Due to food health issues in animal meat production, people are transitioning from animal-based protein foods to plant-based protein foods. This trend has prompted research efforts to consider alternative high-quality protein sources, such as single-cell organisms, especially functional proteins.
In recent years, microalgae have attracted the attention of many researchers as a reliable renewable energy source. They are also being introduced as third-generation biofuels, which can increase energy per area unit by up to 30 times compared to first- and second-generation biofuels. In addition, they have higher growth rates, carbon fixation capabilities, and higher lipid production compared to terrestrial plants.
The main components of interest harvested from microalgae include proteins, lipids, carbohydrates, and small amounts of vitamins, pigments, and sterols.
Microalgae can be cultivated on a large scale in photobioreactors or raceway ponds. After culturing the target strain, the final biomass is harvested through a series of steps, including biomass separation, screening, thickening, dehydration, and drying, and finally a biorefinery to extract the target product.
Optimizing these steps is essential for the efficient production of high-quality algae biomass. Chlorella is one of the most commonly consumed microalgae species and has been cultivated commercially since the 1960s. The default growth conditions for Chlorella are photoautotrophic (light as an energy source). However, it is also capable of heterotrophic growth on a variety of organic carbon sources.
Such heterotrophic growth conditions are often used in large-scale production because higher biomass concentrations, lower operating costs, less pollution, and longer continuous operation times can be achieved.
After the cultivation step, the biomass is often dried to extend the shelf life. Depending on the drying conditions, the colloidal properties of heat-sensitive compounds such as proteins may be affected, resulting in altered protein quality. In addition, Chlorella produces a range of volatile organic compounds that contribute to distinct flavor profiles. Processing, especially thermal treatments, leads to the degradation of existing volatile organic compounds and the formation of new ones, resulting in changes in the sensory profile.
Therefore, evaluating the effects of the drying process is essential to increase the application of microalgae functional ingredients in innovative food products.
Dehydration and drying are important elements in downstream extraction. Therefore, it is important to consider different technologies and their impact on cost and energy, and most importantly the quality of biomass, high-value products and metabolites. Dehydrating microalgae ensures efficient downstream processing by removing most of the water, thus reducing the cost and energy required for the subsequent drying step, although dehydration requires approximately 20-40% of the energy required for the entire microalgae harvesting process.
In addition, in the case of production for human or animal consumption, the risk of contamination needs to be eliminated.
Drying process This is considered a critical step, as the algae slurry obtained from the upstream harvesting process can be fragile. According to some researchers, the drying process requires the most energy, accounting for more than 80% of the total cost of manufacturing algae products such as biodiesel.
Since algae are susceptible to microbial contamination, mechanical damage and adverse storage conditions, which may reduce the quality of the biomass, it is important to dry the algae efficiently for optimal storage. There are many methods for drying algae, such as conventional sun drying, hot air drying, freeze drying, microwave drying, oven drying and spray drying.
Conventional methods, such as sun drying and oven drying, are usually adopted because they do not require high energy and capital inputs. However, these methods may not be the right choice, for example, they are susceptible to contamination from external sources such as birds, insects, and microorganisms when drying in the sun. In addition, this method is highly dependent on weather conditions and may not be feasible for rainy and low-sun regions. Another reason is the degradation of pigments such as chlorophyll due to direct solar radiation.
In addition, oven drying can negatively affect thermally labile metabolites and bioactive compounds.
Processes such as freeze drying and spray drying have become increasingly popular in drying algal biomass. Freeze drying is one of the safe forms of drying that can preserve important byproducts that might otherwise be lost, while the spray drying process can save time and produce high-value products. Disadvantages of these methods include high operating and maintenance costs.
In addition, the spray drying method includes a high-pressure mechanism, which may rupture cells and degrade high-value components such as pigments.
The choice of drying depends on the available capital and energy, as well as the importance of the byproducts that need to be successfully obtained from the algae harvest. Although downstream processes such as dehydration and drying are essential for extracting valuable, high-quality algal biofuels and food supplements or animal feed ingredients, there is a lack of research outlining the importance of these processes.