Characterization of Protein-Based Complexes loaded with anthocyanins from Purple Sweet Potato

The functional characteristics of protein-based complexes for use as carrier systems of Purple Sweet Potato (PSP) Anthocyanins (ACN)were evaluated using rheological, physical and microscopy principles. Four different concentrations of PSP-ACN stock solution (2, 5, 15, and 25 mg) were mixed with 20 μM of protein in NaCl solution (final mix concentration) from three different types of protein: soy protein isolate (SPI, 90% protein), whey protein isolate (WPI, 90-92% protein), and -Lactalbumin (-L, 90%). Stability at two pH 1.35 and 7.45 was also evaluated. Dynamic oscillatory tests on dilute biopolymer solutions were performed at 23°C to determine the relaxation time ()and a critical frequency value (CFV) to characterize the structure of the protein-based complexes and explain why some proteins yield more stable and efficient matrices. Higher concentrations of PSP-ACN yielded lower CFVs and this trend was more significant (P < 0.05) for the WPI samples. The SPI complexes produced smaller structures at the 5-25 mg/L concentration levels (lower CFV values) which were also more sensitive to shear –i.e., frequency. Transmission Electron Microscopy (TEM) was carried out to observe distribution and size of the particles within the protein matrix. These observations correlated well with the entrapment efficiency (EE %) and loading capacity (%LC) of the complexes. The -Lactalbumin samples had higher entrapment efficiency, loading capacity, and PSP-ACN to protein ratios while the soy protein isolate (SPI) complexes yielded the lowest ratios at all concentrations, mainly due to the amount of electrostatically precipitated protein remaining constant and unaffected by changes in the PSP-CAN concentration. The increase in EE% with increasing PSP-ACN concentration resulted in better entrapment ratios for WPI and -L but not for SPI. Results provide new knowledge on the functional and rheological properties of selected protein-based complexes for use as effective purple sweet potato anthocyaninscarriers.

Keywords: rheology; microscopy; encapsulation; structure; stress relaxation; biopolymer; antioxidants; critical frequency value.