Yes, I added the zest from half a grapefruit in the primary on about the third day (allowed to ferment to dryness over 8 days). I put the zest in a sanitized coffee filter and then bunched it up with a rubber band (also sanitized) and suspended it with a sanitized cotton string tied to the bucket handle. I left it in there for three full days (maybe four, can't remember) and then removed it. I plan to check the wine again for acidity in another 4 weeks and if needed, I may repeat that process as it ages. Several people warned me about how much power that zest has so I didn't want it to overwhelm the wine and make it too grapefruity. But so far, it tastes fantastic. There's a nice acidity taste to it and you can really taste the fruitiness of the grapes although I haven't technically measured the acidity so far. My wife and daughters are all looking forward to a full glass. I am too! My son wants a glass but he's 18 and his mother would kill me.