High Moisture Extrusion of Soy Protein: Investigations on the Formation of Anisotropic Product Structure

Abstract

The high moisture extrusion of plant proteins is well suited for the production of protein-rich products that imitate meat in their structure and texture. The desired anisotropic product structure of these meat analogues is achieved by extrusion at high moisture content (>40%) and elevated temperatures (>100 °C); a cooling die prevents expansion of the matrix and facilitates the formation of the anisotropic structure. Although there are many studies focusing on this process, the mechanisms behind the structure formation still remain largely unknown. Ongoing discussions are based on two very different hypotheses: structure formation due to alignment and stabilization of proteins at the molecular level vs. structure formation due to morphology development in multiphase systems. The aim of this paper is, therefore, to investigate the mechanism responsible for the formation of anisotropic structures during the high moisture extrusion of plant proteins. A model protein, soy protein isolate, is extruded at high moisture content and the changes in protein–protein interactions and microstructure are investigated. Anisotropic structures are achieved under the given conditions and are influenced by the material temperature (between 124 and 135 °C). Extrusion processing has a negligible effect on protein–protein interactions, suggesting that an alignment of protein molecules is not required for the structure formation. Instead, the extrudates show a distinct multiphase system. This system consists of a water-rich, dispersed phase surrounded by a water-poor, i.e., protein-rich, continuous phase. These findings could be helpful in the future process and product design of novel plant-based meat analogues.

Effect of Oil Content and Oil Addition Point on the Extrusion Processing of Wheat Gluten-Based Meat Analogues

“…Since micro-CT imaging is a non-invasive method for assessing three-dimensional network structures, it is expected that small differences in anisotropic structure can be visualized more precisely with this non-invasive method than with the invasive method of tearing the extrudates. It is known from the literature that, in high-moisture extrusion, the anisotropic product structure is formed by the multiphase protein matrix being deformed by shear stresses in the cooling die [11,24]. However, it is possible that in our case the added oil leads to wall slip regardless of the addition point and the rheological properties.…”

“…A common process to create these is high-moisture extrusion with an attached cooling die. During the extrusion process, the plant proteins are mixed with water, heated, sheared, and forced through the cooling die, resulting in the formation of an anisotropic structure [4][5][6][7][8][9][10][11][12][13][14][15]. The consumer's expectations for the sensory properties of meat analogues are the same as the expectations for the typical sensory properties of meat.…”

“…During extrusion processing, the protein matrix forms a multiphase system with an inhomogeneous water distribution. In the cooling die, this multiphase protein matrix is deformed and aligned along the flow direction, creating an anisotropic product structure [11]. The review paper by Cornet et al [24] provides a detailed overview of the possible mechanisms found in literature that lead to the formation of an anisotropic structure in meat analogues.…”

Blending Proteins in High Moisture Extrusion to Design Meat Analogues: Rheological Properties, Morphology Development and Product Properties

“…High moisture extrusion (HME) can be used to produce meat analogues from plant-based, protein-rich raw materials that are intended to mimic meat in terms of its unique structure, texture, and mouthfeel [ 1 , 2 ]. The studies on HME, which started in the 1980s [ 3 , 4 , 5 ], can basically be classified into two different categories, which refer to the use of the raw material, either single-component, where only a single protein-rich raw material (e.g., a protein concentrate or isolate) was used [ 3 , 6 , 7 , 8 , 9 , 10 , 11 , 12 ], or multi-component, where two or more raw materials were mixed together [ 13 , 14 , 15 , 16 , 17 , 18 , 19 , 20 , 21 ]. For both categories, it was shown that anisotropic meat-like structures were created in the process.…”

“…In our previous work, we showed that a multiphase system is formed in the single-component extrudates based on soy protein isolate (SPI) in which the phases differ in their water concentration [ 10 ]. According to the model of Tolstoguzov [ 24 ], this multiphase system arises as a result of the thermodynamic incompatibility of the proteins involved.…”