Like you, I’ve spent a lot of time trying to track down this quote made by Calvin. I even got my sister to check out her theological study books on Calvin, but to no avail. However, in the course of my research I found a really interesting document about Calvin’s use of the words “sweet” and “sweetness” with regard to salvation compared to the bitterness of adversity and sorrow:
Romans 8:31 and following was a passage of great encouragement for Calvin. In reference to verse 33—“Who will bring any charge against God’s elect?”— Calvin writes: “The first and chief consolation of the godly in adversity is to be persuaded for certain of the fatherly kindness of God. From this comes both the certainty of their salvation and the calm security of soul by which adversities are sweetened, or, at least the bitterness of sorrow is mitigated.”42 (page 10/20)
That consolation is above all an experience of God’s grace even in the midst of the most difficult times. For we must “learn, even in the midst of our sufferings to perceive the grace of God and let it suffice us when anything severe is to be endured to have our cup mingled with some portion of sweetness lest we should be ungrateful to God, who in this manner declares that he is present with us.”47 (page 11/20)
Here, Calvin speaks of bearing the cross and how thoughts of our deliverance “sweetens all the bitterness of the cross”:
“The sharpest form of adversity, however, is experienced when we are called upon to bear the cross, which, for Calvin, is at the heart of the Christian life.50 Here, a key text for Calvin is 2 Timothy 2:11: “If we have died with him, we will also live with him.” “Who could not fail to be stirred by this exhortation,” Calvin asks, “that we ought not to be borne down by our afflictions, since we shall have such a happy deliverance from them. The same thought abates and sweetens all the bitterness of the cross, since neither pains nor torments nor reproaches nor death should dismay us, seeing that we share them with Christ, and especially since all these things are forerunners of our triumph.”51 “The only consolation which mitigates and even sweetens the bitterness of the cross is the conviction that we are happy in the midst of our miseries, for our patience is blessed by the Lord and will soon be followed by a happy result.”52 However, "nothing sweeter can be imagined for soothing the bitterness of persecution than hearing that the Son of God suffers, not only along with us, but in us.”53 (page 12/20)
Calvin also said the “sacred bread of the Lord’s Supper, is spiritual food, as sweet and delicate as it is healthful for pious worshipers of God, who in tasting it, feel that Christ is their life.” (page 13/20) Source: https://www.calvin.edu/library/database/crcpi/fulltext/ctj/88092.pdf
It is entirely possible that Calvin, in speaking of red communion wine, said “wine combines sweetness and bitterness, which makes a more effective metaphor for the sweetness and bitterness of Jesus' sacrifice on the cross.” He certainly drew a beautiful word picture comparing the bitterness of persecution and sorrow with the suffering of the cross and comparing that to the sweetness of our conviction that our salvation has been secured by what our Lord and Saviour accomplished when he laid down his life so that we might have eternal life.
I was unable to track down the specific quote, but I learned a great deal in the process of looking for it and I have been blessed by what I read.