Cook the pasta in a large pot of boiling salted water until tender but still firm to the bite, stirring occasionally, about four minutes. Drain. Stir 480ml of the cream and the lemon juice in a large heavy frying-pan to blend. Add the butter and cook over medium heat just until the butter melts, stirring occasionally, about three minutes. Remove from the heat. Add the pasta and toss. Add the remaining 120ml of cream, and Parmesan to the cream sauce in the frying pan. Add the lemon zest, nutmeg, salt, and white pepper. Toss the pasta mixture over low heat until the sauce thickens slightly, about one minute.