Drying Heat-Sensitive Products
In general terms, conveyor dryer operation can be broken into two categories: commodity or specialty-product manufacturing. Commodity producers require a wide processing range from their drying equipment. Many times they are forced to accept the lower operating margins that accompany such products. Commodity-style conveyor dryers tend to be lower in initial cost but may face restrictions in the areas of quality control and process flexibility.
Specialty product dryers are customized to a narrow range of products and generally produce higher quality levels. Without extensive modification, specialty product dryers are restricted to the specific products for which they were designed. A classic example of a specialty product dryer is a three-stage onion dryer. Its basic design is found worldwide, and it produces comparable product regardless of origin. The three-stage onion dryer also is a classic example of how a specialty product dryer devolved into a customized commodity product dryer, producing high volumes of a limited number of products of relatively standardized quality levels.
While worldwide demand for most dried commodity products remains strong and is growing at predictable rates, the dried specialty products market has shown the greatest growth rates, typically with higher per-unit margins. Specialty products include fruits, berries, vegetables and customized extruded products such as dog and cat foods, treats and aquaculture feeds. In each case, one or more of the products' critical characteristics is heat sensitive, thus restricting the type of drying system that can be utilized. For example, while a range of fruits and berries currently are conveyor dried, the consumer market has consistently reaffirmed its preference for fruits and berries that are freeze dried. With explosive growth predicted in dried fruits and berries, if conveyor dryer operators' equipment is capable of meeting the stringent quality demands of the marketplace, this may be a niche opportunity.
The Drying ProcessIn the conveyor drying process, a moving stream of heated air removes moisture from products carried through the process airstream on a conveyor bed. The process air's latent heat evaporates product moisture, and the moving airstream carries the water vapor away from the product. The hotter the process airstream, the more rapidly the drying process occurs.
The drying process begins on the product surface and gradually moves to the product's interior. In general terms, product temperature is not related directly to process air temperature in the normal range of drying temperatures. The one exception occurs during the initial portion of the drying process. During initial drying, evaporation of surface moisture provides sufficient product cooling to offset any convective heating of the product by the process air. This cooling effect remains constant until the rate of surface evaporation exceeds the ability of internal moisture to migrate to the surface, at which time the product begins to heat up, accelerating the internal rate of moisture transfer. This point, known as the critical moisture point (point C on figure 1), is necessary to establish control over product quality in heat-sensitive products. The drying period prior to this point is referred to as the constant-rate period (portion B-C of figure 1).
Due to evaporative cooling prior to the critical moisture point during the constant-rate period, product temperature does not increase as process air temperature increases. This permits the use of elevated drying temperatures to accelerate the product's drying rate. Provided that sufficient surface moisture readily is available, temperatures approaching two times the normal temperature may be used even in the most heat-sensitive products. However, once the critical moisture point has been reached, product temperature gradually rises to the level of the process air temperature. This is referred to as the falling-rate period (portion C-D of figure 1). During this period, control must be achieved over the drying process to avoid heat-related quality degradation. The drying rate is determined by the product's ability to migrate moisture from its interior to the surface, which is primarily time dependent. Depending on the product, the falling-rate period consumes one-half to two-thirds of the total drying time.
Later modifications to the one-pass dryer configuration included adding more one-pass dryers in a multistage configuration. The multistage dryer not only provided extended drying time but added the benefit of resetting the product bed during the transfer from one dryer bed to the next. Multistage dryers frequently reached 164' (50 m) in length and were the standard in specialty product dryers for decades.
As conveyor dryer lengths became extended and capital costs excessive, the drying industry introduced the two-pass dryer. This configuration utilized two independently driven conveyors; the second conveyor was situated below the first to reduce the dryer's overall length. This design became the extruded products industry standard for the production of dog and cat foods. Later, three-pass dryers became available.
Within two- and three-pass configuration dryers, heat zones are arranged vertically. For example, in a two-pass configuration, the product enters and exits the dryer at the same end, and the process air temperature used on the initial wet product is the same process air temperature to which the near-dry product is subjected. To avoid overheating the product prior to exiting the dryer, lower process air temperatures are required in both the first and final drying zones. Dryer capacity and energy consumption are affected negatively by this inherent feature, but many times these effects are considered acceptable due to the design's lower capital costs.
Advances in dryer configuration design are providing for the manufacture of high quality, heat-sensitive products. One design stacks a series of one-pass dryers vertically but uses a common top plenum for heaters and fans to provide independently controlled process air temperatures to each conveyor pass. This design provides good control of process air temperatures in multiple heat zones, permitting a close match to the specific drying parameters for a given product. Because each conveyor pass and temperature zone is controlled independently, the dryer provides process flexibility for a range of products, offering increased opportunity for the dryer owner to expand the product mix and increase quality while balancing operating expenses and market demand.
Drying Fruits, Vegetables and Fish FoodIn the fruit and vegetable dehydration industry, it is well established that higher processing temperatures can reduce quality in the critical areas of nutrition, color, aroma, taste, texture and rehydration rates. The relationship between tangible product quality and process air temperature is well established for premium dried fruits and vegetables.
Processed Carrots. In processed carrots, 20% losses in both alpha and beta carotenes, and a 75% loss in vitamin C are found when comparing convection-dried to freeze-dried product. Taste panels have confirmed that fruit and vegetables processed at lower temperatures consistently are preferred to those dried at higher temperatures.
Onions. Onion dehydrators learned long ago that undesirable pinking occurred when product temperatures became elevated, resulting in the use of large, multistaged dryers to avoid temperature rise in onions. Extract studies on products containing volatile aromatic oils confirm that 50% declines in essential oils occur when the product is processed at higher temperatures.
Salmon Feed. Like fruits and berries, salmon feed is a heat-sensitive product. Production involves the addition of highly temperature-sensitive components - either astaxanthin or costaxanthin - to enhance the quality of the fish being fed. Both astaxanthin and costaxanthin are temperature sensitive as a function of the total drying time re-quired, with an accelerating rate of pigment breakdown as the product temperature increases. To reduce pigment deterioration, dryer process air temperatures are regulated below 194?F (90?C), which greatly reduces dryer productivity.