The effects of blueberry and blackcurrant powder addition to oat bran paste on its physicochemical characteristics, and their subsequent in vitro glycaemic response, and cellular anticancer properties : A thesis submitted in partial fulfilment of the requirements for the Degree of Doctor of Philosophy at Lincoln University

Abstract

Cereal foods supplemented with fruit ingredients have attracted the attention of consumers due to their richness of bioactive compounds (phenolic compounds), and potential health promoting effects (reducing the risk of obesity, type 2 diabetes, and cancers). Despite the well-known health benefits of the purified ingredients from berry fruits, few studies have reported the actual use of these ingredients in relation to the nutritional effects of a food matrix and the consumption of cereal foods supplemented with berry fruits. Although using simple purified ingredients may help to identify mechanisms of action, the additive or synergistic effects provided by a food matrix cannot be observed by not including complex food systems, since the functionality of foods is highly influenced by the interactions between all of the components within the food matrix. Oat bran was enriched with 0%, 10%, 15% and 25% (w/w) of blueberry and blackcurrant powder, respectively. The Rapid Viscosity Analyser (RVA) equipment was used to form oat bran pastes which were enriched with blueberry or blackcurrant powder in order to study the effects of the food matrix on physicochemical characteristics, nutritional and functional properties of the pastes. High performance liquid chromatography (HPLC), and an in vitro digestion process, were performed to observe the effects of bioactive compounds from blueberry and blackcurrant powders on the predicted in vitro glycaemic response. Results showed that the oat bran paste containing blueberry powder, or blackcurrant powder, increased the values for total phenolic compounds, and the antioxidant capacity. However, the total monomeric anthocyanin content decreased after the in vitro digestion. Cyanidin, delphinidin, malvidin, pelargonidin, and peonidin in the extracts of blueberry and blackcurrant enriched pastes were identified by the pH differential method. The extent of starch degradation, and the area under the curve (AUC) of reducing sugar released, of blueberry and blackcurrant enriched pastes decreased during the in vitro digestion. The in vitro α-amylase and α-glucosidase inhibitory activities of the extracts from blueberry and blackcurrant enriched oat bran pastes were determined, and the potential mechanisms of the interaction of anthocyanins on digestive enzymes were studied. Addition of blueberry or blackcurrant powder into oat bran paste significantly increased the α-amylase and α-glucosidase inhibitory activities and decreased IC50 values of the pastes (p < 0.01). The extracts of blueberry and blackcurrant enriched pastes were observed to be mixed-type inhibitors against α-amylase, while they were competitive inhibitors towards α-glucosidase. In addition, their enzymes inhibitory activities were hypothesised to be driven by hydrogen bonding. Cyanidin-3-glucoside and delphinidin-3-glucoside in the extracts of blueberry and blackcurrant enriched pastes had stronger binding affinities compared to malvidin-3-glucoside and cyanidin-3-rutinside. The anticancer bioactivity towards HepG2 cell line between the extracts of undigested and intestinal digested blueberry and blackcurrant enriched pastes were compared, and the potential mechanisms on their anticancer properties were also studied. The extracts of undigested blueberry and blackcurrant enriched pastes inhibited the cell growth, and cell invasion, towards HepG2, more significantly than the extracts of digested pastes (p < 0.01). Mechanistic studies suggested that the extracts of blueberry and blackcurrant enriched pastes induced HepG2 cell apoptosis by initiation of the cell cycle arrest and regulation of the expression of apoptotic-related proteins, including Bcl-2, Bax, and caspase-3. The intracellular reactive oxygen species level of extracts treated-HepG2 cells increased significantly (p < 0.01) via regulation of the nuclear factor-like 2 (Nrf2)/heme oxygenase 1 (HO1) signalling pathway. The synergistic effects of this study suggest that enrichment of oat bran with blueberry and blackcurrant powder has the potential to develop a range of functional foods, and there may be a new and effective option to prevent and control chronic diseases in human.