Effect of surfactants on water sorption and barrier properties

of hydroxypropyl methylcellulose films

Ricardo Villalobosa, Pilar Hernandez-Munozb, Amparo Chiraltc,*

aDepartamento de Ingenierıa de Alimentos, Universidad del Bıo Bıo, Casilla 447 Chillan, ChilebInstituto de Agroquımica y Tecnologıa de Aliemntos, CSIC, Apartado de Correos 73, 46100 Burjassot, Valencia, Spain

cDepartamento de Tecnologıa de Alimentos, Universidad Politecnica de Valencia, Apartado de Correos 22012, Camino de Vera s/n, 46022 Valencia, Spain

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

0268-005X/$ - see front matter q 2005 Elsevier Ltd. All rights reserved.

doi:10.1016/j.foodhyd.2005.04.006

* Corresponding author. Tel.: C34 963 877 364; fax: C34 963 879 362.

E-mail address: dchiralt@tal.upv.es (A. Chiralt).

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

Food Hydrocolloids 20 (2006) 502–509

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R. Villalobos et al. / Food Hydrocolloids 20 (2006) 502–509 503

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-Gomez, & Martın-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. Materials and methods

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–surfact

mixture in the formulations

Formulations Methocel E-15 (g) Span 60 (g) Sucros

F1 1.5 3.0 –

F2 3.0 3.0 –

F3 4.5 3.0 –

F4 1.5 2.6 0.4

F5 3.0 2.6 0.4

F6 4.5 2.6 0.4

F7 1.5 2.1 0.9

F8 3.0 2.1 0.9

F9 4.5 2.1 0.9

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

were used.

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 hydro-

philic 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 surfac-

tant 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

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

ants ratio (H/S) and hydrophilic–lipophilic balance (HLB) for the surfactant

e ester P-1570 (g) Total solids, % (w/v) HLB H/S

4.5 4.7 0.5

6.0 4.7 1.0

7.5 4.7 1.5

4.5 6.0 0.5

6.0 6.0 1.0

7.5 6.0 1.5

4.5 8.0 0.5

6.0 8.0 1.0

7.5 8.0 1.5

R. Villalobos et al. / Food Hydrocolloids 20 (2006) 502–509504

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- deBoer

(GAB) model (Eq. (2)). The linear and polynomial regression

analysis was carried out using Microsoft Excel 2000

We ZðW0CawÞ

ð1 KawÞð1 C ðC K1ÞawÞ(1)

We Zw0CKaw

ð1 KKawÞð1 C ðC K1ÞKawÞ(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.

2.5. Water barrier properties

Film-forming solutions were spread onto cellulose

acetate sheets (support film) of high water permeability

(Rayophane 300P, La Cellophane Espanola S.A.) with a thin

layer chromatography spreader set at 800 mm. Films were

dried at room temperature (w25 8C) and relative environ-

mental 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 polymethylmetha-

crylate 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

Methocel

Span

Sucroester

0

0.05

0.1

0.15

0.2

0.25(a)

we

(g w

ater

/g s

.s.)

0

0.05

0.1

0.15

0.2

0.25(b)

0 0.2 0.4 0.6 0.8

aw

we

(g w

ater

/ g s

.s.)

F4

F5

F6

Methocel

Fig. 1. Water sorption isotherms (experimental points and GAB fitted model) of

according to constant HLB: (b) 4.7; (c) 6.0 and (d) 8.0 at 10 8C, with different hy

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 KPw2Þ (4)

where

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

3. Results and discussion

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

F7

F8

F9

Methocel

0 0.2 0.4 0.6 0.8 1

aw

1

pure components (a) composite films (b–d). Film isotherms were grouped

drocolloid–surfactant ratio (C 0.5; , 1.0; 1.5).

0.00

0.04

0.08

0.12

0.16

0.20(a)

We

expe

rimen

tal

F1

F2

F3

We

expe

rimen

tal

0.00

0.04

0.08

0.12

0.16

0.20(b)

F4

F5

F6

We

expe

rimen

tal

0.00

0.04

0.08

0.12

0.16

0.20(c)

0.00 0.04 0.08 0.12 0.16 0.20

F7

F8

F9

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.

R. Villalobos et al. / Food Hydrocolloids 20 (2006) 502–509 505

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 com-

ponents, 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 mono-

stearate 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. Equili-

brium 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 methylcel-

lulose–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,

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

Westjaw Z xa Cyb Ccz (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,

Table 2

BET and GAB parameters obtained from isotherms of different composite

films and pure components and permeance values of the films

Films BET GAB Permeance

(g /day

m2 Pa)W0 C W0 C K

F1 0.019 4.2 0.018 4.9 1.016 0.134

F2 0.020 3.9 0.019 4.7 1.018 0.375

F3 0.021 2.8 0.015 5.3 1.101 0.471

F4 0.018 5.1 0.015 5.3 1.101 0.176

F5 0.018 3.4 0.016 4.1 1.035 0.441

F6 0.022 2.1 0.018 2.7 1.053 0.427

F7 0.017 3.7 0.016 4.1 1.012 0.211

F8 0.018 3.4 0.018 3.5 1.018 0.431

F9 0.022 2.7 0.020 2.9 1.007 0.441

Methocel 0.029 2.1 0.030 2.0 0.998 –

Sucrose

ester

0.016 5.6 0.013 8.2 1.009 –

Span60 0.014 6.3 0.020 3.6 0.894 –

Cellophane – – – – – 1.228

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

R. Villalobos et al. / Food Hydrocolloids 20 (2006) 502–509506

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, Hernandez, Gutierrez, & 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 hydroxyethyl-

cellulose (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

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 (Schar & Ruegg, 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 thermo-

dynamic 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

0 0.5 1 1.5 2

H/S

4.7 6.0 8.00

1

2

3

4

5

6(d)

C

0 0.5 1 1.5 2

H/S

4.7 6.0 8.00

0.005

0.010

0.015

0.020

0.025(c)

Wo

4 6 8

HLB

0

0.005

0.010

0.015

0.020

0.025(a)

Wo

0.5 1.0 1.5

0

1

2

3

4

5

6(b)

C

4 6 8

HLB

0.5 1.0 1.5

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.

R. Villalobos et al. / Food Hydrocolloids 20 (2006) 502–509 507

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 measure-

ment 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

0.0

0.1

0.2

0.3

0.4

0.5

0.1 0.12 0.14 0.16 0.18 0.2

We (g water/gd.s.)

Per

mea

nce

(g/d

aym

2 P

a)

0.5 1.0 1.5

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.

R. Villalobos et al. / Food Hydrocolloids 20 (2006) 502–509508

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. There-

fore, 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 AGL2004-

01009) for financial support. Ricardo Villalobos is grateful

for the concession of a fellowship from Agencia de

Cooperacion Internacional (AECI).

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