Algal protein isolates

Microalgal protein for food comes overwhelmingly from two organisms that are biologically unalike. Spirulina (Arthrospira) is a cyanobacterium — a prokaryote. Chlorella is a green microalga — a eukaryote with a true cell wall. Treating them as one category hides the fact that they present opposite processing problems.

The cell wall decides the process

Chlorella has a rigid wall, in many strains containing cellulose-like polymers and, in some, a highly resistant algaenan layer. Intact, it passes through the human digestive tract with much of its protein unavailable, so digestibility depends on breaking it. Disruption is mechanical — bead milling or high-pressure homogenisation — and it is energy-intensive, because the target is a few micrometres across and the wall is strong relative to its size. Disruption also releases everything else inside the cell, including chlorophyll and lipids, which then have to be separated from the protein.

Spirulina has no such wall; its envelope is a thin peptidoglycan layer, and it ruptures easily. Its protein is accessible without a milling step, which is the main reason spirulina reached food markets first and at lower cost.

Phycocyanin is a pigment with a protein’s fragility

Spirulina’s blue colour comes from phycocyanin, a phycobiliprotein — a light-harvesting pigment covalently bound to a protein. Because the colour depends on the protein’s folded state, it behaves like a protein rather than like a dye: it denatures with heat, and it loses colour outside a narrow pH band around neutrality.

This is why natural blue is difficult in food. Phycocyanin is one of very few natural blues available, and its instability rules out baked, acidified and retorted applications unless it is stabilised. It also means a spirulina ingredient’s colour and its protein content degrade together under the same conditions.

Chlorella’s pigment problem is the opposite: chlorophyll gives a strong green that is hard to remove and hard to formulate around, and the organism carries earthy off-flavours — geosmin and related compounds — that are potent at very low concentrations.

Light is the ceiling on growing them

Photoautotrophic cultivation is limited by light, and the limitation is geometric. As cell density rises, cells near the surface absorb the incoming light and shade those below — self-shading — so productivity per unit of illuminated area plateaus regardless of culture depth. Open ponds are cheap and hard to keep axenic; closed photobioreactors give control and cost far more per unit area.

Heterotrophic cultivation avoids the problem entirely by feeding the alga sugar in a conventional dark fermenter, reaching much higher cell densities. It is no longer “growing on sunlight”, which changes the environmental argument, and not all strains or pigments are produced heterotrophically.

The shared limit

Like all fast-growing microbial biomass, microalgae are high in RNA, and dietary purine load constrains how much whole-cell material can be eaten regularly — the same uric-acid ceiling that governs single-cell protein generally. Protein isolation reduces it; whole-cell powders do not.

Heavy-metal accumulation and, for open cultivation, contamination with cyanotoxin-producing organisms are the recognised safety questions, and both are matters of production control rather than of the species itself.