## 3 Bio-Utilization of Waste from the Agri-Food Industry during SCP Production
Also, it was examined which compound among ammonium chloride, ammonium sulfate, and urea is the best source of nitrogen for the _A. oryzae_ strain. The content of protein produced by the mold was 13.8% for ammonium sulfate, 13.2% for urea, and 13% for ammonium chloride, respectively. On this basis, ammonium sulfate was selected as a protein source, and in the next stage, the best amount of this compound was analyzed and selected, which affected the protein content. Substrates with the addition of 0.2, 0.4, 0.6, 0.8, and 1.0% were prepared, and for the medium with the smallest addition of ammonium sulfate, the protein content was 9.13%. Whereas the sulfate concentration increased, the protein content increased. The highest result (13.49%) was obtained for the medium with the 1% addition of a nitrogen source, but this result was not statistically different from those obtained for the medium with 0.8% addition. Thus, the 0.8% concentration of ammonium sulfate was chosen as the most favorable. In the next stage of the research, it was checked what amount of medium would allow one to achieve the highest protein content. The amounts of 45, 60, 75, and 90 g of substrates were used. The protein content for 75 and 90 g of substrate did not differ statistically and amounted to 15.5 and 15.27%, respectively. Thus, it was decided to use 75 g of substrate. The last factor examined was pH. The highest protein content was obtained for the medium with a pH of 5.5. In the final stage of the research, the amino acid content was checked. As a result, the ratio of essential amino acids to the total amino acids of the SCP produced was 46%. The optimal conditions for SCP production using _A. oryzae_ were determined for 75 g of medium with 0.8% ammonium sulfate and at the pH of 5.5. Since date waste is a rich source of sugars, it does not require hydrolysis, and optimal growth conditions can be easily ensured, as it has the potential to be used on a larger scale. In addition to culturing microorganisms in media containing waste materials, another opportunity to obtain microbial protein is the use of hydrogen-oxidizing bacteria. Such a group of microorganisms is characterized by the ability to use hydrogen as a source of electrons and convert carbon dioxide into microbial biomass with a high protein content. Through electrolysis, protein is produced, the production of which is characterized by significantly lower greenhouse gas emissions than traditional protein sources such as meat and plant protein, as well as a much lower water footprint. Protein extracted using hydrogen is characterized by its high quality. In addition, such a production is independent of the traditional agricultural system and sunlight. Moreover, its energy efficiency is very high compared to most other sunlight-independent food sources, such as crops grown in artificial [70]. Protein obtained from such cultures can contain up to 0.75 g/g d.m., which is much higher than the protein content of soybeans--0.46 g/g d.m. and wheat--0.15 g/g d.m. The concept that carbon dioxide can be converted into valuable feed protein, thus shortening current protein production processes, opens up new possibilities for anaerobic digestion as an important driver for the development of an economy that ensures sustainable feed production on site [71]. SCP can also be synthesized by solid state fermentation, with the simultaneous bio-utilization of waste from the agri-food industry. This fermentation process can be used to obtain enriched animal feed, but microbial protein preparations are not obtained in this way due to the difficulty in separating microbial cells from the fermented waste mass. It was found to be possible to use a medium containing orange pulp, molasses, potato pulp, whey, distilled water, and BSG for the cultivation of _K. maraxianus_, in which the sum of fat and protein concentration (59.2% \(w/w\) dm) was obtained after culture [72].