From pond to powder: the five stages
Commercial spirulina production follows a sequence that, when done well, preserves nutritional value and prevents contamination — and when done poorly, creates a product with elevated microcystin, heavy metals, or degraded phycocyanin:
- Culture medium preparation
- Cultivation in open ponds or closed bioreactors
- Harvesting and filtration
- Drying
- Milling, packaging, and quality testing
Stage 1: Culture medium
Spirulina is cultivated in alkaline aqueous media — most commonly based on the Zarrouk medium or simplified commercial equivalents. The key components are:
- Sodium bicarbonate (NaHCO₃): The carbon source (CO₂ dissolved as bicarbonate) and primary pH buffer. Spirulina grows at pH 9.5–10.5 — this extreme alkalinity inhibits most other microorganisms and is central to contamination resistance.
- Sodium nitrate (NaNO₃): The nitrogen source for protein and phycocyanin synthesis
- Potassium dihydrogen phosphate: Phosphate for ATP and nucleic acid synthesis
- Trace minerals (Fe, Mn, Cu, Zn, Mo):Essential for enzyme function. The iron source determines spirulina’s iron content — iron in the culture medium is incorporated into phycocyanin and ferritin proteins.
The mineral composition of the medium directly determines the mineral profile of the final product — which is why spirulina from different producers has different iron, zinc, and selenium content.
Stage 2: Cultivation systems
Open raceway ponds (ORP)
The most common commercial cultivation system. Shallow (20–30 cm depth) concrete or plastic-lined channels in an oval or raceway configuration. A paddlewheel provides continuous circulation at 15–30 cm/s. The culture is exposed to natural sunlight.
Advantages: Low construction cost; scalable to large production areas; proven for decades; compatible with outdoor natural sunlight.
Disadvantages: Exposed to environmental contamination (dust, insects, other microorganisms); dependent on outdoor temperature and weather; cannot maintain consistent culture conditions year-round in temperate climates; open to heavy metal atmospheric deposition from air pollution.
Closed photobioreactors (PBR)
Transparent tubes or flat panels through which culture media circulates in a closed, controlled environment. Light sources can be artificial (LED) or sunlight through transparent material.
Advantages: Controlled contamination; consistent temperature and light; can operate year-round; better hygiene; easier to certify to higher quality standards.
Disadvantages: Higher capital and operating cost; CO₂ supply required; less scalable than open ponds per unit cost.
Premium spirulina products — particularly those marketed for phycocyanin content or certified organic use — are increasingly produced in closed PBR systems. The extra cost is reflected in the price difference between commodity and premium spirulina.
Stage 3: Harvesting and filtration
When culture density reaches the optimal harvest level (2–3 g/L dry weight), a portion of the culture (20–30%) is removed and processed. Harvesting methods:
- Filtration through stainless steel or polypropylene screens:The most common method. Spirulina’s helical trichome structure (50–300 µm length) allows it to be captured on a screen while the liquid culture medium passes through. The filtered paste (wet spirulina) contains approximately 80–85% water.
- Centrifugation: Higher capital cost; more complete recovery of smaller cells; used in closed systems.
The harvested wet paste is immediately processed to prevent bacterial growth — the high water activity and protein content make it highly perishable.
Stage 4: Drying — the most quality-critical stage
Drying method has the largest single effect on phycocyanin content and overall quality:
Spray drying
The wet paste is pumped into a heated chamber (inlet temperature 150–220°C, outlet 80–100°C) where it is atomised into fine droplets and dried within seconds by the hot air flow. The rapid drying means cell exposure to peak temperatures is brief.
However, phycocyanin begins to denature above 60°C — spray drying inevitably degrades a significant fraction of phycocyanin (estimates range from 20–60% loss depending on exact temperatures used). This is why spray-dried spirulina is often olive-green or brownish rather than deep blue-green — the phycocyanin has been partially destroyed.
Spray drying is by far the most common commercial method due to its speed, scalability, and cost.
Freeze drying (lyophilisation)
The wet paste is frozen at −50°C and placed under vacuum. Water sublimes directly from ice to vapour without passing through a liquid phase — the cells are dried at or below freezing, so heat damage is essentially zero.
Freeze-dried spirulina retains the maximum phycocyanin — bright blue-green colour is preserved. All volatile compounds (including DMS) are also retained, which paradoxically can make freeze-dried spirulina taste stronger (more “fresh marine” note) than spray-dried product.
Freeze drying is approximately 4–8× more expensive than spray drying and is used primarily for premium products and pharmaceutical-grade phycocyanin extraction.
Low-temperature spray drying
An intermediate approach — inlet temperatures of 100–130°C rather than 150–220°C, with slower throughput. Better phycocyanin retention than standard spray drying but higher cost. Some premium commercial products use this method.
Stage 5: Testing, milling, and packaging
Quality spirulina is tested at this stage for:
- Microcystins and other cyanotoxins:ELISA or LC-MS/MS analysis. Regulatory limits vary; WHO guidance is <1 µg/g in spirulina products.
- Heavy metals (Pb, Cd, Hg, As):ICP-MS analysis. Most contamination comes from cultivation water quality and atmospheric deposition near industrial areas.
- Microbiological quality: Total plate count, E. coli, Salmonella, mould/yeast
- Nutritional composition: Protein, moisture, ash, fatty acids; phycocyanin content if claimed
This Certificate of Analysis (CoA) should be publicly available from any reputable supplier and should match the batch number of the product.
Why production origin matters for quality
The vast majority of commercial spirulina (est. 70–80%) is produced in China, India, and Taiwan. Quality within these countries varies enormously between producers — from ISO 22000-certified facilities with rigorous testing to unmonitored open ponds with inadequate quality controls.
Country of origin alone does not determine quality — a CoA from a third-party accredited laboratory is the only reliable quality signal. Premium Hawaiian, French, or European spirulina is typically certified and tested, but also exists at the high end of the price spectrum for comparable reasons.