Complex sentence: Cofactors were discovered by Arthur Harden and William Young in 1906, when they found that the rate of alcoholic fermentation in unboiled yeast extracts, increased when boiled yeast extract was added.
Split into simpler sentences: Cofactors were discovered by Arthur Harden and William Young in 1906. Using unboiled yeast extracts, they found that the rate of alcoholic fermentation increased when boiled yeast extract was added.