Evaluation of physicochemical properties of pectin extracted from eggplant waste using ultrasound technique

Introduction: Pectin is a type of water-soluble hetero-polysaccharide that is present in the primary cell wall of plant and is used as a jellying, thickening and stabilizing agent in various food products. The degree of esterification is the most important determinant of the use of pectin in the food products, according to pectin is divided into two groups: high-esterification pectin (50% degree of esterification) and pectin with degree low esterification (degree of esterification less than 50 %(. Considering the high use of pectin in the food products, researchers are now looking for new sources of pectin extraction, among which the use of food waste has been considered high, because the waste of food factories is a major challenge for food manufacturers. Eggplant (Solanum melongena) belongs to the Solanaceae family, which is used extensively in the world. The plan species is believed to have originated in India, where it continues to grow in southern and eastern Asia.  The skin and warhead of this product, which is discarded as waste, can be used as a valuable source for pectin extraction. The most commonly used methods for pectin extracting are the use of hot water, along with acids, which is a time-consuming process and the waste discard of this method is environmentally problematic. Therefore, the use of new methods such as ultrasound has been considered by researcher to minimize the limitations of the traditional method of pectin extracting. The purpose of this study was to extract pectin from eggplant waste using ultrasound and evaluate its physicochemical properties. Material and methods: The waste of eggplant from restaurant of agricultural sciences and natural resources university of Khuzestan were prepared. The waste was dried in an oven at 60 ºC to reach constant weight. The dried waste was powdered using a grinder and passed through the sieve. The ultrasound was used to extract pectin from eggplant waste (skin and warhead). For this purpose, the effect of ultrasound time (40-80 min) and dry matter /solvent ratio (1:10 – 1:30 g/ml) on extraction efficiency degree of esterification of extracted pectin were investigated. The FTIR (wavelengths scanned 4000-400 cm−1) and rheological behavior were studied to evaluate the performance characteristics of the extracted pectin. Analysis of variance (ANOVA) procedure followed by Duncan’s test using SPSS 16 (SPSS Inc., Chicago, IL, USA) software was applied to determine the significant difference (P < 0.05) between treatment means. Result & discussion: Based on results, increasing the extraction time had a significant effect (P<0.05) on the pectin extraction, so that the increase in extraction time from 40 to 60 min increased the extraction efficiency from 14.05 ±0.21 to 29.35±0.21 (%), which is probably due to the fact that the cavitation causes the cell wall to break down and more solvent penetrates the cell matrix, which results in increased extraction of pectin. The highest efficiency of pectin was obtained in the dry matter /solvent ratio (1:10 g/ml) and 60 min. The highest degree of esterification (84.18 ± 0.1 %) was obtained in the dry matter /solvent ratio (1:20 g/ml) and 60 min. Also, the degree of esterification of the obtained pectin varied from 67.69 ± 0.02 to 84.14 ± 0.1 %), which indicated the high quality of pectin was extracted. Due to the fact that the steric bonds are more unstable than acidic hydrolysis in comparison with glycosidic bonds, the higher degree of esterification indicates less damage to the pectin structure during the extraction process. FTIR showed all of the pectin's specific spectra and abundance of methoxy groups in extracted pectin. The FT-IR spectra show the characteristic absorption of -CH at the ranges of 3000-2800 cm-1 and at 1421 cm-1, while the wide band at 3406 cm–1 was assigned to the -OH stretching vibration. The wide band at the ranges of 1700-1600 cm-1 can be due to the stretching vibrations of the C=O bonds in the backbone of crude polysaccharide because of presence of uronic acid. Existence of a peak at 1200-900 cm-1 indicates that pectin contained multiple vibrations of glycosidic (C–O-C) and pyranoid (C=O) linkages due to the characteristic of the pyranose form of glucosyl residues. The apparent viscosity of the extracted pectin solution decreased with increasing shear rate (0.5 to 10 s-1) while in the higher shear rate (10 to 100 s-1), the apparent viscosity of the pectin solution remained almost constant. This process shows that the produced pectin solution at low shear rate exhibits pseudo plastic behavior, while at highest shear rate exhibits Newtonian behavior. These results indicated that eggplant waste could be used as a good source of high-performance pectin.