Longan seeds used as novel resources to prepare resistant starch: structure, properties and digestion

In the present study, the preparation of resistant starch from longan seeds was investigated, and the properties and in-vitro digestibility of this resistant starch were further determined. Longan seed resistant starch (LRSI, LRS-II, and LRS-III) was successfully prepared, by once retrogradation, twice retrogradation and combining retrogradation and debranching, respectively. All LRS groups had higher amylose content (from 422.1 to 462.4 g/kg) and increased particle size than those of longan seed starch (LSS). Meanwhile, the resistant starch exhibited a B-type crystalline structure, and showed a block, uneven and irregular microstructure. Compared with LSS group, LRS groups exhibited higher crystallinity (from 49.03 to 56.14 %), more of short-range ordered structure (DO: from 0.771 to 0.775, DD: 0.808 to 0.942), and lower molecular weight (Mw: from 2.43 to 2.81 × 10⁵ Da). Moreover, LRS groups showed lower viscosity during gelatinization and weaker binding capability to iodine than those of LSS group. Furthermore, all LRS groups exhibited strong resistance to enzymatic hydrolysis with high content of resistant starch (from 360 to 398 g/kg) and low rate of in-vitro digestion (k: from 23.87 to 33.74, C_∞: from 63.5 to 65.5). All present results suggest that longan seeds show the potential to prepare resistant starch applied in food industry.