Preparation, physicochemical and functional properties of protein and extraction and antioxidant activities of phenolics from pericarp and seeds of bitter melon (Momordica charantia).

摘要

Bitter melon (Momordica charantia L.), a member of cucurbitaceae, is cultivated for its edible fruit as a vegetable and for medicinal purposes. Seeds are a rich source of protein, and may serve as a potential source of functional proteins. In addition to the proteins, phenolic compounds from bitter melon could serve as natural antioxidants which help to scavenge free radicals to prevent an oxidative damage. The objectives of this study were to determine proximate composition, amino acid, and mineral contents of the pericarp (fleshy portions) and seeds from bitter melons harvested at three maturation stages (immature, mature, and ripe), extract protein from the seeds of ripe bitter melon for investigation of its characteristics and functionalities, and enhance its functional properties by glycosylation, and extract phenolics from these tissues using ethanol and water solvent systems for determining total phenolic contents, phenolic acid constituents, and antioxidant activity of the extracts. The proximate analysis of pericarp and seeds of bitter melons harvested at three maturity stages showed that moisture, starch and total dietary fiber contents of immature, mature, and ripe pericarps were significantly higher (P-value 0.05) than those of ripe and mature seeds. Conversely, lipid and protein contents of mature and ripe seeds were statistically higher (P-value 0.05) than those of immature, mature, and ripe pericarp. The proteins are a good source of 7 essential amino acids. Ripe seeds, contained more than 30% of protein, could be a potential protein source as functional ingredients. Major minerals in bitter melon were P, K, Mg, S, and Ca. Osborne procedure was used to fractionate protein fractions from ripe bitter melon seeds. The main protein fraction in the seeds was albumin (49.3%), followed by globulin (29.3%), and glutelin (3.1%), while prolamin was not detected, and 18.3% of the protein was non-extractable. Molecular sizes of all protein fractions in the seeds were about 45 and 55 kDa. Denaturation temperatures of albumin, globulin, and glutelin were at 111.9, 117.3, and 133.6°C, respectively. For applicable reason, one step protein extraction was conducted. Optimization of protein extraction conducted using response surface methodology (RSM) showed that optimum conditions for one-step protein extraction from bitter melon seeds were at a pH of 9.0 and NaCl concentration of 1.3 M. Bitter melon seed protein isolate (BMSPI) prepared using these optimum extraction conditions was compared to soy protein isolate (SPI) because SPI is used in a wide variety of food products. BMSPI was more hydrophobic on the surface than SPI. Electrophoretogram profiles showed that BMSPI was not as complex as SPI. BMSPI was a unique protein that could be considered as a high quality, due to its higher essential amino acid contents that exceeded the requirements for pre-school children for minimum essential amino acids, with exception of threonine. While SPI had two denaturation temperatures, BMSPI had a single higher denaturation temperature (113.1°C) that was higher than those of SPI. Native BMSPI demonstrated lower emulsifying activity (EA), foaming capacity (FC), and foaming stability (FS) in comparison to SPI, but higher emulsion stability (ES). Glycosylation of BMSPI with glucose under varying temps (40/50/60°C) and relative humidities (RHs) (50/65/85%) resulted in varying degree of glycosylation (DG); higher temp (60°C) and RH (85%) produced higher DG as expected. However, the DG of the glycosylated BMSPI was affected more by temperature than RH. Insignificant difference was observed in denaturation temperatures between native BMSPI and glycosylated BMSPI. A negative correlation between the DG and the solubility or surface hydrophobicity was observed, while a positive correlation was observed between DG and emulsifying and foaming properties. This glycosylated BMSPI with better emulsifying and foaming properties could be used in a variety of food products where such properties are required.;Eighty percents of ethanol extracted the highest amount of phenolics from both the pericarp and seeds of bitter melon. Main phenolic acid constituents in the extracts were catechin, gallic acid, gentisic acid, chlorogenic acid, and epicatechin as measured by a high pressure liquid chromatography (HPLC). There were low correlations between total phenolic contents and anti-radical power values of the extracts; the extracts with higher the total phenolic contents did not always had the higher anti-radical power values or antioxidant activities. Bitter melon phenolic extracts are potential natural antioxidant substance sources and could possible be used as natural antioxidant agents in suitable food products.