The short answer
A white or milky precipitate is nearly always magnesium or calcium phosphate: the solubility of phosphate with magnesium and calcium ions falls at high temperature and crystals form. This differs from a black precipitate, which is iron sulfide formed from hydrogen sulfide released by sulfur amino acids reacting with iron; see Black precipitate in fermentation broth after sterilisation. A white precipitate is not toxic in itself, but it means part of the phosphorus, magnesium and calcium has left the solution, which affects growth in defined and fermentation media. A correctly prepared standard test medium should not show one; if it does, check the water first.
Telling the precipitates apart
| Appearance | Most likely | Origin |
|---|---|---|
| Fine white powder, suspends on shaking, settles on standing | Magnesium or calcium phosphate | Phosphate meeting magnesium or calcium |
| White flocs clinging to the glass | Denatured protein | Peptone overheated or pH far off |
| Grey to black | Iron sulfide | Sulfur amino acids plus iron |
| Brown, whole bottle darker | Maillard products | Sugar and amino acids overheated |
A simple test: a white phosphate precipitate dissolves on adding a few drops of dilute hydrochloric acid; protein flocs do not.
Formulations prone to it
- High phosphate together with magnesium and calcium: defined media of the M9 type, and fermentation media that combine dipotassium phosphate, magnesium sulfate and calcium chloride.
- Prepared with tap water: the hardness (calcium, magnesium) in tap water reacts with the phosphate in the formulation. This is the most common cause in testing laboratories; the same powder prepared in purified water is fine.
- Alkaline pH: phosphate precipitates with magnesium and calcium more readily above pH 7.5.
- Concentrates: a 10x concentrate sterilised to save space precipitates more because everything is ten times more concentrated.
How to avoid it
- Prepare with purified or deionised water. This alone removes most laboratory cases.
- Sterilise separately: make magnesium and calcium salts as a separate concentrated solution, sterilise it, and add aseptically after the medium has cooled below 50 °C. Keep the phosphate in the main formulation. This is standard practice for fermentation media.
- Add a chelating agent: tens of milligrams per litre of sodium citrate or EDTA hold magnesium and calcium and stop them crystallising with phosphate. Especially effective in trace element solutions.
- Do not leave the pre-sterilisation pH alkaline: adjust to the target and sterilise; do not overshoot "a little high to allow for the drop".
- Do not over-concentrate: formulations with phosphate and magnesium or calcium together should be split once the concentration factor exceeds about 5x.
Can a medium with a precipitate still be used?
- Standard test media (TSA, PCA, MacConkey and the like): prepared in purified water as instructed, there should be no precipitate. If there is, remake it and check the water and the weighing; cultural response may already be off specification.
- Fermentation media: a small amount of white precipitate, shaken up, can be used, knowing that some phosphorus and magnesium is no longer in solution. If the amount varies from batch to batch, so will performance; move to separate sterilisation.
- Defined media: not recommended. Every component is calculated, and whatever precipitates directly affects growth.
FAQ
The medium was clear in the autoclave and turned white as it cooled. Why? The solubility of calcium and magnesium phosphate changes with temperature and some of it crystallises on cooling. The remedies are the same: purified water, separate sterilisation or a chelating agent.
The instructions for a dehydrated medium say "slight precipitate after heating is normal". What is that? Some formulations containing phosphate and magnesium (certain lactic acid bacteria and yeast media, for example) are slightly opalescent by design and the instructions say so; swirl and dispense. A precipitate that is not mentioned in the instructions is the problem.
Does a precipitate affect reading agar plates? It makes plates hazy, reduces colony contrast when counting, and can obscure colour reactions on selective media. Prepare all plate media in purified water and remake any that precipitate.
What water quality is needed? Deionised or distilled water with conductivity below 10 µS/cm is sufficient for media preparation; ultrapure water is not required. Most national test methods specify "distilled water or water of equivalent purity".