Abstract
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.
Highlights
Freeze-drying in general is dependent on process variables like chamber pressure and product temperature, but these parameters are limited by product properties
The basis to control the structure of a bulk product before freeze-drying is during the freezing process of the material, which can be split into two process steps: (a) supercooling, nucleation of crystals and crystal growth in a scraped surface heat exchanger and (b) the hardening or final freezing in an air blast freezer as known for other products like ice cream manufacturing [4,5]
Where CR is the cooling rate (◦ C/min) and Tc is the temperature of the coolant used (◦ C)
Summary
Freeze-drying in general is dependent on process variables like chamber pressure and product temperature, but these parameters are limited by product properties. In addition to chemical composition, the physical properties of the product are important where structure and other properties like the glass transition temperature and the corresponding collapse temperature of the product are the main drivers. When chemical composition can hardly be changed to positively influence glass transition (e.g., for pure soluble coffee), the elevating screw for faster drying is based on physical parameters. The basis to control the structure of a bulk product before freeze-drying is during the freezing process of the material, which can be split into two process steps: (a) supercooling, nucleation of crystals and crystal growth in a scraped surface heat exchanger (initial freezing) and (b) the hardening or final freezing in an air blast freezer as known for other products like ice cream manufacturing [4,5]
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