Thank you for your inquiry.
In my opinion, ascorbic acid is extremely safe. I have never personally encountered an anaphylactic event due to ascorbic acid, regardless of the amount administered. If such anaphylactic events do occur to this agent, they must be extremely rare because a literature search identified only two potential cases (1, 2), both in the older literature. One was to a mixture of ascorbic acid and vitamin B rather than ascorbic acid alone, so it is uncertain as to the actual cause (2).
So, based upon available evidence, I do not think allergic reactions to the ingestion of ascorbic acid occur with any significant frequency if at all. With that in mind, of course we are unable to answer your question as to a specific level at which time it becomes an allergen, but I think that amounts added to food as an antioxidant would be accepted as safe, and there would be very little if any worry about allergic reactions to ascorbic acid in this setting.
Thank you again for your inquiry and we hope this response is helpful to you.
1. Severova EIa. [Anaphylactic shock due to sensitization to ascorbic acid]. Klin Med (Mosk). 1972 Aug; 50(8):130-1.
2. Rafter D. Anaphylactic shock reaction to intravenous parentrovite. Ir Med J. 1987 Nov; 80(11):329.
Phil Lieberman, M.D.