The short answer
Slight caking (breaks up under a spatula) with a slightly darker colour is usually still usable, provided the pH and appearance of the prepared medium are within specification; for critical tests add a reference-strain check. Hard lumps that will not break, clearly darker colour or spots, or an off odour: discard, and do not gamble a batch of test results to save one bottle. Dehydrated media deteriorate for essentially one reason, moisture uptake, so storage after opening is all about keeping water out.
Why caking and discolouration happen
The peptones, yeast extract and sugars in a dehydrated medium are strongly hygroscopic. Every time the container is opened, a little water vapour is absorbed; the powder goes from free-flowing to tacky to lumps. Once moist, the sugars and amino acids undergo a slow Maillard reaction even at room temperature, turning pale yellow to brown; in media containing dyes or antibiotics, moisture speeds the breakdown of those components. Caking is the evidence of moisture; discolouration is the chemistry that follows.
In humid climates such as Taiwan this is far worse than in dry regions. A 500 g bottle used 10 g at a time and opened dozens of times will almost certainly cake towards the end.
Grading the condition
| Condition | Judgement | Action |
|---|---|---|
| Free-flowing, normal colour | Normal | Use |
| Slight caking that breaks easily, slightly darker | Possible moisture | Check pH and clarity after preparation; use if in specification; add reference strains for critical tests |
| Hard lumps that must be broken, clearly darker | Deteriorated | Discard |
| Spots, odour, mould | Contaminated | Discard; check other bottles of the lot |
Moisture also brings an invisible problem: weighing error. Powder that has absorbed 5% water delivers only 38 g of medium when you weigh 40 g, so nutrient concentration and agar strength both come out low. That is why slightly caked medium is confirmed by pH and cultural response, not by appearance alone.
A quick verification
- Prepare 200 mL as instructed and sterilise.
- Compare clarity and colour with the first preparation from a fresh bottle.
- Measure pH after sterilisation; it must be within the specification sheet range.
- For agar media, check gel firmness: a loop should streak without tearing.
- For critical tests, add growth and reaction checks with reference strains.
These five steps take half a day, far less than the cost of a voided test batch.
Storing an opened container
- Close the container immediately after each use; do not leave it open while weighing the next item.
- Store in a cool, dry place, not the refrigerator: a cold bottle brought into the room condenses moisture on its surface and straight into the powder. Most instructions specify 10 to 30 °C.
- Use desiccant: a zip bag with desiccant around the bottle, or a dry cabinet.
- Write the opening date on the bottle. Recommended use after opening is typically 6 to 12 months, much shorter than the unopened shelf life.
- Split it up: once a 500 g bottle is opened, divide it into several sealed smaller bottles and open one at a time.
- Never use a damp spatula, and never return excess powder to the bottle.
For unopened storage and shelf life, see Storing dehydrated media and shelf life.
What pack size to buy
For a low-volume laboratory, 500 g is far more economical than 5 kg even at a higher unit price. Losses after opening, and the testing risk of continuing to use moist medium to save money, both cost more than the price difference. Three to six months of use is a sensible quantity. STBIO dehydrated media come in 500 g, 5 kg and 25 kg, with 100 g and 200 g for some items; the product page lists the options.
FAQ
Can I grind caked medium in a mortar and use it? Grinding only turns lumps back into powder; the absorbed water and the changes it caused are still there. Slightly caked medium can be ground and verified as above; hard lumps are not recommended.
Unopened, past the expiry date, looks normal. Usable? The expiry date is the period for which the manufacturer guarantees performance. Past it the medium is not necessarily bad, but there is no guarantee. For informal use, verify and use; for formal testing, quality systems usually say no.
Some media cake within a month or two of opening while others last a year. Why? Formulation. Media rich in sugar and yeast extract (MRS, SDA, YPD) absorb moisture fastest; media based mainly on inorganic salts hardly do. Chromogenic media and those containing antibiotics additionally lose active components, so they have the shortest life after opening.
Does the expiry date on the COA refer to before or after opening? Before opening. After opening, follow the "use within" period in the instructions; where none is stated, six months is a reasonable rule.