According to Food and Drink in Medieval Poland, by Maria Dembinska, edited and with recipes by William Woys Weaver, Polish bread was leavened with a 'yeasty substance' that Weaver calls 'thick beer' (p120, p183), which also was used to make beer. Though the recipe included in the book by Weaver calls for a sourdough mixture, I felt that a beer barm might be a good replication of the leavening agent.
(The 'leavening agent' is the part that makes the bread rise. Yeast is the most common leavening agent. Beer barm-- the yeast 'overflow' that rises to the top of a fermenting beer, mead, etc, and can be obtained from the dregs when the beer is racked-- is an excellent source of yeast organisms. In English Bread and Yeast Cookery, it is mentioned that beer barm was the usual source of yeast, and most of the leavened bread product recipes that survive call for beer barm. I theorize that instead of creating sourdough mixtures to trap wild yeast as 19th century housewives and prospectors did, medieval people cultivated wild yeast through beermaking. Note that beermaking provides a semi-cultivated, tested strain of yeast to the user.)
Dembinska also says that wheat, spelt, and rye flours were used in varying amounts; rye, rye/wheat, and whole wheat and 'white' wheat types (p114-15). This would be an approximation the bread described as a 'modern whole wheat' (p.115) by Dembinska.
Leaven/starter: I began with about a cup of beer barm taken from the trub (sediment) when a dark, hopped beer was first racked. (The yeast was 'ordinary beer yeast' according to the brewer.)
I took it home and doubled it, as described in Bake your own bread and in The Little House Cookbook, but using 1 cup of water (rather than milk) and 1/2 cup of rye flour. I also gave it a 1/2 tsp of sugar to make up for not putting in the milk.
After about 1 day, I drew off a cup of starter and made a rye bread with it (following the recipe in Bake your own bread), putting the scraps from the mixing bowl back into the starter.
I let the starter sit for about 5 days, then doubled it using 1/2 cup white flour and 1/2 cup water.
After 24 hours, I drew off 1 cup of starter for the bread.
Sponge: 2 c. white flour, 1.5 cups stoneground whole wheat flour, 1 tbsp honey,* 1.5 c. hot water.
Mix to smooth consistency, cover and let sit about 10 hours.
Dough: punch down sponge and add 1 tbsp sea salt, 1/2 tsp baking soda.** Knead in 2-3 cups whole wheat flour. Knead for 10 minutes. Shape into loaves, slash tops diagonally+, and allow to rise 4-5 hours. (I follow Dworkin's directions, greasing the pans and covering with cornmeal because it works better than the other methods I've tried. The cornmeal does not seem to be incorporated into the bread.)
Brush loaves with water (to approximate the effect of a brick oven swabbed out with a water-soaked broom, Dembinska, p. 181). Bake in an oven heated to 375 for 30 minutes; reduce temperature to 350 and baked for 15 minute or so more, until done.
*Dembinska mentions sweetening with honey. I do not know whether honey was included in bread to improve yeast action and keeping quality. However, as Walter points out, the action of the sweetner was orginally included in breads to improve keeping quality, which would be important only if you did not bake everyday: i.e. when the housewife baked her own bread rather than buying it.
** The baking soda is a non-period addition included by Dworkin. I've found it helpful.
+I don't know if tops of the loaves were slashed in medieval Poland, but I recall pictures of medieval breads with slashed tops. Weaver says that bread stamps or signs of the cross were used (Dembinska, p182)
Food and drink in Medieval Poland. Maria Dembinska. Revised and adapted by William Woys Weaver. (Philadelphia, University of Pennsylvania Press, 1999)
For information about medieval Polish bread baking.
Bake your own bread. Floss and Stan Dworkin (NY: Holt, Rinehart and Winston, 1972)
For sourdough tips and recipes.
The Little House Cookbook. Barbara Walter. (NY: Harper & Row, 1979)
For sourdough instructions.