Triterpene saponins as bitter components of beetroot

Abstract

Many varieties of beetroots (Beta vulgaris L.) are valued for their productivity as well as their content of nutrients and pigments from a group of betalains that have strong antioxidant properties. On the other hand, the strong bitterness of roots of several beet varieties is a frequent reason for their being not accepted by consumers. The hitherto published studies describe too vaguely the diversity of beetroot varieties in terms of their bitter taste. Up to now, it is still not clear, which of the secondary metabolites that naturally occur in beetroots is responsible for their bitter taste and aftertaste. The objective of this study was to determine the group of compounds that caused that beetroots had a bitter taste and bitter aftertaste of a high intensity level. The first stage in the research study was to select the most bitter beetroot cultivars based on the sensory characteristics of fresh beet roots (of their flesh and skin) of six cultivars (‘Nochowski’, ‘Chrobry’, ‘Noe 21’, ‘Rywal’, ‘Opolski’, and ‘Wodan’). The sensory profile of the analysed group of beets showed that the flesh and skin of the ‘Nochowski’, ‘Chrobry’, and ‘Noe 21’ cultivars were characterized by the most intense bitterness. The ‘Rywal’, ‘Opolski’, and ‘Wodan’ cultivars were marked by a relatively low intensity level of the bitterness notes. A mixture of triterpene saponins was isolated from a lyophilisate in the roots of the ‘Nochowski’ cultivar that, according to the sensory evaluation results, was classified into the group with the strongest bitterness traits. The results of sensory analyses of the saponin mixture of the ‘Nochowski’ cultivar, its concentration being C = 1.3515 g/dm3, confirmed that the group of those compounds had a strong bitter taste comparable to that of the quinine solution at a C2 = 6.6 × 10-3 g/dm3 concentration. It was also proved that the beetroot extracts tested were a mixture composed exclusively of saponin compounds, which varied in their chemical structure.