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 were tested: fresh, dried, frozen, and freeze-dried. In terms of physicochemical properties, including the amount of dietary fiber and bioactive compounds and antioxidant status, the freeze-dried form proved most beneficial. Two variants of yogurts were made, one without the addition of cornflower petals (control) and one 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 cornflower petals, 3 batches of bulk cow milk, and 288 yogurt samples were analyzed. Freeze-dried cornflower petals added to yogurt contributed to a significant increase in the content of total protein (by ∼4%), fiber (from 0.00 to 0.56 g/100 g), and selected bioactive compounds, including AA (by ∼2% in total), vitamin C (over 3-fold), and phenolic (by ∼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 AA, whey proteins [especially β-LA], vitamins A, E, and C, or β-carotene) but also in the petals (sulfur AA, vitamin C, and phenolic compounds) contributed to a significant increase in the antioxidant activity of fresh yogurt with cornflower (d 0 of storage; i.e., ferric reducing antioxidant power [FRAP] increased by 18%, 2,2-diphenyl-1-picrylhydrazyl by 50%, and 2,2'-azinobis-(3-ethyl-benzothiazoline-6-sulfonic acid) [ABTS] by 60%), in comparison with the control yogurt. Significant changes in the content of selected bioactive compounds occurred during the 21-d storage, which affected the antioxidant activity (i.e., FRAP and ABTS increased by 20%). In addition, Fourier-transform infrared (FTIR) spectroscopy analyses of the yogurts 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 ∼3,300, 1,640, 1,550, and 1,240 cm^-1, and there were noticeable changes at ∼1,030 cm^-1. The maxima also correspond to the vibrations of C=O groups, amide bands, and the polysaccharide fraction, as well as stretching vibrations of the hydroxylic group. The analyses conducted using FTIR spectroscopy demonstrated that the product was highly stable during the refrigerated storage, as reflected in particular by the vibrations in the spectral region below 1,000 cm^-3.