Sterilization vs pasteurization: what's the difference?

The difference between sterilization and pasteurization is how much they kill. Sterilization aims to kill all contaminants, "both living and currently dormant", and uses pressure as well as heat (GroCycle). Penn State calls pasteurizing mushroom compost "not a complete sterilization but a selective killing" of pests, one that minimizes the loss of good microbes (Penn State Extension).

	Sterilization	Pasteurization
Heat	250°F (121°C) at 15 PSI	160–165°F (71–74°C) water bath for straw
Equipment	Pressure cooker or autoclave	Pot or drum and a burner, or a lime bath
Used for	Grain, supplemented sawdust, Master's Mix, manure	Straw, coir, cardboard

Sources: sterilizing heat from UC Cooperative Extension; straw from UC (160°F) and UF/IFAS (165°F); substrates from GroCycle, a company that sells growing kits and courses, and Cornell. Healthy fresh logs and wood chips need no treatment at all, Cornell says.

Substrates rich in nutrients need sterilizing, and poor ones can be pasteurized. GroCycle's test: is it something a wild animal would eat? Grain, manure, and sawdust supplemented with something molds and bacteria grow on easily get sterilized; straw, coir, logs and cardboard don't need it (GroCycle). Cornell says sawdust mixed with bran or soy hulls needs steam sterilization (Cornell). UC explains why: straw and sawdust are low in nitrogen and "semi-selective" for mushrooms, and adding nitrogen loses some of that selectivity, so other fungi grow too (UC Cooperative Extension).

Some farms treat supplemented sawdust with steam at normal pressure instead. Cornell lists it as a low-cost option, but with more contamination than pressurized steam (Cornell).

• Grain: 90 minutes at 15 PSI, timed from when the cooker reaches pressure (GroCycle). UC sterilizes quart jars twice, 45 minutes each, 24 hours apart, to kill heat-resistant bacterial spores (UC Cooperative Extension).
• Sawdust bags: UC autoclaves them for 90 minutes or more at 15 PSI, by volume and number of bags, and elsewhere gives 2–4 hours by volume (UC Cooperative Extension). For Master's Mix, GroCycle gives 2.5 hours (GroCycle) and Midwest Grow Kits, which sells it, 3.5 hours (Midwest Grow Kits).
• Straw, hot water: 1–2 hours at 160–165°F (71–74°C) (UC Cooperative Extension, UF/IFAS).
• Straw, lime: 24 hours in cold water treated with hydrated lime (GroCycle).

At altitude, pressure cookers need more time: our liquid culture guide gives the adjustment.

• Boiling and calling it sterilized. Boiling doesn't get substrate hot enough to sterilize it (GroCycle).
• Starting the clock too early. Time from when the cooker reaches 15 PSI, not from when it goes on the stove (GroCycle).
• Inoculating carelessly. UC warns that sterilized substrate "may be more susceptible to weed molds" if they get in, because it is a "biological vacuum" (UC Cooperative Extension). Inoculate in a still air box or in front of a flow hood.
• Skipping the grain soak. Bacterial endospores can survive sterilization; a 12–24 hour soak first wakes them so the cooker kills them (contamination guide).

Hypha tracks each strain's contamination rate, and a note on each block can record which cooker load it came from, so a bad batch is easier to trace.