I just bought some grape must - it's a must on spinach salad! I am thinking of turning some of my own homemade concord grape juice into the same. Any advice, other than to reduce by slow boiling? Can I can the results in a hot water bath? Will the acid level change by reducing the liquid?
Based on the wikipedia article, it sounds like you want the pulp and skin as well. It sounds like you won't get the same texture, consistency or sweetness by only reducing grape juice.
However, I found this cool how-to page on how to make grape must jelly. I think the hand-crushing grapes part will be the most relevant to you.
You could use some kind of pressing machine, such as an OJ squeezer, to get the same effect without red hands. I'd stay away from blenders and puree machines, as cutting foods doesn't have the same effect as crushing them.
I don't think I've had grape must, so you'll have to be the judge on how much or how little of the pulp goes into your final product.
When grapes are pressed for wine, the results of the first gentle pressing go into the wine. "Grape must" is the second, more violent pressing, which may include adding a little water to get the "virtues" out. The must has a range of uses. The French convert the best must into a brandy-like distilled drink called "marc", which is rarely exported.
A lot of the Italian grape must goes into a condiment described as "aceto balsamico di Modena IGP" - it is not the true "balsamic vinegar that chefs love, and in my local "German supermarket" three varieties are sold, red, white and brown. Tell me that the brown color owes more to caramel than to balsam, and I will believe you. Whatever, the stuff is no substitute for "real" balsamic vinegar, but if you want to make a salad dressing it works well when involved with mustard and olive oil. I think that's what you've discovered.
More of the Italian grape must goes into fizzy wine-like stuff called Lambrusco. If Lambrusco gets above 9.5% alcohol, it has been made from grape juice. Below that, it usually is mostly fermented must that has been mucked about with to give a product of acceptable taste, and is usually about 5.5% alcohol.
I'm not saying what the Germans do with it beyond asking if you've ever tried Asbach brandy? That's the good stuff, it gets worse.
That's about as much as I must know.
A postscript - the point I was making is that the must is the second pressing juice. What I didn't mention clearly is that the concentrated must is formed in the same way that fruit juices are concentrated for transport, in large industrial plants fitted with pumps etc. to exclude the air.
If you try to reduce your grape juice by gentle boiling, I'm afraid that the flavour components will surely be oxidized.