Production and characterization of hydrophilic and hydrophobic sunflower protein isolate nanofibers by electrospinning method

https://doi.org/10.1016/j.ijbiomac.2018.07.132Get rights and content

Abstract

In the present study, sunflower protein isolate (SPI) was extracted and applied for production of nanofiber using polyvinyl alcohol (PVA) in different SPI:PVA volume ratios (0:100, 20:80, 40:60, 60:40, 80:20 and 100:0) under two voltages (18 and 23 kV) and two flow rates (0.5 and 0.75 ml/h). Nanofibers with average diameters ranging from 304 to 400 nm were achieved. SPI:PVA volume ratio of 40:60 at voltage of 18 kV and flow rate of 0.75 ml/h were determined as the optimum conditions for electrospinning which produced bead-free and uniform nanofibers with average diameter of 398 nm. The effects of heating (150 °C, for 24 h) and calcium chloride (1% W/V) solution solely and together on nanofibers hydrophobicity were also examined. SEM images after 24 h soaking in lactate buffer (pH 5.5) showed that only heat treatment could make nanofibers hydrophobic, which was proved by contact angle data. FTIR analysis showed denaturation of protein and interactions between protein and PVA after heat treatment. Moreover, DSC and TGA results indicated miscibility of protein and PVA, and higher thermal stability of heat treated sample. This study showed the potential application of soy protein as a byproduct for production of both hydrophobic and hydrophilic nanofibers.

Introduction

Electrospinning is a simple and effective method to prepare fibers with diameters ranging from nanometer to several micrometers using solutions of both natural and synthetic polymers [1,2]. This method is preferred over other primitive nanofibers producing techniques such as drawing, template synthesis, phase separation and self-assembly due to producing nanofibers with tunable length, diameter and pure size and different structures, applicable to a wide variety of materials, ease of material combination, ease of fiber functionalization, relatively low start-up cost, mass production capability and possibility of using for large scale fiber production [[3], [4], [5], [6]].

In electrospinning process, an electrical field is applied between positively charged syringe needle and a negatively charged grounded collector plate. By passing voltage from a critical value, electrostatic forces overcome surface tension of the polymer fluid. Thus, the solution forms a Taylor cone at the syringe tip and a fine jet is ejected. Repulsive Columbus forces and solvent evaporation cause continuous elongation of the jet and diameter decrease, respectively. Finally, ultrafine fibers deposit over the surface of collector and lose their charge [[6], [7], [8]].

Different materials were examined for producing electrospun nanofibers. Natural polymers are preferred over the synthetic ones, because of their ‘bio-’ and ‘cyto-’ compatibility, and their availability as byproducts of food and agricultural processing factories [5]. Among natural polymers, proteins, particularly plant proteins, are preferred over the carbohydrates because of their nutritive values and low potential to be immunogenic [9].

Oilseeds meals are the most important source of vegetable proteins. Soy proteins are the main oilseed proteins used as functional ingredients in foods. They are appropriate ingredients, due to high nutritive value, lack of anti-nutritional compounds and well-balanced amino acid composition [10,11].

Proteins alone may not always produce nanofibers mainly due to their complex secondary and tertiary structures. In order to be spinnable, the proteins should be either well dissolved in a random coil fashion, or mixed with an aiding polymer which is spinnable. Polyvinylalcohol (PVA) is a common biocompatible, biodegradable and FDA approved ingredient which dissolves in similar conditions as proteins [5].

Protein nanofibers usually are not stable in aqueous medium, due to their hydrophilic nature which may limit their application. By crosslinking, stability of nanofibers is enhanced whereas their biodegradation rate is reduced [12]. Chemical or physical crosslinking treatments could be used to stabilize the porous structure of nanofibers. Divalent cations such as calcium as a chemical crosslinker could be used for improvement of structural integrity by allowing bridge formation between carboxylic acid and negatively charged groups on side chains of the protein molecules [13]. Heating as a physical treatment also improves hydrophobic interactions by water molecules egressing [14].

In this research, possibility of producing nanofibers from sunflower protein isolate (SPI) was investigated and the effects of calcium chloride and heat treatments solely and together on hydrophobicity and physicochemical properties of nanofibers were studied.

Section snippets

Materials

Sunflower meal was provided by a local oil factory and its protein isolate (SPI) was extracted based on Ayhllon-Meixueiro et al. method [15], briefly 100 g sunflower oil cake was crushed (average particle size 0.1 mm) and suspended in 2 L distilled water. While continuous stirring, solution pH was adjusted to 12 by dropwise addition of NaOH 1 M in order to dissolution of proteins. After centrifugation (500 rpm, 15 min) supernatant was separated and HCl 1 M was added dropwise till reaching pH to

Protein content and yield determination

The amount of protein content of meal and extracted isolate were 28.5% and 81.5%, respectively. However, these values were reported about 31% and 97%, respectively, by Ayhllon-Meixueiro et al. [15]. Differences could be due to sunflower variety along with extraction method. The extraction yields were calculated as 23.75 ± 1.05% and 67.91 ± 2.2% based on dry matter and protein content, respectively.

Properties of polymeric solutions

Viscosity, surface tension and electrical conductivity of prepared solutions were shown in Table 1.

Conclusion

Sunflower proteins are considered as a valuable byproduct with high nutritional value. Till now no industrial applications were suggested due to their dark undesirable color. In the present study, by the aid of PVA, soy proteins were successfully electrospun. The SPI:PVA in the ratio of 40:60 was determined as the suitable solution for electrospinning which led to production of fine and uniform nanofibers with average diameter of 398 nm (voltage: 18 kV and flow rate: 0.75 ml/h). Electrospun

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