Lipid Oxidation Decreases as the Water Active Increases Reaching a Minimum at Low Range Around 0.2 and 0.4. However, Higher Water Activities Accelerate the Lipid Oxidation

7/28/2019 Lipid Oxidation Decreases as the Water Active Increases Reaching a Minimum at Low Range Around 0.2 and 0.4.

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Effect of maltodextrin and gum arabic on water sorption and glass transition

temperature of spray dried chicken meat hydrolysate protein

Louise Emy Kurozawa a,*, Kil Jin Park b, Miriam Dupas Hubinger a

a Department of Food Engineering, Faculty of Food Engineering, State University of Campinas, Street Monteiro Lobato, 80, P.O. Box 6121, Campinas, SP, 13083-970, Brazilb Faculty of Agricultural Engineering, State University of Campinas, P.O. Box 6011, 13084-971, Campinas, SP, Brazil

a r t i c l e i n f o

Article history:Received 20 July 2008Received in revised form 9 September 2008Accepted 12 September 2008Available online 20 September 2008

Keywords:

IsothermsCalorimetryBET modelGAB modelGordon-Taylor modelStability

a b s t r a c t

The water adsorption isotherm and glass transition temperatures (Tg) of chicken protein hydrolysatepowder, with and without maltodextrin or gum Arabic, were studied in order to investigate their stabil-ity. The hydrolysate powder, pure and formulated with10%, 20% and 30% (w/w) of additive, was obtainedby spray drying. The sorption isotherm was determined by the gravimetric method. A differential scan-ning calorimeter was used to determine the Tg of samples equilibrated with several water activities. Asresults, the BET model fitted the data for the sorption isotherm of the protein hydrolysate well. A strongplasticizing effect of water on theTg was found, with a greatreduction in this value with increase in wateractivity. The data for Tg versus solids content gave a satisfactory correlation with the Gordon-Taylormodel. The addition of carrier agents increased the Tg of the hydrolysate, decreasing its hygroscopicityand, consequently, increasing its storage stability.

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1. Introduction

Brazilian chicken meat production increased by 37% in the per-iod from 2002 to 2006, reaching almost 9 million tons in 2006(FAOSTAT, 2008) and becoming the third largest world producer.According to Barbut (2002), novel processed poultry products havebeen introduced onto the market in recent years, due to low rawmaterial prices. In order to be competitive, the poultry industrymust develop new products to satisfy emerging consumer de-mands and increase profitability. Thus the protein hydrolysis ofchicken meat could be an alternative solution to obtain value-added products.

Chicken breast meat has a higher protein content (22 g/100 gmeat) and lower fat content (3 g/100 g meat) than other parts ofthe chicken, such as the drumsticks (18 g protein and 5 g fat/100 g meat) and wings (18 g protein and 18 g fat/100 g meat)(TACO, 2004). In addition, animal protein presents a perfect equi-librium of essential amino acids.

Protein hydrolysates are mainly applied in the nutritional man-agement of individuals who cannot digest whole/intact protein.Hydrolysates rich in low molecular weight peptides, especiallydi- and tri-peptides with as little as possible free amino acids, havebeen shown to have more dietary uses due to their high nutritionaland therapeutic values (Bhaskar et al., 2007). Extensively hydroly-

sed proteins also show reduced immunological reactivity, and canbe used in formulas for hyper allergic infants (Mahmoud, 1994).Furthermore, peptides, being easily absorbed, may be an optimalnitrogen source in sports nutrition, and high biological value pep-tides are attractive as a general protein supplement in a wide vari-ety of diets (lizyte et al., 2005).

Protein hydrolysates are highly perishable due to their highmoisture and protein content, and have therefore been processedto improve their shelf life. Of the various methods employed forpreservation, drying is a process in which the food water activityis reduced by water removal through vaporization or sublimation,minimizing enzymatic and microbiological reactions. Spray dryinginvolves both particle formation and drying, where the feed istransformed from the fluid state into droplets and then into driedparticles, by spraying it continuously into a hot drying medium.This technique is widely used in food manufacturing and presentslow operating cost and a short contact time.

Moisture sorption isotherms, important tools for predictinginteractions between the water and the food components, describethe relationship between water activity and the equilibrium mois-ture content of a foodstuff. Knowledge of water sorption isothermsis important in various food processes, such as drying, storage andpackaging, since they are used to estimate drying time, ingredientbehavior on mixing, packaging selection and modeling moisturechanges that occur during storage.

Recently, the concepts related to water activity have been cou-pled with those of the glass transition temperature, Tg, providing

0260-8774/$ - see front matter 2008 Elsevier Ltd. All rights reserved.doi:10.1016/j.jfoodeng.2008.09.006

* Corresponding author. Tel.: +55 19 3521 4088; fax: +55 19 3788 4027.E-mail address: [email protected] (L.E. Kurozawa).

Journal of Food Engineering 91 (2009) 287296

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an integrated approach to the role of water in foods. The glass tran-sition temperature is defined as the temperature at which anamorphous system changes from the glassy to the rubbery state.Molecular mobility in the glassy state is extremely slow, due tothe high viscosity of the matrix (about 1012 Pa s). Thus the Tg canbe taken as a reference parameter to characterize the properties,

quality, stability and safety of food systems. Structural alterations,such as stickiness, agglomeration, caking and crystallization, occurin amorphous food powders when stored at temperatures abovethe Tg. Foodstuffs with low-moisture contents and Tg value abovethe storage temperature can be considered stable. However, aslight increase in moisture significantly reduces the Tg. Therefore,the moisture sorption and Tg supply critical values for the wateractivity and moisture content at room temperature (Khalloufiet al., 2000; Roos, 1993, 1995; Roos and Karel, 1991a; Shresthaet al., 2007a).

Protein hydrolysates contain low molecular weight peptidesand present low Tg values and, consequently, high hygroscopicityand thermoplasticity. Since the Tg increases with molecular weight,the addition of carrier agents (like maltodextrins and gums) has

been used in the production of powders, reducing the stickinessand wall deposition in spray drying (Bhandari et al., 1993; Roosand Karel, 1991a; Truong et al., 2005). Maltodextrins, products ob-tained by starch hydrolysis, consist of b-D-glucose units and areusually classified according to their dextrose equivalency, DE(Bemiller and Whistler, 1996). The addition of maltodextrin ismainly used in materials that are difficult to dry and has been usedfor mango, West Indian cherry, date palm and aai pulps (Jaya andDas, 2004; Righetto and Netto, 2005; Sablani et al., 2008; Tononet al., 2008). Gum Arabic is a complex heteropolysaccharide witha highly ramified structure, with the main chain formed ofD-galac-topyranose units (Bemiller and Whistler, 1996). It has been used asan encapsulating agent in microencapsulation by spray drying, dueto its good emulsifying capacity and low viscosity in aqueous solu-

tion. Its contribution to the stability of dehydrated foods was stud-ied by Gabas et al. (2007) and Righetto and Netto (2005).The aim of the present work was to evaluate the influence of

maltodextrin or gum Arabic on the water sorption Tg and stabilityof spray dried chicken breast protein hydrolysate. Modeling of thesorption isotherms using selected models from the literature wasstudied (BET and GAB models), and also the Tg using the Gordon-Taylor model.

2. Material and methods

2.1. Material

Frozen chicken breast meat was purchased from Doux Frango-

sul (Montenegro, Brazil). The meat was stored in a cold chamberat 18 C and thawed according to the quantity required to

produce the hydrolysate. The main characteristics of the meat, ob-tained according to AOAC (1995), are summarized in Table 1.

For the enzymatic hydrolysis, the commercial protease Alca-lase 2.4 L (Novozymes, Bagsvaerd, Denmark), which is a serineendopeptidase obtained from Bacillus licheniformis, with a declaredactivity of 2.4 AU/g, was used.

The carrier agents used were maltodextrin Mor-rex 1910 (CornProducts, Mogi-Guau, Brazil), with 9.0 6 DE6 12.0, and gum Ara-bic Instantgum (Colloides Naturels, So Paulo, Brazil).

2.2. Preparation of the protein hydrolysate

The hydrolysis experiments were carried out in a 10 L thermo-statically controlled stirred-batch reactor using the pH-stat proce-dure, as described by Adler-Nissen (1985). The samples weredefrosted overnight. The tendons, nerves, skin and visible fat wereremoved from the meat, which was then fragmented, ground in afood processor and homogenized with distilled water (meat:waterratio 1:3 w/w). The mixture was heated to 52.5 C and the pH ad-justed to 8.00 with 2 N NaOH. The enzyme was added (4.2 g en-

zyme/100 protein) to the mixture and the reaction pHmaintained constant by the continuous addition of 2 N NaOH. After6 h, the hydrolytic process was terminated by heating the mixtureto 85 C for 20 min, assuring inactivation of the enzyme. The pro-cess conditions were established according to the results obtainedby Kurozawa et al. (2008). The resulting slurry was centrifuged at3500 rpm (Beckman Coulter, Allegra 25 R model) for 20 min, toseparate the lipids. The protein hydrolysate was stored in a coldchamber at 18 C and thawed according to the quantity requiredfor spray drying. The main characteristics of the chicken proteinhydrolysate, obtained according to AOAC (1995), are summarizedin Table 2.

2.3. Spray drying

Before the spray drying process, carrier materials maltodex-trin (MD) or gum Arabic (GA) were added directly to the proteinhydrolysate with magnetic stirring, until complete dissolution.Table 3 shows the different formulations of the carrier materials

Nomenclature

aw water activityCBET constant of Eqs. (1) and (2)CGAB constant of Eq. (3)k constant of Eq. (5)KGAB constant of Eq. (3)

n number of adsorbed layersN population of experimental dataTg glass transition temperature (C)Tout outlet temperature (C)VE experimental valueVp predicted value

Xe equilibrium moisture content (g water/g dry matter)Xm monolayer moisture content (g water/g dry matter)w weight fractions (g/g total)

Subscripts

c criticals solidsw water

Table 1

Chemical composition of the chicken breast meat

Analysis Content (%, wet basis)

Moisture 74.10 0.14Proteins 19.36 0.94Fat 1.55 0.12Ash 1.10 0.01

Values represent means of three determinations standard deviations.

288 L.E. Kurozawa et al. / Journal of Food Engineering 91 (2009) 287296


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(10, 20and 30% w/w, which correspondto 0.55, 0.73 and 0.83 g MDor GA/g total solids, respectively).

The spray drying process was performed using a laboratoryspray dryer (B191 model, Bchi, Flawil, Switzerland). The equip-ment was operated concurrently using a spray nozzle with an ori-fice of 0.7 mm in diameter. The protein hydrolysate was fed intothe drying chamber using a peristaltic pump. The inlet air temper-ature was 180 C and the outlet air temperature varied from 91 to102 C for each sample. The feed mass flow rate and air com-pressed volumetric flow rate were 0.2 kg/h and 0.6 m3/h,respectively.

2.4. Sorption isotherms

Sorption isotherms were determined by the gravimetric meth-od. One gram of powder was placed into aluminum vials, weighedand equilibrated over saturated salt solutions (LiCl, CH3COOK,

MgCl2, K2CO3, Mg(NO3)2, KI, NaCl and KCl, providing relativehumidity values of 11.3%, 17.6%, 32.8%, 43.2%, 52.9%, 68.9%, 75.3%and 84.3%, respectively, according to Greenspan (1977)) in desicca-tors at 25 C until equilibrium. Once equilibrium was reached, theequilibrium moisture content of the sample was measured gravi-metrically by drying in a vacuum oven at 70 C for at least 48 h,to determine the solid mass in the sample. The physical appear-ance of the samples was also observed to check whether the pow-der had suffered any transformation such as agglomeration, cakingor collapse.

Several models (empirical, semi-empirical and theoretical) withtwo or more parameters have been used in the literature to de-scribe the sorption isotherms. Equations based on sorption theo-ries, such as BET and GAB models, are usually preferred by most

researchers, since some physical meaning may be attached to theirparameters, aiding in the understanding of the water sorptionphenomena.

Derived by simple extension and generalization of Langmuirstheory of unimolecular adsorption, the classic BET (Eq. (1)) (Bru-nauer et al., 1938) is a two-parameter model assuming the conden-sation of an infinite number n of layers from the vapor phase ontothe adsorbent surface. Unfortunately, this model fails for higherwater activities, aw > 0.5 (Jonquires and Fane, 1998).

Xe XmCBETaw

1 aw1 aw CBETaw1

In their original publication, Brunauer et al. (1938) also derived amodified model, considering a limited number of adsorbed layers,

allowing the modeling for water activities up to 0.9. As expected,the corresponding model yields a three-parameter (Eq. (2))

Xe XmCBETaw1 n 1aw

n nawn1

1 aw1 CBET 1aw CBETawn1

2

Due to lack of fit for high water activities, the BET model (Eq. (1))was not used in this work to fit the experimental data. As a conse-quence, the isotherm models used were the modified BET (three-parameters) and GAB models (Van den Berg and Bruin, 1981)(Eqs. (2) and (3))

Xe XmCGABKGABaw

1 KGABaw1 KGABaw CGABKGABaw3

In order to obtain the model parameters, a non-linear regressionanalysis was carried out using the Statistica 5.0 (Statsoft, Tulsa,USA) software package. The degree of fitness of each model wasevaluated by the determination coefficient and mean relative devi-ation modulus E

E100

N

XN

i1

jVE VPj

VE4

2.5. Glass transition temperature

About 3 mg of protein hydrolysate powder were placed into dif-ferential scanning calorimetry (DSC) aluminum pans (20 ll) andequilibrated over saturated salt solutions in desiccators at 25 Cuntil equilibrium was reached. The samples were then hermeti-cally sealed with lids for analysis and weighed. The mass of eachsample pan was matched in advance with the mass of an emptyreference pan to within 0.1 mg.

The DSC analyses were carried out in a TA-MDSC-2920 (TaInstruments, New Castle, De, USA). For temperatures below70 C, liquid nitrogen was used; otherwise a mechanical refriger-ation system (RCS refrigerated cooling accessory) was applied.Equipment calibration was performed with indium (Tmelting =156.6 C) and verification with azobenzol (Tmelting = 68.0 C). Dry

helium, 25 ml/min, was used as the purge gas. After cooling thesample to70 C, the glass transition temperature was determinedon thermo-analytical curves obtained by heating the sample at10 C/min up to 80 C (or other values for the initial and final tem-peratures, according to the sample). The second scanning of eachsamplewas performed to reduce the enthalpy relation of the amor-phous powder, which appears in the first scan. All analyses weredone in triplicate and the data were treated by the software Uni-versal Analysis 2.6 (Ta Instruments, New Castle, De, USA).

To describethe plasticising effect of water on the proteinhydro-lysate, the glass transition temperature data were fitted to the Gor-don-Taylor model (Gordon and Taylor, 1952)

Tg wsTgs kwwTgw

ws kww5

The Tgw value was taken at 135 C (Johari et al., 1987).A non-linear regression analysis was carried out using the Stat-

istica 5.0 (Statsoft, Tulsa, USA) software package to obtain themodel parameters k and Tgs.

3. Results and discussion

3.1. Sorption isotherms

Fig. 1 shows experimental sorption isotherms for the proteinhydrolysates, pure and formulated with maltodextrin (MD) orgum Arabic (GA), at different concentrations, with their respectivefittings to the BET model.

Thesorption isotherms showedan increasein equilibriummois-ture content with increasing water activity, at constant

Table 2

Chemical composition of the chicken breast meat protein hydrolysate

Analysis Content (%, wet basis)

Moisture 91.32 0.06Ash 0.68 0.03Protein 7.05 0.06Fat 0.08 0.01

Table 3

Formulations of protein hydrolysate with maltodextrin (MD) or gum Arabic (GA)

Formulation Concentration (%, w/w)

Protein hydrolysate Carrier agent

Without carrier agent 100 010%MD or 10%GA 90 1020%MD or 20%GA 80 2030%MD or 30%GA 70 30

L.E. Kurozawa et al. / Journal of Food Engineering 91 (2009) 287296 289


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temperature, and it can be seen that the behaviors of all the curveswere of type III, according to Brunauers classification (Rizvi, 1995).Similarisotherms were observed for proteinhydrolysates fromfish,pineapple, tomate pulp, West Indian cherry and lactose hydrolysedskim milk powders (Aguilera et al., 1993; Gabas et al., 2007; Goulaet al., 2008; Righetto and Netto, 2005; Shrestha et al., 2007b).

Analyzing Fig. 1, considerable differences could be observed be-

tween the isotherms of the pure protein hydrolysate powder andthose formulated with additive. The equilibrium moisture contentsof the samples with maltodextrin or gum Arabic were significantlylower at a given water activity, than that of the pure hydrolysate.Similar results were observed by Gabas et al. (2007) and Righettoand Netto (2005). The presence of additives in the protein hydroly-sate probably modified the balance of hydrophilic/hydrophobicsites, promoting a decreased amount of sorbed water (Prez-Alon-so et al., 2006).

The experimental equilibrium moisture content results werefitted to the BET (three-parameters) and GAB models (Table 4) todescribe the water sorption isotherms. Each model was tested foradequacy and goodness of fit by determining the coefficient R2

and mean relative deviation modulus E. These values and the

parameter models obtained by non-linear regression analysis areshown in Table 4. The results showed that for the different additive

concentrations, the BET model presented a better fit than the GABmodel, with mean relative deviations below 13% and determina-tion coefficients close to unity.

Fig. 1. Water sorption isotherms of chicken meat protein hydrolysates, formulated with: (a) maltodextrin; (b) gum Arabic.

Table 4

Estimated parameter values for the BET and GAB models for protein hydrolysate

powders with and without maltodextrin (MD) or gum Arabic (GA)

Model Sample (%) Constant R2

E (%)Xm CBET n

BET 0% 0.153 3.098 19.080 0.998 6.3210%MD 0.070 4.280 26.381 0.995 8.7420%MD 0.048 8.518 26.163 0.994 9.8630%MD 0.039 9.378 25.934 0.992 12.3210%GA 0.088 3.037 21.703 0.998 7.9020%GA 0.070 4.906 21.518 0.995 11.5030%GA 0.063 3.380 21.382 0.996 9.05

Xm CGAB KGAB

GAB 0% 0.141 5.766 0.326 0.962 16.0510%MD 0.063 6.553 0.305 0.940 22.2220%MD 0.048 6.706 0.291 0.938 19.1430%MD 0.044 6.996 0.260 0.940 18.3910%GA 0.073 6.734 0.304 0.949 15.6120%GA 0.067 6.850 0.282 0.947 17.52

30%GA 0.060 7.285 0.261 0.938 17.98



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Although the GAB model has been widely used for foodstuffs, itdid not present a good fit for the experimental data of the chickenmeat protein hydrolysate (mean relative deviation above 15%).This fact can be explained due to the limiting values for the con-stants CGAB and KGAB as suggested by Lewicki (1997), based onthe mathematical analysis of the model. For sigmoidal type curves,the author stated that the constants should assume values in the

range 0.246

KGAB6

1 and 5.66

CGAB61

, to guarantee a relativelygood description of the isotherms and to fulfill the requirements ofthe GAB model, as well as assuring that the calculated monolayermoisture content values differed by no more than 15.5% fromthe true monolayer capacity. In the present work the curve exhib-ited a non-sigmoidal type that explains the high mean relativedeviation.

The monolayer moisture content (Xm) is the amount of waterthat is strongly adsorbed to specific sites at the food surface andis considered an important value to assure food stability. For pro-tein hydrolysates without additives, the Xm value was 0.153 g wa-ter/g dry matter. Aguilera et al. (1993) and Shrestha (2007b) foundXm values of 0.062 and 0.072 g water/g dry matter for proteinhydrolysates from fish and lactose hydrolysed skim milk, respec-tively. These differences between the Xm values obtained for thechicken meat protein hydrolysates and those obtained in otherstudies could be attributed to compositional differences and tothe degree of hydrolysis of the products. Adding maltodextrin orgum Arabic, the Xm values decreased from 0.067 to 0.038 gwater/g dry matter and from 0.080 to 0.057 g water/g dry matter,respectively, with increasing carrier agent concentrations from10%to 30%. This behavior can explain the encapsulation effect, whichdiminishes the surface exposed to water molecules. Gabas et al.(2007) verified the same behavior with pineapple pulp powderwith (0.060 g water/g dry matter) and without maltodextrin(0.166 g water/g dry matter) or with gum Arabic (0.072 g water/gdry matter). Prez-Alonso et al. (2006) obtainedXm values between0.0696 and 0.0735 for pure maltodextrin 10DE and 0.0811 and0.1100 for gum Arabic, in the range of 2540 C, which are in

agreement with the present work, since samples containing malto-dextrin resulted in lower Xm than samples with gum Arabic.According to Prez-Alonso et al. (2006), these findings can beattributed to a combination of factors, which include the confor-mation and topology of molecule and the hydrophilic/hydrophobicsites adsorbed at the interface.

The physical appearance of the protein hydrolysate powderafter reaching equilibrium at different relative moistures was ob-served. Samples without carrier materials, with water activity,aw, of 0.113, presented free-flowing characteristics. However, liq-uefaction occurred for samples stored at aw above 0.176. For pow-ders formulated with 10%, 20% and 30% of maltodextrin, cakingonly started at aw of 0.529, 0.689 and 0.753, respectively. Collapseand liquefaction were verified in samples (10, 20 and 30%MD)

stored at relative moistures above 0.689, 0.753 and 0.843, respec-tively. The same behavior occurred for samples formulated withgum Arabic, with the exception of the sample with 30%GA, whichcollapsed at a water activity of 0.753.

According to Aguilera et al. (1995), caking is an undesirablephenomenon in which a low-moisture and free-flowing powderis initiallytransformed into lumps, then into an agglomerated solidand ultimately into a sticky material, resulting in loss of function-ality and decreasein quality. The main cause of caking and agglom-eration is water-induced plasticization of the particle surface.These physical changes can be explained by the glass transitionconcept on the basis of Tg. Below the Tg temperature, amorphousfood material exists in a non-equilibrated, stable glassy state. Asthe product temperature exceeds the Tg, so the amorphous food

enters the rubbery state, and some physical transformations, suchas agglomeration, caking and collapse occur.

3.2. Glass transition temperature

The thermograms of protein hydrolysates at various wateractivities are shown in Figs. 24. Generally the glass transition ofthe amorphous materials produces a stepwise change in the heatflow due to changes in the heat capacity, at the phase transitiontemperature.

The glass transition temperatures (Tg) of the chicken meat pro-tein hydrolysates are in good agreement with those reported forfreeze-dried fish protein hydrolysate (Aguilera et al., 1993). TheTg is known to decrease with decreasing molecular weight (Roos,1993). The low Tg value of the chicken meat protein hydrolysatewas due to the presence of low molecular peptides as a result ofthe enzymatic hydrolysis. Hashimoto et al. (2004) observed higherTg values for whole fish muscles. Shrestha et al. (2007b) verifiedthat whole lactose presents higher Tg values than hydrolysed lac-tose. The effect of several water activities on the Tg of osmoticallydehydrated tilapia fillets using binary or ternary solutions wasevaluated by Medina-Vivanco et al. (2007). The glass transitiontemperatures found by these authors were higher than those re-ported in the present study.

The effect of water as a plasticizing can be seen in Figs. 24, inwhich the increase in moisture content caused a significant de-crease in Tg. Similar behavior was observed for several productssuch as osmotically dehydrated tomato, fish muscle and its proteinfractions, abalone, hydrolysed lactose milk and freeze-dried pine-apple (Baroni et al., 2003; Hashimoto et al., 2004; Sablani et al.,2004; Shrestha, 2007b; Telis and Sobral, 2001).

In Fig. 5, the effect of adding maltodextrin or gum Arabic on theglass transition temperature of chicken meat protein hydrolysatescan be observed. Since the Tg increases with the increase in molec-ular weight, the addition of materials such as maltodextrin or gumArabic (in the solution to be dehydrated) contributes positively topowder stability. This behavior was also observed for mango pulpwith maltodextrin; immature West Indian cherry with maltodex-trin or gum Arabic; and date palm with maltodextrin (Jaya and

Das, 2004; Righetto and Netto, 2005; Sablani et al., 2008). How-ever, the increase the maltodextrin/gum Arabic concentration from20% to 30% had no further influence on the glass transition temper-ature. Grabowski et al. (2006), working with hydrolyzed sweet po-tato puree, also observed this same behavior. Using spray dryingconditions of a temperature of 190 C and amylase level of3.75 ml/kg puree, the pure sample and that with 10% of maltodex-trin presented Tg values of 51.75 C and 60.21 C, respectively.However, when the maltodextrin concentration was increased to20%, the Tg value was only 59.89 C.

Shrestha et al. (2007a), studying the spray drying of orangejuice with various levels of maltodextrin, reported that an increaseon maltodextrin proportion from 60% to 75% in orange juice

Fig. 2. Thermogram of pure chicken meat protein hydrolysate powder equilibratedat different water activities.



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resulted in a relatively lesser increase in Tg value, compared to 5060% increase in maltodextrin. For the authors, considering the veryhigh Tg of anhydrous maltodextrin, higher Tg value was expectedfor maltodextrin:orange juice mixture (75:25). Based on the workofRoos and Karel (1991b), in which they predicted a Tg value ofabout 130 C for maltodextrin:sucrose mixture (75:25), Shresthaet al. (2007a) observed that DSC method gives better estimationof Tg values when the material has higher sugar concentration;however, Tg value of the mixture with high maltodextrin concen-tration might have been underestimated. Therefore, these authorsstudied the glass transition temperature behavior of this systemusing the thermal mechanical compression test (TMCT), showingthat the method could measure the phase transition behavior of

amorphous system that has high molecular weight component.According to Shrestha et al. (2007a), the ability of DSC to accurately

measure the Tg is diminished as less defined enthalpic change takesplace in macromolecules.

The experimental glass transition temperatures were fitted tothe Gordon and Taylor (1952) model. The parameters obtainedby non-linear regression analysis are shown in Table 5, and Fig. 5shows the curve predicted by the Gordon-Taylor model.

According to Table 5, the Tgs value for the pure hydrolysate(44.4 C) was significantly lower than that for samples with malto-dextrin (91.9136.9 C) or gum Arabic (94.7125.12 C), showingthat the addition of substances with high molecular weights in-creases the Tg of the product. In the Gordon-Taylor model, theadjustable parameter k, which controls the degree of curvature ofthe Tg composition dependence (in a binary system), can be relatedto the strength of the interaction between the system components

(Gordon and Taylor, 1952). The addition of maltodextrin or gumArabic increased the k value. Silva et al. (2006) found k values of

Fig. 3. Thermograms of chicken meat protein hydrolysate powders equilibrated atdifferent water activities and formulated with maltodextrin: (a) 10%; (b) 20%MD;(c) 30%MD.

Fig. 4. Thermograms of chicken meat protein hydrolysate powders equilibrated atdifferentwater activities and formulated with gum Arabic: (a)10%;(b) 20%; (c)30%.



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5.52 and 3.92 for camucamu powder with and without maltodex-tin, respectively.

3.3. Storage under critical conditions

The critical water content/water activity is the value when theglass transition temperature of the product is equal to the roomtemperature (which was assumed to be 25 C in this work) (Shres-

tha et al., 2007b). All amorphous products are metastable and areliable to caking, collapsing or crystallizing with time during

storage. The stability of these products is strongly associated withthe Tg, which depends on the storage conditions such as wateractivity, humidity or temperature (Roos and Karel, 1991a). Roos(1993) suggested using sorption moisture data with glass transi-tion temperature in order to evaluate food stability. Thereforethe sorption moisture and Tg data were plotted in a single graph(Fig. 6). Analyzing this figure, it is possibleto obtain the critical val-

ues for the water activity and moisture. The water content and Tgvalue were predicted by the BET and Gordon-Taylor models,respectively. The critical Tg and water activity for the powdersare shown by the arrows.

The critical water activity and moisture content for the proteinhydrolysate were 0.1 and 0.04 g water/g solids, respectively at astorage temperature of 25 C. The lower critical water activityand moisture content clearly indicated the vulnerability of thechicken meat protein hydrolysate powder under the processing,handling and storage conditions. When the powder is stored at awater activity of 0.1 (or relative moisture of 10%), it will presenta moisture content of 0.04 g water/g solids and its Tg will be25 C. Therefore, when stored under conditions with a relativemoisture of 10% and temperature above 25 C (or relative moisture

above 10% and temperature of 25 C), the powder will suffer dete-riorative changes such as structural collapse, stickiness and caking.

Fig. 5. Effect of solids content on the glass transition temperature of chicken meat protein hydrolysates with: (a) maltodextrin; (b) gum Arabic as carrier agents.

Table 5

Estimated parameter values for the Gordon-Taylor models of the protein hydrolysate

powders without (0%) and with maltodextrin (MD) or gum Arabic (GA)

Powder Tgs (C) k R2 E (%)

0% 44.43 2.59 0.9966 41.3510%MD 91.90 3.71 0.9907 17.6720%MD 132.95 5.89 0.9983 8.6330%MD 136.94 5.77 0.9925 16.47

10%GA 94.70 4.22 0.9941 8.6920%GA 124.04 5.24 0.9990 4.4630%GA 125.12 5.29 0.9917 17.48



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On the other hand, for the sample with 10% maltodextrin, the

critical water activity and moisture content at 25 C were 0.5 and0.11 g water/g solids, respectively. Therefore, the addition of

maltodextrin resulted in an increase in powder stability. The pro-

tein hydrolysate with 10% maltodextrin can be stored at 25 Cand relative moisture of 50% or with a moisture content of 0.11 g

Fig. 6. Relationship between the water activity at 25 C, water content and glass transition temperature of chicken meat protein hydrolysates: (a) without additive; (b)10%MD; (c) 20%MD; (d) 30%MD; (e) 10%GA; (f) 20%GA; (g) 30%GA.



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water/g solids. These critical values are significantly higher thanthe critical values found for the pure hydrolysate (10% and 0.04 gwater/g solids).

Similarly, analyzing the other samples formulated with malto-dextrin or gum Arabic, one arrives at the critical values shown inTable 6 for these samples. Increasing the carrier agent concentra-tion resulted in a rise in the critical water activity from 0.1 to 0.7and 0.1 to 0.57, for maltodextrin and gum Arabic, respectively, inorder to depress the Tg to ambient temperature (25 C).

Shrestha et al. (2007b) evaluated the stability of whole andhydrolyzed lactose using sorption isotherms and the glass transi-tion temperature. The critical water activity and moisture contentfor whole lactose were 0.39 and 0.08 g/g solids, respectively. Thestability of hydrolyzed lactose was much lower than lactose withlow critical values of aw and moisture content of 0.15 and0.024 g/g solids, respectively.

A low glass transition temperature of a product affects the dry-ing process, since the conditions of the outlet air humidity andtemperature of the spray dryer would be higher than the criticalconditions. The higher the temperature difference (DT= ToutTg),the greater is the degree of stickiness. Visually, it couldbe seen thatspray drying the pure protein hydrolysate resulted in a largeamount of the powder being stuck in the dryer chamber and cy-

clone. This fact occurred because the outlet temperature of thedryer (Tout = 91 C) was higher than the Tg value of pure anhydrousprotein hydrolysate (Tgs = 44.4 C), resulting in a higher DT value.Since the Tg of maltodextrin 10DE (160 C, anhydrous, accordingto Roos and Karel (1991b)) is high, then the addition of the carrieragent increased the Tg of the powder, reducing the DT, which inturn decreased the stickiness behavior. No data for the glass tran-sition temperature of anhydrous gum Arabic was found in the lit-erature. However, Righetto and Netto (2005) found some data forgum arabic at three different values for water activity, that weresimilar to the data for maltodextrin found by Roos and Karel(1991b).

4. Conclusions

The BET isotherm was found to be adequate to describe theexperimental data obtained for the chicken meat protein hydroly-sate powder. The glass transition temperature was determined fordifferent water activities and the effect of water as a plasticizingagent was observed, in which an increase in moisture contentcaused a significant decrease in the glass transition temperature.The data for Tg fitted the Gordon-Taylor model well. The chickenmeat proteinhydrolysatepresented a low Tg and low critical valuesfor moisture content and water activity, indicating its vulnerabilityduring processing, handling and storage. The addition of maltodex-trin or gum Arabic increased the Tg and, consequently, contributedto the stability of the powder. These data can be used to assist inchoosing the proper spray drying operational conditions with re-spect to stickiness and the storage behavior of the chicken meatprotein hydrolysate.

Acknowledgments

The authors gratefully acknowledge the financial support fromthe Fundao de Amparo Pesquisa do Estado de So Paulo (FA-PESP), the Coordenao de Aperfeioamento de Pessoal de NvelSuperior (Capes) and the Conselho Nacional de DesenvolvimentoCientfico e Tecnologico (CNPq).

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Table 6

Critical values for the water activity (awc) and moisture content (Xc) of the protein

hydrolysate pure (0%) and formulated with maltodextrin (MD) or gum Arabic (GA)

Powder awc Xc (g water/g solids)

0% 0.10 0.0410%MD 0.50 0.1120%MD 0.60 0.1130%MD 0.70 0.12

10%GA 0.42 0.1020%GA 0.51 0.1230%GA 0.57 0.12

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