Modification of cereal and tuber waxy starches with radio frequency cold plasma and its effects on waxy starch properties
Introduction
Starch is a polymeric carbohydrate molecule harnessed for food and non-food use. It is the main energy reserve for most green plants and exists as semi-crystalline granules. Amylose and amylopectin are the two major polyglucans that constitute the starch granule. Both are made of chains of α-(1,4)-linked d-glucose units, interconnected through α-(1,6)-glycoside linkages, contributing to branches in the polymer (Vamadevan & Bertoft, 2015).
Starch in its native form has several drawbacks which limits their use in various applications. These drawbacks are mostly due to their unreactive nature, insolubility, retrogradation tendencies and inability to withstand high temperatures and shear during processing (Jayakody & Hoover, 2008). In order to overcome these challenges starch is modified by various means, including the use of chemicals and enzymes as well as physical processes in order to meet the huge market demands for starches with unique properties (Kaur, Ariffin, Bhat, & Karim, 2012). Physical methods of starch modification such as cold plasma technology, is currently being employed because it is considered a green technology devoid of the use of chemicals (Thirumdas, Kadam, & Annapure, 2017). Plasma, which is known as the fourth state of matter, is generated when energy (electric, magnetic, thermal, microwave or radiofrequency) is applied to a gas, resulting in the production of active species such as electrons, ions, free radicals and large numbers of unionized neutral molecules (Thirumdas, Kadam et al., 2017). Plasma is basically divided into high temperature plasma and low temperature plasma. In high temperature plasma all species involved in the plasma reaction are in thermal equilibrium, this is however not the case for low temperature plasma (Bogaerts, Neyts, Gijbels, & van der Mullen, 2002). Plasma chemistry is dependent on the type and composition of gases fed into the plasma unit, humidity, applied power and treatment time (Misra, Pankaj, Segat, & Ishikawa, 2016). The type of gases used may lead to the introduction of hydroxyls, ketones, aldehydes, esters and free radicals as in the case of carbon dioxide and argon gas plasmas (Desmet et al., 2009). Cold plasma has been applied in areas such as the inactivation of microbes (Moreau, Orange, & Feuilloley, 2008) and enzymatic inactivation (Misra et al., 2016).
With regards to starch modification, a low-pressure glow plasma treatment generated using air caused a decrease in molecular weight and radius of gyration of starches (Lii, Liao, Stobinski, & Tomasik, 2002). Wongsagonsup et al. (2014) also reported a reduction in molecular weight upon treatment of starch with a jet atmospheric argon plasma. Thirumdas, Saragapani, Ajinkya, Deshmukh, and Annapure (2016) reported a decrease in cooking time of rice and an increase in the rate at which amylose molecules leach during cooking after treatment with a low-pressure radio frequency cold plasma generated using air. Plasma treatment may either alter or have no effect on gelatinization temperatures and the enthalpy (ΔH) when measured by differential scanning calorimetry. Bie et al. (2016) reported a decrease in enthalpy and an increase in the temperature of gelatinization upon treatment of starches with an oxygen and helium glow plasma. However, Wongsagonsup et al. (2014) reported a decrease in gelatinization temperature. Cold plasma treatment has therefore been proven as an effective way of modifying starches. To the best of our knowledge, there is limited information with regard to the effect of a radio frequency cold plasma generated using a carbon dioxide-argon gas mixture on the properties of waxy starches. We hypothesize that radio frequency cold plasma will significantly alter the properties of cereal and tuber waxy starches. In this present study the morphological, pasting and thermal properties, iodine binding, resistant starch, starch damage, crystallinity, and changes in short range molecular order of waxy rice, maize and potato as affected by a carbon dioxide-argon radio frequency cold plasma treatment was investigated.
Section snippets
Materials
Waxy maize and waxy potato starch used for the experiment were obtained from Ingredion Incorporated (Bridgewater, NJ, USA). Waxy rice was obtained from Remy Industries (Belgium). Potassium iodide and iodine were obtained from Sigma Aldrich (St. Louis, MO, USA). CO2 and Ar gases were obtained from Matheson, Eagan, Minnesota. All chemicals used for the experiments were of analytical grade.
Plasma apparatus and treatment
The plasma unit connected to a vacuum pump, chilling unit and a radio frequency power (RF) supply with a
Results and discussion
The waxy starch samples were treated at a radio frequency power (RF) of 120 W with the native starches as controls. The samples were also exposed to the carbon dioxide-argon gas mixture under vacuum in the plasma unit without RF (0 W). The 0 W samples will be referred to as gas treated waxy maize, rice or potato starch (GTWMS, GTWRS, or GTWPS) respectively.
Conclusion
Carbon dioxide-argon radio frequency cold plasma enhanced the ability of all three waxy starches to withstand retrogradation as shown by the lower setback and final viscosities, a characteristic that can be utilized to prevent staling in bread. Higher enthalpies were observed after DSC analysis in all three starches. There was a decrease in crystallinity in waxy potato, but not rice and maize, as shown by WAXS. The NMR results showed the possible formation of V-type single helices as indicated
Funding
This research did not receive any specific grant from funding agencies in the public, commercial, or non-profit sectors.
Acknowledgements
We appreciate the help given by Dr. Gopinath Tata at the Minnesota NMR Center, Gail Celio at the University of Minnesota imaging center as well as Gabrielle Seliber at the college of biological sciences.
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