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Water Sorption and Glass Transition Temperature of

Spray-Dried Mussel Meat Protein HydrolysateVanessa M. Silva

a , Louise E. Kurozawa

b c , Kil J. Park

c & Míriam D. Hubinger

a

a University of Campinas, School of Food Engineering, Campinas, SP, Brazil

b Department of Food Technology, Rural Federal University of Rio de Janeiro, Seropédica,

RJ, Brazilc University of Campinas, School of Agriculture Engineering, Campinas, SP, Brazil

Available online: 29 Nov 2011

To cite this article: Vanessa M. Silva, Louise E. Kurozawa, Kil J. Park & Míriam D. Hubinger (2012): Water Sorption and GlassTransition Temperature of Spray-Dried Mussel Meat Protein Hydrolysate, Drying Technology, 30:2, 175-184

To link to this article: http://dx.doi.org/10.1080/07373937.2011.628766

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Water Sorption and Glass Transition Temperature ofSpray-Dried Mussel Meat Protein Hydrolysate

Vanessa M. Silva,1 Louise E. Kurozawa,2,3 Kil J. Park,3 and

Mıriam D. Hubinger1

1University of Campinas, School of Food Engineering, Campinas, SP, Brazil2Department of Food Technology, Rural Federal University of Rio de Janeiro, Seropedica, RJ, Brazil3University of Campinas, School of Agriculture Engineering, Campinas, SP, Brazil

The water sorption behavior at 25�C and glass transition tem-perature (Tg) of mussel meat protein hydrolysate powder withoutand with maltodextrin 10 DE or gum arabic at 15 and 30% (w/w)were studied in this work. The sorption isotherms were determinedby the gravimetric method, and the glass transition temperaturewas obtained by differential scanning calorimetry (DSC) after pow-der conditioning at various water activities. Sorption isotherms datawere well fitted by a modified Brunauer-Emmett-Teller (BET)model. Powder without additives showed the highest water adsorp-tion followed by those produced with 15 and 30% of carrier agent,respectively. The Gordon-Taylor model was able to predict theeffect of water on the glass transition temperature. At 25�C, thecritical water content that ensured the glassy state of the musselhydrolysate powder during storage increased from 0.05 to 0.12 gwater/g product and the critical water activity increased from 0.24to 0.60 when the concentration of carrier agents was increased to30%.

Keywords Glass transition; Mussel; Physical stability; Sorptionisotherms

INTRODUCTION

Global demand for flavors and fragrances is projected toincrease 4.3% per year, reaching US$23.5 billion in 2014.Overall advances will be stimulated by gains in food andbeverage processing activity, which still represents the lar-gest market segment, with 47% of aggregate demand, in2009.[1] This is due to the widespread application of flavormaterials in processed foods, snacks, soft drinks, and otheritems such as meat and seafood products, sauces, and con-diments.

High-quality seafood flavors can be added to driedsoups, sauces, instant noodles, snacks, surimi seafoods,and soft drinks, such as tomato juices. Mussel meat pre-sents a unique taste, high-quality raw material, mainlycompared to fish residues, which ensures a good quality

flavor and low fatty content, which avoids the susceptibilityto lipid oxidation.

Most commercial natural seafood flavor extracts areproduced by an enzymatic hydrolysis process. The rawstock is homogenized, pasteurized, the enzyme is added,and the process is carried out until a specific degree ofhydrolysis is attained. Then the enzyme is inactivated andthe solution is filtrated and concentrated or dehydrated.[2]

Most successful seafood flavors produced by this processare made from clam, crab, shrimp, lobster extracts, andmussel meat.[3–8]

Nevertheless, protein hydrolysates in a liquid state arehighly perishable due to their high moisture and proteincontents; therefore, an additional process is necessary toimprove their shelf lives. The most commonly used tech-nique for dry flavorings production is spray drying.[2]

Powdered flavorings are more stable and well as easier tohandle and incorporate into a dry food system.

The glass transition temperature (Tg) is defined as thetemperature at which an amorphous system changes froma glassy to a rubbery state. Structural changes, such asstickiness, caking, and collapse, occur in amorphous foodpowders when stored at temperatures above the Tg. Never-theless, the glass transition concept and state diagrams arenot considered as the only factors that determine thechemical, biochemical, and microbial stability of foodsystems. Glass transition temperatures and the physicalstate of a system are not the only way to clarify complexchemical degradation reactions as enzyme inactivation,nonenzymatic browning, vitamin degradation, proteindenaturation, and oxidation in food systems. Other factors,such as food constituents and constituent concentrations,play a major role in predicting and explaining deteriorationresulting from those chemical reactions.[9] Diffusion-controlled reactions, such as volatile losses, increase withhigher water content and molecular mobility. Therefore,the retention of volatiles is favorable in amorphousmatrices below Tg. To avoid or reduce those problems, it

Correspondence: Mıriam D. Hubinger, University of Campi-nas, School of Food Engineering, Monteiro Lobato Street, 80,13083-862, Campinas, SP, Brazil; E-mail: mhub@fea.unicamp.br

Drying Technology, 30: 175–184, 2012

Copyright # 2012 Taylor & Francis Group, LLC

ISSN: 0737-3937 print=1532-2300 online

DOI: 10.1080/07373937.2011.628766

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is possible to establish and apply critical values of tempera-ture and water content.[10]

Some studies determined the Tg of proteins from aba-lone, tuna, shark, king fish, and horse mackerel[11–15] andfew works on protein hydrolysates from fish and chickenbreast are found in literature.[16,17] The Tg of proteinhydrolysates is lower than the Tg of intact proteins. More-over, it the collapse of fish protein hydrolysate occurs dueto short exposures to high humidity and, consequently, lossof free-flowing properties.[16] This suggests that proteinhydrolysates may be as important as low-molecular-weightsugars with respect to product stability during processingand storage.

Protein hydrolysates contain low-molecular-weightpeptides, are highly hygroscopic, and can either remainas a syrup or stick to the dryer chamber wall during spraydrying, causing wall deposition. Caking can also manifestduring storage.[18,19] The addition of high-molecular-weight additives (carriers) such as maltodextrin or gumarabic to the solution can raise the overall Tg of themixture, avoiding structural changes that affect the vola-tiles retention of the powders during drying and=orstorage steps.

Hydrolyzed starches offer the advantages of being rela-tively inexpensive and low in viscosity at high solids con-tent and can afford good protection against oxidation.They are products of starch hydrolysis, consisting of a-D-glucose units linked mainly by (1! 4) glycosidic bondsand described by their dextrose equivalency (DE), whichdetermines their reducing capacity and is inversely relatedto their average molecular weight.[20,21] The major short-comings of these products are the lack of emulsifyingcapacity and marginal retention of lipophilic volatiles.However, they are very well suited as carriers for hydrophi-lic volatiles.[22]

Gum arabic is a natural plant exudate of acacia treesand consists of a complex heteropolysaccharide with ahighly ramified structure, presenting a main chain formedof D-galactopyranose units joined by b-D-glycosidic bonds(1! 3). It produces low-viscosity solutions at high concen-trations, has excellent emulsifying properties, and providesgood volatiles retention during the drying process.[21–23]

Few studies have reported the use of carrier agents, suchas maltodextrin, gum arabic, and a mixture of soy proteinisolate with gelatin, on the spray-drying of protein hydro-lysates, including studies on black tilapia, chicken breast,and casein, respectively.[24–26] Therefore, it is interestingto study the effect of the type of carrier agent on the Tg

of hydrolysates, a property that differs as a function ofthe protein source as well as the degree of hydrolysisachieved, in order to determine the critical conditions atwhich products can be stored without undergoing deterio-rative changes such as stickiness, caking, and collapse inaddition to volatile losses.

The aim of this work was to study the effect of usingmaltodextrin or gum arabic on the physical stability ofspray-dried mussel meat protein hydrolysate. For this pur-pose, critical storage conditions were determined based onthe water adsorption and glass transition temperature ofpowders conditioned at different water activities.

MATERIALS AND METHODS

Material

Mussel meat, Perna perna, was purchased fromMarepesca Industry Import and Export Ltd. (Imaruı,Brazil). For enzymatic hydrolysis, the commercial proteaseProtamex (Novozymes, Bagsvaerd, Denmark), which is amixture of serine and metallo endopeptidases obtainedfrom Bacillus licheniformis and Bacillus amyloliquefaciens,with a declared activity of 1.5AU=g (density¼ 1,100 kg=m3), was used. The gum arabic Instantgum (Colloids Nat-urels, Sao Paulo, Brazil) and maltodextrin 10 DE (CornProducts, Mogi Guacu, Brazil) were used as carrier agents.

Preparation of the Protein Hydrolysate

The hydrolysis experiments were carried out in a 7-Lthermostatically controlled stirred-batch reactor using thepH-stat procedure, as described by Adler-Nissen.[27] Thesamples were ground and homogenized with distilled water(meat : water ratio 1:2 w=w). The mixture was then heatedup to 51�C and the pH was adjusted up to 6.85 with 1NNaOH. Enzyme was added (4.5 g enzyme=100 g protein)to the mixture and the reaction pH was maintained con-stant by the continuous addition of 1N NaOH. The totaltime of hydrolysis was of 3 h, after which the reactionwas stopped by heating the mixture up to 85�C for15min for enzyme inactivation. The resulting solutionwas centrifuged at 3,500 rpm (model Allegra 25R,Beckman Coulter, Fullerton, CA) for 20min to separatethe lipids. Process conditions were established accordingto the results obtained by Silva et al.[28] The protein hydro-lysate was stored in a cold chamber at ÿ18�C before thespray-drying process. The chemical composition (moisture,protein, and ash) of the mussel meat protein hydrolysatewas obtained according to the Association of Official Ana-lytical Chemists (AOAC)[29] and lipids according to Blighand Dyer,[30] as summarized in Table 1.

Spray Drying

Before the spray-drying process, carrier materials—mal-todextrin (MD) or gum arabic (GA)—were added directlyto the protein hydrolysate at concentrations of 15 and30%—that is, 15% of carrier agents for 85% of hydrolysateand 30% of carrier agents for 70% of hydrolysate (w=w)—with magnetic stirring until complete dissolution. Spraydrying was performed in a laboratory-scale spray dryer(model MSD1, Labmaq, Ribeirao Preto, Brazil) with a

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1.2-mm-diameter nozzle and spray chamber of500mm� 150mm. The mixture (at 25�C) was fed intothe chamber through a peristaltic pump at a feed flow rateof 0.8 kg=h. The drying air flow rate was 36m3=h, com-pressor air pressure was 0.25MPa, and compressor air flowrate was 2.4m3=h. Inlet air temperature was 180�C andoutlet air temperature was 105� 5�C for each sample.The conditions were established after preliminary tests.

Sorption Isotherms

Sorption isotherms were determined by the gravimetricmethod. Eight saturated salt solutions were prepared (LiCl,CH3COOK, MgCl2, K2CO3, Mg(NO3)2, KI, NaCl, andKCl) in order to provide relative humidity values of 11.3,22.6, 32.8, 43.2, 52.9, 68.9, 75.3, and 84.3%, respectively,[31]

at 25�C. Triplicate samples of 1 g of mussel hydrolysatepowder were weighed into aluminum vials and equilibratedover such saturated solutions in desiccators at 25�C. Therequired time for equilibration was 3–4 weeks, based onthe change in sample weight, which did not exceed 0.1%.The equilibrium moisture content was determined in avacuum oven at 70�C until constant weight.[29]

Sorption isotherms are generally described by empiricalmathematical models easily found in the literature. Theclassic Brunauer-Emmett-Teller (BET) model[32] shown inEq. (1), is a two-parameter model that assumes the conden-sation of an infinite number of layers from the vapor phaseonto the adsorbent surface. However, this model fails forwater activities higher than 0.5.[33]

Xe ¼XmCBETaw

ð1ÿ awÞð1þ ðCBET ÿ 1ÞawÞð1Þ

In their original publication, Brunauer et al.[32] alsoderived a modified model, considering a limited numberof adsorbed layers, allowing the modeling of water activi-ties up to 0.9 (Eq. (2)). The isotherm models used werethe BET (two- and three-parameter), Eqs. (1) and (2),

and Guggenheim-Anderson-De Boer (GAB) models,[34]

Eq. (3).

Xe ¼XmCBETaw 1ÿ nþ 1ð Þ awð Þnþn awð Þnþ1

h i

ð1ÿ awÞ 1ÿ 1ÿ CBETð Þaw ÿ CBET awð Þnþ1h i ð2Þ

Xe ¼XmCGABKGABaw

ð1ÿ KGABawÞð1þ KGABðCGAB ÿ 1ÞawÞ½ �ð3Þ

Glass Transition Temperature

The glass transition temperature was determined bydifferential scanning calorimetry (DSC) in a TA-MDSC-2920 (TA Instruments, New Castle, DE) equipped witha mechanical refrigeration system (refrigerated coolingaccessory, RCS). Mussel hydrolysate powder of about3mg was placed into aluminum pans (20 mL) and equili-brated over saturated salt solutions in desiccators at25�C. After equilibrium was reached, samples were her-metically sealed, weighed, and taken for DSC analysis.The powders were heated at 10�C=min from ÿ70 to120�C and an empty pan was used as a reference.Depending on the sample moisture content, differentinitial and final temperatures were used. Two runs wereperformed for each sample, once the second scanningreduces the enthalpy relaxation of the amorphous powder,which appeared in the first scan, thereby enhancing theaccuracy of Tg measurement visible on the DSC thermo-gram. Equipment calibration was performed with indium(Tmelting¼ 156.6�C) and verification was performed withazobenzol (Tmelting¼ 68.0�C). Dry helium, 25mL=min,was used as the purge gas. All analyses were done in trip-licate and data were treated using Universal Analysis 3.9(TA Instruments).

The plasticizing effect of water on the glass transitionwas described by the Gordon-Taylor model,[35] representedby Eq. (4), where Tgw was taken as ÿ135�C.[36]

Tg ¼wsTgs þ kwwTgw

ws þ kww

ð4Þ

where Tg is the glass transition temperature, Tgs is thesolid glass transition temperature, ww is the water frac-tion, ws is the solids fraction, Tgw is the water glass tran-sition temperature, and k is the Gordon-Taylor modelcoefficient.

Statistical Analysis

In order to obtain the model parameters, a nonlinearregression analysis was carried out using Statistica 5.0(Statsoft, Tulsa, OK). The degree of fitness of each model

TABLE 1Chemical composition of the mussel meat and mussel meat

protein hydrolysate

AnalysisMussel meat(% wet basis)

Protein hydrolysate(% wet basis)

Moisture 75.23� 0.15 92.01� 0.02Protein 15.69� 0.14 5.83� 0.03Fat 3.21� 0.15 0.36� 0.03Ash 1.71� 0.01 0.89� 0.07

Values represent means of three determinations� standarddeviations.

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was evaluated by the determination coefficient and meanrelative error (E).

E ¼1

100

X

N

i¼1

Ve ÿ Vp

�

�

�

�

Ve

ð5Þ

RESULTS AND DISCUSSION

Sorption Isotherms

Figure 1 shows the sorption isotherm plots at 25�C ofpure mussel meat protein hydrolysate and powders pro-duced with 15 and 30% of maltodextrin 10 DE or gum ara-bic stored at eight water activities. According to this figure,the equilibrium moisture content increased as wateractivity increased at 25�C.

Experimental data of sorption isotherms were fitted tothe GAB and BET (two- and three-parameter) modelsand the estimated parameters are presented in Table 2.The best results were found for the GAB (R2> 0.989 andE< 13%) and three-parameter BET (R2> 0.993 andE< 9.87%) models. However, Lewicki[37] suggested thatthe GAB model constants should present values rangingfrom 0.24�KGAB� 1 and 5.67�CGAB�1 to show a sig-moidal curve behavior, to be in agreement with BETmodel, and to ensure Xm values close to actual moisturecontent (error� 15.5%). As can be seen, for all powdersthe CGAB constant was outside of this range. Therefore,the three-parameter BET model was chosen as the bestadjustment. The adsorption curves (Fig. 1) were type IIIaccording to Brunauer et al.’s classification.[32] This typeof curve was also observed for freeze-dried fish and chickenmeat protein hydrolysates.[16,17] The effect of carrier typeand its concentration was also evaluated using sorption iso-therms curves. Pure mussel meat hydrolysate was the most

hygroscopic powder because it contains low-molecular-weight peptides. The use of carrier agents reduced thewater adsorption capacity of the powders. The powdersproduced with 30% of carrier agent showed the lowestwater adsorption, followed by those produced with 15%of carrier agent.

The monolayer moisture content (Xm) is the amount ofwater that is tightly bound and cannot act as anaqueous-phase reaction medium and=or the rate of thereaction is so slow in this water as to be negligible in termsof food storage stability. The Xm values for mussel meatprotein hydrolysate ranged from 0.052 to 0.083 g water=gsolids with carrier agents and 0.139 g water=g solids forpure mussel meat protein hydrolysate powders accordingto the three-parameter BET model. Powders produced withmaltodextrin 10 DE presented lower Xm values comparedto powders produced with gum arabic, and an increase inthe carrier agent concentration reduced the Xm value. Thiscan be explained by the composition of the carrier agent.Because gum arabic has a great number of ramificationswith hydrophilic groups, it can easily adsorb moisture fromthe ambient air, whereas maltodextrin 10 DE is less hydro-lyzed, showing less hydrophilic groups, thus adsorbing lesswater and showing low hygroscopicity. Lower powderhygroscopicity was also observed for powders producedwith lower DE maltodextrins of betacyanin pigments,anthocyanins pigments, and acai juice.[38–40] The visualappearance of the powders showed that the pure musselmeat hydrolysate adsorbed more water and, as a result, aliquefaction occurred at water activities higher than0.328. However, powders formulated with both carrieragents presented greater physical stability than pure hydro-lysate, with the appearance of a free-flowing powder atwater activities lower than 0.689.

In relation to water adsorption, for powders producedwith 15% of maltodextrin or gum arabic and 85% of hydro-lysate, Xm values were of 0.074 and 0.083 g water=g solids,respectively (Table 2). Similar values were found byPerez-Alonso et al.[41] of 0.073 and 0.081 g water=g solidsfor pure maltodextrin 10 DE and gum arabic, respectively,reflecting the major influence of carrier agents on sorptionphenomena. As the carrier agent concentration increasedup to 30% and the amount of hydrolysate was reduced to75%, Xm values were reduced to 0.052 and 0.069 g water=g solids for maltodextrin and gum arabic, respectively.

Kurozawa et al.[17] also observed a reduction in Xm ofsamples formulated with 10–30% maltodextrin 10 DEand gum arabic (0.067–0.038 and 0.081–0.057 g water=gsolids, respectively). In addition, the Xm value was of0.153 g water=g solids for pure spray-dried chicken meatprotein hydrolysate. Aguilera et al.[16] reported an Xm valueof 0.062 g water=g solids for freeze-dried fish proteinhydrolysate. In the present work, the mussel meat hydroly-sate powder was less hygroscopic than the chicken meat

FIG. 1. Water sorption isotherms of pure mussel meat hydrolysate pow-

der and formulated with 15 and 30% of maltodextrin 10 DE or gum ara-

bic. The symbols represent the experimental data and the lines the BET

model curves adjusted for each type of microcapsule.

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hydrolysate because the monolayer moisture content waslower; however, both were more hygroscopic thanfree-dried fish hydrolysate. This variation could be attribu-ted to the difference in substrate composition used in thehydrolysis reaction and consequently in the producedhydrolysate, type of enzyme, and degree of hydrolysisachieved as well as the different drying conditions thatinfluence the powder’s physical characteristics.

Glass Transition Temperature

Figure 2 shows the thermograms obtained from DSCanalysis of each powder stored at different water activitiesand at 25�C. The glass transition temperature was taken asthe midpoint of the glass transition. According to Fig. 2,the glass transition temperature decreased with an increasein moisture content due to the plasticizing effect of water.At high water activities, this shift became clearer due tothe increase in sample moisture content, as observed inthe water sorption curves.

The effect of carrier agent maltodextrin or gum arabicon the glass transition temperature of mussel proteinhydrolysate can also be observed. The addition of carrieragents led to an increase in Tg values. For pure mussel meatprotein hydrolysate, the increase in water activity from0.113 to 0.843 led to a reduction in Tg from 36.1 toÿ62.1�C. The same trend was also observed forfreeze-dried fish and spray-dried chicken protein hydroly-sates, where the increase of water activity from 0.12 to

0.64 and 0.113 to 0.843 led to a reduction of Tg from 23.9to ÿ42.8�C and 28.4 to ÿ72.2�C, respectively.[16,17]

Low-molecular-weight peptides are present in hydroly-sates; consequently, they have low Tg values. On the otherhand, food polymers, such as actin and myosin proteins,have a higher molecular weight; therefore, the Tg tends tobe higher. Hashimoto et al.[42] found higher Tg values forbonito muscles after heat treatment and freeze drying, from60 to 140�C, at moisture contents ranging from 11 to 1%.Freeze-dried tilapia presented Tg values from 52.3 up toÿ58.7�C when water activities ranged from 0.11 to 0.85,respectively.[43] Sablani et al.[11] reported a Tg for abaloneranging from 90 to ÿ30�C at water activities between 0.2and 1.0, respectively.

The experimental Tg data were well-fitted to theGordon-Taylor model, showing an average relative errorlower than 3%. The estimated parameters of theGordon-Taylor model (Tgs, k), mean relative error (E),and coefficient of determination (R2) for pure hydrolysateand powders formulated with 15–30% of maltodextrinand 15 and 30% of gum arabic can be seen in Table 3.

In order to increase the powder’s stability, the additionof carrier agents with a high molecular weight (1,800 g=molfor maltodextrin[44] and 47,000–3,000,000 g=mol for gumarabic[45]) increased Tg values of powders produced with15% of carrier agent, because the Tg of anhydrous malto-dextrin 10 DE is 160�C.[44] For gum arabic, no datafor anhydrous material was found. However, there are

TABLE 2Parameters of GAB and BET (two- and three-parameter) models for pure mussel meat hydrolysate and for powders

produced with 15 and 30% of maltodextrin 10 DE (MD) or gum arabic (GA)

Model Concentration Parameters R2 E (%)

GAB Xm CGAB KGAB

0% 0.137 2.27 0.94 0.990 10.0015% MD 0.092 2.27 0.90 0.993 5.4430% MD 0.070 3.54 0.88 0.997 5.3015% GA 0.096 1.43 0.91 0.989 12.9530% GA 0.095 1.96 0.83 0.998 4.80

Two-parameter BET Xm CBET

0% 0.101 3.77 0.975 10.3815% MD 0.058 6.54 0.971 10.7430% MD 0.043 18.70 0.966 10.1015% GA 0.059 3.22 0.971 19.0230% GA 0.046 9.33 0.950 23.40

Three-parameter BET Xm CBET n

0% 0.139 1.35 12.99 0.997 9.8715% MD 0.074 2.51 13.79 0.996 4.5730% MD 0.052 5.82 13.68 0.999 2.1815% GA 0.083 1.16 12.80 0.993 8.8130% GA 0.069 2.15 9.78 0.999 2.15

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indications that the glass transition behavior of gum arabicparallels the Tg curve of maltodextrin DE 10, with tem-perature values about 10�C higher.[46] In addition, Righetto

and Netto[47] determined the glass transition temperature(Tg) of gum arabic and the results varied from 62�C at awof 0.33 to 42.6�C at aw of 0.54. Comparing the results

FIG. 2. Thermograms of mussel meat hydrolysate protein powder equilibrated at different water activities: (a) pure; and produced with carrier agents

(b) 15% maltodextrin, (c) 30% maltodextrin, (d) 15% gum arabic, and (e) 30% gum arabic.

TABLE 3Gordon-Taylor model parameters (Tgs and k) for pure mussel meat hydrolysate and produced with 15 and 30% of

maltodextrin 10 DE (MD) or gum arabic (GA)

Concentration Tgs (�C) k R2 E (%)

0% 64.4 3.60 0.954 2.6915% MD 71.9 2.89 0.949 2.1030% MD 97.9 3.78 0.948 2.3515% GA 69.3 2.62 0.952 1.9830% GA 77.7 3.42 0.959 1.79

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obtained in Fig. 2, the Tg value of pure carrier agents wasreduced about 20�C for aw of 0.33 and approximately 5�Cfor aw of 0.54, respectively, when a maximum amount of75% of the hydrolysate was used. As can be seen, a slightincrease in the Tg of mussel meat hydrolysate powderwas observed with increased carrier agent concentrationfrom 15 to 30%. Kurozawa et al.[17] also observed that anincrease of 10 to 20% of maltodextrin or gum arabic ledto an increase in Tg value; however, an increase in carrieragent concentration from 20 to 30% had less influence onTg values.

Figure 3 shows the Tg values obtained for the differentpowders as a function of equilibrium moisture content,where the water plasticizing effect can be observed by adecrease in the glass transition temperature at low moistureincrement. The k parameter, shown in Table 3, rangedfrom 2.62 to 3.78 according to the Gordon-Taylormodel.[35] This parameter controls the degree of inclinationof the Tg curve in relation to water content (in a binary sys-tem) and is related to the interaction forces between the

system constituents.[35] Its value was 2.59 and 6.8 forchicken meat protein hydrolysate and freeze-dried fish pro-tein hydrolysate, respectively.[16,17] The effect of carrieragents on the Tg value of powders for all water contentscan be observed in Fig. 3 as an increase in value. This resultshowed that the use of maltodextrin or gum arabic wasefficient to reduce the water plasticization of mussel proteinhydrolysate. In addition, slight differences were observedfor carrier agent concentrations of 15 and 30%. This seemsto indicate that the use of carrier agents reduced the mol-ecular mobility of this system due to the reduction in thefree water content; however, there is a limit for freewater-binding that modifies the Tg–water content relation-ship.

According to Table 3, the use of carrier agents increasedthe Tgs values from 64.4 to 97.9�C, increasing the powder’sstability. The same behavior was obtained for chickenmeat hydrolysate with the addition of carrier agents.[17]

Kurozawa et al. obtained a Tgs of 44.43�C for the purechicken meat hydrolysate protein powder, and the additionof 10% maltodextrin or gum arabic led to Tgs values of91.90 and 94.70�C, respectively.

Determination of Critical Storage Conditions

The critical storage conditions, critical water activity(awc) and moisture content (Xc), for pure mussel meat pro-tein hydrolysate and powders formulated with maltodex-trin or gum arabic were found by sorption isothermsdata predicted by the three-parameter BET model and Tg

data predicted by the Gordon-Taylor model at 25�C(Figs. 1 and 4), respectively, and the combination of thesecurves is presented in Fig. 5. The critical water content=water activity is the value at which the glass transitiontemperature of the product is equal to room tempera-ture. Above this temperature, amorphous powders are

FIG. 3. Values of glass transition temperature vs. equilibrium water con-

tent (wet basis) for pure mussel meat hydrolysate powders and formulated

with 15 and 30% of maltodextrin 10 DE or gum arabic (experimental

points and Gordon-Taylor fitted model).

FIG. 4. Glass transition temperature as a function of solid content of

pure mussel meat hydrolysate powders and formulated with 15 and 30%

of maltodextrin 10 DE or gum arabic.

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susceptible to deteriorative changes like collapse, stickiness,and caking, resulting in quality loss.

Table 4 shows the critical values for each powder at25�C. The values were awc¼ 0.24 and Xc¼ 0.05 g=g drysolids for pure hydrolysate, and awc and Xc ranged from0.49 to 0.60 and from 0.09 to 0.12 g=g dry solids, respect-ively, for hydrolysates formulated with carrier agents. Thismeans that when the powder is stored at 25�C, themaximum relative humidity to which it can be exposed is24, 49, to 60% and the moisture content is 0.5, 9, to 12%for each formulation, respectively. However, when storedat relative humidity higher than 24, 49, and 60% (at25�C), respectively, or at a higher temperature (ataw¼ 0.24, 0.49, and 0.60), the powders will suffer physical

FIG. 5. Variation of glass transition temperature (solid line) and equilibrium moisture content (dashed line) with water activity for mussel meat hydro-

lysate: (a) pure; and produced with carrier agents (b) 15% maltodextrin, (c) 30% maltodextrin, (d) 15% gum arabic, and (e) 30% gum arabic.

TABLE 4Critical values of water activity (awc) and moisture content(Xc) for pure mussel meat hydrolysate and produced with15 and 30% of maltodextrin 10 DE (MD) or gum arabic

(GA)

Concentration awc Xc (g water=g solids)

0% 0.24 0.0515% MD 0.49 0.1030% MD 0.60 0.1215% GA 0.51 0.0930% GA 0.50 0.10

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changes as collapse, stickiness, and caking. The use of car-rier agents resulted in an effective increase in powder stab-ility.

In terms of critical storage conditions, Perez-Alonsoet al.[41] observed that the point of maximum stability forpure gum arabic at 25�C was 0.147 g H2O=g solids at awof 0.57, and Fabra et al.[48] reported a value of 0.150 gwater=g sample for maltodextrin (9–12% DE) at aw of0.79. Looking at Table 4, both carrier agents presented adecrease in the critical storage conditions when added tomussel hydrolysate and a higher reduction was observedfor maltodextrin. The critical storage conditions obtainedby Kurozawa et al.[17] for chicken breast hydrolysate wereawc¼ 0.10 and Xc¼ 0.04 g=g solids for pure hydrolysateand awc¼ 0.42 to 0.70 and Xc¼ 0.10 to 0.12 g=g solids forpowders produced with 10, 20, or 30% of maltodextrin10 DE or gum arabic. Pure mussel meat hydrolysate pow-der presented higher stability compared with the purechicken breast hydrolysate powder, both produced withoutadditives. Some reasons for this behavior are the differentphysicochemical composition of the hydrolysates, relatedto the composition of raw material, type of enzyme, differ-ent hydrolysis conditions, and degree of hydrolysisachieved.

CONCLUSIONS

The experimental data were well-fitted to three-parameter BET model for sorption isotherms at 25�C.The Gordon-Taylor model was suitable for predicting thestrong plasticizing effect of water on Tg, with a greatreduction in this value as the water activity increases. Thepure mussel meat protein hydrolysate presented high wateradsorption behavior and low Tg values, which indicate thephysical vulnerability of this powder. The use of carrieragents effectively improved powder stability, increasingcritical storage conditions at 25�C from 0.24 to 0.49–0.60for critical water activities and from 0.05 to 0.09–0.12 g=gdry solids for critical moisture contents. Up to these valuesof relative humidity or at higher temperatures, at the samewater activities, mussel powder hydrolysate can collapseand become sticky. With these values it is possible toevaluate the powder flavor loss in a stability test duringstorage.

NOMENCLATURE

aw Water activityCBET Constant in Eqs. (1) and (2)CGAB Constant in Eq. (3)KGAB Constant in Eq. (3)k Constant in Eq. (5)N Population of experimental datan Number of adsorbed layersTg Glass transition temperature (�C or K)Ve Experimental value

Vp Predicted valueW Weight fractions (g=g total)Xe Equilibrium moisture content (g water=g dry

solids)Xm Monolayer moisture content (g water=g dry

solids)

Subscripts

c Criticals Solidsw Water

ACKNOWLEDGMENTS

The authors gratefully acknowledge the financial sup-port from the Conselho Nacional de DesenvolvimentoCientıfico e Tecnologico (CNPq process number471889=2007-5) and the Fundacao de Amparo a Pesquisado Estado de Sao Paulo (FAPESP process numbers07=56545-4, 07=54520-4, and 09=54137-1).

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