Extraction and Characterization of Pectin from Sweet Potato (Ipomoea batatas) Peels using Alkaline Extraction Method D. N. Abang Zaidel1,a, N. H. Hamidon1 and N. Mat Zahir1 1Department of Bioprocess and Polymer Engineering, Faculty of Chemical and Energy Engineering, Universiti Teknologi Malaysia, 81310 Skudai, Johor
Abstract Pectin is a polysaccharide consists of galacturonic acid (GalA) that is found in the cell wall of plant. Sweet potato (Ipomoea batatas) peel contains pectin, which acts as thickening and gelling agents in food application. In Malaysia, sweet potato peels are mostly waste materials resulting from the sweet potato processing industry and are normally discarded. The aim of this study was to extract and characterize pectin from sweet potato peels using alkaline extraction. Pectin was extracted using 0.05M, 0.10M, 0.15M, 0.20M and 0.25M sodium hydroxide (NaOH) at temperature 90oC for 15 minutes. Pectin was characterized for its yield, GalA profile, degree of esterification (DE) and rheological properties. GalA and DE of the extracted pectin was characterized using Fourier Transfer Infrared Spectroscopy (FTIR). Rheological properties of pectin were analyzed using viscometer to investigate the viscosity changes due to shear rate. In this study, extraction using 0.25M NaOH yields the highest percentage of pectin which is 16.78% while 0.05M NaOH gives the lowest yield (10.98%). The results proved that the sweet potato pectin has low degree of methyl-esterification. Keywords: pectin, sweet potato, alkaline extraction, degree of esterification INTRODUCTION Pectin is a complicated mixture of polysaccharides that makes up about one third of the cell wall preserved substance of plants. The higher proportions of pectin are found in the middle of lamella of cell wall and much smaller proportions are in the cell wall of grass. Commercially, pectin has broad applications in both the food and pharmaceutical industries, where it acts as gelling and thickening agents (Lapasin & Pricl, 1995; Glicksman, 1979), prevents the formation of cheesy milk layer in gelled milk dessert, and regulates the thickness and mouth-feel of fruit drink powder when the powder is dissolved in cold water (Pedersen, 2002). In addition, pectin has proven to have beneficial effects on human health (Yamaguchi et al., 1994; Tian et al., 2008; Inngjerdingen et al., 2008). Pectin has been widely studied and published but is difficult to characterize as a model system due to the heterogeneous nature of the polymer.
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Currently, commercial pectin is almost exclusively extracted from apple pomace and citrus peels under acidic condition. High-methoxyl pectin is easily extracted by mineral acids. However, extraction of low-methoxyl pectin has to be conducted at higher pH since it is not soluble in low pH. Alkalis used in pectin extraction include sodium hydroxide and potassium hydroxide (Massiot et al., 1988; Nurdjanah, 2008). Although pectin occurs commonly in plants, the source of commercial pectin is limited. This is due to the gelling ability depends on molecular size and degree of esterification. Detection of large quantity of pectin is not enough for the plant to be a commercial source of pectin. Many efforts have been try to prepare pectin from another sources like tropical fruits (Simpson, 1984), soy hull (Gnanasambandam & Proctor, 1999), beet and potato pulp (Turquois, 1999) and others. Francis & Bell (1975) have made a review on the commercialization of pectin from various sources such as sugar cane, guava, water hyacinth, kangkong, papaya, coconut, and many more. However, pectin extracted from those sources has poor gelling ability characteristics as compared to apple and citrus pectin. But recently, Byg et al. (2012) reported that industrial potato waste contain appreciable amount of rhamnogalacturonan I (hairy region of pectin). This opens the possibility to investigate the potential use of other crop residue materials, such as sweet potato residue, as pectin source. The use of sweet potato within the food industrial sector, has led to the manufacturing of considerable amounts of waste materials such as sweet potato peels that are normally discarded. These discarded peels may cause an environmental problem, particularly water pollution. Thus, in addition to being fed to animals, the peels can be used in the production of pectin, which would then increase the potential return for the sweet potato processing industry and reduce the pollution. The aim of this study was to investigate the extractability of pectin from sweet potato peels using alkaline extraction with different concentration of alkali. MATERIALS AND METHODS Materials The purple sweet potatoes are collected from fruit market in Taman Universiti, Johor Bahru, Johor. Chemicals and other reagents used in this study are analytical reagent grade. Sample preparation The sweet potatoes obtained were washed carefully with tap water. Then, the potatoes were peeled and the peels were soaked with ethanol for 30 minutes. After that, the sweet potato peels were washed with tap water again and pressed under hand pressure to remove excess water. The peels were grinded by using a Waring commercial blender. The grinded peels were sieved using cheesecloth to separate the residue from the starch milk. The residue was rinsed thoroughly with running water until the water was clear. The residue was dried in cabinet drier for 18 hours. Then, the dried residue was grinded and packed it in air tight polyethylene bags. It was stored until pectin extraction performed. Pectin extraction Firstly, 10g of sample (dried residue) was dispersed in 250mL of distilled water and heated at 90°C for 15 minutes. The suspension was centrifuged using Hettich Universal 320 R Bench Top Centrifuges at 3,000rpm for 15 minutes. 500mL of 0.05M, 0.1M, 0.15M, 0.2M or 0.0.25M sodium hydroxide (NaOH) was added to the filtrate. The mixture was kept for 2 hours at 25°C and then centrifuged at 10,000 rpm for 15 minutes. The supernatant was neutralized by adding 5M hydrochloric acid (HCl) until the measured pH was 7. The same volume of 95% ethanol as the amount of HCl needed to neutralize the supernatant was added into the mixture. The mixture was stirred for 5 minutes and stored at 4°C for 12
hours. Then the mixture was centrifuged again at 10, 000 rpm for 15 minutes to separate the precipitated pectin from the ethanol solution. The precipitated pectin was washed successfully with 70%, 80 %, and 90% ethanol and centrifuged at 10, 000 rpm for 15 minutes. The pectin was dried in a cabinet drier for 18 hours. Lastly, the dried pectin was grinded, packed and stored in a desiccator until further analysis. Analysis of pectin 1. Yield of pectin Pectin yield can be described as the ratio of dried pectin extracted to dried cell wall materials. Formula for calculation of pectin yield as in equation (1): Weight of dried pectin (g)
Pectin yield (g/100g) = Weight of sample powder taken for extraction (g) × 100
(1)
2. Degree of esterification The degree of esterification (DE) of pectin was determined by Fourier transfer Infrared (FTIR) spectrometry (Perkin Elmer, USA). The degree of esterification was calculated from the calibration curve of the pectin standards. The FTIR analysis was conducted to confirm the presence of the functional group in the sweet potato peels. The infrared spectra are obtained by scanning the extracted pectin in the FTIR and through the interpretation of the infrared absorption spectrum of the chemical bonding of the molecules in the extracted pectin was determined 3. Viscosity measurement Viscosity of pectin solution was measured by using viscometer (Brookfield DV-II+ Pro Programmable Viscometer, USA) with spindle number 42, speed number 5rpm until 20 rpm, at shear rate 19-95 s-1. Pectin samples (1% w/v) were prepared in 20mL distilled water at 29°C. The pectin solution (1.0mL) was used to determine the viscosities for all five samples. RESULTS AND DISCUSSION Analysis of extracted pectin 1. Pectin yield Figure 1 shows the graph of percentage yield of pectin against alkali concentration. The yield of pectin from cell wall material of purple sweet potato were extracted using various concentrations of alkali; 0.05M to 0.25M of sodium hydroxide (NaOH). The graph depicts that the percentage yield of pectin increases as the concentration of NaOH increases. The lowest percentage yield is 10.98% which is at 0.05M NaOH. Meanwhile, the highest percentage yield is 16.78% which is at 0.25M NaOH. The result is similar to a previous study by Nurdjanah (2008), where 0.05M NaOH yields 11.1% of pectin. Meanwhile, acid extraction of sweet potato pectin studied by Zaidel et al. (2015) yielded 74.43% of pectin. According to Knee (1973) and Jarvis et al. (1981), significant amounts of pectin were extracted under alkaline conditions as compared with neutral conditions. Nevertheless, alkaline conditions cause instability in the backbone of the pectin molecule (galacturonic acid), and, consequently, the pectin molecule tends to decompose (Albersheim et al., 1960). As a result of the decomposition of pectin molecules, the extracted pectin cannot be precipitated with alcohol. Therefore, the recovery of the extracted pectin tends to be reduced under alkaline conditions resulting low yield of pectin obtained through the study.
Percentage Yield of Pectin (%)
18 16 14 12 10 8 6 4 2 0 0.05
0.1 0.15 0.2 Alkali Concentration (M NaOH)
0.25
Figure 1. Graph of percentage yield of pectin (%) against alkali concentration (M NaOH) 2. Degree of esterification The Fourier Transfer Infrared (FTIR) spectra results of pectin for all concentrations of NaOH are shown in Figure 2. The broad, strong area between 3200 and 3600cm-1 represent the O-H stretching vibration. In pectin samples, absorption in the O-H region was due to vibrational modes of inter and intra-molecular hydrogen bonds of the galacturonic acid polymer (Chen et al., 2014). The absorption peak at 2900 cm−1 (2800 to 3000cm−1) was attributed to C-H vibrations modes. These include CH, CH2, and CH3 stretching and bending vibrations (Santos et al., 2013). The peaks between 1406 and 1637cm-1 are from OC=O bonding and the signal showed at 1097cm-1 is from C-O bonding. The degree of esterification was calculated from absorbance intensities for 1650 and 1750 cm-1 band. Walter (1991) reported that bands in the region between 1000 and 2000 cm-1; and specifically bands in the regions of 1650 and 1750 cm-1 correspond to the free and esterified carboxyl groups, respectively (Gnanasambandam & Proctor, 2000). The FTIR spectroscopy results proved that the sweet potato pectin has low degree of methylesterification.
Figure 2. FTIR spectroscopy results of 0.05M, 0.10M, 0.15M, 0.20M, and 0.25M of NaOH
3. Rheological properties Pectin acts as a thickening agent by increasing the viscosity of a liquid to induce the textural and sensory properties of particular food products (Lapasin & Pricl, 1995). Rheology has been defined as the science of deformation and flow of matter (Lapasin & Pricl, 1995; Steffe, 1996; Bourne, 2002). At 1 atm pressure, deformation and flow of polysaccharide dispersion are normally initiated either by gravity or an applied shear rate. In this study, low concentration of pectin solution was used. The viscosity reading for all different concentrations of NaOH was obtained with constant shear rate and speed for the spindle of 5rpm until 20 rpm for each sample. The results are shown in Figure 3.
1.6 1.4
Viscosity (Pa.s)
1.2 1
0.05M NaOH
0.8
0.10M NaOH
0.6
0.15M NaOH 0.20M NaOH
0.4
0.25M NaOH
0.2 0 -0.2
19
39
59
79
99
Shear Rate (s-1)
Figure 3. Graph of viscosity against shear rate for different concentration of NaOH The flow behavior of a dilute solution of sweet potato pectin was analyzed at 29 °C. The results as shown in Figure 3 depicted that the viscosity of the sweet potato pectin extracted at different concentration of NaOH depends on the shear rate. The viscosity of pectin for all samples decreased rapidly with increasing shear rate from 19 to 38s-1. This was similar to a study by Chen et al. (2014), where the viscosity of okra pectin was found to decrease rapidly with increasing shear rate (1 to 100 s−1) but decreased less rapidly at higher shear rates (100 to 1000 s−1). It was expected that the sweet potato pectin in this study would exhibited a shear thinning behavior, i.e. decreasing viscosity with increasing shear rate under steady-shear conditions. But when the shear rate increased from 38s-1 to 95s-1, the viscosity of pectin increased gradually. The flow behaviors of pectin extracted with different concentration of NaOH were showing a similar trend, i.e. rapid decrease from 19 to 38s-1 and gradual increase from 38 to 95s-1. When comparing the different concentration of extracting solvent on the viscosity of pectin, the lowest concentration of NaOH having the most distinct trend compared to higher concentration.
CONCLUSION As a conclusion, the peels of sweet potato are good sources of pectin which can be extracted using alkaline extraction method. However, the pectin yield and the characteristics are dependent on the extraction process. In this study, the percentage of pectin yield is increasing as the concentration of sodium hydroxide (NaOH) increases. The lowest percentage of pectin yield is 10.98% and the highest percentage is 16.78%. Analysis by Fourier Transfer Infrared (FTIR) spectroscopy determined the main functional group of sweet potato pectin and degree of esterification. The extraction of pectin from sweet potato peels using NaOH resulted in low-methoxyl pectin.
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