Thermal stability of anthocyanins and colourless phenolics in pomegranate (Punica granatum L.) juices and model solutions

Abstract

The present study aimed at a systematic assessment of the factors influencing the anthocyanin (AC) stability and colour retention of pomegranate juices and less complex model solutions with particular focus on the effects of colourless phenolic copigments (CP). The thermal stability of ACs in three pomegranate juices obtained from isolated arils and the entire fruit with and without previous steaming, in model solutions with AC:CP ratios ranging from 1:0 to 1:4 (m/m), and in two purified extracts from pomegranate juices characterised by different phenolic profiles, respectively, was investigated upon heating at 60, 70, 80 and 90°C for 15min to 5h. The thermal impact on the AC and CP contents, and the formation of 5-hydroxymethylfurfural (HMF) and AC degradation products were monitored using HPLC-DAD–MSn. Total phenolic contents, antioxidant capacity and colour properties were determined spectrophotometrically.Heating at 90°C for 5h resulted in total AC losses ranging from 76% to 87% of the initial AC levels in the juices, 78% in both extracts as well as 57% and ∼78% in the model solutions, showing the best stability at an AC:CP ratio of 1:2 and in juices having the highest initial AC contents, respectively. In contrast, the AC stability was independent of total phenolic contents, and low and high molecular pomegranate matrix components (such as organic acids and sugars). Good correlation of the AC contents with red colour (a∗) was observed for all samples at elevated temperatures (70–90°C). The stability of putative health-promoting polyphenols of pomegranate juices was not markedly affected by the thermal treatment. Unexpectedly, the HMF contents only slightly increased upon forced heating. Therefore, the visual appearance does not adequately reflect the quality and storage stability of pomegranate juices.