Baking – Greek yogurt substitute in baking

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).

Greek yogurt is simply strained yogurt. It was only fairly recently that Greek yogurt was widely available in the United States, so prior to wide availability, a simple substitute was to strain normal yogurt.

The type of sauce you are trying to make would definitely use a strained yogurt. Whether you wanted to strain your own or buy strained yogurt (aka Greek yogurt) is really up to you. In Greece, sheep's milk was traditionally used for yogurt, but now there's a lot more cow's milk too, so either would be authentic. I would think that full fat would be the most authentic, but not the healthiest, as I doubt low fat varieties were used traditionally (if they even existed).

The American use of the term Greek yogurt is really just one of branding. Many other areas in the middle east use the same style of strained yogurt. The Greeks also have plenty of normal (unstrained) yogurt.