Using an Instant Pot to sterilize triple mix

Today (Aprl 10, 2022) I did a kitchen experiment to see if I could use our Instant Pot to sterilize triple mix.The reason for the desire to heat-treat the mix is the risk of Jumping Worms (Amynthas ssp.) an invasive species that has recently been reported in my city of Hamilton (Ontario Canada). Although the worms do not survive our winters (most die with the first freeze in autumn) the egg cases / cocoons survive very cold winters (to -35C). Any bagged “soil” product that contains compost or “black earth” is a potential source of jumping worm eggs. Research at the University of Wisconsin Arboretum showed that eggs are killed after three days at only 40C (104F). But, given that we won’t have temperatures hot enough to solarize anything until June or July, I decided to try heating my favourite triple mix indoors with an Instant Pot.

I expected the process would be safe, since Instant Pots are designed to be left on for hours at a time. In fact, the “slow cook” function allows a maximum of 20 hours continuous operation. The electricity used would be low (compared to the cost of running a gas stove or barbecue.) As well, the Instant Pot has a tight-fitting lid which would mitigate any odour.

I used a 7-litre DUO Instant Pot and filled it nearly to the top with CIL/Premier Triple Mix. This is my regular brand and I have alway appreciated it’s consistency and quality. I did try contacting the company’s customer service about how, or if, they mitigated the risk of jumping worm cocoons in the compost portion of their bagged products. I received a cut-and-paste replay (“our products are not know vectors etc etc”) which was not reassuring.

I used the Instant Pot’s “slow cook” function, on the “more” (high) setting. The lid was latched and set to “venting.” I checked the temperature (with a digital meat thermometer) at the bottom and top after 90 minutes and again after 5.5 hours. Here are the results:

90 minutes, temperature near bottom: 190F
90 minutes, temperature near top: 104F
1.5 hours, temperature near bottom: 194F
1.5 hours, temperature near top: 170F

Did it scorch or smell bad? No. In fact, the odour was earthy, and not unpleasant. Perhaps this is because I stored the bag outside all winter, but I really don’t know. Others have reported on the stink associated with sterilizing garden soil in a home oven, so I was surprised.

So, let’s look at the temperature results. Sterilization happens at 180F (enough to kill bacteria and other micro-organisms) and I expect it would kill jumping worm eggs as well. The research at U of Wisconsin showed that eggs were killed after three days at only 104F. I will try to find out if a short time at at 180F is enough to kill them.

The thermometer readings at 5.5 hours showed that the mix at the top (an inch or so down) reached only 170F. I continued heating the mix and, even after an additional hour, the top portion stayed at 170F. So, to be sure that the entire contents got to at least 180, some stirring would be necessary. If the Instant Pot had a cook function hotter than “slow-cook” but not as hot as “saute”, I could possibly increase the temperature to acheive sterilization at the top. Pressure cook is not an option since it requires liquid and the soil particles would clog the vent.

I expect that the mix reached the maximum temperatures (~190F on bottom / 170 on top) before I checked the temperature at 5.5 hours. In subsequent batches I’ll test at 30-minute intervals to see if I can reduce the time required per batch.

For now, though, I’m posting this to give my experiment an online home. This post will be updated as I learn more.

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