Microalgae grow into a nutritious biomass, which can be processed into a wide range of products or ingredients, such as proteins, polyunsaturated fatty acids (PUFAs), antioxidants and other bioactive compounds. But to turn the harvested algae into a safe and long-lasting final product, there is a vital - and heavily energy consuming – step: the drying process.

With a mission to contribute to more sustainable food systems, ProFuture is leading pioneer trials to assess different drying techniques that could help make the processing of microalgae protein ingredients more efficient, affordable and less energy-demanding.

Drying microalgae proteins – freeze drying vs solar drying

Removing the water from food is an important step of food processing. It allows to preserve perishable goods for much longer (months or even years!) and makes them more convenient for transport, by decreasing their volume and weight. Depending on the desired final product, different drying techniques can be used, which differ in cost, energy, time-efficiency and the extent to which they preserve the nutritional value of the product.

Microalgae intended for food and feed applications, are often dried by freeze drying. This technique involves freezing the product and then removing the ice by sublimation with vacuum. One of the downsides of freeze drying is that it demands high operating costs and energy use, resulting in a high financial burden. On the other hand, one of its greatest advantages is that it preserves the nutritional value of microalgae, keeping most of its vitamins and PUFAs (the so called “good fats”). These are essential to create nutritious food and feed products with high quality standards!

In turn, solar drying is an antique form of drying which uses the sun as a source of energy to dry products. Compared to freeze drying, solar drying has lower investment and operating costs and presents as a more energy efficient process by being able to depend only on solar energy. Even if industrial solutions use electronics and accessory equipment to control and speed-up the drying process, the energy use in solar drying is usually lower, and close to zero in sunny days.

Solar drying is commonly used to dry agricultural products, such as fruits, vegetables and herbs. However, ProFuture researchers are now testing the effects of solar drying in microalgae and assess its potential as a more sustainable and efficient form of drying. If solar drying proves to be effective in drying microalgae, the drying procedure would consume less energy and have less costs.

What is a solar dryer and how does it work?

A solar dryer is a device that uses solar energy to dry agricultural products. The solar dryer used in ProFuture consists of two parts:

• a drying chamber (inside) – where the products are spread on trays or racks, and hot dry air passes through them;

• a heat collector (outside) – the black roof and external walls of the dryer collect the solar thermal energy and drive it into the drying chamber through a ventilation system.

ProFuture’s solar dryer prototype (BlackBlock), installed at Necton’s facilities, is a hybrid dehydration system that uses an algorithm designed to run with low energy consumption while assuring the conditions to keep the nutritional quality of the algal biomass.

The system self-regulates the humidity and temperature in the solar dryer, by adjusting the amount of hot air kept inside or released outside through the ventilation system and by turning on and off a dehumidifier and a heater. As a result, the solar dryer keeps the optimal conditions for the drying to run smoothly, allowing the researchers to monitor the process remotely.

© BlackBlock by BBKW, Necton, Portugal.

What are the main challenges and opportunities of the solar dryer?

One of the main advantages of solar drying compared to freeze drying, are the lower investing and operating costs. For example, depending on the type of equipment, a solar dryer can cost five times less than a freeze dryer.

More so, solar dryer can offer a more environmentally friendly option to freeze drying, as it can potentially dry higher amounts of microalgal biomass using less energy, due to its higher capacity (size). However, current trials are still assessing how this potential is converted into numbers.

In fact, using solar drying to dry microalgae is uncharted territory and much is still unknown about the optimal drying conditions for each microalgae species. In other words, the conditions capable of reducing the moisture content below 5% (the quality standard value) in the least possible time, without sacrificing the product’s nutrients, colour, texture or flavour. Plus, each microalgae species has different drying profiles, and even within the same species, different drying conditions might be necessary. This is highly dependent on the dewatering system used to obtain the microalgal paste and on the final density of the paste.

Another challenge that arises from using the solar dryer is the effect of high temperatures during summer, which could potentially cause the oxidation (degradation) of certain vitamins and fatty acids in the microalgal products.

To understand and overcome all of these challenges, ProFuture has now started preliminary studies using the solar drying on three microalgae species: Skeletonema, Tetraselmis and Nannochloropsis.

© Benjamin Schmid, Necton.

What are the preliminary results and what are the next steps?

ProFuture’s research with the solar dryer started in July 2020 at Necton’s facilities. Preliminary studies revealed promising results, showing that the solar dryer has the potential to efficiently dry microalgae. However, this particular technology posed a small challenge: most microalgae pastes would form a thick layer during solar drying, making the water removal (the drying) more difficult.

To overcome this challenge, Necton’s researchers are now investigating efficient methods to achieve a faster drying, such as displaying the biomass in spaghetti-like shapes over nets. The next steps will focus on finetuning the methods to make the spaghetti-shaped biomass and scale up the drying process.

Moreover, seasonal comparisons and biochemical analysis are being done to access the nutritional quality of solar-dried biomass in terms of proteins, lipids and vitamins.

© Benjamin Schmid, Necton.