Question
Inspissation
Answer
Inspissation is the process of solidifying/coagulating a liquid or semi-liquid medium (typically egg- or serum-containing media) by slow heating, used in the preparation of certain enriched bacteriological culture media that cannot be sterilized by ordinary autoclaving (since high-pressure steam sterilization would cause excessive denaturation/damage to the protein-rich medium).
Method: the liquid medium (e.g., a mixture of whole egg and nutrient broth for Lowenstein-Jensen medium, or serum-based broth for Loeffler’s serum slope) is dispensed into tubes/bottles, arranged at a slant (to create the characteristic sloped solid surface), and subjected to slow, controlled heating — typically in a specially designed inspissator device — at a temperature around 80–85°C for a set period (often on successive days for some media) — sufficient to coagulate the egg/serum protein into a solid gel, without the more extreme heat of autoclaving that would degrade essential nutrients or cause uneven, cracked solidification.
Examples of media prepared by inspissation:
- Lowenstein-Jensen (LJ) medium — the standard egg-based medium for culturing Mycobacterium tuberculosis; the coagulated egg provides essential lipids/proteins for mycobacterial growth.
- Loeffler’s serum slope — used for the rapid growth of Corynebacterium diphtheriae, prepared from coagulated serum.
Rationale: since these enriched media are usually contaminated to some degree with heat-resistant spore-forming environmental organisms picked up from the raw ingredients (egg, serum), inspissation is sometimes performed on successive days (similar in principle to Tyndallisation) to allow any surviving spores to germinate between heating sessions and be killed on subsequent heating, achieving effective sterilization despite the lower temperature used.