Ultrasonic-assisted protein extraction from sunflower meal: Kinetic modeling, functional, and structural traits

Highlights

• A new kinetic model for extraction of sunflower protein (SP) was developed.

• Sonication (observably 20/28 kHz) increased the release and diffusion of soluble SP.

• Ultrasonicated SP displayed higher solubility and oil holding efficacy than control.

• AFM indicated the collapse of cross-linkages among SP molecules following sonication.

• FTIR spectra exhibited notable changes in secondary structure of SP after sonication.

Abstract

Effect of mono and dual frequency (20, 28 and 20/28 kHz) ultrasound action on extraction kinetics, functional, and structural traits of sunflower protein (SP) was examined. A simplified model based on Fick's second law was developed, and the mass transfer of soluble SP in a heterogeneous system was successfully described. Findings suggested that ultrasound action significantly increased (p < 0.05) the release and diffusion of soluble SP across the cell wall into extraction solvent, causing enhanced observed rate constant (k) and diffusion-effective coefficient (D_s) values over control. Moreover, dual frequency sonication (20/28 kHz) considerably increased oil holding efficacy and surface charge (by 21.07 and 32.15%, respectively), but reduced water holding efficacy and particle size (by 40.74 and 26.61%, respectively) relative to untreated sample (p < 0.05). Also, ultrasonicated SP displayed excellent solubility under varying pH (2−10), likened to the control (p < 0.05). Scanning electron microscopy (SEM) and atomic force microscopy (AFM) outcomes indicated that ultrasonicated SP showed irregular fragments, heterogenous/ disordered structure and small-sized particles, suggesting that sonication (notably 20/28 kHz) destroyed the cross-linkages among SP molecules. FT-IR spectroscopy exhibited that ultrasonication improved the content of β-sheet from 8.52–18.94% and random coil from 11.63–21.71%, whereas reductions in α-helix and β-turn were noticed, implying limited unfolding of SP structure and decreases in intermolecular interactions. Intrinsic fluorescence analysis revealed that dual frequency treatment was observed to be more efficient in altering the tertiary structure of SP reference to single frequency and control.

Industrial relevance

Sunflower protein, isolated from sunflower residue, is a potentially low-cost resource for food applications. Ultrasonication is reported to improve the extraction of proteins as well as enhancing their functionalities. The existing study displayed that ultrasonication (observably dual frequency treated) was successfully applied to improve the extractability of soluble sunflower protein, and enhanced its functionality (solubility and OH_e) reference to control. The research outcomes may benefit food/ chemical industries in extraction and/or alteration of sunflower protein in new applications.

Introduction

The worldwide demand for protein in human nutrition is predicted to increase in the coming few years. At the same time, current reports indicated that around 25% of the global populace does not have enough proteins supply (Azam, Khan, Ahmad, Khan, & Ali, 2014), however, a huge number of protein resources (e.g. sunflower meal) remain underexploited. Sunflower meal contains high content of protein (300 to 500 g/kg), which makes it a potential source of cheap protein (Dorrell & Vick, 1997). Sunflower protein (SP) is low in anti-nutritional substances and is devoid of toxic compounds, making it an alternate source of protein (González-Pérez & Vereijken, 2007). Additionally, the functional traits of SP are close to leguminous proteins (González-Pérez et al., 2005). Extraction of SP, therefore, is imperative as it may be very useful in food, chemical and pharmaceutical applications as regards supplementing other vegetable and/or animal proteins. Nevertheless, traditional extraction methods come with many limitations such as high power/ solvent consumption, high operation cost, loss in functionality and laborious operation with low extraction efficacy (Wang & Weller, 2006). Consequently, researchers are still looking for alternative technologies to extract SP that will have good functionality, economically/ technically fit for industry.

Presently, there are several novel techniques that can be employed for protein extraction to overcome the limitations of traditional methods, including microwave (Görgüç, Özer, & Yılmaz, 2020; Varghese & Pare, 2019), ultrasonication (Fahmi, Khodaiyan, Pourahmad, & Emam-djomeh, 2011; K. Li, Ma, Li, Zhang, & Dai, 2016) and pulsed electric field (Buchmann, Brändle, Haberkorn, Hiestand, & Mathys, 2019; Zhou, He, & Zhou, 2017). Among them, ultrasonication, a novel physical technology, is widely used to pretreat samples so as to reduce power consumption, shorten extraction time and improve protein extractability and functionality (Elhag et al., 2019; Hadiyanto & Adety, 2018; Mintah et al., 2019). Li, Ma, et al., 2016, Li et al., 2016 reported that ultrasound action increased the extractability of protein from rice flour by 51.15% over conventional method (alkali extraction). Similarly, Ly, Tran, Tran, Ton, and Le (2018) indicated that the extraction rate for ultrasonic-aided-protein extraction from rice bran was 3.48 times higher than that of the traditional approach. Improvement in the protein extractability is mainly attributed to the phenomenon of acoustic cavitation, that is, disturbing cell membranes and improving mass transfer across cell walls. Moreover, Malik, Sharma, and Saini (2017) assayed the effect of high intensity ultrasonication (bath and probe) on functionality of sunflower protein. They reported that ultrasonication, especially probe sonication, considerably enhanced solubility, emulsifying capacity/ stability, foamability and foam stability, whereas water binding efficacy (WB_e) was reduced. Also, Mintah et al. (2020) found that sonication improved dispersibility, oil absorption efficacy and sulfhydryl groups of insect protein and its hydrolysates, whilst reduction in WB_e, particle size, turbidity and surface charge were observed.

Although, some lab-scale studies indicate that ultrasonication has already been employed to improve the extractability of protein. However, most of the existing research works have focused on optimization of the process conditions and qualitative examination of their impacts, with very few quantitative description/ analyses on kinetics behavior of sonication-aided extraction, which is substantial to understand the nature and practicability of the extraction process. It is very important, therefore, to utilize mathematical tool(s) to simulate the extraction process in a heterogeneous system. Recently, many empirical, semi-empirical and theoretical models were developed and established to simulate the diffusion behavior of the solid-liquid extraction from natural raw materials, such as Weibull-type model, pseudo first-order model, swelling/ diffusion model, phenomenological model, two site kinetic model, etc.(Ho, Cacace, & Mazza, 2008). Nonetheless, these models only considered a single process of aggregation or diffusion factors. Additionally, to the best of our knowledge, the mathematical model considering the aggregation and diffusion has not yet been reported to simulate/ describe the real sonication-aided extraction of SP.

Furthermore, native SP does not have desirable functionality (mainly solubility and oil holding efficacy) for food preparation (Malik and Saini, 2017, Malik and Saini, 2018), due to the denaturation of protein during oil extraction. The technique (ultrasonication) applied in treating SP impacts functionality and structural traits of other isolated proteins (Hu et al., 2013; Li et al., 2020; Mintah et al., 2019, Mintah et al., 2020). Nonetheless, the influences of sonication-aided extraction on functionality and structural attributes of SP have not yet been examined in depth. Understanding the impact of different sonication frequency modes on extraction kinetics, techno-functionality and structural attributes of SP may result in operational application of such in various food systems. Thus, the current study aimed at examining the extraction of soluble SP employing a simplified kinetic model considering aggregation and diffusion to deduce the constants which could disclose the efficacy of ultrasonication. Also, alterations in functional and structural traits of SP were studied to assess the effect of sonication on SP.