A process for producing an interferon alpha 5 (IFNa5) protein by expression in an Escherichia coli host cell which comprises: a) providing an IFNa5 producing E. coli host cell; b) culturing the IFNa5 producing E. coli host under conditions effective to express said IFNa5 protein by said recombinant IFNa5 producing E. coli host cell in a fermentation medium, with the addition of a carbon feed solution, wherein - said fermentation medium is free of components from animal origin or yeast origin, and - said carbon feed solution comprises a carbon source and from about 90 to about 111 mg iron chloride heksahydrate, from about 12.15 to about 14.985 mg calcium chloride dehydrate, from about 20.25 to about 24.975 mg zinc sulfate heptahydrate, from about 4.5 to about 5.55 mg manganese sulfate monohydrate, from about 9 to about 11.1 mg copper sulfate pentahydrate, from about 3.42 to about 4.218 mg cobalt chloride heksahydrate, from about 0.9 to about 1.11 mg sodium molybdate dehydrate and from about 2.07 to about 2.553 mg boric acid per liter of added carbon feed solution; and c) isolating, and optionally purifying, the expressed IFNa5 protein, wherein said addition of carbon feed solution results in a concentration from about 28,5 to about 36,9 mg iron chloride heksahydrate, from about 3.8475 to about 4.9815 mg calcium chloride dehydrate, from about 6.4125 to about 8.3025 mg zinc sulfate heptahydrate, from about 1.425 to about 1.845 mg manganese sulfate monohydrate, from about 2.85 to about 3.69 mg copper sulfate pentahydrate, from about 1.083 to about 1.4022 mg cobalt chloride heksahydrate, from about 0.285 to about 0.369 mg sodium molybdate dehydrate and from about 0.6555 to about 0.8487 mg boric acid per liter of said fermentation medium. a) providing an IFNa5 producing E. coli host cell; b) culturing the IFNa5 producing E. coli host under conditions effective to express said IFNa5 protein by said recombinant IFNa5 producing E. coli host cell in a fermentation medium, with the addition of a carbon feed solution, wherein - said fermentation medium is free of components from animal origin or yeast origin, and - said carbon feed solution comprises a carbon source and from about 90 to about 111 mg iron chloride heksahydrate, from about 12.15 to about 14.985 mg calcium chloride dehydrate, from about 20.25 to about 24.975 mg zinc sulfate heptahydrate, from about 4.5 to about 5.55 mg manganese sulfate monohydrate, from about 9 to about 11.1 mg copper sulfate pentahydrate, from about 3.42 to about 4.218 mg cobalt chloride heksahydrate, from about 0.9 to about 1.11 mg sodium molybdate dehydrate and from about 2.07 to about 2.553 mg boric acid per liter of added carbon feed solution; and - said fermentation medium is free of components from animal origin or yeast origin, and - said carbon feed solution comprises a carbon source and from about 90 to about 111 mg iron chloride heksahydrate, from about 12.15 to about 14.985 mg calcium chloride dehydrate, from about 20.25 to about 24.975 mg zinc sulfate heptahydrate, from about 4.5 to about 5.55 mg manganese sulfate monohydrate, from about 9 to about 11.1 mg copper sulfate pentahydrate, from about 3.42 to about 4.218 mg cobalt chloride heksahydrate, from about 0.9 to about 1.11 mg sodium molybdate dehydrate and from about 2.07 to about 2.553 mg boric acid per liter of added carbon feed solution; and c) isolating, and optionally purifying, the expressed IFNa5 protein,