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I make yogurt about twice a week in an instant pot using 2qt of 1% milk and 3 tablespoons of whey from the previous batch as a starter (original starter was 1 Tbsp of Chobani plain non-fat). I normally culture for 8 hours, refrigerate for 8 to 10 hours and then strain for 3 to 4 hours. I usually end up with a quart of yogurt and a quart of whey. yesterday I forgot and left the batch in the pot about 2 hours past the shut off time, refrigerated for 10 hours and strained for 3 hours and got almost 25% less whey separation. I stopped straining after 3 hours because the strainer stopped dripping whey. the batch is as thick and tasty as always but I have 20 to 25% more yogurt. can you tell me why?

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Presumably your culture was still doing its work after 8 hours (and cultures derived from old batches can lose potency so what was achieved in 8 hours a few batches ago may take a little longer now). The shut-off time turns off the heat I assume, but the temperature will drop very slowly from that point. It wouldn't surprise me if this batch was actually a little more tasty due to the longer fermentation.

However there could be another cause, especially if you're in the Northern hemisphere. Even if you did everything exactly the same, including the starting temperature of all ingredients, one possible cause would be a change in the milk. I don't know whether you always buy the same milk (and even then the ultimate source can be different from most shops.)

But then it gets more interesting, even if you get your milk from the same herd of cows: milk isn't a uniform product. See for example Seasonal variations in the composition of Holstein cow's milk and temperature-humidity index relationship.. The paper I link (among many others dating back over 100 years) would appear to suggest that both fat and protein concentration will increase as the weather cools off. More protein in particular would mean more to coagulate.

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    I must admit this is mostly theoretical at the moment, I just happen to have been reading about yoghurt-making recently with a view to having another go. I've made much more cheese than yoghurt. – Chris H Nov 30 '18 at 15:13

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