This process for increasing the reactivity of cellulose-containing materials such as animal feedstuffs involves contacting the material, in a pressure vessel, with a volatile liquid swelling agent e.g. ammonia, which has a vapor pressure greater than atmospheric pressure at ambient temperatures. The contact is maintained for a time period sufficient for the agent to swell the cellulose of the material. The pressure is then rapidly reduced to atmospheric pressure, thereby causing the agent to boil and explode the material. The rapid pressure reduction also causes some freezing of the cellulose. The swelling agent is separated from said cellulose-containing material and recovered for recycling. The process also increases the water holding capacity of cellulose-containing materials.