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Development and characterization of edible films based on modified corn starch and grape juice
Food Chemistry 292 (2019) 6–13
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Development and characterization of edible films based on modified corn starch and grape juice Meral Yıldırım-Yalçına, Mahmut Şekerb, Hasan Sadıkoğlua,c,
T
⁎
a
Department of Chemical Engineering, Gebze Technical University, 41400 Gebze, Kocaeli, Turkey Yenikent, 41400 Gebze, Kocaeli, Turkey c Department of Chemical Engineering, Yildiz Technical University, 34210, Esenler, İstanbul, Turkey b
A R T I C LE I N FO
A B S T R A C T
Keywords: Edible film Grape juice Water vapor permeability Oxygen permeability Cross-linking Sodium trimetaphosphate Citric acid
Chemically modified corn starch with sodium trimetaphosphate (STMP) or citric acid (CA) and grape juice was used to produce edible films. Modification reactions were discussed by results of FT-IR scan, water solubility, swelling power, viscosity and degree of cross-linking properties. Mechanical, barrier, physical (solubility, color, transparency, microstructure) and glass transition temperature properties of films were studied to understand the effects of grape juice and modified starch usage in films. Usage of starch cross-linked with STMP decreased significantly oxygen permeability from 5.82 to 2.51 cm3 µm m−2 d−1 kPa−1, water vapor permeability from 1.89 to 1.38 g mm m−2 h−1 kPa−1, solubility from 0.65 to 0.55 g soluble solid/total solid, percent elongation from 62.96 to 16.47. The chemical reaction between starch and CA affected barrier, solubility and elongation properties of films and values were higher than values of STMP films.
1. Introduction In recent years, increased consumer demands for minimally processed, high quality and safe food has addressed researchers to develop innovative edible films and coatings. Besides being environmentally friendly, edible films and coatings have been proved to effective in extending the shelf life and quality of food by improving the barrier properties; preventing the changes in sensory characteristics and delivering active agents such as antioxidants and antimicrobials. Polysaccharides based films and coatings are widely chosen because of their mechanical and optical properties (Guilbert, Gontard, & Gorris, 1996). Usage of fruit and vegetable purees or juices in edible film formulations have been studied to increase the nutritional content and provide specific sensory characteristics (Rojas-Graü et al., 2006; Sothornvit & Pitak, 2007). Grape (Vitis vinifera) is a widely-grown crop in the world. A common practice in Turkey consists of preserving grapes as fruit leather (pestil) due to their short shelf life. Kaya and Maskan (2003) studied the water vapor permeability properties of grape pestil at different temperatures (15, 25 and 37 °C) and relative humidity conditions. Pestil can also be used as edible film but its mechanical, physical and barrier properties need to be investigated in detail. Starch is the most important polysaccharide used in biodegradable film formulations. Starch-based films were mainly aimed for use as barrier coatings to increase the shelf-life ⁎
of foods. Starch-based coatings have been successfully used to extend the shelf-life of fruits and vegetables such as banana (Thakur et al., 2019); apples, tomatoes and cucumbers (Mehyar, Al-Qadiri, & Swanson, 2014); bakery products (Saraiva et al., 2016) and seafood products (Alotaibi & Tahergorabi, 2018). The hydrophilic character of starch results in films being sensitive to water transfer (Santana et al., 2018; Seligra, Jaramillo, Famá, & Goyanes, 2016). Cross-linking of starch reduces water absorption and solubility characteristics. Detduangchan, Sridach, and Wittaya (2014) reported that cross-linked starch improves water vapor barrier properties of films because reduces the interactions between starch and water. Cross-linking can also reduce oxygen permeability of films by affecting the diffusion path. Sodium trimetaphosphate (STMP) is one of the safe, non-toxic crosslinking agents for starch approved by the Food and Drug Administration (FDA, 2018). Phosphate-based modifiers can enhance the water barrier and other functional properties of films (Detduangchan et al., 2014). Several studies have emphasized the cross-linking potential of citric acid (CA) (Ghanbarzadeh, Almasi, & Entezami, 2011; Menzel et al., 2013; Olsson et al., 2013; Seligra et al., 2016). The effect of CA on the water barrier properties of starch films was examined by Ghanbarzadeh et al. (2011). However, there is no reported study about the effects of STMP or CA content on oxygen permeability of edible films. CA is a harmless and non-toxic organic acid and its unreacted part shows plasticizer and antibacterial effects on the edible films (Menzel et al.,
Corresponding author at: Department of Chemical Engineering, Yildiz Technical University, 34210, Esenler, İstanbul, Turkey. E-mail addresses: [email protected] (M. Yıldırım-Yalçın), [email protected] (M. Şeker), [email protected] (H. Sadıkoğlu).
https://doi.org/10.1016/j.foodchem.2019.04.006 Received 21 November 2018; Received in revised form 15 March 2019; Accepted 1 April 2019 Available online 02 April 2019 0308-8146/ © 2019 Published by Elsevier Ltd.
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M. Yıldırım-Yalçın, et al.
2013; Reddy & Yang, 2010). Acid hydrolysis of starch can take place due to the addition of CA under high temperature and low pH conditions (Menzel et al., 2013). Controlled pH levels and low temperature applications make possible to produce starch films cross-linked with CA without excessive hydrolysis. Previous studies focused on the direct addition of CA to the film forming solutions (Menzel et al., 2013; Olsson et al., 2013; Seligra et al., 2016). Modifying starch in powder form with CA and preparing the film forming solutions has not studied before. The aim of the present work was to determine and compare the barrier, physical, mechanical and thermal properties of edible films based on starch modified with STMP and CA and incorporated with grape juice. The influence of cross-linking reaction between CA and starch, which is taking place before and during film preparation process, was also studied to investigate the effects on film characteristics.
2.2.3. Color analysis The color analysis of starch samples was carried out by using a Minolta CR–400 chroma meter (Konica Minolta, Sensing Inc., Osaka, Japan) according to CIELab system (L*: lightness, a*: greenness or redness, b*: blueness or yellowness).
2. Material and methods
Degree of cross - linking (%) =
2.1. Preparation of grape juice
where A is the peak viscosity of the control sample (native starch), and B is the peak viscosity of the cross-linked starch.
2.2.4. Degree of cross-linking The degree of cross-linking of modified starch was determined from the peak viscosity measured by using a Rapid Visco Analyzer (RVA 4500, Perten Instruments, Kungens, Sweden), according to Koo, Lee, and Lee (2010). Starch slurries were heated for 5 min from 50 to 95 °C, held at 95 °C for 5 min, then cooled from 95 to 50 °C, and held at 50 °C for 4 min. The degree of cross-linking was calculated by using the Eq. (1).
Seedless grapes (Vitis vinifera cv. Sultana) harvested from Manisa region of Turkey were purchased from a market (İstanbul, Turkey). Firstly, grapes (average length and radius 23.01 ± 0.42 mm and 16.84 ± 0.44 mm, respectively) were washed and extracted with fruit juicer (Arçelik K1579, Turkey). The juice was filtrated with cheese cloth to separate the skins, boiled for 5 min to inactivate enzymes and stored at −18 °C until usage.
A−B ·100 A
(1)
2.2.5. Rheological properties of modified starches The rheological measurements of modified starches were analyzed with a Rheometer (Brookfield Rheometer RS4 SST, USA). Starch sample (2 g) and 25 ml of water were mixed and heated at 80 °C for 20 min. Measurements were performed at constant temperature with a parallel plate probe. The shear stress was measured as a function of shear rate from 0 to 300 s−1 for 5 min.
2.2. Preparation and analysis of modified starch powder 2.3. Preparation and analysis of edible films Modification of starch with STMP was conducted according to Lim and Seib (1993) and Seker and Hanna (2005) with some changes. Native corn starch (100 g, Dr Oetker, Izmir, Turkey), 5 g of sodium sulfate (Sigma-Aldrich GmbH, Steinheim, Germany) and different amount of STMP (0.667, 1.25 and 2.5 g; Sigma-Aldrich Inc., St Louis, USA) were added to 100 ml of distilled water, and the pH was adjusted to 10. Starch slurry was dried at 40 °C for 24 h and heated at 130 °C for 2 h. After that, starch was washed with water/ethanol (50/50, v/v) mixture, dried at 40 °C for 16 h, grounded in a laboratory mill and sieved through a 250 µm size mesh. Starch was also modified with CA (Merck KgaA, Darmstadt, Germany) according to Kapelko‐Żeberska, Zieba, Pietrzak, and Gryszkin (2016) with some changes. Certain amount of CA (10, 20 and 30% of the native starch mass) was dissolved in 40 ml of distilled water, and pH was adjusted to 4.0. Then 100 g native starch and 60 ml distilled water were incorporated to each CA solution. The following steps were the same as described method of starch modification with STMP.
Edible films were prepared by dispersing 5% (w/v) native (NS) or modified corn starch in grape juice/distilled water mixture (50/50, v/v) and plasticized with glycerol (15% of starch weight; Sigma-Aldrich GmbH, 87%, Steinheim, Germany). The concentrations, which provided good film properties, were determined according to our preliminary experiments. The solutions were heated in a water bath (Labo SM3, Istanbul, Turkey) at 80 °C for 30 min under constant mechanical stirring at 500 rpm (IKA-20, Staufen, Germany) to accomplish a complete starch gelatinization. After degassing under vacuum (Heidolph Instruments GmbH & Co., Schwabach, Germany) to release all air bubbles, 15 ml of film solution were poured into 12 cm diameter teflon trays and dried at 50 ± 1 °C for 24 h (Memmert Ecocell, München, Germany). The films were peeled off and conditioned at 53 ± 2% RH and 25 ± 2 °C for 48 h in a desiccator containing Mg(NO3)2 saturated solution. Film obtained were named as STMP or CA10, CA20 and CA30 according to amount of CA. Sample abbreviations were given in Table 1. In another set of experiments, the same amount of native starch and CA (10, 20 and 30% of starch mass) were mixed during the film gelatinization and the modification of starch was expected to occur during this step. pH values of solutions were adjusted to 4 to prevent starch hydrolysis (Olsson et al., 2013) and same process was followed. These films were named as NS + CA10, NS + CA20 and NS + CA30 according to amount of CA. Additional effects of CA, apart from starch modification were determined by adding CA to the film solution containing STMP-modified starch (STMP + CA30). In order to eliminate the plasticizing effect of CA and better understand its potential effects another set of films was prepared. Native or STMP modified starch and CA containing film solutions were centrifuged after gelatinization step (named as NS + CA30 + C and STMP + CA30 + C, respectively). Then the supernatant containing unreacted CA was removed, equal amount of water was added to the mixture and same procedure was carried out.
2.2.1. Fourier transform infrared (FT-IR) spectroscopy The FT-IR spectra of modified starches were measured on a Perkin Elmer Spectrum 100 FT-IR Spectrometer (Waltham, MA, USA) to characterize the possible reaction between starch and STMP or CA. The transmittance spectra regions were obtained between 4000 cm−1 and 800 cm−1 with a 4 cm−1 resolution. 2.2.2. Solubility index and swelling power of modified starches Native and modified starch samples (0.2 g) were mixed with 10 ml of distilled water in tubes. Then tubes were heated in a shaking water bath at constant temperature of 80 °C for 30 min and periodically stirred with glass stirrers. After cooling to room temperature, the slurries were centrifuged for 15 min at 1107g. The supernatants were separated from precipitate and tubes containing precipitate were weighed. Separated supernatants and precipitates were dried at 105 °C for 24 h to determine the dry matter in supernatant and precipitate. The solubility index was calculated as the ratio of dried supernatant to the dry starch weight and swelling power (g/g) was determined as the ratio of hydrated precipitate mass to its dry mass (Mandala & Bayas, 2004).
2.3.1. Film thickness measurement Thickness of the films was measured by a micrometer (Fowler, Pro7
Food Chemistry 292 (2019) 6–13
M. Yıldırım-Yalçın, et al.
Table 1 Abbreviations and composition of films. Abbreviations
Starch type
NS STMP STMP + CA30 NS + CA30 + C STMP + CA30 + C CA10 CA20 CA30 NS + CA10 NS + CA20 NS + CA30
native cross-linked cross-linked native cross-linked cross-linked cross-linked cross-linked native native native
with STMP with STMP with with with with
STMP 10% CA 20% CA 30% CA
max, Massachusetts, A.B.D) at five random positions. Average film thickness was 0.17 ± 0.01 mm.
Additional CA (% of the starch weight)
Centrifugation
– – 30 30 30 – – – 10 20 30
– – – + + – – – – – –
TR =
Ir × 100 I0
(2)
where Ir is the light intensity with the specimen in the beam, and I0 is the light intensity with no specimen in the beam.
2.3.2. Water solubility of films The films solubility in water was measured according to Gontard, Guilbert, and Cuq (1992). Samples (17 mm diameter) were dried at 105 °C for 24 h to measure the initial dry matter and immersed into 50 ml of water at 25 °C for 24 h with occasional agitation. Then the insoluble content of film was separated and dried at 105 °C for 24 h to measure the final weight.
2.3.6. Mechanical properties Tensile strength (TS) and percent elongation at break (E) of films were determined following the standard method D882 (ASTM, 2012). Films were cut into strips with a dimension of 15 mm × 75 mm. The test strips were placed into the grips on the texture analyzer with a 50 N load cell (Model TA1, Lloyd Instruments, Hampshire, UK) and clamped. The initial grip length was 50 mm and the crosshead speed were 25 mm min−1. The tensile properties were determined from the stress–strain curve generated by NexygenTM (NexyGen Plus, Lloyd Instruments, Hampshire, UK) software. At least eight samples of each formulation were measured.
2.3.3. Oxygen permeability Oxygen permeability (OP) of films was measured by an Ox-Tran 2/ 21 ML modular system (Modern Controls Inc., Minneapolis, MN, USA) according to standard method D3985 (ASTM, 2010). Each film sample was placed on an aluminum mask with an open testing area of 5 cm2 and placed in the test cell. During the test at 23 °C and 50 ± 1% RH, one side of film sample was exposed to 98% N2 ± 2% H2 flow and the other side was exposed to high purity dry air flow. Oxygen permeability (cm3 µm m−2 d−1 kPa−1) was calculated by multiplying O2 transmission rate by the film thickness and dividing by O2 partial pressure difference.
2.3.7. Microstructure analysis Microstructure of the surface of films was observed by scanning electron microscopy (SEM, XL-30 SFEG, Philips, Eindhoven, Holland). Film samples were freeze dried and the surfaces of films were coated with gold by using Sputter Coater SC7620 (Quorum Technologies Ltd., Lewes, UK) before the analysis.
2.3.4. Water vapor permeability Water vapor permeability (WVP) of films was determined according to modified gravimetric water method (McHugh, Avena‐Bustillos, & Krochta, 1993) based on standard method E96/E96M (ASTM, 2015a). Circular aluminum test cups containing a 29.7 cm2 opening were filled with 20 ml of distilled water to supply high relative humidity inside the cup. Circular shaped film was sealed to the cup with a ring and symmetrically located screws. Average stagnant air gap between the water level in the cup and the film surface was approximately 13 mm. The cups were placed in a chamber at 25 °C containing saturated Mg(NO3)2 solution (53 ± 2% RH) and their weight loss was measured for 24 h. The slope of weight loss versus time plot was found to calculate the water vapor transmission rate (WVTR) of the films. Finally, the WVP (g mm m−2 h−1 kPa−1) was calculated by multiplying the WVTR by the film thickness and dividing by the water vapor partial pressure difference across the film.
2.3.8. Glass transition temperature of films Glass transition temperature (Tg) of films was determined by using a differential scanning calorimeter (Perkin-Elmer Jade DSC, Beaconsfield, UK). Samples of 6 ± 1 mg were weighted in a sealed aluminum pan and an empty pan was used as reference. Sample was cooled to 20 to −40 °C at a rate of 10 °C min−1 and scanned at a heating rate of 10 °C min−1 between −40 and 150 °C under nitrogen gas purge. Tg was determined as the midpoint temperature of the glass transition. 2.4. Statistical analysis The measurements were conducted at least in triplicate and results were given as means followed by standard deviations. Statistical analysis was performed with the SPSS software (version 15 for windows, SPSS, Inc., Chicago, IL, USA). The data was subjected to analysis of variance (ANOVA) followed by Duncan’s multiple range post hoc test. The significance level was set at p < 0.05.
2.3.5. Film color and transparency properties Color values of film samples were measured with the same device described above. Films were placed on a white standard plate (calibration plate CR-A43) and six points in central and lateral locations of three separate samples were evaluated. Film transparency was determined according to standard method D1746 (ASTM, 2015b). The films were placed on the surface of a cuvette after cut into rectangular shapes (10 mm × 40 mm). Transparency of films was measured by triplicate with a UV–Visible spectrophotometer (Lambda 35, Perkin Elmer, Shelton, USA) at 560 nm and percent transparency was calculated as follows:
3. Results and discussion 3.1. Analysis of modified starch powder 3.1.1. FT-IR spectroscopy analysis FT-IR spectra of starch samples (Fig. 1a and b) showed the chemical structure changes after the starch modification reactions. In STMP cross-linked starch films, the peak shown at 1016 cm−1 corresponds to 8
Food Chemistry 292 (2019) 6–13
9
0.07a 0.07b 0.01cd 0.00c 0.01e 0.01de 0.01cde ± ± ± ± ± ± ± 2.96 0.34 0.04 0.03 0.15 0.12 0.09 0.12a 0.12b 0.02c 0.00c 0.01d 0.01cd 0.01cd ± ± ± ± ± ± ±
Values were given as means ± standard deviation, different letters in the same column indicate significant differences between different samples (p < 0.05).
4.92 0.63 0.07 0.05 0.26 0.17 0.13 0.00a 0.04b 0.03c 0.04d 0.01c 0.04e 0.03e ± ± ± ± ± ± ± 0.27 0.12 0.19 0.04 0.18 0.51 0.50 142.37 ± 4.86a 35.75 ± 2.27b 3.04 ± 0.38c 3.80 ± 0.81c 11.08 ± 0.16d 1.58 ± 0.23c 1.28 ± 0.11c 0.04c 0.02b 0.04a 0.08f 0.04e 0.02d ± ± ± ± ± ± 6.74 5.59 4.85 4.73 5.25 5.30 5.64 12.35 ± 0.04d 8.07 ± 0.14b 7.73 ± 0.19ab 7.40 ± 0.01a 11.68 ± 0.26c 13.30 ± 0.17e 13.24 ± 0.20e 4.50 ± 0.71c 3.33 ± 0.29b 2.25 ± 0.35a 2.00 ± 0.71a 7.00 ± 0.50d 9.17 ± 0.29e 12.00 ± 0.71f
99.07 94.66 96.93 96.92 95.69 98.41 98.26
± ± ± ± ± ± ±
0.36a 2.57b 0.99ac 0.49ac 1.03bc 0.49a 0.81a
± ± ± ± ± ± ±
0.02a 0.08b 0.09c 0.04c 0.04a 0.06a 0.02a −1.81 −0.97 −1.42 −1.38 −1.73 −1.70 −1.78
± ± ± ± ± ± ±
0.17a 0.26b 0.10cd 0.03c 0.10bd 0.13b 0.67b
– 98.28 98.68 99.16 89.26 95.39 97.45
Consistency index (K) Degree of cross-linking (%) b* a* L*
Native starch STMP 0.67 STMP 1.25 STMP 2.5 CA 10 CA 20 CA 30
3.1.2. Solubility index and swelling power of modified starches The solubility index and swelling power of STMP modified starch significantly decreased with increasing STMP content when compared to native starch sample (Table 2). This could be explained by an increase of density of cross-links and intermolecular bonding of starch chains. Cross-linking strengths the bonding between starch chains which seemed to made them to resist against water solubility and swelling (Detduangchan et al., 2014; Koo et al., 2010). On the other hand, solubility index and swelling power of starch treated with CA increased significantly with high concentration of CA. Hong, Zeng, Buckow, and Han (2018) stated that solubility and swelling of starch increased with esterification. Also, some researchers founded a positive relationship between water solubility and degradation of starch (Choudhury & Gautam, 1998). Therefore, esterification or degradation reactions can take place during modification with CA, besides crosslinking.
Swelling power (g g−1)
phosphate stretching (PeOeC), the one at 1216 cm−1 to phosphate vibration (P]O) and at 2928 and 1739 cm−1 to CH stretch attached to O and OH bending band (Detduangchan et al., 2014). In Fig. 1b the peak in the range of 1718–1740 cm−1 was characteristic for ester bonding (C]O) due to esterification reaction between starch and CA. The band range from 3200 to 3400 cm−1 showed the vibration stretching of inter- and intramolecular hydrogen bonds of starch (Kahar, Ismail, & Othman, 2012; Ma, Jian, Chang, & Yu, 2008).
Solubility index (%)
Fig. 1. Fourier transform infrared (FT-IR) spectra of cross-linked starch with different sodium trimetaphosphate (a) and citric acid (b) contents.
Sample
Table 2 Solubility index (%), swelling power (g g−1), color (L*, a*, b*), degree of cross-linking and power law properties of starch samples.
Flow behavior index (n)
Viscosity at 100 s−1 (Pa s)
Viscosity at 200 s−1 (Pa s)
M. Yıldırım-Yalçın, et al.
Food Chemistry 292 (2019) 6–13
M. Yıldırım-Yalçın, et al.
a
3.1.4. Degree of cross-linking The degree of cross-linking of modified corn starches with different amount of STMP and CA is given in Table 2. Cross-linking degree increased with increasing concentration of modification agent. Peak viscosity decreased (peak viscosity results were not shown) with increasing cross-linking degree. Detduangchan et al. (2014) stated that cross-linking agent reduce interactions between starch and water molecules resulting in lower peak viscosity and higher cross-linking degree. Ma et al. (2008) studied on substitution of CA on starch nanoparticles and found that CA modification decreased peak viscosity values because of the cross-linking which reduced chain mobility.
b
O2 Permeability cm3 μm m-2 d-1 kPa-1
3.1.3. Color analysis As shown in the Table 2, lightness (L*) values of samples were between 99.07 and 94.66, while greenness (a*) and yellowness (b*) values were between −1.81 and −0.97, and 6.74 and 4.73, respectively. Native starch had the highest L* values, which indicates whiteness of starch (Ačkar, Babić, Šubarić, Kopjar, & Miličević, 2010). The color results of native corn starch were close to earlier work of Teli, Rohera, Sheikh, and Singhal (2009). They reported the L*, a*, b* values as 94.22 ± 0.02, 0.27 ± 3.02 and 8.26 ± 0.20, respectively. Starch modified with 0.67% STMP or 10% CA reduced L* values, but increasing concentrations did not affect L* values (p ≥ 0.05). The increase of STMP content firstly changed and then did not affect significantly the a* and b* values. Moreover, the amount of CA had no effect on a* and b* values (p ≥ 0.05). CA stabilizes the color because of the chelating agent ability (Akubor, 2013).
10
c
9 7
bc
bc
bc
bc
bc
b
b
6 5 4 a
3 2 1 0
3 de
e
de
2.5
WVP (g mm m-2 h-1 kPa-1)
bc
b
8
cd
c b
b
2 a
b
b
a
1.5 1
0.5 0
3.1.5. Rheological properties of modified starches The power law rheological properties through regression analysis according to Ostwald–de Waele model (R2 > 0.99 for all measurements) were calculated. The consistency index (K, Pa sn), flow behavior index (n), apparent viscosity (Pa s) at 100 s−1 and 200 s−1 of native and modified starches are given in Table 2. All samples showed non-Newtonian shear-thinning behavior (n < 1). The reduction in viscosity of modified starch samples is an evidence of the cross-linking. It decreases the interactions of starch and water molecules resulting in lower viscosity (Detduangchan et al., 2014). Cross-linking decreased viscosity, although this decrease was not significant for all STMP and CA concentrations. The pH values of all CA cross-linked samples were greater than 4.0. Therefore, acid hydrolysis of glycosidic bonds in starch molecules could be hindered (Olsson et al., 2013).
Fig. 2. Oxygen (a) and water vapor (b) permeability of film samples. Different letters in the same column indicate significant differences between samples by Duncan test (p < 0.05).
starch films gave rise to greater water solubility because of the plasticizer effect of CA. Hydrophilic plasticizers increase solubility of films in water (Cuq, Gontard, Cuq, & Guilbert, 1997). Starch modified with CA did not show significant changes in solubility. 3.2.2. Oxygen permeability As can be observed in Fig. 2a, STMP modified starch film had the lowest OP. A previous study by Ustunol and Mert (2004) indicated that cross-linked whey protein decreased OP of films due to an increase in tortuosity of the diffusion path after cross-linking. Moreno, Cárdenas, Atarés, and Chiralt (2017) stated that cross-linking reduced OP of corn starch-gelatin based films because of the tighter matrix. Usage of starch modified with CA increased OP of films, but this increase was not statistically significant when compared to NS films (p ≥ 0.05). Therefore,
3.2. Analysis of edible films 3.2.1. Water solubility of films STMP cross-linked starch decreased the water solubility of films as shown in Table 3. However, the presence of CA in STMP-modified
Table 3 Solubility, color (L*, a*, b*), transparency, mechanical properties and glass transition temperatures (°C) of films. Sample
Solubility (g soluble solid/g total solid)
NS STMP STMP + CA30 NS + CA30 + C STMP + CA30 + C CA10 CA20 CA30 NS + CA10 NS + CA20 NS + CA30
0.65 0.55 0.70 0.73 0.66 0.64 0.59 0.64 0.69 0.67 0.70
± ± ± ± ± ± ± ± ± ± ±
0.02bc 0.04a 0.01bc 0.05c 0.11abc 0.02abc 0.02ab 0.01abc 0.03bc 0.04abc 0.01bc
L*
91.77 91.82 90.80 90.40 91.50 91.03 92.11 91.16 90.56 91.50 90.07
a*
± ± ± ± ± ± ± ± ± ± ±
0.22 fg 0.44 g 0.42bcd 0.44ab 0.20ef 0.34cde 0.18 g 0.25def 0.22abc 0.18ef 0.14a
−1.36 −1.12 −1.05 −0.96 −0.80 −0.94 −1.01 −0.83 −0.88 −0.85 −0.75
± ± ± ± ± ± ± ± ± ± ±
0.03a 0.01b 0.06bc 0.07 cd 0.05 fg 0.04cde 0.07bc 0.07efg 0.04def 0.02defg 0.11 g
b*
Transparency (560 nm)
9.87 ± 0.28b 11.64 ± 0.90de 13.50 ± 0.70f 12.33 ± 0.62e 8.72 ± 0.83a 10.37 ± 0.38bcc 10.33 ± 0.30bc 10.39 ± 0.33bc 11.08 ± 0.35 cd 9.48 ± 0.41ab 13.45 ± 0.67f
43.74 32.40 10.95 49.43 23.33 26.74 18.53 30.82 35.66 30.03 28.77
± ± ± ± ± ± ± ± ± ± ±
2.19 h 1.68f 0.60a 1.98ı 0.77c 1.67d 2.03b 1.15ef 1.33 g 3.02ef 1.82de
Percentage Elongation
Tensile Strength (N mm−2)
62.96 ± 3.63de 16.47 ± 2.53a 55.39 ± 8.79 cd 114.40 ± 8.91 g 38.86 ± 7.83b 60.95 ± 10.54de 63.68 ± 13.36de 48.36 ± 7.84bc 67.78 ± 7.02e 67.54 ± 12.76e 85.12 ± 11.89f
0.36 0.38 0.30 0.50 0.36 0.29 0.24 0.11 0.41 0.40 0.37
± ± ± ± ± ± ± ± ± ± ±
0.07bcd 0.15 cd 0.09bcd 0.05e 0.11bcd 0.06bc 0.10b 0.05a 0.07de 0.07cde 0.06 cd
Glass transition temperature (°C)
55.03 58.27 70.06 67.31 65.10 75.90 71.63 67.04 41.90 42.78 38.77
± ± ± ± ± ± ± ± ± ± ±
3.73b 4.75b 0.14cde 5.54 cd 4.32c 1.88e 1.77de 0.35 cd 1.47a 0.16a 2.14a
Values were given as means ± standard deviation, different letters in the same column indicate significant differences between samples by Duncan test (p < 0.05). 10
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STMP, CA30 and NS + CA30 films were shown in Fig. 3. L* values of samples were between 91.82 and 90.07, while a* and b* values were between −1.36 and −0.75, and 13,50 and 8.72, respectively. The L*, a*, b* values of corn starch based edible film was reported as 95.7, −0.5 and 4.1 by Teixeira, Garcia, Takinami, and Mastro (2018). All films had high values of L*, negative values of a* and positive values of b*. STMP cross-linked starch increased (p < 0.05) the a* values of films from −1.36 to −1.12. Also, CA modification increased (p < 0.05) the a* values. The change of a* value was not significant between CA10 and CA20, while it was significant for CA30 (from −1.01 to −0.83). The b* values of films based on cross-linked starch with STMP were significantly (p < 0.05) higher than those of NS films, while films based on cross-linked starch with CA did not modify the b* values (p ≥ 0.05). Gutiérrez, Tapia, Pérez, and Famá (2015) observed no significant differences between a* and b* values of native and crosslinked starch films. Cross-linked starch in edible film reduced the transparency of films significantly (Table 3). This result was in accordance with Wang, Liu, and Wang (2017) who found that yam starch cross-linked with STMP reduced transparency of films. CA addition to NS or STMP cross-linked starch based films also reduced the transparency. Removing unreacted CA molecules from film matrix by centrifugation created more transparent films.
CA addition was not advantageous in terms of improving OP. Modification of starch with CA before or during film formation did not affect OP. CA addition to the STMP cross-linked starch during the film solution making step and removal of unreacted CA before film formation increased OP. The grape-based starch film produced in this study had good oxygen barrier properties compared to other studies. OP values were between 2.51 ± 0.05 and 8.66 ± 0.57 cm3 µm m−2 d−1 kPa−1 while other authors reported values of 12.11 and 78 cm3 µm m−2 d−1 kPa−1 for corn starch and whey protein based films (Ghasemlou et al., 2013; Perez-Gago & Krochta, 2001). There is no information in literature but the OP of other fruit based films has been studied before under test conditions. For apple-based film, the OP value was reported as 10.20 ± 0.91 cm3 µm m−2 d−1 kPa−1 by Rojas-Graü et al. (2007) and for banana based film, Sothornvit and Pitak (2007) found that in between 22 and 40 cm3 µm m−2 d−1 kPa−1. 3.2.3. Water vapor permeability The WVP values of films developed in this study were found lower than for other fruit based and starch films (Jiménez, Fabra, Talens, & Chiralt, 2012; Rojas-Graü et al., 2007). Cross-linking starch with STMP affected WVP positively as shown in Fig. 2b (p < 0.05). Cross-linking restricts the mobility of starch chains in the amorphous region and prevents water absorption, therefore STMP cross-linking reduces WVP of film (Detduangchan et al., 2014). WVP of films based on starch modified with CA were increased for CA10 sample and decreased for the higher increase of CA content (CA20 and CA30) due to the hydrophobic ester groups (Ghanbarzadeh et al., 2011). These values did not show significant differences with NS sample. However, Reddy and Yang (2010) and Seligra et al. (2016) reported that films including starch and CA showed lower VWP parameters. CA addition to STMP cross-linked starch increased WVP because of the plasticizer property of the excess CA (Bertuzzi, Vidaurre, Armada, & Gottifredi, 2007). WVP of the NS + CA films increased when the CA content increased from 10 to 30 because of the plasticizer effect of unreacted CA molecules. When compared to WVP of NS and NS + CA films, there was no positive effect of direct CA addition to NS film solution.
3.2.5. Mechanical properties STMP cross-linking decreased elongation at break sharply and increased TS slightly, but not significantly (Table 3). Detduangchan et al. (2014) also found lower elongation and higher TS results for crosslinked films with STMP when compared to results for native films. Reddy and Yang (2010) indicated that the cross-linking connections cause stronger films because of the interactions between starch molecules, however, if high level of cross-linking and consequently molecular mobility restriction occurs, the TS of film declines. CA incorporation into STMP cross-linked film solution raised the percent elongation value and did not affect the TS significantly. Siew, Heilmann, Easteal, and Cooney (1999) studied about plasticizers effect on sodium caseinate film and they stated that high plasticizer content enhances the elongation and limits the TS. The percent elongation values did not change for CA10 and CA20 and reduced significantly for CA30 when compared to NS film. The elongation values of NS + CA films increased with CA amount. This result also emphasized the
3.2.4. Film color and transparency properties The color values of films were shown in Table 3 and images of NS,
Fig. 3. Images and SEM of films obtained with native starch (a), (e) and chemically modified with STMP (b), (f); CA at 30% (c), (g) and native starch containing citric acid at 30% (d), (h). 11
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References
plasticizing property of CA and demonstrated the weak cross-linking reaction between starch and CA when they were mixed during the film manufacturing process.
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Structural, thermodynamic and digestible properties of maize starches esterified by conventional and dual methods: Differentiation of amylose contents. Food Hydrocolloids, 83, 419–429. Jiménez, A., Fabra, M. J., Talens, P., & Chiralt, A. (2012). Effect of re-crystallization on tensile, optical and water vapour barrier properties of corn starch films containing fatty acids. Food Hydrocolloids, 26(1), 302–310. Kahar, A. W. M., Ismail, H., & Othman, N. (2012). Morphology and tensile properties of high-density polyethylene/natural rubber/thermoplastic tapioca starch blends: The effect of citric acid-modified tapioca starch. Journal of Applied Polymer Science, 125(1), 768–775. Kapelko-Żeberska, M., Zięba, T., Pietrzak, W., & Gryszkin, A. (2016). Effect of citric acid esterification conditions on the properties of the obtained resistant starch. International Journal of Food Science & Technology, 51(7), 1647–1654. Kaya, S., & Maskan, A. (2003). Water vapor permeability of pestil (a fruit leather) made from boiled grape juice with starch. Journal of Food Engineering, 57(3), 295–299. Koo, S. H., Lee, K. Y., & Lee, H. G. (2010). Effect of cross-linking on the physicochemical and physiological properties of corn starch. Food Hydrocolloids, 24(6), 619–625. Lim, S., & Seib, P. (1993). Preparation and pasting properties of wheat and corn starch phosphates. Cereal Chemistry, 70, 137–144. Ma, X., Jian, R., Chang, P. R., & Yu, J. (2008). Fabrication and characterization of citric acid-modified starch nanoparticles/plasticized-starch composites. Biomacromolecules, 9(11), 3314–3320. Mali, S., Grossmann, M. V. E., García, M. A., Martino, M. N., & Zaritzky, N. E. (2006). Effects of controlled storage on thermal, mechanical and barrier properties of plasticized films from different starch sources. Journal of Food Engineering, 75(4), 453–460. Mandala, I. G., & Bayas, E. (2004). Xanthan effect on swelling, solubility and viscosity of wheat starch dispersions. Food Hydrocolloids, 18(2), 191–201. McHugh, T. H., Avena-Bustillos, R., & Krochta, J. M. (1993). Hydrophilic edible films: Modified procedure for water vapor permeability and explanation of thickness effects. Journal of Food Science, 58(4), 899–903. Mehyar, G. F., Al-Qadiri, H. M., & Swanson, B. G. (2014). Edible coatings and retention of
3.2.6. Microstructure analysis Representative SEM micrographs of NS, STMP, CA30 and NS + CA30 films are shown in Fig. 3e–h, respectively. NS film showed compact and homogeneous surface without pores and cracks. Garcia, Martino, and Zaritzky (2000) indicated that plasticized starch film samples showed homogenous surface. Well dispersed starch molecules were more visible in STMP film. The micrograph of the CA30 film showed a smoother appearance than the others but showed some pores. The image of the NS + CA30 film was similar to the NS film indicating good compatibility between CA and starch. Reddy and Yang (2010) reported that native starch film showed similar surface structure to cross-linked one. On the contrary, Garg and Jana (2007) found that cross-linked starch and low-density polyethylene blend films exhibited smoother surface than native starch and low-density polyethylene blend films. 3.2.7. Glass transition temperature of films STMP cross-linked starch based films showed slightly higher but statistically similar Tg values to NS based films, while CA cross-linked starch based films showed higher values (Table 3). Detduangchan et al. (2014) reported that cross-linking enhanced the Tg values because of the decreasing chain mobility. On the other hand, the addition of plasticizer causes Tg depression due to the change in starch molecule structure and flexibility (Mali, Grossmann, García, Martino, & Zaritzky, 2006). Therefore, decrease of the Tg values of NS + CA samples demonstrated the plasticizer effect of CA addition to NS just before the gelatinization. 4. Conclusion Chemical modifications of starch have been studied to determine the mechanical, barrier and physical properties of starch films. This study shows that it is possible to produce edible films by modifying starch with STMP or CA. The decrease of solubility, swelling and viscosity properties of STMP modified starch proved the cross-linking. Solubility and swelling properties of CA modified starch showed that esterification or degradation reactions can take place besides crosslinking. The grape-based edible films were characterized by slight yellowish, transparent and flexible properties. The most noticeable effect of STMP cross-linking was decreasing oxygen and water vapor permeability of films. Moreover, cross-linking reduced the transparency of films. CA addition in NS and STMP film increased WVP because of the plasticizer effect. Also, percentage elongation and Tg values of NS + CA films demonstrated the plasticizer effect of CA addition. The results suggest that grape juice and modified starch-based films would be a suitable approach for dried or instant water-soluble food products. Declaration of interests The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements Meral Yıldırım-Yalçın would like to thank The Scientific and Technological Research Council of Turkey (TUBITAK) for granting PhD scholarship (2211-A). 12
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Saraiva, L. E. F., Naponucena, L. D. O. M., da Silva Santos, V., Silva, R. P. D., de Souza, C. O., Souza, I. E. G. L., ... Druzian, J. I. (2016). Development and application of edible film of active potato starch to extend mini panettone shelf life. LWT-Food Science and Technology, 73, 311–319. Seker, M., & Hanna, M. A. (2005). Cross-linking starch at various moisture contents by phosphate substitution in an extruder. Carbohydrate Polymers, 59(4), 541–544. Seligra, P. G., Jaramillo, C. M., Famá, L., & Goyanes, S. (2016). Biodegradable and nonretrogradable eco-films based on starch–glycerol with citric acid as crosslinking agent. Carbohydrate Polymers, 138, 66–74. Siew, D. C. W., Heilmann, C., Easteal, A. J., & Cooney, R. P. (1999). Solution and film properties of sodium caseinate/glycerol and sodium caseinate/polyethylene glycol edible coating systems. Journal of Agricultural and Food Chemistry, 47(8), 3432–3440. Sothornvit, R., & Pitak, N. (2007). Oxygen permeability and mechanical properties of banana films. Food Research International, 40(3), 365–370. Teixeira, B. S., Garcia, R. H. L., Takinami, P. Y. I., & del Mastro, N. L. (2018). Comparison of gamma radiation effects on natural corn and potato starches and modified cassava starch. Radiation Physics and Chemistry, 142, 44–49. Teli, M. D., Rohera, P., Sheikh, J., & Singhal, R. (2009). Application of germinated maize starch in textile printing. Carbohydrate Polymers, 75(4), 599–603. Thakur, R., Pristijono, P., Bowyer, M., Singh, S. P., Scarlett, C. J., Stathopoulos, C. E., & Vuong, Q. V. (2019). A starch edible surface coating delays banana fruit ripening. LWT-Food Science and Technology, 100, 341–347. Ustunol, Z., & Mert, B. (2004). Water solubility, mechanical, barrier, and thermal properties of cross-linked whey protein isolate-based films. Journal of Food Science, 69(3), 129–133. Wang, L., Liu, X., & Wang, J. (2017). Structural properties of chemically modified Chinese yam starches and their films. International Journal of Food Properties, 20(6), 1239–1250.
potassium sorbate on apples, tomatoes and cucumbers to improve antifungal activity during refrigerated storage. Journal of Food Processing and Preservation, 38(1), 175–182. Menzel, C., Olsson, E., Plivelic, T. S., Andersson, R., Johansson, C., Kuktaite, R., ... Koch, K. (2013). Molecular structure of citric acid cross-linked starch films. Carbohydrate Polymers, 96(1), 270–276. Moreno, O., Cárdenas, J., Atarés, L., & Chiralt, A. (2017). Influence of starch oxidation on the functionality of starch-gelatin based active films. Carbohydrate Polymers, 178, 147–158. Olsson, E., Menzel, C., Johansson, C., Andersson, R., Koch, K., & Järnström, L. (2013). The effect of pH on hydrolysis, cross-linking and barrier properties of starch barriers containing citric acid. Carbohydrate Polymers, 98(2), 1505–1513. Perez-Gago, M. B., & Krochta, J. M. (2001). Denaturation time and temperature effects on solubility, tensile properties, and oxygen permeability of whey protein edible films. Journal of Food Science, 66(5), 705–710. Reddy, N., & Yang, Y. (2010). Citric acid cross-linking of starch films. Food Chemistry, 118(3), 702–711. Rojas-Graü, M. A., Avena-Bustillos, R. J., Friedman, M., Henika, P. R., Martín-Belloso, O., & McHugh, T. H. (2006). Mechanical, barrier, and antimicrobial properties of apple puree edible films containing plant essential oils. Journal of Agricultural and Food Chemistry, 54(24), 9262–9267. Rojas-Graü, M. A., Avena-Bustillos, R. J., Olsen, C., Friedman, M., Henika, P. R., MartínBelloso, O., ... McHugh, T. H. (2007). Effects of plant essential oils and oil compounds on mechanical, barrier and antimicrobial properties of alginate–apple puree edible films. Journal of Food Engineering, 81(3), 634–641. Santana, R. F., Bonomo, R. C. F., Gandolfi, O. R. R., Rodrigues, L. B., Santos, L. S., Pires, A. C. S., ... Veloso, C. M. (2018). Characterization of starch-based bioplastics from jackfruit seed plasticized with glycerol. Journal of Food Science and Technology, 55(1), 278–286.
13
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Food Chemistry journal homepage: www.elsevier.com/locate/foodchem
Development and characterization of edible films based on modified corn starch and grape juice Meral Yıldırım-Yalçına, Mahmut Şekerb, Hasan Sadıkoğlua,c,
T
⁎
a
Department of Chemical Engineering, Gebze Technical University, 41400 Gebze, Kocaeli, Turkey Yenikent, 41400 Gebze, Kocaeli, Turkey c Department of Chemical Engineering, Yildiz Technical University, 34210, Esenler, İstanbul, Turkey b
A R T I C LE I N FO
A B S T R A C T
Keywords: Edible film Grape juice Water vapor permeability Oxygen permeability Cross-linking Sodium trimetaphosphate Citric acid
Chemically modified corn starch with sodium trimetaphosphate (STMP) or citric acid (CA) and grape juice was used to produce edible films. Modification reactions were discussed by results of FT-IR scan, water solubility, swelling power, viscosity and degree of cross-linking properties. Mechanical, barrier, physical (solubility, color, transparency, microstructure) and glass transition temperature properties of films were studied to understand the effects of grape juice and modified starch usage in films. Usage of starch cross-linked with STMP decreased significantly oxygen permeability from 5.82 to 2.51 cm3 µm m−2 d−1 kPa−1, water vapor permeability from 1.89 to 1.38 g mm m−2 h−1 kPa−1, solubility from 0.65 to 0.55 g soluble solid/total solid, percent elongation from 62.96 to 16.47. The chemical reaction between starch and CA affected barrier, solubility and elongation properties of films and values were higher than values of STMP films.
1. Introduction In recent years, increased consumer demands for minimally processed, high quality and safe food has addressed researchers to develop innovative edible films and coatings. Besides being environmentally friendly, edible films and coatings have been proved to effective in extending the shelf life and quality of food by improving the barrier properties; preventing the changes in sensory characteristics and delivering active agents such as antioxidants and antimicrobials. Polysaccharides based films and coatings are widely chosen because of their mechanical and optical properties (Guilbert, Gontard, & Gorris, 1996). Usage of fruit and vegetable purees or juices in edible film formulations have been studied to increase the nutritional content and provide specific sensory characteristics (Rojas-Graü et al., 2006; Sothornvit & Pitak, 2007). Grape (Vitis vinifera) is a widely-grown crop in the world. A common practice in Turkey consists of preserving grapes as fruit leather (pestil) due to their short shelf life. Kaya and Maskan (2003) studied the water vapor permeability properties of grape pestil at different temperatures (15, 25 and 37 °C) and relative humidity conditions. Pestil can also be used as edible film but its mechanical, physical and barrier properties need to be investigated in detail. Starch is the most important polysaccharide used in biodegradable film formulations. Starch-based films were mainly aimed for use as barrier coatings to increase the shelf-life ⁎
of foods. Starch-based coatings have been successfully used to extend the shelf-life of fruits and vegetables such as banana (Thakur et al., 2019); apples, tomatoes and cucumbers (Mehyar, Al-Qadiri, & Swanson, 2014); bakery products (Saraiva et al., 2016) and seafood products (Alotaibi & Tahergorabi, 2018). The hydrophilic character of starch results in films being sensitive to water transfer (Santana et al., 2018; Seligra, Jaramillo, Famá, & Goyanes, 2016). Cross-linking of starch reduces water absorption and solubility characteristics. Detduangchan, Sridach, and Wittaya (2014) reported that cross-linked starch improves water vapor barrier properties of films because reduces the interactions between starch and water. Cross-linking can also reduce oxygen permeability of films by affecting the diffusion path. Sodium trimetaphosphate (STMP) is one of the safe, non-toxic crosslinking agents for starch approved by the Food and Drug Administration (FDA, 2018). Phosphate-based modifiers can enhance the water barrier and other functional properties of films (Detduangchan et al., 2014). Several studies have emphasized the cross-linking potential of citric acid (CA) (Ghanbarzadeh, Almasi, & Entezami, 2011; Menzel et al., 2013; Olsson et al., 2013; Seligra et al., 2016). The effect of CA on the water barrier properties of starch films was examined by Ghanbarzadeh et al. (2011). However, there is no reported study about the effects of STMP or CA content on oxygen permeability of edible films. CA is a harmless and non-toxic organic acid and its unreacted part shows plasticizer and antibacterial effects on the edible films (Menzel et al.,
Corresponding author at: Department of Chemical Engineering, Yildiz Technical University, 34210, Esenler, İstanbul, Turkey. E-mail addresses: [email protected] (M. Yıldırım-Yalçın), [email protected] (M. Şeker), [email protected] (H. Sadıkoğlu).
https://doi.org/10.1016/j.foodchem.2019.04.006 Received 21 November 2018; Received in revised form 15 March 2019; Accepted 1 April 2019 Available online 02 April 2019 0308-8146/ © 2019 Published by Elsevier Ltd.
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2013; Reddy & Yang, 2010). Acid hydrolysis of starch can take place due to the addition of CA under high temperature and low pH conditions (Menzel et al., 2013). Controlled pH levels and low temperature applications make possible to produce starch films cross-linked with CA without excessive hydrolysis. Previous studies focused on the direct addition of CA to the film forming solutions (Menzel et al., 2013; Olsson et al., 2013; Seligra et al., 2016). Modifying starch in powder form with CA and preparing the film forming solutions has not studied before. The aim of the present work was to determine and compare the barrier, physical, mechanical and thermal properties of edible films based on starch modified with STMP and CA and incorporated with grape juice. The influence of cross-linking reaction between CA and starch, which is taking place before and during film preparation process, was also studied to investigate the effects on film characteristics.
2.2.3. Color analysis The color analysis of starch samples was carried out by using a Minolta CR–400 chroma meter (Konica Minolta, Sensing Inc., Osaka, Japan) according to CIELab system (L*: lightness, a*: greenness or redness, b*: blueness or yellowness).
2. Material and methods
Degree of cross - linking (%) =
2.1. Preparation of grape juice
where A is the peak viscosity of the control sample (native starch), and B is the peak viscosity of the cross-linked starch.
2.2.4. Degree of cross-linking The degree of cross-linking of modified starch was determined from the peak viscosity measured by using a Rapid Visco Analyzer (RVA 4500, Perten Instruments, Kungens, Sweden), according to Koo, Lee, and Lee (2010). Starch slurries were heated for 5 min from 50 to 95 °C, held at 95 °C for 5 min, then cooled from 95 to 50 °C, and held at 50 °C for 4 min. The degree of cross-linking was calculated by using the Eq. (1).
Seedless grapes (Vitis vinifera cv. Sultana) harvested from Manisa region of Turkey were purchased from a market (İstanbul, Turkey). Firstly, grapes (average length and radius 23.01 ± 0.42 mm and 16.84 ± 0.44 mm, respectively) were washed and extracted with fruit juicer (Arçelik K1579, Turkey). The juice was filtrated with cheese cloth to separate the skins, boiled for 5 min to inactivate enzymes and stored at −18 °C until usage.
A−B ·100 A
(1)
2.2.5. Rheological properties of modified starches The rheological measurements of modified starches were analyzed with a Rheometer (Brookfield Rheometer RS4 SST, USA). Starch sample (2 g) and 25 ml of water were mixed and heated at 80 °C for 20 min. Measurements were performed at constant temperature with a parallel plate probe. The shear stress was measured as a function of shear rate from 0 to 300 s−1 for 5 min.
2.2. Preparation and analysis of modified starch powder 2.3. Preparation and analysis of edible films Modification of starch with STMP was conducted according to Lim and Seib (1993) and Seker and Hanna (2005) with some changes. Native corn starch (100 g, Dr Oetker, Izmir, Turkey), 5 g of sodium sulfate (Sigma-Aldrich GmbH, Steinheim, Germany) and different amount of STMP (0.667, 1.25 and 2.5 g; Sigma-Aldrich Inc., St Louis, USA) were added to 100 ml of distilled water, and the pH was adjusted to 10. Starch slurry was dried at 40 °C for 24 h and heated at 130 °C for 2 h. After that, starch was washed with water/ethanol (50/50, v/v) mixture, dried at 40 °C for 16 h, grounded in a laboratory mill and sieved through a 250 µm size mesh. Starch was also modified with CA (Merck KgaA, Darmstadt, Germany) according to Kapelko‐Żeberska, Zieba, Pietrzak, and Gryszkin (2016) with some changes. Certain amount of CA (10, 20 and 30% of the native starch mass) was dissolved in 40 ml of distilled water, and pH was adjusted to 4.0. Then 100 g native starch and 60 ml distilled water were incorporated to each CA solution. The following steps were the same as described method of starch modification with STMP.
Edible films were prepared by dispersing 5% (w/v) native (NS) or modified corn starch in grape juice/distilled water mixture (50/50, v/v) and plasticized with glycerol (15% of starch weight; Sigma-Aldrich GmbH, 87%, Steinheim, Germany). The concentrations, which provided good film properties, were determined according to our preliminary experiments. The solutions were heated in a water bath (Labo SM3, Istanbul, Turkey) at 80 °C for 30 min under constant mechanical stirring at 500 rpm (IKA-20, Staufen, Germany) to accomplish a complete starch gelatinization. After degassing under vacuum (Heidolph Instruments GmbH & Co., Schwabach, Germany) to release all air bubbles, 15 ml of film solution were poured into 12 cm diameter teflon trays and dried at 50 ± 1 °C for 24 h (Memmert Ecocell, München, Germany). The films were peeled off and conditioned at 53 ± 2% RH and 25 ± 2 °C for 48 h in a desiccator containing Mg(NO3)2 saturated solution. Film obtained were named as STMP or CA10, CA20 and CA30 according to amount of CA. Sample abbreviations were given in Table 1. In another set of experiments, the same amount of native starch and CA (10, 20 and 30% of starch mass) were mixed during the film gelatinization and the modification of starch was expected to occur during this step. pH values of solutions were adjusted to 4 to prevent starch hydrolysis (Olsson et al., 2013) and same process was followed. These films were named as NS + CA10, NS + CA20 and NS + CA30 according to amount of CA. Additional effects of CA, apart from starch modification were determined by adding CA to the film solution containing STMP-modified starch (STMP + CA30). In order to eliminate the plasticizing effect of CA and better understand its potential effects another set of films was prepared. Native or STMP modified starch and CA containing film solutions were centrifuged after gelatinization step (named as NS + CA30 + C and STMP + CA30 + C, respectively). Then the supernatant containing unreacted CA was removed, equal amount of water was added to the mixture and same procedure was carried out.
2.2.1. Fourier transform infrared (FT-IR) spectroscopy The FT-IR spectra of modified starches were measured on a Perkin Elmer Spectrum 100 FT-IR Spectrometer (Waltham, MA, USA) to characterize the possible reaction between starch and STMP or CA. The transmittance spectra regions were obtained between 4000 cm−1 and 800 cm−1 with a 4 cm−1 resolution. 2.2.2. Solubility index and swelling power of modified starches Native and modified starch samples (0.2 g) were mixed with 10 ml of distilled water in tubes. Then tubes were heated in a shaking water bath at constant temperature of 80 °C for 30 min and periodically stirred with glass stirrers. After cooling to room temperature, the slurries were centrifuged for 15 min at 1107g. The supernatants were separated from precipitate and tubes containing precipitate were weighed. Separated supernatants and precipitates were dried at 105 °C for 24 h to determine the dry matter in supernatant and precipitate. The solubility index was calculated as the ratio of dried supernatant to the dry starch weight and swelling power (g/g) was determined as the ratio of hydrated precipitate mass to its dry mass (Mandala & Bayas, 2004).
2.3.1. Film thickness measurement Thickness of the films was measured by a micrometer (Fowler, Pro7
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Table 1 Abbreviations and composition of films. Abbreviations
Starch type
NS STMP STMP + CA30 NS + CA30 + C STMP + CA30 + C CA10 CA20 CA30 NS + CA10 NS + CA20 NS + CA30
native cross-linked cross-linked native cross-linked cross-linked cross-linked cross-linked native native native
with STMP with STMP with with with with
STMP 10% CA 20% CA 30% CA
max, Massachusetts, A.B.D) at five random positions. Average film thickness was 0.17 ± 0.01 mm.
Additional CA (% of the starch weight)
Centrifugation
– – 30 30 30 – – – 10 20 30
– – – + + – – – – – –
TR =
Ir × 100 I0
(2)
where Ir is the light intensity with the specimen in the beam, and I0 is the light intensity with no specimen in the beam.
2.3.2. Water solubility of films The films solubility in water was measured according to Gontard, Guilbert, and Cuq (1992). Samples (17 mm diameter) were dried at 105 °C for 24 h to measure the initial dry matter and immersed into 50 ml of water at 25 °C for 24 h with occasional agitation. Then the insoluble content of film was separated and dried at 105 °C for 24 h to measure the final weight.
2.3.6. Mechanical properties Tensile strength (TS) and percent elongation at break (E) of films were determined following the standard method D882 (ASTM, 2012). Films were cut into strips with a dimension of 15 mm × 75 mm. The test strips were placed into the grips on the texture analyzer with a 50 N load cell (Model TA1, Lloyd Instruments, Hampshire, UK) and clamped. The initial grip length was 50 mm and the crosshead speed were 25 mm min−1. The tensile properties were determined from the stress–strain curve generated by NexygenTM (NexyGen Plus, Lloyd Instruments, Hampshire, UK) software. At least eight samples of each formulation were measured.
2.3.3. Oxygen permeability Oxygen permeability (OP) of films was measured by an Ox-Tran 2/ 21 ML modular system (Modern Controls Inc., Minneapolis, MN, USA) according to standard method D3985 (ASTM, 2010). Each film sample was placed on an aluminum mask with an open testing area of 5 cm2 and placed in the test cell. During the test at 23 °C and 50 ± 1% RH, one side of film sample was exposed to 98% N2 ± 2% H2 flow and the other side was exposed to high purity dry air flow. Oxygen permeability (cm3 µm m−2 d−1 kPa−1) was calculated by multiplying O2 transmission rate by the film thickness and dividing by O2 partial pressure difference.
2.3.7. Microstructure analysis Microstructure of the surface of films was observed by scanning electron microscopy (SEM, XL-30 SFEG, Philips, Eindhoven, Holland). Film samples were freeze dried and the surfaces of films were coated with gold by using Sputter Coater SC7620 (Quorum Technologies Ltd., Lewes, UK) before the analysis.
2.3.4. Water vapor permeability Water vapor permeability (WVP) of films was determined according to modified gravimetric water method (McHugh, Avena‐Bustillos, & Krochta, 1993) based on standard method E96/E96M (ASTM, 2015a). Circular aluminum test cups containing a 29.7 cm2 opening were filled with 20 ml of distilled water to supply high relative humidity inside the cup. Circular shaped film was sealed to the cup with a ring and symmetrically located screws. Average stagnant air gap between the water level in the cup and the film surface was approximately 13 mm. The cups were placed in a chamber at 25 °C containing saturated Mg(NO3)2 solution (53 ± 2% RH) and their weight loss was measured for 24 h. The slope of weight loss versus time plot was found to calculate the water vapor transmission rate (WVTR) of the films. Finally, the WVP (g mm m−2 h−1 kPa−1) was calculated by multiplying the WVTR by the film thickness and dividing by the water vapor partial pressure difference across the film.
2.3.8. Glass transition temperature of films Glass transition temperature (Tg) of films was determined by using a differential scanning calorimeter (Perkin-Elmer Jade DSC, Beaconsfield, UK). Samples of 6 ± 1 mg were weighted in a sealed aluminum pan and an empty pan was used as reference. Sample was cooled to 20 to −40 °C at a rate of 10 °C min−1 and scanned at a heating rate of 10 °C min−1 between −40 and 150 °C under nitrogen gas purge. Tg was determined as the midpoint temperature of the glass transition. 2.4. Statistical analysis The measurements were conducted at least in triplicate and results were given as means followed by standard deviations. Statistical analysis was performed with the SPSS software (version 15 for windows, SPSS, Inc., Chicago, IL, USA). The data was subjected to analysis of variance (ANOVA) followed by Duncan’s multiple range post hoc test. The significance level was set at p < 0.05.
2.3.5. Film color and transparency properties Color values of film samples were measured with the same device described above. Films were placed on a white standard plate (calibration plate CR-A43) and six points in central and lateral locations of three separate samples were evaluated. Film transparency was determined according to standard method D1746 (ASTM, 2015b). The films were placed on the surface of a cuvette after cut into rectangular shapes (10 mm × 40 mm). Transparency of films was measured by triplicate with a UV–Visible spectrophotometer (Lambda 35, Perkin Elmer, Shelton, USA) at 560 nm and percent transparency was calculated as follows:
3. Results and discussion 3.1. Analysis of modified starch powder 3.1.1. FT-IR spectroscopy analysis FT-IR spectra of starch samples (Fig. 1a and b) showed the chemical structure changes after the starch modification reactions. In STMP cross-linked starch films, the peak shown at 1016 cm−1 corresponds to 8
Food Chemistry 292 (2019) 6–13
9
0.07a 0.07b 0.01cd 0.00c 0.01e 0.01de 0.01cde ± ± ± ± ± ± ± 2.96 0.34 0.04 0.03 0.15 0.12 0.09 0.12a 0.12b 0.02c 0.00c 0.01d 0.01cd 0.01cd ± ± ± ± ± ± ±
Values were given as means ± standard deviation, different letters in the same column indicate significant differences between different samples (p < 0.05).
4.92 0.63 0.07 0.05 0.26 0.17 0.13 0.00a 0.04b 0.03c 0.04d 0.01c 0.04e 0.03e ± ± ± ± ± ± ± 0.27 0.12 0.19 0.04 0.18 0.51 0.50 142.37 ± 4.86a 35.75 ± 2.27b 3.04 ± 0.38c 3.80 ± 0.81c 11.08 ± 0.16d 1.58 ± 0.23c 1.28 ± 0.11c 0.04c 0.02b 0.04a 0.08f 0.04e 0.02d ± ± ± ± ± ± 6.74 5.59 4.85 4.73 5.25 5.30 5.64 12.35 ± 0.04d 8.07 ± 0.14b 7.73 ± 0.19ab 7.40 ± 0.01a 11.68 ± 0.26c 13.30 ± 0.17e 13.24 ± 0.20e 4.50 ± 0.71c 3.33 ± 0.29b 2.25 ± 0.35a 2.00 ± 0.71a 7.00 ± 0.50d 9.17 ± 0.29e 12.00 ± 0.71f
99.07 94.66 96.93 96.92 95.69 98.41 98.26
± ± ± ± ± ± ±
0.36a 2.57b 0.99ac 0.49ac 1.03bc 0.49a 0.81a
± ± ± ± ± ± ±
0.02a 0.08b 0.09c 0.04c 0.04a 0.06a 0.02a −1.81 −0.97 −1.42 −1.38 −1.73 −1.70 −1.78
± ± ± ± ± ± ±
0.17a 0.26b 0.10cd 0.03c 0.10bd 0.13b 0.67b
– 98.28 98.68 99.16 89.26 95.39 97.45
Consistency index (K) Degree of cross-linking (%) b* a* L*
Native starch STMP 0.67 STMP 1.25 STMP 2.5 CA 10 CA 20 CA 30
3.1.2. Solubility index and swelling power of modified starches The solubility index and swelling power of STMP modified starch significantly decreased with increasing STMP content when compared to native starch sample (Table 2). This could be explained by an increase of density of cross-links and intermolecular bonding of starch chains. Cross-linking strengths the bonding between starch chains which seemed to made them to resist against water solubility and swelling (Detduangchan et al., 2014; Koo et al., 2010). On the other hand, solubility index and swelling power of starch treated with CA increased significantly with high concentration of CA. Hong, Zeng, Buckow, and Han (2018) stated that solubility and swelling of starch increased with esterification. Also, some researchers founded a positive relationship between water solubility and degradation of starch (Choudhury & Gautam, 1998). Therefore, esterification or degradation reactions can take place during modification with CA, besides crosslinking.
Swelling power (g g−1)
phosphate stretching (PeOeC), the one at 1216 cm−1 to phosphate vibration (P]O) and at 2928 and 1739 cm−1 to CH stretch attached to O and OH bending band (Detduangchan et al., 2014). In Fig. 1b the peak in the range of 1718–1740 cm−1 was characteristic for ester bonding (C]O) due to esterification reaction between starch and CA. The band range from 3200 to 3400 cm−1 showed the vibration stretching of inter- and intramolecular hydrogen bonds of starch (Kahar, Ismail, & Othman, 2012; Ma, Jian, Chang, & Yu, 2008).
Solubility index (%)
Fig. 1. Fourier transform infrared (FT-IR) spectra of cross-linked starch with different sodium trimetaphosphate (a) and citric acid (b) contents.
Sample
Table 2 Solubility index (%), swelling power (g g−1), color (L*, a*, b*), degree of cross-linking and power law properties of starch samples.
Flow behavior index (n)
Viscosity at 100 s−1 (Pa s)
Viscosity at 200 s−1 (Pa s)
M. Yıldırım-Yalçın, et al.
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M. Yıldırım-Yalçın, et al.
a
3.1.4. Degree of cross-linking The degree of cross-linking of modified corn starches with different amount of STMP and CA is given in Table 2. Cross-linking degree increased with increasing concentration of modification agent. Peak viscosity decreased (peak viscosity results were not shown) with increasing cross-linking degree. Detduangchan et al. (2014) stated that cross-linking agent reduce interactions between starch and water molecules resulting in lower peak viscosity and higher cross-linking degree. Ma et al. (2008) studied on substitution of CA on starch nanoparticles and found that CA modification decreased peak viscosity values because of the cross-linking which reduced chain mobility.
b
O2 Permeability cm3 μm m-2 d-1 kPa-1
3.1.3. Color analysis As shown in the Table 2, lightness (L*) values of samples were between 99.07 and 94.66, while greenness (a*) and yellowness (b*) values were between −1.81 and −0.97, and 6.74 and 4.73, respectively. Native starch had the highest L* values, which indicates whiteness of starch (Ačkar, Babić, Šubarić, Kopjar, & Miličević, 2010). The color results of native corn starch were close to earlier work of Teli, Rohera, Sheikh, and Singhal (2009). They reported the L*, a*, b* values as 94.22 ± 0.02, 0.27 ± 3.02 and 8.26 ± 0.20, respectively. Starch modified with 0.67% STMP or 10% CA reduced L* values, but increasing concentrations did not affect L* values (p ≥ 0.05). The increase of STMP content firstly changed and then did not affect significantly the a* and b* values. Moreover, the amount of CA had no effect on a* and b* values (p ≥ 0.05). CA stabilizes the color because of the chelating agent ability (Akubor, 2013).
10
c
9 7
bc
bc
bc
bc
bc
b
b
6 5 4 a
3 2 1 0
3 de
e
de
2.5
WVP (g mm m-2 h-1 kPa-1)
bc
b
8
cd
c b
b
2 a
b
b
a
1.5 1
0.5 0
3.1.5. Rheological properties of modified starches The power law rheological properties through regression analysis according to Ostwald–de Waele model (R2 > 0.99 for all measurements) were calculated. The consistency index (K, Pa sn), flow behavior index (n), apparent viscosity (Pa s) at 100 s−1 and 200 s−1 of native and modified starches are given in Table 2. All samples showed non-Newtonian shear-thinning behavior (n < 1). The reduction in viscosity of modified starch samples is an evidence of the cross-linking. It decreases the interactions of starch and water molecules resulting in lower viscosity (Detduangchan et al., 2014). Cross-linking decreased viscosity, although this decrease was not significant for all STMP and CA concentrations. The pH values of all CA cross-linked samples were greater than 4.0. Therefore, acid hydrolysis of glycosidic bonds in starch molecules could be hindered (Olsson et al., 2013).
Fig. 2. Oxygen (a) and water vapor (b) permeability of film samples. Different letters in the same column indicate significant differences between samples by Duncan test (p < 0.05).
starch films gave rise to greater water solubility because of the plasticizer effect of CA. Hydrophilic plasticizers increase solubility of films in water (Cuq, Gontard, Cuq, & Guilbert, 1997). Starch modified with CA did not show significant changes in solubility. 3.2.2. Oxygen permeability As can be observed in Fig. 2a, STMP modified starch film had the lowest OP. A previous study by Ustunol and Mert (2004) indicated that cross-linked whey protein decreased OP of films due to an increase in tortuosity of the diffusion path after cross-linking. Moreno, Cárdenas, Atarés, and Chiralt (2017) stated that cross-linking reduced OP of corn starch-gelatin based films because of the tighter matrix. Usage of starch modified with CA increased OP of films, but this increase was not statistically significant when compared to NS films (p ≥ 0.05). Therefore,
3.2. Analysis of edible films 3.2.1. Water solubility of films STMP cross-linked starch decreased the water solubility of films as shown in Table 3. However, the presence of CA in STMP-modified
Table 3 Solubility, color (L*, a*, b*), transparency, mechanical properties and glass transition temperatures (°C) of films. Sample
Solubility (g soluble solid/g total solid)
NS STMP STMP + CA30 NS + CA30 + C STMP + CA30 + C CA10 CA20 CA30 NS + CA10 NS + CA20 NS + CA30
0.65 0.55 0.70 0.73 0.66 0.64 0.59 0.64 0.69 0.67 0.70
± ± ± ± ± ± ± ± ± ± ±
0.02bc 0.04a 0.01bc 0.05c 0.11abc 0.02abc 0.02ab 0.01abc 0.03bc 0.04abc 0.01bc
L*
91.77 91.82 90.80 90.40 91.50 91.03 92.11 91.16 90.56 91.50 90.07
a*
± ± ± ± ± ± ± ± ± ± ±
0.22 fg 0.44 g 0.42bcd 0.44ab 0.20ef 0.34cde 0.18 g 0.25def 0.22abc 0.18ef 0.14a
−1.36 −1.12 −1.05 −0.96 −0.80 −0.94 −1.01 −0.83 −0.88 −0.85 −0.75
± ± ± ± ± ± ± ± ± ± ±
0.03a 0.01b 0.06bc 0.07 cd 0.05 fg 0.04cde 0.07bc 0.07efg 0.04def 0.02defg 0.11 g
b*
Transparency (560 nm)
9.87 ± 0.28b 11.64 ± 0.90de 13.50 ± 0.70f 12.33 ± 0.62e 8.72 ± 0.83a 10.37 ± 0.38bcc 10.33 ± 0.30bc 10.39 ± 0.33bc 11.08 ± 0.35 cd 9.48 ± 0.41ab 13.45 ± 0.67f
43.74 32.40 10.95 49.43 23.33 26.74 18.53 30.82 35.66 30.03 28.77
± ± ± ± ± ± ± ± ± ± ±
2.19 h 1.68f 0.60a 1.98ı 0.77c 1.67d 2.03b 1.15ef 1.33 g 3.02ef 1.82de
Percentage Elongation
Tensile Strength (N mm−2)
62.96 ± 3.63de 16.47 ± 2.53a 55.39 ± 8.79 cd 114.40 ± 8.91 g 38.86 ± 7.83b 60.95 ± 10.54de 63.68 ± 13.36de 48.36 ± 7.84bc 67.78 ± 7.02e 67.54 ± 12.76e 85.12 ± 11.89f
0.36 0.38 0.30 0.50 0.36 0.29 0.24 0.11 0.41 0.40 0.37
± ± ± ± ± ± ± ± ± ± ±
0.07bcd 0.15 cd 0.09bcd 0.05e 0.11bcd 0.06bc 0.10b 0.05a 0.07de 0.07cde 0.06 cd
Glass transition temperature (°C)
55.03 58.27 70.06 67.31 65.10 75.90 71.63 67.04 41.90 42.78 38.77
± ± ± ± ± ± ± ± ± ± ±
3.73b 4.75b 0.14cde 5.54 cd 4.32c 1.88e 1.77de 0.35 cd 1.47a 0.16a 2.14a
Values were given as means ± standard deviation, different letters in the same column indicate significant differences between samples by Duncan test (p < 0.05). 10
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STMP, CA30 and NS + CA30 films were shown in Fig. 3. L* values of samples were between 91.82 and 90.07, while a* and b* values were between −1.36 and −0.75, and 13,50 and 8.72, respectively. The L*, a*, b* values of corn starch based edible film was reported as 95.7, −0.5 and 4.1 by Teixeira, Garcia, Takinami, and Mastro (2018). All films had high values of L*, negative values of a* and positive values of b*. STMP cross-linked starch increased (p < 0.05) the a* values of films from −1.36 to −1.12. Also, CA modification increased (p < 0.05) the a* values. The change of a* value was not significant between CA10 and CA20, while it was significant for CA30 (from −1.01 to −0.83). The b* values of films based on cross-linked starch with STMP were significantly (p < 0.05) higher than those of NS films, while films based on cross-linked starch with CA did not modify the b* values (p ≥ 0.05). Gutiérrez, Tapia, Pérez, and Famá (2015) observed no significant differences between a* and b* values of native and crosslinked starch films. Cross-linked starch in edible film reduced the transparency of films significantly (Table 3). This result was in accordance with Wang, Liu, and Wang (2017) who found that yam starch cross-linked with STMP reduced transparency of films. CA addition to NS or STMP cross-linked starch based films also reduced the transparency. Removing unreacted CA molecules from film matrix by centrifugation created more transparent films.
CA addition was not advantageous in terms of improving OP. Modification of starch with CA before or during film formation did not affect OP. CA addition to the STMP cross-linked starch during the film solution making step and removal of unreacted CA before film formation increased OP. The grape-based starch film produced in this study had good oxygen barrier properties compared to other studies. OP values were between 2.51 ± 0.05 and 8.66 ± 0.57 cm3 µm m−2 d−1 kPa−1 while other authors reported values of 12.11 and 78 cm3 µm m−2 d−1 kPa−1 for corn starch and whey protein based films (Ghasemlou et al., 2013; Perez-Gago & Krochta, 2001). There is no information in literature but the OP of other fruit based films has been studied before under test conditions. For apple-based film, the OP value was reported as 10.20 ± 0.91 cm3 µm m−2 d−1 kPa−1 by Rojas-Graü et al. (2007) and for banana based film, Sothornvit and Pitak (2007) found that in between 22 and 40 cm3 µm m−2 d−1 kPa−1. 3.2.3. Water vapor permeability The WVP values of films developed in this study were found lower than for other fruit based and starch films (Jiménez, Fabra, Talens, & Chiralt, 2012; Rojas-Graü et al., 2007). Cross-linking starch with STMP affected WVP positively as shown in Fig. 2b (p < 0.05). Cross-linking restricts the mobility of starch chains in the amorphous region and prevents water absorption, therefore STMP cross-linking reduces WVP of film (Detduangchan et al., 2014). WVP of films based on starch modified with CA were increased for CA10 sample and decreased for the higher increase of CA content (CA20 and CA30) due to the hydrophobic ester groups (Ghanbarzadeh et al., 2011). These values did not show significant differences with NS sample. However, Reddy and Yang (2010) and Seligra et al. (2016) reported that films including starch and CA showed lower VWP parameters. CA addition to STMP cross-linked starch increased WVP because of the plasticizer property of the excess CA (Bertuzzi, Vidaurre, Armada, & Gottifredi, 2007). WVP of the NS + CA films increased when the CA content increased from 10 to 30 because of the plasticizer effect of unreacted CA molecules. When compared to WVP of NS and NS + CA films, there was no positive effect of direct CA addition to NS film solution.
3.2.5. Mechanical properties STMP cross-linking decreased elongation at break sharply and increased TS slightly, but not significantly (Table 3). Detduangchan et al. (2014) also found lower elongation and higher TS results for crosslinked films with STMP when compared to results for native films. Reddy and Yang (2010) indicated that the cross-linking connections cause stronger films because of the interactions between starch molecules, however, if high level of cross-linking and consequently molecular mobility restriction occurs, the TS of film declines. CA incorporation into STMP cross-linked film solution raised the percent elongation value and did not affect the TS significantly. Siew, Heilmann, Easteal, and Cooney (1999) studied about plasticizers effect on sodium caseinate film and they stated that high plasticizer content enhances the elongation and limits the TS. The percent elongation values did not change for CA10 and CA20 and reduced significantly for CA30 when compared to NS film. The elongation values of NS + CA films increased with CA amount. This result also emphasized the
3.2.4. Film color and transparency properties The color values of films were shown in Table 3 and images of NS,
Fig. 3. Images and SEM of films obtained with native starch (a), (e) and chemically modified with STMP (b), (f); CA at 30% (c), (g) and native starch containing citric acid at 30% (d), (h). 11
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plasticizing property of CA and demonstrated the weak cross-linking reaction between starch and CA when they were mixed during the film manufacturing process.
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3.2.6. Microstructure analysis Representative SEM micrographs of NS, STMP, CA30 and NS + CA30 films are shown in Fig. 3e–h, respectively. NS film showed compact and homogeneous surface without pores and cracks. Garcia, Martino, and Zaritzky (2000) indicated that plasticized starch film samples showed homogenous surface. Well dispersed starch molecules were more visible in STMP film. The micrograph of the CA30 film showed a smoother appearance than the others but showed some pores. The image of the NS + CA30 film was similar to the NS film indicating good compatibility between CA and starch. Reddy and Yang (2010) reported that native starch film showed similar surface structure to cross-linked one. On the contrary, Garg and Jana (2007) found that cross-linked starch and low-density polyethylene blend films exhibited smoother surface than native starch and low-density polyethylene blend films. 3.2.7. Glass transition temperature of films STMP cross-linked starch based films showed slightly higher but statistically similar Tg values to NS based films, while CA cross-linked starch based films showed higher values (Table 3). Detduangchan et al. (2014) reported that cross-linking enhanced the Tg values because of the decreasing chain mobility. On the other hand, the addition of plasticizer causes Tg depression due to the change in starch molecule structure and flexibility (Mali, Grossmann, García, Martino, & Zaritzky, 2006). Therefore, decrease of the Tg values of NS + CA samples demonstrated the plasticizer effect of CA addition to NS just before the gelatinization. 4. Conclusion Chemical modifications of starch have been studied to determine the mechanical, barrier and physical properties of starch films. This study shows that it is possible to produce edible films by modifying starch with STMP or CA. The decrease of solubility, swelling and viscosity properties of STMP modified starch proved the cross-linking. Solubility and swelling properties of CA modified starch showed that esterification or degradation reactions can take place besides crosslinking. The grape-based edible films were characterized by slight yellowish, transparent and flexible properties. The most noticeable effect of STMP cross-linking was decreasing oxygen and water vapor permeability of films. Moreover, cross-linking reduced the transparency of films. CA addition in NS and STMP film increased WVP because of the plasticizer effect. Also, percentage elongation and Tg values of NS + CA films demonstrated the plasticizer effect of CA addition. The results suggest that grape juice and modified starch-based films would be a suitable approach for dried or instant water-soluble food products. Declaration of interests The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements Meral Yıldırım-Yalçın would like to thank The Scientific and Technological Research Council of Turkey (TUBITAK) for granting PhD scholarship (2211-A). 12
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