Fractional sterilization by free steam: Steam exposure at 100 °C for 20 minutes on three consecutive days is known as what process?

Introduction / Context:
Some media and materials are heat-sensitive and cannot withstand autoclaving at 121 °C. Fractional sterilization using free steam at 100 °C across multiple days exploits the germination of surviving spores between cycles, allowing subsequent kills. This classic method is important historically and conceptually in sterilization science.

Given Data / Assumptions:

• Steam at 100 °C (not under pressure) is applied for 20 minutes.
• The exposure occurs on three consecutive days.
• Goal: sterilize without high temperatures that damage constituents.

Concept / Approach:
Tyndallization (fractional sterilization) alternates heating and incubation. Day 1 kills vegetative cells; incubation allows surviving spores to germinate into vegetative forms; Day 2 and Day 3 cycles then kill the new vegetative cells. This staged approach achieves sterilization for materials incompatible with pressurized steam.

Step-by-Step Solution:
1) Recognize the temperature/time pattern: 100 °C, 20 minutes, repeated for 3 days → hallmark of tyndallization.2) Distinguish from inspissation (80–85 °C for serum media, not three-day steam cycles).3) Distinguish from autoclaving (121 °C at ~15 psi for 15–30 minutes; single cycle).4) Confirm answer: tyndallization.

Verification / Alternative check:
Classical microbiology texts describe fractional sterilization as effective for sugar-rich or proteinaceous media that would degrade at autoclave temperatures, highlighting the three-day schedule.

Why Other Options Are Wrong:

• Inspissation: uses lower heat to coagulate serum/egg media; different protocol.
• Autoclaving: higher temperature and pressure; single sterilization cycle.
• Pasteurization: aims to reduce pathogens in foods, not full sterilization.
• None of these: incorrect because the method is a named process.

Common Pitfalls:
Confusing any multi-day heating with inspissation; overlooking the key temperature of 100 °C and the three-day repetition.

Final Answer:
Tyndallization (fractional sterilization).