In Situ Determination of the Physical State of Biological Samples during Freeze Drying

Abstract

The proceeding change of water content in a freeze-drying process, as well as the physical state of cell-sugar suspensions with Lactobacillus paracasei ssp. paracasei, was monitored by means of an in situ weighing system. Due to the pressure and temperature variations in the drying chamber, the weighing system was found to be influenced. This deviation of the weighing system was quantified by means of drying simulations with drying-inert Teflon dummies. This quantified deviation could be used for correction of the following freeze-drying processes. The applicability of this correction method was verified by drying model sugar solutions with known dry matter. Taking the quantified drift of the balance into account, the measured water contents for the model solution were proved to deviate less than 5% from the actual value. Finally, this verified correction method was applied to freeze drying of cell-sugar suspensions. As a result, the drying course of the cell-sugar samples could be depicted in the appropriate state diagram. Thus, the weighing system could be used to monitor the state transitions, which the bacterial suspension experienced during freeze drying, and qualified for specific investigations on the glassy state during freeze drying.

Drying of probiotics to enhance the viability during preparation, storage, food application, and digestion: A review

“…Isomaltose oligosaccharide is a prebiotic and was more effective than fructose‐oligosaccharide, raffinose, stachyose, and chitosan in protecting Pediococcus pentosaceus QK‐1 during vacuum freeze‐drying (Hu et al., 2021). With multiple hydroxyl groups, the water replacement and formation of a glassy matrix are commonly accepted as the protection mechanisms of mono, di‐, and oligosaccharides and polyols (Aschenbrenner et al., 2015). The water replacement hypothesis suggests that hydrogen bonds initially formed between the polar headgroups of phospholipids at the surface of cellular bilayers and water are lost during dehydration but are replaced by protectants with hydroxyl groups, thus preventing the transition of cell membrane into a gel phase (Santivarangkna, Higl, et al., 2008).…”

“…Initially, a liquid mixture suspension of probiotics and cryoprotectants is frozen under atmospheric pressure, and extracellular ice crystals are formed and separated from the residual sample. In the subsequent primary drying step, the frozen solvent that is unbound to cells is sublimated under high vacuum, and the bound water is removed via desorption in the secondary drying (Aschenbrenner et al, 2015;Barbosa-Cánovas et al, 2005). Freeze-dried products are dry, light, and porous and have good reconstitution properties to regain their original shape and texture after rehydration, making freeze drying a popular method of producing dried food products with high quality (Barbosa-Cánovas et al, 2005).…”

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“…In freeze-drying, dehydration is carried out by sublimation of a frozen sample 51 . This has a wide application in encapsulating bioactive compounds from plant sources 52, 53, 54, heatsensitive probiotics 55 . The advantage of freezedrying is that it provides better stability to physicochemical and biological properties of peptides 56 .…”

The Influence of Maltodextrin on the Thermal Transitions and State Diagrams of Fruit Juice Model Systems

“…These diagrams have been of great help in monitoring the progress and development of various employed unit operations, such as freezing, frozen storage, lyophilization, cryoconcentration, dehydration, and spray drying, which are all used to extend shelf life and to generate a range of high-, intermediate-, and low-moisture fruit products, such as whole fruits, cut fruits, juices, purees, jams, marmalades, dried fruits, powders, and leathers [ 1 , 2 , 4 , 5 , 6 , 7 , 8 , 9 ]. For instance, the freezing curves ( ) and the freeze-concentrated unfrozen phase transition temperatures and of products can be used to prevent physical, chemical, and structural changes that take place during the frozen storage of fresh and cut fruits and to avoid the product shrinkage or collapse usually observed during the freeze drying of biological materials [ 5 , 10 , 11 , 12 , 13 , 14 ]. The temperatures and are regarded as reference parameters determining the stability of frozen foods, because maximum ice formation takes place when food systems are stored between these temperatures [ 2 , 15 ].…”

“…The temperatures and are regarded as reference parameters determining the stability of frozen foods, because maximum ice formation takes place when food systems are stored between these temperatures [ 2 , 15 ]. As a general statement, the formation of a glassy vitreous state is then required in frozen storage to prevent molecular motion and further crystallization of water into ice, as well as in freeze drying, because collapse during primary drying will occur when the product temperature exceeds the collapse temperature, which is normally a few degrees above the value [ 10 , 11 , 12 , 13 , 14 ]. On the other hand, the relationship between and in the solid mass fraction domain of has also been regarded in the literature as a reference parameter determining the suitable conditions of drying processes and the storage stability of low-moisture food products.…”