ENGLISH VERSION

ZESZYT 340

Dariusz Góral
Analysis of using the impingement method in the fluidized bed freezing fruits and vegetables

In the recent years, there have been developed mechanical freezing tunnels applying impingement technique for frozen food manufacture. Freezing foods with the air impingement system involves jets of air at high velocity impinging on the product surface that results in substantially increased overall heat transfer between interface of a product undergoing freezing and environment and hence, a rapidly rise of freezing rate. The objective of the present study was to analyze a possibility of the impingement process implementation for vegetable and fruit fluidization freezing as well as assessment of the results of the combined processing technique.
The studies were conducted on the fruit and vegetable samples of approx. 0,5 kg weight cooled in air at impingement fluidization system, while the comparative ones in a cabinet freezer and a liquid nitrogen cryostat at –15 up to 
–70oC temperature range. There were investigated a bed of products, local heat transfer coefficient, freezing and thawing rate and time, mass changes, 
microstructure and chosen attributes of product quality. Local convection heat transfer coefficient was determined for some slice-shaped vegetable samples of 3 × 3 × 0,5 cm dimensions using heat flux sensor. Microstructural changes induced by freezing were analyzed on the grounds of scanning electron microscope images. Product quality after thawing was evaluated through the structure assessment by cutting test performed on a testing machine, whereas organoleptic examination of materials included scores for color, flavor, juiciness and texture rated on a 5-point preference scale.
Application of the combined impingement and fluidization (impingement fluidization) systems for vegetable and fruit freezing allowed to achieve very short processing time of raw material. That benefited from the increased convection heat transfer coefficients (62-195 Wm-2K-1) as compared to classic fluidization. There has been developed a model for local heat transfer coefficient determination, which owing to its simplicity and suitability for a wide range of Reynolds numbers is applicable in computations essential for freezing technique design in the process under study. The major advantage of the investigated freezing method proves to be high quality of products subjected to such a treatment. The fruits and vegetables that underwent individual quick freezing showed only slight tissue damage, attractive appearance and excellent consistency, no frosty surface or color change. This is evidenced not only by scanning electron microscope images of tissue structure or organoleptic analysis but mainly instrumental shear tests which showed limit deformation to be lower by 10% than that in vegetables frozen in a freezer.