Drying in food industry

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1. OBJECTIVE


Drying is defined as the application of heat under controlled conditions, to remove the water present in foods by evaporation to yield solid products. It differs from evaporation, which yields concentrated liquid products. The main purpose of drying is to extend the shelf-life of foods by reducing their in-water activity. Micro-organisms which cause food spoilage and decay and many of the enzymes which promote undesired changes in the chemical composition of the food are unable to grow, multiply or function in the absence of sufficient water (BAT in the Food, Drink and Milk Industries, June 2005).


2. FIELD OF APPLICATION


Typical applications for drying techniques include dairy products (milk, whey, creamers), coffee, coffee surrogates, tea, flavours, powdered drinks, processed cereal-based foods, potatoes, starch derivatives, sugar beet pulp, fruits, vegetables and spices. The water removal from the wet germinated grain is applied in the production of malt, a process which is called kilning. For the malting process, the drying step is essential and is required to create the desired colour and flavour (BAT in the Food, Drink and Milk Industries, June 2005).


3. DESCRIPTION OF TECHNIQUES, METHODS AND EQUIPMENT


  • Principles applied for drying: (BAT in the Food, Drink and Milk Industries, June 2005)


(a) Hot air drying:

Hot air is used as the heating medium and is in direct or indirect contact with the liquid product. The heat transferred from the hot air to the product causes evaporation of the water content.

(b) Surface drying by heat conduction through a heat transfer system (i.e. contact dryers):

The heating medium is not in contact with the wet food but separated from it by a heat transfer surface. The heat is transferred by conduction through the surface, and by convection from the hot surface to the food product for evaporating and removing water from the food. This has two main advantages compared to hot air dryers; less air volume is required and therefore thermal efficiency is higher, and the process may be carried out in the absence of oxygen.


  • Types of drying equipment: (BAT in the Food, Drink and Milk Industries, June 2005 and Operations in Food Processing - the Web Edition, R. L. EARLE, 1983)


Spray dryers:

In spray drying, the material to be dried is suspended in air, i.e. the liquid is converted into a fog-like mist (atomised), providing a large surface area. The atomised liquid is exposed to a flow of hot air in a drying chamber. Air and solids may move in parallel or counterflow. The moisture evaporates very quickly so that this process is very useful for materials that are damaged by exposure to heat for any appreciable length of time. The solids are recovered as a powder consisting of fine, hollow, spherical particles. Heating the drying air can be accomplished by steam or by direct gas-fired air heaters or by indirect heaters fired by gas, liquid or solid fuels. Generally, as an integral part of the process, the exhaust air is passed through cyclones and/or filters to recover particulate materials (dust) which are carried over in the exhaust air. The recovered material is incorporated back in the product.

  • Spray drying parameters:
The dryer body is large so that the particles can settle, as they dry, without touching the walls on which they might otherwise stick. Commercial dryers can be very large of the order of 10m diameter and 20m high. Air inlet temperatures of up to about 250°C or even higher (depending on the type of product) are used, but due to evaporation, the air temperature drops very rapidly to a temperature of about 95°C (outlet temperature of the air). The product temperature will be 20 to 30°C below the air outlet temperature. #
  • Field of application:
Spray drying is applied on a large scale in the dairy industry and for drying coffee.


Roller (or drum) dryers:

The principle of the roller drying process is that a thin film of material is applied to the smooth surface of a continuously rotating, steam heated metal drum. The film of the dried material is continuously scraped off by a stationary knife located opposite the point of application of the liquid material. The dryer consists of a single drum or a pair of drums with or without “satellite” rollers. The applied steam pressure in the drums can vary from 4 to 8bar, depending on the product. Roller drying is applied for example, for milk, starch and potato flakes. Drum drying may be regarded as conduction drying.


Vacuum band and vacuum shelf dryers:

Food slurry is spread or sprayed onto a steel belt (or "band"), which passes over two hollow drum within a vacuum chamber. The food is first dried by the steam-heated drum, and then by steam-heated coils or radiant heaters located over the band. The dried food is cooled by the second water-cooled drum and removed by a doctor blade. The rapid drying and limited heat damage to the food makes this method suitable for heat-sensitive foods.

  • Batch vacuum dryers:
They are substantially the same as tray dryers, except that they operate under a vacuum, and heat transfer is largely by conduction or by radiation. The trays are enclosed in a large cabinet, which is evacuated. The water vapour produced is generally condensed, so that the vacuum pumps have only to deal with non-condensible gases. Another type consists of an evacuated chamber containing a roller dryer.


Tray dryers:

In tray dryers, the food is spread out, generally quite thinly, on trays in which the drying takes place. Heating may be by an air current sweeping across the trays, by conduction from heated trays or heated shelves on which the trays lie, or by radiation from heated surfaces. Most tray dryers are heated by air, which also removes the moist vapours.


Tunnel dryers:

These may be regarded as developments of the tray dryer, in which the trays on trolleys move through a tunnel where the heat is applied and the vapours removed. In most cases, air is used in tunnel drying and the material can move through the dryer either parallel or counter current to the air flow. Sometimes the dryers are compartmented, and cross-flow may also be used.


Fluidized bed dryers:

In a fluidized bed dryer, the food material is maintained suspended against gravity in an upward-flowing air stream. There may also be a horizontal air flow helping to convey the food through the dryer. Heat is transferred from the air to the food material, mostly by convection. With products that are particularly difficult to fluidize, a vibrating motion of the drier itself is used to aid fluidization; it is called vibro-fluidizer which is on springs. The fluidized solid particles then behave in an analogous manner to a liquid, i.e. they can be conveyed. Air velocities will vary with particle size and density, but are in the range of 0.3-0.75 m/s. They can be used not only for drying but also for cooling.


Pneumatic dryers:

In a pneumatic dryer, the solid food particles are conveyed rapidly in an air stream, the velocity and turbulence of the stream maintaining the particles in suspension. Heated air accomplishes the drying and often some form of classifying device is included in the equipment. In the classifier, the dried material is separated, the dry material passes out as product and the moist remainder is recirculated for further drying.


Rotary dryers:

The foodstuff is contained in a horizontal inclined cylinder through which it travels, being heated either by air flow through the cylinder, or by conduction of heat from the cylinder walls. In some cases, the cylinder rotates and in others the cylinder is stationary and a paddle or screw rotates within the cylinder conveying the material through.


Trough dryers:

The materials to be dried are contained in a trough-shaped conveyor belt, made from mesh, and air is blown through the bed of material. The movement of the conveyor continually turns over the material, exposing fresh surfaces to the hot air.


Bin dryers:

In bin dryers, the foodstuff is contained in a bin with a perforated bottom through which warm air is blown vertically upwards, passing through the material and so drying it.


Belt dryers:

The food is spread as a thin layer on a horizontal mesh or solid belt and air passes through or over the material. In most cases the belt is moving, though in some designs the belt is stationary and the material is transported by scrapers.


Freeze dryers:

Freeze-drying is the process of removing water from a product by sublimation or desorption. The aim of the process is to preserve sensitive material that they cannot be dried by evaporation. The material is held on shelves or belts in a chamber that is under high vacuum. In most cases, the food is frozen before being loaded into the dryer. Heat is transferred to the food by conduction or radiation and the vapour is removed by vacuum pump and then condensed. In one process, given the name accelerated freeze drying, heat transfer is by conduction; sheets of expanded metal are inserted between the foodstuffs and heated plates to improve heat transfer to the uneven surfaces, and moisture removal. The pieces of food are shaped so as to present the largest possible flat surface to the expanded metal and the plates to obtain good heat transfer. A refrigerated condenser may be used to condense the water vapour.


Steam bundle dryers:

In steam bundle driers, the heating medium or steam is not in contact with the wet product. A heat transfer surface is used to transfer the heat to the product’s surface for drying. The steam passes through the drier, through cylindrical tubes/bundles which rotate, to avoid local overheating and to improve uniform drying. The drier uses less air volume and subsequently emissions into the air are limited.


Steam dryers:

Steam drying is a special drier design that uses superheated steam produced via a heat-exchanger. The drier consists of a pressure vessel in which the water from the product is driven off, turned into steam and then used to dry more product. This system is used in the sugar industry, on a limited scale, for drying beet pulp. One advantage is the low energy consumption for drying.

Drying in food industry.jpg

Literature: Unit Operations in Food Processing - the Web Edition, R. L. EARLE, 1983, Published by NZIFST (Inc.), http://www.nzifst.org.nz/unitoperations


4. COMPETITIVE TECHNOLOGIES AND ENERGY SAVING POTENTIALS


a) Changes in the process
  • Two- and three-stage drying processes
    (BAT in the Food, Drink and Milk Industries, June 2005 and Operations in Food Processing - the Web Edition, R. L. EARLE, 1983).
Two stage drying process was introduced to improve the traditional single stage drying in terms of product quality and cost of production.
Two-stage dryer:
The two-stage dryer consists of a spray dryer with an external vibrating fluid bed placed below the drying chamber. The product can be removed from the drying chamber with a higher moisture content, and the final drying takes place in the external fluid bed where the residence time of the product is longer and the temperature of the drying air lower than in the spray dryer.
Three-stage dryer:
This principle forms the basis of the development of the three-stage dryer. The second stage is a fluid bed built into the cone of the spray drying chamber. Thus it is possible to achieve an even higher moisture content in the first drying stage and a lower outlet air temperature from the spray drier. This fluid bed is called the integrated fluid bed. The inlet air temperature can be raised resulting in a larger temperature difference and improved efficiency in the drying process. The exhaust heat from the chamber is used to preheat the feed stream. The third stage is again the external fluid bed, which can be static or vibrating, for final drying and/or cooling the powder.
The use of multi-stage dryers results in:
  • higher quality powders with much better re-hydrating properties directly from the drier
  • lower energy consumption
  • increased range of products which can be spray dried i.e., non density, non hygroscopic
smaller space requirements


  • Pre-evaporation, presses and centrifuges
    (BAT in the Food, Drink and Milk Industries, June 2005)
The energy consumption for drying can be less if the dry substance content of the wet material is higher. This can be achieved by pre-evaporation or by using special dewatering equipment such as presses or centrifuges.


  • Use of heat pump dryers
    (BAT in the Food, Drink and Milk Industries, June 2005)
The heat pump dryer consists of a conventional drying chamber with an air circulation system and the usual components of an air conditioning refrigeration system. The drying air is humidified by an evaporator, which is the cooling section of the refrigeration cycle, and reheated by the condenser of the heat pump. The energy efficiency expressed by a specific moisture extraction rate, i.e. kg water removed/kWh energy used, is between 1-4, with an average of 2.5kg/kWh.
Fluidized bed dryers are not suitable for sticky materials or if the shape is irregular. The two driers can be used in series. Dehumidified air from the heat pump is directed first to the fluidised bed with the semi-dried product. The airflow then passes through the cabinet drier. It is reported that by using this combination energy efficiency can be improved by up to 80%.
The use of heat pump driers results in a reduction in energy and water costs. The economic feasibility of the process depends on the price of the fuel in relation to that of electrical power. When heat pump dryers are used, a good heat source is needed in combination with a simultaneous need for heat near the source.


  • Infrared drying
    (Nowak, Dorota, and Lewicki, Piotr P. Infrared drying of apple slices. Innovative Food Science & Emerging Technologies, 5(3) September 2004, pp 353–360,
    Pan, Zhongli, Thompson, F. James, and Godfrey, D. Larry. Infrared Drying of Rice to Improve Energy Efficiency and Disinfestation. Fact sheet 2004. California Energy Commission, PIER Program. CEC-000-2006-000)
Infrared drying is efficient because it only heats what needs to be heated, as opposed to traditional drying methods, which heat substantial amounts of air.
For drying apple slices, a laboratory dryer was equipped with near-infrared radiators with peak wavelength at 1200 nm. The energy efficiency of the infrared dryer was between 35% and 45%. Apple slices were dried with infrared energy and by convection under equivalent conditions. Comparison of infrared drying with conventional drying at equivalent parameters showed that energy costs were lower and the time of the process can be shortened by up to 50% when heating is done with infrared energy. Researchers also found that it was easy to control the material temperature.
For rice drying, a conventional column dryer uses 2,000 Btu of thermal energy to remove one pound of moisture, compared to 1,500 Btu using an infrared dryer. Conventional dryers use 7.7 kWh of electrical energy to dry a ton of rice while the infrared dryer uses on 1.6 kWh. Replacement for a typical 50ton/hr column rice dryer by an infrared dryer is estimated to reduce 152,500 kWh/year of electrical energy consumption, 100 in peak demand and 12,000 therms per year in gas consumption.


  • Radio frequency drying
    (Northwest Food Processing Association)
Radio frequency can be used instead of conventional processes to dry food products. It’s a quicker and less energy consuming process used in fruit and vegetable processing and meat industries.


  • Use of freeze drying
    (Northwest Food Processors Association,
    Analysing research and technology development strategies - the ATLAS project. Energy Efficient Technologies In Industry. Utrecht University January 1997, Report No. 97001,
    Final Report. State Technologies Advancement Collaborative (STAC), Western US Food Processing Resource Efficiency Initiative, November 2006)
According to Southern California Edison (2005), freeze concentration uses less energy, increases product purity and yield, and considerably reduces processing costs that traditional evaporation and distillation techniques.
The theoretical energy savings are given by the difference between the energy needed for the crystallization and the vaporization of water. Crystallization requires 151kJ/kg, where the evaporation requires 1055kJ/kg water. Thus, energy savings of more than 80% are theoretically possible if freeze concentration is used instead of evaporation (ATLAS Project 2005).
Food processing companies have applied freeze concentration to fruit juices, beer, wine, vinegar, milk, and coffee. Among its benefits, freeze concentration is said to provide the following (SCE 2005):
  1. Improved product quality. Low operating temperatures retain the natural flavor, nutritional value, and aroma of the product. This reduces or eliminates the need for flavor and aroma additives.
  2. Lower energy consumption. Crystallizing a pound of water requires about one-eighth the energy needed to vaporize the same amount.
  3. Increased product recovery. Up to 100% of the product can be recovered without generating waste by-products, which can lead to indirect energy and water savings.
  4. Reduced capital and maintenance costs. With less corrosion at lower temperatures, freeze concentration uses inexpensive construction materials and equipment that requires less maintenance.
  5. Lower shipping costs. Products can be concentrated to a fraction of their original volume. For example, concentrated milk takes up less space, weighs less, and remains edible longer than fresh milk.
While there are many types of freeze concentration systems, indirect freeze models are best suited for food processing. A heat-exchange surface keeps the product separate from the refrigerant and a mechanical device scrapes the surface to prevent ice deposits from forming as the components freeze (SCE 2005).
  • Heat pump dehumidifier
    (Northwest Food Processors Association)
Heat pump dehumidifiers (HPDs) offer several advantages over conventional hot-air dryers for the drying of food products: higher energy efficiency, better product quality, and the ability to operate regardless of ambient weather conditions.
In an HPD dryer, the drying takes place in a sealed chamber and the moisture is removed in its liquid state. The source of the heat that is absorbed at the evaporator is the humid air that is drawn from a product during the drying process. The moisture is condensed out of the humid air. The latent heat recovered is released at the condenser of the refrigeration unit and used to reheat the air within the dryer. The system is recirculatory, rather than open. Removal of water in its liquid state allows the latent heat of vaporization to be captured, resulting in higher efficiency. However, because the drying is performed in a sealed chamber, HPD is more suited to batch processing than continuous processing.
Lower temperatures may be used in HPD dryers, which may help to improve product quality.
  • Ohmic heating
    (Lima, Marybeth, Zhong, Tuoxiu and Lakkakula, N. Rao. Ohmic Heating: A Value-added Food Processing Tool. Louisiana Agriculture, 45(4) Fall 2002)
Ohmic heating is a food processing method in which an alternating electrical current is passed through a food sample. This results in internal energy generation in foods. This produces an inside-out heating pattern, which is much faster than conventional outside-in heating. Ohmic heating is somewhat similar to microwave heating but with very different frequencies. The advantage of ohmic heating is that it uniformly heats foods with different densities.
According to Lima at al. (2002), potential applications for ohmic heating include blanching, evaporation, dehydration, fermentation and extraction. Ohmic heating is said to save significant time and energy in hot air and freeze drying of foods and enhances extraction yields in some processing operations. For example, in research conducted at Louisiana State University's AgCenter, sweet potato samples were ohmically heated and freeze-dried, and their freeze drying rate was measured and compared with a control (no heating). Ohmic heating increased the rate of freeze-drying up to 25 percent, a significant time and energy savings for processing (Lima et al. 2002).
  • Pulsed fluid bed drying
    (Northwest Food Processors Association)
The pulsed fluid-bed dryer (PFBD) is a modified conventional fluid-bed where gas pulses cause high frequency vibration of the particle bed. The design is similar to conventional fluid-bed dryers except that the plenum chamber below the supporting grid is divided into sections by vertical partitions. The primary hot air stream from a gas-fired burner flows from both sides of the plenum to a rotating valve-distributor with a disc on each side open on one quarter of its surface. When rotating, this valve-distributor alternately directs the drying hot air stream to the different sections of the chamber. The total air flow rate remains constant as the disc openings are never closed.
The characteristics of the PFBD lead to several advantages:
  • easy fluidization for irregularly shaped particles
  • fluidization of particles with a wide size distribution;
  • fluidization with 30 to 50% less air;
  • improved fluidization uniformity
  • fluidization of fragile particles.
Since commissioning in 1997, materials tested in the prototype included recycled polypropylene and polyethylene, cat litter, granulated fertilizer, pulp and paper primary sludge, wood strands, bark, carrot cubes, chopped onions, cranberries, and chitin (from shrimp residue). Results have indicated that, compared to dryers actually used for the drying of vegetables, PFBD systems will lead to better product quality, and could lead to lower production costs given that they are smaller and more energy efficient (CADDET 2000).


b) Changes in the energy distribution system

No information is available.


c) Changes in the heat supply system

No information is available.



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