Lemon curd serves a vital purpose in my household: it's a stand-in, at times, for the kind of sour candy that’ll burn your tongue off. That longing for zing sometimes ends in the artificial substitute—a handful of Extreme Sour SweetTarts—but, more than often than not, it leads me toward a batch of lemon curd. But lemon curd isn't the only curd, and there’s no need to stop at one sour spread. Recently, instead of enjoying grapefruit segments as an afternoon snack, I’ve been turning them into a fragrant, delightfully sour curd. “Curd works best when there is some sharpness present, to counter the sweetness,” baker and cookbook author Edd Kimber explained in a recent email.