Effects of sodium dodecyl sulphate and sonication treatment on physicochemical properties of starch
Introduction
Starch is an important food ingredient and forms a major constituent of the human diet. Native starch exists as a granular structure and is composed of amylose and amylopectin arranged in amorphous and crystalline regions. Starch granules also contain minimal quantities of minor components like proteins, lipids, pentosans, and minerals (Morrison, 1995). The interactions of these minor components with amylose and amylopectin can influence the properties and functional behaviour of the starch.
Starch granule-associated proteins (SGAPs) are defined as the proteins biologically distinct from plant storage proteins and are tightly bound in and on starch granules (Baldwin, 2001). These protein are mainly starch biosynthetic enzymes and have a molecular weight around 5000–149,000, and possess different polypeptide species such as prolamin, 2S albumin and globulin. The presence of starch granule surface-proteins has been reported in maize starch (Imam, 1989), barley starch (Prentice & Stark, 1992), and in mung bean starch (Oates, 1990). Among the main commercial starches, cereal starches (wheat, maize, barley, and rice) contain more protein (0.25–0.6%, w/w) than tuber (potato, 0.06%), and root (tapioca, 0.1%) starches (Debet & Gidley, 2006). The nature of protein/starch granule interactions is not well characterised but most of the surface-proteins are believed to be adsorbed onto the surface of the starch granule (Baldwin, 2001).
In the enzymatic hydrolysis of starch to produce glucose syrup, the presence of protein layer on the surface would presumably restrict the access of the enzyme, thus reducing the degree of hydrolysis. The removal of the surface protein can enhance the accessibility of enzyme to the granule surface and interior of the granule. For this reason, a number of methods (e.g., salt treatment, rigorous extraction, use of ethanol) have been investigated to improve the hydrolysis of poorly hydrolysable starch granules (Debet & Gidley, 2006). The sodium dodecyl sulphate (SDS) treatment is one of the methods, which can efficiently remove the surface protein of starch granules (Eerlingen, Cillen, & Delcour, 1994).
When a liquid is subjected to the action of ultrasound, rapidly collapsing cavitation bubbles induce high pressure gradients and high local velocities of liquid layers in their vicinity. This in turn may cause shear forces that have no significant influence on small molecules, but are capable of breaking the chains of polymers, provided the chains are longer than a certain limiting value. This is the mechanochemical action of ultrasound on polymers (Czechowska-Biskup, Rokita, Lotfy, Ulanski, & Rosiak, 2005). Wang and Wang (2004) reported that the combination of protease with high-intensity ultrasound greatly improved starch yield to 79.8–86.7%, compared to 62.5–71.8% with protease alone. Even more effective was a combination of high-intensity ultrasound with 0.5% SDS which increased starch yield to 85% with 0.2% residual protein.
Several studies have reported the effect of sonication (Iida, Tuziuti, Yasui, Towata, & Kozuka, 2008), SDS (Debet and Gidley, 2006, Radhika and Moorthy, 2008) or combination of SDS and protease (Wang & Wang, 2004) on the physicochemical properties of starch. However, very little data are available on the combined effect of SDS and sonication on the physicochemical properties of starch. Therefore, the main objective of this study was to evaluate the effects of sodium dodecyl sulphate (SDS), sonication and combination of both treatments on the removal of starch granule-associated proteins of corn and mung bean starch because it contained relatively high protein content. Potato and sago starch were used as comparison because these starches contained very little amounts of protein content. The effect of these treatments on the physicochemical and functional properties of the starches would provide more insight into the possibilities of exploiting these treatments for starch modifications.
Section snippets
Material
Corn, potato and sago starch were procured from the Sims Company Sdn. Bhd, Penang, whereas mung bean flour was from the Pearl Island Packaging Sdn. Bhd, Penang.
Sodium dodecyl sulphate and sonication treatment
Starch (250 g, 40% w/v) was suspended in 625 ml SDS solvent (2% w/v) at room temperature. The starch suspension was stirred for 30 min using a magnetic stirrer and centrifuged (Kubota 5100, Kubota Corp., Tokyo, Japan) at 2328g for 15 min. The supernatant was carefully removed. The pellet was washed three times, re-suspended with distilled
Results and discussion
In the following discussion, the term “native starch” refers to the starch that has not undergone any form of chemical treatment. “SDS-treated starch” refers to starch that has undergone 30 min treatment with SDS. “SDS + sonication starch” refers to starch that has undergone 30 min treatment with SDS and subsequently subjected to 10 min sonication. “Sonicated starch” or “sonication starch” refer to starch that was stirred with distilled water for 30 min at room temperature and subsequently subjected
Conclusion
Four types of selected starches of different botanical origin exhibited different physicochemical properties after SDS and sonication treatment. All starches showed a significant increment in solubility and the overall change in the pasting profile for the treated starches was a reduction in pasting temperature, pasting time accompanied with increase in peak viscosity, except for potato starch. SEM analysis revealed that SDS did not cause major damage to the structure but sonication changed the
Acknowledgements
This work was funded by a Science Fund Grant from the Ministry of Science, Technology, and Innovation (Project No. 05-01-05-SF0347). One of the authors (H.T. Chan) acknowledges the fellowship and postgraduate research grant awarded by the Universiti Sains Malaysia.
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