Is there any potential harm to using warm water in my sourdough starter?

Question

My house is cold. It is regularly around 13 to 15°C (55 to 59°F) and it is only with a lot of (expensive) heating that I get it up to anything approaching "normal" room temperature. My sourdough starter is about three weeks old and is healthy enough (I've made several loaves with a decent texture and taste) but it does seem a little down in the dumps compared to many that I've seen online.

I've tried adding warm water rather than room temperature. I mix water half from my filter jug and half just boiled, which is about 53°C (127°F). I add it to my starter and the combined temperature is about 33°C (91°F).

This, along with several tea-towels for keeping that heat in, seems to help. The starter is significantly more bubbly and active and actually come close to doubling in size like everyone says it should!

But I'm aware that heat can kill yeast cells. Am I putting my starter at risk by this method? What do I need to be wary of?

Answer 1

I would limit the temperature of the water to around 110F.

• 140F is widely cited as the "Death Temperature".
• At 120F the yeast is at it's "injury point" meaning you are harming the yeast, but may not kill it.
• Some claims can be found stating water temperature as high as 95 can produce off flavors from the yeast when making bread. However, this really won't apply to your starter since it will make up such a small percentage of the loaf.

Here is a relevant, although not duplicate, SA post asking when eyast is killed: What temperature of water will kill yeast?

Some sources to skim through on the topic:

• http://www.homebrewtalk.com/showthread.php?t=67272
• https://www.ncbi.nlm.nih.gov/pmc/articles/PMC232826/
• https://www.exploratorium.edu/cooking/bread/yeast_temp.html
• https://www.reference.com/science/temperature-kills-yeast-6cd7b72a3ed2d24b

Answer 2

It may work, but it is risky. It is not just a matter of accidentally killing the yeast with too hot water. A starter is a battlefield of many different bacteria and yeasts, and the environment determines which strain will dominate in the long run. An environment which starts at 33 C, then cools down to 13 over a day, then gets back up to 33 is a very different one than an environment which stays constantly at 20.

It might be still workable if the starter is behaving OK for now. But if you want to be on the secure side, it would be better to provide the right environment for the yeast. There are professional proofing boxes, but you can probably repurpose something else, or build your own. Especially if you are good with circuits, a small isolated box warmed by a single 100 W lightbulb can be easily held at the proper temperature if you rig it to a cheap thermostat that turns the bulb on and off. You don't need very tight control of the temperature, so you don't even need electronics (but you can of course use them if you feel like it). Other lower-tech solutions like the beer cooler sous vide hack should also work well, and keep a much more constant temperature than throwing 33 C directly into the starter.