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 thermograms 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 threedimensional network structure and on the interaction between proteins, plasticizers, and othCorrespondence. 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, email: gwpadua@uiuc.edu.
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% Tg is 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 [1719]. 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 prepared 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 dspacings 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]. Miyazaki and coworkers [23] reported a glass transition at-123 °C for lysozyme crystals containing more than 24%
water. Sobral and co-workers [24] reported Tg values 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 zeim an 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 Tg of 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 Tg for zein plasticized with oleic acid at 41% (w/w). That value was similar to the Tg
predicted 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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(Received: August 30, 2001)
(1st Revision received: April 15, 2002)
(2nd Revision received: August 14, 2002)