intermittent solid crust on top of yogurt
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?
Best Answer
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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