This article is from the Food Preserving FAQ, by Eric Decker firstname.lastname@example.org with numerous contributions by others.
>From Joyce Miller :
This isn't the Roman or medieval method, but it is the Southwestern French
method. I haven't tried this recipe out. When I was still thinking about it
I found bottled verjuice by Roland. This recipe is from Paula Wolfert's _The
Cooking of South-West France_. Let us know how this works out.
"...The grapes - the bourdelois, the gressois, and the farineau - are no
longer grown. Some types can make the process a little tricky. If the grapes
are picked too ripe, their liquor will be too watery; if too green, the
verjus will not taste good. We want grapes in the middle of their ripening,
whose juice can be allowed to ferment slightly.
To make verjus, choose the sourest green grapes available. Holding on to the
thick stem, dip them in bunches into boiling water for three seconds to kill
the yeasts. Remove at once and drain on a towel. Roll the bunches, one by
one, in the towel while removing the grapes from the stems. Discard any
blemished grapes. When dry, place grapes in the workbowl of a food processor
and process 10 seconds; then strain, pressing down on them to extract all the
juice. Let stand for 10 minutes, then ladle juice into a sieve lined with a
damp cheesecloth and strain again. Use at once, or freeze in plastic ice
cube trays. Store the cubes in a plastic bag in the freezer. Use frozen or
immediately upon defrosting for maximum flavor. Keeps 3 months.
Some people add alcohol to their verjus along with vinegar & sugar so it
will keep, but this distorts the flavor. Another way to obtain the sour
taste of verjus is to add a pinch of tartaric acid, which one can find at
a wine-making shop. Don't go over 2 pinches, it is really strong."