Consistent Scale-Up of the Freeze-Drying Process

Research output: Book/ReportPh.D. thesis – Annual report year: 2018Research

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Lactic acid bacteria, are one of the most common examples of starter cultures due to their broad application for the production of fermented food products. After the fermentation and the concentration steps, starter cultures can be supplied in the frozen or in the dried state. Nowadays, dried products are preferred since they allow easier handling, transport, storage and application.
Freeze-drying or lyophilization is a drying process in which water is removed through a sublimation and a desorption step. It results in products with increased shelf stability, without losing its structural and shape characteristics. However, it is an expensive and time-consuming process frequently not operated in a robust and efficient way. Overly conservative freeze-drying cycles and scale-up based on empirical knowledge result in drawbacks not just from an economic standpoint but also from a product point of view. Thus, process development in this area has been focused on minimizing drying times while maintaining product quality.
This process is both dependent on the physical form of the material being dried and on the physical parameters and characteristics of the freeze-drying apparatus. The majority of the studies found in the scientific literature are applied to the pharmaceutical industry, therefore the available information for the freeze-drying of particulate material in open trays, mainly used in the food industry, is limited.
Freeze-drying of frozen particulate matter is characterized by a bimodal pore size distribution based on the intra-particle and inter-particle void space. Pore size distribution of the particle bed is determined by the particle size and how the particles are packed, while pore size distribution inside particles is dependent on the initial solid concentration and on the freezing rate. The ratio between the two pore size distributions plays has an important role for the dynamics of the freeze-drying process.
Local temperature and moisture content in the product are essential for the design of efficient freeze-drying cycles. Consequently, a fundamental understanding of water vapor flows during drying in a freeze-dryer is essential for the construction of an accurate model. In this thesis, the primary drying step during freeze-drying of Streptococcus thermophilus was investigated based on Computational Fluid Dynamics (CFD) techniques. A scale-up approach, going from the small scale of a single particle where only product characteristics are addressed, to the laboratory/pilot scale where both product and equipment characteristics are taken into account, was implemented.
The established model represents a good strategy to determine the effect of different operating conditions during the freeze-drying of pellets in open trays. Additionally, it opens the possibility to better understand scale-up issues and to develop robust freeze-drying cycles that facilitate the transfer between apparatus.
Original languageEnglish
Place of PublicationKgs. Lyngby
PublisherTecnical University of Denmark
Number of pages131
Publication statusPublished - 2018

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