24
FOAM-MAT FREEZE-DRYING OF APPLE JUICE PART 2: STABILITY OF DRY PRODUCTS DURING STORAGE NARINDRA RAHARITSIFA 1,3 and CRISTINA RATTI 2 1 Département des Sciences des Aliments et de Nutrition 2 Département des sols et de génie agroalimentaire Faculté des Sciences de l’Agriculture et de l’Alimentation Université Laval Québec G1K 7P4, Canada Accepted for Publication March 8, 2009 ABSTRACT Sorption isotherms at 5, 20 and 35C and glass transition temperature as a function of water content were evaluated for foamed (3% egg white and 1% methylcellulose) and nonfoamed freeze-dried apple juice by using the static gravimetric method and differential scanning calorimetry, respectively. Equi- librium isotherms were fitted to the Guggenheim–Anderson–de Boer equation, whereas glass transition temperature to the Gordon–Taylor model. After freeze-drying at 20C during 48 h, the dry products were stored at different temperatures (5 and 20C) under ambient conditions or vacuum. The nutri- tional, physical and structural properties were assessed before and at the end of the freeze-drying process and after storage by determining vitamin C content, solubility, color and microstructure. Freeze-dried nonfoamed juice retained more vitamin C and was more soluble than foamed products after freeze-drying. However, foam-mat juice powders presented higher stability during storage at 20C, which agreed with their higher values of glass transi- tion temperature. Freeze-dried juice stored at this temperature collapsed showing a decrease in solubility and a marked color change. PRACTICAL APPLICATIONS Quality of dry products depends largely on how they are stored (relative humidity, temperature, presence of light and vacuum). Degradation of flavor, color and texture occur under adverse storage conditions. Nutritional com- pounds such as vitamins or unsaturated fats could also be negatively affected 3 Corresponding author. TEL: 604-755-3153; FAX: 418-656-3723 (c/o C. Ratti); EMAIL: [email protected] Journal of Food Process Engineering 33 (2010) 341–364. All Rights Reserved. © 2009 Wiley Periodicals, Inc. DOI: 10.1111/j.1745-4530.2009.00517.x 341

FOAM-MAT FREEZE-DRYING OF APPLE JUICE PART 2: STABILITY OF DRY PRODUCTS DURING STORAGE

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FOAM-MAT FREEZE-DRYING OF APPLE JUICE PART 2:STABILITY OF DRY PRODUCTS DURING STORAGE

NARINDRA RAHARITSIFA1,3 and CRISTINA RATTI2

1Département des Sciences des Aliments et de Nutrition

2Département des sols et de génie agroalimentaireFaculté des Sciences de l’Agriculture et de l’Alimentation

Université LavalQuébec G1K 7P4, Canada

Accepted for Publication March 8, 2009

ABSTRACT

Sorption isotherms at 5, 20 and 35C and glass transition temperature asa function of water content were evaluated for foamed (3% egg white and 1%methylcellulose) and nonfoamed freeze-dried apple juice by using the staticgravimetric method and differential scanning calorimetry, respectively. Equi-librium isotherms were fitted to the Guggenheim–Anderson–de Boer equation,whereas glass transition temperature to the Gordon–Taylor model. Afterfreeze-drying at 20C during 48 h, the dry products were stored at differenttemperatures (5 and 20C) under ambient conditions or vacuum. The nutri-tional, physical and structural properties were assessed before and at the endof the freeze-drying process and after storage by determining vitamin Ccontent, solubility, color and microstructure. Freeze-dried nonfoamed juiceretained more vitamin C and was more soluble than foamed products afterfreeze-drying. However, foam-mat juice powders presented higher stabilityduring storage at 20C, which agreed with their higher values of glass transi-tion temperature. Freeze-dried juice stored at this temperature collapsedshowing a decrease in solubility and a marked color change.

PRACTICAL APPLICATIONS

Quality of dry products depends largely on how they are stored (relativehumidity, temperature, presence of light and vacuum). Degradation of flavor,color and texture occur under adverse storage conditions. Nutritional com-pounds such as vitamins or unsaturated fats could also be negatively affected

3 Corresponding author. TEL: 604-755-3153; FAX: 418-656-3723 (c/o C. Ratti); EMAIL:[email protected]

Journal of Food Process Engineering 33 (2010) 341–364. All Rights Reserved.© 2009 Wiley Periodicals, Inc.DOI: 10.1111/j.1745-4530.2009.00517.x

341

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because of oxidation reactions, which could be a function of the dehydrationmethod or the pretreatment used to manufacture the dry product. The deter-mination stability and quality degradation of foamed and nonfoamed applejuice products during freeze-drying and storage of the dry products will com-plete the evaluation of foaming as a potential pretreatment for freeze-drying.

INTRODUCTION

Freeze-drying is a common method for producing stable food products,with the highest quality and long shelf life. This process is largely used to drythermal sensitive products such as antibiotics, bacteria, pharmaceutical prod-ucts, food ingredients or those with high value-added such as coffee, herbs ornutraceuticals (Wolff and Gibert 1988; Palmfeldt et al. 2003). However, theenergy consumption to maintain vacuum during the long processing timesinvolved in freeze-drying is highly expensive, which constitutes the maindisadvantage of this technique. Foaming prior to freeze-drying has beenrecently proposed in order to reduce dehydration time of apple juice (Raha-ritsifa and Ratti 2008) or egg white (EW; Muthukumaran 2007). However,quality changes happening during and after freeze-drying of foamed materialshave not been reported so far, even if the fact that the different structure andhigher porosity of these materials could cause increased deterioration in termsof oxidation, aroma release, rehumidification, etc. Glass transition temperatureas well as sorption isotherms are important parameters that could help in thedetermination of the optimal process and storage conditions of these powders.

Sorption isotherms relate equilibrium moisture content to water activityat a given temperature, and are of particular importance in the determination offreeze-drying end point that ensures economic viability and microbiologicalsafety (McLaughlin and Magee 1998), in selecting appropriate packagingmaterial (Gal 1987), in describing the hygroscopic properties and stabilityduring storage. In addition, the net isosteric heat of sorption (qst), which isusually obtained from sorption experimental data, can be used to estimate theenergy requirements of a dehydration process and provides information on thestate of water in food products (Ratti et al. 1989). The moisture content levelat which the qst reaches the latent heat of vaporization of water is consideredby some researchers as an indication of the bound water in the product (Wangand Brennan 1991; Quirijns et al. 2005). Because of the complex compositionand structure of foods, mathematical prediction of sorption behavior is diffi-cult. The Guggenheim, Anderson and de Boer (GAB) isotherm equation hasbeen widely used in to describe the sorption behavior of foods such as meatproducts (Lind and Rask 1991), vegetables (Kiranoudis et al. 1993), agar-agarand derivates (Iglesias and Bueno 1999), and fruits (Kaymak-Ertekin and

342 N. RAHARITSIFA and C. RATTI

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Gedik 2004; Moraga et al. 2006). The GAB equation is an extension of thetwo-parameter (Mo, C) Brunauer, Emmett and Teller (BET) model, taking intoaccount the multilayer region through the introduction of a third parameter,k (Perez-Alonso et al. 2006).

Glass transition temperature, Tg, is considered a complementary para-meter to determine stability of foods during storage (Del Valle et al. 1998;Sablani et al. 2007). In fact, if the storage temperature is above the glasstransition temperature of the product, important changes in the state of thematerials may accelerate the mechanisms of physicochemical deterioration infoods with the risk of decreasing their shelf life (Slade and Levine 1991; Rooset al. 1996). Tg has been reported in relation to quality changes of sugars(Jovanovic et al. 2000; Ahmed et al. 2005; Foster et al. 2006; Haque and Roos2006), freeze-dried fruits (Sá et al. 1999; Shishegarha et al. 2002; Telis andSobral 2002; Marques et al. 2007), garlic (Madamba et al. 1996; Rahmanet al. 2005; Ratti et al. 2007), etc. The Gordon–Taylor model was found to beuseful in the prediction of the Tg as a function of water content (Roos 1995).

From the considerations mentioned earlier, the main goal of this workwas to characterize the quality after freeze-drying as well as the stabilityduring storage of freeze-dried foamed apple juice, and to compare it withnonfoamed samples, with the following objectives: (1) to evaluate the qualitychanges (nutritional, physical, organoleptic and structural qualities) afterfreeze-drying and storage; (2) to determine their sorption isotherm and the netisosteric heat of sorption; and (3) to obtain their glass transition temperatureand therefore the critical water activity and critical water content.

MATERIALS AND METHODS

Materials

Clarified apple juice (Del Monte, Nabisco, Ontario, Canada) havingpH 3.5 was purchased from the local supermarket and stored at 4C until use.The foaming and stabilizing agents used to foam the apple juice were EWpowder (Newly Weds, Quebec, Canada) and methylcellulose (MC; Methocel65HG, Fluka BioChemika, Buchs Sg, Switzerland).

Foam Preparation

The apple juice was whipped with a mixer at 800 rpm during 5 minwhereas the foaming agent was slowly poured into it, at room temperature.Proper amounts of apple juice and foaming agent were weighed to give finalconcentrations of 1% (w/w) MC and 3% (w/w) EW. Those concentrations

FOAM-MAT FREEZE-DRYING 343

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were found to be appropriate to obtain stable foams as previously reported(Raharitsifa et al. 2006). Nonfoamed apple juice was used as control.

Freeze-Drying

Foams (20 g) were poured carefully with the help of a spatula into250-mL beakers up to a thickness of 4 cm (VWR International Ltd., Montreal,Quebec, Canada). Samples were then placed in a freezer (Sanyo MDF 235,Gunma, Japan) at a temperature of -40C for 24 h and then freeze-dried at 20Cduring 48 h under vacuum. The drying was performed at a pressure of 4 Pa ina Unitop 400 L (Virtis, Gardiner, NY) equipped with a condenser Freezemobile 25EL (Virtis). Quality parameters such as solubility, vitamin C, colorand microstructure of foamed and nonfoamed samples were measured beforeand after freeze-drying.

Sorption Isotherm

Sorption isotherms were determined by using the static gravimetricmethod. The salts (LiCl, CH3COOK, MgCl2, K2CO3) were dissolved in boilingdistilled water and slowly cooled to test temperatures for crystallization toform saturated salt solutions. Freeze-dried samples (approximately 1 g) wereplaced in aluminium dishes, and then in vacuum desiccators containing satu-rated salt solutions providing water activities (Aw) in the range of 0 to 0.5. Theywere stored in an incubator (MIR-153, Sanyo Scientific, Bensenville, IL) at 5,20 and 35C for 10 days. After equilibrium was reached, water activity of thesalt solution was measured with a water activity meter (Aqualab Series 3model TE, Decagon Device, Inc., Pullman, WA) at the test temperature, andthe freeze-dried samples were weighed in an analytical balance (Mettler AE200, Grefensee, Zürich; with an accuracy of 60.0001 g) and transferred to avacuum oven at 50C for 48 h to obtain the dried weight (AOAC 1990). Theequilibrium moisture content in dry basis was then determined and plottedagainst water activity.

In order to represent the water sorption behavior of freeze-dried powders,the GAB model was used:

MM nCA

nA C nAo=

−( ) + −( )( )w

w w1 1 1(1)

where M is the equilibrium moisture content (kg H2O/kg d.b.); Mo is themonomolecular layer moisture content on dry basis (kg H2O/kg d.b.); C isthe surface energy constant and n is a constant correcting the properties of themultilayer molecules with respect to the bulk liquid. Parameters Mo, n and C

344 N. RAHARITSIFA and C. RATTI

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were obtained from experimental data by nonlinear regression using Sigma-Plot software (version 10.0, Systat Software Inc., Richmond, CA).

Isosteric Heat of Sorption (qst)

The net isosteric heat was obtained from the Clausius–Clapeyronequation

∂ ( )∂( )

=∂ ( )∂( )

= −ln lnp p

T

A

T

q

Ro

1 1w st (2)

where p is the equilibrium pressure (Pa); po is vapor pressure of pure water(Pa); Aw is the water activity; T is the absolute temperature (K); qst is netisosteric heat sorption (kJ/mol); and R is the universal gas constant (kJ/mol·K).The net isosteric heat of sorption was calculated from Eq. (2) by plottingln (Aw) against 1/T for a specific moisture content, followed by the determi-nation of the slope of the curve, which equals qst/R. Afterwards, the curve qst

was represented as a function of M.

Glass Transition Temperature (Tg)

A differential scanning calorimeter (DSC Pyris 1, Perkin-Elmer,Norwalk, CT) equipped with a refrigerated cooling system (Intracooler II,Perkin Elmer, Shelton, CT) was used to determine the glass transition tem-perature of freeze-dried samples at various moisture contents. Indium(Tm = 156.6C, DHfus = 28.5 J/g, Perkin Elmer standards) was utilized for tem-perature and heat flow calibration. Samples were weighed and placed inaluminium pans that were sealed hermetically to avoid any moisture lossduring the analysis. An empty aluminium pan was used as reference.

Drying and storage cause a thermal history effect on the Tg, which in turnproduce a structural relaxation in the product. This is seen as an overshoot inthe DSC thermograph during the first scan. Therefore, all the samples werescanned twice, the first scan normally being from -60 to 90C at 25C/min. Thesample was then cooled down to -60C at 40C/min before undergoing a secondscan from -60 to 90C at 5C/min. To avoid condensation of moisture, thesample head was purged with dry nitrogen (30 mL/min). Glass transitionswere recorded as the onset temperature of the discontinuities in the curves ofheat flow versus temperature. The Gordon–Taylor equation (Gordon andTaylor 1952) was fitted to the experimental glass transition temperature data:

Tgw Tg kw Tg

w kw= +

+1 1 2 2

1 2

(3)

FOAM-MAT FREEZE-DRYING 345

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where w1 and w2 are the weight fractions of the solute and water, respectively;k is a constant, Tg1 and Tg2 are the Tg of the anhydrous solute and amorphouswater (-135C from Slade and Levine 1995), respectively. Nonlinear regressionanalyses were performed using SigmaPlot software package (version 10.0,Systat Software Inc.).

Solubility

Solubility was determined using a method adapted from reported studies(Doymaz 2004; Giri and Prasad 2007). Solubility ratio (SR) was determined byimmersing 5 g (initial weight) of dried samples into a Pyrex brand crystallizingdish (80 ¥ 40 mm; VWR International, Mississauga, Ontario, Canada) con-taining distilled water. The crystallizing dish was placed on a Fisher IsotempCeramic Top Hotplate 10″ ¥ 10″ (International Model, Garner, NC), whichwas agitated at constant speed (100 rpm). Two temperatures were controlledfor the experiment, 20 or 100C. The solubility experiment was stopped at 30,60, 120 and 180 s and the solution was filtered. The solids left in the filter weredried for 30 min at 70C (to remove the excess solution) and then weighed(final weight). The accuracy of the balance (Mettler AE 200, Grefensee,Zürich, Switzerland) was 0.0001 g. Solubility ratio was obtained by dividingthe difference of the initial and final weights (before and after dissolution) bythe initial weight.

Vitamin C

The ascorbic acid content in the fresh and freeze-dried products wasdetermined by 2,6-dichlorophenol-indophenol visual titration method (Hughes1983). The reagents used were 3% meta-phosphoric acid (HPO3) solution,ascorbic acid standard containing 0.1 mg L-ascorbic acid in 1 mL of 3% HPO3

and the dye solution containing 50 mg of 2,6-dichlorophenol-indophenol in200 mL of distilled water. All the reactants were bought from Sigma-Aldrich(Canada Ltd., Oakville, Ontario, Canada).

Color Measurement

The CIE parameters L*, a* and b* values were measured with a MinoltaCR-300 (Osaka, Japan) colorimeter. The colorimeter was calibrated with awhite standard calibration plate provided by the manufacturer. The colorvalues were expressed as L (whiteness or brightness/darkness), a (redness/greenness) and b (yellowness/blueness). Luminosity (L*) and total colordifference (DE) were calculated from the following equations:

L L Lo* = − (4)

346 N. RAHARITSIFA and C. RATTI

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ΔE L L a a b bo o o= −( ) + −( ) + −( )2 2 2 (5)

Scanning Electron Microscopy (SEM) Analysis

Microstructure of freeze-dried products was observed by SEM (JEOLmodel 6360LV, Tokyo, Japan). Small amounts of samples were mounted onSEM specimen stubs by using double-sided adhesive tape. The mountedsamples were then coated with gold for 2 ¥ 60 s in a SEM sputter coatingsystem (Nanotech, SEM2, UK) under high vacuum at 2.5 kV. The coatedsamples were viewed with SEM at 30 kV in the secondary electron mode at50 and 350¥ magnifications.

Storage

Plastic 20-cm diameter desiccators with stopcock, internal gasket, perfo-rated floor and drierite adsorbent (VWR International) were used to storesamples after the freeze-drying process. Desiccators were covered to protectthem against light. Samples were stored for 70 days at either 5 or 20C inrefrigerated cabinets, with or without vacuum. Glass transition temperature,vitamin C and solubility were measured after 70 days of storage whereasmoisture content was determined once a week.

Statistics

Measurements were done at least in duplicate and in triplicate when thecoefficient of variation was higher than 10%. Data were subjected to statisticalanalysis by the general linear model procedure of SAS (SAS Institute Inc.,Cary, NC) and a least significant difference test with a confidence interval of95% was used to compare the means.

RESULTS AND DISCUSSIONS

Sorption Isotherms

Water activity values of the saturated salt solutions at 5, 20 and 35C areshown in Table 1, together with values taken from previous literature reports.As can be seen, measured values agreed closely to literature ones.

Figure 1 shows the sorption equilibrium isotherms at 5C for the freeze-dried products. Type II sigmoid-shaped isotherms were obtained for all theproducts. The sorption isotherm of freeze-dried apple juice shows similarvalues as the data for amorphous fructose, sucrose or freeze-dried apple tissue

FOAM-MAT FREEZE-DRYING 347

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found by different researchers (Iglesias and Chirife 1982; Aguilera et al.1998). It is important to note that fructose, sucrose and glucose are the mainsugars in apple juice in approximate proportions of 7:2:1, respectively (Fulekiet al. 1992).

TABLE 1.WATER ACTIVITY VALUE OF DIFFERENT SALT SOLUTIONS AT

DIFFERENT TEMPERATURES

Salt 5C 20C 35C

Exp.* Ref.† Exp.* Ref.† Exp.* Ref.†

LiCl 0.128 0.113‡ 0.110 0.113§ 0.108 0.113‡CH3COOK 0.241 0.291‡ 0.225 0.23¶ 0.211 0.216‡MgCl2 0.334 0.336‡ 0.327 0.33§ 0.32 0.321‡K2CO3 0.439 0.431‡ 0.434 0.432¶ 0.432 0.432¶

* Experimental values (this work).† Literature values.‡ Labuza et al. (1985); Resnik and Chirife (1988).§ Greenspan (1977).¶ Kiranoudis et al. (1993).

5°C

Aw

0,0 0,1 0,2 0,3 0,4 0,5

Moi

stur

e co

nten

t (gH

2O/g

d.b.

)

0,00

0,02

0,04

0,06

0,08

0,10

0,12

0,14

0,16

EW foamMC foamNonfoamedGAB model

FIG. 1. EXPERIMENTAL DATA FOR FREEZE-DRIED FOAM MADE WITH 3% EGG WHITE(EW), 1% METHYLCELLULOSE (MC) AND FREEZE-DRIED NONFOAMED APPLE JUICE

Lines are predicted sorption isotherms using the Guggenheim–Anderson–de Boer (GAB)equation at 5C.

348 N. RAHARITSIFA and C. RATTI

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Foamed freeze-dried samples were more hygroscopic than nonfoamedones. This is probably caused by an increase in the number of active sites forwater sorption because of the chemical and physical changes induced by theaddition of a macromolecule to the juice (Marques et al. 2007). Also, foamsmade with MC showed more facility to adsorb moisture in comparison withfoams made with EW, probably because of their different chemical composi-tion and their different interaction with water.

The effect of temperature on sorption isotherms was similar for all thesamples, as shown in Fig. 2 for freeze-dried foams made with MC. It is widelyreported that the equilibrium moisture content decreases with increasing tem-perature at a constant water activity. This can be explained by the higherexcitation state of water molecules at higher temperature thus decreasing theattractive forces between them (Mohamed et al. 2004).

The constants obtained by fitting the GAB equation (Eq. 1) to experi-mental data are shown in Table 2. The C value shown in this Table, which isrelated to the sorption energy, was higher than 2 for all the samples, indicatingthat the isotherms are type II (Moraga et al. 2006) as shown previously inFigs. 1 and 2.

It has been reported that the value of Mo for several foods fell within therange of 0.04–0.11 gH2O/g d.b. (Karel 1975). In the present study, the esti-mated monolayer moisture contents at 20C (as an example) were 0.061, 0.069

Aw

0,0 0,1 0,2 0,3 0,4 0,5

Moi

stur

e co

nten

t (gH

2O/g

d.b.

)

0,00

0,02

0,04

0,06

0,08

0,10

0,12

0,14

0,16

5°C20°C35°C

FIG. 2. EXPERIMENTAL DATA FOR FREEZE-DRIED FOAM MADE WITH 1%METHYLCELLULOSE (MC) AND PREDICTED SORPTION ISOTHERMS USING THE

GUGGENHEIM–ANDERSON–DE BOER EQUATION AT DIFFERENT TEMPERATURES

FOAM-MAT FREEZE-DRYING 349

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TAB

LE

2.G

UG

GE

NH

EIM

–AN

DE

RSO

N–D

EB

OE

R(G

AB

)PA

RA

ME

TE

RS

AT

DIF

FER

EN

TT

EM

PER

AT

UR

ES

GA

Bpa

ram

eter

s5C

20C

35C

EW

foam

MC

foam

Non

foam

edE

Wfo

amM

Cfo

amN

onfo

amed

EW

foam

MC

foam

Non

foam

ed

Mo

0.06

70.

081

0.05

30.

061

0.07

00.

045

0.05

00.

067

0.05

6C

19.7

2513

.475

37.5

528.

854

7.86

09.

068

5.67

14.

945

2.63

4n

1.01

51.

017

0.99

91.

030

1.05

21.

069

1.09

51.

072

1.15

3R

20.

980

0.96

60.

929

0.95

10.

942

0.91

50.

877

0.89

60.

895

EW

,egg

whi

te;

MC

,met

hylc

ellu

lose

.

350 N. RAHARITSIFA and C. RATTI

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and 0.045 gH2O/g d.b. for 3% EW, 1% MC and nonfoamed (juice), respec-tively, which (as well as for the other tested temperatures) are in the range ofthe literature values. The value of the monolayer (Mo) indicates the amount ofwater that is strongly adsorbed to the matrix. Some authors considered it as theproduct maximal water content for stability (Labuza 1980; Fonseca et al.2000). From Table 2, freeze-dried foams have higher Mo values than freeze-dried juice.

In Figs. 1 and 2, the curves predicted by the GAB model for differentproducts and temperatures are shown together with experimental data. As canbe seen, there is a good agreement between experimental and predicted values.The determination coefficient (R2) for the fitting of experimental data to GABmodel presented in Table 2 was also satisfactory.

Heat of Sorption

Knowledge of heat of sorption is useful in the estimation of the energyrequired for drying. The isosteric heat of sorption as a function of moisturecontent of freeze-dried powders used in the present study is shown in Fig. 3.As observed in fruits, the heat of sorption was found negligible over the rangeof high moisture contents, and increased dramatically as moisture decreasesbelow 10 g H2O/100 g dry basis. This is in agreement with previously reportedresults (Iglesias and Chirife 1976a,b; Goula et al. 2008).

Water content (gH2O/100g solid)

0 5 10 15 20

q st (

kJ/m

ol)

0

10

20

30

40

50

EW foamMC foamNonfoamed

FIG. 3. VARIATION OF NET ISOSTERIC HEAT WITH MOISTURE CONTENT FORFREEZE-DRIED FOAMS MADE WITH 3% EGG WHITE (EW), 1% METHYLCELLULOSE

(MC) AND FREEZE-DRIED NONFOAMED APPLE JUICE

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Because of the stronger interaction between the added macromoleculesand water (Hatakeyama et al. 2000), higher heat of sorption was observed forfoamed than nonfoamed juice. Thus, at moisture contents higher than 5 gH2O/100 g dry solid, more energy was required to remove water from foamedsamples, indicating that drying of foamed juice is not necessarily easier. Atlower water contents, however, all the products presented approximately thesame heat of sorption.

SEM Microscopy

The effect of freeze-drying process and foaming on the final microstruc-ture of the samples were observed under scanning electron microscope(Fig. 4). Freeze-dried foamed samples (Fig. 4a,b) showed a skeletal-like struc-ture with high porosity and stretched pore shape, whereas nonfoamed freeze-dried apple juice (Fig. 4c) presented a compact and less porous structure.

Glass Transition

Figure 5 shows the value of Tg (onset) for the different samples as afunction of moisture content. As expected, Tg value decreased with increasingwater content because of the water plasticization. As the glass transition ofpure water is -138C (Hallbrucker and Mayer 1987), increasing values of watercontent reduces the Tg of biopolymer-water mixtures. As can be seen fromFig. 5, foamed juice powders present higher glass transition temperatures (i.e.,10C higher) than freeze-dried juice, certainly because of the presence of highmolecular weight macromolecules used to form the foams (i.e., MC or eggalbumen). Glass transition temperature is known to increase with increasingmolecular weight (Slade and Levine 1995; Roos et al. 1996).

The Gordon and Taylor (1952) model, shown in Eq. (3), is a reliableexpression to represent glass transition temperature, which has been success-fully applied to several foods such as fruits and vegetables. Experimental data

a 3% EW b 1% MC c Nonfoamed

FIG. 4. SCANNING ELECTRON MICROGRAPHS OF FREEZE-DRIED FOAM MADE WITH(a) 3% EGG WHITE (EW), (b) 1% METHYLCELLULOSE (MC) AND (c) NONFOAMED

FREEZE-DRIED APPLE JUICE

352 N. RAHARITSIFA and C. RATTI

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on glass transition was fitted to Eq. (3); the parameters resulting from thisfitting are shown in Table 3.

As can be seen, the parameter Tgs, which is a measure of the glasstransition temperature of the dry mixture, is much higher for foamed juicepowders as compared with nonfoamed. Figure 5 presents the model predic-tions together with experimental data of foamed and nonfoamed juicepowders, showing a close agreement.

Storage

Roos (1995a) and Levine and Slade (1991), reported that plastification ofbiosolids is a result of the combined effects of temperature and moisture. The

Moisture content (% d.b.)

0 2 4 6 8 10 12 14

Tg

(°C

)

-40

-30

-20

-10

0

10

20

30

EW foamMC foamNonfoamedGordon–Taylor model

FIG. 5. GLASS TRANSITION ONSET TEMPERATURE OF FREEZE-DRIED FOAM MADEWITH 3% EGG WHITE (EW), 1% METHYLCELLULOSE (MC) AND NONFOAMED

FREEZE-DRIED APPLE JUICE, AS A FUNCTION OF MOISTURE CONTENT

TABLE 3.GORDON–TAYLOR (G–T) PARAMETERS OF THE FOAMED

AND NONFOAMED FREEZE-DRIED APPLE JUICE

G–T parameters EW foam MC foam Nonfoamed

Tgs 26.49 23.47 17.45k 4.034 3.722 4.651R2 0.977 0.976 0.948

EW, egg white; MC, methylcellulose.

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relationship between equilibrium moisture content, glass transition tempera-ture and water activity is plotted in Fig. 6 for freeze-dried foamed and non-foamed apple juice at 5C. This figure shows that water content (CWC) and itscorresponding water activity (CWA), which Tg is below the storage tempera-ture (i.e., 5C), can be considered as a critical value for stability. According toRoos (1995), such information shows the combined effects of water activityand temperature on physical state and provides an important tool for theprediction of the behavior of foods in processing, handling and storage. Thevalues of CWA and CWC for each sample at 5 and 20C, temperatures usedduring storage, are given in Table 4. The results presented in this Table indi-cated that, at storage temperature of 5C, freeze-dried foam made with 3% EW,1% MC and nonfoamed samples should be stored at relative humidities lowerthan 21.8, 14.8 and 15%, respectively, corresponding to their value of CWA.Exceeding those values during storage may dramatically induce deteriorationsuch as stickiness or caking of the products (Slade and Levine 1991; Roos1995a,b). For 20C, the values of CWA and CWC for freeze-dried nonfoamedapple juice are practically 0 (zero) as the glass transition temperature for thecompletely dry apple juice corresponds to the storage temperature. This clearlyindicates that freeze-dried apple juice could have a higher risk of deteriorationat storage temperatures equal or higher than 20C. In addition, freeze-driedfoams made with 3%EW presented always the highest CWA and CWC valuesbecause of its higher Tg in comparison with other products, which indicatesthat this product should be more stable during storage.

It is important to note that monolayer values (Mo, Table 2) were higherthan the critical moisture content values for stability (CWC, Table 4). Sladeand Levine (1991) indicated that rates of deteriorative reactions are related toCWC, which include the molecular mobility above Tg, rather than to themonolayer value, Mo, only obtained from sorptional data.

During the storage experiment, no deterioration was detected in anyproduct at 5C. However, after 45 days of storage at 20C, freeze-dried juicecollapsed as shown in Fig. 7. This can be explained by the fact that the storagetemperature exceeded the Tg of the freeze-dried juice, confirming what wasindicated previously (Table 4).

Solubility

Solubility of freeze-dried foamed and nonfoamed apple juice after freeze-drying and after 70 days of storage at 4 and 20C is shown in Fig. 8 as afunction of dissolution time. Figure 8a was obtained just after the freeze-drying process. As can be seen, freeze-dried juice dissolved instantaneously ascompared with foamed freeze-dried products. Foamed freeze-dried applejuice needed more time to dissolve than the nonfoamed one, with a delayed

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EW foam at 5°C

-30

-20

-10

0

10

20

30

0

2

4

6

8

10

12

14

Tg Isotherm

MC foam at 5°C

Tg(

°C)

-40

-30

-20

-10

0

10

20

30

Moi

stur

e co

nten

t (gH

2O/1

00gd

.b.)

0

2

4

6

8

10

12

14

16

Tg Isotherm

Nonfoamed at 5°C

Aw

0,0 0,1 0,2 0,3 0,4 0,5 0,6-40

-30

-20

-10

0

10

20

0

2

4

6

8

10

12

Tg Isotherm

CWC

CWC

CWC

CWA

CWA

CWA

Storage temperature (5°C)

Storage temperature (5°C)

Storage temperature (5°C)

a

b

c

FIG. 6. RELATIONSHIP BETWEEN GLASS TRANSITION TEMPERATURE AND SORPTIONISOTHERM AT 5C FOR FREEZE-DRIED FOAM MADE WITH 3% EGG WHITE (a), 1%

METHYLCELLULOSE (b) AND NONFOAMED FREEZE-DRIED APPLE JUICE (c)CWA, corresponding water activity; CWC, water content.

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dissolution, probably because of the addition of macromolecules (egg albumenor MC) to the juice. Freeze-dried foams made with EW were, however,significantly more soluble than those made with MC (P < 0.01). The higherinsolubility of MC could be because of the degree and distribution of substitu-ents within the anhydroglucose units of MC (Miyamoto et al. 1985). Theeffect of solution temperature (20 or 100C) on solubility was not significant(not shown in Fig. 8).

On the other hand, it was observed a reduction in the solubility of EWfreeze-dried foams during storage, which can be caused by the cross-linking of

TABLE 4.CRITICAL WATER ACTIVITY AND CRITICAL WATER CONTENT VALUES OF THE

FOAMED AND NONFOAMED FREEZE-DRIED APPLE JUICE

EW foam MC foam Nonfoamed

5C CWA (%) 21.8 14.8 15.0CWC (gH2O/g d.b.) 0.072 0.062 0.045

20C CWA (%) 3.8 3.8 –CWC (gH2O/g d.b.) 0.01 0.008 –

EW, egg white; MC, methylcellulose.

Nonfoamed Foamed

b

a

Collapse

FIG. 7. FREEZE-DRIED JUICE (FOAMED AND NONFOAMED) AFTER 7 WEEKS OFSTORAGE AT 20C

(a) Side view. (b) Top view.

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0,2

0,4

0,6

0,8

1,0

EW foamMC foamNonfoamed

Sol

ubili

ty r

atio

(W

i-Wf/W

i)

0,0

0,2

0,4

0,6

0,8

1,0

EW foam_4°CstorageMC foam_4°CstorageNonfoamed_4°Cstorage

t=0

70j/4°C

70j/20°C

Dissolution time (s)

0 20 40 60 80 100 120 140 160 180 200

0,0

0,2

0,4

0,6

0,8

1,0

EW foam_20°CstorageMC foam_20°CstorageNonfoamed20°Cstorage

a

b

c

FIG. 8. SOLUBILITY BEHAVIOR AT 20C OF SAMPLES AFTER THE FREEZE-DRYINGPROCESS (a) AND AFTER 70 DAYS OF STORAGE AT 4C (b) AND 20C (c)

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the proteins (Anema et al. 2006) decreasing their interaction with water.Freeze-dried foams made with EW, and stored at 4C, took longer time todissolve than those measured right after the freeze-drying process. Also, it hasbeen observed that the solubility of the nonfoamed freeze-dried juice was notinstantaneous after 70 days storage at 20C. It is important to note that freeze-dried juice collapsed after 7 weeks of storage.

Vitamin C

The losses of ascorbic acid after the freeze-drying process and after 70days of storage at different conditions are shown in Table 5. All rehydratedsamples contained less ascorbic acid than the fresh product because of acombination of oxidation (especially during foaming) and subsequent rehy-dration (Inyang and Ike 1998). However, the special conditions of the freeze-drying process (i.e., low temperature and the use of vacuum) led to a goodretention of vitamin C (more than 90%) during the process, even in the case offoamed samples with their porous structure.

Concerning the effect of storage, as each dessicator was kept in the dark,the light negative effect was not observed. The combination of low tempera-ture (4C) and vacuum has a very significant effect to improve the retention ofvitamin C during storage (P < 0.01). At 20C, retention of vitamin C fornonfoamed freeze-dried juice was higher than for foamed products, probablybecause of the higher porous structure of the dry foams.

TABLE 5.VITAMIN C LOSSES CAUSED BY THE FREEZE-DRYING PROCESS AND AFTER 70 DAYS

OF STORAGE AT DIFFERENT CONDITIONS

Effect of process Effect of storage

Product Loss (%) Condition Product Loss (%)

EW foam 10 � 0.9 4C (vacuum) EW foam 4.30 � 3.11MC foam 6.6 � 0.6 MC foam 5.30 � 2.68Nonfoamed 6.5 � 0.5 Nonfoamed 3.95 � 0.77

4C (air) EW foam 19.55 � 2.33MC foam 22.50 � 5.93Nonfoamed 21.15 � 7.84

20C (vacuum) EW foam 10.80 � 0.98MC foam 22.40 � 1.69Nonfoamed 14.65 � 1.34

20C (air) EW foam 20.75 � 0.63MC foam 25.00 � 2.40Nonfoamed 14.95 � 0.92

EW, egg white; MC, methylcellulose.

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Color

Color is one of the most important quality parameters in dehydratedproducts. Difference in luminosity (Lo-L = L*) and total color difference (DE)values between the fresh product and dissolved samples after freeze-dryingand after the storage of foamed and nonfoamed samples are presented inTable 6. The influence of freeze-drying affected differently the color change inthe samples. Dissolved freeze-dried foams showed slightly higher luminosityand total color difference than dissolved freeze-dried juice. These results wereconfirmed by visual observations. During storage, negative values of L-Lo wereobserved for freeze-dried nonfoamed and MC foamed powders, which meansthat lightness values decreased as a function of time. Moreover, freeze-driedfoams made with EW presented the highest lightness change. On the otherhand, freeze-dried juice showed the highest DE value after storage. The colorchange was thus more marked with this product in comparison with foamedones, which could be because of the fact that this product presented a lower Tg(Fig. 5) and, after 7 weeks of storage at 20C, not only the structure collapsedas visually observed but also there was nonenzymatic browning in the sample(Fig. 7).

CONCLUSIONS

Freeze-dried apple juice powders (foamed and nonfoamed) presentedclassical type II sorption isotherms. Freeze-dried foamed apple juice was morehygroscopic than nonfoamed samples, probably because of their porous struc-ture and the strong interaction between added macromolecules and water.Also, foamed samples showed substantially higher glass transition tempera-tures. As the nonfoamed product had a lower Tg, their storage at temperaturesabove 20C presented higher risk of collapse. At 20C storage temperature and

TABLE 6.THE EFFECT OF FREEZE-DRYING PROCESS AND THE STORAGE ON COLOR

PARAMETERS OF FOAM AND NONFOAMED SAMPLES

Products Effect of freeze-drying Effect of storage

L-Lo DE L-Lo DE

EW foam 5.21 � 2.02 6.41 � 1.58 7.12 � 3.05 7.21 � 2.99MC foam 4.18 � 3.25 4.98 � 2.86 -2.76 � 2.31 2.91 � 2.17Nonfoamed 1.66 � 0.45 1.71 � 0.58 -2.95 � 2.4 12.36 � 5.74

EW, egg white; MC, methylcellulose.

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in presence of air, nonfoamed freeze-dried product collapsed with markedchange of color, confirming the results found by the glass transition determi-nations. Although freeze-dried foams had a more porous structure than non-foamed ones, their solubility was lower. No marked loss of vitamin C wasdetected after the freeze-drying process. Finally, the best storage conditionfor the freeze-dried foamed and nonfoamed apple juice was found to be 4Cunder vacuum.

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