Commercial production of heterologous proteins by yeasts has gained considerable interest. Expression systems have been developed for Saccharomyces cerevisiae and a number of other yeasts. Generally, much attention is paid to the molecular aspects of heterologous-gene expression. The success of this approach is indicated by the high expression levels that have been obtained in shake-flask cultures. For large-scale production however, possibilities and restrictions related to host-strain physiology and fermentation technology also have to be considered. In this review, these physiological and technological aspects have been evaluated with the aid of numerical simulations. Factors that affect the choice of a carbon substrate for large-scale production involve price, purity and solubility. Since oxygen demand and heat production (which are closely linked) limit the attainable growth rate in large-scale processes, the biomass yield on oxygen is also a key parameter. Large-scale processes impose restrictions on the expression system. Many promoter systems that work well in small-scale systems cannot be implemented in industrial environments. Furthermore, large-scale fed-batch fermentations involve a substantial number of generations. Therefore, even low expression-cassette instability has a profound effect on the overall productivity of the system. Multicopy-integration systems may provide highly stable expression systems for industrial processes. Large-scale fed-batch processes are typically performed at a low growth rate. Therefore, effects of a low growth rate on the physiology and product formation rates of yeasts are of key importance. Due to the low growth rates in the industrial process, a substantial part of the substrate carbon is expended to meet maintenance-energy requirements. Factors that reduce maintenance-energy requirements will therefore have a positive effect on product yield. The relationship between specific growth rate and specific product formation rate (kg product·[kg biomass]−1·h−1) is the main factor influencing production levels in large-scale production processes. Expression systems characterized by a high specific rate of product formation at low specific growth rates are highly favourable for large-scale heterologous-protein production.

The enzyme β-galactosidase was first mentioned in the literature by Beijerinck exactly a hundred years ago. The Department of Microbiology and Enzymology of the Delft University of Technology keeps the memory of Beijerinck, its first professor, alive by maintaining a ‘Beijerinck-room’ in the attic of the building. In addition to manuscripts and laboratory notebooks, this room contains some of his chemicals and biological preparations, and it was here that we recently found a 90-year old lactase preparation. Even after storage under suboptimal conditions, the preparation still exhibited measurable enzyme activities.