Verjuice, agresta, omphacium and oleo gremiale

It's not often you see verjuice in the popular press:

http://latimesblogs.latimes.com/dailydish/2011/09/sour-grape-juice-the-french-call-it-verjus.html

The best substitute is probably the sour grape juice sold in Persian markets, which I suspect is basically the same thing. Otherwise, modern cooks typically use lemon juice or vinegar in its place.

Le Grand claims that it was originally made from the juice of sorrel, 

http://books.google.com/books?id=xY1AAAAAcAAJ&pg=PA146&dq=inauthor:Aussy+verjus&hl=en&ei=Y4SMTrL6JY73sQKLrbS2BA&sa=X&oi=book_result&ct=result&resnum=1&ved=0CDUQ6AEwAA#v=onepage&q&f=false

but in fact there are earlier references to a specific grape called "verjus" and to rents paid in it. The Romans had a similar product called "omphacium" (though that could also refer to a kind of oil made from underripe olives). But this did not seem to be the ancestor of verjuice - Charlemagne, who had his stewards keep garum on hand, says nothing about either omphacium or verjuice. Verjuice does not seem to appear (like trenchers) until about the 12th century.

Though when I first translated my own version of the Viandier I thought of it as an archaic ingredient, in fact it has never fully disappeared. It's mentioned fairly frequently in 18th century cookbooks and still, if more sparingly, in the nineteenth century.

I'm probably not the only one (with Le Grand) to be frustrated by trying to puzzle out what "verjus de grain" referred to. In fact, it appears to be an equivalent to the grape version, but made with beer, as "verjus de pomme" was made with cider. [UPDATE 8/23/13 : Terence Scully says this was probably a kind of jam made with preserves of the same kind of tart young grape used to make the juice; that's a creditable explanation, especially since the one early source for "verjus de grain" itself seems to be period guesswork.]

The word "agresta" is more or less contemporary with verjus and was the southern French or Italian (as well as Latin) word for it; omphacium is the Roman word for something similar, but from an earlier period.

For those who care to puzzle through the Latin, Du Cange cites an author who specifically distinguishes between agresta and omphacium:

http://books.google.com/books?pg=PA150&dq=agresta+omphacium&ei=ioGMTsK6NeilsALCpLCpBA&ct=result&id=eeI-F_-b7xIC#v=onepage&q=agresta%20omphacium&f=false

As I understand, this comes down to using verjuice as opposed to omphacium, depending on the species being cooked.

The Romans also had a kind of olive oil equivalent - oleo gremiali - made from very young grapes. Further research may yet uncover other liquids made from young, tart fruits (yes, the olive is a fruit).