Journal of Food Engineering 134 (2014) 1–4

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Journal of Food Engineering

journal homepage: www.elsevier .com/ locate / j foodeng

Polylactide stereocomplexation leads to reduced migration duringmicrowave heating in contact with food simulants

http://dx.doi.org/10.1016/j.jfoodeng.2014.02.0170260-8774/� 2014 Elsevier Ltd. All rights reserved.

⇑ Corresponding author. Tel.: +46 8 7908271; fax: +46 8 100 775.E-mail address: minna@polymer.kth.se (M. Hakkarainen).

Yasemin Bor, Jonas Alin, Minna Hakkarainen ⇑Department of Fibre and Polymer Technology, School of Chemical Science and Engineering, KTH Royal Institute of Technology, SE-100 44 Stockholm, Sweden

a r t i c l e i n f o

Article history:Received 16 August 2013Received in revised form 18 February 2014Accepted 20 February 2014Available online 27 February 2014

Keywords:PolylactideStereocomplexFood packagingMigrationESI-MS

a b s t r a c t

The effect of stereocomplexation on the stability and migration resistance of polylactide during micro-wave and conventional heating in contact with different food simulants was evaluated. The heatingeffects were followed through mass loss measurements, molecular weight measurements and identifica-tion of the individual migrants by electrospray ionization-mass spectrometry (ESI-MS). Increased masslosses were observed as a function of time and temperature, but approximately 50% smaller mass losseswere always measured for PLA stereocomplex as compared to the corresponding regular PLLA material.The stability of the stereocomplex material was, thus, significantly higher. Microwave heating increasedthe mass loss as compared to the conventional heating at the same time and temperature. This effect wasespecially significant when 10% ethanol was used as food simulant instead of water. The amount ofwater-soluble migrants was in most cases under the detection limits, but when heating temperaturewas increased to 95 �C, ESI-MS revealed the formation of homologous series of linear lactic acid oligo-mers. Results indicate that PLA stereocomplex materials could have potential in single-use microwaveapplications.

� 2014 Elsevier Ltd. All rights reserved.

1. Introduction

As a result of increased plastic waste problems, biodegradableplastic packaging is attracting increased attention (Rhim et al.,2009). Among the most attractive degradable materials are poly-lactide and its copolymers. In addition to being biodegradableand compostable, polylactide can be produced from renewable re-sources such as corn, wood residues, sugar beet and other biomass.Except environmentally aspects, optical, mechanical and physicalproperties of PLA are comparable to those of other plastic materials(Auras et al., 2003). The possibility to tailor physical and mechan-ical properties by adjusting the stereocomposition of L- andD-isomers additionally extents the range of possible applications(Auras et al., 2004). Hydrolytic degradation of polylactide(Hakkarainen, 2002; Hakkarainen et al., 1996; Li et al., 1990) andpolylactide composites (Roy et al., 2012; Azwar et al., 2012) hasbeen extensively studied. A few studies also addressed the migra-tion from polylactide food packaging to food simulants (Mutsagaet al., 2008) or real foods like cheese (Plackett et al., 2006). WhilePLA provides better protection against ultraviolet light than low

density polyethylene (LDPE), its CO2, O2 and water permeabilitycoefficients are lower than those of polystyrene (PS) (Conn et al.,1995). On the other hand, mechanical strength and barrier effectto ethyl acetate and D-limonene are similar to poly(ethylene tere-phthalate) (PET), which is one the most preferred materials in foodpackaging applications (Auras et al., 2006). In a recent study theapplicability of polylactide for cosmetic packages was evaluatedthrough aging studies in paraffin and protic media (Rydz et al.,2013).

Traditional PLA has limitation in high temperature applicationsuch as cups for hot drinks or microwavable packaging due to lim-ited thermal stability. Here PLA stereocomplexes with higher melt-ing temperature and improved thermal and hydrolytic stabilitycould offer an attractive alternative (Tsuji, 2005; Andersson et al.,2010, 2012b). In previous studies we have shown higher migrationresistance for PLA stereocomplex during heating in air (Anderssonet al., 2012a) or in contact with food simulants at room tempera-ture (Bor et al., 2012). Stereocomplex PLA is due to increased ther-mal stability proposed as a possible biobased material formicrowave applications. The aim of this study was, thus, to evalu-ate the microwave heating effects on PLA stereocomplex in com-parison with plain poly(L-lactide).

Fig. 1. ESI-MS spectrum showing migrants from PLLA, heated for 60 min in water at60 �C.

2 Y. Bor et al. / Journal of Food Engineering 134 (2014) 1–4

2. Experimental

2.1. Materials and chemicals

The polylactide materials consisted of Poly(L-lactide) (PLLA)(HM1011, Mn(SEC) = 103,000 g/mol) and stereocomplex poly-(L-lactide)/poly(D-lactide) (PLAS) from Hycail. The PLLA componentin the PLAS was HM1011 and the composition was 50/50 of PLLA/PDLA. 8 g of granules were dissolved in 40 mL chloroform (HPLCgrade-Fisher Scientific) and the solutions were molded on18.5 cm diameter glass petri plates. The casting solvent was evap-orated first in fume hood and then in the vacuum oven at roomtemperature. Water (LC grade, Fisher Scientific) and 10% ethanol(Merck) were used as food simulants.

2.2. Microwave assisted extraction (MAE)

A CEM MES-1000, a multimode microwave solvent extractionsystem with a rotating turntable, was used to heat the PLA samplestogether with the food simulants. PLA films were cut as stripsweighting approximately 110 mg each. The strips were put intomicrowave vessels together with 10 mL of food simulant. The sam-ples were subjected to microwave heating at temperatures 40 �C,60 �C, 80 �C and 95 �C for 10, 30, 60 or 120 min. For comparisonthe samples were also heated conventionally for 60 min at 60 �C.Triplicate samples were tested for each material, time, temperatureand food simulant.

2.3. Mass losses

Mass losses for each sample after microwave heating and con-ventional heating were calculated according to:

Mass loss% ¼W initial �Wdry

W initial� 100 ð1Þ

where Winitial is the original weight of the samples before micro-wave or conventional heating and Wdry is the dry weight after heat-ing and evaporation of the solvent in the vacuum oven. Mass lossesare presented as averages from triplicate samples together with thestandard deviations.

2.4. Electrospray ionization-mass spectrometry (ESI-MS)

Identification of the migrants was performed by ESI-MS(Finnigan LCQ). The solvent was evaporated and the residue wasdissolved into water/methanol (2:1, v/v) (LC grade, Fisher Scien-tific). Filtrated solutions were injected directly into the ESI-MS ata 5 lL/min flow rate. The instrument was set at positive modeand the LCQ ion source was operated at 5 kV. The capillary temper-ature was set at 175 �C.

2.5. Size exclusion chromatography (SEC)

Size exclusion chromatography analysis was conducted byVerotech PL-GPC 50 Plus system equipped with a PL-RI detectorand two PolarGel-M Organic (300 � 7.5 mm) columns from Varian.Samples were dissolved in tetrahydrofuran (THF) (VWR Company),which was also used as a mobile phase (1 mL/min, 35 �C). After fil-tration, the solutions were injected with a PL-AS RT autosamplerinto the SEC system.

2.6. Differential scanning calorimetry (DSC)

Differential scanning calorimetry, coupled with a GC 100(Mettler Toledo, Switzerland) gas controller, was utilized for

thermal analysis. Samples weighting 4–5 mg were placed into40 lL aluminum cups and they were subsequently sealed andpierced with a thin needle. The analyses were carried out undernitrogen atmosphere. PLLA were heated to 200 �C from 25 �C at aheating rate of 10 �C/min and the cooled to 0 �C at 10 �C/min, afterwhich it was again heated to 200 �C at 10 �C/min. Due to highermelting temperature PLAS was heated to 250 �C. The degree ofcrystallinity was calculated with the equation:

wc ¼DHf

DH0f

� 100

where wc is the degree of the crystallinity, DHf is the heat fusion ofthe studied polylactides and DH0

f is the heat fusion for a 100% crys-talline polylactide. DH0

f value used for PLLA was 93 J/g (Fischer et al.,1973) and DH0

f for PLAS was 142 J/g (Tsuji, 2005). The degree ofcrystallinity and melting point were determined from the first heat-ing scan to evaluate the effect of microwave heating on the materi-als. The glass transition temperatures were taken from the coolingscan.

3. Results and discussion

The effect of stereocomplexation on the stability and migrationresistance of polylactide during microwave and conventional heat-ing in contact with different food simulants was evaluated. Theheating effects were followed though mass loss measurementsand identification of the individual migrants by ESI-MS.

3.1. Migrants

ESI-MS analysis was conducted to identify the migrants aftermicrowave heating in contact with water and 10% ethanol. Figs. 1and 2 show mass spectra of migrants detected after 60 min heatingof PLLA at 60 �C in water and 10% ethanol. After aging in water theamount of migrants was still under the detection limit, but after60 min at 60 �C in 10% ethanol some traces of linear and cyclic lac-tic acid oligomers were detected as Na+ adducts. For the PLAS theamount of migrants was under the detection limits even after heat-ing in 10% ethanol indicating higher stability. The linear oligomersin the mass spectra of PLLA appear at m/z = 1 + 72 � n + 18 + 23 andcyclic oligomers at m/z = 72 � n + 23, where 72 is the mass of therepeating unit, 18 is the mass of the end group and 23 is the massof Na+. Our previous study showed that PLLA originally containedsmall amounts of cyclic oligomers (Andersson et al., 2012a). Thesecyclic oligomers are soluble in ethanol, but not in water. This ex-plains the small traces of cyclic oligomers seen in 10% ethanol. Lin-ear oligomers are formed due to hydrolysis of PLLA or the cyclicoligomers during microwave heating process. Increased time and

Fig. 2. ESI-MS spectrum showing migrants from PLLA, heated for 60 min in 10%ethanol at 60 �C.

Fig. 4. Mass losses for PLLA films after microwave heating in contact with water.

Fig. 5. Mass losses for PLLA films after microwave heating in contact with 10%ethanol.

Y. Bor et al. / Journal of Food Engineering 134 (2014) 1–4 3

temperature led to increased mass loss and increasing amounts oflinear oligomers. After 120 min at 95 �C linear oligomers rangingfrom pentamer to pentadecamer migrated both from PLLA andPLAS. However the intensities were lower in the case of PLAS.Fig. 3 shows as an example the migrants from PLLA.

3.2. Mass loss

Figs. 4–7 show the mass loss for the microwave heated PLLAand PLAS in contact with water and 10% ethanol at different tem-peratures and after different heating times. As expected the massloss increased as a function of heating time and temperature. Masslosses were slightly higher when 10% ethanol was used as simulantinstead of water, which is in good correlation with ESI-MS results.Comparison of mass loss for PLLA (Figs. 4 and 5) and PLAS (Figs. 6and 7) shows that stereocomplexation led to significantly lowermass loss and stereocomplex materials could, thus, have potentialas microwavable packaging at least in single-use applications.

3.3. Comparison of microwave heating and conventional heating

Figs. 8 and 9 present a comparison of mass losses for PLLA andPLAS after microwave and conventional heating. It is clearly seenthat microwave heating caused somewhat higher mass loss ascompared to the conventional heating for the same time at sametemperature. The same effect is seen for PLLA and PLAS eventhough in agreement with Figs. 4–7 the mass losses are higherfor PLLA as compared to PLAS.

3.4. Molecular weight

A few samples were also analyzed by SEC to determine the ef-fect of microwaving on the molecular weight. Results presented

Fig. 3. ESI-MS spectrum showing migrants from PLLA, heated for 120 min in 10%ethanol at 95 �C.

Fig. 6. Mass losses for PLAS films after microwave heating in contact with water.

in Table 1 support the mass loss results, showing larger molecularweight decrease after heating in 10% ethanol as compared to heat-ing in pure water. This difference was especially significant aftermicrowave heating. Similar results showing the harsh effect ofmicrowaving in contact with ethanol were previously shown forPET (Alin and Hakkarainen, 2013) and PC (Alin and Hakkarainen,2012). It was not possible to follow the molecular weight of PLASdue to the limited solubility after stereocomplex formation.

Fig. 7. Mass losses for PLAS films after microwave heating in contact with 10%ethanol.

Fig. 8. Comparison of mass losses for PLLA after microwave and conventionalheating for 60 min at different temperatures.

Fig. 9. Comparison of mass losses for PLAS after microwave and conventionalheating for 60 min at different temperatures.

Table 1The effect of microwave and conventional heating on molecular weight of PLLA.

Original Microwave Conventional

Water 10% Ethanol Water 10% Ethanol

Mn 107,000 102,000 77,000 97,000 92,000Mw 226,000 218,000 185,000 214,000 201,000

4 Y. Bor et al. / Journal of Food Engineering 134 (2014) 1–4

4. Conclusions

Stereocomplexation increased the stability and migration resis-tance of PLA during microwave and conventional heating in con-tact with food simulants. Approximately 50% smaller mass losseswere measured for PLA stereocomplex as compared to the corre-sponding regular PLLA material. Microwave heating increased themass loss as compared to the conventional heating at the sametime and temperature. This effect was especially significant when10% ethanol was used as food simulant instead of water. Resultsindicate that PLA stereocomplex materials could have potentialin single-use microwave applications.

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