Hope this helps a little:
Taken from Wikipedia on “Mustum”
In Roman Catholic Eucharistic liturgy, must may be substituted for sacramental wine, on condition that the ordinary has granted permission for the benefit of a priest or lay person who should not, usually because of alcoholism, ingest wine; but in normal circumstances it may not be used in place of wine.
Official Roman Catholic documents define must (mustum in Latin) precisely as “grape juice that is either fresh or preserved by methods that suspend its fermentation without altering its nature (for example, freezing),” and it excludes pasteurized grape juice.
This teaching goes back at least to Pope Julius I (337–352), who is quoted in Thomas Aquinas’s Summa Theologica as having declared that in case of necessity, but only then, juice pressed from a grape could be used. Aquinas himself declared that it is forbidden to offer fresh must in the chalice, because this is unbecoming owing to the impurity of the must; but he added that in case of necessity it may be done.
Aquinas himself declared:
Must has already the species of wine, for its sweetness "Aut dulcis musti Vulcano decoquit humorem"; Virgil, Georg. i, 295] indicates fermentation, which is "the result of its natural heat" (Meteor. iv); consequently this sacrament can be made from must. ... It is forbidden to offer must in the chalice, as soon as it has been squeezed from the grape, since this is unbecoming owing to the impurity of the must. But in case of necessity it may be done: for it is said by the same Pope Julius, in the passage quoted in the argument: "If necessary, let the grape be pressed into the chalice."
The latest document from the Congregation for the Doctrine of the Faith on the matter, issued on 24 July 2003, gave the following norms, which simplify those previously in force:
The Ordinary is competent to give permission for an individual priest or layperson to use mustum for the celebration of the Eucharist. Permission can be granted habitually, for as long as the situation that occasioned the granting of permission continues (e.g., the priest is an alcoholic).
When the principal celebrant at a concelebration has permission to use mustum, a chalice of normal wine is to be prepared for the concelebrants.
Given the centrality of the celebration of the Eucharist in the life of a priest, one must proceed with great caution before admitting to Holy Orders those candidates unable to ingest alcohol without serious harm.
Attention should be paid to medical advances in the area of alcoholism and encouragement given to the production of unaltered mustum.