The impact of drying method and temperature on the colour and functional quality of Illawarra plum (Podocarpus elatus)

Abstract

Illawarra plum (IP) is a fruit native to Australia and is a source of antioxidants including phenolics and anthocyanins which contribute important functions in food preservation and quality. However, the retention of natural antioxidants is influenced by thermal processing due to their sensitivity to heat. This study measured the impact of three common drying methods to identify the best processing conditions, including cost effectiveness on retaining the quality of IP. The colour (L*, a*, b*), levels of total phenolics, flavonoids, proanthocyanidins, anthocyanins and antioxidant scavenging capacity of the samples were measured on dried IP fruits (freeze dried, convection oven 70, 90, 110 °C or vacuum oven 70, 90 or 110 °C). The freeze dried sample observed the best colour (L* 44.18±0.50, a* 16.72±0.15, b* 1.66±0.04) and highest levels of compounds from the methods observed: TPC 679.19±15.04 mM GAE/g DW, TFC 190.25±4.65 mM CE/g DW, proanthocyanidins 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. For maximum retention of quality, freeze drying is recommended, however, it is costly and for a more economical approach, vacuum drying at 90 °C or convection oven drying at 110 °C are identified as suitable conditions for drying IP to prevent unnecessary loss of phenolic quality, however these methods will cause shrinkage and browning in the fruit. These conditions can be applied for drying IP for direct use as a dried product or for further applications in food

Keywords

• Fruit, Drying
• Phenolic
• Antioxidant
• Anthocyanins
• Native, Quality