Before anaerobic fermentation starts, there is an aerobic phase in which the trapped oxygen is consumed. How closely the fodder is packed determines the nature of the resulting silage by regulating the chemical reactions that occur in the stack. When closely packed, the supply of oxygen is limited, and the attendant acid fermentation brings about decomposition of the carbohydrates present into acetic, butyric and lactic acids. This product is named sour silage. If, on the other hand, the fodder is unchaffed and loosely packed, or the silo is built gradually, oxidation proceeds more rapidly and the temperature rises; if the mass is compressed when the temperature is , the action ceases and sweet silage results. The nitrogenous ingredients of the fodder also change: in making sour silage as much as one-third of the albuminoids may be converted into amino and ammonium compounds; in making sweet silage a smaller proportion is changed, but they become less digestible. If the fermentation process is poorly managed, sour silage acquires an unpleasant odour due to excess production of ammonia or butyric acid (the latter is responsible for the smell of rancid butter).