When to harvest: reading culture density
Spirulina should be harvested when the culture reaches the right cell density — dense enough to make harvesting efficient, but not so dense that the culture crashes from self-shading or nutrient depletion.
The Secchi disk / transparency method
The simplest field method: lower a white disk (or a white spoon) into the culture. The culture is ready to harvest when you can no longer see the disk at a depth of 5–8 cm. This optical density corresponds approximately to the optimal harvest density.
A good culture ready for harvest should have the consistency of a dark green smoothie — you should not be able to see your fingers through a 1 cm layer.
The pour test
Pour a small amount of culture onto a white plate. At harvest density, it should leave a dark, opaque green trace. If the plate shows clearly through the layer, the culture is too dilute to harvest efficiently.
Never harvest below 1 g/L dry weight
Below this density, the yield is too low to justify the effort and the culture disruption. Target 2–4 g/L for efficient harvesting.
Harvest frequency
The key principle: harvest a portion of the culture (typically 20–30%), not the entire volume. This is called the “continuous harvest” or “draw-and-fill” method:
- Harvest 20–30% of the culture volume (e.g., 2–3 litres from a 10-litre tank)
- Top up with fresh medium (nutrients + water) to replace the harvested volume
- The remaining culture provides the inoculum to repopulate the tank
In good growing conditions (strong light, 30–35°C), a healthy culture doubles in approximately 3–5 days. This allows harvesting every 3–5 days continuously.
Filtration: separating spirulina from the medium
Spirulina’s filamentous structure makes it relatively easy to filter — it forms a paste rather than passing through coarse mesh. Options by scale:
Cheesecloth or muslin (beginner/small scale)
A double layer of cheesecloth or fine cotton muslin over a colander works for small batches. Pour the harvested culture through and allow it to drain. Gently squeeze the cloth to remove excess medium. The result is a wet spirulina cake.
Limitations: slow for large volumes, the cloth retains some spirulina and is harder to clean thoroughly.
Fine mesh filter bags (75–100 micron)
Nylon filter bags with 75–100 micron mesh are the step up from cheesecloth. They allow faster drainage and are easier to rinse clean. This is the most common home-grower solution for tanks up to 50 litres.
Vacuum filtration (intermediate scale)
A Büchner funnel and vacuum pump setup dramatically speeds filtration and presses the spirulina cake drier, which reduces subsequent drying time and improves product quality. This is appropriate for growers producing 100+ g/month.
Rinsing
After filtering, rinse the spirulina cake briefly with fresh, clean water. This removes residual culture medium (which can affect taste and product quality) and removes some of the salty mineral taste. Two to three rinses with a small volume of water are sufficient — over-rinsing does not improve quality further and increases water waste.
Drying: the critical step for quality
Drying temperature determines phycocyanin retention. This is the most important quality decision in home spirulina production:
Temperature limits
- Above 70°C: rapid phycocyanin degradation begins
- 60–70°C: significant phycocyanin loss (30–50% in most analyses)
- Below 45°C: phycocyanin is largely preserved; drying is slow but quality is maintained
- 35–40°C: optimal balance of drying speed and phycocyanin preservation for home production
Drying methods
- Food dehydrator (35–42°C): The best practical method for home growers. Consistent low temperature, good airflow, and relatively fast drying (6–10 hours for a thin layer). Widely available for €50–150.
- Oven at lowest setting with door ajar: Only if your oven can maintain below 50°C. Most ovens struggle with this — check with a thermometer before use. The main risk is exceeding the phycocyanin degradation threshold.
- Sun-drying: Effective in hot climates, but UV radiation and ambient temperatures above 40°C in direct sun can compromise phycocyanin. Thin shade drying (indirect sunlight) at 30–40°C ambient is acceptable in Mediterranean or tropical climates.
- Freeze-drying: Produces the highest quality powder with maximum phycocyanin retention. Equipment cost (€500+ for a basic unit) is prohibitive for hobby growers but practical for micro-commercial operations.
Applying spirulina for drying
Spread the wet spirulina cake in thin layers (3–5 mm) on non-stick sheets or parchment paper. Thinner layers dry faster and more evenly. Breaking up the cake into small portions speeds drying.
Spirulina is fully dry when it breaks apart into a powder rather than bending or remaining sticky. Moisture content should be below 7% for safe storage.
Storage after harvest
Dried spirulina should be stored immediately in sealed, opaque containers — exposure to light after drying degrades phycocyanin further. Keep at room temperature or below; refrigeration extends shelf life but introduces moisture risk unless the container is truly airtight.
For the full storage guide, see how to store spirulina properly.
Colour as a quality check
After drying, the colour of your spirulina tells you how well the process went:
- Deep blue-green: excellent — phycocyanin well-preserved, drying temperature was appropriate
- Mid-green: acceptable — some phycocyanin loss, but nutritional value largely intact
- Olive or brownish-green: overheated during drying — phycocyanin largely lost, though protein and minerals remain
See the full guide to spirulina colour as a quality signal.
