Scholarly article on topic 'Utilization of Corn Hominy as a New Source Material for Thermoplastic Starch Production'

Utilization of Corn Hominy as a New Source Material for Thermoplastic Starch Production Academic research paper on "Agriculture, forestry, and fisheries"

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Abstract of research paper on Agriculture, forestry, and fisheries, author of scientific article — E.S. Iriani, T.C. Sunarti, N. Richana, D. Mangunwidjaja, A. Hadiyoso

Abstract Corn hominy or ampok (in local language) is a by-product from corn milling industry. It is about 35% in weight of dry milled kernel. Corn hominy contains a small amount of fibre (7.26%), protein (10.2%), fat (8.12%), and a large amount of starch (63.21%) which could be used as an alternative source for making starch based thermoplastics (STP), hence reducing the use of starch. The aim of this research is to investigate the effects of weight ratio of tapioca to corn hominy as well as the glycerol content in formula compositions on mechanical properties, i.e. tensile strength and elongation at break and thermal properties (Tg and Tm) of corn hominy based thermoplastic. The ratio of tapioca to corn hominy composition varied from 0:100; 25:75; 50:50; 75:25 and the glycerol content varied from 25% to 35 wt%. The blends were melt processed using counter rotating twin screw mixer with the barrel temperature of 120, 130 and 120°C at 60rpm for 5min. It has been observed that using corn hominy as a sole material for making thermoplastic was not feasible. The tensile strength, elongation at break, and glass transition point (Tg) were greatly affected by increasing the tapioca content. Thermoplastics starch produced by extruder was then pressed by a hydraullic press, and this showed good characteristics which signaled by homogenous surface of SEM images. The surface morphology of thermoplastic starch was unchange by increasing the glycerol content and corn hominy ratio. However, higher corn hominy ratio gave smoother surface morphology at the cross section of thermoplastic starch.

Academic research paper on topic "Utilization of Corn Hominy as a New Source Material for Thermoplastic Starch Production"

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Procedía Chemistry 4 (2012) 245 - 253

Utilization of Corn Hominy as a New Source Material for Thermoplastic Starch Production

E.S. Iriania'*, T.C. Sunartib, N. Richanaa, D. Mangunwidjajab, A. Hadiyosob

aIndonesian Center for Agricultural Postharvest Research and Development, Jl. Tentara Pelajar No. 12, Bogor 16114, Indonesia hDepartment of Agroindustrial Technology, Faculty of Agricultural Technology, Bogor Agricultural University,

Darmaga, Bogor 16680, Indonesia

Abstract

Corn hominy or ampok (in local language) is a by-product from corn milling industry. It is about 35% in weight of dry milled kernel. Corn hominy contains a small amount of fibre (7.26%), protein (10.2%), fat (8.12%), and a large amount of starch (63.21%) which could be used as an alternative source for making starch based thermoplastics (STP), hence reducing the use of starch. The aim of this research is to investigate the effects of weight ratio of tapioca to corn hominy as well as the glycerol content in formula compositions on mechanical properties, i.e. tensile strength and elongation at break and thermal properties (Tg and Tm) of corn hominy based thermoplastic. The ratio of tapioca to corn hominy composition varied from 0:100; 25:75; 50:50; 75:25 and the glycerol content varied from 25% to 35 wt%. The blends were melt processed using counter rotating twin screw mixer with the barrel temperature of 120, 130 and 120 °C at 60 rpm for 5 min. It has been observed that using corn hominy as a sole material for making thermoplastic was not feasible. The tensile strength, elongation at break, and glass transition point (Tg) were greatly affected by increasing the tapioca content. Thermoplastics starch produced by extruder was then pressed by a hydraullic press, and this showed good characteristics which signaled by homogenous surface of SEM images. The surface morphology of thermoplastic starch was unchange by increasing the glycerol content and corn hominy ratio. However, higher corn hominy ratio gave smoother surface morphology at the cross section of thermoplastic starch.

© 2012 Published by Elsevier Ltd.

Keywords: Corn hominy; thermoplastic starch; biodegradable plastic.

1. Introduction

Our dependence on plastic in our daily life is very high, almost 100 million tons annually [1]. Plastic which is derived from petroleum has some future constraints due to limited resource and difficulty in

* Corresponding author. Tel.:+62-812-911-6088; fax:.+62-251-8321762 E-mail address: evi_savitri@litbang.deptan.go.id

1876-6196 © 2012 Published by Elsevier Ltd. doi: 10.1016/j.proche.2012.06.034

degrading so it can cause environmental problems. In addition, the migration of monomers from the plastic packaging into food products can cause various health problems. In recent years, many researches have tried to solve this problem. Significant progress has been achieved in the development of biodegradable products based on agricultural materials [2,3]. One of the most studied and promising raw materials for the production of biodegradable plastics is starch and cellulose which are natural renewable carbohydrate polymer obtained from a great variety of crops. Starch and lignocellulose are low cost material in comparison to most synthetic plastics. Starch has been investigated widely for the potential manufacture of products such as water soluble pouches for detergents and insecticides, flushable liners and bags, and biomedical delivery systems and devices [4]. Lignocellulose is usually used as a reinforcement or filler for composite polymer. Lignocellulosic fibres are strong, light in weight, abundant, non-abrasive, non-hazardous and inexpensive. They can serve as an excellent reinforcing agent for plastics[5].

Native starch commonly exist ingranular structure with about 15-45% of ciystallinity. During extrusion, starch granules are exposed to high temperature and shear and undergo structural changes such as gelatinization, melting and fractionation. The ability to process starch and the resulting physical properties depend on the extent of structural changes of the starch. During gelatinization, starch molecules are released from the granule structure [6]. The addition of a plasticizer to gelatinized starch allows free starch molecules to behave in a similar fashion to common thermoplastic synthetic polymers [7].

The most commonly plasticizers are polyols (glycerol, sorbitol, etc.) that allowgood destructuration of the starch but they usually induce recrystallization phenomena (or retrogradation) after being stored for a period time because these molecules are small and can easily separate from the starch macromolecular chains [8]. Starch retrogradation can also cause thermoplastic starch to become rigid and brittle [9]. Thermoplastics starch exhibits many disadvantages such as strong hydrophilic character (water sensitivity) and poor mechanical properties compared to conventional polymers [10], which make it unsatisfactory for applications such as packaging materials.

Generally, there are two approaches to mitigate these shortcomings. One approach is to blend starch with biodegradable polymers such as poly lactide, polycaprolactone and poly(propylene carbonate), poly(vinyl alcohol) [11,12]. However, these polymers are very expensive, and starch is immiscible with them at molecular level. The other approach is the reinforcement of thermoplastic starch with mineral, cellulose or starch nanocrystal fillers. A significant improvement in mechanical properties and water resistance of thermoplastic starch has been reported by such reinforcement. Mineral fillers such as kaolin [13] and clay [14, 15] with polar groups were easily associated with thermoplastic starch. Cellulose fillers used as reinforcing agents in thermoplastic starch matrices include cellulose nanocrystals [16], natural fibres [17], cellulose derivatives [18] and commercially regenerated cellulose fibres [19]. The chemical similarities of starch and plant fibres provide a good interaction [20].

Today, many research focused on the use of fibres as reinforcement for thermoplastic starch. The fibres described in the literature for this intention are cellulose micro-fibres [21], natural fibres [22] and commercial regenerated cellulose fibres [19]. Unlike biodegradable polyester, when natural fibres are mixed with thermoplastic starch, their mechanical properties improved significantly, the chemical similarities of starch and of plant fibres providegood compatibility. A significant improvement in water resistance is obtained by adding commercial cellulose fibres [19] or micro-fibre [23]. This behavior is related to the high crystalline 'hydrophobic' character of the cellulose fibres in comparison to starch hydrophilic property. In addition, these authors [21] demonstrated an improved thermal stability due to a higher and longer thermal resistance of cellulose fibres.

Corn hominy or ampok is a by- product of corn milling that contains a number of compounds such as starch (57%), fibre (25%), protein (11%), and fat (5%) [24]. The starch and fibre content is still quite high which allows it to be used as raw materials for bioplastic. The fibre in corn hominy feed can be utilized as

a filler to improve the properties of thermoplastic product. Thus in addition to being inexpensive, non-toxic, and easily recycled, the use of this material contributes to environmental protection [25]. The presence of fibres during biodegradation induces the fast breakdown of foam due to the action of microorganisms attracted by its lignocellulosic components [26].

Utilization of corn by-product such as corn hominy for biodegradable polymer, will provide greater added value and also decrease the enviromental problem. The objective of this research is to examine the effects of different composition ratio between tapioca and corn hominy and added glycerol to mechanical properties, i.e. tensile strength and elongation at break, morphological structure and thermal properties (Tg and Tm) of corn hominy based thermoplastic.

2. Experiment

2.1. Material and equipment

The main material in this research is corn hominy which was gathered from PT Kediri Matahari Prima, a corn flour milling company in East Java, and tapioca starch obtained from PT Budi Makmur Perkasa, a local tapioca industry in Subang. Glycerol was used as a plasticizer while magnesium stearate was purchased from Sigma Aldrich. The equipments used in this research were rheocord mixer (rheomix) 3000 HAAKE with 200-250 g in capacity, microwave (Sharp R-8720M 1000 W), universal testing machine (UTM) Instron, scanning electron microscope (SEM) Zeiss type Evo50, differential scanning colorymeter (DSC) Shimadzu TA-50WSI, centrifuge, digital scale, erlenmeyer, and other laboratory sets.

Table 1. Formula to produce corn hominy based thermoplastic material

Sample (w/w)

Code Corn hominy (mc 25% ) Tapioca (mc 25% ) Glycerol MgSt*

A1B1 100 00 25 2

A2B1 075 25 25 2

A3B1 050 50 25 2

A4B1 025 75 25 2

A1B2 100 00 30 2

A2B2 075 25 30 2

A3B2 050 50 30 2

A4B2 025 75 30 2

A1B3 100 00 35 2

A2B3 075 25 35 2

A3B3 050 50 35 2

A4B3 025 75 35 2

2.2. Sample preparation and characterization

The preliminary research was conducted to rehydrate and reduce the size of the starch and corn hominy to obtain a suitable moisture content and mesh size (200 mesh). The tapioca starch and corn hominy was then analyzed for their characteristic such as moisture content, ash, fat, fibre, protein, and carbohydrate content. Other analysis were amylose and amylopectin content of tapioca starch and corn hominy. The experiment was conducted at the Indonesia Center for Agricultural Postharvest Research and

Development (ICAPRD) and the Agroindustrial Technology Department, Bogor Agriculture University Laboratory.

2.3. Extrusion process for producing thermoplastic starch

The extrusion process were conducted to find out the optimum condition process for thermoplastic product. The combination formula used to produce TPS is shown in Table 1.

Thermoplastic starch was produced by mixing the tapioca starch, corn hominy, glycerol and aquadest using high speeed mixer for 5 min. The samples were then stored is sealed plastic bags for 24 h at 50 °C. After that, the samples were then fed into the rheocord mixer (rheomix) 3000 HAAKE at 60 rpm. The temperatures were set at three different barrel temperatures, i.e. 120-130-120 °C for 5 min. The thermoplastic material produced from this extrusion process was agglomerated so it must be cut into small pieces or pelletized. The pellets were then compressed by hydraulic press at 160 °C for 20 min. The sheets produced were then cut into dumbell shape for mechanical testing.

2.4. Mechanical properties

The tensile tests were performed with an Instron 5569 universal test instrument equipped with a load cell of 200 kgf. The samples, previously conditioned at 53% relative humidity between 24 °C and 25 °C for 10 d, were tested in accordance with the ASTM D638M-96 type II requirements, using a crosshead speed of 50 mm min-1. The tensile modulus was calculated by the instrument software using the slope of the initial portion of the stress strain curves. The mechanical tensile data was averaged over at least five specimens.

2.5. Scanning electron microscopy (SEM)

The morphology of thermoplastic starch pellet were investigated by scanning electron microscopy (SEM) Zeiss type Evo 50. Pellet samples were cut horizontally and the surface was coated with a thin gold film.

2.6. Thermal analysis

Differential scanning calorimetry was performed with a Shimadzu TA-50WSI calorimeter calibrated with indium and zinc. An empty pan was used as a reference. Samples were cut from the plates in circular form and weighed accurately (20 mg) into aluminum pans. Measurements were carried out under a nitrogen flow, from 25 to 300 °C, at a heating rate of 10 °C min-1.

3. Results and Discussion

3.1. Proximate characteristic

The result of proximate analysis of corn hominy and tapioca starch is shown in Table 2. Based on these results, the aquadest was added depending on tapioca and corn hominy ratio to get moisture content of 25%.

Composition of tapioca and corn hominy listed in Table 2 showed that corn hominy has higher fibre, protein and fat content compared to tapioca. On the other hand, tapioca has higher carbohydrate and amylose content than corn hominy. This different condition will affect the melting and rheology

properties and also thermal properties of the thermoplastic produced. Ratio of amylose to amylopectin will have an influence on the final material. Amylose and amylopectin recrystallize in different ways. Amylose will form the more crystalline part of a thermoplastic while amylopectin the amorphous component and consequently will affect the properties of the thermoplastics produced [27].

Table 2. Proximate analysis of corn hominy and tapioca starch

Component Tapioca starch (%) Corn hominy (%)

Moisture content 12.33 8.74

Ash 0.09 2.47

Fibre 1.11 7.26

Fat 0.17 8.12

Protein 0.48 10.2

Carbohydrate (by difference) 84.12 63.21

Amylose 42.4 20.9

Amylopectin 57.6 79.1

3.2. Extrusion process

The thermoplasticization process of starch during extrusion involves a complete and/or the partial destruction of the initial crystalline order. By applying mechanical shear stresses and heat in the presence of suitable plasticizers, the molecular chains gain mobility, favoured by the hydrogen bonding interactions with the plasticizer. The destructing process is usually accompanied by a variation of the rheological properties of the starch/plasticizer mixtures and it can be followed by monitoring the evolution of torque over time during the mixing in a Brabender chamber. When glycerol was used for the mixing experiments, the temperature set for the mixing chamber was usually lower than 120 °C.

In all experiments, the mixing time of 5 min was adequate, since in general the samples were completely destructured after 4 min. Longer periods led to some starch degradation, as suggested by the development of a light brown color. Increasing corn hominy ratio produced a darker thermoplastic starch as shown in Table 3. Thermoplastic starch which was produced from 100% corn hominy was very poor because the melting blends were not homogen, darker and brittle.

3.3. Mechanical properties

As shown in Figure 1a, the mechanical properties of thermoplastic starch blends without tapioca starch were very poor, especially the tensile strength, and was described as unthermoplastic starch granules. Increasing tapioca starch ratio increased tensile strength value. On the other hand, increasing glycerol seem to decrease the tensile strength of the blends. Tensile strength depends on many factors such as granule size, minor component of starch (fat, protein and fibre) [28]. This condition was contradictory with the statement that said that fibre gave significant effect on physical characteristic of starch based biocomposite [29].

The elongation of thermoplastic starch will increase with the increasing of tapioca starch ratio and glycerol content as showed on Figure 1b. Increasing plasticizer content brings about a decrease in the tensile strength of thermoplastic starch, whereas the elongation at break increases. Starch is a natural polymer containing numerous hydrogen bonds between the hydroxy groups in its molecules, and so it manifests substantial tensile strength values. Glycerol, sorbitol, or glycol behave as diluents and decrease the interaction between molecules, and consequently they diminish tensile strength. At the same time they

act as plasticizers, which improves macromolecular mobility and leads to a rise in elongation at break [29,30].

Table 3. Thermoplastic starch visual appearance

Sample Codes Mixture Appearance Color and Flexibility

A1B1 Not homogenous Dark and Brittle

A2B1 Homogenous Dark and Flexible

A3B1 Homogenous Dark and Flexible

A4B1 Homogenous Light and Flexible

A1B2 Not homogenoous Dark and Brittle

A2B2 Homogenous Dark and Flexible

A3B2 Homogenous Dark and Flexible

A4B2 Homogenous Light and Flexible

A1B3 Non homogenous Dark and Brittle

A2B3 Homogenous Dark and Flexible

A3B3 Homogenous Dark and Flexible

A4B3 Homogenous Light and Flexible

The increase in elongation at break with increasing plasticizer content occurs only for some ranges of glycerol content. There was a reduction in the elongation at break with glycerol addition of more than 35%. The molecular interactions become weak by excess of glycerol because some interactions between starch molecules are replaced by interactions between glycerol and the starch molecules [31-35].

Fig. 1. Effect of tapioca and glycerol content to (a) tensile strength and (b) elongation at break of thermoplastic starch

3.5. Scanning electron microscopy (SEM)

The SEM morphologies of thermoplastic starch pellet was investigated by SEM Zeiss type Evo50 which showed an homogeneous surface, indicating that the starch granules were completely disrupted. All the thermoplastic materials produced from all of the treatments showed a smooth surface. It seems that corn hominy, tapioca and glycerol concentration did not effect the surface morphology. When samples were investigated by cross section, there were some differences between the different treatment. SEM images of cross section showed that the images were not as smooth as surface images. This condition was probably caused by surface section that got more pressure and heat from the plate when moulded by

hydraullic press. There were some starch granules that were not fully disrupted. Higher ratio of corn hominy gave smoother surface than tapioca, meanwhile glycerol content did not give the same effect. These results were on contrast with Matthew and Dufresne [36] that if the plasticizer content is increased there will be a strong decrease in melt viscosity, which made the plasticization of starch difficult, due to the decrease in the shear during processing.

Fig. 2. SEM images of thermoplastic starch at surface (top) and cross section (bottom)

3.6. Thermal analysis

Differential scanning calorimetry was performed with a Shimadzu TA-50WSI calorimeter calibrated with indium and zinc. The search for a clear-cut glass transition temperature feature in the DSC tracings was fruitless for A4B1 and A4B3 treatment. In addition the melting point (Tm) of A1B1 and A4B1 was also not detected. Determination of glass transition temperatures of thermoplastic starch by DSC is a hard task, as a result of the fact that the starch is made up of two polysaccharides: amylose and amylopectin. Depending on the starch composition, DSC thermograms can show higher or lower Tg values, or both at the same time. The higher and lower phases were detected in most cases in blends containing less than 30% glycerol [37].

Table 4. Transition glass (Tg) and melting point (Tm) of corn hominy based thermoplastics

Sample Code Tg-1 Tg-2 T J m

A1B1 45.27 99.85 -

A4B1 59.86 - 156.86

A1B3 43.83 102.4 -

A4B3 59.13 - 160.84

As showed in Table 4 and Figure 3a, sample without tapioca starch has two glass transitions, while samples containing more tapioca starch ratio (A4B1 and A4B3) only has one glass transition. This behavior is likely due to the formation of starch rich or glycerol rich regions, which induces partial phase separation. In Table 4, it was also found that treatment A1B1 and A1B3 which contained no tapioca starch

did not have Melting Temperature (Tm). This condition may be caused by high fibre content in corn hominy that could not be melted unlike starch. Melting temperature of A4B1 and A4B3 ranged from 156160 °C. This results showed an agreement with the previous differential scanning calorimetry study that onset melting points were detected at a range of 160-180 °C [38].

Fig. 3. DSC Thermogram (a) A1B1 and (b) A4B3

Colonna and co-workers found two thermal transitions for TPS materials at 25-30% glycerol, of which the low temperature was attributed to glycerol relaxation [39]. Meanwhile Kalichevsky et al. also reported a substantial degree of phase separation at high plasticizer content in amylopectin sugar mixtures[40].

A large amount of plasticizer can induce increased mobility of starch chains and lowers the glass transition temperature. As shown in Table 4, sample A1B3 (glycerol 35%) had a lower Tg (43.83 °C) compare to A1B1 (glycerol 25%) with Tg 45.27 °C. Zeleznak and Hoseney found that the glass transition temperature of wheat starch with 13-18.7% moisture varies between 30 and 90 °C and that Tg is likely to be lower than room temperature if the starch humidity increases to above 20% [41]. Van Soest et al. determined a Tg value of 5 °C for extruded potato starch with 14% moisture content, whereas at higher moisture content the Tg could not be determined [42].

4. Conclusion

Corn Hominy could be utilized as a potential raw material for thermoplastic product because it was cheap, abundant, biodegradable, renewable and can improve some thermoplastic properties. Increasing tapioca ratio will increase tensile strength and elongation at break. Meanwhile increment of glycerol content will increase elongation but decrease tensile strength. Thermoplastic materials produced from tapioca and ampok mixture showed a good result signed by homogenous surface of SEM images. Thermal properties of thermoplastics product showed that increasing the glycerol content will lower the transition glass of thermoplastic product, but increasing ampok ratio will induce non homogenous mixture as shown by two transition glass points.

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