a basic guide to drying fruits and vegetables

A Basic Guideto Drying

Fruits and Vegetables

Donald G. Mercer, Ph.D., P.Eng.Department of Food Science

University of GuelphOntario, Canada

© September 2012

A Basic Guide to Drying Fruits and Vegetables

Contents

Section Topic

1. Introduction

2. Why Do We Dry Mangoes?

3. How Do We Dry Mangoes?

4. Is Quality of the Mangoes Important? 4.1 Overall Appearance 4.2 Feel 4.3 Aroma 4.4 Source 4.5 Variety 4.6 Taste

5. What Are the Initial Preparation Steps?

6. How Should Mangoes Be Sliced?

7. What Happens in the Drying Process? 7.1 The Drying Mechanism 7.2 Factors Affecting Drying 7.3 Basic Design of a Forced-Air Dryer 7.4 Basic Design of a Solar Dryer 7.5 The Role of Air Temperature 7.6 The Role of Air Velocity 7.7 The Effect of Thickness on Drying 7.8 Case Hardening

8. How Do Drying Methods Compare?

9. What Do We Need to Remember for Mango Drying?

10. How Do We Use Dried Mango Slices?

11. Summary Comments

12. Sources of Information

A Basic Guide to Drying Fruits and Vegetables

Donald G. Mercer, Ph.D., P.Eng.Department of Food Science

University of GuelphOntario, Canada

1. Introduction:

One of the most serious problems facinggrowers of fruits and vegetables is how toprevent these products from spoiling andthereby becoming unfit for consumption. There are various methods ofaccomplishing this, such as canning orfreezing. However, one of the mostsuitable methods of preserving most fruitsand vegetables is through drying toremove most of the water content.

It is not possible to provide details aboutthe drying of a wide variety of fruits orvegetables in a short guide such as this. Therefore, we will use mangoes as aspecific example. While the drying ofvarious products will vary to a certaindegree, the basic principles are the samefor mangoes as they would be for mostother fruits or vegetables.

Mango trees grow naturally in many partsof Sub-Saharan Africa (Figure 1). Theirsweet, fleshy fruit is quite nutritious andtasty. Sadly, when the mangoes ripen,they are often so plentiful that there islittle or no significant commercial value tothem. Since they cannot be sold, themangoes may be left to rot where theyhave fallen from the trees.

Once harvested, there is a limited timeduring which the mangoes may be usedbefore they begin to soften and spoil. There are a few ways in which themangoes may be preserved for later use,

including, for example, mango jams andrelishes.

Figure 1: A mango tree in SouthernMalawi

One very convenient way to increase theuseful life of mangoes is by drying them,therefore ensuring their later availability. With proper storage and handling, driedmango slices can be enjoyed severalmonths after fresh mangoes havedisappeared from the local markets.

It is the purpose of this guide to providesome instruction for the drying ofmangoes. We will attempt to cover theentire process from the selection of thefruit for drying, all the way through to thesteps necessary for preparing driedmangoes for consumption. Althoughmangoes are discussed in detail, theprinciples are applicable to most fruitsand vegetables.

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A Basic Guide to Drying Fruits and Vegetables Page 1.

Figure 2 shows mangoes on a tree inNorthern Malawi.

Figure 2: Mangoes on the tree

2. Why Do We Dry Mangoes?

Mangoes, like most fresh fruits orvegetables, consist mostly of water. Typically, a mango may contain around85% water.

In Figure 3, we see a mango that weighs353 grams. Since 85% of its weight iswater, there will be about 300 grams ofwater and only 53 grams of solid material. Of these solids, some are present in theskin or peel, as well as in the stone or pit. Both the peel and stone also containwater. Of course, neither the peel nor thestone will be eaten and will be discarded.

Figure 3: A mango weighing 353 grams

Figure 4 shows just how much waterthere actually is in a mango. Here, wehave placed 300 grams of water in abeaker. A bit of food colouring has beenadded to make the water more visible inthe photo. When you take time to thinkabout it, it is rather amazing how muchwater is present in the foods we eat.

Figure 4: There are 300 grams of waterin the mango shown in Figure 3

Within the ripe fleshy portion of themango, there is a considerable amount ofnaturally occurring sugar. It is the sugarwhich gives the mango its appealingsweetness. The juice of the mango iscomposed of a combination of sugardissolved in water inside the fleshyportion of the fruit. There are othercompounds present which provide thedistinctive flavour and aroma.

Unfortunately, the juice of the mangoes isalso a convenient source of nutrients formicroorganisms like molds. Once moldsor other microorganisms begin to grow,they will make the flesh of the mangoesunfit for human consumption. Looking atFigure 5, we can see how unappetizing amoldy slice of mango appears. The black

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spots are mold colonies. Some of thesehave developed to the stage where theyhave produced a grey fuzzy covering.

Figure 5: A moldy slice of mango

If we remove most of the water from themangoes, there will no longer be enoughmoisture present to support the growth ofmicroorganisms. As a result, themangoes will not spoil as rapidly as theywould if the water was present. Thismeans that we will have a “shelf-stable”product that can be kept for severalmonths without spoiling. Even thoughmost of the water has been removed, thedried flesh of the mango will retain mostof its nutritional properties.

In addition to making the mangoes lastlonger before they spoil, there are someadditional reasons for drying them. Byremoving most of the water that wasoriginally present, we are left with only thesolids and a small amount of water. Thismakes a very large difference betweenthe weight of the fresh mangoes andthose that have been dried.

A benefit of the weight reduction is thatthe mangoes can be shipped at a lowercost since there is no need to transportlarge amounts of water. For example:100 kg of fresh mangoes can be dried togive about 17 kg of dried mango slices. This means that we can ship only thesolids portion of the mangoes and a smallamount of water, thereby avoiding theexpense of having to transport the addedweight of the 83 kg of water that wasremoved.

Removing the water from mangoescauses the fleshy portion of the fruit toshrink in size. Therefore, the driedmangoes take up much less room tostore or transport than fresh mangoes. Figures 6a and 6b show mango slicesbefore and after drying. Hopefully, youcan see the differences in theirappearance and size.

Figure 6a: Mango slices before drying

Figure 6b: Mango slices after drying

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These mangoes were dried with theirpeels left on as part of a drying study.

An added advantage of increasing theshelf-life of the mangoes throughdehydration is that they can be shippedlonger distances (possibly to foreignmarkets). During the considerable time ittakes to reach far-off destinations, freshmangoes could easily spoil. However,this problem can be avoided if driedmango slices are exported.

Taste and texture are two other things toconsider when comparing fresh and driedmangoes. When the water is removedfrom the mangoes, the sugars that weredissolved in the mango juice remain in thedried mango flesh. This means that thedried mango slices will contain a highconcentration of natural sugars and willhave a very pleasant sweet taste and asomewhat stronger flavour than the fresh“wet” mangoes. Drying also tends tomake the mango flesh take on a leathery texture which makes them a chewysnack.

We will look at how dried mangoes canbe used later in this guide.

3. How Do We Dry Mangoes?

The most effective way to dry any fruits orvegetables (or even meats) is to exposethem to heated air. This will removemoisture and leave behind the solids andmaterials which were previously dissolvedin the water.

Sources of heat can include burningvarious fuels, or other moreenvironmentally friendly methods such assun-drying.

Sun-drying can be done in the open air byspreading the selected material on metalracks or mesh screens. If the air is notalready saturated with moisture, thewarmth of the sun and warm gentlebreezes will remove moisture as the airpasses across the surface of the fruits orvegetables being dried. Sun-drying canbe slowed significantly if the air is humid. This reduces the “drying capacity” of theair, which is the ability of the air to removemoisture from a material.

The sun can also be used as a source ofenergy to heat air and dry materials thatare located inside a cabinet. We refer tothese enclosed devices as “solar dryers”. Although the concept is fairly simple,designing an efficient and effective solardryer is a challenge. A great deal ofresearch has been done to improve thedesign and operation of solar dryers. This has made them a very attractivealternative to dryers that rely on burningfuels (wood, oil, gas, etc.) as a source ofheat.

In the remaining portions of this guide, wewill look at various aspects of mangodrying, with particular emphasis on solardrying.

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4. Is Quality of the Mangoes Important?

The importance of selecting the properstarting materials for drying is oftenoverlooked. Only the best quality fruitavailable should be used for drying. Anyprocessing facility that is drying mangoesshould have a set of standards in place todefine the quality of their ideal startingmaterial. Since this is very subjective, wecannot set out any actual standards here. We can, however, look at some of thebasic attributes which need to beconsidered.

4.1 Overall appearance:The mangoes need to pass a visualinspection before they can even be givenadditional consideration. They should befree from blemishes such as bruises andinsect damage. They may need to be ofa certain size or weight to make handlingeasier. Their colour may assist indetermining the desired degree of ripeness. As mangoes ripen, theirsweetness increases and their texturechanges. Both of these are importantfactors that can be partially assessed bya preliminary visual inspection.

If there is any surface contamination,such as visible mold growth or birddroppings, these mangoes and thosewithin the same group should be seriouslyconsidered for rejection. While birddroppings can be removed by washing, itshould be remembered that fecal mattercarries with it a variety of microorganismsthat can be the cause of potential food-borne illnesses.

Figure 7 shows a mango purchased forsome drying trials. It has a slightlydifferent colour than the one shown inFigure 3, even though it is the same

variety. This is due to the fact that it isslightly more ripe

Figure 7: A mango selected for drying

4.2 Feel:The mangoes being selected may needto have a certain degree of firmness thatwill indicate their level of ripeness. Thiswill help verify the initial visual inspection. Mangoes that are overly ripe, or too soft,are hard to peel and slice. Their texturemay not be suitable for processing. Mangoes that are too firm may not havesufficient ripeness to have developed thecorrect level of sweetness. While theyare easy to handle and may drysatisfactorily, the taste of the final driedproduct may not be as sweet as desired.

Feeling the mangoes may also let youknow how juicy they are. A certain levelof moisture is necessary to provide a highquality mango from the point of view oftexture, flavour, and sweetness.

4.3 Aroma:It is often possible to pick the desiredstarting material be examining its odour. Mangoes have a characteristic aroma thatmay help in assessing the quality of thestarting material. In general, any fruit withan uncharacteristic or suspicious odourshould be avoided, since it may bespoiled.

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4.4 Source:Mangoes for drying and other processingshould be bought from trusted suppliers. The fresh mangoes entering a processingfacility can bring many problems withthem. As a processor, you need to knowthe past history of the mangoes you areusing as your raw materials and willultimately be selling to your customers.

You need to know if chemicals were usedduring the growth period of the mangoes. Were they sprayed with insecticides orpesticides? Were any fertilizers used,and if so, were they appropriate? Whatwere the general conditions under whichthe mangoes were grown? Are there anypotential health risks imposed by thegrowing, harvesting, or storage andhandling conditions? Mangoespurchased in the market may not have agood history of how they were grown,harvested, or handled (see Figure 8).

Figure 8: We have no idea how thesemangoes in the market were handled

Another very important factor is themanner in which the mangoes wereobtained. Were they removed from thetrees in a picking process, or were theypicked up off the ground as “windfalls”?

Basically, “windfalls” means that the fruithas ripened on the tree and has beenknocked down by the wind. In othercases, it may simply be too heavy to besupported any longer by the stem whichconnects it to the tree.

Fruit that has fallen from the tree andbeen picked off the ground often hassurface contamination. The causes ofthis contamination are often from being incontact with droppings of animals such assheep, goats, or cattle that have beengrazing in the areas around the trees. “Windfall” mangoes should be avoidedwhenever possible.

4.5 Variety:Certain varieties of mangoes may havebetter properties for drying than others. Some mangoes are large and juicy with ahigh level of sweetness. Others may besmaller with a higher fibre content andlower sweetness level. There is also theneed to understand which varieties willgive you the best finished product to meetthe needs of your customers. You mayneed to process a number of differentvarieties to satisfy the differentpreferences of the consumers.

In areas with only one variety of mangoavailable, this factor will not be an issue.

4.6 Taste:Since your dried mangoes will end up asa food product, it is a good idea to tastethe starting material. Mangoes which arenot sufficiently sweet or have anundesirable texture, or “mouth-feel”,should not be used. This is a difficultpoint to describe or define. However,some basic descriptions outliningexamples of desirable flavour attributescan go a long way to assuring a higherquality finished product.

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5. What are the Initial Preparation Steps?

Once the mangoes have been selected,the basic preparation steps can begin.

Even if the incoming mangoes have beeninspected at their point of purchase, theyneed to be closely examined as they arecoming into the processing plant facilities. This should be done before they areunloaded from the delivery vehicle. Bydoing this, they can be easily removedfrom the processing facility area if theyare rejected. Had they been unloadedprior to inspection, it would be necessaryto reload them onto the delivery vehiclefor removal from the plant. Preliminaryinspection also reduces the risks ofspreading potential contaminants aroundthe processing plant should any be foundon the incoming mangoes.

All accepted fresh mangoes must bethoroughly washed before entering themain processing area. The outer peel ofthe mangoes can be contaminated withpotentially dangerous microorganismsfrom a variety of sources such as birds,insects, rodents, and grazing animals (inthe case of windfalls). There should beminimal delay between acceptance of theincoming mangoes and the time they gothrough the initial washing phase of theprocess. This will reduce the need tostore incoming fresh mangoes beforethey are washed and help maintain aclean production environment.

Washing must be done with potablewater. “Potable water” is water that is fitfor human consumption. This will preventthe incoming fruit from beingcontaminated further, which would be thecase if impure water was used in thewashing stage.

A rinse with a chlorine solution orpassage through a bath of chlorinesolution would help reduce the microbialpopulation on the surface of themangoes. The easiest way to prepare adisinfecting solution of chlorine is by usinga bleach solution. A household bleachcontainer often has instructions on thelabel for preparing such a solution.

The strengths of chlorine solutions canvary depending on their application andthe concentration of the initial bleachsolution. A mixture of 180 mL of bleachand 4 litres of potable water may besufficient for this application. This wouldneed to be followed by a thorough rinsewith potable water.

There is always a risk of bringingundesirable microorganisms and othercontaminants into a processing facility. Tominimize this risk, there needs to be ameans of ensuring separation betweendifferent areas. Raw material receivingareas must be separated from the rest ofthe processing facility. Workers shouldnot be allowed to travel from one area tothe other without safeguards being inplace to prevent contamination of onearea by materials and contaminants fromanother area. You may want to considerthe receiving area as being “dirty” andhave it totally isolated from the remainderof the processing plant.

Once the mangoes have been washed,they may need to be stored for a shortperiod of time before going on to the nextprocessing step. This area could beconsidered as “clean” storage. Holdingtimes should be minimized. The storagearea must be closely monitored to ensurethat the first mangoes to come into the“clean” storage area are the first ones toleave when additional fresh mangoes are

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brought in.

From the “clean” storage area, thewashed mangoes would go to a peelingoperation. Here, the outer peel of themangoes will be removed by using aparing knife or peeling device.

Because the inner surface of themangoes is now exposed to the workerswho are doing the peeling and othersteps, these workers should be wearingproper attire for the job. This wouldinclude hairnets or other suitable headcovering. Where applicable, beard netsmust also be worn. Coats, smocks, orsimilar uniform along with proper safetyequipment (e.g., proper safety shoes orother footwear, gloves, knife guards, etc.)are absolutely essential. Light-coloureduniforms, especially white, are better thandarker coloured uniforms for hygienicpurposes since stains and spills are moreeasily recognized on the lighter-colouredfabric.

Your fingers are the ten mostcommon causes of infection and

food contamination

Hand-washing stations using warmpotable water and equipped with soapdispensers should be within easy accessof workers. Towels for hand-dryingshould be clean and changed regularly. In addition, there should be adequatewashroom facilities which should bemaintained and cleaned according to aregular schedule. Workers should neverreturn to their work stations withouthaving properly washed their hands.

Workers must wash their hands:

C before and after handling foods oreating

C after using the washroom facilities C af ter sneezing, coughing, or

blowing their nose C after touching a cut or open sore C after being outside or touching any

unclean surfaces

Proper hand-washing procedures include:

C wetting the hands with clean warmwater

C applying soap C rubbing the hands together to

create a lather which is to bespread on the front and back ofthe hands, between the fingers,and under the fingernails.

C lathering should continue for atleast 20 seconds

C once thoroughly lathered, thehands should be rinsed with warmpotable water (i.e., water that issafe for drinking).

C a clean fresh towel should be usedfor drying the hands (paper towelsare ideal for this purpose)

C the towel should be used to turnoff the water faucet and then bediscarded in a proper receptacle

Hand-washing is the most effectiveway to stop the spread of illness

and disease

During peeling, workers will be usingknives or peeling devices to remove theouter peel from the mangoes. Sufficientsafety training must be provided to eachworker to prevent personal injuries and toimpress upon them the importance ofcleanliness within the processing facility.

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WORKER SAFETYIS OUR #1 CONCERN

Workers with cuts or open sores shouldnot be allowed in the production facilitywhere there could be a potential threat tothe cleanliness of the area.

Peels from the mangoes must beremoved from the area within areasonable time period and not beallowed to accumulate. The peels mayprovide a risk of contamination eventhough they have been previouslywashed. Since they are biodegradable,they can be sent to a composting area. The peeled mangoes should then besliced for drying.

Figure 9 shows peeled mangoes awaitingslicing. The slight greenish tinge is due tothe fact that very little flesh was removedwith the peel and that the mango may nothave fully ripened.

Figure 9: Peeled mangoes

The condition of the knives and otherutensils is important for worker safety. Blades should be kept sharp and freefrom rust, dirt, and other sources ofcontamination.

All surfaces should be washed withpotable water and disinfected with achlorine in water solution of suitableconcentration after each production batchor shift. All equipment should bethoroughly cleaned according toappropriate standards which are beyondthe scope of this guide.

6. How Should the Mangoes Be Sliced?

Mango slicing can be difficult due to theslippery nature of the mangoes and thepresence of the large stone within themango itself.

As you can see in Figure 10, mangostones tend to be flat. The ones shown inthe photo were scraped to remove mostof the mango flesh. They were then driedin the sun to remove the surfacemoisture. When dry, you can see thefibres sticking out of the stone. These willnot be as noticeable when the stone iscut away from the mango flesh and it isstill wet.

Figure 10: Mango stones (side view and flat surface view)

There are numerous ways to slice amango. The exact procedure will depend

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on personal preference. If you look at theshape of the mango, it is not generallyround. Instead, it is somewhat flattened. You can place the peeled mango on acutting surface so that the longerdimension is aligned vertically. This willorient the mango so that the stone is inthe up-down position as well.

The mango can then be sliced verticallyfrom the outside until the knife hits theside of the stone. Large flat slices cangenerally be obtained from either side ofthe stone, but it will not be possible to getsuch large slices from the mango flesharound the stone. Here, you will getmuch smaller slices. Procedures on howto slice the mangoes should beestablished to ensure that all mangoesare sliced in a uniform manner.

The slices themselves should be about 5to 6 mm thick (approximately ¼ inch). Thickness is a very important factor inmango drying as we will see later.

The mango slices should be placed in aclean container such as a large metalbowl. The bowl needs to be covered andkept in a cool, clean area until the slicesare required for drying. It is a good ideato avoid slicing the mangoes too far inadvance of when they are needed. In thisway, problems with sanitation can bereduced, and there is less risk of flavourchanges.

7. What Happens in the Drying Process?

7.1 The Drying Mechanism:Before we go too far into the details aboutdrying mangoes, we need to look at howmoisture is removed during a typicaldrying process.

Most products dry because moisture(which is really water) is removed fromtheir surfaces. If we have a slicedmango, we can see that the surface tendsto be wet - especially if the mango is ripeand juicy.

It is important that the air used for dryingpurposes is not overly humid, or saturatedwith moisture. When warm dry air isblown across the surface of the wetmango slices, it picks up some of themoisture by the process of evaporation. Evaporation is the change that occurswhen water goes from being a liquid to avapour. So, the warm air now containswater vapour and carries it out of thedryer and away from the slices ofmangoes.

As more and more air blows across thesurface of a mango slice, moisture frominside the mango slice comes to thesurface to replace the moisture that waslost. We call this process of moisturemoving from the centre of the material tothe outer surface “diffusion”. Moisturethat has diffused to the surface is thenevaporated and swept away by themoving air.

At the start of the drying process, thecombination of a wet surface andadditional moisture coming to the surfacemakes the rate at which the water isremoved quite high. Figure 11 showshow the drying process occurs when the

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moist mango slices are first placed in thedryer. The surface is considered to besaturated with moisture which is likehaving a “pool” of moisture there fromwhich evaporation can take place.

Eventually, a point will be reached wherethe surface no longer looks wet, sincethere is no visible moisture on thesurface. Moisture will still be travellingfrom the centre of the slice to the surface,but it will be removed as soon as it getsthere. As more and more moisture isremoved, the rate of water removal getsslower and slower. In Figure 12, we cansee that there is no pool of moisture onthe surface of the mango slice. Slowdiffusion of moisture to the surface hasbecome the only way in which moisturecan be removed.

There are very few things that we can doto try to speed up the drying process. Weare limited by how fast the moisture candiffuse to the surface. This simply cannotbe rushed. You should try to keep thisdescription of the drying mechanism inmind whenever you are drying anything

As we shall see later, reducing thethickness of the slices is one method todecrease drying times. When thethickness is reduced, the distancemoisture has to travel from the centre ofthe slice to the surface is also reduced.

Figure 11: Moisture removal from mango slices early in the drying process

Note: A key to effective drying is to have the warm air as dry as possible.

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Figure 12: Moisture removal from mango slices late in the drying process

Note: The rate of drying is controlled by diffusion of moisture to the surface afterthe surface moisture pool has been evaporated.

7.2 Factors Affecting Drying:

There are three basic factors which havethe greatest effect on mango drying. They are:

i. Temperature of the drying air ii. Velocity of the drying air iii. Thickness of the mango slices

The first two factors are directly linked tothe design of the dryer. If the dryerhappens to be a solar dryer, then weather conditions will play an important part in allof this. While it is not the purpose of thisguide to teach anyone how to build a fooddryer, we should look at some of thedesign features of dryers to understandhow they function.

7.3 Basic Design of a Forced-AirDryer:

Forced-air dryers are usually designed asa closed cabinet with a fan inside to blowair through the unit. There are manyinteresting designs and arrangements tosuit various special drying needs. Figure13 shows a diagram of how a typicalforced air dryer may look.

Air is brought into the dryer by the fanwhich then blows the air across heatingcoils or through gas flames to warm it. Once the air is heated, it has the ability tohold more moisture than cooler air. Theincreased water-holding capacity of theheated air is a key factor in drying.

The heated air then travels to the dryingchamber of the unit where the material tobe dried is located. In most cases, moistfood products are spread on wire meshracks within the drying chamber.

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Figure 13: Diagram of a typical forced-air dryer

When the heated air passes over themoist surfaces, it picks up water throughthe process of evaporation. The air thencarries the water away from the food andeventually out of the dryer. As the airpicks up moisture, it cools and itsmoisture content increases. This reducesits ability to pick up additional moisture asit continues its path through the rest ofthe dryer.

Freshly heated air is continuously blowninto the dryer, while moisture-rich air isconstantly forced out of the dryer by theincoming air. With sufficient time, themoisture content of the food in the dryerwill have reached its desired target level. However, it must be realized that justbecause the outside surface of the fruitlooks and feels dry, there is still amoisture gradient in the individual piecesof fruit. That is, the outer portion isactually drier than the inner portion. Inaddition, because the air is flowing overthe fruit, the fruit at the front of the dryerwhere the air enters will be dryer than thefruit at the exit where the air leaves. Thisis because the temperature of the air

decreases as it picks up moisture. As theair passes through the dryer, it picks upmore and more moisture, as well ascooling, and may become saturated tothe point where it can pick up no moremoisture. Many processors move theracks of fruit from the exit end of the dryerto the inlet end of the dryer part-waythrough the drying process to help reducethis uneven drying tendency.

When the dried food is removed from thedryer, it must be allowed to cool before itis packed for storage or for sale. This willprovide time for the moisture in the fruit toequilibrate and become uniformthroughout the fruit.

It is possible to control the temperature ofthe air in a forced-air dryer by adjustingthe heater’s temperature controls. Thespeed (or velocity) at which the air movesthrough the dryer can be controlled byadjusting the speed at which the fan isoperating. The velocity of the air must besufficiently high to evaporate the moisturefrom the surface of the food in the dryerand sweep this moisture-rich air out of the

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dryer. Air flows that are too low will nothave the desired effect and the efficiencyof the dryer will be disappointing.

It is usually best to have the air flowingacross the surface of the material ratherthan coming in from the bottom of thedryer and travelling upwards through thematerial being dried. This ensures betterair distribution and better exposure of thefood to the drying air.

Dried product should be allowed to coolafter it has been removed from the dryer. It should never be packaged while stillwarm. Even though it appears to betotally dry, there will still be a trace ofmoisture left inside. Warm products may“sweat” in the package. By this, we meanthat small amounts of moisture may stillleave the dried products and collect onthe inside of the cool packaging materialas tiny droplets of water. These waterdroplets can then support mold growth.

7.4 Basic Design of a Solar Dryer:

Now, let’s take a quick look at solardryers.

Solar dryers can have many differentshapes, sizes, and designs - just asforced-air dryers do. The main differencebetween solar dryers and forced-airdryers is the source of energy for heatingthe air. Rather than using fuel such asnatural gas or other combustible materialsto provide the heat, solar dryers relytotally on heat from the sun.

Some solar dryers may have fans in themto assist in circulating the air, sotechnically they may be considered to beforced-air dryers. However, we usuallyclass them as “solar dryers” basedprimarily on their source of energy ratherthan on how they actually function. Othersolar dryers may not have fans in themand rely on the natural circulation of air.

This circulation is caused by the heatedair rising up from the bottom of the dryerthrough the drying chamber. The air thathas picked up moisture on its waythrough the drying chamber leaves thecabinet by means of an exhaust outlet atthe top of the dryer.

A diagram of a basic solar dryer is shownin Figure 14. The surface of the blackmetal heat collector becomes quite warmwhen exposed to bright sunlight. Thiscauses the air inside the heat collector toheat up and expand. This hot air beginsto rise and travels upwards into the dryingcabinet. Food to be dried is spread onwire mesh racks inside the drying cabinet. As the warm air moves across the surfaceof the food, it picks up moisture.

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Figure 14: Diagram of a typical solar dryer

The warm moist air continues to travelupwards and leave the drying cabinetthrough an opening at the top, which isvery much like a chimney.

While the warm air is moving upwards outof the heat collector, more cool air isdrawn into the heat collector. This air iswarmed up and rises into the dryingcabinet. The natural circulation of the airinto the bottom of the drying cabinet andout through the top will continue as longas there is enough heat to warm theincoming air.

Since sunlight is the only source of heatenergy, air circulation will not proceed ifthere is insufficient sunlight available.

Unfortunately, the rate at which the airflows through the drying cabinet cannotbe controlled without the use of fans. Solar-powered fans are a great additionto a solar dryer. By including a solarpanel and battery, constant air flows canbe maintained through short periods of

cloudiness. You can see the solar paneland battery for a solar dryer in Figure 15.

Figure 15: A solar panel and battery on asolar dryer

Figure 16 shows a solar dryer being usedto dry a number of different products inEquatorial Guinea. There are no fansand the dryer relies totally on naturalcirculation of the air (also called

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“convection”) to dry the food materials. The heat collector is the black metalassembly going from the lower left of thephoto into the bottom of the dryer.

The drying cabinet in Figure 16 has aglass front to allow sunlight to enter. Theheat of the sunlight entering the dryingcabinet also assists in raising thetemperature. Care must be taken in thisregard, since some food products canlose a portion of their nutrients in brightsunlight.

Figure 16: A solar dryer on location inEquatorial Guinea.

Moist air will leave the drying cabinetthrough the white tube located at the topof the drying cabinet.

New developments in technology oftenbring opportunities to improve on previousdesigns. Figure 17 shows how the solardryer unit in Figure 16 was modified oncesolar-powered fans were available toassist in moving the air. The solar-powered fans are the two circular devicesnear the top of the dryer.

Figure 17: A solar dryer with solar-powered fans to circulate the air

The two fans blow the moist air out of thetop of the dryer which draws freshlyheated air from the heat collector into thedrying cabinet. This, in turn, pulls freshcool air into the heat collector where it iswarmed up.

In this new version of the dryer, theoriginal two-piece heat collector has beenreplaced by a larger single heat collector. In full sunlight, the black metal surfacemay reach temperatures over 75EC. Airtravelling through the heat collector andgoing into the drying cabinet can reach50EC or more which is ideal for dryingmaterials such as mangoes.

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Recent improvements in design andavailable equipment have enabledadditional modifications to be made tosolar dyers. One significant technologicaladvance has been in the area of solar-panels, or photo-voltaic cells, which canbe used to generate electricity to powerlarger fans and greatly improve theefficiency of the dryer.

Figure 18 shows an example of a moderndryer with a solar panel mounted on thefront and a large heat collector mountedon top of the drying cabinet. This solarpanel and its battery connection wereseen previously in Figure 15.

Figure 18: A modern solar dryerequipped with a solar panel to power fans

Air enters the heat collector on the right-hand side of the photo (inlet openings arenot visible). It is warmed up as it travelsacross to the left side of the photo. Heated air leaves the heat collector and isdrawn by a set of fans into the dryingcabinet. The fans then blow this heatedair across the surface of the food whichhas been sliced and placed on wire meshracks inside the drying cabinet. This solardryer is capable of providingtemperatures of up to 60EC for drying

fruits and vegetables.

In Figures 17 and 18, the solar dryers aremounted on bases which can be turned tokeep the heat collectors pointing directlyat the sun. This is an important featuresince the sun travels across the sky fromeast to west throughout the course of theday.

Basically, solar drying technology islooking for any modifications that canprovide uniform airflow throughout theentire drying cabinet, and give the desiredtemperatures for drying.

7.5 The Role of Air Temperature:

As stated at the beginning of this section,the temperature of the drying air is one ofthe most important factors for the dryingof a food product. Mangoes are noexception.

In order to show the effects oftemperature on the drying of mangoes, aforced-air dryer was used in thelaboratory. Mangoes were washed,peeled, and cut into 6 mm thick slices forthese tests. The sliced mangoes werethen placed on a rack which wasconnected to a balance on top of thedryer. In this way, the weight of themangoes could be recorded during theentire drying process without opening thedryer.

Three different temperatures were used: 50EC; 55EC; and 60EC. The speed ofthe air was kept constant, since we knowthat it can have an effect on drying andwe did not want to introduce anyadditional factors into this set ofexperiments. Figure 19 shows a graph ofthe weight of the mango slices during the

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Figure 19: The effect of air temperature on the drying of mangoes

time the drying tests were run. There is acurve for each temperature.

From Figure 19, it can be seen that theweight of the mangoes decreases fasterwith the air temperature at 60EC than itdoes for 50EC or 55EC. No actual valueshave been placed on the graph since it isthe general trend that we would like toemphasize here.

For most food products, it is best to usedrying temperatures of 50EC to 55EC. The temperature should never exceed60EC. The main reason for this isbecause higher temperatures can destroynutrients within the food itself. Withoutthese nutrients, the food loses much of itsdietary importance.

Regrettably, many people think that thehotter the air, the better it will be fordrying the food material. This is anincorrect assumption that can lead tosome serious problems, in addition todestroying some of the nutrients

contained within the food. We willdiscuss this in a later section under theheading of “Case Hardening”.

Before leaving our discussion of theeffects of temperature on the rate ofdrying, let’s take a slightly different lookat it. If we have identical weights ofmangoes with identical initial moistures,we can re-draw Figure 19 with themoisture content on the vertical axis asshown in Figure 20.

In Figure 20, a horizontal dashed line hasbeen drawn to indicate a constant wetbasis moisture. At points where this linecrosses the drying curves for eachtemperature, a dashed line has beendropped vertically. The three verticaldashed lines meet the horizontal axis for

1 2 3“time” at points labelled t , t , and t .

1t is the time for the sample dried at 60ECto reach a certain moisture content. It is

2shorter than t which is the drying time at

355EC. t is the time taken for drying at

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Figure 20: Times required to reach equal moisture contents at three drying temperatures

50EC, and is the longest time of the three. This clearly shows the impact oftemperature on the drying of the mangoslices.

7.6 The Role of Air Velocity:

In addition to the temperature of the airused for drying, the velocity, or speed, atwhich the air is travelling is also animportant factor in drying.

When there is no movement of air acrossthe surface of the mango slices, air at thesurface becomes saturated with water. As a result, water on the surface cannotbe removed. If this saturated air isdisplaced by fresh unsaturated air, theunsaturated air can pick up more of themoisture from the mango surface. If theair is kept moving, the drying process willcontinue at a satisfactory pace. In dryingterminology, a layer of air clinging to thesurface of a material and preventingefficient moisture removal is called a

“stagnant boundary layer”. It is notmoving, so it is stagnant. Since it isbetween the surface of the material andthe outside air, it is considered to be aboundary layer.

In drying studies, we are often concernedwith how fast the air must be moving tosweep away the stagnant boundary layer. Tests can be done using different airspeeds to determine the effects of airspeed on the rate of drying.

Figure 21 gives us a look at the effects ofair velocity on the drying of mango slicesin a laboratory dryer. The air temperatureused for drying the mango slices in Figure21 has been kept constant at 50EC.

An air speed of 0.2 metres per secondallows the mangoes to dry reasonablywell. However, when the speed isincreased to 0.5 m/s, there is a noticeableimprovement in how fast the dryingoccurs. This is because the faster air flow

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is better at sweeping away the stagnantboundary layer of air around the moistmango slices. The air at the surface thathas picked up moisture is beingcontinuously replaced by freshunsaturated air which is very efficient atremoving moisture.

Figure 21: The effect of air velocity on the drying of mangoes (at 50EC)

7.7 The Effect of Thickness on Drying

In Figures 11 and 12, we showed howmoisture travels from the centre of themango slices to the surface where it isremoved by the air passing through thedryer.

The important thing to recognize here isthat as the slice of mango gets thicker,the distance from the centre of the slice tothe surface increases. When thedistance increases, the time it takes forthe water to travel from the centre to thesurface also increases. This means thatit will take longer for the product to dry.

Figure 22 shows the effects that thethickness has on the drying of appleslices. This graph has been includedsince similar tests have not been done inour lab for mangoes. However, the sametrends as shown for apples have beenobserved for other fruits and vegetablesstudied in this manner.

As can be seen, the 0.8 cm thick slicesdry more slowly than the thinner slices.This is because water at the centre of theslice has further to diffuse and reach thesurface in the thicker slices than it does inthe thinner slices.

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1t is the time required for 0.4 cm thickslices of mangoes to reach a certainmoisture content. It is noticeably shorterthan the times taken to dry 0.6 cm thick

2 3slices (t ), and 0.8 cm thick slices (t ), tothe same moisture content.

In most mango drying work, a thicknessof 0.5 to 0.6 cm (approximately ¼ inch) isused. It is a convenient thickness tovisualize when slicing the mangoes, andit makes the slices easy to handle withouttearing or ripping them. They are alsothin enough to dry reasonably quickly andthe final dried product is not excessivelythick.

Figure 22: The effect of thickness on the drying of apple slices

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7.8 Case Hardening

Care must be taken in the wayinformation is interpreted when dealingwith food drying.

In Figure 19, we saw the effects oftemperature on the drying of mangoslices. However, this graph can bemisleading if the overall effects oftemperature are not fully understood.

Some dryer operators may look at graphslike that shown in Figure 19 andconcluded that if 60EC is a fasttemperature at which to dry food quickly,then 80EC will be faster and better, whichin turn means that they can dry moreproduct in a day. When they try to dryfoods like mangoes at 80EC and find thatthe drying actually takes longer, they areleft wondering what went wrong.

This is where we also need to recall howthe drying process takes place in amango slice. You may recall fromFigures 11 and 12 that moisture isremoved from the surface of the mangoslice first. Moisture that is still inside themango slice then needs to diffuse to thesurface where it will eventually beremoved by the warm air passing throughthe dryer.

As moisture is removed from fruits orvegetables, their cellular structure beginsto collapse and the cells become smaller. The result of this is the shrinking of theproduct as it dries.

At 80EC, moisture is removed quiterapidly from the surface of the mangoes. It may be removed so fast that the hot airstarts to make a hard thick layer on theoutside of the mango slices due to thecollapsing of cells at the surface. This

layer is much like a skin that can trapmoisture inside the mango slices. Itseverely reduces the efficiency of thedrying process and may prevent thistrapped moisture from leaving the mangoslices. We call the development of theskin-like shell of hard material on theoutside of the product “case hardening”.

Once this outer shell has formed, there islittle that can be done to dry the productin a controlled manner. An additionalproblem is that many dryer operators lookat the case hardened mango slices andthink that they are dry. When they feelthem and bend them, the slices actuallyappear dry enough to remove from thedryer and be packaged for sale.

After packaging, moisture at the centre ofthe mango slices may begin to travel veryslowly through the thick layer of collapsedcells at the surface of the slice. It thengoes into the air inside the packagewhere it condenses on the coolerpackaging surfaces to form waterdroplets. These water droplets are nowavailable to support mold growth on themango slices. Of course, any moldgrowth will make the product un-saleableand unfit for eating.

Figures 23 and 24 illustrate how casehardening occurs.

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Figure 23: Drying of mango slices with warm air

Figure 24: Excessively hot air being used to dry mango slices.

Note the dry outer layer around the slice and its shrunken size

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8. How Do Drying Methods Compare?

There are numerous ways of dryingmangoes and other fruits or vegetableswith similar moisture contents.

In some areas, mango slices may simplybe put out in the sun to dry in the open airas shown in Figure 25.

Figure 25: Open-air drying of mango slices

This may be satisfactory in cases wherethe mangoes are for the personal use ofwhomever is drying them. However, it isdefinitely not appropriate from a foodsafety point-of-view, especially if you wantto sell the dried product.

In the following photographs, we can seesome of the problems associated with theopen-air drying of mango slices.

Dirt can easily be blown onto the surfaceof the mangoes (Figure 26).

Figure 26: Dirt on a mango slice drying in the open air

In other cases, there can be unwantedvisitors landing on the food. This wouldinclude insects like wasps (Figure 27) orhouseflies (Figure 28).

Figure 27: Wasp on a mango slice

Figure 28: Housefly on a mango slice

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Undesirable pests also include crawlinginsects such as ants (Figure 29), roaches,beetles, and others.

Figure 29: Ant on a mango slice

Drying products inside a cabinet such asa solar dryer or a forced-air dryer protectsthe product from many of these problems. It also protects the product fromcontaminants in the air and the highlyundesirable effects of birds and theirdroppings.

In addition, drying with an actual dryergives more control over the dryingprocess than can be obtained with open-air drying. Unfortunately, solar drying islimited to sunny weather with sufficientheat and low humidity to drive the dryingprocess. Forced-air drying allows for ahigher degree of controlled drying andpermits around-the-clock drying which isnot limited by the amount of availablesunlight. Temperatures and air velocitiesare among the controllable factors withforced-air drying.

Whatever drying method is selected, it isimportant to understand the dryingprocess and to take every precaution tomake a product that is of the highestpossible quality and that is safe forhuman consumption.

9. What Do We Need to Remember forMango Drying?

When drying any fruits or vegetables it isimportant to prepare a standardized set ofprocedures so that everyone operatingthe equipment will be using the sameapproach and methods. Failure to do thiscan result in non-uniform products frombatch to batch and from one processoperator to another.

All personnel must be thoroughly trainedin the operation of the process. Thisincludes the actual drying of a product, aswell as maintenance and cleaning of theequipment.

One very important aspect that is oftenoverlooked is familiarization of workerswith “cause and effect” relationships. Basically, this means that everyoneworking on the process must know theconsequences of any actions taken toadjust the process, or the effects that anychanges to the process may have on thefinal product.

As an example, operators mustunderstand that thicker slices of productwill take longer to dry than thinner slicesdue to diffusional limitations within theproduct. They also must understand thatthey cannot simply increase thetemperature of the air going into the dryerto compensate for the thicker slices. Ifthey do take this action, they will end upwith a thickly case hardened product thatwill not be acceptable for sale, and willultimately spoil during storage due to theentrapped water.

Once a detailed set of operatinginstructions has been developed and theoperators have been thoroughly trained,there is still a need to monitor their

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performance. This can generally be doneby regularly testing products throughquality control checks and periodicrefresher training courses for the processoperators.

Every aspect of the process must bemonitored. Accurate records need to bekept and retained for future reference. Each batch of product will need a uniquereference or “lot” number to identify it incase problems are identified at a laterdate. This lot number must appear onpackages of finished product that areoffered for sale and must match the lotnumbers on the records. In this way, itwill be possible to match actual productwith records taken during the time it wasproduced. “Traceability”, as it is called, isessential in any commercial processingoperation.

To keep accurate records, it is necessaryto design data sheets that operators canuse to collect information during theprocessing of each product batch. Allcritical operating parameters must berecorded. These would include suchthings as the temperature of the airentering the dryer, the temperature insidethe dryer, and a host of otherobservations.

Training manuals and “trouble-shooting”guides are also important resources in aproduction environment. If a situationpresents itself during a production runand the operator is unsure of how to copewith it, a trouble-shooting manual canoffer some assistance. It could take theform of a list of problems which might beencountered, accompanied by a numberof potential remedial actions for dealingwith them.

There will be a need to have standardized

assessments of the acceptability of rawmaterials. What constitutes a “quality”mango versus an unacceptable mangothat should be rejected rather than used? Providing a quality check-list will go a longway to ensuring that sub-standard rawmaterials are not used for dryingpurposes.

Overall, high standards for all aspects ofproduction, packaging, storage, anddistribution must never be compromised. Failure to follow procedures at any stage of the processing chain must never betolerated.

10. How Do We Use Dried MangoSlices?

Whatever fruits or vegetables are dried,their characteristics will be substantiallychanged compared to their undriedstarting form.

Let’s consider tomatoes as an example.Fresh tomatoes contain up to 95%moisture by weight which makes themquite soft and juicy. Dried tomatoescontain only 10% moisture, and tend tobe dark in colour and somewhat leathery. As a result, dried tomatoes cannot beused in fresh salads or other similarapplications which require these attributesof fresh tomatoes. However, driedtomatoes are ideal for soups or stewswhere they can be heated in water torehydrate them and increase theirmoisture content. Since moisture isneeded to rehydrate the dried tomatoes,it is necessary to add additional water tothe soup or stew to compensate for therequired extra moisture.

Teaching consumers the way in which touse dried products can be a challenge.

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Providing instructions in the form ofsample recipes for such things aschutneys can go a long way towards thisgoal. Recipes can be incorporated intothe packaging or labels on the finishedproduct. Demonstrations at a local levelor instructions given in newspapers andradio announcements can be effective infamiliarizing consumers with how to usedried products, as well.

When providing instructions on the use ofdried fruits or vegetables, do not forgetthat they may be consumed in their driedform as snacks. Consumers may not befamiliar with this application, so they mayneed to be urged to expand their eatinghabits to discover the appeal of suchthings as dried mangoes, which aresweet, chewy snacks.

Since dried fruits have most of the waterremoved, the natural sugars which theycontain become concentrated after theyhave been dried. This means that 100grams of dried fruit such as mangoes willcontain more sugar than 100 grams offresh mango slices. This also means that100 grams of dried mangoes will containmore calories than 100 grams of freshmangoes. Consumers need to be madeaware of this fact in such a way that theydo not view it as a negative attribute ofdried mangoes. Dried mangoes are alsoless filling than the equivalent weight offresh mangoes so their intake needs tobe monitored. For this reason, theinclusion of recommended serving sizeson package labels would be appropriate.

11. Summary Comments

We sincerely hope that this brief guidewill provide assistance to drying fruits andvegetables. Drying is an extremelyversatile approach to preserving foodproducts.

Please keep in mind that whateverproducts are being dried, it is important tounderstand every aspect of the processfrom the selection of raw materials rightthrough to distribution of the finishedproduct to the consumer.

12. Sources of Information

The following two book chapters areavailable on-line:

Donald G. Mercer, “Solar Drying inDeveloping Nations: Possibilities andPitfalls”, Chapter 4, in “Using FoodScience and Technology to ImproveNutrition and Promote NationalDevelopment”, G.L. Robertson and J.R.Lupien, editors. Published by TheInternational Union of Food Science andTechnology, 2008. ISBN 978-0-9810247-0-7

Donald G. Mercer and Robert Myhara, “Improving the Operation of a CommercialMango Dryer”, Chapter 6, in “Using FoodScience and Technology to ImproveNutrition and Promote NationalDevelopment”, G.L. Robertson and J.R.Lupien, editors. Published by TheInternational Union of Food Science andTechnology, 2008. ISBN 978-0-9810247-0-7

In addition, there are numerous otherinternet resources available on the topicof food dehydration and drying.

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a basic guide to drying fruits and vegetables

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