and purification of proteins from microalgae is not studied widely and scalable downstream processes of microalgae for efficient extraction of proteins are still in very high demand. Diversity in microalgal species, variation in the cell structure, variation in the intracellular protein content, release of protein degrading (protease) enzymes from the cells are major obstructions for up-scaling of the protein extraction process. Some novel extraction techniques such as pulsed electric field, microwave-assisted extraction and ultrasound-assisted extraction are employed for successful extraction of proteins from microalgae.

Manipulation in growth conditions can enrich microalgae with high amount of carbohydrate or polysaccharide molecules. Major components of the cell wall of algae are cellulose and hemicellulose. Other than the cell wall, algae also store polysaccharide molecules also in the cytoplasm. Marine algae produce complex sulfated cell wall polysaccharides, which have many biomedical applications (Domozych et al., 2012). Some cyanobacterial strains (e.g. Nostoc sp., Spirulina sp., Porphyridium sp.) are surrounded by a matrix of polymeric substance mainly constituted by polysaccharides, which form a protective layer between the cell and the immediate environment (Colica and Philippis, 2013). Biotechnological potential of the cyanobacterial extracellular polymeric matrix are attracting increasing attention to the pharmaceutical, bio plastic and food industries. Conventional extraction of extracellular polymeric matrix from cyanobacterial strain Porphyridium sp. involves alcoholic precipitation followed by dialysis and membrane filtration (Patel et al., 2013). However, research for the scalable operations to extract polysaccharides from microalgae is in urgent need to facilitate the applications of microalgae for the industries. Novel extraction technologies such as enzyme-assisted extraction, microwave assisted extraction, ultrasound assisted extraction, supercritical fluid extraction, and pressurised liquid extraction are currently being applied for the extraction of bioactive molecules from microalgae (Kadam et al., 2013). These extraction technologies are attracting interest from the industries because of its advantages (higher yield, reduced treatment time, and lower cost) compared to the conventional solvent extraction techniques.

Recent studies had shown that microalgal biofuel is still not profusely producing at commercial level despite its high potential due to high production cost (Gendy and El-Temtamy, 2013). In an elongation of previous statement, several molecules extracted from microalgae (e.g. astaxanthin, β-carotene, omega-3-fatty acids etc.) have high demand in the market and there are opportunities for additional new products like nutraceuticals, pharmaceuticals (e.g. lutein, zeaxanthin), biopolymers and bio plastics, which can help the microalgal products based industry to flourish. In addition to its irreplaceable value as a substitute of fossil fuel, global market for this high value algae product (cosmetics, nutraceuticals and pharmaceuticals) is worth around US$ 2 billion (www.oilgae.com, 2016).

Technological development of efficient downstream processing of microalgal products is one of the key factors that determine the economic viability of algae based products. In this direction, a number of review articles are reported based on lipid derived biodiesel production (Roux et al., 2017, Scott et al., 2010). However, limited articles focused on the downstream processing of microalgal products based carbohydrate, protein and pigments. Hence, this review article focuses on: i) microalgae based products (based on carbohydrate, protein and pigments) at commercial scale with market demand and leading industry involved; ii) detailed technological advances for downstream processing of microalgal products; iii) extensive search for patented invention have been reported in relation to product specific extraction; iv) current scenario on global algal biomass market with future perspectives.