It is a problem with old recipes because the grains they used don't exist anymore and the malting has changed
Not nearly as much as you might think, at least if we're talking 13th century or later. The biggest change is that most malt is rapidly air-dried, whereas medieval malt was direct-fire kilned while it was still wet. This would likely produce something akin to a lightly crystallized high-kilned malt. The earliest known textual description of proper malting comes from
Le Tretiz, a mid-13th century poem by Walter de Bibbesworth.
Here's the original Anglo-Norman: (source:
https://prospectbooks.co.uk/samples/WalterofBibbesworth.pdf)
Ore le fraunceis pur breser brece e bracer cerveise
Puis ki desore suffist
Le fraunceis qe vous ai dist,
Ore ferreit bien a saver
Cum l’en deit breser e bracer
A la manere ke hom fest serveise
Pur fere nos noces bien a ese.
Allumés, auncele, une frenole.
Quant averas mangé de kakenole,
En une cuve large e leez
Cel orge la enfondrez,
E quant il est bien enfondré,
E le eauwe seit descouelé,
Mountez dunc cele haut soler,
Si le facez bien baler,
E la coucherez vostre blé
Taunt cum seit bien germee,
E de cele houre apeleras
Breez qe einz blé nomaz.
Le breez de vostre mein movez
En mounceus ou en rengez,
E puis le portés en une corbail
Pur enseccher au torrail,
Car corbail ou corbailloun
Vos servirunt tut la foisoun.
Puis serra le brez molu
E de eauwe chaude bien enbu.
Si le lessez descoure ataunt
Hors de keverel meintenaunt,
Taunt cum la bresceresce entent
Ki ele eit bersil a talent.
E puis le berzize prendra
De forment ou orge ki ele a,
E par le geeste e le berzille
Dunt home plus se sutille
Par dreit dever de bracerye.
Mes tut diviser ne sai jeo mie,
Mes tut issint de art en art
Attirez chescune part
Deskes vous eez bone serveise,
Dount home devient si ben a eise
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And here's the English translation:
Now the French for roasting malt and brewing ale:
Now it would be as well to know how to malt and brew
As when ale is made to enliven our wedding feast.
Girl, light a fennel-stalk(after eating some spice-cake);
Soak this barley in a deep, wide tub,
And when it’s well soaked and the water is poured off,
Go up to that high loft, have it well swept,
And lay your grain there till it’s well sprouted;
What you used to call grain you call malt from now on.
Move the malt with your hands into heaps or rows
And then take it in a basket to roast in the kiln;
Baskets, big or little, will serve you in plenty.
When the malt is ground and well steeped in hot water;
You let it drain sufficiently, now outside the mash-tub,
Until the breweress knows she has enough wort;
Then she’ll take the grout* that she has, of wheat or barley.
Thus with the barm and wort that people use so cleverly
In the proper process of brewing – I can’t describe it all –
From skill to skill you must perform each process
Until you have good ale and people are so pleased with it
*"Grout" is a reference to a coarse, herbed grain meal. It's the origin of our modern concept of "gruit," and likely derives from Viking and Anglo-Saxon brewing techniques - which probably used an herbed hardtack-like product as their "malt." It would accomplish something similar to "gruit."