The utilisation of grape pomace as a value added ingredient: The role of grape phenolic compounds on functional foods : A thesis submitted in partial fulfilment of the requirements for the Degree of Doctor of Philosophy at Lincoln University

Grape pomace, the residue obtained after making wine, contains large quantities of polyphenols, dietary fibre, and has a high antioxidant capacity. The contents of the valuable compositions in grape pomace are believed to be much higher than the ones extracted from the grape to wine. The main polyphenols detected in grape pomace include hydroxycinnamic acids, hydroxybenzoic acids, flavan-3-ols, flavonols, and stilbenes. Cereal and cereal-based products are popular foods all over the world. However, they have a high carbohydrate content and a low polyphenol and antioxidant content. In addition, too high an intake of these highly digestible starch foods is believed to be involved in global issues of obesity and high blood sugar levels. Consumers’ demand for healthier and natural origin products has been increasing in recent decades, along with the awareness of the importance of “fresh” products. Hence, the use of plant parts to fortify cereal-based products has attracted much research, resulting in proven benefits to human health. Grape pomace has been added to many types of product, including starchy food and dairy products. Overall, cereal-based products have been confirmed to be an apparopriate food matrix to apply grape pomace based on its nutritional and physical characteristics. In this study, three white grape pomaces (Sauvignon Blanc, Pinot Gris, and Gewürztraminer) and two red grape pomace (Merlot and Pinot Noir) obtained from a New Zealand winery were the main objects. Analysis of compositions of these grape pomaces showed a high content of total phenolic compounds, in which the red grape pomaces had higher concentrations than the white ones. Between varieties, Pinot Noir had the highest content of phenolic compounds, followed by Merlot, Pinot Gris, and Gewürztraminer, while Sauvignon Blanc possessed the lowest value. The results also showed that the concentration ranges of total phenolic content found in New Zealand grape pomaces were similar to the values found in grape pomaces obtained elsewhere such as Brazil, France or Italy. Further analysis by HPLC found that catechin and epicatechin were the most abundant phenolics in all pomaces. A significant concentration of malvidin-3-o-glucoside was found in Merlot and Pinot Noir pomaces, but was not detected in white pomaces. The antioxidant capacity was reflected in the total phenolic content of each pomace with the red pomaces showed a better antioxidant capacity than the white pomaces.The red pomaces had a significantly higher content of both soluble and insoluble dietary fibre content. In contrast, white pomaces had a higher sugar content than the red pomaces. Replacement of wheat flour with different levels (5 %, 10 %, and 15 %) of grape pomace powder was performed to investigate the change in physicochemical and nutritional properties of cookies. Inclusion of pomace powder caused a significant reduction (p < 0.05) of both the thickness and diameter of the cookies. The cookies hardness decreased significantly (p < 0.05) with increased supplement levels, but there was no difference (p > 0.05) in the reduction between grape varieties. The colour of cookies also changed significantly (p < 0.05), they became darker with higher levels of addition (L* value decreased), the increase of a* value and decrease of b* value indicated that the final products got redder and bluer. The total phenolic content of cookies increased with increasing of pomace addition. The results of scavenging capacity determined by 2,2-diphenyl-1-picrylhydrazyl (DPPH) and Ferric ion reducing antioxidant power (FRAP) were in agreement with the phenolic content of cookies. The amount of sugar released during the in vitro digestion of cookies was attenuated due to the fortification with grape pomaces. Samples with the addition of 15% Pinot Noir and Merlot showed the greatest reduction in reducing sugar release. All other samples reduced the content of sugar in comparison with the control sample. Fortification of rice starch using grape pomace powder was conducted by using a rapid visco-analyser (RVA) to investigate their impacts in another type of food matrix, namely a paste. Pasting properties of paste significantly changed (p < 0.05) with the addition of pomace powder. The peak viscosity, breakdown, final viscosity, and setback of the pastes decreased significantly (p < 0.05), and the pasting temperature increased. The pasting profile of starch is defined by the apparent ratio of amylose to amylopectin; hence the inclusion of grape pomace powder changed this ratio and the final pasting profile. In an in vitro digestion, the general trend of reducing sugar released was similar to the control paste and pastes with pomace powder, but the actual amounts of reducing sugars released were lower than the control. A higher reduction was observed in samples with Pinot Noir powder. Such reductions were attributed to the content of polyphenols and dietary fibre. The influences of polyphenols from grape pomaces were tested in the mean of extracts instead of traditional powder. The changes in pasting profile were similar to the changes observed with grape pomace powder, means that peak viscosity, breakdown, final viscosity and setback decreased with the increase of added level of polyphenol extracts. The temperature increased but the range of variation was much lower than the ones with pomace powder inclusion. The extracts showed the similar impacts to reduce sugar released during in vitro digestion process in comparison with pomace powder enrichment. In addition, the total phenolic content and antioxidant capacity before and after digestion were in similar trend with pomace powder samples did. The α-amylase inhibitory was also tested on these samples, with the highest inhibition rate was attributed to Pinot Noir (64.3%)