Effect of surfactants on water sorption and barrier properties of hydroxypropyl methylcellulose films

Food Hydrocolloids 20 (2006) 502–509 www.elsevier.com/locate/foodhyd

Effect of surfactants on water sorption and barrier properties of hydroxypropyl methylcellulose films Ricardo Villalobosa, Pilar Herna´ndez-Mun˜ozb, Amparo Chiraltc,* a Departamento de Ingenierı´a de Alimentos, Universidad del Bı´o Bı´o, Casilla 447 Chilla´n, Chile Instituto de Agroquı´mica y Tecnologı´a de Aliemntos, CSIC, Apartado de Correos 73, 46100 Burjassot, Valencia, Spain c Departamento de Tecnologı´a de Alimentos, Universidad Polite´cnica de Valencia, Apartado de Correos 22012, Camino de Vera s/n, 46022 Valencia, Spain b

Received 10 August 2004; revised 2 March 2005; accepted 19 April 2005

Abstract Moisture sorption isotherms and water barrier properties of films made from hydroxypropyl methylcellulose (HPMC) containing surfactant mixtures of sorbitan monostearate (SPAN 60) and sucrose palmitate (sucrose ester P-1570) were evaluated at 10 8C. The effect of hydrocolloid/surfactant ratio (H/S) (0.5, 1.0 and 1.5) and the hydrophilic/lipophilic balance of the mixture (HLB: 4.7, 6.0 and 8.0) was analysed. GAB and BET sorption models were tested to fit the experimental data. The equilibrium moisture content of films increased dramatically above awZ0.6. Films with greater H/S ratio have greater water binding capacity. For a specific hydrocolloid/surfactant ratio, equilibrium moisture content of the coatings decreased as the HLB of the surfactant mixture increased. Equilibrium moisture contents of the composite films could be predicted from the corresponding values of pure compounds and the respective component mass fractions by a linear model, in the water activity range of 0.11–0.75. At the high relative humidity at which water permeability of the films was evaluated, the water vapour barrier was effective when the films reached a critical amount of surfactants (H/SZ0.5). q 2005 Elsevier Ltd. All rights reserved. Keywords: Hyxroxypropyl methylcellulose; Surfactants; Edible films; Moisture sorption isotherms; Water vapour transfer

1. Introduction In the last few years, the increased awareness for environmental conservation and protection has promoted the development of edible coatings and films from biodegradable materials to maintain the quality of both fresh and processed fruits and vegetables (Baldwin, Nisperos-Carriedo, & Baker, 1995; Nussinovitch & Lurie; 1995; Park, 1999; Wong, Tillin, Hudson, & Pavlath, 1994; Krochta & Mulder-Johnston, 1997). Such films can act as moisture barriers preventing the water loss in fresh products and therefore extending their shelf-life (Avena-Bustillos, Krochta, & Saltveit, 1997; Baldwin, Nisperos, Chen, & Hagenmaier, 1996; D’Aquino, Piga, Agabbio, & Tribulato, 1996; Erbil & Muftugil, 1986). Generally, edible films and coatings are formed by polymeric agents, proteins and polysaccharides, serving as * Corresponding author. Tel.: C34 963 877 364; fax: C34 963 879 362. E-mail address: [email protected] (A. Chiralt).

0268-005X/$ - see front matter q 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.foodhyd.2005.04.006

a structural matrix and providing mechanical resistance. Lipid compounds such as fatty acids, natural waxes, surfactants and resins are frequently incorporated into the hydrocolloid matrix when a barrier to water is desired (Hernandez, 1994; Kester & Fennema, 1986). Lipids can also have emulsifier and plasticiser properties that improve flexibility through increased mobility of the adjacent hydrocolloid chains. Much research into edible films and coatings has been focussed on different aspects that affect water barrier properties: (a) the effect of the film components’ hydrophilic–lipophilic nature, (b) physical state, quantity and molecular size of the lipid components, (c) plasticiser addition and conditions of film preparation and formation (Debeaufort, Quezada-Gallo, & Voilley, 1998; Donhowe & Fennema, 1993; Hagenmaier & Shaw, 1990, 1991; Kester & Fennema, 1989; Martin-Polo, Mauguin, & Voilley, 1992). The permeability of a film involves solubilization and diffusion of molecules through the film matrix. Due to the inherent hydrophilic nature of film-forming polymers obtained from polysaccharides and proteins they are plasticized by water modifying their macromolecular

R. Villalobos et al. / Food Hydrocolloids 20 (2006) 502–509

503

ester P 1570, Mitsubshi-Kasei Foods Corp., Tokyo, Japan) were used.

structure and thus both solubility and diffusion coefficients which become highly dependent on the relative humidity conditions during testing. The uptake of water vapour by these materials depends on both the chemical structure of the film and also on its morphology. Water sorption isotherms provided information on the water binding capacity of the films at a determined environmental relative humidity, and are a useful tool for the analysis of water plasticizing effects and their effect on water permeability. There is great potential in the use of cellulose derivatives as edible films and coatings intended for regulating moisture transfer in food systems. Cellulose ether films cast from aqueous or aqueous-ethanol solutions of methylcellulose, hydroxypropylmethylcellulose, hydroxypropylcellulose and carboxymethylcellulose tend to have moderate strength, are resistant to oils and fats, and are flexible, transparent, odourless, tasteless, water-soluble and moderate barriers to oxygen and moisture (Krochta & Mulder-Johnston, 1997). Several studies have been carried out evaluating water barrier properties of these films in combination with lipids. However there is little work regarding moisture sorption properties of these films (Chinnan & Park, 1995; Debeaufort, Voilley, & Meares, 1994; Gocho, Shimizu, Tanioka, Chou, & Nakajima, 2000; Velazquez, Herrera-Go´mez, & Martı´n-Polo, 2003) and their correlation with water barrier properties. In this work we have prepared composite edible films of HPMC (support matrix) and surfactant mixtures of sorbitan monostearate (Span 60) and sucrose palmitate (sucrose ester P-1570) having low and high HLB, respectively. The effect of surfactant mixtures of different hydrophobicity on the moisture sorption properties and water vapour permeability of the obtained films was studied.

2.2. Experimental design Nine film-forming solutions were obtained using HPMC at 1.5, 3.0 and 4.5% (w/v) and Span 60-sucrose ester P-1570 mixtures at 3.0% (w/v). The hydrophobic surfactant (Span 60, hydrophilic–lipophilic balance, HLBZ4.7) and the hydrophilic surfactant (sucrose ester P-1570, HLBZ15) were mixed in different ratios to obtain three overall HLB levels (4.7, 6.0 and 8.0) for each level of hydrocolloid concentration (Table 1). In turn, solutions and dispersions of pure components were prepared to study their sorption properties. 2.3. Preparation of hydroxypropyl methylcellulose films Hydrocolloid and hydrophilic surfactant were dispersed and dissolved in 150 ml of deionised water at 90 8C with constant stirring. After 10 min, melted hydrophobic surfactant was added and the mixture was stirred for 10 min more. The mixture was then emulsified using an Ultra-Turrax T-25 homogeniser (Janke and Kunkel, Germany) at 12,500 rpm for 5 min. Subsequently, 50 ml of deionised water was added and the mixture allowed to cool to room temperature (w25 8C) while maintaining slow stirring. Finally, the total solid concentration in formulation was adjusted by adding the required amount of water. Film-forming solutions were spread on Petri capsules and were placed in an air-circulating oven at 65 8C for 1 day. Then, the films were transferred to a vacuum oven at 60 8C for another day to obtain dried-structured films. 2.4. Moisture sorption isotherms

2. Materials and methods

Each dried film specimen (in triplicate) was placed inside six hermetic glass jars containing saturated salt solutions (ASTM E 104-85, ASTM, 1996): LiCl, CH3COOK, MgCl, NaBr, NaCl, KCl) at 10 8C to maintain 11.3, 23.4, 33.5, 62, 75.7 and 86.8% relative humidities (Greenspan, 1977). The film specimens were weighed periodically (0.00001 g precision) until they reached constant weight, where

2.1. Materials Hydroxypropyl methylcellulose (DSZ1.9, MSZ0.23, Methocel E15 Food Grade, Dow Chemical Co., Midland, USA) sorbitan monostearate (Span 60, ICI Surfactant, Cleaveland, UK) and sucrose palmitate (sucrose

Table 1 Composition of edible film formulations (g/100 ml solution), hydrocolloid–surfactants ratio (H/S) and hydrophilic–lipophilic balance (HLB) for the surfactant mixture in the formulations Formulations

Methocel E-15 (g)

Span 60 (g)

Sucrose ester P-1570 (g)

Total solids, % (w/v)

HLB

H/S

F1 F2 F3 F4 F5 F6 F7 F8 F9

1.5 3.0 4.5 1.5 3.0 4.5 1.5 3.0 4.5

3.0 3.0 3.0 2.6 2.6 2.6 2.1 2.1 2.1

– – – 0.4 0.4 0.4 0.9 0.9 0.9

4.5 6.0 7.5 4.5 6.0 7.5 4.5 6.0 7.5

4.7 4.7 4.7 6.0 6.0 6.0 8.0 8.0 8.0

0.5 1.0 1.5 0.5 1.0 1.5 0.5 1.0 1.5

504

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equilibrium was assumed. Finally, the equilibrium moisture content was determined using a vacuum oven at 70 8C and 50 Torr for 6 h. Experimental sorption isotherms were fitted to two extensively used equations: BET molecular model of adsorption (Eq. (1)) and the Guggenheim-Anderson- deBo¨er (GAB) model (Eq. (2)). The linear and polynomial regression analysis was carried out using Microsoft Excel 2000 ðW0 Caw Þ We Z ð1 K aw Þð1 C ðC K 1Þaw Þ We Z

(1)

solution was used to control the relative humidity on the other side of the film 75.7%. During WVT testing, the cellophane side of the film was placed in contact with the part of the test cup having the lowest relative humidity. Water vapour transmission rate measurements (Eq. (3)) were performed at 10 8C and finally expressed as permeance (Eq. (4)) WVT Z G=tA Z ðG=tÞ=A

(3)

Permeance Z WVT=ðPw1 K Pw2 Þ

(4)

where

w0 CKaw ð1 K Kaw Þð1 C ðC K 1ÞKaw Þ

(2)

In Eqs. (1) and (2), We is the equilibrium moisture content on dry basis, W0 is the monolayer moisture value, aw is water activity, C and K are the equation parameters, both are temperature dependent and related to the interaction energy between water and film.

WVT water vapour transmission rate, g/h m2 G/t slope of the plotting of amount of water lost over time, g/h A area of the film, m2 Pw1 partial pressure of water vapour on the film’s underside, Pa Pw2 partial pressure of water on the film’s upper surface, Pa

2.5. Water barrier properties 3. Results and discussion

(a)

0.25

we (g water/g s.s.)

Film-forming solutions were spread onto cellulose acetate sheets (support film) of high water permeability (Rayophane 300P, La Cellophane Espan˜ola S.A.) with a thin layer chromatography spreader set at 800 mm. Films were dried at room temperature (w25 8C) and relative environmental humidity (w30%) in natural convection for one day. The water vapour transfer (WVT) through dry coating films was measured according to the ‘water method’ of the ASTM E-96-95 (ASTM, 1995), using polymethylmethacrylate cups following the design proposed by Gennadios, Weller, and Gooding (1994). Deionised water was used inside the testing cup to achieve 100% relative humidity on one side of the film, while a saturated sodium chloride

0.2

Composition of edible film formulations (F1–F9) and variables of interest, such as the hydrophilic–lipophilic balance (HLB) of surfactant and hydrocolloid/surfactant ratio (H/S), are shown in Table 1. The low surface energy of hydroxypropyl methylcellulose allows the incorporation of hydrophobic additives in different quantities, being possible to obtain films from all the formulations prepared. 3.1. Moisture sorption isotherms Fig. 1 shows the water sorption isotherms of surfactants Span 60 and sucrose ester P-1570; and films made from F1 F2 F3 Methocel

Methocel Span

Sucroester

0.15 0.1 0.05

(b)

0.25

we (g water/ g s.s.)

0

0.2

F5

0.15

F6

F7

F4

F8 F9 Methocel

Methocel

0.1 0.05 0 0

0.2

0.4

0.6

aw

0.8

1

0

0.2

0.4

0.6

0.8

1

aw

Fig. 1. Water sorption isotherms (experimental points and GAB fitted model) of pure components (a) composite films (b–d). Film isotherms were grouped according to constant HLB: (b) 4.7; (c) 6.0 and (d) 8.0 at 10 8C, with different hydrocolloid–surfactant ratio (C 0.5; , 1.0; 1.5).

R. Villalobos et al. / Food Hydrocolloids 20 (2006) 502–509

a non-interactive contribution of each component is assumed and moisture content was obtained from the equilibrium values of pure components and their proportion in the mixture West jaw Z xa C yb C cz

(5)

where Westjaw equilibrium moisture content of the film at each aw . x, y, z mass fractions of HPMC, sorbitan monostearate and sucrose palmitate in the film. a, b, c equilibrium moisture content of pure components at each aw. Fig. 2a–c shows experimental vs. estimated equilibrium moisture content of films made from F1 to F9 formulations,

(a) 0.20

We experimental

F1

0.16

F2 F3

0.12 0.08 0.04 0.00

(b) 0.20

We experimental

F4

0.16

F5 F6

0.12 0.08 0.04 0.00

(c) 0.20

We experimental

either pure HPMC or formulations F1–F9. Edible films and pure components presented type III sorption isotherms with a negligible convexity at low aw (Brunauer, Deming, Deming, & Teller, 1940). Among the three pure components, HPMC films showed the highest water affinity due to the large amount of hydrophilic groups present in their structure. Equilibrium moisture for HPMC films increased slowly between 0 and 0.62 aw; from this value up, higher water activities implied a substantial water gain in the film owing to the effect of the solubilisation phenomenon. Sorption isotherms for HPMC films were similar to those obtained by Chinnan et al. (1995) for films of pure methylcellulose and hydroxypropylcellulose. However, lower water binding capacity was obtained for HPMC films than for HPMC powder (Spiess & Wolf, 1983), which can be explained by the smaller specific surface of the film as compared with the particle bed. Both sorbitan monostearate and sucrose palmitate showed flatter isotherms compared to pure HPMC films in the complete range of aw. No differences in water sorption capacity among the three components were detected at low aw. Above 0.35 aw, films made from pure HPMC gained greater amounts of moisture than sorbitan monostearate or sucrose palmitate. Equilibrium moisture content for both surfactants was similar despite the more hydrophilic character of the sucrose ester (James & McGregor, 2000). In Fig. 1b–d, the sorption isotherms of the composite films, corresponding to HLB of 4.7, 6.0 and 8.0, respectively, can be observed. The sorption isotherm of pure HPMC has also been plotted in each figure for visual comparison. Sorption isotherms of studied films were similar to those reported by Rico-Pena and Torres (1990) for films made from methylcellulose and palmitic acid. For all HLB levels, it can be clearly observed that at aw above 0.62 there was an increase in the water sorption capacity of films as the H/S ratio increased. When HLB increased from 4.7 (formulations F1–F3) to 8.0 (formulations F7–F9), differences in water holding capacity of films associated to H/S ratio were less notable (see Fig. 1d). On the other hand, for a constant H/S ratio, the water sorption capacity decreased in line with the HLB increase, despite the increase in the hydrophilic character of the surfactant mixture. This effect could be attributed to possible hydrogen bond interactions between hydroxyl groups of HPMC and sucrose ester, reducing the number of active sites for water adsorption. These results are in agreement with that reported by Okhamafe and York (1983). These authors found that the solubility coefficient of water in hydroxypropyl methylcellulose–polyethylene glycol films was lower than in hydroxypropyl methylcellulose–polyvinyl alcohol films which was attributed to the higher number of hydroxyl groups per molecule of polyethyleneglycol. To analyse the component interactions in the films and their possible effect on sorption properties, the experimental equilibrium moisture content at each aw is compared with the value obtained by applying Eq. (5). In this equation,

505

F7

0.16

F8 F9

0.12 0.08 0.04 0.00 0.00

0.04

0.08

0.12

0.16

0.20

We Predicted Fig. 2. Comparison between experimental values of equilibrium moisture content and those predicted by the linear model (Eq. (5)) for the different films with HLB 4.7 (a), (b) 6.0 and (c) 8.0 with different H/S ratio.

506

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films have been grouped according to HLB levels. The obtained points lie on the diagonal for low and intermediate aw levels, which indicates low interaction between components in agreement with their separation in independent phases as observed during the film drying (Villalobos, Chanona, Herna´ndez, Gutie´rrez, & Chiralt, 2004). This phenomenon has also been observed in films containing surfactants by Debeaufort and Voilley (1995) and McHugh (1996). At high levels of water activity, it can be observed that points in Fig. 2, do not fall on the diagonal, possibly as a result of the different interactions between the water molecules and the polar groups of the film depending on film composition. Differences due to specific interactions among film components have also been observed in the sorption isotherms at high water activity, where the solubility phenomena take place. These effects seem to become more acute as the HLB of surfactant mixture in the films increases. As water activity of the film increases, changes in the supra-molecular structure of the polymer may occur, as reported by Seo and Kumacheva (2002) for hydroxypropylcellulose (HPC), thus modifying polymer– lipid interactions. Regarding polymer–surfactant interactions in aqueous solution, hydrophobic interactions are the main driving force for polymer-surfactant complexation and in a minor extent the existence of specific attractions between the polymer segments and the surfactant hydrophilic moieties (Avranas & Ilion, 2003). Generally, the interaction between a water-soluble non-ionic polymer and non-ionic surfactants is weak. It has been shown that poly(vinylalcohol) (PVA), poly(ethyleneoxide) (PEO) and polyvinylpyrrolidone (PVP) do not interact with polyethoxylated non-ionic surfactants. Neutral polymer such as poly(acrylic) acid and non-ionic surfactants form complex through hyprophobic interactions and hydrogen bonding between the carboxylic groups and the oxygen of the ethyleneoxide chain. Other studies showed also significant interaction in aqueous neutral polymer and non-ionic surfactant systems of hydroxyethylcellulose (HEC) and polyethylene oxide nonyl phenyl ether; PEO and nonylphenol polyethylene glycol; poly(propylene oxide) (PPO) with sugar-containing non-ionic surfactant n-octyl thioglucoside (OTG); and the formation of complexes between hydroxypropylcellulose (HPC) and two neutral surfactants: n-octyl-B-D-thioglucopyranoside (OTG) and n-octyl-B-D-glucopyranoside (OG) (Lindman & Thalberg, 1993). Sorption isotherm behaviour could indicate that, at low relative humidities, the HPMC matrix adsorb water molecules regardless of the presence of lipids which also interact with water molecules independently. As the relative humidity rises, the water increasingly penetrates the HPMC network, partially dissolving them to form a gel, where there is a greater molecular mobility and component interactions are promoted, thus affecting sorption behaviour. The GAB and BET equations were used to fit the water adsorption data of the films and pure components. Table 2

Table 2 BET and GAB parameters obtained from isotherms of different composite films and pure components and permeance values of the films Films

F1 F2 F3 F4 F5 F6 F7 F8 F9 Methocel Sucrose ester Span60 Cellophane

BET

GAB

Permeance (g /day m2 Pa)

W0

C

W0

C

K

0.019 0.020 0.021 0.018 0.018 0.022 0.017 0.018 0.022 0.029 0.016

4.2 3.9 2.8 5.1 3.4 2.1 3.7 3.4 2.7 2.1 5.6

0.018 0.019 0.015 0.015 0.016 0.018 0.016 0.018 0.020 0.030 0.013

4.9 4.7 5.3 5.3 4.1 2.7 4.1 3.5 2.9 2.0 8.2

1.016 1.018 1.101 1.101 1.035 1.053 1.012 1.018 1.007 0.998 1.009

0.134 0.375 0.471 0.176 0.441 0.427 0.211 0.431 0.441 – –

0.014 –

6.3 –

0.020 –

3.6 –

0.894 –

– 1.228

W0: (g H2O/g d.s.).

summarises the constants for the GAB equation (obtained considering the entire range of water activities) and for BET (only for water activity up to 0.62). To fit the GAB model, the second-degree polynomial equation was used for the regression analysis. Due to the high degree of correlation among the three GAB parameters (Scha¨r & Ru¨egg, 1985), their physical meaning is not considered, although predicted values are used to plot isotherms in Fig. 1. On the other hand, the BET equation constants, which have a thermodynamic base, were used to carry out a physical interpretation of the interaction of the components with water molecules and the effect of HLB and H/S variables, as shown in Fig. 3. The monolayer moisture content for the pure HPMC film (2.9 g/100 g d.s.) was very similar to the value reported by Debeaufort et al. (1994) for a methylcellulose film (3.1 g/100 g d.s.), but relatively low when compared with the average values for hydrophilic food components (5–8 g/100 g d.s.) (Iglesias & Chirife, 1982). As can be observed in Fig. 3a monolayer moisture content, W0, did not change considerably when the HLB in the surfactant mixture increased, although in films having a higher amount of surfactant mixture, the W0 values tended to decrease when the HLB increased. Constant C related to the water–substrate interaction energy did not present a clear variation when there was a change in HLB, either (Fig. 3b). As shown in Fig. 3c, a slight increase in the monolayer moisture content was observed when the proportion of hydrocolloid in the film increased. This is attributable to the greater number of sites available for water adsorption provided by the HPMC, as can be seen from the higher W0 values of HPMC as compared with those of the pure lipids (Table 2). The W0 values underwent a more marked increase in those films in which HLB is 6.0 and 8.0, where there is

R. Villalobos et al. / Food Hydrocolloids 20 (2006) 502–509

(b) 6

0.020

5

Wo

(a) 0.025

507

4

0.015

C 3

0.010

2 0.005

0.5

1.0

1

1.5

0.5

4

6

8

4

1.5

6

HLB

8

HLB

(c) 0.025

(d) 6

0.020

5

Wo

1.0

0

0

4

0.015

C 3

0.010

2 0.005

4.7

6.0

1

8.0

4.7

6.0

8.0

0

0 0

0. 5

1

1. 5

H/S

2

0

0. 5

1

1. 5

2

H/S

Fig. 3. Effect of HLB and hydrocolloid/surfactant ratio on BET parameters. In (a) and (b) symbols represent H/S ratio and in (c) and (d) symbols represent HLB values.

sucrose ester. As to C parameter, a significant drop is produced when the H/S ratio increases (Fig. 3d). This seems to indicate that as the films become hydrophilic the water molecules are adsorbed with less energy in the active sites. This behaviour is coherent with the results shown by the pure components, where the HPMC presents the lowest adsorption energy level as compared with both sucrose palmitate and sorbitan monostearate compounds (Table 2). 3.2. Water barrier properties The barrier properties to water vapour transfer of the different coatings on a cellulose acetate support film are shown in Table 2. For comparative purposes, permeance has been used as an indicator of the barrier properties since film thickness was around 60.5 mm in all cases. To measure the barrier properties of the pure cellophane film, the film was previously moistened with distilled water and then dried in order to imitate the conditions occurring during the casting and drying of the HPMC film. Water vapour transfer was greater in the previously moistened cellophane than in the non-moistened cellophane. Cellophane when moistened is structurally modified due to the strong interaction with water molecules which induces an increase in the cellulose fibre separation, thus presenting a more open structure that facilitates even greater water transport. The permeance of cellophane (C), shown in Table 2, corresponds to previously moistened cellophane. Values of permeance obtained for coatings on cellophane support were lower than those for the supports, which indicates that these coatings can be useful as water vapour barriers.

The relative humidity conditions used during measurement of permeance (100:75 relative humidity gradient) were established trying to reproduce the behaviour of coatings for chopped vegetables stored at 10 8C. At high relative humidity water acts as a strong plasticizer in hydrophilic polymers decreasing their barrier properties. Nevertheless, the level of plasticization produced by water molecules is not the only determining factor in water barrier properties of coatings, since the presence of low polarity substances can limit the permeation process when their ratio in the coating reaches a critical level. As can be observed in Table 2, permeance increased as the amount of surfactant mixture in the coating decreased from H/SZ0.5–1, whereas no notable differences are observed between films with H/SZ1.0 and 1.5. This behaviour can be explained considering that the film is structured as a continuous hydrocolloid matrix with disperse surfactant particles inside and on the film’s surface as has been observed in previous studies through film microscope images (Villalobos et al., 2004). In this model, diffusion of water molecules is expected to take place through the continuous hydrophilic phase. In this sense, the coatings that presented the lowest permeance values were those where the proportion of surfactant mixture reached a critical value at which the films showed the lowest equilibrium moisture content at high water activity levels. Fig. 4 shows the relationship between permeance and equilibrium moisture content (estimated from GAB model) of the films at awZ0.87, which has been considered as the mean aw value in the film according to the relative humidity gradient used in the experimental measurements. It can be observed that differences in permeance seem to be related with the critical level of the H/S ratio rather than with the

R. Villalobos et al. / Food Hydrocolloids 20 (2006) 502–509

Permeance (g/daym2 Pa)

508 0.5 0.4 0.3 0.2 0.1 0.5

1.0

1.5

0.0 0.1

0.12

0.14

0.16

0.18

0.2

We (g water/gd.s.) Fig. 4. Relationship between film equilibrium moisture content at awZ0.87 and their permeance at 10 8C. Symbols represent H/S ratio and the different HLB values are represented by colours: grey: 4.7; black: 6.0 and white: 8.0.

different moisture contents of the films in the range studied. The effect of BHL for a determined H/S ratio did not have a significant effect on permeance values.

4. Conclusions The equilibrium moisture content of HPMC films depends on the percentage and chemical nature of the surfactant compounds incorporated into the hydrocolloid matrix. The addition of surfactants decreased the equilibrium moisture content of the films; the higher the polarity of the surfactant mixture the greater the decrease. This can be attributed to hydrogen bond interactions between hydrocolloid and polar groups of surfactant, thereby reducing the number of polar groups available to interact with water molecules. At the high relative humidity at which the water transmission rate of the films was measured, water permeance is controlled by a critical ratio of surfactant without significant effect of the equilibrium moisture content probably due to the high level of plasticization of the hydrocolloid matrix. Therefore, HPMC coatings containing the highest level of surfactant, could be effective moisture barriers in high moisture products such as minimally processed vegetables.

Acknowledgements The authors thank the Spanish MEC (project AGL200401009) for financial support. Ricardo Villalobos is grateful for the concession of a fellowship from Agencia de Cooperacio´n Internacional (AECI).

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