Spirulina growing: harvest techniques.

Harvesting timeline and cell density

• Optical density (OD680) threshold: Spirulina is typically harvested when optical density at 680 nm reaches 0.8–1.0. This corresponds to ~0.4–0.6 g dry weight per litre (g/L). At OD680 <0.6, the culture is too dilute; water loss during harvest is high and yield is poor. At OD680 >1.2, cells begin to aggregate, wall stress increases, and spontaneous lysis occurs (culture crashes within 24–48 hours). Optimal window is 0.8–1.0 OD680.
• Harvest frequency: In continuous paddlewheel raceways, harvest every 3–5 days (remove ~20–30% of culture volume) to maintain optimal density. In batch static cultures (100–500 L tanks), single harvest at 0.8–1.0 OD680, or partial harvest (remove 30–50%) and re-inoculate with nutrient-rich medium for second growth cycle. Partial harvest allows faster turnaround (2–3 week second cycle) than starting from stock culture.
• Daily growth rate and harvest scheduling: Under optimal light (200–300 µmol/m²/s) and temperature (35–37°C), spirulina doubles approximately every 2–3 days. Monitor OD680 daily with a simple spectrophotometer (basic models cost £100–£300); plot on graph to predict harvest window. This is more reliable than visual colour assessment (colour is subjective; OD680 is quantitative).

Wet harvest and dewatering

• Fine mesh screening: Spirulina cells are retained on fine mesh (100–200 µm pore size). Standard cheesecloth (50 µm) is too fine and clogs; aquarium filter mesh (150–200 µm) is ideal. Gently pour or pump culture through mesh into a collection pan. The retained spirulina paste (~15–20% dry weight) collects on the mesh; filtrate (culture medium) drains through. Repeat rinse with deionised water or distilled water once to remove residual salt (reduces bitter taste in final product).
• Cloth press (manual dewatering): Wrap the wet spirulina paste in clean cotton cloth (old t-shirt or cheesecloth layers work). Hang over a bowl or sink for 2–3 hours; gravity drains to ~40–50% moisture. Alternatively, wrap in cloth and gently squeeze (hand-press) to remove bulk water, then hang. Do NOT wring aggressively (ruptures cells and releases phycocyanin into the liquid). The goal is gentle dewatering, not complete dryness.
• Screen drain (passive dewatering): Place wet spirulina on a flat stainless steel screen or food-grade plastic screen suspended at an angle (30–45°). Liquid drains downward via gravity. Cover with cheesecloth to prevent dust and insect contamination. Leave overnight (8–12 hours) for ~30–40% moisture. This is passive and requires no labour; good for larger batches.

Drying methods

• Sun-drying (tropical method): Spread wet spirulina 2–3 cm thick on clean screens or trays in direct sunlight. In tropical climates with ambient temperature <25°C and low humidity, drying takes 6–12 hours (noon to sunset, then complete overnight or next morning). Temperature must remain <35°C to avoid phycocyanin destruction (>40°C causes rapid degradation). Vivid blue colour indicates intact phycocyanin. When moisture drops to ~5–8% (brittle texture, breaks easily), collect into airtight container immediately. Do NOT over-dry (<3% moisture is overly fragile). High humidity or ambient >25°C makes sun-drying impractical; use oven or freeze-dry instead.
• Oven-drying: Spread wet spirulina ~1 cm thick on food-grade trays. Place in oven at 40–50°C (NOT higher; >50°C causes phycocyanin loss). Leave oven door cracked open 2–3 cm (allows moisture escape and prevents temperature spike). Drying time: 8–12 hours. Stir gently every 2–3 hours to ensure even moisture removal. When moisture reaches ~5–8%, remove and cool to room temperature on a clean cloth, then transfer to airtight container. Oven-dried product is vivid blue if temperature was controlled; if colour is brownish-green, temperature exceeded 50°C.
• Freeze-drying (laboratory/small-scale premium method): Freeze wet spirulina paste at −20°C (or −40°C for faster freezing). Transfer frozen block to freeze-dryer chamber. Set pressure <0.1 mbar, temperature 0–5°C (shelf). Primary drying: 4–6 hours (ice sublimates under vacuum). Secondary drying: 2–3 hours at slightly elevated temperature (10–15°C) to remove residual bound water. Final product: <2% moisture, vivid bright blue, completely intact phycocyanin. Cost is high (£2,000–£10,000 for home-scale freeze-dryer); yield is ~50–70 g per batch. Used by premium producers and supplement manufacturers.

pH control during harvest

• Harvest pH and phycocyanin stability: Phycocyanobilin (the light-absorbing chromophore in phycocyanin) is stable at pH 9.5–10.5 (alkaline, matching spirulina culture pH). As culture medium pH drops below 9.5 (during dense culture when photosynthesis slows, respiration dominates, and CO⊂2 is retained), phycocyanin begins to degrade. Check culture pH before harvest; if <9.5, add small amount of food-grade sodium hydroxide (0.1 M) to bring pH back to 9.5–10. This preserves phycocyanin colour and bioavailability in the final product. Acidic (low pH) spirulina powder appears brownish-olive, indicating phycocyanin loss.

Post-harvest handling and storage

• Immediate storage after drying: Once dried to ~5–8% moisture, allow product to cool to room temperature (still in trays, uncovered, ~30 min). Transfer to airtight, food-grade containers (glass jars, vacuum-seal bags, or plastic tubs with airtight lids). Remove as much air as possible (vacuum-sealing is ideal). Exposure to air causes oxidation and phycocyanin fading within weeks.
• Room temperature storage: In cool dark place (15–20°C, <50% humidity), airtight container: shelf life ~6–12 months. Colour fades from vivid blue to dull blue-green over this period (phycocyanin oxidative degradation). Taste remains stable. Use within 6 months for premium colour and phycocyanin content.
• Refrigerated storage: In airtight container at 4–8°C (“refrigerated”), shelf life >2 years. Colour remains vivid blue; phycocyanin bioavailability is largely preserved. Freezing (−20°C) extends shelf life indefinitely but is impractical for powder (moisture migration in freeze-thaw cycles can reduce quality). Stick to refrigeration for home-scale storage.
• Indicator of spoilage: Spirulina does not spoil like meat or dairy (bacterial/fungal growth is rare due to alkalinity and low water content). Colour loss (fading to green-brown) indicates phycocyanin oxidation (normal age-related degradation, not spoilage). Musty or ammonia-like smell suggests mould (improper drying or storage humidity >60%); discard immediately. Off-flavour (bitter, rancid) suggests lipid oxidation; product is still safe but has lost freshness.

Yield calculations

• Wet-to-dry conversion: Fresh wet spirulina paste (~20–25% dry weight) yields approximately 1 kg dry powder from 4–5 kg wet harvest. From a 100 L culture at OD680 0.8–1.0 (~0.4–0.6 g/L dry weight), you harvest ~40–60 g dry spirulina. After drying loss (~5% final moisture), final yield ~38–57 g per 100 L batch.
• Annual production estimate (continuous raceway): A small 50 L paddlewheel raceway harvesting 20% volume every 3 days (maintaining 0.8–1.0 OD680) yields ~40–50 kg dry spirulina per year (assuming 300 days operational in temperate climate with seasonal downtime). A 500 L system yields ~400–500 kg/year.