Modification of cereal and tuber starches using cold plasma technology to improve its structure and functionality

Abstract

Starch is a versatile biopolymer with multiple uses in the food and non-food industries. In the food industry, starch is used as a thickening, binding, gelling, stabilizing, and encapsulating agent amongst others. In the non-food industry, starch can be used as a sizing agent to add strength to paper and textiles, as an adhesive, and used in the manufacture of bioplastics. Starch needs to be modified to enhance its stability, solubility, heat, and shear tolerance for industrial applications. Starch is mostly modified using chemical reagents due to their effectiveness in altering starch physicochemical and functional properties. However, there is increasing concerns about the chemical waste generated during chemical starch modification. There is also an increasing demand for food products with “clean labels”. To meet these demands, there is a push to find eco-friendly alternatives such as cold plasma technology. Cold plasma is obtained by applying electrical energy to a gas, which leads to ionization of the gas to produce reactive oxygen and nitrogen species (RONS), ultraviolet radiation, and free radicals. Cold plasma can be applied directly to starches or in the form of plasma-activated water (PAW). In the case of PAW, RONS, ultraviolet radiation, and free radicals are generated in water upon treatment with atmospheric cold plasma. These reactive species can induce changes in the structure and functionality of starches, without generating virtually any waste. The main aim of this research was to modify cereal and tuber starches with a carbon dioxide-argon radio frequency cold plasma for the first time. In addition, plasma-activated water (PAW) at different temperatures was utilized to modify high amylose and waxy starch properties. In objective one, carbon dioxide-argon radio frequency cold plasma was used to modify waxy starches. A 3 X 3 factorial design with three waxy starches (maize, rice, and potato) and three treatment levels (no treatment, 0 W and 120 W) was used for the experiment. The starch samples were treated at 0 W (gas treatment) to investigate whether gas treatment alone without ionization could induce changes in the starches. Solid state 13C nuclear magnetic resonance, differential scanning calorimetry, micro-visco-amylograph, wide angle x-ray scattering, scanning electron microscopy and optical light microscopy were used to investigate the effect of cold plasma on waxy starch properties. Starches modified with cold plasma were more stable when subjected to heat and shear. This was indicated by the lower breakdown values obtained for maize (189.0 as compared to 212.8 BU) and rice (24.5 as compared to 32.0 BU) after treatment. The tendency for retrogradation to set in also decreased as indicated by the lower setback and final viscosity values for maize (44 as compared to 65.8 BU; 209.3 as compared to 295.3 BU), rice (48.3 as compared to 89.5 BU, 184.5 as compared to 311.3 BU) and potato (76.5 as compared to 147.3 BU, 386.3 as compared to 516.0 BU) after treatment. However, the morphology of the starches remained unchanged. The crystallinity of waxy potato starch reduced by 2.8% and 5.5% after 0 W and 120 W treatment, respectively, but was unaffected in rice and maize. The ability of starches to form inclusion complexes with iodine were unaffected after plasma or gas treatment. We also, observed V-type single helices in maize and rice starches which could be attributed to the formation of starch-lipid complexes. In objective two, the effect of radio frequency cold plasma on starch fine structure (unit and internal chain distribution) was determined using the High-Performance Anion Exchange Chromatography (HPAEC), since studies have shown that alterations at the fine structural level impacts starch functionality. Also, the occurrence of cross-linking was investigated using Fourier transform infrared spectroscopy-attenuated total reflectance (FTIR-ATR). A 3 X 2 X 2 factorial design with three waxy starches (maize, rice, and potato) two starch forms (granular and non-granular) and two treatment levels (no treatment and 120W) was used. Radio frequency plasma resulted in negligible changes at the fine structural level. FTIR-ATR confirmed the ability of cold plasma to induce cross-linking in these starches which explains why these starches were more stable during heat and shear treatment (objective 1). In vitro digestibility studies showed that cold plasma treatment increased the amount of slowly digestible starches (5.62%; 10.24%) and resistant starches (0.28%; 85.66%) in non-granular waxy maize and granular waxy potato starches respectively. In objective 3, plasma-activated water (PAW) at different temperatures was utilized to modify high amylose and waxy starches. A 4 X 4 factorial design with four starches (high amylose maize, high amylose potato, waxy maize, and waxy potato) and four treatment temperatures (no treatment, 25°C, 60°C and 80°C) was used. The X-ray photo electron spectroscopy was used to determine the ability of PAW at different temperatures to induce any changes in the elemental surface composition of these starches. The effect of PAW at different temperatures, on functionality and the thermal profile of starches was also investigated. PAW at 60°C and 80°C increased the solubility and water absorption capacity of the starches. Both treated and untreated starches were mostly made up of carbon and oxygen with trace quantities of nitrogen or silicon. Modification of starches with PAW at 60°C increased the gelatinization temperatures and enthalpies. Overall, this study demonstrated that radio frequency cold plasma and PAW significantly influenced starch properties. These changes were however not seen at the molecular level as observed in the fine structure analysis.