Abstract :
[en] The previous studies of sweet potato leaf polyphenols were mainly focus on the non-flavonoids part (phenolic acids), there weren’t enough study on the flavonoids part. Chemically, flavonoids have the specific structure of a 15-carbon skeleton, which consists of two phenyl rings (A and B) and a heterocyclic ring (C), and can be abbreviated as C6-C3-C6 structure.
Firstly, ultrasonic-microwave synergistic extraction (UMSE) was used to extract flavonoids from sweet potato leaves (SPL) by response surface methodology (RSM). The optimal conditions for flavonoids extraction were 1:40 (g/mL) of solid-liquid ratio, 57 °C of extraction temperature, 76 s of extraction time and 72 % (v/v) ethanol for 2 times of extraction, and the highest flavonoids yield from SPL was 5.1 %. After purification, the flavonoids purity reached up to 76.1 (%, DW). The result of high performance liquid chromatography (HPLC) revealed 11 compounds including astragalin, quercetrin, 4,5-chlorogenic acid, isoquercitrin, tiliroside, quercetin, 3,4,5-chlorogenic acid, caffeic acid, kaempferol, myricetrin and rhamnetin in sweet potato leaf flavonoids (SPLF), which possessed good antioxidant activity compared to soy isoflavones, Ginkgo biloba extract and propolis flavone.
Secondly, the effect of heat treatment, high hydrostatic pressure (HHP) treatment, pH, light, temperature and simulated digestion on the stability of SPLF was studied. Heat treatment at 75 °C for 90 min or HHP treatment at 600 MPa for 30 min didn’t cause significant effect on SPLF. Heat treatment at 100 °C for 60 min and 90 min led to a decrease in antioxidant activity by 20 % and 25 % respectively, while pH 7.0 and 8.0 significantly decreased amount of SPLF by approximately 75 %, decreased antioxidant activity by about 30 % and 47 % separately. Light treated samples recorded a decrease in SPLF by 52 % and antioxidant activity by 24 %. No significant effect on SPLF was observed for samples stored at -18, 4 °C or room temperature (RT) (≈ 20 °C). The retention of flavonoids and antioxidant activity was 45 % and 56 %, individually in SPLF after simulated digestion.
Thirdly, nanoparticles of SPLF were prepared by freeze-drying encapsulation using 2 % (g/mL) maltodextrin, which showed the highest stability with high absolute zeta potential (-41.6 mV) and encapsulation efficiency (EE) (59.0 %), low mean particle size (277.4 nm) and polydispersity index (PDI) (0.417). Confocal laser scanning microscopy (CLSM) and infrared spectrum provided the evidence. After simulated oral, gastric and intestinal digestion, nanoparticles additionally reserved 16, 31, 28 % of flavonoids and 16, 16, 10 % of antioxidant activity, separately, compared with the sample without encapsulation, which could be intuitively demonstrated by scanning electron microscopy (SEM).
Finally, SPL was used to substitute 1, 2, 3.5 and 5 % (w/w) of wheat flour to make bread. 1 % SPL could change the bread color significantly, which could be clearly distinguished by human eyes. The hardness and chewiness of the crumb increased with increased SPL level. When adding 5 % of SPL to the flour, specific volume of the bread shrank in half, compared with the control. Pores in the crumb became deteriorating and disintegrating when the adding of SPL was higher than 3.5 %. Total polyphenols content (TPC) and total flavonoids content (TFC) of the bread increased 6-fold and 10-fold, and antioxidant activity enhanced 10-fold, separately when adding 5 % SPL to the flour. The addition of SPL won’t alter volatile compounds of the bread, but suppressed the generation of typical bread odors significantly. Overall, SPL (no more than 1 %) could be supplemented to the flour to make bread with high TPC (2-fold), TFC (2.5-fold), excellent antioxidant activity (3-fold), and no significant adverse effect on the physical characteristics and flavor.
