aging effects on sorbitol- and non-crystallizing sorbitol-plasticized tapioca starch films

Doungjai Thirathumthavorna, b

Sanguansri Charoenreina

a Department of Food Scienceand Technology,Faculty of Agro-industry,Kasetsart University,Bangkok, Thailand

b Department ofFood Technology,Faculty of Engineeringand Industrial Technology,Silpakorn University,Nakhon Pathom, Thailand

Aging Effects on Sorbitol- and Non-CrystallizingSorbitol-Plasticized Tapioca Starch Films

The effect of aging on the properties of tapioca starch films plasticized with either sor-bitol (S) or non-crystallizing sorbitol (NCS) was investigated in this study. Tapiocastarch, plasticizer and deionized water were mixed, heated, cast on high-densitypolyethylene plates and dried at ambient conditions. The results showed that S wasmore effective in plasticizing fresh starch film than NCS. However, sorbitol crystal-lization was observed in S-plasticized starch film after one month of storage, whilethere was no crystallization observed in NCS-plasticized starch film after two monthsof storage. Mechanical properties of both S- and NCS-plasticized starch films changedsignificantly with time, but with less change in the NCS-plasticized films. Tensilestrength, elastic modulus and toughness increased over time; conversely, elongationdecreased. Additionally, the water vapor transmission rate decreased as storage timeincreased. The fact that mechanical properties of both S- and NCS-plasticized filmschanged is likely due to an increase in crystallinity of the starch in the films with time.

Keywords: Starch films; Sorbitol; Non-crystallizing sorbitol; Aging effects; Tapiocastarch

Starch/Stärke 59 (2007) 493–497 493

1 Introduction

Starch films and coatings are odorless, tasteless, color-less, non-toxic, biologically absorbable [1], semi-perme-able to carbon dioxide [1-3], good oxygen barriers [4, 5]and possess many characteristics, i.e. tensile strength andelastic modulus, similar to plastic films [6, 7]. However,starch films and coatings are tacky at high relative humidityand rather brittle at low humidity [8]. Barrier and mechan-ical properties of starch-based films are affected by fac-tors such as amylose content [6, 9, 10], molecular weight ofstarch [9, 11, 12] and type of plasticizer [9, 13-16]. Plasti-cizers are used to overcome starch film brittleness andimprove flexibility and extensibility [13, 14, 17, 18].

Sorbitol (S) is one of the plasticizers commonly used toimprove the flexibility of starch films. It has the advantageof improving mechanical properties, with less increase inpermeability compared to other plasticizers [19-22]. Themain disadvantage of S as a plasticizer is its tendency tocrystallize over time [15, 16], with resulting reduction infilm flexibility and eventual loss of film continuity. Use ofcommercial non-crystallizing sorbitol (NCS) is a way toavoid this problem. NCS is the partial hydrolysis productof corn, wheat or potato starch with subsequent hydro-genation under pressure. The end product is an ingredientcomposed of sorbitol, maltitol and higher hydrogenated

saccharides. Commercial sorbitol syrups includeRoquette’s 70/100 (Roquette America, Inc., Keokuk, IA),which is 75% sorbitol, SPI Polyols’(New Castle, DE)Sorbo 70% and A-625 which contain 70% sorbitol andADM’s (Decatur, IL) sorbitol syrup, called Sorbitol Solu-tion, Non-crystallizing which is 70% sorbitol (http://www.caloriecontrol.org/hydrosta.html).

Starch itself normally crystallizes with aging and could alsocause an adverse effect on film properties. There weremarked changes in the stress-strain behavior [13, 15, 23,24] and gas permeabilities [25] of starch films after storage.It was found that these changes related to the crystal-lization and/or reorientation of amylopectin and amylose[25-29], with resulting effects in glass transition tempera-ture [27], water content [27] and plasticizer migration [15].

We hypothesized that aging would affect the properties ofplasticized starch film, but that the effect would be lessusing NCS. Thus, this research had the objective of com-paring the effect of aging on the tensile and moisture-barrier properties of S- and NCS-plasticized starch films.

2 Materials and Methods

2.1 Materials

Tapioca starch was obtained from Tapioca DevelopmentCo., Ltd. (Bangkok, Thailand). Sorbitol was purchasedfrom Fischer Scientific, Inc. (Fair Lawn, NJ). NCS wasprovided by SPI Phama, Inc. (New Castle, DE).

Correspondence: Sanguansri Charoenrein, Department of FoodScience and Technology, Faculty of Agro-Industry, Kasetsart Uni-versity, Bangkok, Thailand, 10900. E-mail: [email protected].

© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.starch-journal.com

DOI 10.1002/star.200700626

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494 D. Thirathumthavorn and S. Charoenrein Starch/Stärke 59 (2007) 493–497

2.2 Preparation of films

A native tapioca starch suspension was prepared at aconcentration of 2.5% (w/w) in deionized water. Theamount of plasticizer was 40% (w/w) of total solids. Thestarch suspension and plasticizer were mixed and heatedup to 95 6 57C for 30 min to gelatinize the starch; thendegassed in vacuum and cast on high-density poly-ethylene plates. All casting plates were kept on a leveledsurface at ambient conditions (23 6 37C and 35 6 5%RH) until films could be peeled off from plates. Prior totesting all films were stored in a cabinet with controlledconditions (RH 50 6 5% and temperature 23 6 27C) forthree days (marked as fresh films), one and two months,respectively. Five replications were done in this experi-ment.

2.3 Film thickness and moisture content

Thickness values of the films were determined using amicrometer (No. 7326, Mitutoyo Manufacturing Co., Ltd.,Japan) to 6 0.0001 inch (0.00254 mm) at six randompositions. The average film thickness was used to calcu-late water vapor permeability (WVP) for each film replicate.The average thickness of films was 0.0478 6 0.005 mm.

The moisture content of starch films were determined bydrying in a vacuum oven at 707C for 24 h. The moisturecontent of S- and NCS-plasticized starch films was10.23 6 0.48% and 10.32 6 0.63%, respectively.

2.4 Tensile properties

All samples were tested with an Instron Universal TestingMachine (Norwood, MA) set at crosshead speed of 12.5mm/min. Tensile properties were measured using filmsamples of 8 mm width and 50 mm initial length, with fivesamples for each replication. For all tests, the room envi-ronment was adjusted to 23 6 27C and 50 6 5% RH.

2.5 Water vapor permeability test

A modification of the ASTM E96-92 gravimetric methodwas used to determine water vapor transmission rate(WVTR) [30]. Distilled water (6 mL) was dispensed intoflat-bottom Plexiglas® cups coated with high-vacuumgrease (Dow Corning, Midland, MI, U.S.A.) around the rimto ensure good sealing. Starch film was sealed to eachcup with a four-screw ring cover. The cups were placed in0% RH cabinets containing anhydrous calcium sulfate(W.A. Hammond Drierite Co., Xenia, Ohio, U.S.A.) and anair-circulating fan. Cup weights were taken periodicallyafter steady state was achieved and used to calculate the

film WVTR. From the WVTR, film thickness and differencein water vapor partial pressure across the film, the watervapor permeability (WVP) was calculated:

WVTR = P?D? ln[(P-p2)/(P-p1)]R?T?Dz

Where: P Total pressureD Diffusivity of water through air at 257CR Universal gas constantT Absolute temperatureDz mean stagnant air gap heightp1 water vapor partial pressure at

solution surfacep2 corrected water vapor partial pressure

at film inner surface in cupWVP = WVTR ? thickness

p2 – p3

where: p3 water vapor partial pressure at filmouter surface in the cabinet

2.6 Oxygen permeability

The oxygen permeability of the starch films was meas-ured at 237C and 50 6 1% RH using an Ox-tran 2/20 MLmodular system (Modern Controls, Inc., Minneapolis,MN) according to the American Society of Testing andMaterials Standard Methods [31]. Starch films were con-ditioned and stored in the chamber at 23 6 27C and50 6 5% RH using magnesium nitrate.

2.7 Statistical analysis

The experiments were a completely randomized designand the difference in means was determined by DuncanNews Multiple Range Test. The data were analyzed usingSPSS 10.0 for Windows.

3 Results and Discussion

3.1 Tensile properties

Both S and NCS plasticizers made tapioca starch filmsmore flexible, but in fresh films the plasticizing effect of Swas greater than that of NCS. Tensile strength and elasticmodulus of S-plasticized film samples were lower (Figs. 1aand 1b), while elongation was greater for the S-plasticizedfilms than for NCS-plasticized film samples (Fig. 1c).

However, Figs. 1a and 1c clearly show that the tensileproperties of S-plasticized starch films were stronglyaffected by storage time. There were significant changes


Starch/Stärke 59 (2007) 493–497 Aging Effects on Sorbitol-Plasticized Tapioca Starch Films 495

Fig. 1. Development of tensile strength (elastic modulus, elongation, toughness) of sorbitol- (S) and non-crystallizing sor-bitol (NCS) plasticized starch films on storage. Statistical differences are denoted by different letters.

in tensile strength, elongation and elastic modulus of S-plasticized starch films over time. The elongation(Fig. 1c) decreased rapidly from 144% to 80% and 44%during storage for one and two months, respectively.Tensile strength (Fig. 1a) and elastic modulus (Fig. 1b)also changed significantly (P,0.01), indicating that thefilms became stiffer and less extendable. These effectscould be explained by the phenomenon of sorbitolcrystallization (Fig. 2) and starch crystallization in agedstarch films, with resulting decrease in plasticizationefficiency.

Tensile strength of aged samples plasticized with NCS(Fig. 1a) increased somewhat during storage for a monthand then leveled off. However, elongation of NCS-plasti-cized films did no change significantly after storage forone month (Fig. 1c). It is also clear that starch films plas-ticized with NCS were still flexible after storage for twomonths (Fig. 1b). Overall, Fig. 1d shows that the tough-ness (the amount of energy that a material can absorbbefore rupturing) of NCS-plasticized films was signifi-cantly greater than that of S-plasticized films after twomonths of storage. Also, contrary to S-plasticized films,NCS-plasticized films did not show any crystallization.

From these results, it was concluded that NCS is apotentially useful plasticizer for starch films.

Because NCS did not crystallize, the change in tensileproperties of NCS-plasticized starch films in time is likelydue to increase in crystallinity of starch in the films with

Fig. 2. Sorbitol crystallization found in sorbitol-plasti-cized tapioca starch film after storage for a month (pho-tograph with Digital Camera, Canon PowerShot A70).


496 D. Thirathumthavorn and S. Charoenrein Starch/Stärke 59 (2007) 493–497

Tab. 1. Water vapor transmission rate (WVTR) and water vapor permeability (WVP) of S- and NCS-plasticized starch films.

Sample Storage time WVTR1/ [g h-1m-2) WVP [g mm kPa-1 h-1 m-2]

S-plasticized starch films fresh 60.06 6 4.56a2/ 1.11 6 0.07ns

1 month 58.41 6 2.02 ab 1.13 6 0.062 months 54.91 6 1.42 b 1.35 6 0.37

NCS-plasticized starch films fresh 57.97 6 4.77a 1.06 6 0.06ns

1 month 52.12 6 2.58b 1.12 6 0.192 months 52.02 6 1.31b 1.15 6 0.08

1/ The values reported in mean 6 standard deviation.2/ A significant letter in the same column of S- or NCS-plasticized starch films indicates the differ-

ence in each film sample (P,0.05).ns There is no significant difference (P.0.05) among storage time of each plasticized-starch film.

time [23, 28, 32]. Starch crystallization likely also occurredin S-plasticized films, but had less effect than the S crys-tallization.

3.2 Water vapor permeability

Effects of storage time on water vapor transmission rate(WVTR) and water vapor permeability (WVP) of starchfilms are shown in Tab. 1. Results show that aging did nothave a significantly different effect on WVP of S- andNCS-plasticized starch (P.0.05). However, WVTR of bothS- and NCS-plasticized starch film samples decreasedsignificantly (P,0.05) as storage time increased (Tab. 1).This decrease was likely due to the development overtime in both types of films of starch crystalline domains,which are not permeable to water vapor.

3.3 Oxygen permeability (OP)

The OP values for fresh film samples plasticized with S orNCS tested at 50% RH were below the detection limit ofthe Oxtran 2/20 ML (0.05 cm3 mm m-2 d-1 kPa-1). Thus, theOP of these starch film samples tested at 50% RH couldbe reported as , 0.05 cm3 mm m-2 d-1 kPa-1. This indicatesthat these starch films are excellent oxygen barriers at lowto intermediate RH. However, the OP of starch film isexpected to be higher at increased RH, due to the hydro-philic nature of the starch components. Increasing RH hasan effect on OP of hydrophilic films [19, 32], due to theabsorption of water molecules that plasticize the filmstructure. Thus, polymer chain mobility increases, result-ing in increased OP [34].

4 Conclusions

From this study, it can be concluded that NCS could beused to plasticize starch film without concern aboutcrystallization of S over time. The result is that there is littleor no change in mechanical properties of NCS-plasticizedduring storage or use. However, preventing starch crys-tallization during film storage will require additional study.

Acknowledgement

This research work was supported by the GraduateSchool, Kasetsart University and the Ministry of UniversityAffairs, Thailand. The authors would like to thank Prof. Dr.John M. Krochta for his guidance and reviewing thismanuscript.

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(Received: April 18, 2007)(Revised: July 25, 2007)(Accepted: August 9, 2007)


aging effects on sorbitol- and non-crystallizing sorbitol-plasticized tapioca starch films

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