At AimGrow, we treat sodium citrate as a process tool whose role depends on how and where it is used. In phycocyanin workflows, citrate may help control pH during aqueous extraction, affect ionic strength during salting-out, or serve as a phase-forming salt in PEG-based downstream purification. These are distinct process functions: sodium citrate is not automatically a stabilizer or extraction enhancer in every formulation. We also distinguish citric acid from citrate salts because their effects can differ.

1. How can citrate help control extraction pH?

C-phycocyanin is sensitive to its surrounding conditions, so pH control can help limit unwanted changes during extraction and handling. A citric acid–citrate buffer can resist pH drift as cellular material is released and process conditions change. Because citric acid has three dissociation steps, with pKa values near 3.1, 4.8, and 6.4, citrate buffers can be formulated across acidic-to-near-neutral conditions. The appropriate target depends on the algal source, extract composition, temperature, and light exposure; there is no single pH that applies to every process.

Research on Spirulina phycocyanin commonly reports good stability in a mildly acidic range around pH 5.5–6.0. A separate study of Cyanidioschyzon merolae phycocyanin estimated a 486.9-day half-life in 50 mM citrate buffer at pH 5 under ordinary daylight conditions. The same study found much faster degradation under stronger illumination and at pH 7. That figure describes a particular strain, buffer, and light condition; it is not a shelf-life estimate for every Spirulina extract or finished product. See the Spirulina extraction and stability review and the study of light and citrate-buffer conditions.

2. Does sodium citrate always stabilize phycocyanin?

Not necessarily. Earlier research has proposed that citrate ions can interact with phycocyanin’s protein component and influence its hydration environment. This is a possible mechanism, not a guarantee that adding sodium citrate will protect color in every formulation. The salt form, pH, concentration, and extract source all matter.

A 2026 study illustrates why citric acid and citrate salts should not be treated as interchangeable. In a C-phycocyanin extract tested at 25 °C, with 2.5 mM additives in a phosphate-buffered system, the estimated half-life was 77 days without an additive, 208 days with citric acid, and 82 days with citrate. The citric-acid sample’s pH shifted from 7.0 to 6.2. Thus, the reported 269% increase applied to citric acid under those study conditions—not to sodium citrate generally. These laboratory results are context-specific, and the effect should be verified in the intended extract and food matrix. See the 2026 C-phycocyanin study.

3. How is sodium citrate used in purification?

At higher concentrations, sodium citrate can also be evaluated as a salting-out agent: changing the solution’s ionic environment can encourage phycocyanin and other proteins to precipitate, after which the desired fraction is recovered and redissolved. One published laboratory study using Chaohu algae reported selected sodium-citrate conditions of 0.5 M for a one-step process and 0.9 M for a two-step process. Those values came from that specific feedstock and protocol; they should not be assumed to be optimal for Spirulina. Salt concentration can change both purity and recovery, and downstream salt removal may be needed.

Sodium citrate has also been investigated as the salt component in PEG-based aqueous two-phase systems. In these systems, phycocyanin partitions between polymer-rich and salt-rich phases; the outcome depends on the organism, polymer, salt concentration, pH, and phase ratio. A study of phycobiliproteins from Anabaena variabilis and Nostoc examined sodium citrate alongside other phase-forming salts. In a separate extraction approach, citric acid—not sodium citrate—has been studied as the aqueous medium for microwave-assisted recovery of phycobiliproteins from Spirulina. Microwave heating can speed release, but time and temperature still need to be controlled to limit pigment degradation. See the PEG/salt partitioning study and the citric-acid microwave-extraction study.

Our practical approach is to treat citrate chemistry as a set of process variables to optimize—not as a universal recipe. We compare pH, citrate concentration, salt level, temperature, extraction time, and light exposure while tracking phycocyanin recovery, purity, color retention, and residual salts. The best choice depends on the raw material and the downstream use.