Thermoplastic Starch Production

Disruption or melting of granular starch can be accomplished by a process predominantly regulated by thermal energy input, such as film casting (essentially heat application with low mechanical input), or regulated by the combination of thermal and mechanical energy input, such as conventional plastic processing techniques (e.g., extrusion, kneading, and injection molding). Extrusion is the most common industrial process used. Because melting temperature (220–240ºC) is higher than the degradation temperature range of starch (200–220ºC, approximately), some plasticizer is necessary in order to process granular starch (Shogren, 1993). Plasticizers are low molecular weight substances used to enhance the flexibility and processability of a polymeric compound by decreasing the hydrogen bonding of the polymeric chains, which leads to an increasing free volume or molecular mobility of polymers. Hence, the properties of TPS mainly rely on the hydrogen bond-forming abilities between plasticizers and starch molecules. The amount and type of plasticizer strongly influence the physical properties of the processed starch by controlling its destructuration and depolymerization and affecting the final properties of the material, such as its glass transition temperature (Tg) and elastic modulus (Avérous, 2004).

Water is a primary plasticizer of starch. In addition, the most commonly used plasticizers are polyols; mono-, di-, or oligosaccharides; fatty acids; lipids; and derivatives. Many studies have demonstrated the plasticization effect of water on starches and also the various techniques for analyzing Tg have been compared (Zeleznak and Hoseney, 1987; Kalichevsky et al., 1992; Shogren, 1993; Mathew and Dufresne, 2002). On a molecular level, moisture-induced plasticization of a polymer leads to increased intermolecular distances (free volume), decreased local viscosity, and increased backbone chain segmental mobility (Slade and Levine, 1991). The addition of low molecular weight plasticizers to an amorphous matrix has basically the same effect as increased temperature on molecular mobility. Due to the hydrophilic nature of starch, the water content of starch-based materials is dependent on air relative humidity (RH) during processing and storage, which, in turn, directly affect their physical properties. The water content of starch increases from 7% to 24% (w/w), when the relative humidity increases from 20% to 90% (w/w), whereas a decrease in material Tg from 140ºC to 18ºC was observed (Shogren, 1993). Similar behavior was observed in relation to Tm; its value dropped from 126ºC to 72ºC, as the water content of corn starch increased from 10% to 60% (Souza and Andrade, 2002).

Glycerol has also been extensively employed as a plasticizer for obtaining TPS materials. The presence of small amounts of glycerol can promote a classic antiplasticizing effect on starch films (Forssell et al., 1997; Avérous et al., 2000; Altskär et al., 2008). Due to strong interaction between glycerol and starch, a hydrogen-bonding network is formed and a reinforced material is obtained. As the plasticizer contents increase, interactions between plasticizer and starch become stronger, resulting in swelling and a plasticization effect (Lourdin, Bizot, and Colonna, 1997; Chang, Karima, and Seow, 2006). It was also observed that 20% (w/w) glycerol seems to be the maximum that can act as a plasticizer. Above this percentage, phase separation occurs and, because glycerol is hygroscopic, the amount of adsorbed water increases as it binds to starch film as well as to “free’’ glycerol (Godbillot et al., 2006). At very high plasticizer content (above 50% w/w), starch materials become soft and behave more like a gel or paste.

Several other compounds have also been shown to be useful as plasticizers. In general, monohydroxyl alcohols and high molecular weight glycols failed to plasticize starch, whereas shorter glycols and sorbitol were effective (Gaudin et al., 1999; Mathew and Dufresne, 2002; Da Róz et al., 2006). The antiplasticizing effect is observed in compress molding of corn starch sheets plasticized with 1,4-butanediol, diethyleneoxide glycol, and sorbitol (Da Róz et al., 2006). Studies demonstrated that starch films obtained with plasticizers containing amide groups are less sensitive to aging effects than those plasticized with glycols. However, the former films present little internal flexibility, which negatively affects their mechanical properties (Ma and Yu, 2004).

The main difference between the conversion of starch granules into TPS by casting or by other conventional thermomechanical processes is the amount of water or plasticizer utilized during the gelatinization or melting of the granular starches.

A casting process is often employed to produce edible films for packaging and coating of food products (Liu, 2005). When a casting technique is used, starch is processed by heating under moderate shear in the presence of excess water or other plasticizer, which causes irreversible swelling of the granules. According to the classical gelatinization model of starch, swelling is accompanied by disruption of the native crystalline structure and solubilization of amylose. The structural changes that take place during gelatinization include simultaneous crystallite melting and doublehelix unwinding, absorption of water in the amorphous growth ring, changes in shape and size of granules, dispersion of blocklet-like structures, and leaching of amylose from the granule. In the last decade, due to the development of experimental techniques, several researchers have reported many studies on the gelatinization of starch and have put forward the classical starch gelatinization model (Jenkins and Donald, 1998; Vermeylen et al., 2006; Peng, Zhongdong, and Kennedy, 2007).

The mechanism of starch film formation depends, among several factors, on the solid concentration and amylose content. Generally, aggregation and packing of swollen granules dominate film formation of starch dispersions with a relatively high solid concentration. In relation to dilute starch solutions, the film formation follows the order of helical formation, aggregation or reorganization of aggregates; the former is primarily driven by cooling and the latter by dehydration (Liu and Han, 2005). Preparation conditions, such as temperature, relative humidity, heating period, shear rate, drying procedure, and composition, starch source, and plasticizers are important to the film structure.

In the processing of a TPS by extrusion, a common plastic processing technique, the temperature and shear stress are higher compared to those employed in a casting technique. In most cases, water content is lower than 20% w/w and this kind of processing leads to differences in starch melting due to water content, morphology of the materials, and differences in aggregation and crystallization behavior of the amylose and amylopectin compared to cast films.

During thermomechanical processing by extrusion, the first step is the compaction and heating of the starch granules. Then, granules are partially transformed at the same time by heat (internal granular disorganization) and by mechanical processing (granular fragmentation). Fragmentation itself induces only partial loss of crystallinity. Small fragments finally are melted due to a local temperature increase, contributing to interparticle friction. Susceptibility to fragmentation is related to the mechanical properties of starch granules associated with their architecture and botanical origin. This difference could be related to differences in granular organization, that is, the presence of brittle areas or defects (Barron et al., 2001). Under temperature and shearing, starch is destructured, plasticized, and melted, but also partially depolymerized (Avérous, 2004). The transformations of starch that occur during extrusion are influenced by extruder geometry as well as by processing variables such as extrusion temperature, screw speed, feed rate, and moisture content (Lai and Kokini, 1991; Souza and Andrade, 2002).

In general, the processing of starch into TPS is accompanied by a decrease in molecular weight, although the depolymerization is significantly less pronounced for the cast films (Altskär et al., 2008). It is accepted that, in the heating–shearing process, starch degradation increases as water content decreases and temperature or rotation speed increases (Chinnaswamy and Hanna, 1990; Silva et al., 2004). Nevertheless, some studies indicated that an optimum in breakdown occurs at a certain moisture content, leading to an inversion of the expected behavior (Govindasamy, Campanella, and Oates, 1996; van den Einde et al., 2004a). Although only the mechanical contribution associated with specific mechanical energy has been accounted for in the evaluation of starch molecular breakdown during thermomechanical processing (Della Valle et al., 1995; Martin, Averous, and Della Valle, 2003; van den Einde et al. 2004b), it was suggested that thermal breakdown also plays an important role in this process (van den Einde et al. 2004b).

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