Dissolve yeast in 1/2 cup warm water. Boil remaining water and pour into a large stone crock. Add pitted halves of lemons, sugar, and raisins and mix until sugar dissolves. Cool to lukewarm. Add beer and yeast mixture. Cover with cheesecloth and ferment at room temperature for 24 hours. Strain liquid through muslin. Store in sealed glass jars or bottles. Keep for 3 to 4 days before serving.