you're 21, I assume you're parents are older than you (usually a safe assumption), therefore you're all adults
Talking about it openly and honestly really is the best route, they need to accept you're not a small child any more, as a young adult they have to leave you to make your own mistakes and be there when you need them.
Hiding it, concealing it and ultimately lying to your parents will just lead to more stress, arguing and bad feelings between you all
it may seem difficult now but is explaining that you lied to them about something they clearly feel strongly about will be a lot more difficult. It doesn't have to be a fight, just tell them how you feel/what you need in a calm & reasonable manner, if they say something to upset you don't ride on the anger/hurt, rationally think about what it is that made you angry, consider that they love you when they say something stupid to annoy you, and bare that in mind while you take on what they said.
The volume of your argument is inverse to the validity of your argument, the louder someones got to shout the less strength what they say has.
if you really need to hide it, pretty much the sprays etc. advised above and never smoke to close to when you'll be around them for periods of time like dinner etc. give it 30-45 minutes, they'll pick up on your "altered" behaviour