Sulfates MAY licitly be added to wine for preservation:
From Mont La Salle’s (formerly Christian Brothers) website FAQ at montlasallealtarwines.com/faq.php#qq1_1
Is our current Altar Wine approved for Sacramental use?
We at Mont La Salle like to state that we cannot determine how other suppliers make their wine or if it is made according to the requirements of Canon Law. We know with absolute certainty that Mont La Salle altar wine is made according to Canon Law and, in addition, our label states “approved for sacramental use”. As far as we know, there is no other wine offered for commercial sale that makes this statement.
Then Mont La Salle has this to say about sulfates:
Sulfites in Wine - (Why does the wine label state ‘contains sulfites’? And, what are sulfites?) Sulfites in wine are nothing new since the yeast cells during fermentation produce a small amount of sulfites and the better winemakers have made tiny additions of sulfites for many centuries. This miniscule amount of added sulfites acts as an anti-oxidant and as a yeast inhibitor which preserves the natural good condition of the wine and retards spoilage. For the information and protection of those few people who are extremely sensitive or allergic to sulfites, U.S. Federal Law, for some time now, requires that wine containing (10) or more parts per million of sulfites be labeled “contains sulfites”. This labeling requirement is mandatory for all wines produced in the U.S. The wines are the same as they were before such labeling was required. Substances that assist in making sound wine and that remain in the wine in minute traces, such as sulfites, have been and are considered by canonists and theologians to be acceptable for the Eucharist. One such approval, as reported in the Sacramentary, was the Holy Office Decree of 2nd August, 1922."
In reality Mont La Salle relies on the following (and not Canon Law) to add sulfates to their sacramental wines:**
"Mass Wine: Treated with Sulphurous Anhydride, Etc. (Holy Office) Private.
The Holy Office was asked by the Archbishop of Tarracona: Whether in the Sacrifice of the Mass, wine may be used which is made from the juice of the grape, treated with sulphurous anhydride or with potassium bisulphite.
Reply. In the affirmative.
(Private) ; Holy Office, 2 Aug., 1922.
Not published in the AAS; cf. Il Monitore, Oct., 1923, p. 289."