Bacteriostatic water — usually shortened to BAC water — is the standard solvent for reconstituting lyophilized research peptides. The name comes from its key property: it inhibits bacterial growth in the reconstituted solution, extending stability from hours to weeks.
Composition
BAC water is sterile water containing 0.9% benzyl alcohol as a preservative. The benzyl alcohol does the bacteriostatic work — it doesn’t kill bacteria outright (that would be bactericidal), but it inhibits their growth and reproduction, keeping the reconstituted peptide solution stable for 30–60 days under refrigeration.
The water itself is USP-grade — meaning it meets United States Pharmacopeia standards for sterility, particulate, and endotoxin levels. The benzyl alcohol is added at the manufacturing stage, before vial sealing.
BAC Water vs. Sterile Water vs. Saline
| Solvent | Composition | Post-Reconstitution Stability | Typical Research Use |
|---|---|---|---|
| Bacteriostatic Water | Sterile water + 0.9% benzyl alcohol | 30–60 days refrigerated | Standard for most lyophilized peptides |
| Sterile Water for Injection (SWFI) | Sterile water, no preservatives | ~24 hours refrigerated | Single-use protocols where preservative may interfere |
| 0.9% Saline (Sodium Chloride) | Sterile water + 0.9% NaCl | Compound-dependent | Specific peptides or assay-specific protocols |
For most research-grade peptide work, BAC water is the default. Use sterile water without preservative only if your specific assay or compound is documented to be sensitive to benzyl alcohol — and if you do, plan to reconstitute and use within 24 hours.
Standard Reconstitution Ratios
The most cited reconstitution ratio in published peptide research methods:
- 2 mL BAC water : 10 mg peptide → 5 mg/mL concentration
- 1 mL BAC water : 5 mg peptide → 5 mg/mL concentration
- 3 mL BAC water : 5 mg peptide → 1.67 mg/mL (lower concentration, easier syringe-unit math)
The choice of concentration is driven by the syringe units you’ll be drawing for your protocol. The reconstitution calculator at matteprotocol.com/reconstitution/ converts any (volume, mass, target dose) combination into the corresponding syringe unit on a U-100 or U-50 insulin syringe.
How Long BAC Water Lasts (the Vial Itself)
An unopened BAC water vial is shelf-stable at room temperature for 24+ months — check the lot expiration on the vial.
Once you puncture the rubber stopper to draw water, the vial’s “in-use” stability depends on the manufacturer’s labeling. Most BAC water vials are labeled for 28-day in-use stability after first puncture, though some research-grade vials extend this to 90 days. After the in-use stability period, discard any remaining water — the bacteriostatic preservative degrades over time and repeated punctures introduce small particulate.
Storage
- Unopened: room temperature in original packaging, away from light and heat extremes
- After first puncture: room temperature is fine; refrigeration optional but doesn’t extend in-use stability beyond labeled limit
- Reconstituted peptide solutions should be refrigerated at 36–46°F (2–8°C), not the BAC water vial itself
Vial Sizes
Common research-supply sizes:
- 3 mL — single small reconstitution, common for one-time studies
- 10 mL — standard size, good for several reconstitutions over the in-use period
- 30 mL — larger format for high-volume research
Matte Protocol stocks BAC water in 3 mL and 10 mL sizes. Both are USP-grade with 0.9% benzyl alcohol in sealed amber glass.
Troubleshooting Reconstitution
Cake doesn’t dissolve
If the lyophilized cake won’t dissolve after a gentle swirl:
- Wait — some peptides need 5–10 minutes to fully dissolve, especially at higher concentrations
- Warm the vial gently in your hand (body temperature only, never above 37°C / 98°F)
- Try a higher solvent volume — too little water for the mass of compound can leave undissolved residue
If after 30 minutes the cake still hasn’t dissolved, the compound is likely degraded — contact the supplier.
Solution looks cloudy
A few peptides (notably highly hydrophobic ones) produce a slightly cloudy solution at certain concentrations. Documented in the published method? It’s expected. Not documented? Could be a signal of improper purity.
Foaming during reconstitution
Injecting BAC water directly onto the cake (rather than down the vial wall) can foam the peptide, especially for surfactant-class compounds. Foam isn’t degraded peptide — it’s just suspended air bubbles. Let it settle 10 minutes; the foam dissipates.
The Compliance Note
Bacteriostatic water itself is a regulated medical product when sold for human use. When sold for laboratory research as a reagent — which is how Matte Protocol sells it — it is provided strictly for research, with the same Research Use Only labeling and disclosures that apply to all peptide products.
Research use only. Not for human consumption, veterinary use, or as food additives. Statements have not been evaluated by the US FDA.