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Aroma Barrier Properties of Sodium Caseinate-Based Films Maria José Fabra,† Alicia Hambleton,‡ Pau Talens,† Fréderic Debeaufort,*,‡,§ Amparo Chiralt,† and Andrée Voilley‡ Food Technology Department-Institute of Food Engineering for Development, Universidad Politécnica de Valencia, Camino de Vera s/n. 46022, Valencia, Spain, ENSBANA-EMMA, Université de Bourgogne, 1 esplanade Erasme, 21000 Dijon, France, and IUT-Génie Biologique, Boulevard Dr. Petitjean, B. P. 17867, 21078 Dijon Cedex, France Received December 10, 2007; Revised Manuscript Received February 9, 2008
The mass transport of six different aroma compounds (ethyl acetate, ethyl butyrate, ethyl hexanoate, 2-hexanone, 1-hexanol, and cis-3-hexenol) through sodium caseinate-based films with different oleic acid (OA)/beeswax (BW) ratio has been studied. OA is less efficient than BW in reducing aroma permeability, which can be attributed to its greater polarity. Control film (without lipid) and films prepared with 0:100 OA/BW ratio show the lowest permeability. OA involves a decrease in aroma barrier properties of the sodium caseinate-based films due to its plasticization ability. Preferential sorption and diffusion occurs through OA instead of caseinate matrix and/or BW. The efficiency of sodium caseinate-based films to retain or limit aroma compound transfers depend on the affinity of the volatile compound to the films, which relates physicochemical interaction between volatile compound and film. Specific interactions (aroma compound-hydrocolloid and aroma compound-lipid) induce structural changes during mass transfer.
1. Introduction In recent years, edible films and coatings have received increasing attention from researchers and industry as an interesting alternative of food packaging.1–5 Functional properties of edible films strongly depend on their composition. Indeed, protein-based and polysaccharide-based packaging have good organoleptic, mechanical, and barrier properties to noncondensable gases (O2, CO2, N2) and aroma. The degree of hydrophobicity of the amino acid residues in a protein controls the influence of moisture on mass transport properties of the protein films.6 Most edible films are quite moisture-sensitive, but this inherent hydrophilicity makes them excellent barriers to nonpolar substance such as oxygen and some aroma compounds. Commercial caseinates like sodium caseinate (NaCas) has a satisfactory thermal stability and can easily form films from aqueous solutions due to its random coil nature and ability to form extensive intermolecular hydrogen, electrostatic, and hydrophobic bonds.7 Edible films of milk protein are usually flavorless, tasteless, and flexible. Barrier properties of sodium caseinate-based films seem interesting to control moisture, oxygen, and aroma compound.5,8,9 The transfer of aroma compounds into the packaging induces a modification of the organoleptic properties of foods during storage. Loss of volatile compounds diminishes flavor intensity, thus changing the aromatic note of the food product. Moreover, absorbed off-flavors permeating through packaging from the environment can modify sensory characteristics of the food. Interactions between packaging and food aroma compounds have an impact on food quality.10–12 A reduction of food quality may result from the oxidation of aroma compounds due to the ingress of oxygen, or it may be the result of the loss of specific * To whom correspondence should be addressed. Phone: +33 (0) 3 80 39 68 43. Fax: +33 (0) 3 80 39 66 11. E-mail: frederic.debeaufort@ u-bourgogne.fr. † Universidad Politécnica de Valencia. ‡ Université de Bourgogne. § IUT-Génie Biologique.
aroma compounds into the packaging material or environment. Thus, permeability of volatile compounds is very important in storage and distribution. Effective control of the aroma mass transfer across the packaging material can prevent undesirable off-flavor and sensory alteration during storage.5 The efficiency of an edible coating to retain or limit transfers of aroma compounds is highest when it has low affinity for volatile compounds and low diffusivity. Physico-chemical characterization of volatile compounds influences the film permeability; an aroma compound’s shape and size affect its diffusivity, whereas solubility is influenced by the compound’s nature polarity and ability to condense.13 The objective of this work is to better understand the physicochemical interactions between aroma vapors and edible films based on sodium caseinate containing glycerol and lipid mixtures of oleic acid (OA) and beeswax (BW) during permeation.
2. Materials and Methods 2.1. Materials. Alanate 110 sodium caseinate (Llorella, S. A. Barcelona, Spain) was used as film-forming component of the hydrophilic continuous phase for emulsion-based edible films. BW (Brillocera, S. A., Valencia) and OA (Panreac quimica, S.A. Castellar Del Vallés, Barcelona, Spain) were used as the hydrophobic dispersed phase. Glycerol (Panreac quimica, S.A. Castellar Del Vallés, Barcelona, Spain) was used as plasticizer. Aroma Compounds. The volatile compounds selected were 99.5% ethyl acetate (Aldrich Chemical Co., Inc.), 99% ethyl butyrate (Aldrich Chemical Co., Inc.), 98% ethyl hexanoate (Aldrich Chemical Co., Inc.), 98% 2-hexanone (Aldrich Chemical Co., Inc.), g98% 1-hexanol (Merck-Schuchardt), and 98% cis-3-hexenol (Aldrich Chemical Co., Inc.). Their physicochemical characteristics are summarized in Table 1. They are chosen because they are present in many food products such as confectionery, dairy products, and biscuits and they allow to compare different chemical groups and chain length.14 The hydrophobicity is expressed by log K (where K is the partition coefficient of the aroma compound between octanol and water). Ethyl
10.1021/bm701363p CCC: $40.75 2008 American Chemical Society Published on Web 03/25/2008
Aroma Barrier Properties of NaCas-Based Films
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Table 1. Physicochemical Characteristics of Aroma Compounds
a
Ref 14.
b
Ref 15. c Ref 16.
d
Ref 17.
e
Ref 18. f Ref 19.
g
Ref 20.
h
Ref 21. I Ref 22. j Ref 23. k Ref 24.
acetate, ethyl butyrate, 2-hexanone, and cis-3-hexenol can be considered as a hydrophilic compound (log K < 2), whereas ethyl hexanoate and 1-hexanol are hydrophobic ones (log K > 2). 2.2. Solution Preparation Method. The film-forming aqueous dispersions of the control film, contained 8% (w/w) sodium caseinate and the amount of plasticizer required to obtain the predetermined protein/plasticizer ratio. Dispersion was homogenized for 1 min at 13500 r.p.m, followed by 3 min at 20500 rpm using an Ultra-Turrax homogenizer (Ultraturax T25, Janke & Kunkel, Germany). For sodium caseinate-lipid emulsion films, lipid fraction was composed of OA and BW in different OA/BW ratios (100:0; 70:30; 50:50; 30:70; 0:100). After glycerol was added to aqueous solutions of sodium caseinate, the amount of BW required was melted in the hot solution and was also homogenized for 1 min at 13500 rpm, followed by 1 min at 20500 rpm. The temperature of homogenization was 85 °C. The emulsions were cooled at room temperature and OA was added in the amount required for each film composition. Each emulsion was homogenized again with a vacuum high-shear probe mixer (Ultraturax T25, Janke & Kunkel, Germany) for 2 min at 20500 rpm. The film-forming dispersions were degasified at room temperature with a vacuum pump. The films prepared without BW (100:0 OA/BW ratio) were homogenized as explained previously but at room temperature. 2.3. Film Formation Method. Films were prepared by weighing the amount of the degasified film-forming dispersion/emulsion that would provide 2 g of total solids on a Teflon casting plate resting on a leveled surface. The solution was spread over a whole surface area and films were dried for approximately 24 h at 45% RH and 20 °C. Dry films could be peeled intact from the casting surface. 2.4. Aroma Permeability. A dynamic measurement method of aroma vapor fluxes through films were used. The apparatus has been described by Debeaufort and Voilley.11 The permeation cell was composed of two chambers divided by the film to be studied (Figure 1). The film area exposed to transfer was 15.9 cm2. The two chambers were continuously swept by an approximately 30 mL min-1 nitrogen flow. The aroma concentrations in the vapor phase on the upper side of the cell were obtained by mixing two flows: one containing the volatile compound and the other dry nitrogen. Flows containing vapors were obtained from bubbling dry nitrogen through pure compounds. The volatile compounds passing across the film were swept by the carrier gas (N2) and carried out up to an automatic injection valve through a transfer line heated at 190 °C to prevent adsorption. A total of 1 mL of the carrier gas was automatically injected at periodical time in the gas chromatograph. Moisture and inorganic compounds could be analyzed first by the Thermal Conductivity Detector (TCD), but not in this study, whereas organic volatiles were analyzed with a flame ionizing detector (FID). The two detector are mounted in series to allow
Figure 1. Dynamic system to measure aroma permeability through sodium caseinate-based films.
the simultaneous analysis of both moisture and flavor compounds. External calibration have been done using dilute aqueous solutions of the aroma (from 50 ppm up to 90% of solubility limit in water) inject with a syringe. Films were equilibrated at 30% relative humidity at 25 °C before permeability determinations. Permeation measurements were carried out at 25 °C. The highest concentration of aroma in the vapor phase (saturation) was obtained by bubbling the carrier gas through pure aroma at 25 °C and atmospheric pressure, and its concentration was measured by the gas–liquid chromatography (GLC; Chrompack CP 9000, Varian, France). The column was a 20 m and 1/8′ internal diameter of a Carbowax packing. The injector and FID-TCD detectors temperatures were 190 and 200 °C, respectively. The analyses were conduct at isotherm temperatures of the oven, at 90 °C for the ethyl acetate and ethyl butyrate, at 100 °C for the 2-hexanone, and at 110 °C for the 1-hexanol and cis-3-hexenol. Detection threshold of aroma permeability measured by FID of the ethyl acetate, ethyl butyrate, ethyl hexanoate, 2-hexanone, 1-hexanol, and cis-3-hexenol are 0.000007, 0.00005, 0.00045, 0.00003, 0.005, and 0.0004 g mm/Pa m2 s, respectively. Each measurement was been done at least in triplicate. 2.5. Statistical Analysis. Statistical analysis of data was performed through analysis of variance (ANOVA) using Statgraphics Plus for Windows 5.1 (Manugistics Corp., Rockville, Md.). Fisher’s least significant difference (LSD) procedure was used.
3. Results 3.1. Aroma Permeability of Ethyl Esters. The basic mass transport mechanisms of low-molecular weight compounds in polymer films include sorption and diffusion. The sorption and dissolution of a permeant in the polymer is a thermodynamic property depending on the temperature and the vapor pressure of the permeant. When the solubility coefficient (S) is independent
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Table 2. Ethyl Acetate, Ethyl Butyrate, and Ethyl Hexanoate Permeability of Sodium Caseinate-Based Filmsa
a
OA/BW ratio
permeability ethyl acetate (g mm/Pa m2 s)
permeability ethyl butyrate (g mm/Pa m2 s)
permeability ethyl hexanoate (g mm/Pa m2 s)
control 100:0 70:30 50:50 30:70 0:100
0.006 ( 0.001a 36.1 ( 8.5b 26.4 ( 6.7b,c 18.7 ( 1.6c 0.13 ( 0.02a 0.0061 ( 0.0005a
0.19 ( 0.05a 1732.69 ( 341.17b 349.50 ( 17.73c 238.79 ( 0.74d 1.07 ( 0.15a 0.08 ( 0.01a
