Pectin extraction from eggplant peel using microwave and evaluation of its properties

Pectin is a complex mixture of polysaccharides in the primary cell wall of plants which is a polymer of α-galacturonic acid, to which neutral sugar is connected to form aside chain. Pectin is a natural food additive used extensively in the food industry as thickener, texturizer, emulsifier, stabilizer and gelling agent. In 2018, the world market demand for pectin was in excess of 60,000 tons and Europe was estimated to have the largest market with 31,000 tons (valued about US$420). In Iran, about one hundred tons of pectin is consumed annually in the food and pharmaceutical industries, all of which are supplied from abroad, and due to its price in the world market is of considerable magnitude. Therefore, the study of its production within the country could be of great importance. During processing and canning of eggplant, its cap and skin are discarded as waste which can be considered as a valuable ingredient in the food industry. The main objective of this study was to develop an MAE (Microwave assisted extraction) of pectin from eggplant peel and investigate the effect of process variables (microwave power, irradiation time and pH) and the response (extraction yield, degree of esterification, galacturonic acid content and emulsifying activity); and to obtain optimum conditions for maximum extraction yield of pectin from eggplant peel. The response surface methodology (RSM) using Box-Behnken design was employed in this study.

Eggplant peels were provided by Yek-o-Yek factory as an unwanted by-product. Then, the peels were divided into small pieces and dried in hot air oven at 65 °C until it attains constant weight. The peels were then grinded and passed through a 40-mesh sieve to obtain powdered sample. The independent variables examined were microwave power (360–450–540 W), irradiation time (1–2.5–4 min) and solution pH (1–2–3). MAE of pectin was performed in an ordinary household microwave oven with a total of 17 treatments according to RSM. About 5 g of dried eggplant peel powder was weighed and placed into a 250 ml beaker, 75 ml distilled water (the liquid-solid ratio 15:1 v/w) containing different pH values was added and exposed to microwave radiation at different powers for the selected irradiation times. After microwave heating, the mixture in the beaker was allowed to be cooled down and filtered using filter paper (Whatman no-1). The filtered extract was centrifuged and the supernatant was precipitated with an equal volume of 98% (v/v) ethanol. The coagulated pectin mass was washed with 98% (v/v) ethanol for two times and dried in oven at 60 ° C until it attains constant weight. The pectin extraction yield was calculated by dividing the weight of dried pectin to the weight of dried eggplant peel powder. Galactoronic acid content of pectins was determined using the meta-hydroxydiphenyl method. The esterification degree of the pectins was determined by the titrimetric method with minor modifications. The emulsifying activity (EA) of the eggplant peel pectins were analyzed according to the method by Dalev & Simeonova (1995). Optimum extraction conditions to achieve maximum extraction efficiency, degree of esterification, galacturonic acid content and emulsifying activity were determined. Then, the stability of the oil-based emulsion prepared by mixing 0.5% w/w solution of pectin extracted in optimal conditions and corn oil, were examined at 4 and 23 ° C. In addition, the behavior of pectin extracted under optimal conditions (at concentrations of 0.1, 0.2, 1 and 2%) and its spectra using a Fourier transform infrared spectrometer (FTIR) were investigated.

The results indicated that the extraction efficiency, esterification degree, the amount of galacturonic acid, and the emulsifying activity of the pectins extracted were 2.20- 17.16%, 20.20- 36.13%, 51.3- 74.7%, and 1.87- 21.64%, respectively. With increasing microwave power, irradiation time and decrease of solution pH the extraction efficiency of pectin extracted increased, while esterification degree decreased. The amount of galacturonic acid and emulsifying activity showed an upward trend up to microwave power of 450 watts and the irradiation time of 3 minutes after which demonstrated a downward trend. The optimum conditions for reach to the maximum extraction yield, galacturonic acid content and emulsifying activity were in microwave power of 360 W, irradiation time of 4 min and pH of 1 that at these conditions, the extraction yield of 16.17, galacturonic acid content of 70.81 and emulsifying activity of 2.68 were predicted. Also, these observations indicated that with increase in concentration, the flow behavior of pectin solutions was changed from Newtonian to pseudo-plastic. In addition the stability of pectin-stabilized emulsion at 4°C was more than 23°C. In general, this study showed that the microwave method could be used as a novel and high-performance method for extracting pectin from eggplant peel.