FAQs

Post-autoclave medium abnormalities are among the most common quality complaints in laboratory microbiology. Causation is rarely singular and requires a structured investigation.

Step 1 — Classify the abnormality

White flocculent precipitate: Most commonly calcium/magnesium phosphate precipitate (reaction between divalent cations and phosphate nutrients), or incompletely dissolved agar particles. Test: add a small amount of the precipitate to dilute hydrochloric acid — if it dissolves, it is a phosphate salt (unrelated to agar quality); if insoluble, consider incomplete agar dissolution.

Overall haze (smoky appearance): Most often produced by high-ash agar in phosphate-containing media, or by excessive raw material impurities.

Colour deepening (amber/brown): Over-sterilisation (temperature >125°C or duration >20 min) causing Maillard-type polysaccharide degradation, or an inherently dark raw material lot of lower purity.

Step 2 — Systematic root-cause investigation (operator → water quality → raw material)

① Operator level: Confirm autoclave parameters were within 121°C / 15–18 min (large-volume bottles may extend to 20 min, but beyond this degradation accelerates sharply). Confirm agar was pre-soaked in cold water for 10–15 min before heating (skipping this step commonly leaves undissolved material).

② Water quality: Laboratory water hardness (total Ca + Mg) should be <10 ppm (deionised or ultrapure water). Mains or poorly treated purified water is the leading cause of phosphate precipitation — independent of agar quality.

③ Raw material: Compare ash figures across recent batch COAs; run a parallel autoclave test with the suspect lot against a reference lot under identical conditions — if the reference is clear, the raw material is implicated.