Mixing and segregation of wheat bran and vegetable pieces binary mixtures in fluidized and fluid-spout beds for atmospheric freeze-drying

ABSTRACT

The segregation of binary mixtures of nonfood wheat bran and vegetables at different levels of dryness was studied by simulating different stages of the atmospheric freeze-drying by immersion in an adsorbent material process. It was characterized using a new set of four indices, which allow to evaluate not only the segregation level but also the segregation pattern. The mixing performance was evaluated in a fluidized bed (considering also the effect of air superficial velocity) as well as in a spout-fluid bed; peas, carrot disks, and potato slabs were used as food products. In general, it was found that food material segregates toward the bed bottom during the first stages of the drying process in fluidized beds, which translates in a poor contact product-adsorbent. On the contrary, uniform mixing patterns were observed in the spout-fluid bed, for the beginning and final stages of the process. On the other hand, despite the very low cost of nonfood wheat bran and its compatibility with food materials, it presents a channeling fluidization as a consequence of the cohesive properties of its particles. This behavior was also characterized by video analysis in two different fluidized beds. A channel generation and collapse general cycle were highlighted, whose frequency and channel characteristics depend on the air superficial velocity. In addition, three main types of food particle transports were identified: passive (downward), active (upward), and particle-blocking effects. These findings allowed to explain segregation phenomena in these kinds of binary mixtures.

KEYWORDS:

• Atmospheric freeze-drying
• binary mixtures
• cohesive powder
• segregation
• spout-fluid bed

Acknowledgments

The present work was based on the PhD thesis of the corresponding author. In addition, the authors wish to thank Massimo Curti and Giorgio Rovero for their practical help and recommendations in the fluidization field and equipment operation.

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