1-s2.0-s0924224497000071-mvsvain

This paper gives an update of recent developments in thedry conservation of foodstu�s, focusing on fruits and vege-tables. Technological progress in microwave and radio fre-quency assisted drying processes will be presented. Thee�ects of dehydration on product quality will be discussed.As these technological developments are driven byconsumer demand for healthy, fresh-like and convenientfood, the e�ects of dehydration will be discussed. Increas-ingly, the link between the process and product quality canbe elucidated by quantifying the mobility of complex foodmatrices. The role of the glass transition temperature on thequality and shelf-life of dehydrated foods will be described.# 1998 Elsevier Science Ltd. All rights reserved.

Consumer demand has increased for processed productsthat keep more of their original characteristics. Inindustrial terms, this requires the development ofoperations that minimize the adverse e�ects of proces-sing. In the particular case of food drying this indicatesloss of volatiles and ¯avours, changes in colour andtexture, and a decrease in nutritional value. Further-more, residual enzyme activity and microbial activity indried foods are essential parameters that a�ect productquality and shelf-life. Along with these interests therehydration properties of dehydrated foodstu�s are of

crucial importance. In order to optimize the quality ofdehydrated fruits, vegetables and aromatic herbs, novelor improved dry conservation procedures should bedeveloped.

Mass production of dried foods is often accomplishedthrough the use of convective dryers. This drying pro-cess su�ers from quality losses regarding colour, ¯avour(taste and aroma), and texture, while rehydration isoften poor. Case hardening (the formation of a hardouter shell) and shrinkage are the main problems. Inrecent years, improvement of quality retention by driedproducts (rehydratability, etc.), by altering process con-ditions and/or pretreatments, has been a major researchgoal [1].

Freeze-drying is far more expensive than convectivedrying, and is used for the production of a minoramount of high-value arable and horticultural produce.However, it is well known that freeze drying producesthe highest-quality dried food product. This is largelybecause the structure of the food is not severelydamaged as in other preservation procedures. There-fore, technological progress in freeze-drying will con-cern two major ®elds:

. the reduction of running costs by increasing theprocess e�ciency (reduction of the freeze-dryingtime, optimization of the operating parameters)while maintaining the quality of the ®nal product[2];

. the processing of new products that require a high®nal quality.

Electromagnetic energy (EME)-based drying pro-cesses may produce materials at acceptable expense,which more readily rehydrate than hot air driedproducts due to a less degraded (collapsed) structure [3].High drying rates could be established (an example isgiven in Fig. 1).

The quality of dehydrated foods is dependent in parton changes occurring during processing and storage.Some of these changes involve modi®cation of thephysical structure. These modi®cations a�ect texture,rehydratability and appearance. Other changes are dueto chemical reactions, but these are also a�ected by phy-sical structure, primarily due to e�ects on di�usivities ofreactants and of reaction products. The elucidation ofthe time dependence of mobility changes induced by

0924-2244/98/$19.00 Copyright # 1998 Elsevier Science Ltd. All rights reservedPI I : S0924-2244 (97 )00007-1

Approaches toimproving the

quality of driedfruit and vegetables

H.H. Nijhuis, H.M. Torringa,S. Muresan, D. Yuksel, C.

Leguijt and W. KloekAgrotechnological Research Institute (ATO-DLO),

PO Box 17, NL 6700 AA Wageningen,The Netherlands (fax: +31-317-475347;

e-mail: [email protected])

Review

Trends in Food Science & Technology 9 (1998) 13±20

glass transition seems to be a promising route to optimizeand control dehydration processes [4].

This paper gives an overview of recent developmentsin the dehydration of fruits and vegetables, basicallyfocusing on three aspects:

. technological developmentsÐmicrowave andradio-frequency drying;

. food matrix mobility and its relation with productqualityÐglass transition temperature;

. the e�ect of dehydration on product qualityÐ¯a-vour and sensorial properties.

Radio frequency dryingTheoretical aspects

The principle of radio-frequency (RF) dielectric heat-ing relies on the fact that heat is generated in mostdielectric materials when a high-frequency electric ®eldis applied across them. In many cases, this heat is simplydue to a resistance to the ¯ow of high-frequency electriccurrent through the material, and is a similar mechan-ism to that which causes electric heating elements toheat up when they are connected to the 50-Hz mainssupply. In other cases, the heat arises from a resonantabsorption mechanism whereby the frequency of theapplied electric ®eld is such that the electric dipoles,within the material have di�culty in reorienting them-selves with respect to the continually reversing electric®eld.

Whether the heating mechanism is due to electricalresistance or due to the reorientation of dipoles (or acombination of both), dielectric losses within the materialcan be characterized through an e�ective dielectric lossfactor, "00r .

In RF dryers, as in microwave units, direct electricalcontacts do not have to be made to the products. RFapplicators can be extremely simple; the most basicdesign consists of two parallel metal plates or electro-des, with the product being placed between them.

State of the artRF energy has been used in the processing of foods

for many years. The best known application in this sec-tor of industry is for the post-baking of biscuits, crack-ers and cookies, where it is used as a ®nal dryer after aconventional, normally gas-®red, baking oven [5]. Itsuse in the process industries is based on its ability toheat water irrespective of where within the productbody this water is held. In the case of post-baking theaction of the convective and radiant heat transfer in themain oven is to dry the outer layers and leave theremaining water as a concentrated band in the middle,which RF energy can deal with very e�ectively [6]. Theindustrial use of RF energy for drying sheets, webs andthick materials is well established and present e�orts aredirected to ®nding new applications and developing newequipment. The application of RF energy is promisingbecause of the potential for automatic moisture levellingas a result of higher power absorption in more moistregions and the acting as a booster for hot-air con-vective drying processes. RF heating can promote mass¯ow within the product, preventing case hardening andenabling hot-air drying to operate more e�ectively thanit otherwise would.

In these types of operations the product has alreadybeen through a heating process which has established adryer outer layer, a desirable feature for biscuits andcrackers but, in the case of most other foodstu�s,exactly what needs to be avoided. If RF is used from thestart of a drying process, case hardening will not occur,as has been demonstrated in the processing of certaintypes of industrial boards, impregnated webs and food-stu�s [7]. In some circumstances the e�ect of using theseso called volumetric heating techniques is to create awater ®lm on the surface, driven there by the establish-ment of a pressure di�erence. When this happens andthe water contains ionic material, as it does in the caseof foodstu�s, a conducting path can be establishedduring RF heating, leading to the possibility of elec-trical surface conduction and product damage. It istherefore important that the surface ®lm be dealt with ina suitable manner. A good example is the pre-drying offrench fries with a combination of hot-air and RFenergy. Dielectric heating is applied to obtain a homo-genous moisture distribution within the product andavoidance of case hardening can be achieved by balancingthe energy ¯ows of surface and volumetric heating [8].To understand the problem and the potential solutions itis necessary to appreciate the principles of RF and energytransfer mechanisms that occur at these frequencies.

Fig. 1. The moisture content of carrots during microwave and hotair drying [3]. *, Microwave with speci®c power input of 5 W/g;&, hot-air drying, air temperature 80�C; moisture content on drymatter basis.

14 H.H. Nijhuis et al./Trends in Food Science & Technology 9 (1998) 13±20

Technological progressAs has been indicated above, RF heating by itself will

not meet the quality demanded. It will therefore benecessary to combine it with some means of ensuringthe surface moisture is kept to a level that prevents theestablishment of a conducting path yet at the same timedoes not cause case hardening and product degradation.The obvious ®rst approach is to pass air over the product.This will have two e�ects: to keep the surface `dry' andto act as a means of transporting the moisture fromthe drying bed. However, experience has shown that thebalance between air `drying' as a means of reducing thesurface moisture content and producing an acceptableproduct is very di�cult to achieve.

The use of humidity-controlled heated air ¯ows inRF-drying is part of a European research program [9].

Microwave dryingTheoretical aspects

Microwaves are de®ned as electromagnetic waves inthe frequency range 300MHz±300GHz. There are twonarrow bands allocated for use in the microwave range,915MHz and 2450MHz.

Microwave energy is not a form of heat. Heat is asecondary e�ect of an electromagnetic ®eld interactingwith matter, such as food. The microwave ®eld changesdirection millions of times per second in microwaveovens. The conversion of microwave energy into heat isexplained by basically two phenomena:

. Molecules, with a permanent dipolar moment,rotate in the rapidly changing electric ®eld. Whenmolecules rotate in a ®eld that changes polarity ata frequency of many millions of times per second,heat is evolved because of friction forces betweenthe molecules.

. Charge drift under the action of the ®eld (ionicconduction). When the ions drift, due to the elec-tric ®eld, they collide with other molecules in abilliard ball fashion and heat is evolved because offriction.

Water molecules are polar, i.e. the centre of charge isdisplaced. This means that they can rotate under thein¯uence of an alternating electrical ®eld. Foodstu�susually contain 50±97% water. Thus food is very wellsuited for heating and drying with microwave energy. Inhot air drying, the product surface becomes dry ®rst andthe dried food layer is a poor conductor of heatthroughout the dehydration process. Microwaves, how-ever, are able to penetrate a dry surface layer and heatthe food throughout all high-moisture regions. Thispromotes mass transport and increases drying rate.

State of the artToday microwave drying is used mainly for drying of

pasta and post-baking of biscuits. However, severalother industrial microwave drying applications exist(Table 1). Microwave drying of fruits and vegetables ishardly carried out at an industrial scale. However, atthe research level a number of successful microwavedrying attempts have been conducted. In the dehydra-tion of sliced bananas and mashed bananas, which arefoamed by blending with a foaming agent, drying ratescould be increased by a factor of 16 using microwaveenergy instead of hot air alone [11]. A similar increase inthe drying rate was observed in the industrial produc-tion of raisins by sun drying, whereas chemical pre-treatment decreases skin resistance to di�usion andmicrowave pre-drying creates a partial pu�ng of thestructure, in this way enhancing the internal moisturedi�usion during sun drying [12, 13]. Microwave drying

Table 1. Outline of microwave drying applications throughout the world [10]

Product Process details Scale CountryBacon Pre-cooking I USAMeat balls Under vacuum, with hot air P GermanyChicken parts With air P JapanEgg yolk With infrared I JapanVegetables With air, under vacuum I UK and JapanOnions With hot air I USAPotato slices With air I Germany and UKFrench fries Pre-drying P NetherlandsPotato chips Followed by frying I No longer usedPasta With hot air I USA and ItalyBread crumbs Bake, dry, grind I USARice With hot air I UK, USAPet-food Preceded by extrusion I UKSnacks Pu�ng I USAGrain, peanuts Under vacuum P USA, CanadaCocoa, co�ee Roasting I USABiscuits Post-baking I Worldwide

I=industrial, P=pilot-plant

H.H. Nijhuis et al./Trends in Food Science & Technology 9 (1998) 13±20 15

of mushrooms at low power density in combinationwith heated air resulted in improved moisture di�usity,better rehydration properties and improved ¯avourretention [14].

High drying rates could also be established withcombined vacuum-microwave drying, e.g. in the dryconservation of parsley [15]. This process may be opti-mized using pulsed microwaves. The re-distribution ofmoisture by di�usion during the power o� time resultsin an e�cient use of energy. For cranberries, micro-wave-vacuum drying improved the color quality andresulted in a softer product than the conventionallydried product [16]. Pre-treatment with fructose corn-syrup retarded enzymatic browning and reduced thetartness of the cranberries.

The advantages of using microwave energy in thedrying of carrots were described by Torringa et al. [3]The more homogeneous dehydration and the developedinternal pressure with microwaves results in less reduc-tion in the volume of the microwave dried product(Fig. 2). Also the volume of air within the product ishigher in the microwave-treated carrot than in air driedproducts with the same bulk volume.

Technological progressThe drawbacks of microwave heating in general are:

. The problem of uneven heating. Too high tem-perature in the edges and corners of products maylead to overheating and irreversible drying out.The customer will experience this as low quality ofthe product [17].

. A way of controlling the mass transport by con-trolling the power input that is needed, as toorapid mass transport may cause damage to thefood texture by `pu�ng'.

. High start-up costs have prevented many potentialusers of microwave energy from investing in thenew technology.

. Previously, operational costs were considerable,with expensive magnetrons needing to be replacedtoo frequently [18].

Knowledge about heating uniformity in microwaveovens is increasing. Controlling heating uniformity isimportant to ensure the microbial safety and high qual-ity of microwave dried foods [18]. There is a lack ofgeneral models for predicting the heating pattern, andmoisture and vapour distribution during the dryingprocedure based on the dielectric properties, water dis-tribution, density, food composition (e.g. salt content)and ®eld and oven geometry. In addition, reliable dataon products (e.g. moisture and vapour di�usivities) arestill missing. Another problem is the dependency of thedielectric loss factor on the moisture content changingthe rate of energy absorption in the course of the dryingprocess, which requires the power to be adjustedthroughout the process. Therefore, the calculation ofthe power dissipation within the product from micro-wave ®eld distribution and calculation of the resultingheat and mass transport are problematic due to thechanging product properties during the process [19, 20].Also more knowledge is needed about the in¯uence ofgeometry, the size of the object being dried and phenom-ena like shrinkage, pu�ng and stress-cracking. Ensuringoptimum design and performance in combined andconventional heating ovens is a major challenge forresearchers in this area.

Inter-relationship between matrix mobility andquality parameters

Fresh foods can be regarded as a matrix consisting ofcarbohydrates, proteins, fats, water and componentsdissolved in water. In fresh foods, the molecular mobi-lity of compounds within the water phase is high, andtherefore they are sensitive to chemical, enzymatic,microbial and physical deterioration. Drying of freshfoods leads to a reduced water content, and therefore aconcentration of dissolved components such as sugarsdue to sublimation or evaporation of water, dependingon the drying method. During the concentration, theamorphous components of the concentrated solutionwill turn into a glass when the maximum concentrationis reached. If the matrix is cooled su�ciently fast, and/or crystallization inhibitors are used, components thatwould naturally crystallize may also remain in theamorphous, glassy state. A glass can be characterized byits phase transition, which occurs in a range of tem-peratures, between the so-called onset and the endset. Inorder to use a single value, it is common to use theaverage between the onset and endset, as measured in a

Fig. 2. Progress in bulk density of carrots for di�erent drying pro-cesses [3]. *, Microwave with speci®c power input of 5 W/g; &,


di�erential scanning calorimetry (DSC) scan, as theglass transition temperature (Tg). This second-ordertransition is accompanied by an increased molecularmobility and a drastic drop of the elastic modulus andcan be measured by means of DSC (as a change in heatcapacity), dynamic mechanical spectroscopy (as achange in elastic modulus) or magnetic resonance spec-troscopy (as a change in molecular mobility). A typicalstate diagram (Fig. 3) of sugar±water mixtures (as amodel for the water phase of fruits), shows that the glasstransition temperature decreases with increasing watercontent: water acts as a plasticizer/softener for sugars.Drying of foods is a way to increase the glass transitiontemperature. Sugars in their turn can act as a plasticizerfor higher molecular weight carbohydrates. The glasstransition temperatures of anhydrous carbohydratesincrease with increasing molecular weight, which pro-vides a tool for manipulating the glass transition tem-perature of food products, i.e. increasing it by addinghigh-molecular weight carbohydrates [21].

If a glass of concentration equal to or higher than Cg0

is stored above Tg0, several collapse phenomena can

occur, as indicated in Fig. 3. It can become sticky [23]and will undergo structural transformations such ascollapse of the physical structure, and crystallization,which results in a release of encapsulated compoundssuch as ¯avours [22]. The collapse will result in adecrease of the porosity, and shrinkage of the product,which can have an important e�ect on the rehydrationproperties of the dried product.

Water activity remains an important concept in driedproducts, particularly for the analysis of productstability in relation to packaging and storage condi-tions. The water activity (aw) is de®ned as the vapourpressure in the food p divided by the vapour pressure ofpure water (p0) at the same temperature. Sorption iso-therms describe the relation between the water activityand the moisture content, and are normally determinedby storing dried samples over saturated salt solutions

and measuring the steady-state water content. There canbe considerable hysteresis between the adsorption iso-therm and desorption isotherm which is mainly due tothe non-existence of equilibrium of the vapour pressureinside a food and its surroundings.

By combining information from sorption isotherms(water activity versus moisture content) and state dia-grams (glass transition temperature versus moisturecontent/sugar content) it is possible to construct foodstability maps. These maps are useful in determiningstorage conditions for foods at which glass transitionsare avoided or in determining how far a product shouldbe dried in order to keep it stable under given storageconditions. One should keep in mind that the glassystate is a meta-stable state and that therefore the physi-cal properties and the composition of the glassy matrixdepend on the way the glass was obtained [24]. Gen-erally, fast cooling or heating minimize crystallizationand lead to a matrix with minimal water mobility andthus higher stability. The faster drying achieved withEME methods will certainly lead to a di�erent glassymatrix compared with convective air drying and thiscould correspond to better stability of the matrix.However, there are no data available on matrix stabilityin relation to drying methods and this would deservefuture research.

Sensory qualityThere is often a decrease in the quality of the dried

products because most conventional techniques usehigh temperatures during the drying process. Processingmay also introduce undesirable changes in appearanceand will cause modi®cation of the natural `balanced'¯avour and colour. This is not in agreement with theincreasing demand of consumers for the highest-quality®nished products. Therefore, dehydration technologiesshould be focusing on the production of dried productswith little or no loss in their sensory characteristicstogether with the advantages of added convenience.

E�ects of dehydration on ¯avourThe properties of dried vegetables are in¯uenced by

chemical and physical changes. Chemical changesmainly a�ect sensory properties such as colour, tasteand aroma, whereas physical changes mainly in¯uencethe handling properties such as swelling capacity andcooking time. Heat treatment of fruits and vegetablesoften reduces the number of original volatile ¯avourcompounds, while introducing additional volatile ¯avourcompounds through the autoxidation of unsaturatedfatty acids and thermal decomposition, and/or initiationof Maillard reactions. As an example, the change inaroma pro®le of mushrooms due to hot-air drying ispresented in Fig. 4.

The loss of volatile compounds during drying hasbeen extensively studied for both spray drying and

Fig. 3. Schematic state diagrams of sugar±water mixtures. Tg andTm indicate the glass transition temperature and the ice meltingtemperature, respectively. Cg

0 is the composition of the glass atconditions of maximum ice formation [22].


freeze drying, in both model systems and in a variety offoods. After drying, volatile compounds are partiallyretained in the dried product. Two major theories havebeen suggested for this phenomenon: the selective di�u-sion theory of Thijssen and Rulkens [26], and the for-mation of micro-regions as proposed by Flink andKarel [27]. The ®rst concept is based on the fact that thedi�usion coe�cient of water decreases to a much lesserextent than the di�usion coe�cients of other substances,as water content decreases. The second theory includesthe physical encapsulation of volatile compounds bymacromolecules upon the formation of micro-regionsduring the drying process. Several factors such as theinitial content of solids, type of solids, sample thickness,sample matrix, initial volatile concentration and dryingrate are considered to control the retention of volatiles[28].

The loss of volatile ¯avour compounds after dehy-dration may be due to the inactivation of volatile-forming enzymes as well as loss of the precursors. Thealliinase activity of fresh onions decreased by up to 90%after hot-air drying, while freeze-dried onions stillshowed 45% activity [29].

With respect to the introduction of new compoundsduring dehydration, two major chemical reactions areimportant: autoxidation and the Maillard reaction. The¯avour characteristics of food products containingunsaturated fatty acids can be drastically a�ected whenthese lipids are in direct contact with oxygen. Initiatorssuch as light, metallic ions and heat can initiate auto-xidation reactions. Karel and Yong [30] summarized the

e�ects of the water content on lipid oxidation. Theysuggested that the water content is the major factorcontrolling lipid oxidation in dehydrated foods, but itse�ects are rather complex.

The Maillard reaction is also an important source ofvolatile ¯avour compounds, which can have a consider-able e�ect on the ¯avour of dried vegetables and fruits.Nursten [31] has classi®ed these volatiles into threegroups based on their origin: sugar dehydration/frag-mentation products, amino acid degradation products(Strecker degradation products), and compounds pro-duced by further interactions.

Sensory pro®ling techniquesAlthough air drying is the most popular commercial

drying technique to preserve perishable products, as it isrelatively inexpensive, this technique is considered togive less satisfactory product quality than other con-servation procedures [32], both with respect to thenutritional value and to the sensory properties such astexture, colour and ¯avour. However, the quality maybe improved by pretreatments such as direct osmosis oroptimization of the processing conditions.

Though quality improvement by innovations in dry-ing technology have been reported, extensive sensorystudies on dehydrated fruits and vegetables are scarce.The ¯avour and textural characteristics of hot-air-driedand freeze-dried French beans, peas and spinach werecompared by means of ¯avour and texture pro®ling bySinesio et al. [33]. These authors found that the sensorypanel clearly detected bene®ts of the freeze drying

Fig. 4. Aroma pro®le for fresh and hot-air dried mushrooms (Agaricus bisporus) [25].


process over traditional air drying of the vegetables. Thesamples were rehydrated before presenting to thetrained assessors. The freeze-dried products, whichreceived high scores for juiceness and fruity aroma, werenatural in colour and texture. The air-dried productswere characterized by an unnatural colour, were wrin-kled, cracked, tough with a less-juicy texture, and had aburnt and bitter ¯avour. Sinesio et al. reported that theair-dried products were less acceptable than the freeze-dried ones.

Leino [34] evaluated the impact of processing onodour characteristics of dried chive. Freeze-dried chiveobtained higher scores from the assessors for fresh-green odour than did the air-dried samples. On thecontrary, hay-like odour sharply increased in the chiveproducts after air drying. Onion-like odour was lower inthe air-dried products.

Yuksel et al. [35] explained how rehydration a�ectedthe ®nal quality of freeze-dried sweet bell pepper pieces(Fig. 5).

With respect to RF and microwave drying processes,only total sensory quality data from preference tests andconsumer acceptance are available. Combined micro-wave±vacuum drying of banana slices resulted in higherquality products than freeze-drying. However, no sub-stantial di�erences were observed in colour, taste,aroma, texture and rehydration capacity [11].

Other studies also reported the high quality ofmicrowaved products. In a separate study, the ¯avourof ripe bananas dried with microwaves was preferredover a warm-air-dried product [36]. Microwave-assistedair-drying of carrot cubes resulted in a substantialdecrease in drying time, and the product quality wasbetter than for conventionally air-dried samples [37].

ConclusionsAs has been said for 30 years, the potential for use of

ElectroMagnetic Energy (e.g. RF and microwaveenergy) for high-speed and high-quality drying andheating is enormous. An overview of possible advan-tages and disadvantags of the alternative dry conserva-tion techniques is presented in Table 2.

RF applicators are extremely simple, whereas inrecent years generators of microwaves have becomemore reliable due to better regulation devices. Moreknowledge about the basic theory of electromagnetic®elds, their interaction with dielectrics with high lossfactors such as water-containing foodstu�s and therelated development of equipment will most certainlylead to the drying process industry investing in `new'technology.

This development will be supported by the need tooptimize drying processes both with respect to processeconomics and product quality (nutritional value, col-our, taste and aroma). The control of the quality para-meters requires more knowledge about the mechanismof the e�ects of the drying conditions on these qualityaspects. However, most of the published studies refer toconsumer acceptance or total sensory quality only, withlittle or no reference to the sensory pro®ling techniquessuch as QDA1, Flavour Pro®le1, Texture Pro®ling1

and Free Choice Pro®ling.

References1 Cohen, J.S. and Yang, T.C.S. (1995) `Progress in FoodDehydration' in Trends Food Sci. Technol. 6, 20±25

Fig. 5. Sensory odour pro®le of freeze-dried green bell pepper(Capsicum annuum) before and after rehydration [35].

Table 2. Relative advantages and disadvantages of several dryconservation processes for fruits and vegetables

Characteristic Freezedrying

Microwave Radiofrequency

Drying rate ÿ + +Flexibility ÿ + +Start-up time 0 0 0Reliability 0 ÿ 0/ÿInvestment costs ÿÿ ÿ 0/ÿEnergy consumption + 0 0Running costs + 0 0Colour ++ + +Flavour ++ + +Nutritional value ++ + +Microbial stability ÿ + +Enzyme inactivation ÿ + +Mechanical stability ÿÿ ÿ ÿRehydration cap. ++ + +Crispiness ++ + +Fresh-like appearance(after rehydration)

++ + +

Ratings: ÿÿ strongly negative; ÿ negative; 0 no e�ect; +positive; ++ strongly positiveAll ratings indicate the e�ect of using an alternative technologyinstead of hot air drying on a speci®c process or quality char-acteristic


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