Abstract
A large quantity of longan fruits (Dimocarpus longan Lour.) produced annually are processed into many products, one of which is black longan, from which the dried, dark-brown meat has been used medicinally in traditional medicine, while the starch-containing seeds are discarded. In this study, starch samples (BLGSs) were isolated from seeds of black longan fruits prepared using varied conditions. The in vitro digestibility was determined in comparison with those extracted from fresh (FLGS) and dried (DLGS) seeds. Scanning electron microscopy (SEM), X-ray diffraction (XRD), differential scanning calorimetry (DSC), and Fourier transform infrared (FTIR) spectroscopy were employed to evaluate the starch properties. The results showed that the yields of FLGS, DLGS, and BLGSs were 20%, 23%, and 16–22% w/w, respectively. SEM images showed starch granules of mixed shapes, with sizes up to 15 µm in all samples. XRD patterns confirmed an A-type crystallinity for FLGS and DLGS, with strong refraction peaks at 2θ = 15°, 17°, 18°, and 23°, while BLGSs also showed detectable peaks at 2θ = 10° and 21°, which suggested V-type structures. Thermal properties corroborated the changes by showing increases in peak gelatinization temperature (Tp) and enthalpy energy (ΔH) in BLGSs. The paste viscosity of BLGSs (5% w/w) decreased by 20–58% from that of FLGS. The FTIR peak ratio at 1045/1022 and 1022/995 cm−1 also indicated an increase in ordered structure in BLGSs compared to FLGS. The significant increase in the amounts of slowly digestible starch (SDS) and resistant starch (RS) in BLGSs compared to FLGS, especially at a prolonged incubation time of 20 (4.2×) and 30 days (4.1×), was proposed to be due to the heat-induced formation of starch inclusion with other components inside the seed during the black longan production process. Thus, black longan seed could be a new source of starch, with increased RS content, for potential use in the food and related industries.
Highlights
Longan (Dimocarpus longan Lour.) is a popular tropical fruit grown in many parts of Asia
BLGS samples exhibited two additional visible peaks at 2θ, of 10◦ and 20◦, which were not detected in X-ray diffraction (XRD) of fresh longan seeds (FLGS) and dried longan fruits (DLGS)
BLGS samples exhibited two additional visible peaks at 2, of 10° and 20°, which were not detected in XRDs of FLGS and DLGS
Summary
Longan (Dimocarpus longan Lour.) is a popular tropical fruit grown in many parts of Asia. In the production process of dried longan, the whole fruits were placed in a drying chamber of an oven, with controlled temperature and hot air circulation. While the moisture was gradually taken out from the flesh (aril), the seed inside the fruit was subjected to heat via the air and the moisture in the flesh This created an environment of the heat-moisture treatment (HMT), which is known to alter the thermal and digestibility properties via changes in starch granular arrangement [5]. The conditions employed in the production of this so-called black longan (BLG), of which the darkish brown to black meat has been used medicinally in the practice of Traditional Chinese Medicine (TCM) [2], could potentially affect the properties of starch, including physicochemical, digestibility, and functional properties. The results were discussed in comparison with starches extracted from seeds of fresh fruits (FLGS) and seeds of oven-dried fruits (DLGS)
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