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Biomacromolecules 2008, 9, 658–663
Effect of Amylose Content on Physical and Mechanical Properties of Potato-Starch-Based Edible Films Riku A. Talja,*,† Marko Peura,‡ Ritva Serimaa,‡ and Kirsi Jouppila† Department of Food Technology, PO Box 66 (Agnes Sjöbergin katu 2) and Department of Physical Sciences, PO Box 64 (Gustaf Hällströmin katu 2), University of Helsinki, FI-00014 Helsinki, Finland Received June 12, 2007; Revised Manuscript Received November 9, 2007
The present study investigated the amylose content and the gelatinization properties of various potato starches extracted from different potato cultivars. These potato starches were used to prepare edible films. Physical and mechanical properties of the films were investigated. The crystallinity of selected native starches and edible films made of the same starches were determined by X-ray diffraction. The amylose content of potato starches varied between 11.9 and 20.1%. Gelatinization of potato starches in excess water occurred at temperatures ranging from 58 to 69 °C independently of the amylose content. The relative crystallinity was found to be around 10–13% in selected native potato starches with low, medium, and high amylose content. Instead, films prepared from the same potato starches were found to be practically amorphous having the relative crystallinity of 0–4%. The mechanical properties and the water vapor permeability of the films were found to be independent of the amylose content.
Introduction Biopolymers, such as polysaccharides and proteins, are often used to prepare edible films which maintain and/or control the quality of food or pharmaceutical products. For many biomaterials in low moisture foods, water acts as a plasticizer causing structural failures thus affecting the stability and quality of such foods.1,2 Edible films could be used to improve the stability and quality of low moisture foods or pharmaceutical solids by coating as they may act as a barrier layer between the material and the surroundings. Thus, it is important to study various properties of edible films. For example, the mechanical, thermal, and permeability properties of the films change as the type and content of the plasticizer in the film and the storage conditions change.3–7 If the plasticizer crystallizes, the brittleness and tensile strength of the films increase with a concurrent decrease of the elongation at break.6 However, crystallization of plasticizing polyols was not observed when a binary mixture of polyols was used to plasticize starch films.7,8 Also, the crystallization of amylose and amylopectin in starch films has been reported to affect the mechanical properties by increasing the rigidity.9 Potato starch is mainly composed of biopolymers amylose and amylopectin, but it also contains smaller quantities of phosphate, protein, and lipids as reviewed, e.g., by Jobling.10 Starch granules are composed of lamellar structures in which crystalline and amorphous regions alternate. Amylopectin is a highly branched molecule, the side chains of which form double helices resulting in crystalline regions.11,12 The less ordered regions in starch granules contain rigid amorphous branching zones of amylopectin11 and amorphous amylose and amylopectin.13 Amylose is mostly amorphous in the starch granules, but it can also form double helices with amylopectin side chains and crystallize.11 Starch granules are insoluble into cold water. When heated in excess water, the starch granules swell and the partially * Corresponding author. E-mail:
[email protected]. Tel: +358 9 191 58301. Fax: +358 9 191 58460. † Department of Food Technology. ‡ Department of Physical Sciences.
ordered structures are disrupted at the gelatinization temperature range, resulting in an increase in viscosity.14 The amylose content and the gelatinization temperature of potato starch have been suggested to vary with the environmental conditions during the growth of potato tubers.15 In amylose and starch gels, initial crystallization at 26 °C has been reported to occur at a similar rate within the first 48 h as a result of amylose crystallization.16 Moreover, crystallization in the starch gel continued for weeks because of the slow crystallization of amylopectin.16 The effect of amylose content on the properties of starchbased films has been studied previously. In these studies, films have often been prepared from a physical blend of amylose and amylopectin which is plasticized with various polyols. The amylose content has been shown to affect the crystallinity of the starch film, which is often linked to the mechanical properties.17 It has been observed that crystallization occurs in the film to some extent during the film formation and that the crystallization continues during the storage.9,17 The crystallinity of the film depends on the relative vapor pressure (RVP) and temperature conditions during the film formation.18 The main crystallizing component of starch films is amylose,19 which crystallizes to B-type polymorph during the film formation and storage.20 Moreover, it has been proposed that amylose forms cocrystallites with amylopectin, stimulating the crystallization of amylopectin.20,21 Films prepared from physical blends of amylose and amylopectin were more crystalline than a film made of native starch and amylose blend.20 The crystallinity in films prepared from blends of amylose and amylopectin has been shown to increase up to 24% amylose.20 The content of amylose and amylopectin and the crystallinity in the film had no effect on the water vapor permeability (WVP) of the films.17 However, Young’s modulus and the tensile strength increased as the crystallinity in the starch films increased.22 The purpose of this study was to investigate the effect of amylose content on the physical and mechanical properties of starch-based films. The amylose content and the gelatinization properties of various native potato starches extracted from different potato cultivars were determined. These potato starches
10.1021/bm700654h CCC: $40.75 2008 American Chemical Society Published on Web 01/01/2008
Effect of Amylose Content on Starch Film Properties
Biomacromolecules, Vol. 9, No. 2, 2008 659
Table 1. Potato Starches Extracted from Various Potato Cultivars Used in the Present Study, The Water and Amylose Contents and the Relative Crystallinity (X) (na equals not analyzed) as Well as the Onset (To), peak (Tp) and Endset (Te) Temperatures and the Temperature Range of Gelatinization (∆T, difference between Te - To) and the Enthalpy of Gelatinization (∆H) of the Starches Measured by DSC (mean values ( standard deviation from three measurements) water amylose content X content (%) (% of solids) (%)
cultivar bulk 2002 (B1)b bulk 2003 (B2) Kardal 2002 (K1) Kardal 2003 (K2) Posmo 2002 (P1) Posmo 2003 (P2) Saturna 2002 (Sa1) Saturna 2003 (Sa2) Seresta 2003 (Se1) Tanu 2002 (T1) Van Gogh 2002 (V1)
16.7 ( 0.1 10.0 ( 0.1 13.0 ( 0.1 14.0 ( 0.1 13.3 ( 0.4 13.1 ( 0.6 13.3 ( 0.1 13.6 ( 0.1 13.1 ( 0.8 13.5 ( 0.1 13.0 ( 0.6
14.2 ( 0.2 16.6 ( 1.5 19.7 ( 0.6 12.3 ( 1.4 16.8 ( 0.4 11.9 ( 1.0 13.7 ( 0.8 15.5 ( 0.3 20.1 ( 0.6 18.4 ( 0.6 15.6 ( 1.1
13 na na na na 10 na na 12 na 10
gelatinizationa To (°C)
Tp (°C)
Te (°C)
∆T (°C)
∆H (J g-1)
58.1 ( 0.9 a 58.4 ( 1.4 abc 59.5 ( 0.2 abc 60.6 ( 1.8 c 59.4 ( 0.2 abc 60.0 ( 0.4 bc 60.4 ( 0.6 c 59.6 ( 0.5 abc 60.4 ( 2.3 c 58.0 ( 0.4 a 59.1 ( 0.6 abc
62.3 ( 0.4 ab 64.2 ( 0.9 cd 62.8 ( 0.7 abc 63.6 ( 1.4 abcd 62.8 ( 0.6 abc 63.7 ( 1.8 abcd 64.6 ( 0.2 d 64.0 ( 0.4 bcd 64.3 ( 2.0 cd 62.1 ( 0.7 a 63.2 ( 0.5 abcd
66.8 ( 1.2 a 67.5 ( 0.2 ab 66.4 ( 1.8 a 69.1 ( 1.4 b 67.0 ( 0.3 a 67.0 ( 2.5 a 67.8 ( 0.3 ab 66.5 ( 1.6 a 67.6 ( 0.2 ab 66.5 ( 0.6 a 67.8 ( 0.8 ab
8.7 ( 0.3 a 9.0 ( 1.2 a 6.9 ( 1.9 a 8.6 ( 1.6 a 7.5 ( 0.4 a 7.0 ( 2.4 a 7.4 ( 1.2 a 7.0 ( 1.5 a 7.2 ( 2.5 a 8.4 ( 1.0 a 8.7 ( 0.8 a
10.8 ( 3.2 a 16.4 ( 5.0 abc 17.5 ( 6.0 abc 20.0 ( 10.2 bc 10.3 ( 2.3 a 12.7 ( 1.0 ab 13.4 ( 2.8 ab 18.0 ( 4.5 abc 17.1 ( 1.1 abc 14.6 ( 3.1 abc 21.2 ( 2.7 c
a Values in the same column followed by the same letter are not significantly different from each other at confidence level of 95% (Tukey test). b Potato starch used in our previous studies.6,7
were used to prepare edible films plasticized with a binary mixture of xylitol and sorbitol. The crystallinity of selected starches and starch films was determined with X-ray diffraction (XRD). The water sorption of selected films was determined and modeled to obtain water sorption isotherms to study the effect of RVP on the film-water interaction. WVP was determined to study the water vapor barrier properties of the films. The Young’s modulus, tensile strength and elongation at break were determined to study the dependence of the mechanical properties of the films on the amylose content.
Materials and Methods Materials. Eleven native potato starches (Table 1), donated by Evijärven Peruna Ltd. (Evijärvi, Finland), were used to prepare edible films. A mixture of food grade xylitol (Xyrofin, Kotka, Finland) and sorbitol (Cerestar, Krefeld, Germany) was used as a binary plasticizer at a ratio of 1 to 1 (w/w). The water content of the starches, determined gravimetrically after drying in an oven at 105 °C for 4 h, is shown in Table 1. Amylose Content. The amylose and amylopectin contents of potato starch were analyzed with an enzymatic method (amylose/amylopectin assay kit, Megazyme International Ireland Ltd., Bray, Ireland). Reagents for the analysis were prepared according to the instructions enclosed with the amylose/amylopectin assay kit. A potato starch sample (10–15 mg) was dissolved in a test tube containing dimethyl sulfoxide (DMSO, Riedel-de Haën, Seelze, Germany). The dissolved starch sample was divided into two solutions to determine the amylose and total starch contents. Amylopectin was precipitated with concanavalin A (Con A, enclosed to the amylose/amylopectin assay kit) to produce an amylopectin-free solution for the determination of the amylose content. The carbohydrates in the solutions were hydrolyzed enzymatically into glucose molecules with a mixture of amyloglucosidase and R-amylase enzymes (enclosed to the amylose/amylopectin assay kit). The glucose molecules in the solution were oxidized with a glucose oxidase/ peroxidase reagent, resulting in a color change of the solution. The absorbance at the wavelength of 510 nm of the oxidized glucose solutions was determined with a spectrophotometer (Perkin-Elmer, ¨ berlingen, Germany). Three UV-vis spectrometer, Lambda 2, U replicate potato starch samples and a reference sample with an amylose content of approximately 70% enclosed to the amylose/amylopectin assay kit were analyzed. The absorbances of solutions with dilution factors were used to calculate the amylose and amylopectin contents. Thermal Properties. A differential scanning calorimeter (TA4000 DSC30, Mettler-Toledo AG, Greifensee, Switzerland) was used to determine the gelatinization properties of the starches. The onset, peak, and endset temperatures and the enthalpy of gelatinization were
determined. The differential scanning calorimeter was calibrated using the melting temperatures and enthalpies of n-pentane (-129.7 °C; 116.7 J g-1), n-hexane (-95 °C; 151.8 J g-1), mercury (-38.8 °C; 11.4 J g-1), distilled water (0.0 °C; 334.5 J g-1), gallium (29.8 °C; 80.0 J g-1), and indium (156.6 °C; 28.5 J g-1). The starch samples (approximately 1 mg) were mixed with water to obtain a solid content of 5% (w/w), and the mixtures were hermetically sealed in 40 µL aluminum pans. Triplicate samples were scanned from 15 to 90 °C at a heating rate of 5 °C min-1. The differential scanning calorimeter (DSC) measuring cell was purged with N2 gas at a flow rate of 50 mL min-1. Film Formation. Edible films were prepared using suspensions of the mixture of xylitol and sorbitol (at a ratio of 1:1), potato starch, and distilled water. Xylitol and sorbitol (total 30% of solids) were weighed and dissolved into distilled water. Starch was added to obtain the filmforming suspension, in which the starch concentration (70% of solids) was 5% (w/w) of the overall water content. The film-forming suspension was heated with continuous mixing by a magnetic stirrer (RH Basic, Ika Works, Inc., Wilmington, NC) and at short intervals by hand with a glass rod to above 90 °C and kept at that temperature for 5 min before allowing it cool down to 50 °C. Air bubbles formed during the heating were removed by placing the film forming solution into a desiccator under vacuum until there was no bubble formation. The film-forming solution was casted on a Teflon-coated plate by a spreader with a gap of 0.8 mm. Starch-based films were obtained by evaporating water in an oven at 35 °C for at least 4 h. X-ray Diffraction of Starches and Starch Films. Starches with a low, medium, and high amylose content, Posmo 2003 (P2), Van Gogh 2002 (V1), and Seresta 2003 (Se1), respectively, and bulk starch 2002 (B1) with a medium amylose content, which was used in our previous studies,6,7 were selected for X-ray diffraction (XRD) measurements. Starch tablets, diameter 15 mm and thickness about 0.6–0.7 mm, for XRD measurements were compressed using a pressing cylinder and a piston. The experiments were carried out using the symmetrical transmission geometry with Cu KR1 radiation (wavelength 1.5408 Å) from a sealed X-ray tube monochromatized using a Ge(111) monochromator in the incident beam. The intensities were measured with NaI(Tl) detector (Quartz&Silice, France) at scattering angles (2θ) from 10 to 50° using an angular step of 0.01°. Films prepared from the same starches (P2, V1, Se1, and B1) were selected for XRD measurements. Film stripes, with dimensions 50, 10, and 0.05 mm, were folded five times to obtain thicker samples for the measurements. The samples were stored at RVP of 54% at 21 °C for 1 week prior to the measurements. The measurements were made at 28 °C with a closed sample chamber to prevent dehydration. The scattering patterns of the films were measured with another X-ray diffraction setup using the perpendicular transmission mode. The setup
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consisted of a rotating anode X-ray tube (Rigaku, Japan), a totally reflecting focusing mirror, a bent Si(111) crystal monochromator, and a MAR345 image plate detector (Mar Research, Germany). The Cu KR1 radiation was used, and the beam was focused on the detector. The attenuation of the X-ray beam through the sample was determined from the image of the beam on the detector after a semi-transparent beamstop. The scattering angle scale was calibrated by measuring the diffraction patterns of silver behenate (AgC22H43O2) and silicon powder. The measured scattering patterns were isotropic, and the intensity curves at the scattering angles (2θ) from 4 to 55° were obtained by integration over the azimuth angle. At least two replicate samples were measured, and almost identical intensity curves were obtained. The intensity curves of the films were corrected for air scattering and absorption and in the case of the powder samples also for the polarization of the radiation. The relative crystallinity (Cr) of the samples was calculated by determining the ratio of the total intensity of the amorphous background (IA) to the total intensity of the diffraction pattern (IE) using eq 1. For the powder samples, a measured intensity curve of Paselli WA-4 starch was used as an amorphous background.23
Cr ) 1 -
IA IE
(1)
Water Sorption. Pieces of the films, approximately 0.5 g, were placed into glass vials (20 mL). The glass vials were placed in a freezer at -20 °C for at least 2 h before placing them in a -80 °C freezer overnight. The frozen film samples in the vials were placed in a freezedryer (Lyovac GT 2, Amsco Finn-Aqua GmbH, Hürth, Germany) and dried for at least 48 h (p < 0.5 mbar). Drying of the film samples was completed in a vacuum desiccator over P2O5 (Merck, Darmstadt, Germany) for 7 days. The water sorption properties of the films prepared from B1, B2, P2, and Se1 starches were determined after storage of the freeze-dried samples in vacuum desiccators over saturated salt solutions of LiCl, CH3COOK, MgCl2, K2CO3, Mg(NO3)2, NaNO2, NaCl, and KCl (Merck, Darmstadt, Germany), giving RVP of 11, 24, 33, 44, 54, 66, 76, and 86% at 25 °C, respectively.24 The water content was obtained by weighing the samples as a function of time. Three replicate samples at each RVP were analyzed. The water sorption behavior was modeled with the Brunauer-Emmett-Teller (BET) and the Guggenheim-Anderson-de Boer (GAB) models, given in eqs 2 and 3, respectively, in which m is the experimental steady-state water content, mm is the monolayer water content, aw is the water activity ()RVP/100 at equilibrium), and K and C are constants.25 Water activity ranges of 0.11-0.4426 and 0.11-0.86 were used to model water sorption isotherms of polyol plasticized films with BET and GAB equations, respectively.
Kaw m ) mm (1 - aw)[1 + (K - 1)aw]
(2)
KCaw m ) mm (1 - Caw)[1 + (K - 1)Caw]
(3)
Water Vapor Permeability (WVP). Granular (
