Single-cell protein for feed

Single-cell protein is the biomass of a cultured microorganism harvested and dried as a feed ingredient. Its attraction is that the nitrogen and carbon it is built from need never pass through a plant: growth is decoupled from arable land, from photoperiod and from the seasonal cycle, and a bioreactor’s productivity per unit area is orders of magnitude above a soy field. The constraints are not agronomic, and they are unusually specific.

The nucleic acid ceiling

A fast-growing cell is a cell dominated by protein synthesis machinery. Ribosomes are largely RNA, so RNA content scales with growth rate, and microbial biomass grown quickly can carry roughly 6–15% of its dry mass as nucleic acid, against a fraction of a percent in conventional feedstuffs. In ruminants this is not a problem — rumen microbes degrade nucleic acid and the purines are recycled. In monogastrics it is. Purine catabolism in birds and in pigs ends at uric acid, which is poorly soluble; sustained high purine intake raises urate load and risks deposition, and in poultry this is the recognised limit rather than an amino acid deficiency. The consequence is that inclusion rates for monogastric diets are set by nucleic acid rather than by protein quality, unless the process includes an RNA reduction step — typically a thermal shock that activates endogenous ribonucleases and lets nucleotides leach out, which costs yield and adds a unit operation.

The cell wall

The protein is inside a wall that the animal did not evolve to digest. Yeast walls are β-glucan and mannoprotein; bacterial walls are peptidoglycan; both are far less accessible than the protein they enclose, and measured digestibility of intact biomass is consistently below its crude protein figure. Mechanical disruption, autolysis or enzymatic treatment lift it, and each is a real cost. The wall components are not wasted — β-glucans and mannans have their own documented effects on gut immune signalling — but a protein claim based on crude protein alone overstates what the animal receives.

The substrate decides everything

Microbial protein is a way of moving a carbon and energy source into feed, so the economics reduce to the price of that source and the efficiency of the transfer. Methanotrophs grown on natural gas take a cheap, widely distributed substrate and a well-understood organism, and the limiting engineering problem is gas–liquid mass transfer: methane and oxygen are both poorly soluble, so the reactor must dissolve two gases fast enough to feed a dense culture, which is what loop and pressurised designs address. Hydrogen-oxidising bacteria fixing CO2 with electrolytic hydrogen have a far better carbon story and a far worse energy one — the whole process is a route for converting electricity into protein, and its viability is a statement about electricity price and electrolyser efficiency, not about microbiology. Fungal biomass on industrial side-streams sits between the two, and inherits the variability of its feedstock.

Safety framing is separate again: the organism, the substrate and any residual carryover are what regulators assess, so an approval is specific to a process and does not generalise across substrates.