The traditional way to preserve forage is to dry it into hay, but in rainy regions mold and decay are common. Virtanen showed that lowering the pH below 4 suppresses harmful bacteria and developed the “AIV method,” which uses formic acid or its salts. Acidification lets lactic acid bacteria dominate and keeps nutrients and vitamins intact. Livestock farmers gained a stable feed supply and prevented drops in milk yield or meat quality. The technique also helped alleviate food shortages after World War II.

Virtanen combined fermentation chemistry with microbiology to evaluate in detail the threshold acidic conditions that prevent spoilage even at high water activity in fodder. He quantified the relationship among added formate, moisture and sugar content, demonstrating that species such as Clostridium and Enterobacter cannot grow around pH 4. By coupling natural lactic fermentation with chemical acidification, he minimized energy loss and protein degradation. This process became the foundation of modern silage production and has been adapted to high-moisture corn storage. He also analyzed the stability of vitamins C and B during storage, paving the way for micronutrient management in animal nutrition.

The AIV method adds 0.5–1.5 % (w/w) formic acid or ammonium formate to rapidly reach pH 3.5–4.0, inhibiting butyric fermentation by Clostridium tyrobutyricum and associated protease activity. Virtanen applied the Henderson–Hasselbalch equation to calculate acid requirements even for highly buffered leguminous forages. Near-infrared spectroscopy studies show a 15–25 % reduction in crude protein degradation compared with hay and improved retention of soluble sugars and β-carotene. In Finland’s high-latitude, low-insolation conditions the method increased dairy productivity by over 20 %, mitigating energy deficits. Today the AIV system is referenced as a low-drug, feed-hygiene compliant technology under EU Regulation (EC) No 183/2005.