Termodinámica

 

Estudio termodinámico y cinético de la adsorción de agua en proteína de suero de leche

 

Thermodynamic and kinetic study of water adsorption on whey protein

 

E. Azuara–Nieto* y C. I. Beristain–Guevara

 

Instituto de Ciencias Básicas, Universidad Veracruzana, Av. Dr. Rafael Sánchez Altamirano s/n, Col. Industrial–Animas, Apdo. Postal 575, Xalapa 91000, Veracruz, México. * Autor para correspondencia: E–mail: eazuara@uv.mx Tel. : (228) 841 89 00; Fax: (228) 841 89 32

 

Recibido 5 de Noviembre 2007
Aceptado 23 de Noviembre 2007

 

Resumen

El objetivo de este trabajo fue relacionar la cinética de adsorción de vapor de agua en proteína de suero de leche (PSL) con el equilibrio termodinámico que se obtiene a diferentes actividades de agua, para determinar los mecanismos que controlan el proceso. La ecuación de D'Arcy–Watt modeló correctamente los datos experimentales de adsorción de agua sobre PSL. El módulo de desviación relativa (P) fue 3.3, 1.3 y 3.7 % para 15, 30 y 45 °C respectivamente. La compensación entalpia–entropía (criterio termodinámico) mostró dos zonas: la primera fue controlada por la entropía (Temperatura isocinética (T_B) = 56.2 ± 1.4 K) y se apreció desde humedad cero hasta la humedad correspondiente a la mínima entropía integral (MEI), mientras la segunda fue controlada por la entalpía (T_B = 407.1 ± 6.8 K) y se observó desde la MEI hasta actividades de agua cercanas a 1.0. La teoría del bloqueo de poro (criterio cinético) sugirió que inmediatamente después de alcanzar la MEI, las moléculas de agua bloquearon la boca de los microporos formando una resistencia que disminuyó la velocidad de adsorción de agua.

Palabras clave: adsorción de agua, proteína de suero de leche, mínima entropía integral, bloqueo de poro, compensación entalpía–entropía.

 

Abstract

The objective of this work was to relate the water vapor adsorption kinetics on whey protein (WP) with the thermodynamic equilibrium obtained at several water activities, in order to determine the driving mechanisms of the process. The D'Arcy–Watt model was found to agree very well with the experimental data of water adsorption on WP. The mean relative deviation modulus value (P) was 3.3, 1.3 and 3.7% for 15, 30 and 45°C respectively. Enthalpy–entropy compensation (Thermodynamic criterion) showed two zones: the first was entropy–controlled (Isokinetic temperature (T_B) = 56.2 ± 1.4 K) and appeared from zero moisture to the moisture content corresponding to the minimum integral entropy (MIE), whereas the second was driven by changes in the enthalpy of water (T_B= 407.1 ± 6.8 K) and was observed from the MIE until water activities close to 1.0. Theory of pore blockage (kinetic criterion) suggested that immediately after reaching the MIE, the water molecules blocked the micropores mouth forming a resistance it which diminished the water vapor adsorption rate.

Keywords: water adsorption, whey protein, minimum integral entropy, pore blockage, enthalpy–entropy compensation.

 

DESCARGAR ARTÍCULO EN FORMATO PDF

 

Referencias

Aguerre, R.J., Suarez, C. y Viollaz, P.E. (1986). Enthalpy–entropy compensation in sorption phenomena: application to the prediction of the effect of temperature on food isotherms. Journal of Food Science 51, 1547–1549.         [ Links ]

Azuara, E. y Beristain, C.I. (2006). Enthalpic and entropic mechanisms related to water sorption of yogurt. Drying Technology 24, 1501–1507.         [ Links ]

Beristain, C.I. y Azuara, E. (1990). Maximal stability of dried products. Ciencia(México) 41(3), 229–236.         [ Links ]

Beristain, C.I., Azuara, E. y Vernon–Carter, E.J. (2002). Effect of water activity on the stability to oxidation of spray–dried encapsulated orange peel oil using mesquite gum (Prosopis juliflora) as wall material. Journal of Food Science 67, 206–211.         [ Links ]

Beristain, C.I., Díaz, R., García, H.S. y Azuara, E. (1994). Thermodynamic behavior of green whole and decaffeinated coffee beans during adsorption. Drying Technology 12, 1221-1233.         [ Links ]

Beristain, C.I., García, H.S. y Azuara, E. (1996). Enthalpy–entropy compensation in food vapor adsorption. Journal of Food Engineering 30, 405–415.         [ Links ]

Bhatia, S. K., Liu, F. y Arvind, G. (2000). Effect of pore blockage on adsorption isotherms and dynamics: anomalous adsorption of iodine on activated carbon. Langmuir 16(8), 4001–4008.         [ Links ]

Brunauer, S., Deming, L.S. y Teller, E. (1940). On a theory of van der Waals adsorption of gases. Journal of the American Chemical Society 62, 1723–1732.         [ Links ]

Diosady, L.L., Rizvi, S.S.H., Cai, W., y Jagdeo, D.J. (1996). Moisture sorption isotherms of canola meals, and applications to packaging. Journal of Food Science 61, 204–208.         [ Links ]

Domínguez, I.L., Azuara, E., Vernon–Carter, E.J. y Beristain, C.I. (2007). Thermodynamic analysis of the effect of water activity on the stability of macadamia nut. Journal of Food Engineering 81(3), 566–571.         [ Links ]

Fletcher, A.J. y Thomas, K.M. (2000). Compensation effect for the kinetics of adsorption/desorption of gases/vapors on microporous carbon materials. Langmuir 16, 6253–6266.         [ Links ]

Gabas, A.L., Menegalli, F.C. y Telis–Romero, J. (2000). Water sorption enthalpy–entropy compensation based on isotherms of plum skin and pulp. Journal of Food Science 65, 680–684.         [ Links ]

Hill, P.E. y Rizvi, S.S.H. (1982). Thermodynamic parameters and storage stability of drum dried peanut flakes. Lebensmittel–Wissenschaft und–Technologie 15, 185–190.         [ Links ]

Hill, T.L., Emmett, P.H. y Joyner, L.G. (1951). Calculation of thermodynamic functions of adsorbed molecules from adsorption isotherm measurements: Nitrogen on graphon. Journal of the American Chemical Society 73, 5102-5107.         [ Links ]

Krug, R.R., Hunter, W.G. y Grieger, R.A. (1976). Enthalpy–entropy compensation. 2–Separation of the chemical from the statistical effect. Journal of Physical Chemistry 80, 2341–2351.         [ Links ]

Lang, K.W., McCune, T.D. y Steinberg, M.P. (1981). A proximity equilibration cell for determination of sorption isotherm. Journal of Food Science 46, 670–672, 680.         [ Links ]

Leffler, J.E. (1955). The enthalpy–entropy relationship and its implications for organic chemistry. Journal of Organic Chemistry 20, 1202–1231.         [ Links ]

Lomauro, C.J., Bakshi, A.S., y Labuza, T.P. (1985). Evaluation of food moisture sorption isotherm equations. Part I: Fruit, vegetable and meat products. Lebensmittel–Wissenschaft und–Technologie 18, 111–117.         [ Links ]

Nunes, R. y Rotstein, E. (1991). Thermodynamics of water–foodstuff equilibrium. Drying Technology 9, 113–117.         [ Links ]

Othmer, D.F. (1940). Correlating vapor pressure and latent heat data. A new plot. Industrial and Engineering Chemistry 32, 841–856.         [ Links ]

Rizvi, S.S.H. y Benado, A.L. (1984). Thermodynamic properties of dehydrated foods. Food Technology 38, 83–92.         [ Links ]

Sonwane, C.G. y Bhatia, K. (2000). Characterization of pore size distributions of mesoporous materials from adsorption isotherms. Journal of Physical Chemistry B 104, 9099–9110.         [ Links ]

Wexler, A. (1976). Vapor pressure formulation for water in range 0 to 100 °C. A Revision. Journal of Research of the National Bureau of Standards. A. Physics and Chemistry 80, 775–785.         [ Links ]