isotermas de harina de arroz

7/29/2019 Isotermas de Harina de Arroz

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Dynamic vapor sorption

(DVS) techniques are cur-rently used to determine thesorption characteristics offood products. When com-

pared with the static gravi-metric technique (SGT), e-tensively used as a classicalmethod in the eisting scien-tific literature, DVS tech-niques have shown many ad-vantages, including shorterequilibration time. As a con-sequence, no antimicrobialagents are needed at elevatedwater activities to prevent mi-

crobial growth and sampledeterioration during eperi-mentation. The use of antimi-crobial agents could alter iso-therm performance and thereliability of the results ob-tained (Yu et al., 2008; San-doval et al., 2011). Amongothers, DVS techniques havethe advantage of allowing de-termination of adsorptionrates under constant or vary-

Keywords / Ai I / I Cv / ric F / wa Acivi /Received: 05/30/2011. Modied: 11/08/2011. Accepted: 11/09/2011.

Aia J. sava. Agroindus-trial Engineer, Universidad delos Llanos Ezequiel Zamora(UNELLEZ), Venezuela. M.Sc.in Food Science, USB, Vene-zuela. Ph.D. in Food Scienceand Technology, University ofReading, UK. Professor, USB,

Venezuela. Address: Departa-mento de Tecnologa de Pro-cesos Biolgicos y Bioqumi-cos, USB. Apartado 89000.Caracas 1080-A, Venezuela.e-mail: [email protected]

0378-1844/11/11/848-05 $ 3.00/0

ing hydration conditions withthe same sample, and furthercalculation of apparent water

diffusion coefficients, as il-lustrated by Romn-Gutirrezet al. (2003).

A large number of moisturesorption isotherm models have

been proposed in the literaturefor different food products.They include those derivedtheoretically and based onthermodynamic considerationssuch as the BET (Brunauer,Emmett and Teller; Brunaueret al., 1938) and the GAB(Guggenheim-Anderson-DeBoer; Van den Berg and Bru-

in, 1981) models; and addi-tional semi-empirical and em-

pir ical models (Peleg, 1993;Viollaz and Rovedo, 1999).Many other empirical two-pa-rameter isotherm models havealso been proposed for differ-ent aw ranges. Although at

present there is not a uniquemodel to accurately representmoisture sorption data in thewhole range of water activity,

due mainly to the complesorption mechanisms involved,the GAB equation has been

recognized as the most versa-tile sorption model. Quirijns etal. (2005) underlined the GABsorption isotherm ability todeduce the thermodynamicstate of the different types ofwater (bound and free) in afood system, as well as thetransition among them. How-ever, it has been claimed thatthe GAB model does not holdon rough or irregular surfacessuch as those of grains andother starchy powders, andthat it is more appropriate for

homogeneous planer surfaces.An isotherm describing themultilayer adsorption on frac-tal surfaces, based on BETtheory, was presented byAguerre et al. (1996) and suc-cessfully applied to amaranthstarch by Calzetta-Resio et al.(1999). On the other hand,Viollaz and Rovedo (1999)

proposed an etension of theGAB model to correlate sorp-

tion data for aw values thatinclude the range above 0.9,not considered in the original

GAB model. This model hasalso been successfully appliedto starchy and powdery prod-ucts in our previous work(Perdomo et al., 2009; Brett etal., 2009; Barreiro et al.2010).

Rice our is frequently usedas an ingredient in manystarchy processed food prod-ucts. Knowledge of the sorp-tion characteristics of riceour is important to establishthe packaging requirementsand stability of products con-

taining this ingredient, but fewpapers have been found in thereviewed literature. Durakovaand Menkov (2004) studiedthe sorption characteristics ofrice flour in a temperaturerange of 10 to 30C, in a wa-ter activity range of 0.11 to0.85 that ecluded elevatedwater activities. Sigmoidaltype-II isotherms were found.Brett et al. (2009) presented

determInAtIon oF moIsture AdsorptIon Isotherms oF

rICe Flour usIng A dynAmIC VApor sorptIon teChnIque

Aleida J. Sandoval, Jos A. Barreiro and Alejandro J. Mller

SUMMARY

Moisture adsorption isotherms for rice our were determined

in a temperature range of 5 to 45C using a dynamic vaporsorption technique. A water activity range from 0 to 0.945 wasstudied. Typical type-II isotherms were obtained. The equi-librium moisture content (EMC) decreased with increasingtemperature over the whole water activity range studied. The

EMC did not exhibit isotherm cross-over with temperature aspreviously reported for rice our using thymol as an antimi-

crobial agent. The adsorption data obtained was tted to vari -

ous isotherm models. It was found that EMC data was betterdescribed by the modied GAB model. Moisture sorption dataobtained in this study can be considered more reliable and ac -curate than those previously reported for this product using

static gravimetric techniques since no antimicrobial agentswere used during experimentation.

J A. Bai. Chemical En-gineer, Universidad Central deVenezuela. M.Sc. and Ph.D. inFood Science, Louisiana StateUniversity, USA. Retired Pro-fessor, USB, Venezuela. Cur-rently with J. A. Barreiro &Assocs., Venezuela.

Aja J. m. MaterialsEngineer, USB, Venezuela. M.Sc. in Chemistry, InstitutoVenezolano de InvestigacionesCientcas, Venezuela. Ph.D. inPhysics, Bristol University, UK.Professor, USB, Venezuela.


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sorption isotherms for riceour at temperatures of 5, 23

and 45C using the SGT for awater activity range from 0.08to 0.98; these authors usedcrystalline thymol as an anti-microbial agent at elevatedwater activities. Isothermcrossover with temperaturewas reported at elevated wateractivities.

In view of the above infor-mation, the aim of this workwas to determine moisturesorption isotherms for riceour at different temperaturesusing a DVS technique not

requiring the use of antimi-crobial agents that could alterisotherm performance andresults reliability.

maia a m

Raw material

Native rice our with initialmoisture content of 14.39%

(w.b.), manufactured by KelIndustries (Venezuela), was

provided by Alfonzo Rivas yCia., Caracas, Venezuela. Pro-imate composition of this lot ofrice our (g per 100g, wet ba-sis standard deviation) wasdetermined in a previous study(Sandoval et al., 2009) andshown to be: moisture 14.390.09; protein 8.64 0.21; fat0.31 0.00; ash 0.92 0.02;carbohydrates (from difference)75.74 0.15.

Adsorption isotherms

Moisture adsorption iso-therms of native rice flourwere determined in a dynam-ic vapor sorption (DVS) ap-

pa ra tus. The epe rim en tswere carried out using anIGASorp moisture sorptionanalyzer (Hiden IsochemaLtd., UK), consisting on acontrolled atmosphere micro-

balance, in which the relative

humidity and the change insample weight were continu-

ously monitored. The equip-ment has a real time proces-sor using IGASorp SystemSoftware V6.50.42 (HidenAnalytical Ltd.). Equilibriumrelative humidity (ERH) val-ues used to build the isothermwere dened by the user. Thesoftware determined the mi-ing ratio of water vapor anddry ultra high purity nitrogenrequired to obtain each of thedesired ERH values. In this

process, sample weight datawere analyzed to determine

kinetic parameters for the pre-diction of equilibrium uptakein real-time. In order to deter-mine the whole isotherm, this

process was repeated for allERH values set. The mai-mum operational water activ-ity that can be obtained inthis equipment is 0.95. Theequipment is able to regulatesample temperature. Results

obtained were eported to anMS Excel le and plotted.

Isotherms were obtained byadsorption by setting thirteenERH values, covering a rangefrom 0 to 94.5%. An amountof 10 1mg of rice flour was

placed inside the balance anddried in a zero relative humid-ity ambient for 3h at 30C, us-ing ultra high purity nitrogen.A maimum time of 720minfor complete data collection ateach RH-level set (timeout) wasused if the equilibrium criteri-on was not already reached.The equilibrium criterion for

stability was defined by thetime the weight had relaed towithin 1% of the equilibriumuptake. The isotherms wereobtained in triplicate at tem-

peratures of 5, 23 and 45C.

Adsorption data analysis

Adsorption data obtainedwere adjusted to various sorp-

determInACIn de IsotermAs de AdsorCIn de humedAd de lA hArInA de ArroZ usAndo unAtCnICA dInmICA de sorCIn de VAporAleida J. Sandoval, Jos A. Barreiro y Alejandro J. Mller

RESUMEN

determInAo de IsotermAs de Adsoro de umIdAde dA FArInhA de ArroZ usAndo umAtCnICA dInmICA de soro de VAporAleida J. Sandoval, Jos A. Barreiro e Alejandro J. Mller

RESUMO

Las isotermas de adsorcin de humedad en la harina de arrozfueron determinadas en un intervalo de temperatura entre 5 y45C usando una tcnica de sorcin de humedad dinmica. Seestudi un intervalo de actividad de agua entre 0 y 0,945 y seobtuvieron isotermas tipo II. En toda la extensin de activida-des de agua estudiada, el contenido de humedad de equilibrio(CHE) disminuy con incrementos de temperatura. Al contrariode lo reportado en estudios previos para la harina de arroz,usando timol como agente antimicrobiano, el CHE no exhibi

As isotermas de adsoro de umidade na farinha de arroz

foram determinadas em um intervalo de temperatura entre 5 e45C usando uma tcnica de soro de umidade dinmica. Es -tudou-se um intervalo de atividade de gua entre 0 e 0.945 e

se obtiveram isotermas tipo II. Em toda a extenso de ativida-des de gua estudada, o teor de umidade de equilbrio (TUE)diminuiu com incrementos de temperatura. Ao contrrio dorelatado em estudos prvios para a farinha de arroz, usandotimol como agente antimicrobiano, o TUE no exibiu cruze de

cruce de isotermas con la temperatura. Los datos de adsorcinobtenidos fueron ajustados a varios modelos de isotermas. Seencontr que los datos de CHE fueron mejor descritos por elmodelo modicado de GAB. Los datos de sorcin de humedadobtenidos en este trabajo pueden ser considerados ms cona-bles y exactos que los reportados previamente para este produc-to usando una tcnica gravimtrica esttica, ya que no se usa-ron agentes antimicrobianos durante las determinaciones.

isotermas com a temperatura. Os dados de adsoro obtidos

foram ajustados a vrios modelos de isotermas. Encontrou-seque os dados de TUE foram mais bem descritos pelo mode -lo modicado de GAB. Os dados de soro de umidade obti -dos neste trabalho podem ser considerados mais conveis eexatos que os relatados previamente para este produto usandouma tcnica gravimtrica esttica, j que no se usaram agen-tes antimicrobianos durante as determinaes.


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tion models used for starchyproducts: BET, Peleg, GABand modied GAB (see TableI). The goodness of fit wasevaluated by determining thecorrelation coefficient (R2)and the root mean square er-ror (RMSE). For this purpose,non linear regression was car-ried out using Matlab ver-sion 6.5. The monolayer water

content was estimated fromthe BET and GAB models.

The Clausius-Clapeyronequation (Eq. 5) was used todetermine the effect of tem-

perature on the sorpt ion dataand to calculate the net iso-steric heat of sorption. Its e-

pression is

(5)

where T: absolute temperature(K), q

st

: net isosteric heat ofsorption (Jmol-1), and R: uni-versal gas constant(8.314Jmol-1K-1).

r a dici

The adsorption isothermsfor rice flour and the error

bars (one standard deviation)for the EMC values obtainedare shown in Figure 1. A sig-moidal shape characteristic oftype II isotherms, based onBrunauers classification

(Brunauer et al., 1940; Al-Muhtaseb et al., 2002) wasobtained. The isotherms for

rice flour presented similarbe havi or to thos e of ot he rstarch-rich foods (McMinnand Magee, 1999; Al-Muhtaseb et al., 2004; Dura-kova and Menkov, 2004; Brettet al., 2009; Perdomo et al.,2009).

The effect of temperatureon moisture adsorption iso-therms can be appreciated inFigure 1. As epected, for agiven water activity, the cor-responding EMC decreaseswith increasing temperature.This behavior has been attrib-uted to the higher energy lev-

els and lower stability of wa-ter molecules at higher tem-peratures, favoring their sepa-

ration from the binding sitespres en t in the food ma tr i(Palipane and Driscoll, 1992).

No cross-over was observeddue to the effect of tempera-ture, as reported by Brett etal. (2009) for this product us-ing the SGT. These authorsused thymol as an antimicro-

bial agent for water activit ies>0.80. The presence of iso-therm cross-over due to theeffect of temperature was at-tributed to the use of thymolat elevated water activities(Sandoval et al., 2011). Themoisture sorption data ob-

tained in the present workusing a DVS technique can beconsidered more reliable and

accurate than those deter-mined by means of SGT re-quiring the use of antimicro-

bial additives at elevated wa-ter activities.

Adsorption data obtainedusing the DVS technique forrice flour at temperatures of5, 23 and 45C were tted tothe isotherm models presentedin Table I. The results for the

constants of the various iso-therms tested, as well as thecorresponding R2 and RMSEvalues, are shown in Table II.Excellent tting to the experi-mental data was obtained forall the models tested. How-ever, Peleg and modied GABmodels showed the best re-sults. The modified GABmodel was selected because itis able to describe the wholeisotherm. This model ehib-ited elevated R2 values (0.999)and low RMSE values (0.001-0.002). The modified GABmodel was used to draw theisotherms shown in Figure 1.The value of constants A, B,C and D presented in Table IIfor the different temperaturesstudied were substituted inEq. 4.

The monolayer water con-tent calculated using the BETand GAB models is repre-sented by constants A in Ta-

ble II. The rice f lour mono-layer water content values in

a temperature range of 5 to45C were lower for the BETmodel (0.069 to 0.055g

Figure 1. Adsorption isotherms of rice our for a temperaturerange of 5 to 45C. Open squares: 5C, open circles: 23C,open diamonds: 45C. Modified GAB fits are shown on thegraph (dashed line: 5C, dotted line: 23C, solid line> 45C).

TABLE IISOTHERM MODELS FOR ExPERIMENTAL DATA FITTING AND THEIR RANGE OF APPLICABILITY

CONSTANTS RELATED TO THE TEMPERATURE EFFECT

Model Mathematical epressionaw range ofapplicability

Equation

BET (Brunauer, Emmett and Teller, 1938)


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water/g dry solids) thanfor the GAB model (0.079to 0.066g water/g dry sol-ids). Values reported byRahman (1995) for cereal

pr od uc ts ra nged from0.051 to 0.086g water/gdry solids. These valuesare similar to those deter-mined herein for riceflour. The value of the

monolayer water contentdecreased with increasingtemperatures regardless ofthe model used (Table II).

Values for the mono-layer water content of riceflour have been reported

by Durakova and Menkov(2004) as 0.0714-0.0680gwater/g dry solids in atemperature range of 10-40C using the BETequation. These valuesare in agreement withthose reported in the presentwork (Table II). Torul andArslan (2006), working withBaldo type rice, reported ab-normally high monolayer wa-ter content values using theGAB model, and from 0.0937to 0.0968kg water/kg dry sol-ids (25-45C) using the ad-sorption data with the BETmodel. These values are high-er than those obtained in thisstudy. Brett et al. (2009) pre-sented monolayer water con-tents (5-45C) ranging from

0.064 to 0.080g water/g drysolids when the BET equa-tions were used. These valuesare of the same order of mag-nitude as those obtained inthis work.

Monolayer water contentvalues reported by Brett et al.(2009) using the GAB modelwere higher than those ob-tained in this study. This was

probably due to EMC devia-tions at elevated water activi-ties reported by these authorsshowing isotherm cross-over

with temperature. These ab-normal data were taken intoaccount when tting the GABmodel in order to obtain themonolayer water content.

Figure 2 shows the varia-tion of the net isosteric heatof sorption with moisture con-tent. As epected, net iso-steric heat of sorption de-creased with increasing mois-

ture content; being this rela-tionship described by

qst= 23.38e-0.137MC R2= 0.996

(6)

with the moisture content val-ue (MC in Eq. 6) in g/100gof dry solids.

As shown in Figure 2, thenet isosteric heat of sorptionvar ied f rom 0.5 to10kJmol -1 in a mois turecontent range of 6 to 24%

(d.b.). Durakova and Men-kov (2004) reported valuesfor rice our (2.3-22.3kJmol-1)in an MC range of 10-22%(d.b.); while Stripatrawan and

Jantawat (2006) found valuesfor Jasmine rice crackers of0.1-24.9kJmol -1 in a range ofmoisture content of 1 to 28%(d.b.) for the isosteric heat ofsorption.

Cci

Adsorption isotherms ofrice our were determined attemperatures of 5, 23 and45C using a dynamic sorp-tion vapor technique. Typical

type II isotherms were ob-tained. No isotherm cross-over with temperature wasevidenced for these productsas reported by other authors

using thymol as antimicrobialagent. Adsorption data were

better described by the modi-ed GAB model in the wholetemperature range studied,evidenced by elevated R2 val-ues and low RMSE values for

bo th pr od uc ts . The wate rmonolayer contents deter-mined using BET model(aw


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isotermas de harina de arroz

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