How freeze-drying works
Lyophilisation removes water through sublimation — bypassing the liquid phase. Ice converts directly to water vapour under vacuum without melting:
- Freezing phase:Spirulina paste is frozen rapidly to −40°C or lower. Rapid freezing creates smaller ice crystals that cause less cell membrane damage during sublimation.
- Primary drying (sublimation):The chamber is evacuated to 0.05–0.5 mbar. The condenser (at −50 to −80°C) captures water vapour as ice. The shelf temperature is raised slightly (still below 0°C for primary drying) to supply sublimation energy without melting. This phase removes ~95% of water.
- Secondary drying (desorption):Temperature is raised to 0–20°C under continued vacuum to remove bound water. Final moisture content: 1–3% vs 10–15% for dehydrator-dried spirulina.
Because spirulina never exceeds 0°C during primary drying, phycocyanin (degrading above 40°C) is completely preserved in the freeze-drying process.
Home freeze dryer options
- Harvest Right (most accessible home unit):Small unit: batch size 2.3 kg wet weight, £1,800–2,200. Medium: 4.5 kg, £2,200–2,800. Cycles take 24–40 hours per batch. These are genuine lyophilisers, not dehydrators — they include a vacuum pump and condenser that reach the temperatures required.
- Scientific/laboratory lyophilisers:Used units can be sourced from laboratory equipment auction sites. A benchtop model (e.g., VirTis, Labconco) can process small batches. Cost: £500–2,000 used depending on condition.
- Commercial freeze-drying services:Some food processing companies offer contract freeze-drying services — if your production volume is large enough (>5 kg/batch), sending wet paste to a contract dryer may be cost-effective without the capital outlay.
Pre-freeze preparation
Preparation of spirulina paste before freeze-drying affects the final product:
- Harvest spirulina by filtration normally, then press the filter cake as dry as possible — lower water content reduces cycle time significantly. Target paste moisture: 70–80% (pressing wet paste through a ricer or pressing between two boards produces appropriate consistency).
- Spread paste in a thin, even layer (5–10 mm) on the freeze-dryer trays. Thicker layers slow the drying cycle and can result in uneven drying — outer layers dry while the centre retains moisture.
- For long-term storage quality: add a small amount of ascorbic acid (vitamin C) to the paste before freezing (0.1% by weight) — it acts as an antioxidant that further protects phycocyanin during the drying cycle and storage.
- Pre-freeze the trays in a conventional freezer (−18 to −20°C) for 2–4 hours before loading into the freeze dryer. This reduces the time the machine must spend on the initial freezing phase and can shorten total cycle time by 4–8 hours.
Cycle settings for spirulina
- Initial freeze: −40°C for 2 hours minimum (pre-freezing in conventional freezer beforehand allows this to be skipped on units without cryogenic capability)
- Primary drying shelf temperature: −10 to 0°C (for phycocyanin-preserving drying; do not exceed 0°C during primary drying)
- Secondary drying: 20°C maximum — well below phycocyanin’s 40°C threshold
- Vacuum level: 0.1–0.3 mbar throughout (standard freeze dryer operating range)
- Total cycle time: 24–32 hours for a 5–10 mm paste layer
Storage after freeze-drying
- Remove trays immediately from the freeze dryer and seal powder into airtight containers within 30 minutes — freeze-dried powder is highly hygroscopic (absorbs moisture from air rapidly)
- Best storage: vacuum-sealed foil pouches or Mylar bags with oxygen absorber at room temperature or refrigerated. Shelf life: 2–5 years vs 6–12 months for dehydrator-dried spirulina.
- Powder colour: bright blue-green from preserved phycocyanin. If the powder has a dull dark green with no blue tint, phycocyanin degradation occurred during the cycle (most likely from shelf temperature exceeding 0°C during primary drying).
Freeze-drying vs dehydrator: comparison
- Phycocyanin retention:Freeze-drying 95–98%; food dehydrator at 38–40°C 85–95%; spray drying 40–80%; sun drying <40%.
- Shelf life:Freeze-dried 2–5 years; dehydrator-dried 6–12 months (sealed, refrigerated).
- Cost per gram:At home, dehydrator is far cheaper to run (<£0.01/g electricity vs £0.05–0.15/g energy amortised over freeze-dryer capital cost). For a home grower producing 50 g/week, a dehydrator is probably the right choice; for 200 g+/week, freeze-drying economics become more viable.
- Particle structure:Freeze-dried spirulina reconstitutes more easily in water than dehydrator-dried. The porous lyophilised structure rehydrates in seconds; dehydrated pressed powder requires blending.
