Starch Aldehyde–Theaflavin Conjugate: Synthesis, Structure, and Antioxidant and Antimicrobial Activities

In the present study, potato starch (PS) was functionalized with theaflavin (TF). Potato starch aldehyde (DPS)-theaflavin (DPS-TF) conjugates were prepared by conjugating TF with DPS. The synthesized DPS-TF conjugates were characterized via UV-visible spectroscopy, Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), proton nuclear magnetic resonance (¹H-NMR) and scanning electron microscopy (SEM) analysis and tested for antioxidant and antimicrobial activities. The UV-vis spectrum results demonstrated that DPS-TF conjugates exhibited the characteristic absorption peaks of theaflavin at 280 nm, which can be attributed to the benzotropolone structure present in theaflavin. The absorbance values of the peaks progressively intensified as the concentration of grafted theaflavins increased. FTIR confirmed the depletion of the aldehyde groups and the presence of TF-specific vibrations in the conjugates in DPS-TF. ¹H-NMR demonstrated that the conjugation occurred between the H-6, H-8, H-6', and H-8' positions of theaflavin and the aldehyde groups of starch aldehyde. XRD demonstrated that the DPS-TF conjugates were in the amorphous state. SEM observation demonstrated that DPS-TF exhibited a mixed morphology of flakes and lumps, which differed from that of native starch and starch aldehyde. The scavenging activity of DPS-TF against DPPH and ABTS radicals was significantly higher than that of DPS (<i>p</i> < 0.05), with the antioxidant activity increasing in line with the concentration of theaflavins. In comparison with PS and DPS, DPS-TF conjugates demonstrated superior antimicrobial activity against <i>Escherichia coli</i> and <i>Staphylococcus aureus</i>. Furthermore, an elevated grafting ratio corresponds to a heightened level of these functional properties. This study highlights the promise of the starch aldehyde-theaflavin conjugates for use as a viable antioxidant and antimicrobial agent for food applications.