ABSTRACT

Drying is a technique that involves removal of moisture using heat energy. This heat affects the protein components in foods especially the thiosulphide groups, which causes hydrophobic bond break that has been attributed greatly to denaturation during drying. Safe moisture content (SMC) is the extent to which moisture can be withdrawn from food crops during drying that such crops can be considered safe for storage with minimal loss of nutritional qualities. Several reported minimum moisture contents, and bulk nutrients’ levels of crops were collected for the purpose of this review, and scattered plot graph was employed to determine the levels of bond interaction between moisture content and each bulk nutrients in the various dried food categories. The moisture contents in grains, root and tuber crops, fruits and vegetables, and cash crops formed a SMC threshold boundary within the 6% - 14%; 0% - 10%; 0% - 22%; and 0% - 30% respectively. Crude fibre and most especially the ash content played the most crucial role by providing the strongest bond interaction with migrating moisture during drying of all the food crops’ categories, and are of utmost important in the determination of SMC.

Keywords:Preservation; Safe Moisture Content; Scattered Plot; Threshold Boundary; Bonds

1. Introduction

Food preservation is the process of treating and handling food to stop or slow down spoilages like loss of quality, edibility or nutritional value. Thus, food preservation allows for longer storage [1] . Preservation usually involves preventing the growth of bacteria, yeasts, fungi, and other micro-organisms as well as retarding the oxidation of fats, which cause rancidity. It can also include processes that inhibit visual deterioration which can occur during food preparation, such as the enzymatic browning reaction in apples after they are cut. Preserving fruit, by turning it into jam, for example, involves boiling (to reduce the fruit’s moisture content and to kill bacteria, yeasts, etc.), sugaring (to prevent their re-growth) and sealing within an airtight jar (to prevent recontamination) [2] . Some of the commonly used preservation methods been used include: irradiation, food processing, drying, canning, cooling (e.g. cold room storage, control atmosphere storage, and refrigeration), modified atmosphere packaging and so on [2] .

The technique of drying is probably the oldest method of food preservation ever practiced by mankind. Drying is a method of food preservation that works by removing water from the food. The removal of moisture prevents the growth and reproduction of microorganisms responsible for decay and reduces the moisture mediated deterioration reactions. Drying brings about substantial reduction in weight and volume of products minimising packaging, storage and transportation costs. It enables storability of products under ambient temperatures [3] . Drying is excellent for most fruits and vegetables. The merit of this procedure is that the dried product weighs very little and the size is reduced considerably for easy storage. Also, a plus is that the food retains almost all of its nutrients. There are several ways to dry food which includes sun drying, oven drying and drying in a dehydrator. The first two methods are difficult. Sun drying can be done with window screens that are washed thoroughly. Oven drying is done in a shallow baking pan at a very low temperature for several hours. A dehydrator is the best, most effective and easiest way to dry food.

Drying food using sun and wind to prevent spoilage has been practiced since ancient times, and was the earliest form of food curing [2] . Water is usually removed by evaporation (air drying, sun drying, smoking or wind drying) but, in the case of freeze-drying, food is first frozen and then the water is removed by sublimation. Bacteria, yeasts and molds need the water in the food to grow, and drying effectively prevents them from surviving in the food. Drying is quite simple compared with other methods. In fact, most of the equipment required may be ready on hand. Dried foods keep well because the moisture content is so low that spoilage organisms cannot grow. Food dehydrators are less expensive to operate but are only useful for a few months of the year. A convection oven can be the most economical investment if the proper model is chosen. A convection oven that has a controllable temperature starting at 120 degrees F. and a continuous operation feature rather than a timer-controlled one will function quite well as a dehydrator during the gardening months. For the rest of the year, it can be used as a table-top oven. People in warm, dry climates have found it easy to preserve their foods simply by properly spacing their produce out and letting the air take the moisture out of the food. Properly dried fruits and vegetables will have 80 - 90 percent of their water removed. Because drying does not violently heat food, it does not destroy as many of the nutrients as canning or cooking. Dried foods can be reconstituted by adding water or often simply consumed to dry. Although there are different drying methods, the guidelines remain the same [2] .

2. Principles of Drying: Heat and Mass Transfer

Dried, desiccated, or low moisture foods are those that generally do not contain more than 25% moisture. Drying involves the application of heat to vaporise water and a means of removing water vapour after its separation from food tissues. Hence, it is a combined/simultaneous heat and mass transfer operation for which energy must be supplied [3] . A current of air is the most common medium for transferring heat to a drying tissue. The two important aspects of mass transfer are:

1) Transfer of water to the surface of material being dried.

2) The removal of water vapour from the surface.

Dehydration prevents problems due to sugar accumulation during storage and eventual browning on-frying [4] . One of the most important considerations in preventing fungal spoilage during storage of dried foods is the relative humidity of the storage environment. Properly packed dried foods should be stored under conditions of low humidity to prevent moisture re-absorption. The following are the factors influencing drying process.

a) Surface area: Generally, the fruits and vegetables to be dehydrated are cut into pieces or thin layers to speed heat and mass transfer. This subdivision speed up drying for two reasons:

i) Large surface area provided more surface in contact with the heating medium (air) and more surface from which moisture can escape;

ii) Smaller particles or thinner layers reduce the distance through which moisture in the centre of the food must travel to reach the surface and escape.

b) Temperature: The greater the temperature differences between the heating medium and the food, the greater will be the rate of heat transfer into the food which provides the driving force for moisture removal.

c) Atmospheric pressure and vacuum: If food is placed in a heated vacuum chamber the moisture can be removed from the food at a lower temperature than without a vacuum. Alternatively, for a given temperature, with or without vacuum, the rate of water removal from the food will be greater in the vacuum. Lower drying temperature and shorter drying times are especially important in the case of heat sensitive foods [3] .

3. Types of Drying

3.1. Sun-Drying

This is the conventional drying method, it involve the spreading of commodity in the sun on a suitable surface, hanging it from eaves of buildings, trees etc. Whilst it is a technique which requires little in the way of capital or expertise, one can give an acceptable quality in a reliable climate. It is ideal for products where little or no value is added. Sun drying does have many limitations, loss  of moisture can be intermittent and irregular, and the rate of drying is generally low so increasing the risk of spoilage during the drying process, the final moisture content of the dried product can be high because of low air temperature and nutrients level (e.g. vitamins) are greatly reduced in the dried product. Crops are also liable to theft and there is need for labour to be on hand to scare away would be predators. In most part of Africa, harvested crops are dried by being left in the field for a period of about six weeks [5] . At this point, in most instances infestation through rats, goats, weevils, etc would have begun. This would arise from the high humidity and rainfall, causing the moisture content to fall around 17% for grains at the time of harvest. The drying method is usually unhygienically practice. It is open to contamination by dust, microbes, rodent and pest infestation. It is totally dependent on good weather. This makes the drying inconsistent and gives it very slow drying rates with danger of mould growth. Sun drying may be able to dry produce to a sufficiently low level of moisture to prevent mould growth. In case of open sun drying, sudden rains and strong winds might completely ruin the whole batch. Sun-drying often cause foods to lose some texture and taste. Foods dried by this method generally require an airtight storage container that includes an oxygen absorber. If dried too quickly, the outer cell layer of the food product may harden, causing the product to become tough and rubbery. Sun-dried foods may lose various nutrients, including vitamin C [6] .

3.2. Oven Drying

Oven drying is the simplest way to dry food because there is no need for special equipment. It is also faster than sun drying or using a food dryer. But oven drying can be used only on a small scale. An ordinary kitchen oven can hold only 4 to 6 pounds of food at one time. The oven is set to the lowest possible setting and preheats to 60 C. Some gas ovens have a pilot right, which may keep the oven warm enough to dry the food. It is important to keep the oven temperature at 60˚C to 70˚C. An oven thermometer can be put on the top tray about half way back where it can be easily seen and the temperature checked half hourly [7] . Drying food in the oven of a kitchen range, on the other hand, can be very expensive. In an electric oven, drying food has been found to be nine to twelve times as costly as canning it. A commercial or homemade food dryer or convection oven provides automatically controlled heat and ventilation. Most households will not need a dryer unless they dry large quantities of food. A food dryer takes less electricity than drying the same amount of food in an electric oven. However, the temperature is usually lower (about 120 degrees F. or 50 C.), so drying takes a little longer than in an oven [8] .

3.3. Solar Drying

Solar drying relies as does sun drying on the sun as its source of energy, it differ from drying in that a structure often of very simple construction, is used to enhance the effect of the insolation. It is a sensible alternative to sun drying [9] . There are mainly three types of solar dryers [10] .

3.3.1. Direct Absorption Dryers

The absorption or hot box type dryers in which the product is directly heated by sun, Direct drying consists of using incident radiation only, or incident radiation plus reflected radiation. Most solar drying techniques that use only direct solar energy also use some means to reflect additional radiation onto the product to further increase its temperature.

3.3.2. Dryers with Solar Air Heaters

The indirect or convection dryers in which the product is exposed to warm air which is heated by means of a solar absorber, or heat exchanger. Trays are arranged so as to induce air circulation. Measuring devices and control may also be incorporated to facilitate air renewal in the system. Fans and internal recycling are usually placed inside the chamber produce to allow the air flux to be uniform. The drying system is equipped with solar collectors made of blackened iron plate, protected by a glass cover, which also helps to decrease heat losses

3.3.3. Combined Direct and Indirect Dryers

Dryers combining the principles of the above two, where the product is exposed to the sun and a stream of pre-heated air simultaneously. Many types of drying systems utilize both direct and indirect solar radiation. In these types of systems, radiant energy from the sun falls directly onto the product being dried; however, in addition, a preheater also is used to raise the air temperature, which in turn, accelerates the drying rate. Acceleration of drying rate can occur in two ways: hot air can transfer some of its heat to the product being dried, thus raising its vapour pressure causing a faster moisture loss; or as temperature of air mass increases, the water-holding capacity also increases [10] .

Solar drying does generate higher air temperature and thus lower relative humidities which are both conducive to improve drying rates, and better lower final moisture content of the dried crops [9] . Hence, the risk of spoilage is reduced both during the actual process and in subsequent storage. The higher temperatures are also a deterrent to insect and microbial infestation. Additionally, protection against dust, insects and other animals is enhanced by drying in an enclosed structure. All these factors contribute to an improved and more consistent product quality. Solar drying is faster because inside the dryer it is warmer than outside. It is labour saving as the product can be left in the dryer overnight or during rain. The quality of the product is better in terms of nutrients, hygiene and colour [11] . However, solar drying is limited in the developing countries because of cost, which may be very expensive. The initial investment capital may be too high for small scale processors. The dryers are often too complicated and so require expertise in management [12] . Solar drying has a tendency to over-dry the products been dried since the internal temperature that cannot be control is always very high. Drying air temperatures as high as 70˚C - 100˚C may be reached with these dryer, which are excessive for most products [13] . Further major drawbacks for natural convection solar dryers are the poor moist air removal which reduces drying rate [13] .

3.4. Freeze Drying

The freeze-drying method as the names implies involve the combination of freezing process and the drying process. The process is usually subdivided in three main steps; the prefreezing, the sublimation drying and the secondary drying [14] .

a) Prefreezing step: here the product is chilled to normal deep freeze temperature, the major part of the water content will freeze-out as ice crystals and the total water content is immobilized in its location within the product.

b) Sublimitation drying step: the ice crystals are sublimated into water vapour at the low sublimation temperature and waters vapour removed from the product, this step involve two main transport process;

i) Heat transfer: here heat is transferred to the product surface through the products layers to the sublimation front (point where the temperature equals that sufficient for the water to vapourise). The driving force is the temperature difference from the heater surface to the sublimitation front. The transport means are the sum of the heat conduction, the heat convection and the heat radiation. The highest temperature will be found on the product surface which is closer to the heater surface.

ii) Water-vapour transport: the water vapour being produced at the sublimation front has to be transported first to the product surface and then away from the product surface, otherwise the vapour will rise and the temperature at sublimation front will also rise so that melting of the frozen product will occur.

c) Secondary drying step: The sublimation drying of the ice crystals leaves a system of pores in a matrix of the product. The rest of the water content of the product has not been frozen out as ice crystals and is still distributed within the matrix material. Hence, the secondary drying is aimed at removing the bulk of the remaining moisture. It is similar to the normal drying process.

When freeze drying is been used to dry food, the normal operation is that sublimation drying and the secondary drying takes place simultaneously in different parts of the product and naturally in the same piece of equipment [14] . The method is advantageous  in that; The nutritive and organoleptic quality of freeze dried foods are comparable with quick frozen foods because the qualities of the products such as vitamins, taste, flavour and appearance have been preserved basically in the same way. When compared to quick frozen foods, the freeze dried products have the advantages that they have reduced weight and that they keep their quality at normal ambient temperature. When compared to hot air dried products, the main advantages of freeze dried foods are appearance, flavour retention and rehydration ability. Generally, the fresh colour of the product is maintained and the shape is only slightly changed, so that the product can easily be recognised for what it is. However, the most recognized feature of the freeze drying process is the amazing quality of aroma retention, the aroma components are distributed within the matrix structure. The clean water of the ice crystals can escape without entraining the volatiles, the volatile aroma components are trapped because they diffuse much more slower through the matrix material than the water vapour, so in the final product the aroma component are practically locked in. The disadvantage of the method is that it is an expensive preservation process, compared to hot air drying or quick freezing [14] . The long period of time required for the drying operation is a major disadvantage of the freeze drying method. Also, during the freeze-drying process, some foods must be pretreated to prevent damage such as loss of color [15] .

3.5. Osmotic Drying

This is a recent method of drying in which sugar is used for the removal of water by osmosis. It is often used prior to other methods of drying [16] . The process of osmosis which involves transfer of water from lower concentrated solution to higher concentrated solution is used to remove water from a dilute solution contained within a semi-permeable membrane by surrounding the membrane with a more concentrated solution. Transfer of water by osmosis is applicable to fruits since they contain sugars in solution and their cellular surface structure acts as an effective semi-permeable membrane [17] . By immersing fruits and vegetables in a concentrated sugar solution water can be removed to the extent of over 50% of the initial fruit weight. Some of the advantages of osmotic drying are;

1) The material is not subjected to a high temperature over an extended time, so heat damage to colour and flovour are minimized2) A high concentration of sugar surrounding the material prevents discoluration by enzymic browning.

3) Some of the fruit acid is removed along with it as water is been removed. The lower acid content combined with the small amount of sugar added to the fruit during osmosis, produces a sweeter product than ordinary dried fruit.

4) Osmotic drying retained the sensory qualities of dehydrated samples better than air-drying of guavas [18] . Osmotic drying has twice ability to preserve the red color appearance, flavor, and texture of guavas than airdrying [18] .

Some of the disadvantages are:

1) The decrease in acidity mentioned above may be disadvantages in certain products.

2) A thin film of sugar is left on the surface of the fruit after drying, which may be undesirable. However, this can be reduced by a quick rinse in water3) Sugar treated fruit which has been dried to a very low moisture content can become rancid after staring for several weeks, which may be caused by the greater retention of flavour oils in osmotically treated fruit.

3.6. Vacuum Drying

The basic aim of this drying method is to shorten the drying time at a reduced temperature, which helps in retaining the basic quality of food products. Drying under vacuum is especially beneficial to crops, especially the fruits and vegetables. Vacuum drying is especially effective for heat sensitive materials and allows for closed system benefits where solvent recovery is required. A vacuum dryer is typically used for separating a volatile liquid by vaporization from a powder, cake, slurry, or other moist material. This process is fundamentally thermal and doesn’t involve mechanically separating the liquid from the material, such as in filtration or centrifugation [19] . The method of drying is widely applied in food industry, paper industry, wood industry (as part of timber processing) and for washing powder. It plays an important role in our life [19] .

The various vacuum drying processes are divided into two categories: the continuous vacuum drying process, in which the vacuum is maintained continuously throughout the drying process; and the discontinuous vacuum drying process, in which phases of convection heating at atmospheric pressure alternate with phases under vacuum. The continuous vacuum drying kilns include hot platen, superheated-steam, air-steam mixture and highfrequency heated kilns. This article deals primarily with the continuous vacuum class of dry kilns [20] .

Principles of the Vacuum Drying Process

Vacuum kilns dry lumber in an airtight chamber maintaining an air pressure lower than the normal atmospheric pressure. The boiling point of water is reduced by these conditions, which increases the rate of evaporation at the surface and results in temperature and total pressure gradients favourable to the flow of humidity through the thickness of the wood. Wood-drying in oxygen-free or oxygen-reduced environments also has other advantages. In particular, the discoloration of wood is partly or completely blocked. The main difference among the various continuous vacuum drying kilns relates to the way the wood is heated. The superheated steam and air-steam mixture method involves usinq fans to circulate superheated steam or air-steam mixture through lumber piled on, stickers (convective heating). The hut platen method (no stickers) involves direct contact with the wood (conductive heating). High-frequency vacuum drying involves the use of a radio-frequency heating field. The rapid changes in the direction of the electric field cause the dipolar water molecules in the wood (no stickers) to oscillate many millions of times per second [20] . This result in heating, due to molecular friction, and thus a rise in temperature and vapour pressure in the wood (heat transfer by electromagnetic radiation).

The advantages of boiling point reduction brought about by using low pressure are realized in treating materials which are damaged or decompose at temperature above 40˚C, or even 15˚C. Vacuum drying had been used with conventional heating for many years, but is exceptionally slow because convection is virtually absent in a vacuum oven. Also, the decompression condition designed by vacuum reduces the boiling point of products to be dried, which gives it ability to dry thermo sensitive materials [21] . There's much less oxygen content in low pressure than in atmospheric pressure which can avert samples from getting oxidized and also deteriorating [21] . Once vacuum drying is usually performed at low pressure, water is evaporated at low temperature (the boiling point of water at 101.3 kPa is 100˚C, whereas at 12.3 kPa, it is 50˚C) and consequently the drying can be conducted at low temperature [22] .

Vacuum drying is ideal for drying materials which will be damaged or changed when exposed to high temperatures. The vacuum removes moisture while preventing the oxidation or explosions that can occur when certain materials combine with air. There are occasions when a solvent must be recovered or materials must be dried to very low levels of moisture, and this is suitable for us to use vacuum drying [19] . Drying at lower temperatures reduces energy consumption. It allows costly solvents to be recovered, and eliminates expenses for additional air pollution control, and minimizes product loss caused by atmospheric contaminants, dusting, oxidation, discoloration, and chemical change. The economics of vacuum drying have become more favorable, especially for drying thick, refractory, high-value species [23] .

Vacuum drying is faster than conventional processes since it is operated under an effective hydraulic conductivity of the material (ease with which water circulates through food material) increases at lower pressure, resulting in a greater internal mass transfer in a vacuum [20] . Vacuum drying in an oxygen-free or oxygen reduced environment, reduces the incidence of chemical discoloration problems caused by the oxidation of certain natural compounds in the produce [20] . Energy consumption of vacuum dryers is, in general, less than that of conventional dryers. However, lower consumption does not necessarily mean lower costs since shorter drying times and drying lower temperatures minimize heat loss from vacuum drying chambers. Since the drying operation is fully enclosed there is no need to introduce and heat fresh air throughout the process, as is the case in conventional dryinq [20] . Vacuum drying technology also offers a highly nutritious instant properties, better flavour, and less hydroscopic final product [24]

Vacuum drying also exhibits its own limitations. Unless the food is packed and maintained at low humidity, the process may not totally inhibit the microorganisms that cause food spoilage, or the toxigenic and infectious organisms that cause food poisoning. In some cases, the end product may crumble easily and lose aroma. Since this process does not change the shape or size of the food item, the finished product requires more space to store than other types of dehydrated food [6] .

3.7. Microwave Drying

The terms dielectric heating and microwave heating are used interchangeably. However, it is generally accepted that dielectric heating is electromagnetic radiation in the frequency range 1 and 100 MHz and microwave heating use the radiation in the range from 300 MHz to 300 GHz. The basic principles of heating and drying by dielectric and microwave frequencies are the same but the generation methods and equipment differs [25] . Microwave heating and drying for industrial purposes is allocated to the range 915 - 2450 MHz. In many industrial heating applications, the frequency is 915 MHz while almost all microwave ovens are at 2450 MHz [25] .

Microwave drying is based on the so called dielectric heating, in which energy is absorbed by ions or molecules that are either induced or permanent dipoles. As the electromagnetic wave travels through the medium the electric and the magnetic field oscillates about zero at each location. The frequency remains unchanged but the wave length is influenced by the medium. The periodic variation of the electric and the magnetic field causes a stress on ions, atoms and molecules, which is converted to heat [25] . When exposed to the microwave field, polar molecules and ions align themselves with the rapidly changing direction of the field, during which heat is generated throughout the material. The energy conversion occurs by two mechanisms, which are ion conduction and dipole rotation. The latter is the most dominant in a large number of materials. Ion conduction designates the phenomena that ions are accelerated by the electric field. When the ions move they collide with other bodies by which kinetic energy is transferred. The collisions are repeated and extended to other bodies, in the field that is changing polarity many millions of times per second.

The electric energy is converted to an ordered kinetic energy which is in turn converted to disordered kinetic energy that can be regarded as heat. Many molecules are dipoles and others become induced dipoles when the molecule is exerted to an electrical field. In a liquid the molecules are normally randomly oriented. However, in the electric field they tend to align with the field-dipole rotation. As the field decays to zero the dipoles returns to the random orientation and are then pulled into the other direction as the field builds up again during each period [25] . With the fluctuations of the field this alignment and relaxation occurs many million times per second. The electric energy is first converted into potential energy which then converts into random kinetic energy (i.e. into heat). In general, polar substances absorbs more energy than nonpolar, however the intensity of vibration depends on the molecular structure, viscosity, temperature and intermolecular bonding. Hence, the energy absorbed depends on the dielectric constant of the material and on a parameter called the “loss tangent”. Four distinct regions can be identified in terms of drying rate versus time: initial warming-up, constant drying rate, and two falling-rate periods [25] . During the first falling-rate period a significant quantity of free water is present and hence the power is selectively absorbed and a high drying-rate prevails. As the drying progresses, the moisture content decreases and the solid phase gradually absorb more and more of the energy. In the second falling-rate period, the total absorbed power is low. Agitation of the material improves the drying rate [25] .

Microwave and dielectric drying possesses potential advantages over the conventional drying method. They are more rapid processes with more uniform heating that can lead to improve quality and higher yield, higher energy efficiency, better and more rapid process control, less floor space and more selective heating [25] . Microwave drying can be used for extremely thermolabile materials with no damage to the product. The microwave drying can be used as a pretreatment, as additional heating during the falling rate period or as a finish drying. Microwave heating can be combined with freeze drying and vacuum drying, even though negative effects of the microwaves at least previously had to be encountered at low pressures. The operational cost of microwave vacuum drying is said to be between that of spray and freeze drying [25] . Microwave irradiation can interact with substances, which are in the liquid, solid and gaseous states. The absorption of microwave energy is observed when polar liquids and solutions are irradiated [26] . This absorption is especially high in the case of water and aqueous solutions. When one irradiates solids with microwaves, the microwaves can be reflected, absorbed, or simply pass through the sample with no absorption occurring. The penetration of microwaves is dependent on the nature of the substance and specifically its dielectric properties [26] .

During the microwave drying process in many cases the material being dried does not absorb or absorb a low level of microwave irradiation but the water molecules associated with it do. Thus the microwave drying process is caused by the property of water to absorb microwave irradiation. Consequently, removing of the water molecules from the drying materials eliminates the heating effect. In such a way the microwave drying appears to be self-regulative process. Finally selective heating of polar molecules of water results in a great time and energy saving [26] . Microwave Drying has a big advantages compared with conventional drying, because in microwave drying, heat is generated by directly transforming the electromagnetic energy into kinetic molecular energy, thus the heat is generated deep within the material to be dried [24] . Microwaves drying offer advantages on dielectric materials since it there is high conversion of microwave energy into thermal energy, independence on thermal conductivity, selectivity, and due to its self-regulation during drying [26] . The main disadvantages of microwave drying are that the cost per energy unit is rather high and the capital cost is high [26] .

3.8. Infrared Radiation Drying

The water molecules in the food evaporate because they absorb infrared energy, which causes them to become thermally excited. The infrared energy penetrates into the sample, rather than having to be conducted and converted inwards from the surface of the sample [27] . Infrared radiation is applied to several dryers because it has advantages of increased drying efficiency and space saving [28] . Decreased drying time, high energy efficiency, high quality product, uniform temperature in the product and reduced necessity for air flow across the product are some of the advantages provided by the use of infrared radiation technology in dehydrating foods [28] . Infrared radiation drying has the unique characteristics of energy transfer mechanism [29] . During Infrared radiation, the energy in the form of electromagnetic wave is absorbed directly by the product without loss to the environment, leading to considerable energy savings.

It is commonly known that infrared radiation is divided into three classes according to the wavelength, i.e., near-infrared radiation (NIR), midinfrared radiation (MIR) and far-infrared radiation (FIR), which have wavelength in the ranges of 0.78 to 1.4, 1.4 to 3 and 3 to 1000 μm, respectively. During infrared radiation the absorbed energy may induce changes in the electronic, vibrational and rotational states of atoms and molecules of a product. The infrared dried product quality is also improved when compared to the case of hot air drying. For example, lower losses of vitamin C, volatile components and flavours, lower loss of β-carotene and better color retention have been reported [29] . Although infrared radiation can accelerate a drying process, foods and agricultural materials, which are heat-sensitive in nature, may be damaged or degraded if radiation intensity, which is the main factor influencing the product temperature, is not properly applied. The drying process should be performed at a lower temperature in order to speed up the drying process without deteriorating the dried product quality [29] .

3.9. Drum Drying (Roller Drying)

Drum drying is a technique where a heated surface is used to provide the energy and aspirators draw the vapor outside the room. This method of drying involves slowly rotating hollow a steel drums that are heated internally by pressurised steam to 120˚C - 170˚C. A thin layer of food is spread uniformly over the outer surface by dipping, by spraying, by spreading or by auxiliary feed rollers. Before the drum has completed 1 rev (within 20 s - 3 min), the dried food is scraped off by a “doctor” blade, which contacts the drum surface uniformly along its length [30] . The capacity of a drum dryer is a function of the drying rate of the thin layer of material and the amount of product that adheres to the drum surface. The drying rate depends on the type of feed device, steam pressure within the drum, and the drum speed. Preheating and pre-concentration of the feed can reduce the drying load. Properties that affect drum adherence are viscosity, surface tension, and wetting power. The four variables that govern the operation of drum dryers are steam pressure, rotational speed, film thickness, and feed material characteristics. The steam pressure, or heating medium temperature, will regulate the drum’s temperature. The rotational speed of the drum determines contact time. The wet material is applied to the drum from below by splash feeders that splash the product onto the drums by rotary blades, by dip feeders where the drum dips into a tank and the concentrated material adheres to the drum, or from below by a pendulum feed pipe. Dryers may have a single drum, or double drums or twin drums. The products dries as the drums rotate and is then scraped off by the knives. The drum drying method is advantageous because it can handle a wide range of products, it has high production rates, and it has low labour requirements. The method is highly applicable to heat-sensitive liquids and pastes, which can be quickly rehydrated from the resulting flakes or powders.

The layer of material on the heated drum is thin and presents no restriction to the vaporizing water. Three stages of heat transfer occur in this thin layer: the first stage consist of heating the thin layer up to its boiling point, the second stage when the water is vaporized and the material gradually changes from a liquid to a solid sate, and a final stage where the temperature of the product approaches that of the drum. Drum drying involves heat transfer from condensing steam through the metal drum to the product layer. The rate of heat transfer depends on the resistance to the removal of water at lower moisture contents and product characteristics [30] . Drum dryers have high drying rates and high-energy efficiencies. The main disadvantages of using the drum drying method is due to the high capital cost of the machined drums, and heat damage to sensitive foods from high drum temperatures [30] . Drums may however be enclosed in a vacuum chamber to dry food at lower temperatures, to improve the sensory and nutritional qualities of dried food. High velocity air may be applied to increase the drying rate or chilled air may be applied to cool the product during drying [30] .

4. Controlling the Quality of Dried Food Products