Effect of melt-processing and ultrasonic treatment on physical properties of high-amylose maize starch

Ultrasonics Sonochemistry 17 (2010) 637–641

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Effect of melt-processing and ultrasonic treatment on physical properties of high-amylose maize starch Felipe F. Lima, Cristina T. Andrade * Universidade Federal do Rio de Janeiro, Instituto de Macromoléculas Professora Eloisa Mano, Centro de Tecnologia, Bloco J, P.O. Box 68525, 21945-970 Rio de Janeiro, RJ, Brazil

a r t i c l e

i n f o

Article history: Received 18 June 2009 Received in revised form 29 November 2009 Accepted 3 January 2010 Available online 7 January 2010 Keywords: High-amylose maize starch Melt-processing Ultrasonic treatment Intrinsic viscosity 1 H NMR spectrometry X-ray diffraction

a b s t r a c t High-amylose maize starch (Hylon VII) was submitted to melt-processing in an internal mixer at 100 °C and 40 rpm for 8 min. Glycerol was used as a plasticiser at different polymer/glycerol ratios. Torque and temperature curves were obtained. After glycerol extraction with ethyl alcohol, the samples were dispersed at 5 g/L, and treated by ultrasound radiation at the same conditions for 30 min. Samples were characterised by 1H NMR spectrometry, viscosity measurements, and X-ray diffractometry. The results revealed that both glycerol and water had an important role on the crystallinity properties of the resulting products. Melt-processed and sonicated samples showed similar 1H NMR spectra. Ultrasound treatment caused a significant reduction in intrinsic viscosity for the sample previously processed with the highest glycerol content, probably because of its higher solubility in water. Ó 2010 Elsevier B.V. All rights reserved.

1. Introduction Regular starches are basically composed of 70–80% highly branched amylopectin and 30–20% essentially linear amylose [1]. Both starch polymers are arranged in semicrystalline granules, which occur mainly in grains, roots and tubers. Genetic mutants of some plant species are known with altered compositions. Mutant maize starches are widely used in different industrial sectors. This is because their functional properties are determined according to differences in amylose/amylopectin ratio. The amylose content in waxy starches is very low, whereas amylose-rich starches may reach high contents of this component [2]. The gelatinisation temperatures of regular and waxy starches in excess water vary between 62 and 72 °C. For high-amylose starches, swelling begins below 100 °C, but temperatures higher than 130 °C are required for gelatinisation [3]. High-pressure treatment of 30 wt.% starch aqueous suspensions has revealed that waxy maize starch is completed gelatinised at 20 °C within a few minutes. Addition of high-amylose maize starch led to a decrease in the gelatinisation degree. A slight degree of granules gelatinisation (10.8%) was observed for high-amylose maize starch submitted to the same high-pressure treatment [4]. Native granular starches can be converted into an essentially amorphous material by conventional processing methods in the presence of plasticisers. Melt-processing of regular starches has * Corresponding author. Tel.: +55 2562 7208; fax: +55 2270 1317. E-mail address: [email protected] (C.T. Andrade). 1350-4177/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.ultsonch.2010.01.001

been reported by several authors. The influence of amylose/amylopectin ratio on the properties of pasted starches, plasticised with 30 wt.% glycerol, was investigated for extruded samples. Highamylose materials showed superior strength and stiffness compared to the high amylopectin-rich materials [5]. Ultrasonic treatment is known as a non-conventional, environmentally friendly, and effective method used to promote chemical modifications of polymers [6,7], inactivate microorganisms [8], and disrupt phospholipid membranes [9,10]. Exposing a polymer solution to high intensity ultrasonic radiation seems to have as a primary effect the reduction of molar mass. The process had been reported as non-random, and cleavage would occur preferentially near the middle of the chain, without altering the chemical structure of the repeating unit [11,12]. Particularly for starch, the molar mass of its components has significant influence on many properties, among them water-absorption and solubility. In the case of starch, sonication has been reported to promote deaggregation of retrograded starch molecules [13], disruption of swollen granules [14], decrease in viscosity of starch systems [15]. Ultrasonic treatment of suspensions of regular, waxy maize starches, and amylomaize V in the granular form at 30 wt.% concentration led to the increase in swelling power, solubility, and gelatinisation temperatures. A decrease in the viscosity of these starches was observed [16]. Recently, granular waxy rice starch suspensions at 5 wt.% concentration were submitted to different ultrasonic treatments. The pasting behaviour of the sonicated suspensions was evaluated, and the lower peak viscosity and final viscosity of starch dispersions, sonicated at temperatures near the

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gelatinisation temperature, were attributed to solubilisation of starch aggregates [17]. In this work, the effect of sonication on melt-processed glycerol-plasticised high-amylose maize starch was investigated. The properties of the resulting products were evaluated by viscosity measurements, high-resolution proton magnetic resonance, and X-ray diffraction. 2. Experimental 2.1. Materials High-amylose maize starch (HS) composed of 70 wt.% amylose and 30 wt.% amylopectin, and with 8.5 wt.% moisture content, was supplied by National Starch and Chemical Industrial Ltd. (Jundiaí, SP, Brazil). Analytical grade glycerol was purchased from Vetec Química Fina Ltda. (Rio de Janeiro, RJ, Brazil) and was used as received. 2.2. Preparation of samples HS and glycerol (G) were premixed in a conventional mixer (Ika Works, Wilmington, NC, USA) for 10 min, and maintained in tightly sealed bags for 48 h at 4 °C before processing. Different HS/G compositions were prepared, which varied from 75/25 to 60/40 wt.%.

for 3 h. The intrinsic viscosity was determined according to the equations of Huggins and Kraemer (Eqs. (1) and (2), respectively)

gsp =C ¼ ½g þ kH ½g2 C ðln gr Þ=C ¼ ½g þ kK ½g2 C

ð1Þ ð2Þ

where gsp = gr  1, C is the concentration, kH and kK are the Huggins’ and Kraemer’s constants, respectively. Average values were considered. The average viscosimetric molar mass was calculated according  a , where the to the Mark–Houwink–Sakurada equation, [g] = K M v constants K and a were given elsewhere as 1.18  103 mL/g and 0.89, respectively [18]. 2.7. 1H NMR spectrometry 1

H NMR spectra were acquired in a Varian Mercury VX 300 (300 MHz for 1H) (Palo Alto, USA), equipped with a 5 mm Indirect Detection probe. Samples (15 mg) were dissolved in deuterated dimethyl sulfoxide (DMSO-d6) (Cambridge Isotope Laboratories, Andover, USA), by stirring at room temperature for 3 h. Some drops of water were added to decrease viscosities. Spectra were recorded at 60 °C with 90° pulse, which corresponded to a pulse width of 9.3 ls, 1 s of delay time, and an acquisition time of 3.6 s. At least, 48 scans were acquired. 2.8. X-ray diffraction (XRD)

2.3. Melt-processing Plasticised HS/G mixtures were processed in a Haake Rheocord 9000 system (Karlsruhe, Germany), equipped with a Rheomix 600 internal mixer for 8 min. The screw speed and the temperature were maintained constant at 40 rpm and 100 °C, respectively. Torque and temperature curves were registered as a function of time. Processing gave rise to samples denoted HSG25, HSG30, HSG35, and HSG40, according to the glycerol content. 2.4. Glycerol extraction After processing, the samples were extensively milled, and glycerol was extracted with ethyl alcohol at room temperature for 24 h. The samples were recovered by filtration, and dried in an oven at 50 °C. The resulting samples without glycerol were denoted HSWG25, HSWG30, HSWG35, and HSWG40. 2.5. Ultrasound processing After glycerol extraction, the samples were milled again, and dispersed at 5 g/L concentration in Milli-Q water from Direct-Q3 System (Millipore Corporation, Billerica, MA, USA) at 50 °C for 3 h. The aqueous dispersions were submitted to sonication for 30 min at 40% wave amplitude in a Sonics & Materials Inc. (Newtown, CT, USA) ultrasonic processor, 750 W model, operating at 20 kHz, and equipped with a standard probe 13 mm in diameter. The reaction vessel was immersed in an ice-salt-water bath (0 °C) to maintain the samples at a low temperature (10 °C). After sonication, the samples were recovered in ethyl alcohol, filtered, and dried at 50 °C. The corresponding samples were denoted by HSS. 2.6. Viscosity measurements Viscosity measurements were carried out in triplicate at 25 °C in a Ubbelohde viscosimeter (capillary tube with 0.58 mm in diameter). The samples HSWG25, HSWG35, HSWG40, HSS25, HSS35 and HSS40 were dissolved in 1 M KOH under magnetic stirring

Diffractograms were obtained for native Hylon VII, melt-processed samples before (HSG samples), and after glycerol extraction (HSWG samples), and for sonicated samples (HSS samples) with a Miniflex diffractometer (Rigaku Corporation, Osaka, Japan) operating at Cu Ka wavelength of 1.542 Å. The samples were exposed to the X-ray beam with the X-ray generator running at 30 kV and 15 mA. Radiation was detected at ambient temperature in the angular region (2h) of 5–40° at a rate of 1°/min and a step size of 0.05°. Diffractograms were smoothed (Savitsky–Golay, polynome = 2, points = 7), and the baseline was corrected. 3. Results and discussion To obtain typical glycerol-plasticised samples, processing of HS was carried out in an internal mixer. The different parameters that could interfere with melt-processing were varied. Temperatures higher than 100 °C, associated with rotation speeds higher than 50 rpm, led to degradation of HS. Water was not a good plasticiser for HS, probably because of evaporation during processing. Contrarily, in the presence of glycerol, homogeneously plasticised pastes were obtained. Torque and temperature values were monitored during the experiments. Fig. 1 shows torque and temperature curves as a function of time for HS compounded with different glycerol contents. For water-plasticised regular starches, the appearance of two peaks had been reported. The first peak was attributed to loading, whereas the second was associated with starch gelatinisation and melting [19]. In Fig. 1a, loading peaks were of low intensity. After a slight decrease, torque reached a plateau after 90 s for HSG25, HSG30, and HSG35 (samples with 25, 30, and 35 wt.% glycerol, respectively). For HSG25 (trace I), torque values continued to rise at 210 s processing, up to the end of the experiment. This result indicates a steady increase in viscosity for this sample, probably because of the complete rupture of the granular structure. For HSG40, torque stability was reached at 210 s, and reflects the low viscosity of the molten product. This behaviour was expected because HS was at the lowest concentration. In Fig. 1b, the variation in temperature is shown as a function of time for the same experiments. Final temperatures were within a

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70

a

Torque (Nm)

60 50

I

40 30

II

20

III

10

IV

0 0

2

4

6

8

Time (min)

Temperature (ºC)

130

b

120

I

110

II III IV

100 90 80 70 0

2

4

6

8

Time (min) Fig. 1. Torque (a) and temperature (b) curves for high-amylose maize starch with glycerol at 25 wt.% (trace I), 30 wt.% (trace II), 35 wt.% (trace III), and 40 wt.% (trace IV).

low temperature range, below 130 °C. Under this temperature condition, significant degradation of starch molecules was not expected. Before sonication and physicochemical characterisation, glycerol was extracted from the samples. Table 1 shows results of intrinsic viscosity in 1 M KOH at 25 °C for some of the samples, before (HSWG) and after sonication (HSS). Since melt-processing combines simultaneous effects of heating and shearing, both of them should cause reduction of molar masses of starch macromolecules [20]. At higher contents of plasticiser, melt-processing is expected to occur under milder conditions, with decreasing stresses. However, the results of Table 1 indicate a more complex behaviour. Although the highest torque value was developed for the sample plasticised with the lowest content of glycerol, the highest value of intrinsic viscosity was determined for HSWG25. After melt-processing, the lowest value of intrinsic viscosity was determined for HSW40. These results may reflect the role of glycerol on the reduction of starch molar mass during melt-processing. Recently, the role of water on starch breakdown during extrusion was attributed to three different effects [21]. As expected, increasing moisture content decreased the stresses. Moreover, increasing moisture content was shown to increase the degree of thermal breakdown, and to destabilize main chain covalent bonds; smaller stresses would be required for mechanical breakdown. Similar effects may be acting on starch molecules in the presence of glycerol. Table 1 also shows intrinsic viscosity data for sonicated samples. Unlike chemical or thermal degradation, the effect of ultrasound waves is considered a non-random process, preferentially over large molecules. As a consequence of limiting molar mass, distribution of molar mass would be reduced [22]. For mixtures of carboxymethyl cellulose samples, the higher the initial dynamic

Table 1 Viscometric data for HSWG and HSS productsa in 1 M KOH at 25 °C.

a

Sample

[g]H (dL/g)

R2

kH

[g]K (dL/g)

R2

kK

 v (105) M

HSWG25 HSS25

1.69 1.46

0.9856 0.9947

0.35 0.32

1.68 1.45

0.9879 0.9895

0.15 0.16

6.21 5.27

HSWG35 HSS35

1.63 1.45

0.9987 0.9896

0.43 0.26

1.64 1.43

0.9759 0.9849

0.13 0.19

5.97 5.23

HSWG40 HSS40

1.17 0.38

0.9991 0.9993

0.43 0.42

1.18 0.39

0.9915 0.9972

0.12 0.13

4.11 1.16

HSWG, melt-processed sample after glycerol extraction and before sonication; HSS, melt-processed and sonicated sample.

Fig. 2. 1H NMR spectra in DMSO-d6 at 60 °C for granular HS (a), and for melt-processed sample with glycerol at 35 wt.% before (b) and after (c) sonication.

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a

I II III IV V

10

20

2θ (º)

30

40

b

I II III

IV V

10

20

2θ (º)

c

30

40

I II III IV V

10

20

2θ (º)

30

40

Fig. 3. X-ray diffractograms for granular HS (traces I in a, b, and c), and for meltprocessed samples (a) with glycerol concentration at 25 wt.% (trace II), 30 wt.% (trace III), 35 wt.% (trace IV), and 40 wt.% (trace V), and for the samples before (b) and after (c) ultrasound treatment.

viscosity, the faster was the degradation. When the initial dynamic viscosity of the polymer solutions was similar, the sonolytic degradation was dependent on the molar mass, and on the polymer concentration [23]. In the present work, the samples were not soluble in water, and were sonicated at a constant initial concentration, as 5 g/L dispersions. According to earlier studies, the sample with the lowest intrinsic viscosity (sample HSWG40) should be the least degraded. However, such result was not observed, and this sample had the most significant reduction in molar mass caused by sonication (Table 1). This result may be explained by the fact that, for HSWG40, a higher number of starch macromolecules, with low molar masses, might be solubilised during dispersion and sonication. This might have contribute to increase the viscosity, and the effectiveness of ultrasound waves on molar mass reduction. Fig. 2 shows 1H NMR spectra in DMSO-d6 solution at 60 °C for granular HS, and for the sample melt-processed with 35 wt.% glycerol, before and after sonication. Similar spectra were obtained for

the other melt-processed and sonicated samples. High viscosity and different chemical environment usually leads to uncompletely resolved peaks in 1H NMR spectra of polysaccharides. Because of the electron-withdrawing effect of the ring and anomeric oxygen atoms, hydrogen atoms linked to C-1 (H1) are more deshielded, and consequently resonate at lower field strength [24]. In Fig. 2a, non-resolved peaks may be observed for H1 centered at 5.34 ppm, for H6 and H60 at 3.70 ppm, and for H4 and H5 at 3.38 ppm. For the sample processed with 35 wt.% glycerol, before (sample HSWG35) and after sonication (sample HSS35), 1H NMR spectra are shown as traces b and c, respectively. These spectra are very similar, which reveals that there was no modification in the chemical structure of starch molecules. Contrarily to granular HS, the anomeric hydrogens in both processed samples were better resolved. This behaviour is consistent with decreased molar mass and less aggregated macromolecules. Fig. 3 shows X-ray diffractograms for HS, for melt-processed and sonicated materials. The amylose content in maize starch granules has a significant effect on their crystalline properties. A transition of crystalline type from A through C to B, together with a decrease in degree of crystallinity, was observed for granular maize starches as the amylose content was increased from 0% to 84% [25]. The diffractogram for granular HS (Fig. 3a, trace I) closely resembles previously reported data [25,26]. The most intense peak at 17° (2h) characterise B-type crystals. This structure is described as a column of water molecules surrounded by six double helices [27]. On the other hand, melt-processing led to changes in diffraction patterns, and to samples with V-type crystallinity [28]. The complete disruption of granular B-type crystallinity was observed only for the HSG25 sample processed with 25 wt.% glycerol. For this sample, no residual peak at 17° (2h) was detected (Fig. 3a, trace II). For the other samples, at least residual peaks at this diffraction angle may be observed (Fig. 3a, traces III–V). The appearance of a peak at 21.5° (2h) for HSG25 and HSG30 may be associated with single-helical amylose VA-type crystals. This structure is generally found in thermoplastic starches conditioned under low relative humidity (less than 10%) [28,29]. The shoulder visualised at around 20° (2h) for the sample plasticised with 30 wt.% glycerol was intensified for the samples with higher glycerol contents. This V-type crystallinity is much more frequently observed [30], and is related to VH-type crystals. The origin of V-type crystallinities was appointed to differences in hydration of crystal unit cell [31]. The results seem to indicate that addition of glycerol at contents above 30 wt.% favoured water-absorption, and consequently led to the formation of the VH-type structure. The same samples were analysed by X-ray diffraction after extraction of glycerol with ethyl alcohol. Fig. 3b shows that the typical VA-type crystallinity may be observed for the samples. This behaviour is reasonably explained by the removing of water from crystals during glycerol extraction. However, even for these samples, it is worth noting the appearance of the peak at 17° (2h), characteristic of B-type crystallinity. The effect of sonication on the crystalline structure of these materials is shown in Fig. 3c (traces II–V). While V-type peaks disappeared, B-type crystallinity was increased. Dispersion in water during sonication for 30 min, followed by a relatively long drying period might have had a significant role on the samples final crystallinity.

4. Conclusion The aim of this work was to evaluate the structural changes of a commercial sample of high-amylose maize starch (HS) as a consequence of melt-processing and ultrasound treatment. Melt-processing was carried out at the same conditions of temper-

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ature and rotation speed, with glycerol as plasticiser at different HS/glycerol ratios. Ultrasound treatment was performed at the same conditions on aqueous dispersions of previously melt-processed samples. No change in the chemical structure was detected by 1H NMR spectrometry for the samples submitted to melt-processing and ultrasound treatment. 1H NMR clearly showed the deaggregating effect of ultrasound. Slow drying probably had an important role on amylose retrogradation, as observed by X-ray diffraction. The highest reduction in the viscosimetric-average molar mass, after melt-processing and after ultrasound treatment, was observed for the sample processed with the highest content of glycerol. Within the relatively low range of values for torque and temperature reached by this mixture, it may be concluded that glycerol influenced the reduction in molar mass caused by the thermomechanical process. Viscosity measurements revealed a significant reduction in amylose molar mass achieved by sonication. A better solubility in water, promoted by the deaggregating action of ultrasound radiation, may explain the strongest effect of ultrasound waves on the sample with the lowest molar mass. Acknowledgements The authors thank Conselho Nacional para o Desenvolvimento Científico e Tecnológico (CNPq) and Fundação de Amparo à Pesquisa do Estado do Rio de Janeiro (FAPERJ) for financial support. References [1] W.R. Morisson, B. Laignelet, An improved colorimetric procedure for determining apparent and total amylose in cereal and other starches, J. Cereal Sci. 1 (1983) 9–20. [2] C.J. Slattery, I.H. Kavakli, T.W. Okita, Engineering starch for increased quantity and quality, Trends Plant Sci. 5 (2000) 291–298. [3] S.E. Case, T. Capitani, J.K. Whaley, Y.C. Shi, P. Trzasko, R. Jeffcoat, H.B. Goldfarb, Physical properties and gelation behavior of a low-amylopectin maize starch and other high-amylose maize starches, J. Cereal Sci. 27 (1998) 301–314. [4] W. Blaszczak, J. Fornal, V.I. Kiseleva, V.P. Yuryev, A.I. Sergeev, J. Sadowska, Effect of high pressure on thermal, structural and osmotic properties of waxy maize and Hylon VII starch blends, Carbohydr. Polym. 68 (2007) 387–396. [5] J.J.G. van Soest, P. Essers, Influence of amylose – amylopectin ratio on properties of extruded starch plastic sheets, J. Macromol. Sci. – Pure Appl. Chem. A34 (1997) 1665–1689. [6] M. Aliyu, M.J. Hepher, Effects of ultrasound energy on degradation of cellulose material, Ultrason. Sonochem. 7 (2000) 265–268. [7] S. Baxter, S. Zivanovic, J. Weiss, Molecular weight and degree of acetylation of high-intensity ultrasonicated chitosan, Food Hydrocoll. 19 (2005) 821–830. [8] M. Cameron, L. McMaster, T. Britz, Electron microscopic analysis of dairy microbes inactivated by ultrasound, Ultrason. Sonochem. 15 (2008) 960–964. [9] C.T. Andrade, L.A.M. Barros, M.C.P. Lima, E.G. Azero, Purification and characterization of human hemoglobin: effect of the hemolysis conditions, Int. J. Biol. Macromol. 34 (2004) 233–240.

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