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I make 14 cups of yogurt at a time. Occasionally one of them will have a firm layer on top that you can lift off in one piece. There is usually a dark ring around the edge.

I'm using whole milk and adding dried milk. For a starter, I'm using Lactobacillus johnsonii 456.

Instead of boiling the milk, I'm using a UV-c sterilizer. The sterilizer is also used on all bowls, cups, lids, stirring equipment, etc.

What is causing the firm layer that lifts off?

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    Hi, aside from the direct answer, you might want to rethink the no-boiling part. The boiling is not only for sterilization, it changes the milk proteins' structure, giving a silkier texture to the yogurt. Maybe try it out once and see if the difference is enough for you to make it worth switching.
    – rumtscho
    Commented Jul 5, 2021 at 6:47
  • Thanks for the suggestion. Most of the yogurt comes out very nice and silky using the Uv-C sterilizer. UV-c sterilizer is much easier than the whole boiling and then cooling gig. I think I figured out what the problem is. The lid on the yogurt maker develops condensation. I usually wipe that off a couple of times during the eight-hour process. I'm not doing that often enough so contaminated water is dripping into one jar. I need to sterilize the lid and wipe it off more often. Commented Jul 6, 2021 at 16:44

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I sometimes get a solid crust. I frequently use unhomogenized yogurt, and if it has creamed out, the fat melts during the scalding, then floats on top of the glass while incubating, and ends up a solid disc afterwards.

I cannot say if it is the absolute same mechanism with you, but it is one possible explanation. Especially since you don't boil the milk, your fat may not be melted out in real fatty eyes. It should certainly soften, I suppose you are incubating somewhere in the mid-40s, and pure butterfat melts somewhere in the lower 30s, so even if the emulsion in the milk behaves differently, it won't be exactly like cold milk.

You can try finding out if this is fat by trying to melt the crust. If it starts looking like melted butter, then this is a very likely explanation.

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  • Thanks for the suggestion. Most of the yogurt comes out very nice and silky using the Uv-C sterilizer. UV-c sterilizer is much easier than the whole boiling and then cooling gig. I think I figured out what the problem is. The lid on the yogurt maker develops condensation. I usually wipe that off a couple of times during the eight-hour process. I'm not doing that often enough so contaminated water is dripping into one jar. I need to sterilize the lid and wipe it off more often. – Commented Jul 6, 2021 at 16:45
  • I solved the problem by moving the yogurt to a cooler part of the refrigerator. Commented Feb 21, 2022 at 1:15

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