Arrangement according to their decreasing importance is penicillin, biogas, citric acid and curd. Penicillin is the most important product. It is an antibiotic. It has been very helpful in curing many diseases. Next important is biogas, which is a clean and economical source of energy. Next in this order is citric acid, which is used as a food preservative. Least important in the list is curd, which is a food item obtained by the action of Lactobacillus on milk.