Physicochemical and thermal properties of gamma-irradiated sago (Metroxylon sagu) starch

doi:10.1016/j.radphyschem.2014.12.003

Radiation Physics and Chemistry

Volume 109, April 2015, Pages 48-53

https://doi.org/10.1016/j.radphyschem.2014.12.003 Get rights and content

Highlights

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Properties of irradiated sago starch at doses 0, 6, 10 and 25 kGy were studied.
•
Apparent amylose content and swelling power was reduced by irradiation
•
Irradiation decreased relative crystallinity but did not alter the crystalline type.
•
SEM of irradiated starch granules up to dose of 25 kGy showed no physical damage.
•
Thermal properties of irradiated sago starch were slightly affected.

Abstract

Sago (Metroxylon sagu) starch was exposed to gamma-irradiation in air at doses 6, 10 and 25 kGy. Subsequent changes in the physicochemical and thermal properties were measured. The apparent amylose content and swelling power of irradiated sago starch was significantly reduced while reducing sugars and starch solubility were significantly increased due to degradation. X-ray diffraction studies showed that radiation did not affect the crystalline type but induced a decrease in the degree of crystallinity, indicating the destruction of the ordered distribution of neighboring polysaccharide chains in the starch granules, in particular of the amylopectin component, which is responsible for starch crystallinity. Differential scanning calorimetry (DSC) of irradiated sago starch showed a small but significant increase in the onset and peak transition temperatures at 10 and 25 kGy dose; the conclusion temperature and gelatinization enthalpy was not affected. SEM and particle size analysis produced no evidence of physical damage to sago starch up to 25 kGy dose radiation treatment since the granular appearance and size distribution was retained.

Introduction

Sago starch is obtained from the trunk of sago palm (Metroxylon spp.) through the process of extraction and purification. It is an important agricultural commodity of Malaysia, ranking fifth highest in terms of agricultural revenue after pepper, palm oil, cocoa and rubber (Abd–Aziz, 2002). Malaysia is the world’s largest exporter, exporting about 44,000 t per year of sago products to countries including Japan, Europe, America and Singapore. Apart from its use in food, sago starch can also be utilized to produce adhesives for paper, textiles, and plywood; as stabilizer in pharmaceuticals and converted using acid/enzyme to produce glucose, high fructose syrup, monosodium glutamate, etc. New and potential applications of sago could be in the production of biodegradable plastics, bio-fuel and ethanol. However, application of sago starch in bioconversion is limited because of high paste viscosity and resistance of the raw granule to enzyme digestion.

Like other native starches, sago starch needs to be modified to obtain the required functional properties to meet industrial needs and extend its range of potential applications. Research on sago starch modification has been reviewed by Singhal et al. (2008). Chemical modification by hydroxypropylation cross-linking and acetylation loosen the granular structure, lower gelatinization enthalpy and increases the thermal stability of sago starch (Aziz et al., 2004). Zainal et al. (2005) synthesized carboxymethylated sago starch which exhibit excellent dispersibility and cold-water solubility. Enzyme-modified sago starch with a higher amount of linear long-chain dextrin suitable for use as coating for fresh fruits and vegetables, thereby inhibiting browning has also been studied by Wong et al. (2007). At present, limited studies have been reported on the ionizing irradiation of sago starch. Pimpa et al. (2007) observed changes in some of the physicochemical properties of electron beam irradiated sago starch while Mohd Adzahan et al. (2009) evaluated the pasting and leaching properties using different radiation techniques.

Ionizing radiation (gamma, X-ray or electron beam) is a physical treatment which has been used to modify starch through cross-linking (Nagasawa et al., 2004) and degradation (Sokhey and Hanna, 1993). Gamma radiation generates free radicals which can induce molecular changes and fragmentation of starch. Degradation of both the amylose and amylopectin occur by random cleavage of the glycosidic chains, to yield smaller fragments of dextrin that may either be electrically charged or uncharged (Ciesla et al., 1991).

The effect of gamma-irradiation on starch properties and food quality has been of interests to food processors. Research has been reported on irradiation of starches of rice (Yu and Wang, 2007), wheat (Ciesla and Eliasson, 2003), maize (De Kerf et al., 2001) and potato (Ciesla and Eliasson, 2002). The properties of irradiated starches have been reviewed by Tomasik and Zaranyika (1995) and Sokhey and Hanna (1993). The effect of irradiation varies with the botanical origin of starch. Crystallinity of irradiated starch increases with increasing radiation dose in wheat starches (MacArthur and D’Appolonia, 1984) and decreases in potato starch (Ciesla and Eliasson, 2002). Retrogradation tendency was lower in irradiated Lasco triticale but higher in Grana wheat starches at 2 kGy dose (Bachman et al., 1997).

The present study was conducted to determine the effects of gamma-irradiation on sago starch that could possibly impart different functional properties for new applications. Here we report the physicochemical and thermal changes induced in sago starch by gamma-irradiation at doses of 6, 10 and 25 kGy.

Section snippets

Materials

Sago (Metroxylon sagu) starch was purchased from commercial sago supplier in Sarawak (as fine white powder, food grade). The moisture content determined using oven-drying at 105 °C until constant weight was 13.14±0.2 g/100 g which is within the typical range for commercial sago starch. The sample was used as received without further treatment.

Irradiation of sago starch

Sago starch (50 g) was sealed in polyethylene bags (13 cm×13 cm) and gamma irradiated in air at Synergy Health plc radiation facility, Swindon, England.

Effect of irradiation on granule morphology and size

SEM micrograph (Fig. 1) shows that native sago granules are predominantly ovoid with some having a spherical shape, but a significant property is the truncated end. Irradiation up to a dose of 25 kGy showed no evidence of physical damage since the smooth granular appearance is retained, with no splitting/fissures observed. Since granule size contributes to starch pasting and retrogradation properties, the changes in sago granule size with irradiation were measured. Fig. 2 displays a unimodal

Conclusion

The physicochemical properties of sago starch were affected by gamma-irradiation, the extent of changes depended on dose levels. The apparent amylose content and swelling power of irradiated sago starch was significantly reduced while reducing sugars and starch solubility increased due to degradation. Thermal properties, onset and peak temperatures, were slightly changed. Radiation induced a decrease in the relative crystallinity but did not modify the crystalline type. However, there was no

Acknowledgements

The authors thank the Ministry of Science, Technology and Innovation (MOSTI), Malaysia for providing the financial support and to Synergy Health plc, Swindon, England for conducting the irradiation of sago starch.

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Physicochemical and thermal properties of gamma-irradiated sago (Metroxylon sagu) starch

Highlights

Abstract

Introduction

Section snippets

Materials

Irradiation of sago starch

Effect of irradiation on granule morphology and size

Conclusion

Acknowledgements

J. Biosci. Bioeng.

Carbohydr. Polym.

Carbohydr. Polym.

Int. J. Biol. Macromol.

Radiat. Phys. Chem.

Radiat. Phys. Chem.

Carbohydr. Res.

Int. J. Pharm.

Food Chem.

Carbohydr. Polym.

Carbohydr. Res.

Carbohydr. Polym.

Food Res. Int.

Food Chem.

Effect of γ-irradiation on some physicochemical and thermal properties of cowpea (Vigna unguiculata L. Walp) starch

Food Chem.

Hydroxypropylation and acetylation of sago starch

Malays. J. Chem