This study was conducted because of the limited knowledge of polyphenols that are bound to pectin in blackcurrant juice. A dialyzed ethanol-precipitated fraction isolated from blackcurrant juice was found to contain carbohydrate (78% w/w), uronic acid (21% w/w), protein (4.8% w/w), anthocyanin (3.9% w/w), and calcium (2.2% w/w). The pectin-rich fraction had a pK a value of 1.67, a ζ-potential of −23.1 mV (pH 4.8), and a degree of esterification of 65.2%. Constituent sugar analysis showed mostly galacturonic acid, rhamnose, arabinose, and galactose, and NMR spectroscopic analysis showed that it was rich in rhamnogalacturonans with arabinogalactan side chains. This fraction was highly pigmented, with cyanidin 3-O-rutinoside being the major anthocyanin. Liquid chromatography showed anthocyanins retained in the fraction that failed to be extracted by methanol, suggesting the presence of bound anthocyanins. Multiangle laser light scattering data showed the presence of two fractions, ∼283 kDa present at 14.6% w/w and ∼97 kDa at 85.5% w/w. The latter also produced a higher UV 280 nm signal, signifying that proteins and/or polyphenols were present mainly in the second fraction. The existence of bound anthocyanins and proteins in pectin indicates the complexity of this biopolymer, which can help to elucidate the instability issues that can arise when blackcurrant juice is mixed with other ingredients in food systems.
A complexation study between blackcurrant pectin (BCP) and whey protein (WP) was carried out to investigate the impact of bound anthocyanins on pectin–protein interactions. The effects of pH (3.5 and 4.5), heating (85 °C, 15 min), and heating sequence (mixed-heated or heated-mixed) were studied. The pH influenced the color, turbidity, particle size, and zeta-potential of the mixtures, but its impact was mainly significant when heating was introduced. Heating increased the amount of BCP in the complexes—especially at pH 3.5, where 88% w/w of the initial pectin was found in the sedimented (insoluble) fraction. Based on phase-separation measurements, the mixed-heated system at pH 4.5 displayed greater stability than at pH 3.5. Heating sequence was essential in preventing destabilization of the systems; mixing of components before heating produced a more stable system with small complexes (<300 nm) and relatively low polydispersity. However, heating WP before mixing with BCP prompted protein aggregation—producing large complexes (>400 nm) and worsening the destabilization. Peak shifts and emergence (800–1200 cm−1) in infrared spectra confirmed that BCP and WP functional groups were altered after mixing and heating via electrostatic, hydrophobic, and hydrogen bonding interactions. This study demonstrated that appropriate processing conditions can positively impact anthocyanin-bound pectin–protein interactions.
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