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ELSEVIER

Journal of Food Engineering 37 (1998) 437-4490 1998 Elsevier Science Limited. All rights reservedPrinted in Great Britain0260-8774/98/ - see front matterPII: SO260-8774(98)00094-6

Sorption Isotherms and Drying Characteristics of MulberryTorus alba

Medeni Maskan & Fahrettin GiigiisFood Engineering Department, University of Gaziantep, 27310 Gaziantep, Turkey

(Received 28 January 1998; accepted 2 July 1998)ABSTRACT

Dryi ng charact eri st i cs of M ulberry (M . alba) i ncl uding sorpt i on isotherms anddry i ng ki net i cs w ere i nvest i gated. Adsorpt i on and desorpt i on i sotherms at 10, 20and 30 and i sost et i c heat s of sorpt i on w ere det erm i ned. A t higher w at eracti vi t i es, as t he t emperat ure w as i ncreased, a crossing of t he i sotherm curvesw as det ected. Some hysteresis eff ect decreasing w i t h hi gher t emperat ure w asobserved. M ulberry w as dried in a pil ot plant tr ay drier w it h a const ant airvelocit y of 1.2 m SK at 60, 70 and 80. Onl y al l i ng rat e dry i ng peri ods (threefal l i ng rat e peri ods) w ere observed i n t he mul berr y dry i ng experi ment s. Thedij fusivi t y val ues changed from 2.32 x lo- t o 2.76 x lop9 m2 s-t w i t hin t hegiv en t emperat ure range. Efi ect of temperat ure on the di ffusiv i t y w as expressedby an Ar rhenius rel ati on w i t h an act iv ati on energy v alue of 21.2 Wmol -.0 1998 Elsevi er Science Li mi t ed. A l l ri ghts reserv ed.

INTRODUCTIONMulberry has been cultivated in the Northern hemisphere for centuries on accountof its wide use for many purposes. Three types of mulberry; white (M. alba), black@4. nigm) and red (M. ru ru are grown in Turkey. It is consumed as fresh or dried.)The syrup prepared from mulberry is believed to have medicinal properties (Anony-mous, 1986). Granulated mulberry is used as a flavouring material for a culturedmilk drink manufactured in Armenia (Aganova, 1989). In addition, it is widely usedin mulberry pekmez (a Turkish concentrated mulberry juice product) production(Anonymous, 1986), canned mulberries, mulberry jams and juices (Snapyan et al .,1981), mulberry pulp, paste, marmalade and jelly (Khatiashvili et al ., 1979). It is alsoused in alcoholic beverages such as wine production (Alian & Musenge, 1977). Wills*Author to whom correspondence should be addressed.E-mail: maskan@alpha.bim.gantep.edn.tr

437

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438 M . M askan, G@iQet al. (1987) studied the chemical composition of mulberry and reported theapproximate composition as per 100 g edible portion. They found that it contains89.3% water, 2.2% protein, 0.2% fat, 2% glucose, 2.3% fructose, 2.2% dietary fiber,0.19% malic acid, 0.59% citric acid, 0.8% ash, 121 kJ energy, 10 mg vitamin C,0.01 mg thiamin, 0.01 mg riboflavin, 0.7 mg niacin, 0.01 mg p-carotene, 310 mg K,6 mg Na, 20 mg Ca, 12 mg mg, 0.3 mg Fe and 0.2 mg Zn.Since mulberry is consumed as fresh, dried and as an ingredient in prepared foodsand formulations, its sorptional equilibrium and drying kinetics are the most import-ant information needed for dryer simulation and design. Sorption isotherms, whichrepresent the functional relationship between water activity and equilibrium mois-ture content of a foodstuff at a given temperature, characterise the state of water infoods and are of primary interest for several food science and technology applica-tions. From a drying standpoint, sorption isotherms are required to define the endpoint of the process. A knowledge of the sorptional equilibrium is also important forpredicting stability and quality changes during packaging and storage of dehydratedfoods and formulations (Pezzutti & Crapiste, 1997).Drying is one of the most important steps of the post-harvest handling of foodsand used to stabilise solid products by lowering their water activity (a,.,). It allowsfoods to be marketed and consumed outside their traditional season. The berry is awidely consumed dried food that traditionally is obtained by sun-drying the berriesin the open air. The main disadvantage of this process is that it is subject to widevariations in climatic and environmental conditions, and is thus poorly controllable.The final product has a widely varying moisture content and organoleptic properties.Because of these reasons, sun-drying is being replaced by drying in a hot-air convec-tion chamber, in which drying conditions can be selected (Holdsworth, 1971). Airdrying of solid foods involves vaporisation of water contained by the food, andremoval of the vapour in a stream of air. The phenomenon is one of simultaneousheat and mass transfer, and is affected by several internal and external conditions,such as food properties and air conditions. Many porous structural materials andsome food commodities show two main periods during drying: the constant rateperiod and the falling rate period. The falling rate period is the most important ina food dehydration process and the mechanisms in this period are more complex.Ficks Law of Diffusion has been used to describe this period (Vergara et al., 1997).Although there are extensive studies on sorptional (Chirife et al., 1992; Singh &Singh, 1996; Lomauro et al., 1985a,b) and drying (Diamante & Munro, 1991; Sal-gado et al ., 1994; Mazza, 1984; Mowlah et al ., 1983; Xiong et al ., 1991; Mulet et al .,1989; Khraisheh et al., 1997) properties of food materials, a survey of recent litera-ture shows that the available information on sorptional equilibrium and dryingcharacteristics of mulberry is rather scarce.The aim of this work was to determine the moisture sorption isotherms of mul-berry together with the drying characteristics of this potentially valuable crop andthe effect of drying temperature.

MATERIALS AND METHODSMaterialsMulberries were freshly harvested from a mulberry tree. Salts (NaOH, LiCl, KAc,MgC12, K2C03, Na2Cr207, Mg(NOs)2, Na2Cr04, NaN02, NaN03, NaCl,

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Sorpt i on hot herms and dryi ng charact eri st i cs of mul beny (Moms alba) 439(NI-I&S04, KCl, Sr(N03)2, NaBr03, KN03) used to obtain constant water activityenvironments were of reagent grade (Merck) and the water used was double dis-tilled water.MethodsM oistur e sorpt ion isotherm det ermi nati onA static gravimetric method was employed for the determination of sorption iso-therms of mulberry (M alba) at 10, 20, and 30C. Nine saturated salt solutions wereprepared corresponding to a range of water activities from 0.07 to 0.92 (Labuza,1984). Each solution, 100 ml, was transferred into separate glass jars and the tripodwas also placed in each jar. Triplicate samples of about 1.0 g of mulberry wereweighed and. placed on tripods in the jars which were then tightly closed, and keptin ovens (NUVE, ES 500) at 10, 20, and 30C for equilibration.The required equilibration time was 40-50 days based on the change in weightexpressed on a dry basis which did not exceed 0.1%. The weight was less than1 mg g- dry solids for three consecutive weighings at intervals of minimum 5 days(Biquet & Labuza, 1988). At high values of a, (greater than 0.85) equilibrium wasconsidered to have been reached when the moisture content did not fluctuate bymore than 10 mgg- per day over three consecutive samplings. This assumptionwas made on the basis that, at higher water activities, where more sorption sites areexposed to the water vapour, the deviations are more pronounced than at loweractivities (Esguerra-Samaniego et al ., 1991). When the samples reached equilibrium,their moisture contents were determined by the vacuum oven method at 50C and40 mmHg for 10 h.Before starting the adsorption process, samples were pre-dried in a vacuum ovenat 70C to dryness (Lopez et al ., 1995). Desorption isotherms were determined onfresh samples containing 82.2% (wet basis) moisture content. At high water activi-ties (a,., > 0.70) a small amount of toluene was placed in a capillary tube fixed in thejar to prevent microbial spoilage of the mulberry (Labuza, 1984).ying experimentsThe drying experiments were performed in a drying pilot plant tray dryer (UOP 8tray dryer, Armfield Ltd, UK), which was operated at an air velocity of 1.2 m s-l,temperatures 60-80C and relative humidities 6-12%. Samples were placed as asingle layer of 0.8 cm thickness completely covering the base of the drying pan which

had dimensions of 27.5 x 18.5 x 1.0 cm3. Therefore, drying took place only from thetop surface. Air was blown into the dryer by means of a centrifugal fan withadjustable flow rate parallel to the drying surface of the sample. Dry bulb and wetbulb temperatures were monitored in the drying chamber. The drying data (averagemoisture content X vs. time t) were obtained by periodic weighing of the sampleswith an Avery Berkel CC62D balance, placed on the top of the dryer. Mulberrieswere dried until equilibrium was reached.

RESULTS AND DISCUSSIONThe adsorption and desorption isotherms of mulberry (M alba) at 10C are shownin Fig. 1. In the range investigated, mulberry isotherms were of the typical sigmoidal

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440 M. Ma m, R G@iQshape curves of type II common to dehydrated foods (Labuza, 1984). Equilibriummoisture content increases slowly at low water activity and shows a steep rise at highwater activities, which is typical behaviour for high sugar substances.It can be seen (Fig. 1) that sorption isotherms of mulberry exhibited the phenom-enon of hysteresis, in which the equilibrium moisture content was higher at aparticular water activity for desorption curve than for adsorption. Similar resultswere obtained by Pezzutti and Crapiste (1997) for garlic, Tsami et al. (1990) forraisins, currants, figs, prunes and apricots, and Giigti~ et al. (1998) for Turkishdelight. The hysteresis was significant throughout the a, studied at 10, 20 and 30C.There is a clear effect of temperature on the hysteresis of the mulberry isotherms.The differences between the equilibrium moisture contents of water sorption iso-therms are greater at 10C compared to higher temperatures. Several theories havebeen formulated to explain the phenomenon of hysteresis. According to one ofthese theories, which is favoured most and explains hysteresis in our study, the polarsites in the molecular structure of the material at its original wet condition arealmost entirely satisfied by adsorbed water. Upon drying and shrinkage, the mole-cules and their water-holding sites are drawn closely enough together to satisfy each

0.0 0.2 0.4 0.6 0.8 1.0a

Fig. . dsorption and desorption isotherms of mulberry (M. ah) at 10C.

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Sorpt i on sotherms and dryi ng charact eri st i cs of mul berry (Morus alba) 441other producing a meta-stable state. This reduces the water-holding capacity of thematerial upon subsequent adsorption (Rockland, 1969; Palipane & Driscoll, 1992).Figure 2 shows the typical relationship of high-sugar foods, with equilibriummoisture contents increasing sharply at high water activities, due to the dissolving ofmulberry sugars. This dissolution was visually observed at 20 and 30C at wateractivities greater than 0.85 on adsorption and desorption branches together withextensive browning of mulberries occurring on the adsorption branch only. Theslight sigmoid shape of the first part of the isotherms may be caused by the watersorption of the biopolymers. The sharp increase in moisture content at high wateractivities is due to the sorption by the sugars as reported by Saravacos et al. (1986).Temperature had the expected negative effect on equilibrium moisture content atlow water activities, but the reverse effect was observed at higher water activities.For the water activity values greater than 0.65 there was an inversion (crossing) ofthe effect of temperature. This may be due to the dissolution or leaching of thesugars in water. Figure 2 clearly shows the intersection point and the inverse effectof temperature on mulberry sorption curves. Similar results have been found forraisins in the temperature range of 20-35X (Saravacos et al., 1986) pistachio nut

0 10C20A 30C

0.0 0.2 0.4 0.6 0.8 1.0a,Fig 2 Adsorption isotherms of mulberry (M alba) at 10, 20 and 30C.

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442 M . M askan, E G@i i~paste (Maskan & Gogus, 1997), various dried fruits (Tsami et al., 1990; Esguerra-Samaniego et al ., 1991), and Turkish delight (Giigii~ et al ., 1998).The isosteric heats of sorption play an important role in both the determinationof drying times and in the energy requirements for drying. So, it is necessary toestimate heat of sorption for drying materials prior to processing. The heats ofsorption for mulberry were calculated at selected moisture contents by applying theClausius-Clapeyron equation using sorption data at 10, 20 and 30C. Figure 3 showsthe isosteric heat of sorption as a function of the moisture content. It can be seenthat isosteric heat varies inversely with the amount of water vapour sorbed by thesolid; the higher the moisture content, the less energy is required to remove watermolecules from the solid. This means that water bound in the lower region (mono-layer region) is tightly bound than water occurring in the higher region (condensedregion) of the isotherms (Cadden, 1988). The results also show that isosteric heat ofdesorption was higher than isosteric heat of adsorption. This is probably due to theextensive heat treatment that the mulberry received during drying of sample pre-paration for adsorption process, which could have damaged the sorption sites andmakes the removal of water easier. The isosteric heat increased until maximum and

40

30

20z36 10

0

10

Adsorption

I 1 1 I I I 110 20 30 40 50 60 70 80Moisture Content (g water/l00 g dry solid)Fig. 3. Plot of isosteric heat of sorption of mulberry (M . alba).

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Sorption isotherms and dving characteristicsof mulberty (Moms alba) 443then decreased with the increase in moisture content. Similar results were obtainedfor apple pectin, pineapple, grapefruit, rice, raw and cooked chicken, raw beef, yeast(Iglesias et al., 1989), yoghurt and concentrated yoghurt dried by various methods(Kim & Bhowmik, 1994), and cookies and corn snacks (Palou et al., 1997). Themaximum heats of sorption were obtained in the moisture range of &-lo% (drybasis). The increase in isosteric heat at low moisture content can be explainedconsidering that the sorption of water by the dry matrix led to swelling of the foodpolymers, resulting in the exposure of sorption sites of higher binding energies notpreviously available. After the maximum, the decrease in the isosteric heat with theamount of water sorbed can be qualitatively explained considering that initially,sorption occurs on the most active available sites given rise to greatest interactionenergy. As these sites become occupied, sorption occurs on the less active sites givenlower heats of sorption (Palou et al., 1997).Drying runs were conducted at 60, 70, 80C and constant air velocity and drierloads. The relative humidities used were those of the ambient air heated to60-80C. Drying was carried out until equilibrium moisture contents reached ateach temperature. The equilibrium moisture contents estimated experimentally were0.17, 0.12 and 0.06 kg water per kg dry solid at 60, 70 and 80C respectively, and thedried product had a puffy structure with brown colour at all temperatures.Drying curves for experiments conducted at three different temperatures areshown in Fig. 4. It can be seen that the increase in temperature reduces the timeneeded to reach equilibrium moisture content. Similar behaviours were observed byVergara et al. (1997) for osmotically dehydrated apples, Moyls (1981) for applepurees, Vaccarezza et al. (1974) and Salgado et al. (1994) for sugar beet root andsugar beet pulp, respectively.A constant rate drying period was not observed in any of the experiments of thiswork. The entire drying process for mulberry occurs in the range of the falling rateperiod and the air temperature was an important factor that influenced dryingkinetics, as it has been commonly found in biological products (Salgado et al., 1994).In most studies carried out on drying, diffusion is generally accepted to be the mainmechanism during the transport of moisture to the surface to be evaporated. Thesolution to Ficks equation, which expresses the diffusion of liquid in a slab shapedsolid in terms of dry basis moisture content, can be written as follows (Aguerre etal., 1982):

(X-XX,) = s exp (- rc2Deff tlL2)( I-XeJ 7c2 (1)where X is the average moisture content (kg water per kg dry solid), X, is theequilibrium moisture content (kg water per kg dry solid), X0 is the initial moisturecontent (kg water per kg dry solid), L is the thickness of the slab (m) for dryingfrom one side, Deff is the effective moisture diffusivity (m SK) and t is the dryingtime (s).Effective diffusivities of mulberry were determined from experimental dry-ing curves. Figure 5 shows a typical plot of ln[(X-X,)/(X0--X,)] vs. time for thedetermination of diffusivity. The non-linear shape of the drying curves indicate avariable moisture diffusivity. It can be seen from these curves that the dryingtemperatures have a pronounced influence on the drying rate and as a consequence,markedly affect the value of the diffusion coefficient. As was expected, the dryingrate increased greatly with increasing temperature. Each curve consists of three

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444 M . M askan, E Gt i&approximately linear falling rate periods as were determined by Karatag and Battal-bey (1991) for drying of pistachio nut meat in the temperature range of 3%60C.They explained this to be due to the capillary property and cell structure of pis-tachio nuts. However, two falling rate periods have been determined by Vaccarezzaet al. (1974) for sugar beet root drying, Diamante and Munro (1991) for drying ofsweet potato and Mowlah et al. (1983) for drying of banana, and Moyls (1981) hasfound both constant and two falling rate periods in drying apple puree.A linear regression analysis was employed to calculate the diffusion coefficientsfrom the slopes of the straight lines. Values of D,rr for different temperatures andfalling rates are presented in Table 1 where DenI, DeffZ and DeW represent diffusiv-ity constants for first, second and third falliyf rate drying periods. The diffusivityvalues for the runs changed from 2.32 x lo-the drying temperature. to 2.76 x 10F9 m2 s- dependin on10-12 m2 s-1

These values of diffusivity in the range of lo- toare comparable to 1 to 3 x lo- m2 SK for drying of apricots in atemperature range of 50-80C 4.1 x 10P1 for raisins under vacuum, 1.1 x 10W9 fordrying of apples, 5 x lo- m2 s-l for sugar beet drying at 50C (Abdelhaq &Labuza, 1987), 8.33 x 10- for drying of banana at 60C (Mowlah et al., 1983), 0.34

0 200 400 600 a00 loo0 1200

Time (min)Fig. 4. Effect of temperature on drying behavior of mulberry (M . alba).

0 80A 7OC0 WC

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Sorpt i on sot hemt s and dy ing charact eri st ics of mul berry (Morus alba)

1

0

-1

2 -2

k;cz:-4

3

200 400 600 803 1ooo 1200Time (min)

Fig. 5. Logarithmic drying curve at various temperatures for mulberry (M. alba).

TABLE 1Effective Diffusivity Values (m s- ) for Drying Mulberry

445

60C 70C 80Cffl 2.32 x lo- 2.84 x lo- 3.58 x 10 - I03 0.9963 0.9931 0.9808L fQ 5.03 .9720lo-O 6.68 .9922lo-O 8.43 .992510 - lo

9 2.63.96301O-9 2.30.99771O-9 2.76.99661O-9

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446 M . Maskan, R Gc@Qto 3.45 x lo- for drying of garlic at temperature range of 25-65C (Pezzutti &Crap&e, 1997) and 1.59 to 8.48 x 10- m2 s-l for potato drying (Yusheng &Poulsen, 1988).It can be seen from Table 1 that diffusivity values increase as drying progresses ateach temperature. Mazza (1984) has stated that, in the low moisture range, thedrying is so slow that the cooling effect of evaporation is insignificant and dryingmaterial assumes the dry bulb temperature of the air. In this phase of drying, therate of moisture movement to the surface of the material increases with tempera-ture. Therefore, even at very low moisture content, the drying rate is appreciablygreater. This is the reason that we assume for high value of diffusivity at the thirdfalling rate period. Another reason for the greater diffusivity at low moisture con-tent may be due to the cell wall destruction, because wet bulb temperature of thesamples approaches the dry bulb temperature and therefore decreases the resistanceto the moisture diffusion within the sample. This means that heating leads tochanges in the physical properties of tissue, among them destroying the semi-permeability of the cell membranes (Vaccarezza et al., 1974).A theoretical explanation of the value of the activation energy is rather difficult inthis work because of a complex mechanism (three falling rate periods) for mulberry

Fig 6 Relationship between diffusivityand reciprocal of absolute temperature.

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Sorpti on i sot hetms and dryi ng characteri sti cs of mul berry MONS alba) 447drying. Therefore, in the present study we are concerned with the first falling rateperiod in order to estimate activation energy (Ea) for diffusion. This was calculatedby plotting ln(Deff) vs. the reciprocal of the absolute temperature as presented inFig. 6. The slope of the straight line is -EaIR, where R is 8.314 kJ mol- K-,assuming that the Arrhenius equation (eqn (2)) applies;Deff = DO exp [ - Eal( RT)] (2)The activation energy for diffusion was calculated to be 21.2 kJ mol- and Do was4.87 x lo- m2 s-. The Eu value lies within the range of 15-40 kJ mol- forvarious foods (Rizvi, 1986) and can be compared with 22.6 kJ mol- obtained fortapioca root, 29.7 kJ mol- for fish muscle, 18.0 kJ mol- for tobacco leaf and28.8 kJ mol- for sugar beet reported by Vaccarezza et al (1974). Pezzutti & Cra-piste (1997) found 27.8 and 16.9 kJ mol- for garlic drying at the wet and dry zonesof sample, respectively. All these values correspond to the so-called first falling rateperiod.

The heat of desorption of mulberry at low moisture content (Fig. 3) is higher thanthe activation energy of drying. Since the evaluation of heat of sorption through theClausius-Clapeyron equation involves the slope of the desorption isotherm, anysmall discrepancy in the shape of the fitted isotherm, especially at low moisture, mayreflect in a large discrepancy in Qs (Xiong et al., 1991). Because of this discrepancyin Qs, it is difficult to compare with activation energy of drying.

CONCLUSIONThe most important characteristics of mulberry required for simulation and optimi-sation of the drying, storage and packaging processes were studied. It can beconcluded that, at low water activities, soluble sugars, such as glucose and fructose,adsorb very little water, and adsorption is mainly due to the other polymericmaterials. At low and intermediate water activities, mulberry becomes less hygro-scopic with increase in temperatures. However, at high water activities, because ofphase transitions of the soluble sugars, higher temperature isotherms are above thelower temperatures (crossing).From the drying curves at three different temperatures, effective coefficients forwater diffusion and activation energy were obtained. Three falling rate dryingperiods were observed. At each particular temperature, the effective diffusivity valueincreased as drying progressed to low moisture contents. This was attributed to thedamage occurring in the cell wall due to the temperature effect. The activationenergy obtained was 21.2 kJ mol-, which is comparable with the reported values offood materials.

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