Dilek Ercili‐Cura, Anna Häkämies, Laura Sinisalo, Pasi Vainikka and Juha‐Pekka Pitkänen introduce a new bacterial source of protein that requires only CO ₂ from air, water and minerals as raw materials and uses renewable electricity as an energy source .

According to the World Meteorological Organisation 2019 report^[ 1^] , the past decade has been the warmest on record and it ended with an increase of 1.1°C of global average temperature compared to the pre‐industrial era.

Green house gas (GHG) concentrations in the atmosphere reached new heights in 2018 for carbon dioxide (CO₂), methane (CH₄) and nitrous oxide (N₂O). Food production, when considering the whole chain, is responsible for 26% of global anthropogenic GHG emissions, around 32% of global terrestrial acidification and 78% of eutrophication^[ 2^] .

The livestock industry delivers almost half of the GHG emissions from the food chain. It is also timely to mention outbreaks of zoonotic viruses spread mostly due to animal farming, such as swine flu, avian flu and the most recent COVID‐19, which put human health and food security at great risk.

If current value chains are sustained, the food industry will continue to be a major threat for climate stability and ecological biodiversity and will be the biggest driver of environmental degradation^[ 3^] .

Meanwhile, the world's population continues growing, which demands a dramatic increase in food production. Given the crucial role of protein in the human diet and the environmental impact of its main livestock source, the food revolution must start by securing a more sustainable protein supply.

Fortunately, today we are witnessing exponential growth in cutting‐edge, multidisciplinary research efforts to create alternative, sustainable protein sources that can be produced without depleting earth's natural resources but by creating a carbon‐neutral food chain. Solar Foods is a Finland‐based start‐up company that utilises CO₂ captured from air, water and some minerals as sole raw materials to produce a protein powder using renewable electricity.

Branded as Solein®, the product is a microbial protein (also referred to as single cell protein, SCP) obtained by growing proprietary bacteria harvested from nature, in specially designed bioreactors using gas fermentation.

This model disrupts the food chain as production of the microbial protein is not dependent on agriculture or climate; it does not occupy arable land and is not affected by geopolitical interferences in the food chain.

The production is decentralised and can happen in the desert, arctic, other unfertile land and even in space. In fact, the basic idea for producing protein from CO₂, electricity and water comes from NASA research in the 1960s^[ 4^] .

Moreover, microbial protein production can have a much higher solar‐to‐biomass conversion efficiency than agricultural crops, where the theoretical efficiency is limited to 6%. Solar‐to‐biomass efficiencies of up to 10% have been reported for bacteria grown using gas fermentation^[ 5^] .

Given the crucial role of protein in the human diet and the environmental impact of its main livestock source, the food revolution must start by securing a more sustainable protein supply.

Solein production process

Solein is a microbial protein, more specifically a bacterial protein. Examples of other microbial proteins on the market today include yeast (e.g. spent brewer's yeast extract powders, spreads), microalgae (spirulina and chlorella powders, supplements) and mycoprotein (fungal biomass branded as Quorn).

Solar Foods uses a proprietary microorganism cultivated in bioreactors by a fermentation process similar to production of e.g. baker's yeast or lactic acid bacteria. Most commercially available SCP products require sugar as a carbon source for microbial growth. In the Solein process, the microorganism is a hydrogen (H₂)‐oxidising bacterium, also called Knallgas bacteria, which can utilise CO₂ from air as a carbon source, similar to photosynthetic organisms.

Unlike plants or algae, which need sunlight for catalysis of growth, Knallgas bacteria utilise H₂ as source of energy to generate ATP. They use H₂ as an electron donor and O₂ as an electron acceptor to fix CO₂ into biomass. H₂‐oxidising bacteria are abundant in nature and thousands of species can be isolated from the environment. The production process of Solein is described in Figure 1.

In Solein production, H₂ is supplied by electrolysis of water onsite. High purity O₂ and H₂ (>99%) are produced by an electrochemical process which operates 100% on renewable electricity. CO₂ supplied to the bioreactor can be obtained either by direct air capture (DAC), which is an emerging technology for capture and storage of CO₂ directly from the atmosphere, or by capture of flue gases from CO₂ emitting industries. This technology offers a promising advancement in reduction of GHG emissions by valorising excess CO₂ emitted to the atmosphere by converting it into food or feed (Figure 2). In fact, production of microbial proteins from H₂‐oxidising bacteria is not a new concept as mentioned earlier^[ 4, 6^] . However, as both water electrolysis and direct CO₂ capture processes are highly energy intensive, the concept of food manufacturing has not been feasible in the past. Today, the growing capacity and declining production costs of wind and solar power and advancements in storage of oversupply energy make this technology economically viable for the production of the most sustainable protein on earth.

Four chemical elements; carbon, oxygen, hydrogen and nitrogen, constitute 95% of the human body. These elements can be sourced from air. The remaining 5% originates from various minerals. Minerals in the form of inorganic salts provide sulphur, phosphorus, calcium, magnesium and other minerals necessary for biomass production. Nitrogen for bacterial protein production can be provided in the form of ammonia (NH₃) or directly as nitrogen gas (N₂) as some autotrophic/heterotrophic bacteria are able to fix atmospheric nitrogen^[ 7^] . NH₃ synthesis from H₂ and N₂ (Haber‐Bosch process) is one of the most energy‐intensive processes as the reaction requires enormous temperatures and pressures (around 500°C and 20MPa). In addition, it is a direct source of massive CO₂ emissions as the energy required to produce H₂ is supplied from natural gas, coal or oil. During the production of Solein, part of the renewably‐generated H₂ can be directed to ammonia production keeping the whole process carbon‐neutral.

Bioreactor cultivation takes place in optimised and controlled conditions to guarantee the maximum growing efficiency and safety. The down‐stream processing consists mainly of processes that further ensure the safety, technological and nutritional quality of the final product. The bacterial biomass Solein is a dried powder which contains 65‐75% protein, carbohydrates, lipids, and micronutrients (Figure 3).

Solein powder

Nutritional quality

The nutritional profile of bacterial biomass is advantageous as it is a protein source (50–80% protein) that is low in carbohydrate and lipid contents. The nutritional quality or biological value of a protein is determined by its amino acid composition, the abundance of essential amino acids and also the digestibility scores defined by Protein Digestibility Corrected Amino Acid Score (PDCAAS) and Digestible Indispensable Amino Acid Score (DIAAS). In general, SCPs possess favourable amino acid compositions which compare well with FAO/WHO recommended intakes for essential amino acids. They are typically limited by their content of sulphur‐containing amino acids, methionine and cysteine, and rich in lysine. Bacterial biomasses, have shown higher methionine levels compared with other microbial proteins^[ 10^] . In fact, various bacterial species produced by gas fermentation (methanotrophic bacteria) have successfully been established as fish feed due to their high biological value^[ 10^] . The amino acid profile of bacterial proteins, including H₂‐oxidising bacteria, is between those of plant and animal‐sourced proteins in terms of essential amino acid contents providing high biological value^[ 8, 9^] . Volova et al . have reported that the amino acid composition and the protein digestibility score of a H₂‐oxidising bacterial biomass were closer to animal‐origin proteins (casein) than to plant proteins (wheat)^[ 8^] .

The nutritional profile of bacterial biomass is advantageous as it is a protein source (50–80% protein) that is low in carbohydrate and lipid contents.

Solein has also shown favourable composition of essential amino acids. Preliminary results have revealed that a smaller quantity of Solein would be sufficient to meet the daily amino acid requirements of an adult compared to soy proteins and mycoprotein (unpublished data). Digestibility of microbial proteins may be hindered by the cell membrane structures, which differ greatly between different types of biomass e.g. algae, yeast, fungi and bacteria. Downstream processing is crucial for addressing challenges regarding nutritional, biological value as well as the safety of the final protein product. Typically, high nucleic acid contents, presence of lipopolysaccharides and bacterial endotoxins associated with Gram negative bacterial cell walls constitute the main food safety and toxicology related challenges for bacterial biomasses. They need to be addressed precisely with safe levels ensured via proper upstream and downstream processing.

Besides being a source of protein, microbial biomasses are also rich sources of bioactive compounds that are vital for health. Most SCPs (fungi, algae, yeast and bacteria) accumulate B‐group vitamins including e.g. riboflavin, niacin, choline, folic acid and cobalamine (vitamine B12), carotenoids, such as asthaxanthin, minerals such as iron and magnesium, and essential fatty acids. Some bacterial species, e.g. yellow‐pigmented H₂‐oxidising bacteria, are associated with carotenoid formation and distinguished by their quinone systems and ubiquinone compositions. In addition to being a rich protein source, these characteristics render bacterial biomasses a perfect replacement for animal‐origin food products, which constitute the main source of Vitamin B12 and iron in human diets. Studies on bioavailability and antioxidant capacity of bacterial‐sourced bioactive compounds are currently being carried out by Solar Foods.

Bacterial biomass produced by gas fermentation is likely to take off in every sector of the food industry replacing unsustainable and poorly functional ingredients.

Technological quality for food applications

The technological quality of a protein ingredient refers to its performance as a structure forming/stabilising or enriching agent during industrial production of foods and also its pleasantness in colour and flavour aspects. Different techno‐functional properties are expected from protein ingredients depending on the characteristics of the food matrix. For example, high moisture products, like drinkable or spoonable applications, benefit from high dispersibility, solubility and also emulsifying, foaming and gelling abilities. Medium or low moisture products, like bakery and extruded products, require different water and fat binding capacities and also texturising properties. SCP products on the market for direct consumption today include yeast extract spreads (Marmite and Vegemite), yeast extract powders (source of protein and umami flavour for vegan formulations), spirulina or chlorella extracts (as nutritional supplements) and Quorn®, which is a fungal biomass marketed in various forms as meat substitutes.

Preliminary characterisation of Solein as a food ingredient have revealed that it has high potential as a protein ingredient in a wide range of food applications from high to low moisture content. It has shown comparable properties to e.g. pea protein isolate in terms of water and oil binding properties and foaming and emulsifying capacity. It performs well during high moisture cooking or in hybrid matrices with plant or animal‐origin proteins and provides a rich protein and micronutrient source for enriching diets without compromising taste and mouthfeel.

Bacterial biomass produced by gas fermentation is likely to take off in every sector of the food industry replacing unsustainable and poorly functional ingredients. Moreover, the technology offers remarkable solutions to fight global malnutrition and supply nutritional and palatable food to parts of the world that are in need.

Regulatory status

Currently there are no food products made of H₂‐oxidising bacteria. The closest product on the market that does not rely on a sugar feedstock, i.e., agriculture, is a group of bacterial proteins used as fish feed. Companies like UniBio and Calysta use gas fermentation technology to convert methane to animal feed protein using methanotrophic bacteria^[ 10^] . To be considered as a food ingredient within the European Union (EU), bacterial biomass produced by gas fermentation needs to acquire Novel Food status (Novel Food Regulation (EU) 2015/2283), which assures the safety of the ingredient for human consumption. The European Food Safety Authority (EFSA) conducts the scientific risk assessment on the safety of novel foods at the Commission's request. Any food which has not been consumed within the EU before May 15, 1997 is classified as novel food and needs authorisation from the European Commission before it can be placed on the EU market for human consumption.

Application for authorisation is a thorough characterisation of the novel food including production process, detailed composition, scientific evidence demonstrating that the novel food does not pose a safety risk to human health and a proposal for the conditions of intended use as well as specific labelling requirements that do not mislead the consumer. Different regulations exist in other parts of the world. In the USA Generally Recognized as Safe (GRAS) status is approved by the US Food & Drug Administration (FDA). In Australia and New Zealand, novel foods require an assessment done by Food Standards Australia New Zealand (FSANZ) and in China novel foods are regulated by the National Health and Family Planning Commission (NHFPC) under the China National Center For Food Safety Risk Assessment (CFSA).

Sustainable food production

The European Union (EU) has set ambitious targets towards cutting GHG emissions by 50% by 2030 and reaching net‐zero carbon by 2050. Climate change is rapidly threatening water availability, agricultural productivity and biodiversity. Partially replacing current protein sources with more sustainable ones and creating alternative food chains that do not rely on agriculture are two emergent actions. Microbial proteins offer immediate advantages in terms of land and water use and lowering of GHG emissions caused by the food chain^[ 9^] .

Microbial proteins offer immediate advantages in terms of land and water use and lowering of GHG emissions caused by the food chain.

In a recent study, Sillman et al .^[ 5^] compared the land and water needs of H₂‐oxidising bacteria‐sourced proteins produced using a DAC system and 100% renewable energy sources to those of soybean protein. The land requirement for microbial protein production was shown to be 30–60 times less than for soybean proteins when the area needed for solar panels or wind turbines was included. No arable land is occupied for Solein production. The total water requirement of microbial protein production was found to be approximately one tenth of that for soybean protein production^[ 5, 9^] .

According to preliminary results of an unpublished life cycle assessment (LCA) study, the land and water efficiency of Solein is around 30‐300 and 35‐120 times higher respectively than that of protein from beef and dairy cattle (global average)^[ 2^] . Solein values were calculated based on mixed energy sources and a stress‐weighted water use accounting method.

The LCA revealed that GHG emissions from Solein production are 25 times lower than that of beef cattle production even if mixed energy sources are used. When based on 100% renewable hydro energy, the value reaches 100 times lower emissions (un‐published data). NH₃ production is responsible for over 1% of global yearly CO₂ emissions because 50% of food production today relies on NH₃ as a fertiliser^[ 11^] .

As mentioned earlier, in Solein production, NH₃ is produced with zero CO₂ emissions. Microbial protein production is a promising route to mitigate high anthropogenic nitrogen input due to the inefficiency of the soil‐to‐plant system and the decrease in nitrous oxide emissions and related eutrophication and ecosystem damage^[ 9^] .

Some of the benefits of producing microbial proteins, such as Solein, for foods are shown in Table 1. We expect microbial proteins produced from air and water to have a big impact on the food industry and on how food is produced and consumed. A sustainable food can be defined as a product:

1. with a production process that promotes environmental sustainability in terms of mitigating GHG emissions, water and land efficiency, biodiversity and soil productivity; or

2. offering solutions to ensuring global food sufficiency and security by providing healthy, affordable and palatable food. Solein addresses both these aspects of sustainability.

Conclusions

Bacterial protein is likely to find opportunities in rapidly growing alternative protein markets as a sustainable protein source. On September 2015, the Food and Agriculture Organisation of the United Nations adopted 17 sustainable development goals to guide international actions until 2030 and beyond. Cellular agriculture (or farming microbes) with all its elements, will revolutionise the food chain in the near future and contribute greatly to securing a safe and sufficient food supply while restoring earth's natural resources.