What types of dryers are used in the food industry?

There are many types of dryers used in the food industry. According to the drying principle, material characteristics, product requirements (such as morphology, nutrient retention, rehydration) and production scale, they can be mainly divided into the following categories:

This is the most widely used category.

Belt dryer

• Principle: The material is evenly spread on the conveyor belt, and the hot air passes through the material layer vertically or horizontally for drying. It can be designed as a single layer or multiple layers.
• Features: Continuous operation, high degree of automation, and large processing capacity. The drying process is gentle and uniform, and the temperature and wind speed can be controlled in sections.
• Applicable: Vegetable slices (potatoes, carrots, onions), fruit slices (apples, bananas), grains, noodles, pet food, shrimp strips, instant breakfast cereals and other flakes, granules, strips.
• Variations: Multi-layer belt dryer (space saving), through-flow belt dryer (hot air is forced to penetrate the material layer, which is more efficient).

Tunnel dryer

• Principle: The material (usually placed on a tray or a trolley) passes through a long tunnel continuously or intermittently, and the hot air flows in the tunnel (cocurrent, countercurrent or mixed flow) to dry the material.
• Features: The structure is relatively simple, with strong adaptability, and can handle materials of various shapes and sizes (such as whole fruits, tubers, and large pieces of meat). The drying time is relatively long.
• Applicable: Raisins, dried plums, dates, mushrooms, fungus, sausages, large pieces of dehydrated vegetables, etc. Commonly used in preserved fruits, edible fungi, and some meat products.

Drying oven

• Principle: The material is placed on a tray, which is stacked on a movable trolley or fixed rack and pushed into a drying box. The hot air is forced to circulate in the box.
• Features: Intermittent operation, low equipment investment, flexible operation, suitable for small batches, multiple varieties or trial production. The drying uniformity is relatively poor.
• Applicable: Small batches of fruits and vegetables, medicinal materials, test samples, high-end or special drying conditions.

Spray dryer

• Principle: Liquid or slurry materials are atomized into extremely fine droplets through an atomizer (centrifugal, pressure, airflow), and instantly contacted with hot air (cocurrent, countercurrent or mixed flow) in the drying tower, and the water evaporates rapidly to obtain a powdered product.
• Features: The drying speed is extremely fast (seconds), and the product is a powder with good fluidity, and the solubility/rehydration properties are usually good. The equipment investment and energy consumption are relatively high.
• Applicable: Milk powder, soy milk powder, coffee companion, egg powder, vegetable protein powder, instant soup, spice extract, enzyme preparation, microencapsulated products (such as oil powder), etc.

Fluidized bed dryer

• Principle: Hot air passes through the porous distribution plate from bottom to top, so that the granular material is suspended in the airflow, forming a state similar to a fluid, and realizing efficient heat and mass transfer between gas and solid phases.
• Features: Fast drying speed, high thermal efficiency, uniform temperature, and less particle breakage. There are single-layer, multi-layer, vibrating fluidized bed and other forms.
• Applicable: Granular and crystalline materials, such as monosodium glutamate, sugar, glucose, instant beverage granules, granular seasonings, instant soup granules, dehydrated vegetable granules, etc.
• Variants: Vibrating fluidized bed dryer (suitable for materials that are easy to stick or have a wide particle size distribution), spouted bed dryer (suitable for large particles).

Rotary drum dryer

• Principle: The material is picked up and sprinkled by the lifting plate in the slowly rotating inclined cylinder, and is dried in contact with the hot air (or flue gas) flowing in the cylinder.
• Features: Simple structure, reliable operation, large processing capacity, and can adapt to a certain range of material properties (such as containing some viscosity). The thermal efficiency is relatively low.
• Applicable: Starch, distiller's grains, feed raw materials, some grains, potato slices, etc.

Drum dryer

• Principle: One or two heated metal drums rotate, and the slurry or paste material forms a thin layer on the drum surface. The water evaporates through conduction heat transfer and is scraped off by a scraper after drying.
• Features: Fast drying speed, high thermal efficiency, and the product is in the form of thin flakes. There are requirements for material viscosity and film-forming properties.
• Applicable: Pregelatinized starch, instant oatmeal, baby rice cereal, mashed potato slices, fruit pulp slices (such as banana slices), yeast paste, concentrated drying of some seasoning sauces, etc.

Vacuum dryer

• Principle: Vacuum in a closed container to reduce the boiling point of water, and the material is dehydrated at a lower temperature (usually 40-90°C). The heat source can be a jacket, heating plate or infrared, etc.
• Features: Low-temperature operation can retain heat-sensitive ingredients (vitamins, pigments, flavors), color and shape to the greatest extent, and has good rehydration. The equipment is complex, the cost is high, and the cycle is long.
• Application: high-end fruit and vegetable chips (such as mango, strawberry), high-end seasonings, instant tea powder, biological products, functional food raw materials, etc. Common forms include vacuum ovens, vacuum belt dryers (continuous, more efficient), and double-cone rotary vacuum dryers (batch type, suitable for powder particles).

Freeze dryer

• Principle: First, deep freeze the material (-30°C to -50°C), and then under high vacuum (<100 Pa), the ice crystals in the material are directly sublimated into water vapor and discharged.
• Features: The product is dried at low temperature, which can well retain the color, aroma, taste, shape and heat-sensitive nutrients (close to 95% or more), and has excellent rehydration. High porosity and crisp texture. The equipment investment and operating costs are extremely high, and the drying cycle is very long (more than ten to dozens of hours).
• Applicable: high-end ready-to-eat food (freeze-dried fruits/vegetables/noodles/soups), instant coffee, high-end seasonings, aerospace food, biopharmaceutical products, high-end pet food, etc.

Microwave dryer

• Principle: Use microwaves (usually 2450MHz or 915MHz) to penetrate the material, so that the water molecules inside the material vibrate and rub at high speed to generate heat, and heat and dry from the inside out.
• Features: Fast heating speed, selective heating (moisture priority), good drying uniformity (better when combined with hot air), can improve the quality of certain products (such as puffing). The equipment cost is high and it is easy to overheat if not properly controlled.
• Applicable: Combined with hot air/vacuum for fruit and vegetable crisps (puffing effect), rapid drying of noodles/macaroni, sterilization and drying of seasonings, rapid drying of rehydrated food, etc.

Infrared dryer

• Principle: Use the specific wavelength of infrared rays emitted by the infrared radiator to be absorbed by the material and converted into heat energy for drying.
• Features: High heat transfer efficiency, fast drying speed, relatively simple equipment. Commonly used for surface drying or combined with other methods (such as hot air).
• Application: Surface drying of baked goods (biscuits, bread), curing of coating surfaces (such as chocolate coatings), auxiliary drying of grains, etc.

Superheated steam dryer

• Principle: Use superheated steam (temperature higher than the saturation temperature under its pressure) as the drying medium.
• Features: High heat and mass transfer efficiency, great energy-saving potential (recoverable steam latent heat), no oxidation and combustion risks, and can sterilize. High equipment requirements (pressure resistance, sealing), complex control system.
• Application: A developing technology, suitable for materials with high hygiene requirements and sterilization requirements, such as beet pulp, distiller's grains, and some food waste residues. It is still being explored and promoted in the food industry.

Extrusion dryer

• Principle: The water inside the material is vaporized by pressurized heating, and then suddenly released by decompression. The water inside the material instantly vaporizes and expands to form a porous structure, and then further dried and shaped.
• Features: The product is crispy and has excellent rehydration.
• Application: puffed food (potato chips, shrimp strips, popcorn), puffed fruit and vegetable crisps, cereal breakfast, etc. Often combined with extruders or air puffing equipment.

• Material properties: initial form (liquid, slurry, granules, blocks), heat sensitivity, viscosity, whether it is easy to agglomerate, and the maximum allowable drying temperature.
• Product requirements: final moisture content, product form (powder, granules, flakes, blocks), rehydration, color, flavor, nutrient retention requirements, and hygiene standards.
• Production scale and continuity: small batch/experimental use (box type), large batch continuous production (belt type, spray, fluidized bed).
• Energy consumption and cost: equipment investment cost, operating cost (energy consumption, maintenance).
• Environmental protection requirements: exhaust gas emissions, dust control, noise.