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Drying and Cooling of Food Products

February 4, 2014
Citric acid fluid bed
Vitamin A fluid bed dryer
Figure 1
Figure 2

Food processing requires focus on characteristics critical to consumer appeal. Drying of food products has more effect than reduction of moisture content. The application of warm and cold air must produce an end product appetizing and palatable to the consumer market. A multi-zone vibratory fluid bed dryer is a proven solution for the processing of food products.
    A multi-zone vibratory fluid bed dryer is capable of producing a homogenized product while preserving product integrity. The utility of the multi-zone vibratory fluid bed can be demonstrated by the advantages of a vibrating fluid bed dryer for and the advantages for multiple zones in the dryer unit.

Application of Vibratory Fluid Beds for Food Products
Fluidization homogenizes the drying of the food particles. Particles are levitated by surrounding the particle by the passing drying or cooling air. All of the food particles are completely dried by the envelopment of drying air according to the counter/cross flow principle of fluidization (figure 1). The food product is normalized by the consistency of the drying air. During drying, food product reaches an adiabatic state where constant evaporation takes place. Constant evaporation occurs until a decreasing margin rate eventuates as the particles become drier. Typically, this event initiates as the food particles achieve 1.5% moisture and less. Increasing the fluidization of the particles blends the particles which prevents stratification. Stratification of the product can and often does occur with the utilization of a belt dryer and results in inconsistent drying with downstream complications.

See Figure 1

    Food products are often fragile and often comprised of friable agglomerations. The preservation of product shape requires minimized contact and delicate handling during the production process. Products such as salt, milk powder, bread crumbs, potato granules, powdered sauces, and “instant” type foods require minimized contact to preserve product integrity. Because fluid bed dryers elevate products with air flow, surface contact of delicate product to metal parts is minimized.
    Adding Vibration to the fluid bed dryer provides plug flow of the fluidized product. Once the product is elevated through fluidization, gentle vibration moves the product consistently inside the dryer. The steady and gentle vibratory action creates movement that assures consistent and repeatable residence time. The vibration also creates movement for particles not fluidized by the pressure drop of the air flow. These particles could consist of nonconforming particles (oversized and irregular particles) or the food product is general sticky and subject to buildup and blocking in a static fluid bed dryer or cooler. Using vibration to ensure the movement of all particles prevents over-drying or burning. Vibration ensures consistency of the process even for the irregular or inconsistent particles.

    Fluid bed dryers also have other advantages that support food products processing. Fluid bed dryers minimize working and moving parts.  Sanitary conditions result from few moving parts, continuous and radius welds, as well as the ability to have highly polished surfaces. The open interior access of the fluid bed dryer improves ease of cleaning and the quality of cleanliness. Clean in Place (CIP) systems can be implemented for automated and repeatable clean procedures to occur and be documented (GMP). Fluid bed dryers provide the opportunity to achieve cleanliness levels that meet pharmaceutical qualifications. In addition to cleanliness, the process footprint of a fluid bed dryer is relatively compact and capable of delivering drying solutions in a fraction of space requirements for other technologies. The technology of fluid bed dryers also allows for predictable stability and control of temperature and optimum energy efficiency. The control of the inlet air also allows fluid bed dryers to have humidity control. Control of humidity conditions allows for unique processing opportunities. Dehumidified air supports cheese drying by lowering the dew point to below freezing dew points. These additional advantages provide several opportunities for fluid bed dryers to improve food processing applications.

Vibratory Fluid Bed Dryer for Fragile Food Agglomerations:
* Homogenized and Consistent Product Drying
* Minimized Contact with Delicate Products to Preserve Agglomerations
* Vibration Ensures Repeatable Residence Time
* Sanitary Conditions through Minimized Product Contact, Few Working Parts, Polished Surfaces, and Continuous Radius Welds
* Construction of Fluid Bed Allow Access for Cleaning or CIP
* Temperature Stability – Tightly Controlled
* High Performance Optimum Energy Utilization
* Humidity Control

Advantages of a Multi-Zone Dryer
Food products often have a number of value-added heating and cooling phases. Common process phases include heating, drying, roasting, sterilization, and subsequent cooling. Proper process control for the application and timing of heat directly impacts taste and color characteristics of food products. By utilizing a multi-zone scheme, each section of the vibratory fluid bed dryer has independent temperature and air flow control (figure 2). Each zone can autonomously support a different heating/cooling phase, or multiple zones can be used to support a single phase with increased cycle residence time. This allows food products to pass a number of independently controlled zones and complete any number of heating and cooling process phases. Each section of a multi-zone dryer can be adjusted in coordination with the product requirements. This further reduces product contact by a factor of the number of process phases. This elimination of transfer exchanges, such as from a stand-alone dryer and a stand-alone cooling system, protects product integrity. Simultaneously, product temperature and product residence time can be closely monitored and controlled because food products need not to be transferred between equipment to perform multiple process operations. Simple temperature controls and control loops ensure reliability of a multi-zone fluid bed design.

See Figure 2

Examples of Food Products and Process Heat Fluidization Levels

Product    Process    Fluidization
Soybeans    Dry    Highly Fluidized
Soybeans    Roasting    Low Fluidization
Rice    Dry    Sub Fluidized
Spices    Dry    Highly Fluidized
Potato Granules    Dry    Highly Fluidized
Instant Products    Dry    Highly Fluidized
Nuts    Roasting    Low Fluidization
Nuts    Dry    Highly Fluidized
Milk Powder    Dry    Highly Fluidized
Bread Crumbs    Dry    Low Fluidization

Product Tempering or Cooling
Food products often need “setting” after an initial heat process. Sometimes referenced as “tempering,” setting involves maintaining the heat temperature for an extended duration and/or decreasing product temperature at set intervals. Some food processes require holding a temperature at a set point for a determined time for decontamination or even sterilization. The use of steam in a fluid bed is also possible to support the setting process and can act as the fluidization gas inside the dryer. Multiple independent temperature control zones in a fluid bed dryer are ideal for this process. Controlled cooling also can require multiple zones, with control of input temperature to each zone independently. Multi-zone fluid bed dryers provide the opportunity for several configurations. The use of heat exchangers and chillers combined with demisters, or the use of desiccant wheels and other schemes such as air sterilization and dehumidification using lithium chloride scrubbers are common practice.

Advantages of Multiple Zones
* Capacity to Consolidate Multiple Process Steps into Single Stage
* Simultaneous Multiple Process Steps Reduces Transfer Handling of
Fragile Produces
* Independent Control to Allow Stability or Adjustment
* Product Residence Time Can Be Adjusted and Sustained through Multiple Zones
* Multiple Zones Allow the Product to Be Set or Tempered Directly after the Heating Cycle
* Scope of Multiple Zones Provides Opportunity for Multiple Configurations.

    For more information, visit www.almoprocess.com or www.allgaier.de.

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