This invention provides a robust fermentation process for the expression of a capsid protein of a bacteriophage which is forming a VLP by self-assembly, wherein the process is scalable to a commercial production scale and wherein the expression rate of the capsid protein is controlled to obtain improved yield of soluble capsid protein. This is achieved by combining the advantages of fed-batch culture and of lactose induced expression systems with specific process parameters providing improved repression of the promoter during the growth phase and high plasmid retention throughout the process.