The present invention provides a process for periplasmic expression of a bacterial toxoid comprising the steps of: a) growing a culture of a gram negative host cell in a fermentation medium, wherein the host cell is transformed with a polynucleotide, and wherein the polynucleotide encodes the bacterial toxoid and a periplasmic signal sequence or providing a gram negative host cell wherein the host cell is transformed with a polynucleotide, the polynucleotide encodes the bacterial toxoid and a periplasmic signal sequence and wherein the gram negative host cell comprises the bacterial toxoid expressed in the periplasm a(i)) inducing expression of the bacterial toxoid b) maturing the host cell, wherein the maturing step comprises: I) subjecting the host cell to a pH shock II) incubating the host cell with no feed addition and/or III) subjecting the host cell to a temperature below &minus20° C. and c) extracting the bacterial toxoid from the host cell wherein the extraction process comprises osmotic shock.
