Exploring the Nutritional and Functional Properties of Two Understudied Australian Endemic Plants: Diploglottis bracteata and Syzigium aqueum ^†

Abstract

Despite the growing national and international interest for Australian traditional plant foods, information on nutritional and functional properties of many endemic species is limited. This restricts their incorporation in food, beverage and nutraceutical applications. This pilot study explored the phytochemical profiles and antimicrobial activity of two Australian endemic fruits: Diploglottis bracteata and Syzigium aqueum, to explore their marketability. Profiling of major bioactive phytochemicals showed the presence of 3 anthocyanins in S. aqueum (delphinidin-3-glucoside, cyanidin-3-glucoside and peonidin-3-glucoside) and four carotenoids in D. bracteata (lutein, zeaxanthin, β-cryptoxanthin and β-carotene). The total carotenoid content was comparable to that of orange coloured carrots, an important dietary source of carotenoids. Elemental analysis revealed that, compared to blueberry (a popular dietary source), D. bracteata had 4.5 times higher potassium, and 3.7 times higher magnesium content. Calcium levels of S. aqueum was 4.9 times higher than blueberry. Methanol, acetone and water extracts of both fruits were analysed for total phenolic content (TPC) and antimicrobial activity. TPC of D. bracteata and S. aqueum methanolic extracts were 2.9 and 1.4 mg gallic acid equivalents/g FW which is in the same range as blueberry, a popular dietary source of bioactive phenolic compounds. Antimicrobial analysis showed methanol, acetone and water extracts of both fruits have strong inhibitory effects against both Gram positive (Staphylococcus aureus) and Gram negative (Escherichia coli) bacteria, but no effects against fungi (Candida albicans). These promising initial results, diverse phytochemical profiles and strong antibacterial activity, warrant further investigation.