A facile strategy for plant protein fiber formation without extrusion or shear processing

Plant-based food is evolving, with plant-based meat sales seeing upwards of 46% growth between the 2019 to 2020 calendar years (Bushnell, Formanski, Gaan, & O'Donnell, 2022). While industry experts predicted continued exponential growth, sales in this sector have stalled (Bushnell et al., 2022). This does not mean that consumers reject plant-based products but rather that the products do not meet the complex sensory, health and price points necessary for mass adoption. New technologies are required to create healthier, sustainable, and affordable plant-based food products. When considering the global ecosystem, agriculture, specifically food, accounts for over 25% of global greenhouse gas emissions(GHG)(Ritchie, Roser, & Rosado, 2020). In this sector, animal-based products have the most significant contribution, totalling 10–100 kg CO2/ kg of product (Ritchie, 2020). While these figures are astonishingly high, GHG emissions will continue to rise due to the growing population and demand for food. Plant-based foods are sustainable alternatives to animal-based products, the common ingredients, such as soy protein, pea protein, and starches, have much lower associated greenhouse gas emissions, usually lower than 1.0 kg CO2/ kg of product (Ritchie, 2020). Within the plant-based sector, meat analogues use sustainable plant-based food ingredients and transform them to have more indulgent organoleptic properties.

Meat analogues can be separated into two main categories, 1) texturized vegetable protein (TVP) composites and 2) whole-muscle analogues. The TVP category includes foods such as burgers, nuggets, and sausages (Monteiro et al., 2019). Structurally, these products are similar to concrete, a composite material composed of fine and coarse aggregates bonded with a fluid (cement) that hardens or cures over time or upon thermal treatment. The aggregates are particles of texturized protein commonly produced through extrusion (Ismail, Senaratne-Lenagala, Stube, & Brackenridge, 2020; Vatansever, Tulbek, & Riaz, 2020). The fluid cement binding the protein aggregates together include various starches, gluten, cellulosics and other hydrocolloids, ultimately achieving the required processed meat properties (Vatansever et al., 2020). Products in these categories fill the grocery store shelves with various companies adding different flavours and producing “new” ways to bind the aggregates together in attempts to resemble processed animal meat products.

In contrast, whole-muscle meat analogues are not as common or available. It is much more complicated to replicate the structure and properties of a whole piece of animal muscle. The current processes include extrusion or shear cell processing, both of which require expensive equipment, high pressure, shear rates ranging from 5 to 160 rpm and heat to induce shear banding (Dekkers, Emin, Boom, & van der Goot, 2018; Krintiras, Göbel, van der Goot, & Stefanidis, 2015; Zhang, Chen, Kaplan, & Wang, 2022). An understanding of the mechanisms responsible for the formation of fibrous structures is also lacking. However, somewhat of a general picture, revolves around the incompatibility between polymeric phases. The incompatibility between polysaccharide-rich phases, protein-rich phases, oil-rich phases and even air, can induce separation, cause demixing within matrices, which can then result in banding of these regions, particularly the protein-rich ones responsible for fiber formation (Ubbink & Muhialdin, 2022; Zhang, Chen, et al., 2022). It would seem that phase separation is an essential part of fiber formation (Ubbink & Muhialdin, 2022; Zhang, Chen, et al., 2022). While many researchers have managed to create fibrous structures, there are limitations regarding the final product characteristics, such as size, moisture content, and texture (Zhang, Chen, et al., 2022). The results can lead one to question how much processing is too much. Our objective was to take a step back, look at the existing qualities of specific plant proteins, and understand how we can better implement the properties to create whole-muscle analogues. Zein is the primary storage protein found in corn; it is rich in glutamic acid, proline, leucine and alanine, giving it an overall hydrophobic nature (Shukla & Cheryan, 2001). Zein is isolated from corn gluten, an abundant “waste product” from corn oil or starch isolation (Shukla & Cheryan, 2001). It is usually used as a low quality animal feed or as a natural herbicide (Shukla & Cheryan, 2001). What the protein lacks in nutritional quality, it makes up for its ability to provide structure (X. Zhang, Dong, Hu, Gao, & Luan, 2021). The glass transition temperature of the amorphous protein under dry conditions is 150 °C (Madeka & Kokinii, 1996); however, with the addition of plasticizers, such as water and acid, this can be lowered below 40 °C (King, Taylor, & Taylor, 2016; Madeka & Kokinii, 1996; Oguntoyinbo, Taylor, & Taylor, 2018). The plasticization of zein allows the protein to be manually pulled or extended into protein fibrils (King et al., 2016; Madeka & Kokinii, 1996; Oguntoyinbo et al., 2018; X. Zhang et al., 2021). This unique fiber-forming ability of zein has found a few applications in plant-based foods, including plant-based cheese, where zein's viscoelastic properties were utilized to enhance extensional flow (Mattice & Marangoni, 2020a). Applications were also suggested for meat analogues, since zein fibers could be formed via electrospinning, antisolvent precipitation or cutting of plasticized zein into thin strips (Mattice & Marangoni, 2020b). The different forms of zein fibers were then manually and strategically place in to protein gels with the hopes of matching animal meat structures (Mattice & Marangoni, 2020b).

Our research objective was to exploit the viscoelastic properties of plasticized zein and enhance its fiber-forming qualities by supporting and separating zein fibrils with an appropriate external continuous matrix. Our previous research efforts identified that the ideal supporting matrix comprised thermally inhibited (TI) rapid swelling starch and pea protein isolate (PP1) (Dobson, Laredo, & Marangoni, 2022). Here we report on the successful creation of stable zein fibers in a plant-protein filled starch network without the need for high temperature, pressure or shear processing, thus increasing the nutritional value of the food, reducing production costs, and improving the environmental footprint of plant-based meat analogues.

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