Straw Pasteurization Calculator: Lime, Water and Time
Choose a cold lime soak or a hot water bath for your straw. For lime, enter the soak water and you get the hydrated lime to weigh out. For the bath, you get the temperature and the hold time the published method uses.
Result
120 g of hydrated lime
for 60 L of soak water, at 2 g per litre
- Dose
- 2 g per litre
- Soak
- 16 to 24 hours
- Soak water pH
- 11 or higher
- Lime type
- Hydrated lime (calcium hydroxide)
Handle the lime with care
Hydrated lime is caustic. Wear gloves, safety glasses and a dust mask when you handle the powder, and never use quicklime or garden lime.
Pasteurized straw is not sterile. It still needs clean draining and cooling before spawn goes in.
How it works
Lime by the Litre, Heat by the Hour
The lime method treats the straw with alkaline water at pH 11 or higher and uses no heat. The calculator sets the dose per litre of water, so the straw's weight does not change it. At the 2 g per litre the Central Texas Mycological Society gives for cold water pasteurization, 60 L of water takes 120 g of hydrated lime. Hydrated lime is caustic, and the society tells you to wear gloves, safety glasses and a dust mask when you handle the powder.
The hot water method from UF/IFAS starts with wheat straw cut into 4 to 5 inch pieces and soaked overnight. You then hold it in a 165 F (74 C) bath for one to two hours, checking the water with a thermometer. After that, you drain and cool the straw before spawning.
Both methods pasteurize, which lowers the competing microbes without killing everything. Some survive, so the straw drains on a clean surface and cools before the spawn goes in. Straw left to drain on a dirty bench can pick up new microbes and undo part of the work.
Other growers' pages print higher and lower lime doses. The calculator uses only the one dose we read in its source, so every result matches a page you can open and check.
Your lime soak, worked through
lime (g) = water (L) x 2 g per litre
- 60 L x 2 g = 120 g of hydrated lime
- Stir the lime into the water, then submerge the straw for 16 to 24 hours.
- The soak water should read pH 11 or higher. Drain the straw before adding spawn.
Limits
What this tool cannot do
- It cannot measure your pH or your bath temperature. A pH strip and a thermometer do that.
- It only covers hydrated lime (calcium hydroxide), never garden lime or quicklime.
- It does not make straw sterile or promise a clean bag.
Questions
Can I use garden lime instead of hydrated lime?
Garden or agricultural lime is calcium carbonate, and it does not raise the water to the pH the method needs. The Central Texas Mycological Society also warns against quicklime, which is much more dangerous to handle. Look for the words hydrated lime or calcium hydroxide on the bag before you buy.
Does the straw weight change the lime dose?
The straw weight does not change it, because the dose follows the soak water. The same 60 L of water takes 120 g whether it covers 3 kg or 5 kg of straw. Use enough water to cover all the straw, then size the dose to that water.
Can I pour boiling water over straw in a cooler instead?
When you pour boiling water over straw, it starts hot and then cools as the water, container and straw trade heat. The UF/IFAS method holds a measured 165 F bath for one to two hours. Without a thermometer reading over that time, you cannot show the straw stayed hot enough for long enough.
Sources2
- Central Texas Mycological Society, The Easy Guide to Cold Water Pasteurization for Straw 2 g of hydrated lime per litre, a 16 to 24 hour soak, pH 11 or higher, and the gloves, glasses and dust mask warning. Checked 2026-10-20.
- UF/IFAS, Grow Your Own Oyster Mushrooms at Home (SS662) Wheat straw cut to 4 to 5 inches, soaked overnight, held at 165 F for one to two hours, drained about an hour and cooled before spawning. As summarized in our straw substrate guide, checked 2026-10-20.