Tannin – starch interplay: Investigating the impact of grape tannins on the properties of various starches : A dissertation submitted in partial fulfilment of the requirements for the Degree of Master of Science in Food Innovation at Lincoln University

Abstract

Starch is a vital macronutrient found in different concentrations across various foods. The structure and physicochemical properties of starch can vary from one source to other, directly or indirectly influencing how it interacts with other bio-macromolecules in the food systems. Interaction of starch with bio-molecules such as tannins can significantly enhance the overall textural-cum-nutritional properties of starch. Tannins, a rich source of phenolic compounds and exhibits antioxidants activity, can interact with various molecules, including starch and proteins in foods, thereby enhancing the nutritional profile of foods. The interaction between starch and tannin molecules in foods has enhanced properties like texture, gelatinization, retro-gradation and breakdown of starch into smaller sugars. This improvement has enabled food scientists to produce foods with a lower glycemic index. This study aimed to develop starch - tannin complexes using starches from three different botanical sources and grape tannins derived from both skin and seeds. All the three normal starches (5% w/v), with amylose content nearly 30%, were cooked with grape skin and grape seed tannins (10 % by starch weight). Various characterizations were carried out to analyze the effect of tannins on starch properties. The quantity of bound and unbound tannins was quantified through methyl cellulose precipitation method, which revealed nearly 95% of tannins were bound to starch molecules in the solution. Furthermore, to detect the phenolic compounds in tannins and starch – tannin complexes, HPLC was performed. A decrease in the proportion of phenolic compounds present in grape skin and grape seed tannins occurred after complexation with starch, indicating development of possible molecular interactions among starch and tannins. Starch-iodine complexes were developed to further elucidate the mechanism of interaction between starch and tannins. A decrease in the absorbance values near 500-540 nm and 540-660 nm gave an idea regarding tannin interaction with amylose and amylopectin chains. In case of pea starch-tannin complexes, a decrease in the absorbance values within spectral range of 540-660nm proved the binding of tannins with amylose chains. Particle size distribution also supported the development of molecular interactions between these two biomolecules. For pea starch-tannin complexes, the particle size was constrained to the range within 40-70 µm in contrast to pea starch control samples, for which the particle size distribution ranged within 20-200 µm. It reveals that the particle size of complexes formed during conjugation of tannins with starch molecules was ranging within 40-70 µm. Back extrusion textural properties were analyzed to notice the effect of tannins on the starch properties such as firmness, consistency, cohesiveness and index of viscosity. For corn and pea starch-tannin complexes, a decrease in the firmness and consistency and an increase in the other two properties were observed while an opposite trend was noticed for wheat starch-tannin complexes in contrast to the starch control samples. These properties provide an insight about the textural properties of starch, which can further help in improving the sensory profile of foods containing starch-tannin complexes. Tannins exhibit antioxidant properties and foods rich with antioxidants are believed to boast nutritional profile of such foods. In FRAP assay, starch bound tannin molecules were able to reduce the Fe3+ ions before and after four hours of incubation at 37°C, thus, highlighting the potential contribution of starch-tannin complexes to the antioxidant ability in foods. In-vitro starch digestion revealed the capability of tannins to prevent the digestion of starch molecules among all the three starches. A significant RDS, SDS and TDS decrease was observed in starch-tannin complexes, which promises the potential of these tannins to improve the overall sensory-cum-nutritional profile of starchy foods.