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Harvesting and drying spirulina at home.

Harvesting spirulina at home requires two steps: separating the biomass from the culture liquid (filtration + pressing) and reducing moisture content to a stable level for storage (drying). The single most important decision in home drying is temperature: phycocyanin, spirulina’s most valuable component, denatures above 40°C. Everything above that temperature produces green powder without phycocyanin.

spirulina growing harvest and drying

When to harvest

  • Optical density target: Harvest when optical density at 680 nm (OD680) reaches 0.8–1.2 in a 1 cm path length cuvette (typical for home monitoring via visual comparison charts or cheap colorimeters). At this density the culture is dark green, opaque, and at or near peak exponential growth — the ideal harvest point for protein and phycocyanin content.
  • Harvest volume: Remove 20–30% of culture volume at each harvest for semi-continuous production. Removing more than 40% at once risks culture crash (insufficient inoculum to re-establish exponential growth quickly). Replace harvested volume immediately with fresh Zarrouk medium.
  • Harvest time: Early morning (before peak light) is the traditional recommendation: carbohydrate content is lower in the morning (photosynthate has been partially consumed overnight) and pH is at its daily minimum. In practice, harvest timing has minimal effect on protein and phycocyanin content for home producers.

Filtration and pressing

  • Filter mesh: Pour culture through a 50–100 micron monofilament nylon mesh (available from homebrew or aquaculture suppliers). Spirulina filaments (200–500 µm long trichomes) are retained; culture medium passes through. The filtered medium can be returned to the culture tank — it retains nutrients.
  • Pressing: After filtration, the spirulina biomass retains approximately 90–95% moisture. Press by wrapping in clean cloth or cheesecloth and pressing against a clean surface to remove free liquid. Target moisture content after pressing: approximately 70–80% (the biomass forms a firm, cohesive green paste that holds its shape when compressed). Do not press through a fine cloth that ruptures cells — gentle pressing only.
  • Fresh paste use: The pressed paste at 70–80% moisture is fully bioavailable and contains intact phycocyanin. It can be used immediately in smoothies, salad dressings, or sauces. Store in a sealed container at 4°C for up to 3 days. The paste will darken slightly at the surface (oxidation) but remains palatable throughout. Fresh paste has the highest phycocyanin content of any home-produced format.

Drying methods: phycocyanin comparison

  • Food dehydrator at 35–38°C (best home method): Spread paste in a thin layer (3–5 mm) on silicone dehydrator trays or baking paper. Set temperature to 35–38°C. Drying time: 8–16 hours depending on layer thickness and dehydrator airflow. The finished powder should be vivid blue-green with substantial phycocyanin preservation (estimated 50–70% of fresh paste phycocyanin retained). Crumble dried sheets and store in an airtight container away from light.
  • Oven drying at minimum setting: Most domestic ovens have a minimum setting of 50–80°C — too hot for phycocyanin. If your oven’s minimum setting is ≤40°C (some modern ovens have a 40°C proof setting): spread paste thinly, leave oven door ajar 5–10 cm for moisture escape, and dry 8–12 hours. Most home ovens set to 50°C+ will destroy a significant fraction of phycocyanin in 2–4 hours; acceptable for colour and protein powder but not optimal for phycocyanin.
  • Freeze-drying (best quality, specialist equipment): Freeze-drying removes water by sublimation at sub-zero temperatures under vacuum. Phycocyanin is preserved at >90% of fresh paste values. This is the method used for premium commercial spirulina. Accessible at home only with a personal freeze-dryer (£500–2,000); most home growers use food dehydrators instead.
  • Sun-drying (not recommended): Traditional sun-drying of spirulina at >40°C in direct sunlight destroys phycocyanin within 1–2 hours. UV radiation additionally damages phycocyanin. Sun-dried spirulina has negligible phycocyanin and significantly degraded nutritional profile compared to low-temperature dried product. Avoid sun-drying for home use.

Storage

  • Moisture target: Below 7% moisture content for long-term storage stability. At >8% moisture, mould growth risk increases significantly. Test by checking whether the powder flows freely (dry) vs clumps in a ball (too moist).
  • Container and light: Store in an airtight dark container (amber glass jar or opaque container with tight lid). Phycocyanin degrades on UV and visible light exposure even in dried powder. Refrigerate for storage beyond 1 month; freeze for beyond 3 months (phycocyanin is stable at −20°C for 12+ months in dry powder).
  • Shelf life indicators: Vivid blue-green powder with a slight marine smell: optimal. Pale olive-green or yellow-green powder: nitrogen stress at harvest or excessive drying temperature. Brown or grey tinge: oxidation or very old product. Loss of smell can indicate oxidation of omega fatty acids.

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