During heat sterilization there is always the possibility of destroying ingredients in the medium. Apart from the degradation of heat-labile components, also contributes to the loss of nutrient quality during sterilization. A common phenomenon is the occurance of the Maillard-type browing reactions which cause discoloration of the medium as well as loss of nutrient quality. These reactions are normally caused by carbonyl groups, usually from reducing sugars, interacting with amino groups from amino acids and proteins. Separate sterilization of the carbohydrate component of the medium may be necessary to prevent such reactions.
Filter sterilization is often used for all components of nutrient solutions which are heat sensitive. Sugars, vitamins, antibiotics or blood components are examples of heat-labile components which must be sterilized by filtration.
Most nutrient media are presently sterilized in batch volumes in the bioreactor at 121 degrees Celcius. Approximate sterilization times can be calculated from the nature of the medium and the size of the fermentor. Not only the nutrient media, but also the fittings, valves and electrodes of the fermentor itself must be sterilized. Therefor, actual sterilization times are significantly longer than calculated ones and must be empirically determined for the specific nutrient solutions in the fermentor. Smaller fermentors are sterilized in an autoclave while larger fermentors are sterilized by indirect or direct steam injection.
STERILIZATION OF FERMENTATION AIR
Most fermentations are operated under high aeration and the air supplied to the fermentor must be sterilized. The number of particels and microorganisms in air varies greatly depending on the location, air movement, and previous treatment of the air