Illawarra Plum (Podocarpus elatus): Characterisation, Pretreatment & Potential Application as a Natural Food Additive

The Illawarra Plum (IP), Podocarpus elatus, is a fruit with a long history of culinary use in Australia. This study firstly outlines the nutritional and functional benefits of Australian native fruits, identifying there is limited research on the Illawarra Plums, which are a rich source of anthocyanins and phenolic antioxidants. There is evidence in the literature of Australian native fruits possessing antimicrobial properties, anti-inflammatory action. They are a source of natural colours and unique flavours, thereby offering potential to be utilised as a food additive to increase food quality or sensory properties. For the fruits including Illawarra Plum to be used in this way, more studies in its postharvest treatment, and the fruits impact on food quality should be conducted. This study aimed to 1) characterise morphological properties, phytochemicals, and optimise antioxidant properties of the fruit and 2) establish the impact of pre-treatment conditions on the fruit quality to identify the optimal conditions for drying fruit and 3) evaluate its potential use as a natural food additive in food products.

The morphological properties of the fruits were characterised for three identified ripening stages based on the internal flesh colour; unripe (green), almost ripe (blushing) and ripe (deep red). The texture of ripe fruit is soft like a plum: mucilaginous, starchy, and gives a slimy mouthfeel. The ripe fruit The Illawarra Plum is approximately the diameter and weight of a large blueberry, and 4-5 times smaller in diameter than an apple. It measures 14.78 ± 3.1 (h) x 17.45 ± 2.7 (w) x 14.69 ± 3.0 mm (d) and weighs 2.5958 ± 1.2 g and its size in each ripening stage is not significantly (p<0.05) different by height, width, or weight. Its internal pip is approximately 50 % of the height and 12-15 % of the width of the cross-section. Solid-liquid extraction can be a beneficial pre-treatment to avoid seed removal, and enhance fruit properties, however we identified solvent selection, and concentration significantly affected the bioactive content. Of the green solvents evaluated (ethanol 95 %, ethanol 50 % and water. The most effective solvent was 50% ethanol and gives 5 times more phenolics than African and American plums. Fourteen peaks were isolated from IP by HPLC-PDA with three putatively identified as chlorogenic acid, epicatechin and p-coumaric acid. Correlation confirmed the anthocyanin content as the main group of phenolics that contributes to the antioxidant scavenging activity.

Following from fruit characterisation and solvent extraction, the best drying method for IP was determined. For optimal colour, (brightness and red-blue levels), freeze-drying (FD) gave the best results (L* 44.18 ± 0.50, a* 16.72 ± 0.15, b* 1.66 ± 0.04) but is the costliest drying method compared to vacuum- or convection drying (VD or CD). Colour is significantly impacted in VD and CD drying because of the formation of Maillard reaction products, browning and caramelisation by-products and fruit shrinkage. While VD at 90 °C or 110 °C and CD at 110 °C showed comparable scavenging antioxidant levels to FD plums (p>0.05), the highest measured levels of phytochemicals were in FD IP: TPC 679.19 ± 15.04 mM GAE/g DW, TFC 190.25 ± 4.65 mM CE/g DW, proanthocyanins 422.46 ± 21.52 mM CE/g DW, DPPH 562.51 ± 40.69 mM TE/g DW, monomeric anthocyanins 402.58 ± 3.97 μg C3G/L DW and 57.97 ± 2.84 % polymeric anthocyanins. The VD 90 °C fruit had comparable scavenging activity and higher anthocyanins than the other CD or VD conditions compared, thus 90 °C VD and FD samples were selected for application to a food product. It was important to understand the benefit of extracting as an additional processing step to understand if it delivers benefit to the end product, compared to simple whole fruit drying and powdering as a preservation method. To compare the dried powdered fruit against fruit extracts, VD and FD IP was added to yoghurt or extracted (VD ext., FD ext.) then added to Greek-style set yoghurt at 0.05 % w/v. Changes in quality parameters, including colour, pH, titratable acidity (TA), syneresis, total solids (TS) and qualitative phytochemical content were monitored over a 28-day storage period.

FD whole fruit, or FD extract (FD ext.) yoghurts had a brighter, more distinct pink colour yoghurt (as measured by changes in L* a* b* colour space) compared to the VD and VD ext. yoghurts. VD ext. yoghurt had negligible changes to colour, or phytochemical content, and had increased syneresis compared to the negative control throughout its storage in chilled conditions. In comparison, VD IP yoghurt was a pale pink, observed a reduction of syneresis during the storage period, and was stable in terms of acidity, solids, and colour. The FD ext. yoghurt had significantly better colour lightness, but had slightly less content in terms of phenolics, flavonoids, proanthocyanins and monomeric anthocyanins to the FD yoghurt. While VD fruit held functional promise for IP compared to FD fruit, this was not proven and FD fruit function was found to be superior for colour, quality parameters and bioactives of yoghurt, which was also superior to FD ext. fruit. From the physical and physicochemical results in this study, the cost benefit of incorporating solvent extraction in the process stream was not justified and the FD IP could be applied to achieve similar outcomes for colour, acidity, reduction in syneresis throughout storage and total solids compared to its extract. This study proves that pre-treated IP can successfully improve the properties, namely consistency, and colour, without detrimental effect to acidity or syneresis when used as a natural additive in yoghurt. Further work is required to explore the addition of IP to other food products.