Yogurt with cornflower (Centaurea cyanus L.) petals as a source of antioxidant compounds and dietary fiber: physicochemical and spectroscopic research during storage

An extensive study was undertaken using the petals of cornflower (Centaurea cyanus (Cyani flos)) flowers in yogurt production as a source of dietary fiber and antioxidant compounds, and as a natural dye, taking into account a 21-d refrigerated storage period. An additional aim of the study was to use cornflower petals in natural form rather than an extract. Four forms of cornflower petals: fresh, dried, frozen, and freeze-dried, were tested. In terms of physicochemical properties, including the content of dietary fiber and bioactive compounds, and antioxidant status, the freeze-dried form proved most beneficial. Two variants of yogurts were made, i.e., without the addition of cornflower petals (control) and with cornflower. The yogurts were stored for 21 d and analyzed every 7 d (at 0, 7, 14 and 21 d). In total 4 forms of petals of cornflower flowers, 3 batches of bulk cow milk and 288 yogurt samples were covered by research. Freeze-dried cornflower petals added to yogurt contributed to a significant increase in the content of total protein (by approx. 4%), fiber (form 0.00 to 0.56 g/100 g), and selected bioactive compounds, including amino acids (by approx. 2% in total), vitamin C (over 3-fold), and phenolic (by approx. 16% in total) and mineral (on average by 8%) compounds, as well antioxidant activity. The presence of antioxidant compounds not only in the processed milk (sulfur amino acids, whey proteins (especially β-lactoglobulin), vitamins A, E, and C, or β-carotene) but also in the petals (sulfur amino acids, vitamin C, and phenolic compounds) contributed to a significant increase in the antioxidant activity of fresh yogurt with cornflower (day “0” of storage), i.e., FRAP – by 18%, DPPH – 50%, and ABTS – 60%, in comparison with the control yogurt. Significant changes in the content of selected bioactive compounds occurred during 21-d storage, which effected on the antioxidant activity, i.e., increased of FRAP and ABTS by 20%. In addition, FTIR infrared spectra were taken for the yogurts. The spectroscopic analyses showed mainly changes in the intensity of vibrations characteristic of this type of product, associated with polysaccharide structures, as well as vibrations associated with protein structures contained in dietary fiber. The regions with the most pronounced changes in the intensity of the bands were ~3300, 1640, 1550, 1240, and noticeable at ~1030 cm−1. The maxima also correspond to the vibrations of C = O groups, amide bands, and the poly