Microalgae have drawn the attention of many researchers in recent times as a reliable and renewable source of energy (Ma et al., 2022). They have also been introduced as the third-generation biofuel and have been known to yield 30 times more energy per area unit compared to first- and second-generation biofuels (Chamola et al., 2019). Furthermore, they have a higher growth rate, carbon fixing ability and elevated lipid production compared to terrestrial plants (Show and Lee., 2014).
The major components of interest harvested from microalgae include proteins, lipids, carbohydrates and minor concentrations of vitamins, pigments, and sterols (Shaikh et al., 2021). These valuable products can be used to produce feedstocks for food and feed supplements, nutraceuticals, pharmaceuticals, and as chemicals (Lammens et al., 2012).
Microalgae can be cultivated at a large scale in photobioreactors or raceway ponds (Shi et al., 2018). This involves various stages, starting with the cultivation of the specific strain followed by harvesting through a series of steps including biomass separation, screening, thickening, dewatering, and drying and lastly biorefinery for extracting the products of interest (Show and Lee., 2014). It is crucial to optimize these steps for the efficient production of high-quality algal biomass. As dewatering and drying are vital elements in downstream extraction, it is important to consider the different techniques and their impacts on the overall production energy and cost and more importantly on the quality of biomass, products, and metabolites.
Dewatering the algae harvest efficiently ensures effective processing in downstream drying processes. Reduction of water through dewatering ensures lower cost and energy needs in overall drying steps as it aims to remove most of the water from the algae harvest (Show et al., 2015b). Dewatering requires approximately 20–40% of the energy requirements of the whole microalgal harvesting process (Yazdanabad et al., 2021). Although it is achieved through various mechanical processes such as filtration and centrifugation, a major setback has remained to be poor economic feasibility (Sharma et al., 2013). Additionally, contamination risks need to be eliminated in case of production for human or animal consumption (Yazdanabad et al., 2021).
The drying process is also important as the dewatering of microalgae. It is also considered a crucial step as the algae slurry achieved from the upstream harvesting processes can be fragile. According to (Patil et al., 2008) drying process requires the most energy accounting for over 80% of the total cost of algal products such as biodiesel production. As algae can be susceptible to microbial contamination, mechanical damage and adversary storing conditions, which may lower the quality of products, it is important to dry algae efficiently for optimal storage (De Farias Neves et al., 2020). There are many ways of drying algae including conventional sun drying, hot air drying, freeze-drying, microwave drying, oven drying and spray drying (Brennan Owende, 2010).
The most traditional method of drying algae is sun-drying. The dewatered microalgae biomass is kept outside until the water content decreases to an acceptable limit. Oven drying utilizes high-temperature exposure for long periods of time to remove moisture from the algae biomass (Badmus et al., 2019). While freeze-drying is conducted by introducing the feed to lower temperatures to dry out and separate moisture from the biomass. Spray drying involves droplet/gas mixing, liquid atomization, and drying liquid droplets. Water droplets are atomized and sprayed down into a vertical tower that contains hot gases. The dried algae biomass is collected from the bottom of the tower and is available within a few seconds of the drying process (Hosseinizand et al., 2017).
Conventional methods such as sun drying, and oven drying are usually pursued as these methods do not require high energy and capital input. But these methods may not be preferred for reasons such as susceptibility to contamination from outside sources such as birds, insects, and microorganisms in the case of sun-drying (Show et al., 2015a). Furthermore, the method heavily relies on the weather, as it may not be feasible for areas with high rains and low sunlight zones. Another reason is the degradation of pigments such as chlorophyll due to direct solar radiation (Wang et al., 2019). Additionally, oven drying can negatively impact the heat-labile metabolites and bioactive compounds (Kadam et al., 2015). On the other hand, processes such as freeze-drying, and spray drying have become more common for drying the algae biomass. Freeze drying can be one of the safest forms of drying in terms of retaining important byproducts which may be lost otherwise. While spray drying process can be time efficient and produce high-value products (Molina Grima et al., 2003). The disadvantages of these methods are high operational and maintenance costs. Additionally, the spray drying method includes high-pressure mechanisms, which may rupture cells and degrade high value-added components such as pigments (Hosseinizand et al., 2018).
In a study conducted by (Ryckebosch et al., 2011) spray and freeze-drying of microalgae were conducted to investigate the effect on lipid and carotenoid stability. It was found that algae paste that was freeze-dried showed signs of lipolysis when stored, leading to lower lipid content. On the other hand, microalgae that were spray-dried were more susceptible to oxidation as there was the breakdown of carotenoids. While Morist et al., (2001) investigated spray drying influence on the fatty acid profile of Arthrospira plants, no significant changes were found in the content of common fatty acids such as linoleic acid, palmitic acid and palmitoleic acid so on. In another study, freeze and air drying were compared for future commercialization prospects. Two microalgae strain Chaetoceros sp. and Phaeodactylum tricornutum were used and lipids were profiled. It was found that air drying led to the loss of almost 70% of the lipids compared to freeze-drying (Esquivel et al., 1993). Another effect that was noted by Oliveira et al., (2010) of air drying was oxidation of the biomass.
Finally, the choice of drying depends on the capital and energy sources available and on the importance of byproducts that need to be successfully attained from the algae harvest (Chen et al., 2015). Even though downstream processes such as dewatering and drying can be vital in the extraction of valuable high-quality algal biofuels and feedstocks for food supplements or animal feed, there is a lack of research outlining the importance of these processes (Ryckebosch et al., 2011).
This study aims to investigate the effect of five different drying techniques on the nutritional quality of the microalgae biomass. For that purpose, an assessment of the Chlorophyll, proteins, lipids, and FAME content was performed to select the most suitable drying technique leading to maintaining high-quality biomass that can be used ultimately for animal-feed production therefore it will enhance the viability of the large-scale biomass production under arid climate.