The effects of black and red goji berry enrichment on physicochemical properties, glycaemic response and anti-diabetic abilities as functional food : A thesis submitted in partial fulfilment of the requirements for the Degree of Doctor of Philosophy at Lincoln University

Abstract

The chronic diseases, such as type 2 diabetes and cardiovascular disease, are the main health issues worldwide and result in a large financial cost (Burton-Freeman, 2010; Sheehan & Ulchaker, 2010). Black goji berry fruits (Lycium ruthenicum) and red goji berry fruits (Lycium barbarum) are rich in dietary fibres and bioactive compounds, and they are widely consumed as super foods (Amagase & Farnsworth, 2011). While there is a lack of research on the physicochemical properties, glycaemic response and anti-diabetic abilities of goji berry enriched foods. Goji berry offers significant potential health benefit as a functional food ingredient. In this study, wheat-based biscuits were enriched with 5%, 10%, and 15% black goji berry powder and red goji berry powder respectively and analysed raw goji berry powder and its enriched biscuits before and after in vitro digestion. Both goji berry powder was high in insoluble dietary fibre (IDF), soluble dietary fibre (SDF), and total dietary fibre (TDF), and low in protein content. These nutrients led higher moisture content and water holding capability (WHC) in goji berry enriched biscuits. With increasing enrichment, the hardness of biscuits deceased, and the spread ratio increased. Due to the large amount of phenolics in goji berry powder, the colour of enriched biscuits was significantly variable, and it was darker than control biscuits. The total phenolic content, individual phenolic substances, and antioxidant capacity of undigested extracts, gastric digesta and intestinal digesta from in vitro digestion of raw goji berry powder and goji berry enriched biscuits were investigated. The goji berry enriched biscuits had higher total phenolic content, total flavonoid content and total carotenoid content, and higher DPPH, FRAP and ABTS values than the control biscuits. During the digestion, more phenolic compounds from goji berry enriched biscuits were protected from degradation, which led to higher bio-accessibility of phenolics and stronger antioxidant activity. Both black goji berry fruits (Lycium ruthenicum) and red goji berry fruits (Lycium barbarum) were adopted and black goji berry had greater performance on antioxidant capacity. When the effect of black and red goji berry and their enriched biscuits on inhibiting α-amylase and α-glucosidase, which are digestive enzymes playing crucial roles in postprandial blood glucose level and diabetes control were evaluated, the black and red goji berry enriched biscuits enhanced the inhibition by 19.16% (IC50 = 2.11 mg/mL) and 9.81% (IC50 = 3.31 mg/mL) on α-amylase and by 12.33% (IC50 = 56.24 mg/mL) and 21.15% (IC50 = 36.58 on α-glucosidase compared to biscuits without goji berry. The inhibition types of all goji berry powder and enriched biscuits were mixed inhibition with increasing maximum velocity (Vmax), decreasing Michaelis constant (Km), lower competitive inhibition constant (Kic), and higher uncompetitive constant (Kiu). Anti-diabetic ability of black and red goji berry powder and their enriched biscuits were also investigated by both in vitro glycaemic response and glucose consumption in HepG2 cells. The results showed that goji berry effectively reduce release rate and content of reducing sugar during in vitro digestion compared to wheat flour and control biscuits, which obtained through glycaemic glucose equivalent (GGE) and area under curve (AUC). In addition, the glucose consumption was increased by treatments of goji berry powder and its enriched biscuits in insulin resistant HepG2. These results and findings of this thesis suggest that the goji berry enriched biscuits are potentially functional food with acceptable physical properties and provide research basis for development and application of goji berry as a functional ingredient in health and food industry.