Complex sentence: It was at this time that the St. Paul flagship brewery was traded to the Stroh Brewing Company, Stroh's continued to operate the brewery until 1997 when it closed the operation ending a 137-year brewing tradition on the site.
Split into simpler sentences: It was at this time that the St. Paul flagship brewery was traded to the Stroh Brewing Company, Stroh's continued to operate the brewery until 1997. When it closed, the operation ended a 137-year brewing tradition on the site.