1 Introduction

Global warming is a major problem, with the main contributors being greenhouse gas emissions from agriculture (6.2 Gt CO2-eq per year) and forestry and other land use practices (5.8 Gt CO2-eq per year). Together, agriculture, forestry, and land use account for approximately 23% of all greenhouse gas emissions [1]. The primary cause of global climate change is the livestock industry’s agricultural practices, which account for between 12 and 18% of greenhouse gas emissions [2]. It is evident that consuming meat and meat products has a negative impact on the environment, even though they are a cheap source of nutrients for the human diet, such as vitamins, iron, and proteins [3]. Cattle farming contributes significantly to greenhouse gas emissions as well as water pollution, water scarcity, and water usage [4]. Thus, concerns related to environment raise the need to focus on a sustainable and healthy alternative to animal meat.

Plant-based meat substitutes made from plant proteins show great promise as alternatives to animal meat, as they can closely replicate the texture, appearance, and sensory experience of traditional meat [5]. Table 1 highlights few of the plant proteins along with their processing condition that was able to replicate animal meat characteristics. Several advantages have been observed from the consumption of a plant-based diet, for instance, weight management, the presence of nutritional elements such as vitamins, micro-, and macro-nutrients in high amounts, and regulation of blood pressure and cholesterol [6]. The ability of plant-based diets to lessen the effects of environmental degradation and the risk of diseases linked to diet is a fact [7]. Meat analogues are commonly formulated with processed and unprocessed protein, lipids, carbohydrates, flavor enhancers, and colors [8].

Table 1 Different raw materials, types of extruder used, and processing conditions in high moisture extrusion of meat analogue
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Many techniques are now used to produce meat analogues, including Couette cell technologies, electrospinning, and extrusion [5]. The methods couette cell technologies and electrospinning face limitation in terms of replicating the fibrous structure of meat, raw material and scaling up of these methods for commercial use is quite difficult. Extrusion is widely considered the most efficient method for producing plant-based meat, offering two distinct categories: high moisture extrusion (moisture content > 40%) and low moisture extrusion (moisture content ≤ 40%) [9]. The low moisture extrusion method results in a porous structure, while the high moisture extrusion technique yields a fibrous texture that resembles meat [10]. However, there is little industrial promotion of these techniques. Although the high moisture extrusion technique imitates the meat structure, there are concerns related to nutritional value and other physical sensations that need to be addressed [11]. The innovative technique known as three-dimensional (3D) printing holds potential in food production as it allows for the modification of product structure and texture. Moreover, it enables the creation of customized food shapes while enhancing the nutritional aspects of the final product [12]. The 3D printing technology is the gradual layer wise deposition of materials, often referred to as inks, to construct intricate and complex three-dimensional structures [13]. An advanced version of 3D printing is the 4D printing technology. It allows for the creation of products that have the ability to transform their features or functionalities on exposure to external triggers, such as time, electricity, temperature, humidity, and other stimuli. This dynamic behavior adds an extra dimension of adaptability and functionality to the printed objects [14].

In conducting this review, we utilized a comprehensive information search strategy to ensure the inclusion of relevant and high-quality studies. The primary databases searched were Google Scholar, Scopus, and Web of Science. While there was no specific cutoff for the searching period, we focused on literature published within the last 30 years to capture both foundational and contemporary research in the field. Our selection criteria prioritized articles that closely aligned with our research focus, ensuring that the studies included were directly relevant to the topic at hand. This approach allowed us to compile a robust and focused body of literature that supports the objectives of our review. Thus, the purpose of this review is to offer a thorough examination of high-moisture extruded meat analogues, with a particular focus on emerging techniques like 3D and 4D printing.

2 Meat analogue

“Meat analogues” refer to plant-based meat replacements that are designed to closely replicate the texture, flavor, and appearance of animal meat. They are created using plant-based ingredients to provide a meat-like experience for individuals who follow vegetarian, vegan, or flexitarian diets [5]. The main ingredients typically found in meat analogues include: water (comprising 50–80% of the total composition), non-textured proteins (making up 4%–20%), textured vegetable proteins (typically ranging from 10 to 25%), flavorings (usually accounting for 3%–10%), binding agents such as wheat gluten, soy protein isolate, milk proteins, carrageenan, xanthan gum etc. (making up around 1%–5%), fat (typically ranging from 0 to 15%), coloring agents (used in minimal quantities, usually ≤ 0.5%). These ingredients are combined to achieve the desired texture, taste, and appearance that mimic animal meat in meat analogues [15]. The components work together to create meat analogues with acceptable sensory attributes. Producing meat substitutes involves using plant protein sources like protein isolates and concentrates [16]. Two different methods widely used for production of protein isolates and concentrates are wet fractionation and dry fractionation. Protein isolates are produced through wet fractionation processes which include alkaline, neutral or acid extraction followed by acid precipitation, resulting in a product with higher protein content. Typically, protein isolate contains approximately 75–90% protein. On the other hand, protein concentrates are obtained through dry fractionation methods where plant based raw materials are milled and the particles are then air classified based on size and density into protein rich fraction, and they generally have a lower protein content, ranging from about 48–65% [17]. To enhance the texture of meat analogues or to provide texturization to the base ingredients, minor additives or chemicals may be employed. Additionally, ingredients such as soy protein concentrates and isolates, egg whites, wheat gluten, and other binding agents like hydrocolloids and starches are utilized to improve water retention, texture, and emulsification properties. The overall consumer acceptance of meat analogues is greatly influenced by the texture, flavor, and appearance of the final product [18].

3 Processing

3.1 High moisture extrusion

The extrusion technique is a sophisticated and multifaceted thermo-mechanical process that involves the continuous mixing followed by shearing, and heating of dense formulations within a heated barrel. To achieve the required viscosity, formulations are typically prepared using single/twin screw extruders. The cooked mass thus obtained is cooled to solidify the extrudate fibers. To achieve the desired textural pattern in meat analogues, it is crucial to carefully control factors such as viscosity, temperature, moisture content, and pressure during the extrusion process. These parameters play a vital role in determining the final texture of the product [19]. When proteinaceous materials are subjected to mechanical and thermal energy, the native proteins undergo a process known as denaturation. Denaturation involves the unfolding and disruption of the protein’s secondary, tertiary, and quaternary structures. As a result, a continuous, viscoelastic mass is created. This viscoelastic material is aligned, cross-linked, reshaped, and transformed into an expandable structure with a chewy texture in the extruder [16].

Despite nearly three decade of development, the high moisture extrusion (HME) technology, which emerged in the 1990s as an evolution of low moisture extrusion (LME) technology, is still primarily in the research stage [20]. During the HME process, raw materials undergo transportation, mixing, shearing, heating, shaping, and cooling under conditions of high moisture (above 40%), temperature, pressure, and shear. These processes induce various transformations, namely the breakdown and reformation of proteins, unfolding and crosslinking of proteins, fragmentation and complex formation of proteins, lipid oxidation, gelatinization and degradation of starch, as well as degradation of phytochemicals, antinutrients, and vitamins [21]. Therefore, in the manufacturing process of meat analogues, important factors that impact the texture and structure of the final product include the temperature and pressure of the extruder, cooling temperature, feed rate, and screw speed. Specifically, the extruder temperature and pressure were identified as critical parameters in the development of a HME meat substitute using soy protein concentrate. These variables play a significant role in achieving the desired texture and overall quality of the meat analogue [22]. The high moisture extruder (twin screw) as shown in Fig. 1 consists of 5 essential components, namely the (i) pre-conditioning system, (ii) feeding system, (iii) screw or worm, (iv) barrel and (v) die [23]. Additionally, the screw configuration has a considerable impact on how materials are transformed, fill levels, and input energy [24]. The composition of the fibrous, high-moisture meat mimic made from isolated soy protein, maize starch, and wheat gluten depended heavily on the screw speed [25]. Figure 2 depicts the twin-screw extruder screw elements and combination types. There are four different screw configurations classified based on screw location and rotating direction, such as: (i) co-rotating non-intermeshing, (ii) co-rotating intermeshing, (iii) counter-rotating non-intermeshing, and (iv) counter-rotating intermeshing [23]. The HME meat analogue employs a co-rotating intermeshing twin-screw extruder long cooling die [26].

Fig. 1
figure 1

An illustration of a common twin-screw extruder for high moisture. The screw, which is a form of construction block, has an outer diameter of 36 mm, a barrel length of 24 D, which is divided into six parts, each of which is 4 D long and is made up of six different types of screw pieces that are ordered and joined [30]

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Fig. 2
figure 2

Twin-screw extruders with their combination sorts of screw parts. a The screw elements are of three different types: the forward conveying element, the reverse conveying element, the kneading; b distinct twin-screw configurations based on varied meshing intensities and rotating directions; c the cross-section of twin screws that are revolving in unison or counter-unison. The red arrow indicates the direction of rotation

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The first stage of extruder is the feeding zone where the raw material is fed and pushed in the forward direction under mild shear stress. To maintain the desired conditions, the temperature of the barrel in the feeding zone is typically adjusted at around 80 °C [27]. Under high temperature and high shear conditions, the combined material cooks uniformly to form a molten body, with a stronger physicochemical response in the melting zone, having a barrel temperature greater than 130 °C [20, 28]. The melting zone temperature for the protein component usually falls within the range of 130 to 180 °C, with an approximate residence time of 150 s [20].

The high-moisture extruder die is made up of two areas: mouth of the die and the moulding area. Extruders are outfitted with die that gives the finished products their shape [29], the long cooling dies provides a structure that resembles fibrous flesh [30]. The material must be able to pass through the die mouth at a temperature over 100 °C in order to create a fibrous structure [31]. To maintain a laminar flow state of the material during the molding process, it is necessary for the molding region temperature in the cooling long die to below than 75 °C [25]. According to reports, a 70 °C temperature was maintained by running water in an extended cooling die for high moisture meat mimic produced from peanut protein powder [7]. The product expansion by superheated water evaporation could be prevented by a substantial lowering of the mass core temperature. The temperature variation between the molten core and the die wall would increase shear flow, this facilitates the cross-linking of the structures to create the fibrous structure [32, 33]. Any changes in die diameter or length have a significant impact on the extrusion process [34]. When the temperature of the molten material decreases below 75 °C while passing through the cooling die channel, the plasticized mass undergoes a process of hardening. This hardening of the material leads to a significant change in viscosity and flow behavior of the molten mass. This transformation is crucial for the formation of fibers in the final product [35]. Figure 3 shows typical low and high moisture extruders.

Fig. 3
figure 3

Low-moisture extruder (a) and high-moisture extruder (b). LME low-moisture extrusion, HME high-moisture extrusion

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3.2 3D printing

The process of building three-dimensional objects using successive layers of material is known as additive manufacturing (AM), and one of the main types is 3D printing, also known as fused deposition modelling (FDM). Other types of AM include selective laser sintering (SLS) and stereo lithography (SL) [36]. One advantage of 3D printing over traditional ways of making things is that it can make customized objects in large quantities. This includes making customized consumer goods, like chocolate [37], as well as customized nutritional products, where more control over texture and nutrition can help meet the needs of specific consumer needs and categories [38].

Among the materials that can be utilized in 3D printing are biomaterials, ceramics, metals, and polymers. It has been determined that this kind of additive manufacturing is a potential production procedure in a number of industries, including the food industry [39]. In the mid-1980s, the development of computer and control systems introduced this new technology [40]. With the use of computer-aided design and manufacturing (CAD/CAM) software, a digital manufacturing device known as 3D printing creates three-dimensional items [41]. Most 3D printing is done by extrusion, binder jetting, inkjet printing, or bioprinting [42]. Typically, meat products are printed by extruding fibrous meat components through a nozzle to produce 3D structures. Although alternative methods are still being developed, the extruder type, which includes a screw conveyor or syringe system that can also control the temperature, has a lot of potential for the 3D printing of meat products with the right design [13]. This technique involves layer-by-layer material extrusion through a nozzle to create geometric 3D structures. Shahbazi et al. [43] used the 3D slicing engine Slic3r software to transform virtual 3D models into a workable 3D soy protein-based low-fat meat imitation [43]. They raised the nozzle tip by the same amount (1.1 mm) after each layer fabrication because the layer height was fixed at 1 mm (Table 2). The reduced fat meat analogue produced with the use of a biopolymeric surfactant acts as an oil replacer produced meat analogue with a firm, fibrous and juicy texture with the desired chewiness and a fatty aftertaste. Using large-volume extrusion-based 3D printing and pea protein isolate, Wang et al. created a 3D model of a chicken nugget. The printing process was carried out at room temperature with a printing speed of 15 mm/s and a 100% infill density (Table 2) [12].

Table 2 Protein source, type of printing technique and printing conditions used for meat analogue
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3.3 4D printing

In 2013, Professor Tibbits of Massachusetts Institute of Technology put forth the idea of 4D printing [44]. A “space–time axis” is added to a 3D print as a result of 4D printing technology, which is an extension of 3D printing [45]. Table 3 highlights the difference between 3 and 4D printing. At the moment, 4D printing technology is used to make food products by combining different food ingredients (recommended foods) according to a given formula and structure. Printing of food in four dimensions is significant and has several interesting advantages, such as shape, color, taste, texture, nutrition, and other aspects of printed goods that can vary predictably in response to particular stimuli, such as water, heat, magnetic fields, light, pH, and others [46]. Researchers are currently interested in how 4D printing can be used to make food that looks different, has different nutrients, is easy to swallow, etc.

Table 3 Difference between 4 and 3D printed foods
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The term “4D printing” refers to the ability of a material system or object to change shape or function after printing in response to outside stimuli, such as those caused by people or the environment, changing the chemical or physical properties of the goods that are made [47]. The proper kind of stimulus-responsive materials is the primary prerequisite for the effectiveness of self-transformation and changing the characteristics and functionality of the component in 4D printing [48]. Food products made from various cereals, muscle meat paste, dairy products, fruits, and vegetables are just a few examples of the various food components that have been used in 4D printing. Consumers are currently more interested in developing cuisine that satisfies their appetites while also being health-conscious [49]. The need for unique food products and the resulting shift in customer attitudes could both be addressed by 4D printing. Researchers have recently looked into 4D printing, including studies on the shape transformation [50] and color transformation of 3D products [51]. Hence, 4D printed meat analogues may provide the desirable color, flavor, and texture of conventional meat and, by reciprocating the cooking characteristics of animal meat, could find high demand in the market.

4 Structure

4.1 Protein conformation

The protein structure during extrusion goes through four key stages of configurational changes (Fig. 4), which may happen when the protein is oxidized or destroyed [52]. In the feeding zone, the protein structure does not markedly alter. The amino acids (hydrophobic) trapped within the molecular chain are unwrapped as the molecular chain of the protein expands in the mixing zone along the flow direction [30]. Because there are more interactions between proteins and between proteins and water in the melting zone, it is easier for proteins to join together, or assemble. This is made worse by the high temperature and the resistance of the kneading components [53]. Protein chains may also fragment under the high shear stress in the melting zone. Proteins tend to aggregate under low-intensity shear, but when subjected to high-intensity shear, the size of the protein particles is broken and the molecular weight is decreased [54]. When shear force is perpendicular to the extrusion direction, phase separation is seen in the continuous protein phase in the die zone [27]. The increased cooling rate of the extrudate promotes laminar flow in the cooling zone [24]. The protein molecules will then rearrange and crosslink in the cooling zone forming a fibrous structure [20].

Fig. 4
figure 4

Protein conformation during various stages of extrusion

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4.2 Protein interactions

The primary forces that keep organization of proteins in place are electrostatic interactions, hydrophobic contacts, hydrogen bonds, dipolar interactions, and disulfide bonds. Studies from the past revealed that extrusion mostly changes the weak bonds between proteins, such as hydrogen (H–H) bonds [55]. A number of studies have shown that disulfide bonds, hydrogen bonds and hydrophobic interactions were the main forces that maintained the structure of protein fibers during the cooling stage of HME [56]. The principal mechanisms for preserving the meat-like structure in the creation of meat analogues based on peanut protein were the formation of hydrogen bonds and disulfide bonds in the cooling zone [7]. Non-covalent H–H bonds, Van der Waals interaction, hydrophobic interactions and electrostatic interactions formed during cooling, causing protein molecules and tiny aggregates to align and solidify in a three-dimensional cross-linked network, resulting in a layered and fibrous structure [57]. By increasing the moisture content of feed during the manufacturing of HME meat analogues, the hydrophobic effect might be strengthened, although this would decrease the degree of protein agglomeration among different zones [58]. A more fibrous structure was created by a synergistic interaction amongst the chemical bonds (hydrogen and disulfide bond) and when the moisture of the meat analogue increased a hydrophobic effect was observed [59]. Also, changing the helical structure by raising the extrusion temperature and increasing the sulfhydryl-disulfide bond (SH–SS) interchange helped to increase the gluten aggregation [60]. Due to the disruption of intermolecular bonds at elevated temperatures, the protein molecules broke down into lower molecular weight fragments in the melting zone. The newly created intermolecular disulfide connections would hasten the polymerization of the broken fragments to create a bigger aggregate [61], but when the extrusion temperature exceeded 150 °C, the protein molecules’ disulfide linkages would be broken, increasing the free thiol content [62]. In comparison to 135 °C extrusion, 115 °C was more favorable for the formation of disulfide connections among peanut proteins [55]. Similarly, as screw speed is increased, the strong shearing, stretching, and extrusion processes may cause additional sulfhydryl groups to be exposed in the protein molecule [63]. After that, a disulfide link was formed between the sulfhydryl group and oxygen, creating the protein aggregates with a high molecular mass [64]. Marsman et al. [65] found that when there is high shear, the disulfide linkages and non-covalent links between soybean protein molecules can change into covalent crosslinking.

5 Customization

Convenience, price, packaging, and taste are seen as differentiators in the food industry. As a result, there is a growing market demand for food products that can be customized and made to fit the needs of the individual. Using 3D and 4D printing, food can be digitally changed to have different shapes, textures, and nutritional profiles. These processes involves the layer-by-layer deposition of food-grade materials, often referred to as ‘inks,’ which can be adjusted in terms of viscosity, texture, and nutritional content. For example, different plant-based proteins can be blended with fats, fibers, and flavorings to create a product that closely mimics the taste and texture of traditional meat. The ability to customize the ingredients and their proportions allows for tailored nutritional profiles, catering to specific dietary needs or preferences. Chen et al. [66] used textured soyabean protein along with hydrocolloids to produce streak like food by the use of 3D printing. Lille et al. [67] created a 3D-printed tailored nutritious snack using materials rich in protein, starch, and fiber, which are considered nutritive functional components. These customized meals for health are the next logical step in improving quality of life through personalization. In reality, food personalization will only be a complete success when meals are made to be healthy and tasty at the same time. The customization of the meat-analogue in terms of taste, texture and nutrition quality is the need of the market. When meat analogy through printing is combined with a nutrition model, users can calculate the specific data of calories and taste of manufactured products. This allows users to have a perfect understanding of nutritional distribution. They can control their diet by using a digital method to choose the ingredients they want and match the settings for making the food for example there is a growing need for low fat food with all essential nutrients present. As a result, 3D/4D meat analogue printing offers a practical solution to for people to have a control on exactly what and how much calories they eat and enable them to manage their diet better.

6 Conclusion

To sum up, the research on plant-based meat alternatives shows a lot of promise for the future because they can resemble traditional animal meat in terms of flavor, appearance, and texture. As these substitutes become more popular due to their ability to reduce environmental impact and encourage healthier eating habits, continued technological advancements will probably make them more appealing and effective. In order to develop these meat substitutes, extrusion cooking-specifically, high-moisture extrusion, or HME—has been widely used. But investigating cutting-edge techniques like 3D and 4D printing presents intriguing opportunities to get past present constraints, like obtaining more accurate textures and better nutritional profiles. Even though there are still obstacles to overcome, such as improving these technologies and increasing production, the development of plant-based meat alternatives has enormous potential to revolutionize the food sector and promote a more sustainable future.