Keptinis – The brewing and fermentation characteristics of a traditional farmhouse ale from Lithuania

Abstract

Keptinis is a historical beer style from Lithuania which is brewed from a mash that is baked in parts or in total in a special oven. According to historical sources, one viable method to brew Keptinis is to mash-in with small amounts of brewing liquor at temperatures of approximately 65 °C and to bake the whole mash afterwards at ambient temperatures ranging from 280 to 300 °C in a so-called Duonkepis, a traditional bread oven. Maillard reactions, caramelisation products and eventually pyrolysis of the remaining starch can therefore be expected. The baked mash is then mashed in again in brewing liquor and lautered. The collected wort is not boiled but fermented subsequently with top-fermenting yeast strains. Available data on the brewing and fermentation characteristics of Keptinis is scarce. Hence, in a reproducible experimental design to reconstruct one method to brew this historical beer style, three brews and one control were conducted. For each experimental brew a grain bill of 6 kg Pilsener malt was mashed in at a grist-to-water ratio of 1:3 and mashed for 60 minutes at 65 °C. The mash of the three brews then was baked at 300 °C for 30, 60 and 120 minutes in a combination steamer using dry air. The mash of the control brew was not baked. The wort of all four brews was fermented with top-fermenting ale yeast at 20 °C. Prolonged baking time affected the pH of wort and beer, the degree of fermentation and the beer colour. The apparent degree of fermentation of the control was 91 %, whereas the prolonged baking time of the mash lowered it to 85 % (30 minutes), 84 % (60 minutes) and 82 % (120 minutes). EBC beer colour units were measured with 4 EBC in the control, 15 EBC units in the beer gained from the mash baked for 30 minutes, 35 EBC units (60 minutes) and 88 units (120 minutes) respectively. In addition, the thiobarbituric acid index (TBI) and the free amino nitrogen (FAN) of the final beers were determined.