I believe there are a couple of species of grapes often referred to as “Frost grapes” One of them is vitis riparia (also known as riverbank grape), the other is Vitis Vulpina. To me those look like Vitis riparia due to the leaves and the fact that the grapes appear to have red flesh. I have made wine and pyment out of Vitis riparia and, they were much sweeter after the frost, although a wine made without any added water was still way too acidic, even after 3 years of aging. The sugar content was really high as well (if I recall correctly the brix was around 26). With the grapes I used there was a very high skin to juice ratio, especially after the frost, so it made a shockingly concentrated, viscous wine. The downside is that it is a very intensely acidic, one-note grape and the wine had very little complexity other than an over the top minerality and harsh acidity. There was a slight appealing earthy savoriness about it, but with absolutely no supporting fruity or floral notes, it just wasn’t enjoyable. Back then I was still using lab cultured yeast to make wine, and the yeast I used (Red Star Premier Cuvée) was a reliable fermenter, but not very interesting, so maybe with a different yeast (like 71B as pvtpublic suggested) or a native ferment it would have been better. The Pyment on the other hand was wonderful because the honey added complexity and smoothed out those rough edges. Even though I only fermented it on 2-3 lbs of grapes per gallon of mead must, they still contributed a surprising amount of color, body and acidity. The pyment was also surprisingly good with less than a year of aging.