2 results
Now showing1 - 2 of 2
- Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: with files , Comparison of Knudsen diffusion and the dusty gas approach for the modeling of the freeze-drying process of bulk food products(Multidisciplinary Digital Publishing Institute (MDPI), 2022-03-11); ; ; ; Freeze-drying is generally used to achieve high quality products and preserve thermal sensitive components; however, it is also considered as a high energy and costly process. Modeling of the process can help to optimize the process to reduce these drawbacks. In this work, a mathematical model is presented to predict the heat and mass transfer behavior for freeze-drying of porous frozen food particles during freeze-drying to optimize the process. For the mass transfer, a comparison between Knudsen diffusion and the more complex dusty-gas approach is performed. Simulation results of a single particle are validated by experiments of single-layer drying to extend the usage of this model from a single particle to a particle bed. For the moisture transfer, adaption parameters are introduced and evaluated. A comparison shows a good agreement of the model with experimental results. The results furthermore suggest a strong correlation of the drying kinetics with pore size and particle porosity. An increase in the pore diameter strongly improves the overall mass transfer rates and hence is a suitable parameter for an effective increase of the drying rates in freeze-drying.Publicationtype: Journal ArticleScopus© Citations 1 85 205 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: with files , Influence of freezing parameters on the formation of internal porous structure and its impact on freeze-drying kinetics(Multidisciplinary Digital Publishing Institute (MDPI), 2021-07-23); ; ; The main objective of this study was firstly to investigate the influence of freezing process parameters on the formation of the internal structure of frozen coffee granules. It was investigated how these frozen internal structures affect the drying kinetics during freeze-drying. A design of experiment study was carried out using the response surface method to quantify the influence of the freezing step that occurs in a scraped surface heat exchanger (SSHE). Therefore, the coffee extract at a concentration of 30% w/w is entering the SSHE as a liquid and gets partially crystallized up to a weight-based ice content of 0.364. During this step, the influence of factors like cooling temperature, scraper rotation speed and temperature cycles on ice crystal structure was investigated. In a second freezing step, the influence of freezing rates during hardening of the product by air-blast freezing is investigated, where the freezing rate is significantly affected by the cake thickness. The produced frozen granules were freeze-dried in single layer experiments. During drying the influence of internal structure on the drying kinetics was investigated. Results show that all factors have a significant impact on structure parameters for 30% w/w coffee solutions. A lower degree of supercooling during freezing in an SSHE, a higher number of temperature cycles (2 to 8 times) and lower freezing rates during hardening (2 °C/min to 10 °C/min) were leading to increased crystal size. This increase accelerates the primary drying rate and decreases the total drying time. A higher number of temperature cycles leads to a significant increase of crystal size and therefore larger pore size at the end of the primary drying. Furthermore, in combination with temperature cycles in the SSHE, it was found that high freezing rates during air blast freezing generally lead to a second nucleation step of ice crystals.Publicationtype: Journal ArticleScopus© Citations 7 131 399