How to make pie filling which tastes like yogurt icecream? I have some ripe papayas and decided to create a new pie recipe with them. The papaya part turned out quite well. I used pureed papayas, dark sugar beet syrup, creme fraîche, egg, and some cognac. The taste is very earthy, somewhat buttery, and while it was nice, I didn't want the whole pie to taste like that. So I decided that a second filling would be nice. I want to make a filling with contrasting taste and color, and lightly mix both in the shell, like marble cake. 

For the taste, I think that the typical Italian yogurt gelato taste will give the perfect combination - quite sour, somewhat sweet, no distracting fruity notes to compete with the papaya. The easiest thing to try was to just bake some yogurt. So I mixed 3.5% and 10% yogurt, added some sugar, vanila and cream of tartar, and mixed in dissolved starch. But after baking the pie, the taste is nowhere near what I need. The biggest problem is that it isn't sour enough. 

Any ideas how to approximate frozen yogurt or yogurt icecream taste? I prefer a baked filling, but if all else fails, I'd settle for a cold-setting filling to be put into a blindbaked shell. Then I'll have to remove the egg from the papaya and think of a way to thicken it.
I'm not much of a pie baker, but I am something of a chemist.  So, I can tell you that the yogurt is pretty sour (pH around 4.4.-4.7), but the acidity comes from lactic acid, which tastes different than the acetic acid of vinegar or the citric acid in citrus fruits.  

I think it's the flavor of citric acid you want, and are missing, not the actual acidity (with cream of tarter in there it should be quite acid).  In this case, lime is a natural partner to papaya. Rather than flavoring the yogurt with vanilla, why not use a healthy jigger of lime juice and its zest?  The juice has a pH of around 2, so it will also increase the acidity some.  The zest gives the essential oils of the fruit, giving a stronger lime smell and flavor (without actually impacting the acidity).