According to Catholicism, does Welch’s Grape Juice constitute valid matter for the consecration of the Mass?

Question

According to Catholicism, does the usage of Welch’s Grape Juice constitute valid matter for the consecration of the Eucharist at a Catholic Mass?

Some priests who are alcoholics may obtain a dispensation to use Mustum at Mass.

The question of the validity of the use of “mustum,” or grape juice, for priests suffering from alcoholism or for some other medical reason was finally resolved by the Congregation for the Doctrine of the Faith in 1994 in a letter signed by then Cardinal Joseph Ratzinger. Among other things this letter stated:

“A. The preferred solution continues to be communion ‘per intinctionem,’ or in concelebration under the species of bread alone.

“B. Nevertheless, the permission to use ‘mustum’ can be granted by ordinaries to priests affected by alcoholism or other conditions which prevent the ingestion of even the smallest quantity of alcohol, after presentation of a medical certificate.

“C. By ‘mustum’ is understood fresh juice from grapes or juice preserved by suspending its fermentation (by means of freezing or other methods which do not alter its nature).

“D. In general, those who have received permission to use ‘mustum’ are prohibited from presiding at concelebrated Masses. There may be some exceptions however: in the case of a bishop or superior general; or, with prior approval of the ordinary, at the celebration of the anniversary of priestly ordination or other similar occasions. In these cases the one who presides is to communicate under both the species of bread and that of ‘mustum,’ while for the other concelebrants a chalice shall be provided in which normal wine is to be consecrated.” - Use of Mustum at Mass

Here is an interesting note on the subject of Welch’s Grape Juice and the Eucharist in the Methodist Church.

The method of pasteurizing grape juice to halt the fermentation has been attributed to a British–American physician and dentist, Thomas Bramwell Welch (1825–1903) in 1869. Welch was an adherent to the Wesleyan Methodist Connexion which strongly opposed "manufacturing, buying, selling, or using intoxicating liquors" and advocated the use of unfermented grape juice instead of wine for administering the sacrament of the Eucharist, or communion, during the church service. A few years earlier, Welch had relocated to Vineland, New Jersey, a town started in 1861 by Philadelphia land developer Charles K. Landis (1833–1900) to create his own alcohol-free utopian society, a "Temperance Town" based on agriculture and progressive thinking. Landis declared that he was "about to build a city, and an agricultural and fruit-growing colony around it." The population reached 5,500 by 1865. Landis determined the potential in growing grapes and named the settlement "Vineland", and advertised to attract Italian grape growers to Vineland, offering 20 acres (81,000 m2) of land that had to be cleared and used to grow grapes. Welch had moved to the region following his sister who was one of Vineland's earliest residents and began to produce an "unfermented wine" (grape juice) from locally grown grapes that was marketed as "Dr. Welch's Unfermented Wine". This product became "Welch's Grape Juice" in 1893 when Welch and his son Charles E. Welch (also a practicing dentist) had decided to incorporate in 1893 as the Welch's Grape Juice Company at Westfield, New York. - History of Welch's Grape Juice

The reason I wish to bring this up is that I have come across priests using Welch’s Grape 🍇 Juice at Mass. The chalice was subsequently offered to the faithful present.

Has the Catholic Church pronounced on such a scenario as being valid or not?

Answer 1

In the article "Why Grape Wine?", written for the website catholic.com, David Lang quotes from a clarification to the CDF document you mention:

THE CONSEQUENCE OF MATTER

So, how does the Church address the possible problem of alcoholism? Could grape juice from a supermarket shelf be sacramentally adequate? No: A process of maturation or a “time-element” appears necessary from Scripture. After all, Christ came to earth and underwent his Passion in the “fullness of time” (D-R), or the “right time” and “proper time” (RSV), or the “fitting time” (NAB)—see Ecclesiastes 3:1, 8:6; Wisdom 18:14-15; Romans 5:6; Galatians 4:4; 1 Timothy 2:6; Hebrews 1:1-2.The

The prefect [of the Congregation for the Doctrine of the Faith] at the time, Joseph Cardinal Ratzinger, issued a more precise clarification of mustum in a circular letter (June 19, 1995) addressed to presidents of episcopal conferences. He elaborated that for grape juice to qualify as mustum, there must be no interference with its inherent tendency to ferment, even if the actual fermentation process is “arrested at an early stage.” This restriction would, for instance, exclude pasteurization, which entails enzymatic destruction. (So much for supermarket grape juice.)

Thus the priests you saw were not confecting a valid Eucharist, and were possibly committing a sin. (If they had found grape juice bottled straight off the vine they would have been fine.)

Answer 2

I have come across priests using Welch’s Grape 🍇 Juice at Mass.

That's very clearly an invalid Mass.

Canon law makes it very clear that wine is the necessary matter:

Can. 924 §1. The most holy eucharistic sacrifice must be offered with bread and with wine in which a little water must be mixed.
[…]
§3. The wine must be natural from the fruit of the vine and not spoiled.

See p. 1117 §"Alcoholic Priests" of New Commentary on the Code of Canon Law, which cites St. Thomas Aquinas Summa Theologica III q. 74 a. 5 arg./ad 3:

Objection 3:
Further, just as the clarified wine is drawn from grapes, so also are the juice of unripe grapes and must. But it does not appear that this sacrament may be made from such, according to what we read in the Sixth Council (Trull., Can. 28): "We have learned that in some churches the priests add grapes to the sacrifice of the oblation; and so they dispense both together to the people. Consequently we give order that no priest shall do this in future." And Pope Julius I rebukes some priests "who offer wine pressed from the grape in the sacrament of the Lord's chalice." Consequently, it seems that wine from the grape is not the proper matter of this sacrament.
[…]
Reply to Objection 3:
The juice of unripe grapes is at the stage of incomplete generation, and therefore it has not yet the species of wine: on which account it may not be used for this sacrament. Must, however, 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. Nevertheless entire grapes ought not to be mixed with this sacrament, because then there would be something else besides wine. It is furthermore 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."