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OSMOTIC DEHYDRATION OF APPLES BY IMMERSION INCONCENTRATED SUCROSE/MALTODEXTRIN SOLUTIONSEBNER AZUARA' and CESAR I. BERISTAI"

Instituto de Ciencias Bh ic asUniversidod VeracnuanaApdo. Postal 575Xalapa, Veracruz, M&icoAND

GUSTAVO F. GUTI~RREZ

Departamento de G r a d d o s en AlimentosApdo. Postal 42186CP 06470,M&ico. D.F., M&ico

ENCB-IPN

Accepted for Publication July 25,2002

ABSTRACTThe effect of sucrose:maltodextrin (S:MIOO) ratios on solidr gain (SG) ndwater loss (WL) was investigated during the osmotic dehydration of apple disks.Concentrated solutions were prepared at 4OC with 1OO:O. 90:10, 0:30, 50:50,30:70and I0:W ratiosof (S:MlOO). The highest score in sensory evaluation wasachieved with 90 : lO ratio of (S:MlOO). Three stages of osmotic dehydration canbe observed by plotting the shrinkage ofapple disks vs the moisture content. Theresult is that a higher maltodextrin concentration favors volume loss andenhances water loss, increasing the duration of stage 2 osmotic dehydration.

INTRODUCTIONOsmotic dehydration removes water from fruits and vegetables when they

are immersed in a hypertonic solution for a specified time and temperature. Theconcentration gradient between the solution and the food acts as a driving forcewhen removing water from the food. Various osmotic agents such as sucrose,glucose, fructose, corn syrup and sodium chloride are often used for osmoticI Correspondence: Ebner Azuara-Nieto, Institute de Ciencias Bhicas, Universidad Veracruzana,Aparrado Postal 575, Xalapa-Veracruz. Mtxico. E-mail: yma@bugs.invest.uv.mxJournal of Food Processing hesewation 26 (2002) 295-306.Al l Rights Reserved."Copyright 2002 by Food & Nutrition Press, Inc.. Trumbull. Connecticut. 295

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296 E.AZUARA. C.I. BERISTAIN and G.F. GUT&RREZ

dehydration (Contreras and Smyrl 1981; enart and Lewicki 1987;Lazarides efal. 1995). Generally, sugar solutions are used for fru its and salt solutions forvegetables, according to their final use. In such a system , the ratio of solid foodto osmotic agent (w/w), approximation to equilibrium, the initial osmoticpressure differential between the food and the osmotic agent, and the relativediffusion rates of water and solute determine the extent of water and weight loss.Osmotic dehydration is a gentle process compared to hot air drying, as itprovides a product with better color, texture, and flavor. Osmodehydrationinhibits the action of polyphenol oxidase and retards the loss of volatile flavorconstituents during dehydration (Ponting 1973;Dixon ef al. 1976).Despite the recognized advantages of the osmotic treatment, its commercialapplications are still quite limited. Problems associated with management andmicrobial stability of the osmotic solution, together with the difficulties tocontrol undesirable solute uptake are probably the main reasons for slowindustrial development (Lazarides ef al. 1995;Raoult-Wack 1994).The characteristics of the final dehydrated food are largely influenced bythe extent of osmotic solids gain. Sugar uptake during osmotic dehydration offruit modifies the composition (sugar to acid ratio) and the taste of the finaldehydrated food (Ponting 1973). This so-called "candying effect" is sometimesdesirable, as it improves the taste and acceptability of the final product. Inmostdehydration cases, however, extensive solute uptake is undesirable, because ofits negative impact on the nutritional profile of osmotically dehydrated fruits,which can no longer be marketed as "natural".Lazarides ef al. (1995) studied mass transfer kinetics during osmoticdehydration of fruit aimed at minimal solid uptake. They observed that by usingcorn syrup solids of large molecular size, it is possible to obtain high waterremoval rates with minimal solid uptake. Corn syrup solids of varying dextroseequivalents (ranging between 18 and 42 DE)were tested under conditions thatfavored large water loss/solid gain ratios. Treatments with corn solids, exceptfor 42 DE, gave loss of solids indicating that the am ount of sugar gained wasmarginally less than the amount of leached fruit solids. The 42 DE reatmentgave a slightly positive value of solids gain and a large water loss/solid gainratio (23.7 /g).The objective of this work was to study the influence of mixed blends ofsucrose/maltodextrin MlOO (10DE) on solids gain and water loss during theosmotic dehydration of apples.

MATERIALS AND METHODSMaterials

Golden Delicious apples and refined sugar (sucrose) were purchased in a

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DEWATERING OF APPLE BY USING MALTODEXTIUN 291

local market and employed in all experiments. MaltodextrinMlOO (10 DE) wasobtained from a local processing plant. The apples were sliced in disk-shapedpieces of 46 mm diameter and 4 mm thickness, and immediately immersed intoa 20 g/L solution of ascorbic acid for 5 rnin to control browning.Procedure

The weight ratio of the osmotic medium for the apple disks was at least20: 1 to avoid significant dilution of the medium. Concentrated solutions wereprepared by blending various proportions of sucrose and maltodextrin MlOOformulated with 100:0, 90:10, 70:30, 5 0 5 0 , 30:70 and 10:90 ratios ofsucrose:maltodextrin. The solids concentration of osmotic solutionwas adjustedto 700 g/kg at 4OC. Apple disks were withdrawn at periodic intervals during3 h. Immediately after the apple disk withdrawal, the disks were blotted withdrying paper to remove excess solution. According to the continuous method(Azuara er al. 1998) each apple disk was weighed and its thickness measured inat least three different points by means of a Mitutoyo micrometer. After themeasurements, the disks were returned to the osmotic solution to continue theprocess. At the end of the experiment moisture content was determined in eachapple disk by using a vacuum oven for 24 h at 70C (AOAC 1984).Sensory Evaluation

Sensory evaluations of the dehydrated apple disks were conducted by usinga 9point hedonic scale ranging from 1 (dislike extremely) to 9 (like extremely).The panel was composed of 20 nontrained judges.Mathematical Model

Azuara ef al. (1992) calculated water loss and solids gain during osmoticdehydration through equations with two parameters obtained from massbalances.

s,tSG,SG =-+s2twhere t = time, s, = a constant related to water loss, s, = a constant relatedto solids gain, WL = amount of water lost by the foodstuff at time t (fraction,

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298 E. AZUARA,C.I. BERISTAIN and G.P.GUTIlhREZ

percent, grams or kg), SG = amount of solids gained by the foodstuff at timet (fraction , percent, grams or kg), WL,= amount of water lost at equilibrium,and SG, = amount of solids gained at equilibrium.

Weight loss (ML) during osmotic dehydration is equal to water loss (WL)minus solids gained (SG) t the same time.ML = WL - SG (3)

According to the continuous method (Azuara el al. 1998)if we plot t/MLvs t, we can define b as the intercept and p the slope of the resulting straightline, then the following equations arise:

Subscript m means that W L nd SG are determined at the last point of theexperiment, through the equations of Beristain et al. (1990):

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DEWATEFUNG OF APPLE BY USING MALTODEXTRIN 299

where M, = initial weight of the foodstuff (t = 0).M, = weight of the food attime t , C, = initial moisture of the foodstuff (wet basis), and C, = finalmoisture of the foodstuff (wet basis) at time t.If s,, s2, WL, and SG, are determined experimentally, it is possible tocalculate WL and SG at any time. Cfcan also be predicted through he followingequation:

Azuara ef a l. (1992)proposed the following equation to calculate the effectivediffusion coefficient for foodstuffs cut in a flat slab geometry:

where F = WL--d/WL_p,WJi,-- = value for

(1 1)

WL, obtained from Eq. (l),WLZq = value for WL, experimentally obtained, 1, = half thickness of theapple disk at time 0.Statistical Analysis

All experiments of osmotic dehydration and analysis of the apple disks wererun in triplicate. The model was estimated from the experimental results usingthe nonlinear regression analysis. These analyses were accomplished with theKaleidaGraph 3.08 software (Synergy Software, PCS Inc., Reading, UK). Allparameters were reported from the mean f standard deviation.

RESULTS AND DISCUSSIONFigure 1 shows a linear relationship between water loss (WL/Mo) andsolids gain (SG/Mo) for the osmotic dehydration of apples at 1OO:O. 90:10,70:30 and 50:50 ratios. When graph representing water loss (WL/Mo) versus

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300 E. AZUARA, C.I.BERISTAIN and G.F. GUT&RREZ

solids gain (SG/Mo) is a straight line, the rate: (WL/SG) will be constant andwill also be an important criterion to determine whether dehydration (WLISG> 1)or impregnation (WL/SG < 1)predominates along the process. The slopein this plot was in all experiments > 1. A nine-fold increase in the value of theslope was obtained when a fraction of the sucrose was replaced by maltodextrinsand lower increments were observed when the maltodextrin content in themixture was increased by rising the sucrose:maltodextrin ratio to 30:70 (Table1). WL/SG ratio showed a sudden increase when 90% of the sucrose wasreplaced by maltodextrin. Table 1also presents that the highest score in sensorytesting was given to apple disks dehydrated with mixtures prepared with ratioof 90:10. The sucrose able to penetrate the apple tissues prevented the loss oforganic solids from inside the fruit tissues (Heng ef al. 1990). Mixtures withhigh maltodextrin content favor the loss of those organic compounds responsiblefor the characteristic of fruit flavor, and therefore, such products received thelowest sensory scores. In contrast, the solutions made with sucrose onlyproduced apple disks with excessive sweetness due to a high sucrose content(SG =29 X).

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Changes in volume and weight occurring in the fruit during osmodehydra-tion, are closely linked with solids gain and water loss. Evolution of volume loss(VL) and weight loss (ML) of apple disks are presented in Fig. 2 for fourexperiments at selected ratios of sucrose:maltodextrin. When maltodextrin wasadded to the osmotic solution, VL and ML of apple disks increased noticeablyand reached equal values after 2 h of treatment (VLzMLz0.8). This isprobably due to the fact that solids gain (SG) s smaller than water loss (WL)and to the fact that in this conditions apple losses only water (density = 1g/cm3). The relatively large molecular size of the maltodextrins is an obstaclefor its penetration into the apple tissue. Increments in VL and M L suggest thatsolutes of high molecular weight such as maltodextrins form a dense layer onthe surface of the apple disk, which produces a concentration gradient andincreases water migration from apple disks to the concentrated solution. It ispossibleto evaluate these macroscopic changes by plotting changes indimension-less volume (VtNo) vs dimensionless moisture content (Cf/Co) of the appledisks.

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DEWATERING OF APPLE BY USING MALTODEXTRIN 303

Figure 3 shows three stages caused by changes in volume and moisture inthe apple disks during osmotic dehydration. Saurel et al. (1994a) showed thatcell plasmolysis in apples occurs solely when low molecular weight solutes arepresent in the osmotic solution.On he other hand, when using high molecularweight solutes, cells did not show evident plasmolysis. This may indicate thatshrinkage associated with initial dehydration did not only occurred solely in thevacuole, but begins at the cell wall. Microscopic changes have an effect onmacroscopic changes such as variations in the moisture content and shrinkageof the apple. During the first stage of the process (Fig. 3), a decrease of about20%of the initial volume was observed. This was independent of the molecularweightof the solutes. However, moisture content decreases in higher proportionwhen only sucrose is used. This may be due to the fact that, in spite of the highamount of water lost, a high sucrose content is gained. In Fig. 3, it is alsoobserved, that when the osmotic solution contains only sucrose, stage 2 is

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304 E. AZUARA. C.I. BERISTAIN and G.F. GUT&RREZ

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associated with a small (7%) decrease of volume and an increment (20%) ofwater loss. These changes may be due to the increase in the porosity of the solid(LeMaguer and Yao 1995). On the other hand, when maltodextrin is added tothe solution plasmolysis of the cells at the solid-liquid interface is inhibited(Saurel e?al. 1994a, b). Neither porosity nor solids gain are increased and stage2 is characterized for a pronounced diminution of change of volume andmoisture content (approx. 40%). When the maltodextrin content in the osmoticsolution is increased, the stage 2 is characterized by a greater loss of volumeand moisture when compared to the losses of a solution made of 100% ucrose.Stage 3 of the process when using sucrose:maltodextrin occurs almost with nochange of volume (decreases approx. 5% ) , but moisture content decreases inabout 40 % . When sucrose is the only component of the solution, stage 3 showsconsiderable changes of volume and moisture (decrease approx. 40%).The time course of changes in the diffusion coefficient for water (Dw) ispresented in Fig. 4. As can be observed in the experiments at ratios 90:10,70:30 or 5050, including maltodexuins in the osmotic solutions results in ahigher Dw than he Dw for osmotic solutions prepared with 100% sucrose. Thevalues of Dw obtained confirm that the presence of high molecular weightsolutes in the osmotic solution enhances water loss.

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DEWATERING OF APPLE BY USING MALTODEXTRIN 305

CONCLUSIONSSolutionscontaining sucrose:maltodextrins used for the osmotic dehydration

increased water loss and decreased solids gain of apple disks as compared tosolutions prepared with 100% sucrose. The highest score in sensory testing wasobserved in apple disks dehydrated with solutions of sucrose:maltodextrin ratioof 90: 10. Our study suggest that the addition of high molecular weight solutesto the osmotic solutions can be used to achieve a twofold transformation of thefruit: (1) control the incorporation of specific solutes of low molecular weightinto the tissue, to obtain osmodehydrated products with better sensorialcharacteristics; and (2) enhance the water loss of fruit.

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transfer kinetics during osmotic preconcentration aiming at minimal soliduptake. J. Food Eng. 25, 151-166.

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LE MAGUER, M. nd YAO, Z.M. 995. Mass Transfer during OsmoticDehydration at the Cellular Level. In Food Preservation 6y MoistureControl, Fundamentals and Applications, (G.V. arbosa-Chovas and J.Welti-Chanes, eds.) pp. 325-350, Technomic, Lancaster, Penn.LENART, A. and LEWICKI, P.P.1987. Kinetics of osm otic dehydration of theplant tissue. In Drying '87, A.S. Mujumdar, ed.) pp. 239-248, Hemi-sphere Publishing C o p , New York.PONTING, J.D. 1973. Osmotic dehydration of fruits- ecent modificationsand applications. Roc. Biochem. 8, 18-20.RAOULT-WACK, A.L. 1994. Recent advances in the osmotic dehydration offoods. Sci. Technol. 5 , 255-260.SAUREL, R., RAOULT-WACK, A.L., RIOS, G. and GUILBERT, S. 1994a.Mass transfer phenomena during osmotic dehydration of apple I. Freshplant tissue. Intern. J. Food Sci. Technol. 29, 531-542.SAUREL, R., RAOULT-WACK, A.L., RfOS, G. nd GUILBERT,S. 1994b.Mass ransfer phenomena during osmotic dehydration of apple 11. Frozenplant tissue. Intern. J. Food Sci. Technol. 29, 543-550.