2.4.2 Production of single cell protein

Figure 2.4.1-1. Chemical composition in the raw material, Norway spruce and birch.

Increasing the cellulose accessibility favors a high sugar yield from the enzymatic hydrolysis. However, carbohydrate degradation is undesired during pretreatment, as the degradation products may inhibit further biological processing of the pretreated material.

The results from the first study to produce SCP showed that cell growth was observed in only 3 of the 8 pretreatment liquids. Best growth was in the media containing mild hot water liquid extraction of birch (11.8g/L), the second best grew on extensive hot water liquid extraction solution from spruce (7.8g/L) and the third pretreatment liquid which resulted in biomass growth stemmed from the mild hot water extraction of spruce (4.0g/L, see Fig. 2.4.2-1A). The protein concentration in the dry biomass were similar to the biomass concentration, with the exception that extensive hot water liquid extraction solution of spruce resulted in slightly higher protein concentration than birch treated by mild hot water liquid extraction (Fig. 2.4.2-1B).

Fig. 2.4.2-1. Production of single cell protein using the pretreatment liquid as starting material. (A) Cell growth and (B) Protein concentrations. HWE is an abbreviation for the hot water extraction, Ext means extensive or severe and Mild refers to the milder extraction conditions.

The second study showed that it was possible to scale up the protein production to 50L. For the protein optimization study (Study 3), it was found that the protein concentration increased with decreased cultivation time (Fig. 2.4.2-5). The protein concentration also increased with decreasing pH and decreasing biomass concentration (Fig. 2.4.2-6). A protein content of 62% was achieved for the best cultivation experiment at lab scale which proves that it is possible to reach protein concentrations over 60%. Nevertheless, repeating it at larger scale and in combination with high yield and biomass concentration remains to be proven.

Figure 2.4.2-5. Protein concentration over time. Samples were collected daily and the protein concentration was measured. The protein concentration is reported as % g/g dry weight.

Figure 2.4.2-6. Protein concentration versus biomass at different pH. The dashed lines are for visual guidance.

In summary, these materials are easy to work with to produce single cell protein and many of them give a high cell biomass and also a high protein content. A next step with these starting materials might be to use low pH and fast-growing cells, for example in a continuous mode (where new growth media is continuously being added and cells