It isn't on the allowed list, but then again, it's a 'total meal replacement diet' so the allowed list is very small! Be cautious about using carb free/ low sugar replacements for things though, you still need to watch the calories and part of the diet is about portion control, not turning to sweet things as comfort etc. Also sweetners are detected by your body in the same way as sugar, causing an insulin response.
If it doesn't say citric acid (or lactic acid) on the label it should be ok as an occasional thing but as I said above. I would advise against eating anything not on the allowed list unless absolutely necessary.