This may just be something I've encountered because I live in a warm climate (sunny Perth!).
I make yogurt weekly for my kids, and I have been frustrated about a little bit of milk leaking down the sides of the Thermomix despite the seal being in place.
Finally I found a simple way to STOP THE LEAKING. It has been working for the past several weeks, even when I make two batches of yogurt in a row, so I thought I would share.
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When I make yogurt, I generally follow the EDC recipe, heating to 90C first, then cooling in a cold water bath to 37C.
After the cold water bath, I heat the milk to 37C for 3 minutes just to make sure it's at the right temperature.
Then I add my yogurt starter 4 seconds speed 4, and heat for 10 minutes on 37C on speed 1 to finish cooking. <-- It is during these two steps that I used to encounter milk leaking out from under the seal and down the sides of the Thermomix. I don't have any other leaking issues at any other time using the Thermomix, so I know it isn't a faulty seal ring.
My solution is simple,
when I am cooling the milk from 90C to 37C, I remove the seal ring from the lid, and pop both the lid and the seal ring in the cold water bath next to the TM bowl full of hot milk.
Cooling the seal ring makes the ring less flexible/pliable/prone to leak. It takes no extra time to cool the lid and seal ring, and I haven't had a leak since I started doing this.
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Hopefully no one else is having this problem, but if you are, maybe this will help!