Bioactive compounds via Glycosylation reaction from Longan Pulp (Dimocarpus longan) for functional food ingredients

Abstract

This research aimed to evaluate an adaptive food process corresponding the glycosylation or Maillard reaction that was simply applicable to produce a new type of longan product (black longan pulp). This reaction provided to the bioactive substances in longan pulp such as sugar-protein conjugates (SPC) and rare sugars. The rudimentary study on physicochemical and biological qualities of four different Edor longan pulps were investigated: (1) Fresh longan pulp (FLP), (2) Golden dried longan pulp (GDLP), (3) Dried whole longan pulp (DWLP), and (4) Dried whole longan pulp of one-year storage (DWLP-1Y). The conjugated compounds, rare sugars (D-psicose or D-allulose and D-allose) and bioactivities especially the antioxidant and anti-cholinesterase (acetylcholinesterase; AChE and butyrylcholinesterase; BuChE) were exhibited in high quantity for DWLP-1Y. Owing to drying process and long-term storage, it generated the SPC and rare sugars formation, resulting in the bioactive efficiency improvement. Consequently, the related factors during long-term storage of dried longan product could be developed as an experimental model for black longan production under specialized conditions, which temperature, relative humidity and time were conducted to accelerate the glycosylation in longan pulp. Dried whole fruits were produced to the black longan product under the moist-dryheating condition: (1) dried whole longan (DWL) and (2) dried longan without peel (N-DWL). The aging temperature and time were conducted at 60ºC under control the relative humidity at 75% for 10 days. The results showed that only the moisture content of black longan pulp was significantly difference (p≤0.05), while other physicochemical qualities were not significant difference (p>0.05). Thus, all forms of dried whole longan could be used as a raw material to produce the black longan with nonsignificant difference on the chemical content. Black dried longan pulp (BDLP) by aging dried whole longan at 60ºC under control relative humidity at 75% for 5-50 days was investigated. The longest aging time of black dried longan pulp showed higher antioxidant activities, a lower sucrose level, and more rare sugar formation than the untreated dried longan pulp. The amount of reducing sugar gradually increased with the aging period whereas it slightly decreased after 30 days of incubation. The rare sugars (D-psicose and D-allose) illustrated in all black dried longan pulp samples and D-mannose also gradually increased during reaction time. The appearance of high molecular weight band on top of SDS-PAGE gel confirmed the presence of conjugated compounds corresponding to the degree of glycation value of black dried longan pulp at 20 days of aging time. Additionally, this condition provided a low level of 5-Hydroxymethylfurfural (HMF) in pulp in order to put in a safe quantity for consumers. The suitable temperature of black longan pulp production was studied in different aging temperature at 50, 60, 70, and 80°C under controlled the relative humidity approximately 70-75% for 20 days. The increasing of temperature provided the physicochemical changes on longan pulps such as color, saccharides component (sucrose, D-fructose, D-glucose, D-mannose, D-psicose, and D-allose). However, the using of high heating temperature during aging process might generate the undesirable components from the final stage of Maillard reaction. The HMF content extremely increased in sample aged more than 60°C. Therefore, the selected condition of black dried longan production was 60°C of aging temperature and 20 days of aging time, which the HMF quantity not ranged over the tolerable daily intake. The flavor profile study in 3 types of dried longan products included of (1) Dried whole longan pulp (DWLP), (2) Golden dried longan pulp (GDLP), and (3) Black dried longan pulp (BDLP) that were analyzed the volatile compounds measurement by using GC-MS and the generic descriptive analysis method by panelists. Overall results from MS spectral library, chemical elements, physical qualities and sensory attributes of dried longan products were demonstrated their relation with a principal component analysis (PCA). Their characteristics could divide into three groups, which the black dried longan pulp was separately illustrated from the golden dried longan pulp and dried whole longan pulp indicating the explicit specific descriptive terminology and chemical compositions. Optimization of extraction conditions for black dried longan pulp was performed by using the high-power ultrasonic processor (500-Watt, 20 kHz) and water as solvent. Three extraction factors on black dried longan pulp were examined that consisted of the amplitude level (20-40%), extraction time (5-15 min), and extraction ratio between water and pulp (50-200 mL/10 g pulp). The optimized extraction conditions were 31% of amplitude level, 11 min of extraction time, and 50 mL/ 10 g pulp of extraction ratio. The crude black dried longan pulp and dried whole longan pulp extract were produced from the optimum extraction condition, which they were studied on the functional, cytotoxicity, and bioactive properties. Focusing on functional properties, the foam and emulsion qualities were investigated by mixing longan crude extracts in food system model. Due to the extracts contained mainly saccharide components than protein, which they were unable to generate their properties by themselves. Thus, the study on foam and emulsion properties was used a soy protein isolated as major protein source in system model test of acid and neutral condition. The crude extracts from dried whole longan pulp and black dried longan pulp slightly improved the ability of foam and emulsion at pH 7.0, while the stability of foam was not differenced. The mean particle size diameter of emulsion samples at pH 3.5 and 7.0 were ranged of 1,292-1,398 nm and 275-324 nm, respectively with the particle size distribution of all emulsion samples exhibited in a moderate monodispersity. The cytotoxicity test of black dried longan pulp and dried whole longan pulp extracts showed no toxic for human cells, safety for consumption, and no effect of inhibition toward the liver cancer cells. The antioxidant activities significantly increased (p≤0.05) when compared to untreated longan pulp extract. In addition, AChE and BuChE inhibition of black longan pulp extract exposed the better inhibitory activity than the dried whole longan pulp extract, describing IC50 value of black dried longan extract concentration approximately 1.51 and 1.34 mg/mL for anti-AChE and anti-BuChE, respectively. However, anti-microbial activity of both longan extracts on Staphylococcus aureus and Escherichia coli showed no inhibition effect against these bacteria. Eventually, the production protocol of black longan extract for food ingredient was achieved. A continuous process of crude extract was performed to produce a black longan syrup (~50°Brix), which contained the biochemical components (rare sugars and sugar-protein conjugated compounds). This syrup product also exhibited the antioxidant activities and cholinesterase inhibition activity. Hence, all information of black dried longan product supported that this type of longan could be used as alternative functional food ingredients in food products.