Abstract: A process and apparatus for the preparation of citric acid by fermentation of carbohydrate-containing nutrient substrates carried out in a fermentation chamber and having two separate air supply systems is disclosed wherein highly sterile air is preheated through coarse-fine-absolute filters, UV gate, and overheating to 120.degree. C, and fed into the fermentation chamber during the germination phase of a fermentation of carbohydrate-containing nutrient substrate in a quantity sufficient to maintain the chamber at a slightly positive pressure; the fermentation chamber is heated separately and independently of the air supply during the germination phase; the feeding of the highly sterile air and the heating of the fermentation chamber are discontinued during the fermentation phase, and fresh, normally filtered, unconditioned air is introduced in a quantity sufficient to maintain the fermentation solution at an optimum fermentation temperature.