Yingtai: Application of Vacuum Freeze-Drying Technology in Plankton Preparatio

Yingtai: Application of Vacuum Freeze-Drying Technology in Plankton Preparatio  

Due to health concerns associated with meat production, there is a growing shift from animal-based protein foods to plant-based alternatives. This trend has prompted research into alternative high-quality protein sources, such as single-celled organisms, particularly functional proteins. In recent years, microalgae have garnered significant attention as a reliable renewable energy source. They are also recognized as third-generation biofuels, capable of yielding up to 30 times more energy per unit area compared to first- and second-generation biofuels. Additionally, microalgae exhibit higher growth rates, carbon fixation efficiency, and lipid production than terrestrial plants. The primary components of interest harvested from microalgae include proteins, lipids, carbohydrates, and minor amounts of vitamins, pigments, and sterols.  

Cultivation of Microalgae Species  

Microalgae can be cultivated on a large scale in photobioreactors or open pond systems. After cultivating the target strain, the final biomass is harvested through a series of steps, including biomass separation, screening, thickening, dehydration, and drying, followed by biorefining to extract the desired products. Optimizing these steps is crucial for the efficient production of high-quality algal biomass.  

Chlorella is one of the most commonly consumed microalgae species and has been commercially cultivated since the 1960s. The default growth condition for Chlorella is photoautotrophic (using light as an energy source). However, it can also grow heterotrophically on various organic carbon sources. Heterotrophic growth conditions are often employed in large-scale production due to their advantages, including higher biomass concentration, lower operational costs, reduced contamination risks, and extended continuous operation times.  

Following cultivation, the biomass is typically dried to extend its shelf life. Depending on drying conditions, the colloidal properties of heat-sensitive compounds (such as proteins) may be altered, potentially affecting protein quality. Additionally, *Chlorella* produces a range of volatile organic compounds (VOCs), which contribute to its distinct flavor profile. Processing, particularly heat treatment, can degrade existing VOCs and generate new ones, thereby modifying the sensory characteristics. Therefore, evaluating the effects of drying processes is essential for enhancing the application of microalgae functional components in innovative food products.  

Dehydration of Microalgae  

Dehydration and drying are critical steps in downstream processing. Thus, it is important to consider different technologies and their impacts on cost, energy consumption, and—most importantly—the quality of the biomass, high-value products, and metabolites. Dehydration removes a significant portion of water, facilitating efficient downstream processing and reducing the energy and cost required for subsequent drying steps. However, dehydration alone accounts for approximately 20–40% of the total energy consumption in the microalgae harvesting process. Moreover, in cases where the product is intended for human or animal consumption, contamination risks must be minimized.  

Drying of Microalgae  

The drying process is considered a critical step because the algal slurry obtained from upstream harvesting is often fragile. According to some researchers, drying is the most energy-intensive step, accounting for over 80% of the total cost in producing algal-based products such as biodiesel. Given that microalgae are susceptible to microbial contamination, mechanical damage, and unfavorable storage conditions—all of which may degrade biomass quality—effective drying is essential for optimal storage.  

Various drying methods are available for microalgae, including conventional sun drying, hot air drying, freeze-drying, microwave drying, oven drying, and spray drying. Traditional methods such as sun drying and oven drying are often used due to their lower energy and capital requirements. However, these methods have limitations. For instance, sun drying is prone to contamination from external sources (e.g., birds, insects, and microorganisms) and is highly dependent on weather conditions, making it unsuitable for regions with frequent rain or limited sunlight. Additionally, direct solar radiation can degrade pigments such as chlorophyll. Oven drying, on the other hand, may adversely affect heat-sensitive metabolites and bioactive compounds.  

Advanced drying processes such as freeze-drying and spray drying have become increasingly common for microalgae biomass. Freeze-drying is one of the safest drying methods, preserving important byproducts that might otherwise be lost, while spray drying offers time efficiency and high-value product yields. However, these methods have drawbacks, including high operational and maintenance costs. Furthermore, spray drying involves high-pressure mechanisms that may rupture cells and degrade high-value components such as pigments.  

The choice of drying method depends on available capital, energy resources, and the importance of the byproducts to be obtained from the algae. Although downstream processes like dehydration and drying are crucial for extracting valuable, high-quality algal biofuels, food supplements, or animal feed ingredients, there is a lack of comprehensive studies highlighting the significance of these processes.