**1.4 Protein from methane: Single-cell protein (SCP)**

Protein is a vital part of our diet. It can be found in plants and meat. For several decades, there has been interest in single-cell protein (SCP), that is, protein derived from yeasts, algae, and bacteria [31, 32]. Yeast cells have been used to make beverages and bread for over 4500 years [33], and also algae have a long history of food. Bacterial SCP is of particularly high interest for commercial production as will be outlined below.

There exit proteinaceous methanotrophs, which contain high amounts of protein made from methane. Bacterial SCP, also called BPM (bacterial protein meal), can be obtained from methane with methanotrophs, in pure and mixed cultures, see Refs. [34]. Continuous aerobic fermentation [35, 36] is generally preferred for higher yield.

A mixed culture with *Methylococcus capsulatus (Bath)*, complemented by *Ralstonia sp, Brevibacillus agri*, and *Aneurinibacillus sp.* [37] was described to yield SCP. Also, SCP production from methanol has been reported [38], using methylotrophs.

Applications of SCP are mainly in the feed and food sector. In 1997, the United Nations "Protein-Calorie Advisory Group" discussed the safety of SCP for Animal and Human Feeding [39], apart from other commissions and panels [40–42].

Twenty-three years earlier, in November 1974, the European Association of Single-Cell Protein Producers (Association européenne des producteurs de protéines unicellulaires) UNICELPE had been founded [43].

SCP produced on a commercial scale has been reported to be deployed in these areas:


For food, another application is to boost the nutritional value, for example, of baked food items, ready-made meals, and soups. Single cells have also been used in the food industry as starter cultures (e.g., in bread, beer, and wine making by various yeasts) [33, 44, 45].

SCP has been described as antiobesity food [46]. A patent [47], EP 79.1641475, discloses the use of lipids from methanotrophs for cholesterol reduction. The use of biogas as feedstock for methanotrophs is detailed in Refs. [48, 49]. In Ref. [50], the simultaneous production and use of SCP and PHB bioplastics are discussed.
