Quality Characteristics of Meat Analogs through the Incorporation of Textured Vegetable Protein: A Systematic Review

Meat analogs produced through extruded products, such as texture vegetable protein (TVP) with the addition of various plant-based ingredients are considered the products that have great potential for replacing real meat. This systematic review was conducted to summarize the evidence of the incorporation of TVP on the quality characteristics of meat analogs. Extensive literature exploration was conducted up to March 2022 for retrieving studies on the current topic in both PubMed and Scopus databases. A total of 28 articles published from 2001 to 2022 were included in the data set based on specific inclusion criteria. It appears that soy protein is by far the most used extender in meat analogs due to its low cost, availability, and several beneficial health aspects. In addition, the studies included in this review were mainly conducted in countries, such as Korea, the USA, and China. Regarding quality characteristics, textural parameters were the most assessed in the studies followed by physicochemical properties, and sensory and taste attributes. Other aspects, such as the development of TVP, the difference in quality characteristics of texturized proteins, and the usage of binding agents in various meat analogs formulations are also highlighted in detail.


Introduction
Meat analogs are quality products that resemble meat in appearance, taste, texture and up to an extent, nutritional values; they can be meat-free or partially replaced with a minor amount of meat [1]. Meat analog can also be referred to as imitation meat, faux meat, meat substitute, or mock meat [2]. Steady growth has been seen in demand for plant-based meat analogs in recent years due to a change in consumer behavior that is largely attributed to consumers' consciousness toward healthy choices in food selection [3] as well ethical and sustainability factors related to meat consumption [4]. According to nutritional values, meat plays a vital role in human nutrition, and red meat consists of highly valued biological proteins with vitamins, iron, zinc, and other micronutrients [5]. However, it has been evident from previous literature that consumption of red or processed meat for a prolonged period causes type 2 diabetes, cardiovascular complications, and some forms of cancers [6,7]. Additionally, the major issues linked with the production of meat include excessive use of land and water resources, high risk of animal diseases, negative impact on terrestrial and aquatic biodiversity, emission of greenhouse gases, and other environmental hazards [8][9][10]. Additionally, the global production and consumption

Materials Methods
The current systematic review was strictly conducted following the PRISMA (Preferred Reporting Items for Systematic Reviews and Meta-Analyses) guidelines [30].

Data Sources and Searches
An extensive literature exploration of studies evaluating the quality characteristic of meat analogs after partial or complete replacement with texture vegetable protein was conducted in PubMed and Scopus. The literature was explored in at least two different scientific repositories that widen the inclusion criteria of articles found in both databases [31,32]. The following query was used to retrieve potential studies to be included in the review: (texture vegetable protein OR TVP) AND (meat) AND (alternat* OR substitut* OR analog* OR fake OR mock OR faux OR imitat*). The literature search was focused on collecting studies that were published up to March 2022.

Study Selection
Two independent investigators conducted the study research while keeping in mind pre-specified inclusion criteria. The main criteria for study selection were: (i) articles should be published in peer-reviewed journals written in the English language; (ii) studies should evaluate at least one quality characteristic (texture, physicochemical properties, sensory properties, taste attributes); (iii) clinical studies evaluating the effects of textured vegetable protein on human health were excluded; (iv) studies should not be a survey conducted to evaluate consumer acceptance; (v) studies should focus on textured vegetable protein quality rather than any other parameters; (vi) studies that developed new technologies to assess textured vegetable protein quality were excluded. The study selection process is detailed in Figure 1.
conclusions. Therefore, the main objective of this systematic review was to add new evidence to the literature by summing up the current knowledge on the incorporation of TVP on the quality characteristics of meat analogs.

Materials Methods
The current systematic review was strictly conducted following the PRISMA (Preferred Reporting Items for Systematic Reviews and Meta-Analyses) guidelines [30].

Data Sources and Searches
An extensive literature exploration of studies evaluating the quality characteristic of meat analogs after partial or complete replacement with texture vegetable protein was conducted in PubMed and Scopus. The literature was explored in at least two different scientific repositories that widen the inclusion criteria of articles found in both databases [31,32]. The following query was used to retrieve potential studies to be included in the review: (texture vegetable protein OR TVP) AND (meat) AND (alternat* OR substitut* OR analog* OR fake OR mock OR faux OR imitat*). The literature search was focused on collecting studies that were published up to March 2022.

Study Selection
Two independent investigators conducted the study research while keeping in mind pre-specified inclusion criteria. The main criteria for study selection were: (i) articles should be published in peer-reviewed journals written in the English language; (ii) studies should evaluate at least one quality characteristic (texture, physicochemical properties, sensory properties, taste attributes); (iii) clinical studies evaluating the effects of textured vegetable protein on human health were excluded; (iv) studies should not be a survey conducted to evaluate consumer acceptance; (v) studies should focus on textured vegetable protein quality rather than any other parameters; (vi) studies that developed new technologies to assess textured vegetable protein quality were excluded. The study selection process is detailed in Figure 1.

Data Extraction
Data related to the studies included in the systematic review were collected by one investigator, then a second investigator checked carefully these data for accuracy. The following characteristic related to the studies were extracted: authors and year of publication; location of the included study; the type of textured vegetable protein used; quality characteristic evaluated; whether or not the TVP was incorporated totally or partially; when partial incorporation, what type of meat was included in the formulation; the binding agent used or not and finally the type of final product manufactured. When any disagreements happened during the data collection, a third investigator was consulted to resolve the issue.

Data Synthesis and Analysis
A meta-analysis was not conducted in the current review because no studies conducted had the same formulation. Thus, the authors did not find it suitable to pool results from studies with this level of dissimilarity. Indeed, there was a plethora of binding agents as well as different types of components included in the formulations of the meat analogs (gelatin, starch, molasses, mushrooms, etc.). Therefore, as recommended by the Cochrane handbook for systematic reviews of interventions only a qualitative synthesis of the data extracted from the studies was conducted [33]. The "webr" package of the R software (v.4.0.3) (R Core Team, 2020, R Foundation for Statistical Computing, Vienna, Austria) was used to draw the pie donut charts based on the data collected from studies. Bar plots and world maps were drawn using Tableau Desktop 2021 (Tableau software, 2003, Salesforce, Seattle, WA, USA).

Study Selection Workflow
A total of 143 records were identified after consulting PubMed and Scopus databases. After removing 10 duplicates, excluding five reviews, and excluding 22 other articles consisting of book chapters or conference abstracts without full text, 106 articles remained. Thereafter, title and abstract screening were performed and led to 37 articles eligible for full-text screening. After carefully reading full texts and excluding articles not meeting the inclusion criteria, 28 articles published from 2001 to 2022 were finally included in the systematic review. Table 1 shows the detailed characteristics of the articles selected for the systematic review. It appears that the majority of studies focused on the use of soy protein only (n = 14). In addition, some other studies evaluated the combination of soy protein with other proteins, such as pea (n = 4), insect (n = 1), rice (n = 1), and wheat (n = 1). Concerning the chronological apparition of the studies, the oldest article included in the review was dated 2001 and the newest was released in 2022 ( Figure 2). Moreover, the highest number of articles were published in the years 2021 (n = 11) followed by 2018, 2019, and 2022 each year (n = 3), respectively. Geographically speaking, the preponderance of studies was conducted in Asia and North America ( Figure 3). More precisely, the highest number of studies were conducted in Korea (n = 10) and the USA (n = 7).

Study Characteristics
An in-depth analysis of the different study designs revealed that 43% of the studies used at least one binding agent in their meat analog formulation while 57% did not. Figure 4 shows that the majority of studies using binding agents were using only soy protein (75%). In addition, some other studies evaluated the combination of soy protein wit teins, such as pea (n = 4), insect (n = 1), rice (n = 1), and wheat (n = 1). Con chronological apparition of the studies, the oldest article included in the review 2001 and the newest was released in 2022 ( Figure 2).   An in-depth analysis of the different study designs revealed that 43% of the studies used at least one binding agent in their meat analog formulation while 57% did not. Figure  4 shows that the majority of studies using binding agents were using only soy protein (75%).     Finally, patties (58.3%) and sausages (frankfurter) (25%) were more or less considered as the final product when a binding agent was used in the formulation (Figure 6).  Finally, patties (58.3%) and sausages (frankfurter) (25%) were more or less considered as the final product when a binding agent was used in the formulation ( Figure 6). One of the main criteria for study inclusion was related to the evaluation of at least one quality characteristic. Indeed, Figure 7 presents the overall proportions of studies assessing the different categories of parameters. One of the main criteria for study inclusion was related to the evaluation of at least one quality characteristic. Indeed, Figure 7 presents the overall proportions of studies assessing the different categories of parameters.
Taste attributes were the least parameters reported by authors (28.57%) in all of the included studies. Textural profiles were evaluated in nearly about (85.71%), whereas sensory and physicochemical properties were reported (92.86%) and (85.7%), respectively, in all included studies.
Physiochemical characteristics were the second most evaluated set of parameters with a total of 83.3% of studies including them in their results. Finally, sensory and taste attributes were the least reported by authors with 46.7% and 43.3% of studies assessing them. Taste attributes were the least parameters reported by authors (28.57%) in all of the included studies. Textural profiles were evaluated in nearly about (85.71%), whereas sensory and physicochemical properties were reported (92.86%) and (85.7%), respectively, in all included studies.
Physiochemical characteristics were the second most evaluated set of parameters with a total of 83.3% of studies including them in their results. Finally, sensory and taste attributes were the least reported by authors with 46.7% and 43.3% of studies assessing them.

Quality Assessment
Quality assessment of current articles was performed based on standard established guidelines by Kmet, et al. [59]. The results of the quality assessment are presented in Table  2. The objective, study design sample size, analytic methods, results reported, and control confounding of the outcome assessor was reported in all the studies. The interventional random allocation, interventional and blinding investigators, and interventional blinding subjects' outcome misclassification bias were not applicable in the included articles since these biases are related to randomized control trials in clinical studies. Moreover, the sample size appropriateness was partially described in the majority of studies. However, methods, control confounding, results, and conclusions were described in detail in most of the studies.

Quality Assessment
Quality assessment of current articles was performed based on standard established guidelines by Kmet, et al. [59]. The results of the quality assessment are presented in Table 3. The objective, study design sample size, analytic methods, results reported, and control confounding of the outcome assessor was reported in all the studies. The interventional random allocation, interventional and blinding investigators, and interventional blinding subjects' outcome misclassification bias were not applicable in the included articles since these biases are related to randomized control trials in clinical studies. Moreover, the sample size appropriateness was partially described in the majority of studies. However, methods, control confounding, results, and conclusions were described in detail in most of the studies.

Discussion
TVP is a dry bulky commodity and is notably extracted from leguminous crops, such as soybeans, pea, and lupine which are produced by the cooking extrusion process. During the production of TVP, the protein is subjected to thermal and mechanical stresses by heating and applying high pressure to produce fibrous TVP [57]. The TVP production has been categorized into types based on water addition during the extrusion process. Low moisture extrusion (dry; <35% of water) and high-moisture (wet; >50% of water) types [57]. Sufficient literature is available on the production of TVP with high and low moisture extrusion [46,60,61]. Similarly, due to extensive agriculture practices, human nutrition has evolved from time to time consequently based on recent trends in agricultural production TVP has obtained popularity due to its outstanding quality characteristics concerning red meat. The most popular TVPs are produced from soy, pea, wheat, and rice protein. TVP can be used to replace meat or can be served in combination with meat at a certain level [62]. The USDA approves up to 30% of TVP in school lunch as mixed with ground beef patties. The TVP showed a resemblance with meat chewiness and flavor [9,63]. TVP has been used as a meat replacer with many economic and functional benefits [64]. Current literature indicates promising outcomes for partial or complete replacement of meat with TVP [19,20]. Previously, Samard and Ryu [45] and Bakhsh et al. [20] evaluated different types of meat, such as beef, pork, and chicken as compared to TVP-based meat analogs.
The results indicate that TVP-based meat analogs were nearer to chicken and pork meats, respectively. Moreover, the different proteins had different quality characteristics, such as soy [65,66], and oat or pea dry fractioned textured isolate proteins which have been shown different implications [6,67,68].
It appears the majority of studies included in this systematic review were conducted in the Asian region, particularly Korea and China. Alternatives to meat as a protein source have existed for millennia, with traditional products, such as tofu and tempeh (made from soybeans) and seitan (made from wheat protein) used as affordable, functional, and nutritious protein sources as early as 965CE and originating in China [25,69].
The consumption of TVP in meat analogs in different regions of the world is based on cultural and religious reasons [7]. Vegetarian dishes including alternative proteins were frequently consumed in the Buddhist religion [70]. In 1960 the invention of TVP led to the modernization of meat analogs as TVP was used as a prime ingredient in the vegan version of meat alternatives [11,71]. The consumption of meat analogs and TVP production is relatively recent in western countries. However, due to personal preference, religious beliefs, and health awareness, the vegetarian and flexitarian population increased tremendously in recent years, particularly in western countries. Although, the consumer approval and demand for these meat alternatives are considerably low. Nevertheless, the production new generation of meat analog-like beyond burgers and impossible burgers has cleared the way to reach the table of western families [11,21]. This could be the reason that Asian countries, such as China and Korea, and Indonesia had more interest in academic research in meat analogs as compared to western countries. Likewise, the research related to meat analogs has surged in 2021 as compared to previous years.
As depicted in Figure 4, soy-based TVP was the most commonly used in studies of the current review. The demand for meat alternatives is growing in contrast to animal-derived proteins. Soy texturized protein, such as TVP, and TSP (texturized soy protein) has been used commonly in meat analogs due to tremendous emulsifying, fat absorption, and gelling qualities [23]. Soy products have better yields, easier handling, lower transportation, and preservation charges as compared to meat. A TVP-based diet is economically feasible and the high protein ingredients provide a variety of choices [72]. Furthermore, the appearance, flavors, and health impact of different proteins are different. Therefore, it is important to select a quality protein for the production of TVP and formulation of meat analogs. Nowadays several types of plant-based proteins are available for the manufacture of meat analogs. However, soy and pea protein are considered the best options due to the possession of some characteristics that resemble meat. The advantage of soy and protein-protein isolate, apart from the high protein purity, is its light color and bland flavor compared to the other protein. Soy-based TVP comprises its richer profile it includes 35% to 40% high-quality protein with a well-balanced composition of amino acids, 15% to 20% fat, 30% carbohydrate, and 10% to 30% moisture also rich in fiber, iron (Fe), calcium (Ca), zinc (Zn), and B vitamins [73]. Additionally, Soy-based TVP S consists of low saturated fat, a high concentration of essential amino acids, low calories, and is cholesterol-free [74].
This review highlighted that in their trials, researchers tend to prioritize texture, and physicochemical properties more than any other quality characteristics (Figure 7). Indeed, in the formulation of meat analogs with the inclusion of TVP, the food producers face the biggest hurdle in the development of adequate texture and taste [75]. In meat analog formulation, the inclusion of simple protein does not guarantee the quality of taste, texture, and appearance. The reason behind the inclusion of extruded products, such as TVP and TSP is to achieve the desired taste and especially fibrous texture and visible appearance. The substitution of texturized proteins in meat analogs occurs in two ways. The first one is through blending and mixing with texturized proteins including meat and the other way is the complete incorporation of meat by TVP to form a meat analog [72]. Generally, the inclusive properties of meat analogs including taste, texture, and appearance are not improved with meat extender alone, however, mixed with meat they expand the overall functional characteristics of meat analogs manifolds. Furthermore, the quality and texture of raw materials can be enhanced by adding chemicals and ingredients through a process of texturization [72,76]. Soy protein isolates and concentrates, wheat gluten, egg white, and other binding agents, such as gelling agents and starches, are supplied to enhance the water-holding capacity, texture, and emulsification characteristics. As the texture and flavor are considered as primary factors which affect the consumer decision in the selection of meat analogs [26].
In the current study, a majority of studies were incorporated with binding agents. Binding agents in meat analogs can be ingredients of animal or plant origin that serve both as water and fat binder. Such substances include wheat gluten, milk proteins, eggs white, carrageenan, methylcellulose, xanthan gum, and other ingredients. Depending on the quantities added, some ingredients can interact as both binders and as extenders. Elevated water holding capacity, and protein network formation can be observed as the main function in the ingredients that have a higher amount of protein. Nevertheless, ingredients that have less or zero protein content, such as flours and starches, act as fillers, despite their water and fat binding traits through the physical entrapment [23]. The concentration level of the binding agents impacts the characteristics of the ultimate product. Besides protein binders, polysaccharides, such as pectin, guar gum, carrageenan, and methylcellulose, are recommended for use in meat-analog products as binders and extenders [77]. For instance, the gelling and thickening functions of polysaccharides, improvement of rheological properties, and water binding capacity have created a promising ingredient that can be implemented in the meat analog industry [78].

Limitations and Challenges
The current study is associated with some limitations. Indeed, based on inclusion criteria our study is limited to physicochemical, textural, and sensory characteristics of meat substitutes, whereas the nutritional and human health aspects as well the consumer acceptance of TVP-based meat analogs products are excluded in this systematic review. Additionally, we did not conduct a meta-analysis in the current study due to the dissimilarities between study designs.
The main purpose and ultimate challenge of making meat analogs is the production of a sustainable product that recreates conventional meat in all of its physical sensations. Normally, the meat substitute acquired from the current TVP-based extruded products is different from muscle tissue because it is deficient in terms of the fibrous structure, tenderness, and meaty sensation as real meat. Consequently, the major challenge for meat analog production is to acquire the texture and taste of real meat, which may necessitate exceptional designs for meat alternative formulations and the optimization of processing conditions. Seeking more low-cost premium plant protein sources and combinations of selected food ingredients for the preparation of meat-like products is crucial.

Conclusions
In conclusion, this study summarized the available evidence concerning the incorporation of TVP in the formulation of meat analogs. The various types of TVPs were discussed in relation to physicochemical, Textural, and sensorial characteristics. However, further studies pooling effect size via a meta-analysis approach should preferably be conducted to fill the gap in the current knowledge.