A Basic Guideto Drying

Fruits and Vegetables

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

University of GuelphOntario, Canada

© October 2011

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. How Do We Dry Mangoes? 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

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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.

Figure 2 shows mangoes on a tree inNorthern Malawi.

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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 leatherytexture 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 ofripeness. 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 how itsmells. Mangoes have a characteristicaroma that may help in assessing thequality of the starting material. In general,any fruit with an uncharacteristic orsuspicious odour should be avoided,since it may be spoiled.

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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 eere theypicked up off the ground as “windfalls”?

Basically, this means that the fruit hasripened 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 of

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cleanliness within the processing facility.

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 free

from 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)

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There are numerous ways to slice amango. The exact procedure will dependon 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. How Do We Dry the Mangoes?

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 themoist mango slices are first placed in the