**1. Introduction**

The increasing demand for high-quality food-grade proteins exhibiting acceptable nutritional and functional properties represents a challenge. Animal proteins are considered to be high-quality proteins but are expensive to produce, with an energy input to protein ratio of 14:1 [1]. Plant proteins may be an economical option to meet the increasing scarcity of protein resources [2]. However, the exclusive use of plant-derived protein is not optimal from a nutritional perspective, as essential amino acid requirements are not met. Nonetheless, mixing plant protein with animal protein can sustain the nutritional requirements of humans. Increasingly, the combination of plant and animal proteins is found to result in a synergistic benefit, which makes the design of foods with both plant and animal proteins a research hotspot [3].

Mixed proteins can be classified into two forms, protein blends (BL) and protein co-precipitates (Co) [4]. The differences in the preparation methods used to generate Co

**Citation:** Zhou, X.; Zhang, C.; Cao, W.; Zhou, C.; Zheng, H.; Zhao, L. A Comparative Functional Analysis of Pea Protein and Grass Carp Protein Mixture via Blending and Co-Precipitation. *Foods* **2021**, *10*, 3037. https://doi.org/10.3390/foods10123037

Academic Editor: Stefania Masci

Received: 31 October 2021 Accepted: 4 December 2021 Published: 7 December 2021

**Publisher's Note:** MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

**Copyright:** © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

and BL are presented in Figure 1. BL refers to the blending of isolated proteins. Mixtures of pea protein isolate (PPI) and cod protein isolate as well as the combination of soy protein isolate and whey protein isolate exhibit synergistic emulsifying effects; however, when the proportion of plant protein replacing animal protein is increased, the synergistic effect is weakened, indicating that BL application has certain limitations [5,6]. Some studies suggest that the emulsifying gelation of directly mixed plant and animal proteins has an obvious antagonistic effect [7,8]. Concerning the preparation of Co, heterologous proteins are processed via isoelectric solubilization/precipitation (ISP) [9]. Driven by pH, proteins from different sources are simultaneously dissolved and precipitated to promote interaction between heteroproteins; generate additional disulfide bonds; change the subunit composition [10]; and alter the surface charge, solubility, and surface hydrophobicity of the proteins, thus effectively improving their functional properties [11]. Studies have found that legume–rapeseed Co has better nutritional and functional properties than single protein [12], and soy–whey protein Co has high water-holding and gel formation capacity [13]. Previously, we investigated the functional properties of soy–tilapia Co and found that the solubility and emulsifying properties of the Co were better than those of the single protein [14–16]. However, there are few comparative studies of BL and Co.

**Figure 1.** Schematic diagram of the preparation of the pea protein isolate (PPI), grass carp protein isolate (CPI), protein blends (BL), and protein co-precipitates (Co).

> Peas (*Pisum sativum* L.) are one of the main legumes grown worldwide, with an annual global output of approximately 14.2 million tons [17]. Peas containing 20–30% protein are rich in lysine but deficient in methionine. Moreover, compared with soybean, pea protein is not associated with a risk of sensitization or transgenic origin. PPI has better emulsifying properties than soybean protein isolate under most pH values and protein concentrations [18]. Grass carp (*Ctenopharyngodon idellus*) is one of the four major carps in China; it is widely consumed in Asian countries and is rich in methionine. Grass carp protein isolate (CPI) can inhibit the flocculation of oil droplets and oxidization of lipids. Therefore, CPI is a potential emulsifier. However, it has poor stability and must be modified or mixed with other proteins for various industrial applications [19,20]. In order to conduct a broader study, pea and grass carp mixed proteins were selected, based on the cost of raw material and the functional properties of the proteins.

> The properties of proteins are easily influenced by environmental conditions. pH is the primary factor affecting the properties of proteins, and it regulates electrostatic interactions. It drives the adsorption, reorientation, alignment, and formation of viscoelastic films on the interface, thus changing the stability of the interface [21]. This study evaluated the effects of blending and co-precipitation on the functional properties of pea and grass carp mixed protein by analyzing the solubility, foaming, and emulsifying properties of BL and Co at

different pH values. The goal was to provide a basis for the subsequent development of products containing plant and animal proteins.
