Andrianaivo M. Rakotonirainy

Ying Wang,

Andrianaivo M. Rakotonirainy,

Graciela W. Padua

Department of Food Science and Human Nutrition, University of Illinois at Urbana-Champaign, Urbana, IL, USA

Thermal Behavior of Zein-based Biodegradable Films

In previous work, zein was plasticized with oleic acid to obtain flexible films. However, it was observed that film properties were affected by the preparation method employed. Our objective was to investigate the effect of processing methods on thermal behavior of zein films by differential scanning calorimetry (DSC). Films containing 41% oleic acid were prepared by casting ethanol solutions of its components on flat surfaces or by extrusion of zein resins prepared by cold water precipitation of ethanol solutions of zein and oleic acid. Extrusion was carried out in single-screw or twin-screw extruders. Zein films were finished by hot rolling or heated in a Carver press. DSC ther-mograms showed large oleic acid melting peaks for cast films, smaller peaks for resin films, and no apparent peaks for heat-treated samples. It was suggested that the resin formation process enhanced zein-oleic acid interactions and promoted plasticization. All samples showed glass transitions at low temperatures.

Keywords: DSC; Zein; Biodegradable films

1 Introduction

The development of biobased polymers for packaging and other applications is of worldwide interest due to foreseen environmental benefits and expected impact on agricultural economics [1, 2]. Proteins have been used empirically to make edible and biodegradable packaging materials. Collagen and gelatin are good examples of such materials. Cuq and co-workers [3] classified the technologies used for preparation of protein-based materials in two broad groups: "wet (or solvent) processes" based on the dispersion or solubilization of proteins in a solvent medium and "dry processes" based on the ther-moplasticity of proteins at low moisture content. Zein, the prolamine of corn, is recognized for its ability to form films [4]. Most reports describe film preparation by a "wet or solvent process" [5, 6] employing polyols or fatty acids as plasticizers [7-9]. Films were also prepared by a combination of "wet" and "dry" processes [10] involving the preparation of a moldable resin of zein and oleic acid, which was later formed into a film.

Cuq and co-workers [3] considered that macroscopic properties of protein-based materials, including mechanical properties, water absorption, and barrier properties, depend on their three-dimensional network structure and on the interaction between proteins, plasticizers, and oth-

Correspondence. Graciela W. Padua, Department of Food Science and Human Nutrition, University of Illinois at Urbana-Cham-paign, 382-D Agricultural Engineering Sciences Building, 1304 W. Pennsylvania Ave., Urbana, IL, USA. Phone: +1-217-333-9336, Fax: +1-217-333-9329, e-mail: .

er functional agents. Protein-plasticizer interactions have been investigated by differential scanning calorimetry (DSC). Kokini and co-workers [11] studied thermal properties of cereal proteins including gliadin, glutenin, and zein and generated physical state diagrams based on DSC measurements and dynamic rheological properties. Madeka and Kokini [12] measured the glass transition temperature (Tg) of zein at various moisture contents and reported its decrease from 139 °C to 47 °C when water content increased from 0 to 6.6%. They indicated that at a moisture content of ~30% Tgis below the freezing point of water and therefore it could not be measured due to formation of ice during the cooling of zein. Other workers [13-15] measured the glass transition of anhydrous zein at 162-165 °C.

di Gioia and Guilbert[16] studied the plasticization of corn gluten meal, a byproduct of cornstarch rich in endosperm proteins, with various polar (water, glycerol) and am-phiphilic (octanoic and palmitic acids, dibutyl tartrate and phthalate, and diacetyl tartaric acid esters of mono-diglyc-erides) plasticizers. Plasticization was achieved by a hot-mixing procedure. Glass transition temperatures of the blends were measured by modulated differential scanning calorimetry as functions of plasticizer type and content (0-30% db). They reported that the first amounts of added plasticizer (<10%) were the most effective at lowering Tg. However, at higher plasticizer content (between 10 and 30%) plasticization effectiveness slowed down, more markedly so for amphiphilic than for polar plasticizers. Changes in plasticization effectiveness were attributed to an increasing difficulty for the plasticizers to diffuse into the polymer matrix, di Gioia and Guilbert [16]

stressed the importance of admixing procedures when preparing biopolymer based resins.

Previous work on plasticization of zein with fatty acids to form flexible films was reported in [17-19]. Film preparation consisted of stirring fatty acids (0.5-1 g fatty acid/g zein) into aqueous ethanol (70%) solutions of zein. Subsequent addition of cold water precipitated the plasticized zein as a moldable compound, which was collected and kneaded into a cohesive and elastic mass. This extensible resin was stretched by hand over the rims of cylindrical containers and allowed to dry at room conditions. Dried films (~0.030 mm thick) were flexible and ductile at room conditions. Lai and Padua[17] employed DSC to investigate protein-plasticizer interactions in zein films. They observed that film-forming methods affected DSC response. Thermograms of films cast from zein and oleic acid solutions showed a melting peak corresponding to oleic acid indicating phase separation between film components. Melting of oleic acid was not apparent in thermograms of films formed out of moldable resins. Apparently, oleic acid had been adsorbed onto the surface of zein and resisted phase separation. The objective of this work was to further investigate the effect of processing methods on thermal behavior of zein films plasticized with oleic acid. Resins were processed by single- and twin-screw extrusion.

2 Materials and Methods

2.1 Materials

The following materials were used: Corn zein, regular grade (F4000, Freeman Industries, Inc., Tuckahoe, NY); oleic acid (C18:1) 90%, as plasticizer (Aldrich Chemical Co., Milwaukee, Wl); phosphorous pentoxide 97% (Aldrich Chemical Co., Milwaukee, Wl).

2.2 Preparation of zein films

Granular zein was dissolved in 75% ethanol at 60 °C to a concentration of 16% (w/v). Oleic acid was gradually stirred into the solution at a final ratio of 41 % (w/w). Cast films were prepared by pouring the solution, cooled down to room temperature, on a flat surface covered with Mylar® (PET) film. Zein-oleic acid resins were obtained as a soft mass by precipitating the above solution in 7-fold volumes of chilled water (4 °C). Resins were collected and extruded at room temperature either in single-screw (Model EPL-V501, C.W. Brabender, Hackensack, NJ) or twin-screw (Model ZSK-30, Werner and Pfleiderer, Ramsey, NJ) extruders. Extruded samples were collected as ribbons (2.54 cm in width and 0.5 mm in thickness) which were allowed to dry in air and stored at room temperature and away from light. Non-extruded resin films were pre-

pared by stretching resins before extrusion into films that were allowed to dry in air and stored as described above.

2.3 Differential scanning calorimetry (DSC)

DSC measurements were carried out in a Perkin-Elmer DSC-7 (Perkin-Elmer Cetus, Norwalk, CT). Calibration was based on pure indium. An empty pan was used as reference. Prior to analysis, samples were placed in a desiccator containing phosphorous pentoxide as desic-cant for 17 days or longer. Samples (0.033 ± 0.003 g) were scanned at a rate of 10 °C/min. Glass transition temperatures were determined from resulting thermograms as the midpoint between onset and end temperatures of step changes in heat flow observed during heating and identified as second-order transitions.

3 Results and Discussion

The DSC thermogram for oleic acid (Fig. 1) shows a melting peak centered at about 29 °C. Also, a second smaller peak was observed at -2.5 °C. Cedeno and co-workers [20] attributed a similar observation to a solid-solid phase transition prior to melting. They explained that fast solidification of oleic acid originates a crystalline solid, which transforms into a high temperature phase before melting. The oleic acid thermogram also showed a second order transition with mid-point at-129 °C (Fig. 2) that was interpreted as a Tg.

Fig. 1. DSC thermogram of oleic acid showing a melting peak centered at 29 °C.

Fig. 2. DSC thermogram of oleic acid showing a second order transition with mid-point at -129 °C

DSC thermograms for cast and extruded films (Fig. 3) showed endothermic peaks centered at about 27 °C, which were attributed to melting of oleic acid. Melting peaks in Fig. 3 were larger for cast films than for any other samples. Zein to oleic acid ratios were assumed to be the same for all samples. Ha [21] measured oleic acid losses during resin preparation and reported only a 1% loss after resin precipitation in cold water. Peaks corresponding to unextruded resin, single-screw extruded film, and twin-screw extruded film were comparable in size. Heat-treated samples showed no apparent peaks. Differences in peak size between cast and unheated resin films were attributed to the resin formation process, which could have promoted binding of oleic acid to zein thus reducing phase separation and melting out. Additional heat-treatment possibly induced zein denaturation, which could have increased oleic acid binding and prevented melting out. Lai and co-workers [19] obtained small-angle X-ray scattering (SAXS) patterns for cast films and resin films. SAXS revealed the development of a layered structure in resins films that was not apparent in cast films. It is believed that the layered structure of resins is able to bind larger amounts of oleic acid than cast films. Layers were no longer observed in SAXS patterns after heat-treatment of films [22]. This change in structure was attributed to zein denaturation. Wide-angle X-ray scattering [19] showed d-spacings of 10.6 Å and 4.9 Å for the three samples, suggesting that the helical configuration of zein was not disturbed by film forming processes.

Fig. 3. DSC thermograms of zein films showing endothermic peaks centered at about 27 °C. (a) cast film, (b) unextruded resin, (c) single-screw extruded film, (d) twin-screw extruded film, (e) hot rolled film, and (f) film heated under pressure.

DSC thermograms showed second order transitions at -80 °C (Fig. 4) for all samples. Transitions were interpreted as Tg values and taken as evidence of plasticization. Low temperature glass transitions have been observed for other plasticized protein systems [23, 24]. Miyazakiand co-workers [23] reported a glass transition at-123 °C for lysozyme crystals containing more than 24% water. Sobral and co-workers [24] reported Tgvalues below -50 °C for edible films prepared from myofibrillar proteins plasticized with glycerol. Fig. 5 shows glass transition temperatures for zein containing 0-50% (w/w) water calculated according to the Gordon and Taylor equation [25]:

where subscripts 1 and 2 refer to polymer and plasticizer, respectively, and 7"is measured in K. vindicates mass fraction, Tg1 = 139 °C was taken from Madeka and Kokini[12], and Tg2, the glass transition of hyperquenched water, was taken as -135 °C [26]. k zeiman empirical constant related to the polymer was taken as 6.24 [12]. Fig. 5 also shows

Fig. 4. DSC thermograms of zein films showing second order transitions, (a) cast film, (b) unextruded resin, (c) single-screw extruded film, (d) twin-screw extruded film, (e) hot rolled film, and (f) film heated under pressure.

glass transition temperatures for zein at 0-12.9% (w/w) water content, measured by Madeka and Kokini [12], being fitted by the Gordon and Taylor equation. The application of the Gordon and Taylor equation for zein plasticized with oleic acid at 41% (w/w), employing Tg1 = 139 °C and kzejn=

6.24, as shown above, and using Tg2 = -129 °C from data in Fig. 2, yields a Tg value of -79 °C. The calculated value was in good agreement with the experimental Tg (-80 °C) for zein films at 41 % oleic acid shown in Fig. 4.

4 Conclusions

Zein was effectively plasticized by oleic acid as evidenced by the lowered Tgof resulting films. Low temperature Tg values measured in this study were in the vicinity of those reported for other proteins at high plasticization levels. It appeared that the Gordon and Taylor equation was able to predict Tgfor zein plasticized with oleic acid at 41% (w/w). That value was similar to the Tgpredicted by the same equation for zein plasticized with water at 41% moisture content. Processing methods for film formation and finishing affected binding of oleic acid to zein. The resin formation process increased binding with respect to film casting. Heat treatment of extruded samples increased binding over unheated films. Thermal properties of resins were not affected by room temperature extrusion.

Acknowledgements

This work was supported in part by the Illinois Corn Marketing Board and the Illinois Agricultural Experiment Station.

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Fig. 5. Tgvalues of zein plasticized with water, calculated according to the Gordon and Taylor equation (solid line). Graph also shows experimental Tgvalues for zein, reported by Madeka and Kokini [12].

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(Received: August 30, 2001)

(1st Revision received: April 15, 2002)

(2nd Revision received: August 14, 2002)