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food and bioproducts processing 9 0 ( 2 0 1 2 ) 667–675

Contents lists available at SciVerse ScienceDirect

Food and Bioproducts Processing

j our nal ho mepage: www.elsev ier .com/ locate / fbp

ffect of spray drying conditions and feed composition onhe physical properties of black mulberry juice powder

ahboubeh Fazaeli, Zahra Emam-Djomeh ∗, Ahmad Kalbasi Ashtari, Mahmoud Omidransfer Properties Lab (TPL), Department of Food Science and Technology, Faculty of Agricultural Engineering and Technology,niversity of Tehran, 31587-11167 Karadj, Iran

a b s t r a c t

In this study, the effects of some processing parameters on moisture content, water activity, drying yield, bulk density,

solubility, glass transition temperature (Tg), and microstructure of spray dried black mulberry (Morus nigra) juice

powders were investigated. A pilot-scale spray dryer was employed for the spray drying process and maltodextrin

with different dextrose equivalent (6, 9, and 20DE) and gum Arabic were used as carrier agent. Independent variables

were inlet air temperature (110, 130, and 150 ◦C), compressed air flow rate (400, 600, and 800 L/h), concentration of

drying aids (8, 12, and 16%) and percent replacement of maltodextrin (6 and 9DE) by gum Arabic and maltodextrin

20DE (25, 50, and 75%). Between the different drying aids, maltodextrin 6DE shows the best effect on the properties

of black mulberry juice powders. The process drying yield ranges from 45 to 82%. The highest drying yield (82%) and

solubility (87%) refer to the blend of maltodextrin 6DE and gum Arabic. The lowest moisture content powders (1.5%)

produced at the compressed air flow rate of 800 L/h. Inlet air temperature negatively influenced the bulk density due

to the increase of powder’s porosity. The lower the bulk density, the higher the solubility of powder is. With regard

to morphology, powders produced with maltodextrin and gum Arabic presented the smallest size.

© 2012 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.

Keywords: Black mulberry; Spray drying; Glass transition temperature; Yield; Bulk density; Solubility

vailing in spray dryers, they tend to stick to the walls of the

. Introduction

pray drying is one of the techniques used extensively in foodelated industries and it is used under optimal processingonditions to obtain powders. Fruit juice powders have manyenefits and economic potentials over their liquid counter-arts such as reduced volume or weight, reduced packaging,asier handling and transportation, and much longer shelfife. Besides, their physical state provides a stable, natural,nd easily dosable ingredient, which generally finds usage inany foods and pharmaceutical products such as flavoring

nd coloring agents (Shrestha et al., 2007).Black mulberry (Morus nigra) is popular edible fruit, which

riginates from Iran (Ercisli and Orhan, 2008). Black mulberryuice is proven to be a very concentrated source of polyphe-ols and anthocyanins (Suh et al., 2003). Fruits and vegetablesith high phenolic content have been studied extensively for

ts cancer preventing properties (Hertog et al., 1997). Many

pidemiological studies showed that red color juices such as

∗ Corresponding author. Tel.: +98 261 2248804; fax: +98 261 2248804.E-mail address: emamj@ut.ac.ir (Z. Emam-Djomeh).Received 4 April 2011; Received in revised form 3 April 2012; Accepted

960-3085/$ – see front matter © 2012 The Institution of Chemical Engittp://dx.doi.org/10.1016/j.fbp.2012.04.006

those of pomegranates, grapes, and different berries have ben-eficial effects on human health due to their high anthocyanincontent and antioxidant activity (Lin and Tang, 2007).

Fruit juice powders obtained by spray drying may presentsome problems in their properties, such as stickiness, hygro-scopicity, and low solubility. According to Bhandari et al.(1997), the sticky behavior of sugar and acid-rich materials isattributed to low molecular weight sugars such as fructose,glucose, and sucrose and organic acids such as citric, malicand tartaric acid, which constitute more than 90% of the solidsin fruit juices and purees. These materials have low glass tran-sition temperature (sucrose: 62 ◦C, fructose: −5 ◦C, glucose:32 ◦C), so molecular mobility of them is high when the tem-perature of the spray-dried particle is greater than 20 ◦C abovethe glass transition temperature. They are very hygroscopicin their amorphous state and loose free flowing nature at highmoisture content. While drying at temperatures normally pre-

17 April 2012

dryer and finally give a paste like structure instead of powder

neers. Published by Elsevier B.V. All rights reserved.

668 food and bioproducts processing 9 0 ( 2 0 1 2 ) 667–675

(Masters, 1979). The glass-transition temperature is the singlemost important parameter for assessing the ability of sugar-rich materials to be spray-dried (Imtiaz-Ul-Islam Md Langrish,2009). One method to avoid stickiness is to spray-dry at tem-peratures lower than the Tg + 20 ◦C. However, this approach isusually not economically feasible. The other method is the useof carriers; the high molecular weight of drying aids increasesthe glass transition temperature of the product (Cabral et al.,2009).

The carrier agents normally used in the spray drying of fruitjuices are maltodextrins and gum Arabic mainly due to theirhigh solubility and low viscosity, which are important con-ditions for the spray drying process (Quek et al., 2007). GumArabic is an effective carrier agent because of its emulsifyingproperties, since it has a little protein content in its compo-sition. Moreover, it exhibits high solubility and low viscosityin aqueous solution, which facilitates the spray drying pro-cess (Pitalua et al., 2010; Frascareli et al., 2012). Blend of gumArabic and maltodextrin proved to be more efficient than theuse of them separately in spray drying (Krishnan et al., 2005;Fernandes et al., 2011).

The quality of spray-dried food is quite dependent on thespray-dryer operating parameters. The spray-drying conditionwas the best way to explain the change quality factors of theproduct. Bhandari et al. (1993) carried out different tests toobtain powder from some concentrated juices. According totheir experiments, the best results were obtained for a juice tomaltodextrin ratio of 65/35 for blackcurrant, 60/40 for apricotand 55/45 for raspberry at inlet air temperatures between 90and 160 ◦C. Jumah et al. (2000) concluded that at high inlet airtemperature, particle size of jameed powder increase and bulkdensity decrease. Goula and Adamopoulos (2005a) studied theeffect of inlet air temperature and compressed air flow rate onthe characteristics of spray dried tomato powder. They con-cluded that drying yield increased with increases in inlet airtemperature and in compressed air flow rate. Shrestha et al.(2007) show that an increase in maltodextrin concentrationcauses a decrease in bulk density of orange juice powder; onthe other hand, Chegini and Ghobadian (2005) concluded thatbulk density decreased with increases in inlet air tempera-ture. They also found that production of orange juice powderwithout any carrier agent was impossible but using maltodex-trin increased drying yield to 18–35%. Fazaeli et al. (2011) usedan artificial neural network to predict the physicochemicalproperties of black mulberry juice powder. The model was val-idated by experimental tests on a pilot spray dryer and wasproved able to accurately predict the most important proper-ties of the powder, such as drying yield, browning index, bulkdensity, anthocyanin content and antioxidant activity.

The objective of this study was to evaluate the effects ofinlet air temperature, compressed air flow rate, concentra-tion of the carrier agents and using binary blends of dryingaids on the physical properties of black mulberry juice pow-der. These include moisture content, water activity, dryingyield, bulk density, solubility, glass transition temperature,and microstructure of spray dried black mulberry juice.

2. Materials and methods

2.1. Sample preparation

Fruits of black mulberry (M. nigra) at a commercially maturestage were purchased from a local market (Karaj, Iran). The

fruit was homogeneously and carefully selected in terms ofshape and ripeness, and then mixed with a blender. A sievewas used to eliminate the seeds, and the extract was thenpressed softly in order to increase the yield. Finally, fresh juicewas clarified using a spiral ultrafiltration system with a molec-ular weight cutoff equal to 40 kDa (Osmonic, USA). This stepwas also necessary to avoid a heat load, and is commonly usedfor sterilization and native enzyme inactivation. The cold,sterile, single-strength clarified juice with 16% of total solu-ble solid (TSS) was rapidly cooled and frozen at −25 ◦C andused for further experiments.

2.2. Spray drying

A Büchi mini spray dryer (Model B-191, Büchi Laboratoriums-Technik, Flawil, Switzerland) equipped with two-fluid nozzlewas used in the spray-drying process. Forty-eight differentexperiments were conducted in triplicate. In all experimentsthe aspirator rate, the feed temperature, the feed rate, and theatomizer pressure were kept at 925 N/m2, 20 ◦C, 150 mL/h, and4.5 bar, respectively. Once the juice total solids were adjusted(11%, w/w), 6 dextrose equivalent maltodextrin (Roquette,France), 9 and 20 dextrose equivalent maltodextrin (TongaatHulett starch, South Africa) and gum Arabic (Merk, Germany)at three concentration levels of 8, 12, and 16% (w/w) andmicro-crystalline cellulose (Merk, Darmstat, Germany), at theconcentration of 1.5% (w/w) were added. In our previous work(Fazaeli et al., 2012) the response surface methodology (RSM)was used to find the optimal conditions of black mulberry juicespray drying which resulted in air inlet temperature of 130 ◦C,carrier agent concentration of 8%, and compressed air flowrate of 800 L/h. In order to investigate the effect of the mixtureof different drying aids 25, 50, and 75% replacement of mal-todextrin (6 and 9DE) by gum Arabic and maltodextrin 20DEwere studied (Table 1). Three inlet air temperatures (110, 130and 150 ◦C), and three compressed air flow rates (400, 600, and800 L/h) were used. Distilled water was fed to the dryer for10 min before and after the spray-drying process. The powdersobtained by the spray drying process were weighed, sealed inbottle, and stored in the dark.

2.3. Analytical methods

2.3.1. Drying yieldSpray drying yield was evaluated by the determination of theproduct recovery given by the percentual ratio between thetotal mass of product recovered by the mass of extract fed tothe system (dry basis).

2.3.2. Moisture content and water activityThe moisture content was determined based on AOAC method(AOAC, 1990). Triplicate samples of black mulberry powder(20 mg) were weighed and then dried in a vacuum oven at70 ◦C. The drying and weighing processes were repeated untilconstant weight was obtained. Measurement of water activ-ity was carried out using a water activity meter (Novasina AwSprint TH-500, Switzerland). Triplicate samples were analyzedand the mean was recorded.

2.3.3. Bulk densityBulk density (g/mL) was determined by gently adding 2 g ofblack mulberry powder into an empty 10 mL graduated cylin-

der and holding the cylinder on a vortex vibrator for 1 min.The ratio of mass of the powder and the volume occupied

food and bioproducts processing 9 0 ( 2 0 1 2 ) 667–675 669

Table 1 – Carrier agent concentrations of liquids used forspray drying.

Carrier agent concentration

% MDa % GAb % MCCc

6DEd 9DE 20DE

8 0 0 0 1.512 0 0 0 1.516 0 0 0 1.5

0 8 0 0 1.50 12 0 0 1.50 16 0 0 1.50 0 8 0 1.50 0 12 0 1.50 0 16 0 1.50 0 0 8 1.50 0 0 12 1.50 0 0 16 1.52 0 6 0 1.54 0 4 0 1.56 0 2 0 1.52 0 0 6 1.54 0 0 4 1.56 0 0 2 1.50 2 0 6 1.50 4 0 4 1.50 6 0 2 1.50 2 6 0 1.50 4 4 0 1.50 6 2 0 1.5

a MD, maltodextrin.b GA, gum Arabic.c MCC, micro-crystalline cellulose.d DE, dextrose equivalent.

iA

2SCtcf3toa

2TdoDIptpbsadtst

Fig. 1 – Drying yield of black mulberry powders containingvarious percentage of maltodextrin 9DE, in different inlet

n the cylinder determines the bulk density value (Goula anddamopoulos, 2005b).

.3.4. Solubilityolubility was determined according to the method used byano-Chauca et al. (2005), where 100 mL of distilled water were

ransferred into a blender jar. The powder sample (1 g) wasarefully added to the blender which operates at 15,000 rpmor 5 min. The solution was placed in a tube and centrifuged at000 × g for 5 min. An aliquot of 25 mL of the supernatant washen transferred to pre-weighed Petri dishes and immediatelyven-dried at 105 ◦C for 5 h. The solubility (%) was calculateds the weight difference.

.3.5. Tg point measurementhe glass transition temperature (Tg) of all spray dried pow-ers was determined by a Differential Scanning Calorimeterr DSC (a 2010 Modulated DSC, TA Instrument, New Castle,E, USA). The purge gas used was dry nitrogen (25 mL/min).

ndium and zinc (Perkin-Elmer standards) were used for tem-erature and heat flow calibration. The samples were cooledo desired temperature (−25 ◦C) by fast cooling to reach tem-erature equilibrium at this temperature. Two to four mg oflack mulberry juice powders were scanned in a hermeticallyealed 50 �L DSC aluminum pans (Perkin-Elmer). An emptyluminum pan was used as a reference. The tests were con-ucted −50 ◦C to 200 ◦C with a heating rate of 10 ◦C/min. Theransfer of samples from container to DSC pan was done in a

ealed “Dry box” containing silica gel with regular N2 flushing,o avoid moisture absorption by the sample.

air temperature and compressed air flow rate of 800 L/h.

2.3.6. Scanning electron microscopy (SEM)The microstructure of the black mulberry juice powderswas examined using a scanning electron microscope (XL-30,Philips, Amsterdam, The Netherlands). To obtain SEM images,small amount of powders were taken from well mixed pow-der samples and coated with very thin layer of gold underhigh vacuum conditions, to provide a reflective surface forthe electron beam. Gold coating was carried out in a sput-ter coater BIO-RAD E-5200 (Bio-Rad Laboratories Ltd., London,UK) under a low vacuum in the presence of inert argon gas.The gold-coated samples were subsequently viewed under themicroscope.

2.3.7. Statistical analysisAll experiments were conducted in triplicate and an analysisof variance was performed. The least significant difference atp < 0.05 was calculated using the Duncan Multiple Range Teston Minitab software (Minitab 15; Minitab Inc., Minneapolis,USA). The data were expressed as mean ± SD.

3. Results and discussion

3.1. Drying yield

Fig. 1 shows the effects of carrier agent concentration andinlet air temperature on the yield of spray-dried powders.Inlet air temperature showed a positive effect on processyield, which can be attributed to the greater efficiency ofheat and mass transfer processes and decreasing the prob-ability of hitting the inadequate drying particles to the dryingchamber wall when higher inlet air temperatures are used.The same results were shown by Tonon et al. (2008), andGoula and Adamopoulos (2005a). However, Papadakis et al.(2006) showed that by increasing the inlet air temperaturethe process yield of raisin juice decreased. It is due tostickiness problems and it means that drying temperatureis above their glass transition temperatures. Increasing ofcarrier agent concentration in black mulberry juice signif-icantly increased the process yield (Fig. 1). It is related toincreasing the Tg values of the amorphous fractions in themixtures that are rich in low Tg components. This is inagreement with the results of Shrestha et al. (2007), Papadakiset al. (2006), and Quek et al. (2007). Although Tonon et al. (2008)showed that increasing maltodextrin concentration decreased

the process yield due to increasing the mixture viscosity.Increasing compressed air flow rate led to higher process yield

670 food and bioproducts processing 9 0 ( 2 0 1 2 ) 667–675

Fig. 2 – Drying yield of black mulberry powders containingvarious carrier agents (8%), in different compressed air flowrate and inlet air temperature of 130 ◦C.

Fig. 3 – Moisture content of black mulberry powderscontaining various percentage of maltodextrin 9DE, indifferent inlet air temperature and compressed air flow rateof 800 L/h.

Fig. 4 – Moisture content of black mulberry powders

(Fig. 2). This is in agreement with the results published byGoula and Adamopoulos (2005a), working with spray dryingof tomato pulp. Higher compressed air flow rate caused adecrease in droplet diameters and because of the smaller par-ticle size, the spray is narrower and denser and the dropletsstrike the wall at lower parts of the drying chamber, wheretheir moisture content is much lower. Product formation onthe walls falls into two categories. Semi-wet deposits causedby droplets, which are not sufficiently dry before hitting thewall, and sticky deposits caused by the nature of the prod-uct at the drying temperature (Masters, 1979). So decreasingthe process yield of samples by decreasing the compressed airflow rate refers to the first category of deposit formation. Asshown in Fig. 2, by using different carrier agents, powders withvarious drying yield are produced. The drying yield of pow-ders was increased with decreasing DE value of maltodextrin.Papadakis et al. (2006) reported similar results. They foundthat by decreasing DE, the recovery of spray dried raisin juicepowder was increased. In this study the highest process yield(82%) refers to powders produced with 6% maltodextrin 6DEand 2% gum Arabic at inlet air temperature of 130 ◦C and com-pressed air flow rate of 800 L/h, and the lowest drying yield(45%) refers to powders produced with 8% maltodextrin 20DEat inlet air temperature of 110 ◦C and compressed air flow rateof 400 L/h.

3.2. Moisture content and water activity

Water activity is different from moisture content as it mea-sures the availability of free water in a food system thatis responsible for any biochemical reactions, whereas themoisture content represents the water composition in a foodsystem. High water activity indicates more free water avail-able for biochemical reactions and hence, shorter shelf life(Quek et al., 2007). The average water activity of powdersin this study ranged from 0.15 to 0.32 thus can be consid-ered quite microbiologically stable. The effects of independentvariables on water activity are in agreement with the effectsof them on moisture content. The results showed the mois-ture content of the spray-dried powders decreased with theincrease in inlet air temperature (Fig. 3); similar results wereobserved for water activity of black mulberry powders. Thisis because at higher inlet temperature, the rate of heat trans-

fer to the particle is greater, providing greater driving force formoisture evaporation. Consequently, powders with reduced

moisture content are formed. Similar results were reportedby Goula and Adamopoulos (2005b), Chegini and Ghobadian(2005), Rodriguez-Hernandez et al. (2005), Ersus and Yurdagel(2007), and Kha et al. (2010) in tomato, orange juice, cactuspear juice, black carrot, and Gac juice powder, respectively.The results also showed that the moisture content of blackmulberry powder decreased when the carrier agent concen-tration increased from 8% to 12 or 16% (Fig. 3). These findingscould be explained by the fact that additional concentrationsof drying aid resulted in an increase in feed solids and areduction in total moisture for evaporation. It is in agreementwith results reported by Abadio et al. (2004), Grabowski et al.(2006), and Kha et al. (2010). However, Goula and Adamopoulos(2010) showed increase in moisture content with an increasein maltodextrin concentration. Goula and Adamopoulos (2010)concluded the presence of larger maltodextrin moleculesmade it difficult for water molecules to diffuse. They used highconcentrations of maltodextrin (25, 50, 100 and 400%) for pro-ducing orange juice powder and because of this increasingcarrier concentration caused an increase in moisture con-tent. Fig. 4 shows higher compressed air flow rate causeda decrease in moisture content of powders. It has similareffect on water activity of black mulberry powders. Particlesize decreases inversely with compressed air flow rate, soan increase in surface area per unit droplet volume resultsin an increase in drying rates and a decrease in moisturecontent.

containing various carrier agents (8%), in differentcompressed air flow rate and inlet air temperature of 130 ◦C.

food and bioproducts processing 9 0 ( 2 0 1 2 ) 667–675 671

Fig. 5 – Bulk density of black mulberry powders containingvarious percentage of maltodextrin 9DE, in different inleta

hitc856a

simgb6jtdatistttt2tTdrbciT6(

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Bttoi

Fig. 6 – Solubility of black mulberry powders containingvarious percentage of maltodextrin 9DE, in different inletair temperature and compressed air flow rate of 800 L/h.

Fig. 7 – Bulk density of black mulberry powders containingvarious carrier agents (8%), in different compressed air flow

water, making for uneven wetting and reconstitution. Cheginiand Ghobadian (2005), and Quek et al. (2007) reported that

Fig. 8 – Solubility of black mulberry powders containing

ir temperature and compressed air flow rate of 800 L/h.

Besides inlet air temperature, the outlet air temperatureas an important effect on the powder properties. By decreas-

ng compressed air flow rate or inlet air temperature, the outletemperature is decreased and because of that the moistureontent of powder is increased. At compressed air flow rate of00, 600, and 400 L/h, the outlet air temperature is 57, 54, and0 ◦C, respectively (at inlet air temperature of 110 ◦C), and 67,1, and 55 ◦C (at inlet air temperature of 130 ◦C), and 82, 75,nd 68 ◦C, respectively (at inlet air temperature of 150 ◦C).

Effect of different carrier agents on moisture content is alsohown in Fig. 4. Higher DE maltodextrins causes an increasen powder moisture content, because lower molecular weight

altodextrins contained shorter chains and more hydrophilicroups (Cai and Corke, 2000). Similar results were reportedy Goula and Adamopoulos (2010). They studied the effect of, 12 and 21DE maltodextrins on moisture content of orangeuice powders and concluded increasing maltodextrin dex-rose equivalent has a positive effect on moisture content. It isue to the fact that high-DE maltodextrins develop stickinessnd reach a state of non-adhesion slower than low-DE mal-odextrins. The stickier a material is, the lower the drying rates. However, Rodriguez-Hernandez et al. (2005), working withpray drying of cactus pear juice, verified higher moisture con-ent values for the powders produced with maltodextrin 10DEhan for those produced with maltodextrin 20E, in contrast tohe results obtained in this work. Their conditions for produc-ion of cactus pear juice powder were inlet air temperature of00–225 ◦C and maltodextrin concentration of 18–23%, both ofhese conditions were higher than those we used in this study.he authors attributed such difference to the polymerizationegree of each agent and they concluded that the moistureetention was greater for the maltodextrin 10DE due to itsetter binder properties. The results show that the moistureontents of the powders were significantly reduced by increas-ng percent replacement of maltodextrin 6DE by gum Arabic.he lowest water activity refers to 2% maltodextrin 6DE and% gum Arabic. It is in agreement with results of Moreira et al.2009).

.3. Bulk density and solubility

ulk density of the black mulberry powders varied from 0.35o 0.55 g/mL. Increasing inlet air temperature caused a reduc-ion in bulk density (Fig. 5) and an increase in the solubility

f spray-dried black mulberry powders (Fig. 6). There is an

nverse relation between the bulk density and solubility of

rate and inlet air temperature of 130 ◦C.

powders. This is consistent with the findings of a number ofstudies (Al-asheh et al., 2003; Chegini and Ghobadian, 2005;Goula and Adamopoulos, 2010; Kha et al., 2010). At very hightemperatures, as evaporation rates are faster, products dry toa more porous or fragmented structure and implying a lowershrinkage of the droplets, and so a lower density of the pow-der. Increasing the drying air temperature generally producesan increase in particle size (Walton, 2000). Large particles maysink, whereas small ones are dustier and generally float on

various carrier agents (8%), in different compressed air flowrate and inlet air temperature of 130 ◦C.

672 food and bioproducts processing 9 0 ( 2 0 1 2 ) 667–675

Fig. 9 – Micrographs of black mulberry powder particles produced with 8%: (a) maltodextrin 6DE, (b) maltodextrin 9DE, (c)gum Arabic, and (d) maltodextrin 20DE at 130 ◦C and 800 L/h conditions.

increasing inlet air temperature causes a decrease in solubil-ity of orange and watermelon juice powder. They found atvery high inlet air temperature, a hard surface layer mightbe formed over the powder particle. This could prevent watermolecules from diffusing through the particle. Consequently,decreased the wettability of the particle and reduced the dis-solution of the powder.

Bulk density shows a decrease with an increase in carrieragent concentration (Fig. 5). This may be due to lower moisturecontent of the products or the higher air trapped in the par-ticles, as maltodextrin is a skin-forming material. Goula andAdamopoulos (2010) reported similar results. Increased carrieragent concentration causes an increase in powder solubility(Fig. 6). This may be attributed to the fact that maltodextrin hassuperior water solubility and is mainly used in process of spraydrying due to its physical properties, such as high solubility inwater (Cano-Chauca et al., 2005; Grabowski et al., 2006; Goulaand Adamopoulos, 2010). Increasing compressed air flow ratecauses an increase in powder bulk density (Fig. 7) and a reduc-tion in solubility (Fig. 8). Increase in compressed air flow ratecauses a decrease in particle size (Goula and Adamopoulos,2005b). Smaller particles produced with higher compressed airrates are also denser and so further increase bulk density anddecrease solubility. This is in agreement with the results ofGoula and Adamopoulos (2005b) for spray drying of tomatopulp.

The effect of different carrier agents on the powder bulkdensity is shown in Fig. 7 and on the solubility of black mul-berry powders is also shown in Fig. 8. The particles producedwith maltodextrin 6DE showed the lowest bulk density, fol-lowed by those produced with gum Arabic and maltodextrin9DE. The sample produced with maltodextrin 20DE showedthe highest bulk density. An increase in maltodextrin dextroseequivalent leads to an increase in bulk density. This can be

attributed to the fact that the higher the maltodextrin DE, thelower its glass transition temperature and, as a consequence,

the lower the Tg value is, the more stickier the mixture is(Adhikari et al., 2004; Goula and Adamopoulos, 2010). Mal-todextrin DE increases lead to a decrease in powder solubility.The effect of maltodextrin DE on powder solubility depends onits effect on powder moisture content. This can be attributedto the fact that low-moisture content seems to be associatedwith fast rehydration (Goula and Adamopoulos, 2008), sincethe lower the moisture content the less sticky the powder isand, thus, the higher will be the surface area in contact withthe rehydration water. The blend of 2% maltodextrin 6DEand 6% gum Arabic show the highest solubility of about 87%.By increasing percent replacement of maltodextrin by gumArabic, the water solubility was increased. Similar resultswere reported by Moreira et al. (2009).

3.4. Microstructure and glass transition temperature

Fig. 9a–d shows the SEM micrographs of the 8% maltodextrin(6, 9, and 20DE), and gum Arabic at inlet air temperature of130 ◦C and compressed air flow rate of 800 L/h. The micro-graphs of the highest and lowest yield are also shown in Fig. 10.

In the microstructure of powders produced with maltodex-trin 20DE, it was verified that particles were larger, amorphous,all pilled up and with a strong attraction from each other(Fig. 9d), while when the dextrose equivalent of maltodextrinis decreased or by using gum Arabic, the particles tended tobecome more spherical, more scattered. This can be explainedby the molecular structure of carrier agents. The average par-ticle size of powders ranges from 4 to 13 �m (Table 2). Spraydried black mulberry powders produced with maltodextrin6DE has higher Tg point due to its larger molecule compar-ing three other carriers (Table 2). The dextrose equivalencyof maltodextrin determines their reducing capacity and is

inversely related to their average molecular weight. Increasein maltodextrin dextrose equivalent lead to lower powder Tg

food and bioproducts processing 9 0 ( 2 0 1 2 ) 667–675 673

Fig. 10 – Micrographs of black mulberry powder particles produced with: (a) 6% maltodextrin 6DE and 2% gum Arabic at130 ◦C, 800 L/h conditions (Max yield), (b) 8% maltodextrin 20DE at 110 ◦C, 400 L/h conditions (Min yield).

Fig. 11 – DSC profile for spray dried black mulberry juice powder produced with (a) 6% maltodextrin 6DE and 2% gumArabic, (b) 8% gum Arabic at 130 ◦C and 800 L/h conditions.

Table 2 – Glass transition temperature (Tg) and particle size of spray dried black mulberry juice powders containingdifferent carriers.

Composition of carriers in powder Inlet airtemperature(◦C)

Compressedair flow rate(L/h)

Tg (◦C) Particle size (�m)

8% maltodextrin 6DE 130 800 73.8 ± 1.2 5.06 ± 0.108% maltodextrin 9DE 130 800 65.7 ± 1 6.65 ± 0.158% gum Arabic 130 800 63.3 ± 1.1 7.2 ± 0.206% maltodextrin 6DE + 2% gum Arabic (Max yield) 130 800 76.4 ± 1.1 4.4 ± 0.08

(t

aotmsflbTpiflamr

4

Tc

8% maltodextrin 20DE (Min yield) 110

Table 2) because lower molecular weight maltodextrins con-ained shorter chains (Kasapis, 2005).

The Tg is the main characteristic transformation temper-ture of the amorphous phase. The glass transition eventccurs when a hard, solid, amorphous sugar undergoes aransformation to a soft, rubbery, liquid phase. The instru-

ent method used for the determination of Tg is differentialcanning calorimetry (DSC) that detects the change in heatow of the amorphous component between glass and rub-ery states occurring over the transition temperature range.he DSC technique was used to obtain heat flow versus tem-erature thermograms (Fig. 11). Glass transition temperature

s observed as an endothermic stepwise change in the heatow (Jaya and Das, 2009). In this study the highest drying yieldnd Tg point refers to the powder produced with the blend ofaltodextrin 6DE and gum Arabic. It is in agreement with the

esults of Krishnan et al. (2005).

. Conclusion

he effect of spray drying conditions, i.e. inlet air temperature,ompressed air flow rate, carrier agent concentration on black

400 40.2 ± 1.2 12.35 ± 0.23

mulberry powder drying yield, moisture content, water activ-ity, bulk density, and solubility was studied. It was observedthat:

• Higher inlet air temperature causes an increase in processyield and solubility and a decrease in bulk density, moisturecontent and water activity.

• Increasing carrier agent concentration or decreasing mal-todextrin DE causes an increase in drying yield andsolubility and a decrease in bulk density, moisture content,and water activity.

• Increasing the compressed air flow rate has a positive effecton drying yield and bulk density and a negative effect onsolubility, moisture content and water activity.

The results obtained in the present work indicate mal-todextrin 6DE to be the best carrier agent for spray dryingof black mulberry juice as compared to the maltodextrin 9and 20DE and gum Arabic. As far as the blends were con-cerned, the blend of maltodextrin 6DE and gum Arabic proved

to be more efficient than the other blends even better than100% maltodextrin 6DE. All the black mulberry juice powders

674 food and bioproducts processing 9 0 ( 2 0 1 2 ) 667–675

produced by spray drying with different carrier agents exhib-ited low moisture content (1.52–2.59%) and water activitybelow 0.32. With regard to glass transition temperature thelowest stickiness and the highest drying yield refer to thepowders which produced with the blend of carrier agents.In general it can be concluded that using gum Arabic andmaltodextrin 6DE as carrier agents leads to better physicalproperties such as yield, bulk density, solubility and powdermorphology.

References

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