Functional Properties of Yeast Mannoproteins—Current Knowledge and Future Perspectives

Abstract

Mannoproteins are structural components of the yeast cell wall exhibiting extensive functionality applicable to the food, feed, and medical industries. They are characterized mostly by immunostimulatory, prebiotic, antimicrobial, antibiofilm, antioxidant, and emulsifying properties. The bioactive properties of mannoproteins underscore their significance in functional food production, therapy, and animal husbandry. This review critically examines the literature on yeast mannoproteins, focusing on their chemical characteristics, biological activity, and potential applications. Considering gaps in the literature data regarding detailed chemical characterization and mechanisms of action of mannoproteins, future research should aim at precise structural analysis, particularly of mannoproteins derived from nonconventional yeast, to uncover new potential industrial and health applications.

1. Introduction

Yeast cells are surrounded by a cell wall, which constitutes 15 to 30% of the cell’s dry mass. The components of the yeast cell wall include mannoproteins, beta-glucans, and chitin, as schematically illustrated in Figure 1. These polymers form distinct layers that not only protect the cell from physical and chemical stresses but also provide it with structural support. Their inherent bioactive properties have promising applications in the food, feed, and medical industries. Beta-glucans and chitin ensure the rigidity and structural stability of the cell wall [1,2]. In contrast, mannoproteins, which are located in the outer layer, are primarily responsible for the adhesive properties of yeast cells to various surfaces—including living cells—and act as a molecular sieve for chemical substances [3,4].

Mannoproteins isolated from the yeast cell wall exhibit a wide range of interesting properties, including immunostimulatory effects, the ability to adsorb mycotoxins, support for wound healing processes, and antioxidant and emulsifying actions [5,6,7,8,9,10,11]. Additionally, they possess prebiotic properties while demonstrating antimicrobial activity against pathogenic bacteria, contributing to the positive modulation of gut microbiota [12,13]. Mannan, a component of mannoproteins, is utilized by lactic acid bacteria in fermentation processes, leading to the production of short-chain fatty acids such as butyric acid, which positively affects gut health [14]. A graphical summary of the examples of the main functional properties of mannoproteins is presented in Figure 2.

The composition and localization of the individual components constituting the yeast cell wall are influenced by growth conditions, the cell cycle, and the growth phase, which may vary among different yeast species [1,13,15]. Each yeast species is characterized by a distinct proportion of cell wall polymers and the chemical structure of these components, directly impacting their functional properties. Precisely elucidating these structural and chemical attributes is essential to fully understand and harness their bioactivities. Only then will it be possible to further refine isolation methods and optimize yeast cultivation conditions to maximize the synthesis yield and efficacy of these preparations.

Yeast of the genus Saccharomyces, considered conventional yeast, face significant challenges due to their poor tolerance to environmental stress and limited ability to utilize diverse carbon sources [16]. Additionally, the exclusive use of these yeast in fermentation processes restricts the sensory characteristics of the products, thereby complicating the adaptation to diverse consumer preferences [17]. The scientific literature extensively describes these yeast, including their cell wall mannoproteins, in contrast to other yeast species referred to as nonconventional ones.

The group of nonconventional yeast includes other species known as non-Saccharomyces. These yeast have been observed to easily adapt to adverse environmental conditions [18,19,20,21], exhibiting tolerance to high temperatures and low pH, as well as the ability to metabolize atypical carbon sources. They demonstrate the capacity for the biosynthesis of products with desirable properties, such as polyalcohols, enzymes, proteins, ethanol, squalene, and lycopene [22,23,24,25,26]. Currently, the group of non-conventional yeast utilized on an industrial scale includes, for example, Debaromyces hansenii, Kluyveromyces marxianus, Cyberlidnera jadinii (formerly Candida utilis), Yarrowia lipolytica, and Pichia kudriavzevii. It is worth noting that 17 species of nonconventional yeast are listed under the Qualified Presumption of Safety (QPS) status in the European Union [27,28,29,30]. Characterization of mannoprotein properties isolated from non-conventional yeast cells is of particular interest, because they may potentially exhibit even more intriguing properties compared to those obtained from S. cerevisiae.

This review was conducted as a narrative synthesis based on literature identified through structured searches in Google Scholar and PubMed. The search strategy involved combinations of keywords such as “yeast mannoproteins,” “yeast cell wall,” “bioactivity,” “nonconventional yeast,” “prebiotic,” “antioxidant,” “antimicrobial,” and “emulsifying properties.” Publications were selected based on relevance to the topic, scientific quality (peer-reviewed journals), and language (English). Approximately 180 articles were initially screened, of which about 100 were selected and critically evaluated. The primary focus was directed on non-Saccharomyces yeast; however, due to the limited availability of data on these species, relevant findings concerning Saccharomyces cerevisiae were also included for comparative purposes.

Previous reviews on yeast mannoproteins have primarily focused on conventional species, particularly Saccharomyces cerevisiae, discussing their chemical structure, extraction techniques, and industrial uses. While some publications have addressed selected functional aspects—such as sensory contributions in winemaking or animal feed supplementation—comparative evaluations involving both conventional and nonconventional yeast species (e.g., Yarrowia lipolytica, Debaryomyces hansenii, Kluyveromyces marxianus) remain limited. Moreover, there is a lack of integrated analyses linking the molecular structure of mannoproteins with their biological activity, particularly in lesser-studied non-Saccharomyces species. This review aims to provide a critical synthesis and structured overview of the existing literature on yeast-derived mannoproteins, with particular emphasis on their chemical structure, biological activity, and potential applications in the food, feed, and pharmaceutical industries. Special attention is given to nonconventional yeast species, whose mannoprotein-related properties remain insufficiently explored. This review also identifies key knowledge gaps—particularly in the context of structure–function relationships—and outlines directions for future research.

2. Characteristics of Mannoproteins

Mannoproteins are highly glycosylated proteins that can be classified into two groups based on the type of glycosylation: N-glycosylated and O-glycosylated. A schematic overview of N-linked and O-linked glycosylation types is presented in Figure 3.

N-glycosylated mannoproteins consist of approximately 10% protein and about 90% carbohydrates, with the carbohydrate portion comprising between 50 and 200 mannose units. The main mannose chain is built through α-1,6-glycosidic bonds, with short, branched side chains containing α-1,2- and α-1,3-glycosidic linkages. The carbohydrate chain is attached to the asparagine residue of the protein via an N-glycosidic bond. Furthermore, two N-acetylglucosamine molecules, linked by a β-1,4 glycosidic bond, serve as a bridge between the carbohydrate main chain and the protein. O-glycosylated mannoproteins have a higher protein content, around 50%, and possess a short and unbranched carbohydrate chain (up to five mannose units). An α-1,2-glycosidic bond exists between the first two mannose molecules, followed by α-1,3-glycosidic bonds between subsequent mannose residues. The branching of the chain occurs through an α-1,6-glycosidic bond.

Some mannose residue chains can undergo phosphorylation, forming mannose-6-phosphate. Phosphorylation of mannose residues is an example of a cellular response to oxidative stress. This process contributes to the negative surface charge of the cell, which plays a significant role in protecting yeast cells. It helps maintain a more rigid cell wall structure and enhances hydrophilic properties, thereby increasing oxidative stress tolerance [31,32,33,34,35,36,37].

Proteins comprising the yeast cell wall (commonly referred to as cell wall proteins, or CWPs) can associate with the polysaccharide layer either covalently or non-covalently. They are classified into three groups based on the type of bonds, the first two of which are 1. PIR-CWPs: connected by bonds sensitive to alkaline compounds, and 2. GPI-CWPs: linked via an interaction with other cell wall proteins. This classification reflects the diversity in the nature of the bonds that integrate these proteins into the cell wall structure [1,8,38,39].

The first group includes PIR-CWPs (with internal repeats). These proteins are connected to β-1,3-glucan by bonds sensitive to mild bases. PIR-CWPs appear to be evenly distributed in the inner layer of the cell wall skeleton. In the analyzed yeast cell walls, it has been determined that PIR-CWPs are present in the cell walls of yeast, except for Schizosaccharomyces pombe. The number of genes encoding these proteins varies depending on the yeast species. The physiological role of PIR-CWPs is not entirely understood. They are not essential for the proper functioning of the cell, but their damage or complete removal from the cell wall disrupts its osmotic stability, increases sensitivity to harmful environmental factors, and causes changes in cell morphology [38,39,40].

The next group is the GPI-CWPs. These proteins are linked to β-1,6-glucan via a glycosylphosphatidylinositol (GPI) anchor at the C-terminus. They are further distinguished by three additional features: the presence of a serine/threonine-rich region, a hydrophobic tail, and an N-terminal secretion signal peptide [41]. About 60–70 types of GPI-CWPs have been identified in Saccharomyces cerevisiae cells. About 40 of them are attached to the yeast cell membrane, where they perform specific functions, while the rest are covalently linked to β-1,6-glucan [3]. The best-known GPI-CWP is Sag1, which is involved in agglutination during mating [3]. Some GPI-CWPs retain enzymatic activity, while others serve primarily structural roles [1]. Their presence on the cell wall can impart hydrophobic and antigenic properties to the yeast surface. They are involved in the biosynthesis and remodeling of the cell wall and may be responsible for adhesive properties and virulence. GPI proteins isolated from S. cerevisiae cells include flocculins, adhesins, and proteins designated as CWPs. The latter term refers to proteins that are likely to have a non-enzymatic function. These typically small proteins are enriched in threonine and serine residues, which indicates a high degree of O-glycosylation. Examples of such proteins include Cwp1p, Ssr1p, Tir1p, Tip1p, Ccw12p, and Sed1p [42].

The final group of proteins includes those that are linked to other proteins by non-covalent bonds or disulfide bridges, such as Bgl2p. These proteins are classified as secretory proteins with an N-terminal signal peptide and are likely to undergo mannosylation [38]. The presence of proteins connected by disulfide bridges has been confirmed using reducing agents (e.g., 2-mercaptoethanol). These glycoproteins form a protective barrier around the yeast cell wall, preventing the entry of external glycosylhydrolases [1].

The structural features of mannoproteins—including their overall structure, total charge, and charge distribution—may vary considerably among yeast species. For instance, mannoproteins in the cell wall of Schizosaccharomyces pombe typically feature linkages between β-1,3-glucan and pyruvylated galactose residues [43]. In Pichia pastoris, these are diester bonds with mannose phosphate. In contrast, Kluyveromyces lactis and S. pombe lack mannose phosphate in their cell wall composition [44].

Consequently, the diverse structure and chemical composition of mannoproteins—which vary by yeast species—may play a crucial role in determining their functional properties. They facilitate interactions with pathogenic bacterial cells through mannose-specific fimbriae located on bacterial cells. These specific fimbriae can bind to mannans present in the yeast cell wall, potentially limiting the ability of bacterial cells to interact with intestinal epithelial cells. Furthermore, mannan can be degraded by the periplasmic enzyme α-mannosidase into mannose residues, which can serve as a prebiotic substance for lactic acid bacteria [4].

3. Functional Properties of Mannoproteins and Mannoprotein-Rich Preparations

3.1. Anti-Biofilm Properties

Mannoproteins exhibit antimicrobial properties that include the inhibition of bacterial biofilm formation—a significant challenge in various industries, such as food production. Mannoproteins derived from extracellular structures are responsible for inhibiting the formation and dispersion of bacterial biofilms [41].

Table 1. Antibiofilm properties of selected preparations isolated from yeast.

Action	Preparation	Dosage of Preparation	Structure and Chemical Composition	Literature
Reduction in formation of biofilm Staphylococcus aureus ATCC 29213 (biofilm inhibition of 12–87%)	Yeast cell wall isolated from S. cerevisiae from Tokaj wine production	10%	Protein content in the raw extract: 7.2%; 15–25 kDa, no information about the polysaccharide fraction	[45]
Reduction in formation of biofilm Listeria monocytogenes (biofilm inhibition of 52.6–79.5%)	CFS—cell free supernatant of S. cerevisiae	2 mL (undiluted CFS)	No characteristics of the preparation, presumably the supernatant containing mannoprotein fractions	[46]
Reduction in formation of biofilm Pseudomonas aeruginosa and Staphylococcus aureus (biofilm inhibition: P. aeruginosa—51.8%, S. aureus—19.7%)	Mannoproteins isolated from the cell wall of the yeast S. cerevisiae BY	200 mg/mL	Protein to mannan ratio 87.1%, Characterization of preparations using FT-IR	[47]
Reduction in formation of biofilm S. aureus ATCC 29213 (maximum biofilm inhibition 63.4% at 4%)	Mannoproteins isolated from the cell wall of the yeast S. cerevisiae 102	2–10% (max effect at 4%)	Protein concentration: 31.7%; carbohydrate concentration: 65%	[48]

presents a summary of the anti-biofilm properties of yeast mannoproteins as evidenced in the literature. A reduction in biofilm formation by 12–87% was observed when 10% mannoproteins from Saccharomyces cerevisiae were added to the culture medium [42]. In another study, mannoproteins (at concentrations ranging from 2% to 10%) derived from S. cerevisiae 102 were shown to notably inhibit the biofilm formation of Staphylococcus aureus ATCC 29213, with the highest inhibition (63.4%) observed at a 4% concentration [43]. The pathogenic bacteria Staphylococcus aureus ATCC 29213 were used as a model organism. Other studies evaluated the anti-biofilm effect of the cell-free supernatant (CFS) from Saccharomyces cerevisiae cultures against Listeria monocytogenes. The CFS was found to significantly inhibit biofilm formation, with inhibition rates ranging from 52.6% to 79.5% [44]. Despite the lack of precise characterization of the supernatant’s composition, it is presumed to contain mannoproteins released from the cells. A mannoprotein preparation derived from the cell wall of Saccharomyces cerevisiae BY, when added to bacterial cultures at a concentration of 200 mg/mL, was observed to exert inhibitory effects on the formation of Pseudomonas aeruginosa (51.8%) and Staphylococcus aureus (19.7%) biofilms [45].

3.2. Antimicrobial Properties

A summary of the available literature data indicating the antimicrobial effect of yeast mannoprotein preparations is presented in

Table 2. Antimicrobial properties of selected preparations isolated from yeast.

Action	Preparation	Dosage of Preparation	Structure and Chemical Composition	Literature
Inhibition of the growth of Pseudomonas aeruginosa	Biosurfactant from the yeast S. cerevisiae	Not specified (100 μL applied in agar well diffusion assay)	No detailed characteristics	[50]
Growth inhibition of Corynebacterium urelyticum (inhibition zone 18 mm)	Biosurfactant from the yeast S. cerevisiae	Not specified (biosurfactant activity evaluated using oil spreading and E24 tests)	Protein concentration: 0.0535 mg/mL; carbohydrate concentration 0.08839 mg/mL; molecular mass 89, 100 kDa	[51]
Inhibition of the growth of Escherichia coli and Bacillus subtilis (37% inhibition of E. coli at 5% concentration, 80% inhibition of B. subtilis at 3% concentration)	Mannoproteins isolated from the cell wall of the yeast S. cerevisiae	5% (E. coli) 3% (B. subtilis)	No detailed characteristics	[49]
Inhibition of the growth of Pseudomonas aeruginosa and Staphylococcus aureus (zone of inhibition: 16.2 mm (P. aeruginosa), 13.7 mm (S. aureus) at 200 mg/mL)	Mannoproteins isolated from the cell wall of the yeast S. cerevisiae BY	50–200 mg/mL	Protein-to-mannose ratio 87.1% Characterization of preparations using FT-IR	[47]
Reduction in the growth of P. aeruginosa ATCC 27853, P. mirabilis ATCC 27853, and S. Enteritidis ATCC 13076 (reduction in bacterial growth ranged from 77% to 95%)	Mannoproteins isolated from Saccharomyces cerevisiae ATCC 7090	2–6%	Molecular weight: 65, 14, 1.9 (Mw/kDa), nitrogen: 6.96%, carbon: 31.9%, hydrogen: 4.42%, protein: 42.3%, total sugars: 55.7% Characterization of preparations using NMR and FT-IR	[48]
Reduction in the growth of S. aureus ATCC 25923 and E. coli ATCC 25922 (inhibition: 93.6% (S. aureus), 91% (E. coli) at 6% concentration)	Mannoproteins isolated from the cell biomass of Metschnikowia reukaufii WLP 4650	2–6%	Molecular weight: 150, 13, 1.9 (Mw/kDa), (nitrogen: 6.21%, carbon: 30.38%, hydrogen: 4.12%, protein: 37.8%, total sugars: 60.0% Characterization of preparations using NMR and FT-IR	[48]
Reduction in the growth of P. aeruginosa ATCC 29212 and E. coli ATCC 25922 (inhibition: 84.4% (P. aeruginosa at 6%), 45.5–70.5% (E. coli at 2–6%))	Mannoproteins isolated from cell biomass of the yeast Wickerhamomyces anomalus CCY 38-1-13	2–6%	Molecular weight: 84, 20, 1.9 (Mw/kDa), nitrogen: 5.41%, carbon: 28.77%, hydrogen: 3.93%, protein: 32.9%, total sugars: 52.8% Characterization of preparations using NMR and FT-IR	[48]
Inhibition of the growth of E. coli F4ac (ETEC) in the intestinal lumen of pigs by binding and excretion with the components of SENTIGUARD	SENTIGUARD product—containing beta-glucans, mannoproteins, oligosaccharides and yeast bile salts—Nutriad, Turnhout, Belgium	0.5%, 5%, 10% (w/v)	No detailed characteristics	[52]
Inhibition of the growth of Clostridium perfringens (reduction in growth rate and maximum growth level; effect was time- and dose-dependent)	The cell wall of the yeast S. cerevisiae	0.5–2% (w/v), min 1.25 mg/mL	Mannan content: 13–26.8% DM; protein content: 17.5–23.8% DM	[53]
Inhibition of the growth of Salmonella cells in the feces of female large white turkey chicks (reduction from 3.99 log CFU/g (control) to 2.60 log CFU/g (treated group) after 21 days)	S. cerevisiae yeast cell wall—containing mannan–oligosaccharides composed by Quality Technology International, Inc., (QTI) Elgin, IL, USA	0.5 g/kg feed	No detailed characteristics	[54]
Reduction in the number of Campylobacter cells in piglets	MRF (mannan-rich fraction from yeast cell wall)—Actigen™	800 mg/kg feed	Mannan 12%; raw fat 1.85%; protein 33.5%; total ash 6.78%	[55]
Reduction in the number of E. coli in vitro after 24 h of fermentation in intestinal juice	Three fractions of α-mannan (LZ-MPS; MC-MPS, G-MPS) isolated from Kluyveromyces marxianus	1% (w/v)	LZ-MPS: yield 71.1 mg/g, sugars—mannose, sugar content: 97.1%, protein: –; MC-MPS: yield 84.7 mg/g, sugars—mannose, sugar content: 96.88%, protein: –; G-MPS: yield 77.3 mg/g, sugars—mannose, sugar content: 91.13%, protein: –	[13]

. For instance, one study observed an antimicrobial effect against Pseudomonas aeruginosa using a biosurfactant obtained from S. cerevisiae (exact composition not provided) [45]. In another investigation, a pure biosurfactant consisting of mannoproteins isolated from S. cerevisiae demonstrated a more pronounced inhibitory effect—evidenced by an inhibition zone diameter of 18 mm—against Corynebacterium urelyticum compared to a partially purified extract [46]. Mannoproteins isolated from the cell wall of S. cerevisiae yeast by the thermal–alkaline method demonstrated inhibitory capacity against E. coli (37%) at a preparation concentration of 5% (m/v) and against B. subtilis (80%) at a concentration of 3% (m/v) [49]. A preparation of mannoproteins derived from the cell wall of the yeast S. cerevisiae BY demonstrated inhibitory capacity against the bacteria P. aeruginosa and S. aureus, with zones of inhibition observed at mannoprotein concentrations ranging from 50 to 200 mg/mL [47]. Mannoproteins isolated from the cell biomass of the yeast Saccharomyces cerevisiae ATCC 7090 demonstrated a dose-dependent reduction in the growth of the bacteria P. aeruginosa ATCC 27853, P. mirabilis ATCC 27853, and S. Enteritidis ATCC 13076, with inhibition rates of approximately 77–95%. In the case of mannoproteins isolated from the yeast Metschnikowia reukaufii WLP 4650, the greatest inhibition of the growth of Staphylococcus aureus ATCC 25923 and Escherichia coli ATCC 25922 was observed in cultures containing 6% of the preparation (93.6% and 91% inhibition, respectively). Conversely, a mannoprotein preparation derived from the biomass of the yeast Wickerhamomyces anomalus CCY 38-1-13 demonstrated a reduction in the growth of P. aeruginosa ATCC 29212 (84.4%) when the preparation was added at a dose of 6%, and of E. coli ATCC 25922 (45.5–70.5%) depending on the preparation dose in the range of 2 to 6% [48].

Specifically, YCW preparations decrease enterotoxigenic Escherichia coli (ETEC) adhesion to intestinal villi, as confirmed by scanning electron microscopy, which revealed that ETEC cells adhere to the yeast cell wall rather than to the host tissue [52]. Furthermore, all examined YCW preparations inhibited the growth of the pathogen Clostridium perfringens by reducing its growth rate and maximum growth level while prolonging the lag phase. The extent of these effects was both time- and dose-dependent [53]. In vivo, studies on female White Leghorn chickens have shown that supplementation with a YCW preparation (0.5 g/kg feed) for 21 days reduced the abundance of Salmonella in chicken feces (2.60 log CFU/g) compared to the control group (3.99 log CFU/g). Similar positive outcomes have been observed in pigs. In these studies, incorporating yeast cell wall-derived mannan into the diet at 800 mg/kg feed was associated with a 16.8% increase in the height of the small intestinal villi and a 21.5% increase in average daily feed intake when assessed using 16S rRNA gene sequencing. Although these changes did not significantly impact overall animal growth performance, piglets supplemented with mannan exhibited a reduced number of Campylobacter bacteria in the lumen of the large intestine [54,55].

It has been observed that the synergistic effects of a mannoprotein preparation in conjunction with certain antibiotics are present. The combination of mannoprotein preparation isolated from the cell wall of S. cerevisiae BY yeast and antibiotics (tetracycline, gentamicin, ampicillin, and ciprofloxacin) (1:1) was investigated at a concentration of 1 mg/mL. The results demonstrated a reinforcement of the inhibition zones by 26, 29, 33, and 36% against P. aeruginosa, while the reinforcement was 20, 33, 40, and 14%, respectively, in the case of S. aureus [47].

3.3. Prebiotic Activity

The qualitative and quantitative composition of the human gut microbiota plays a crucial role in maintaining proper health. Dysbiosis in the microbiome has been linked to a number of diseases, including inflammatory bowel disease, multiple sclerosis, diabetes (type 1 and 2), allergies, asthma, autism, and cancer. It is noteworthy that both the qualitative and quantitative composition of the microbiome depend on genetic predispositions, diet, lifestyle, antibiotics, and other factors. Consequently, there is no single representative composition for the human gut microbiome; its profile varies considerably among individuals and even within the same individual over time [56,57,58]. The most prevalent bacterial families in a healthy individual (i.e., one who is not afflicted by any diseases) include Bacteroidaceae, Clostridiaceae, Prevotellaceae, Eubacteriaceae, Ruminococcaceae, Bifidobateriaceae, Enterobacteriaceae, Saccharomycetaceae, and Methanobacteriacea [57]. The composition of the gut microbiota can be influenced by a multitude of external factors. One such factor is mannoproteins, which possess prebiotic properties and stimulate the growth of selected lactic acid bacteria (LAB) and bifidobacteria, as well as probiotic bacteria. Moreover, mannoproteins enhance the survivability of these beneficial bacteria in the gastrointestinal environment, promoting their successful colonization of the intestines. Furthermore, mannan, a component of mannoproteins, is broken down into mannose residues by the α-mannosidase enzyme, which undergoes fermentation by the gastrointestinal microflora, resulting in the production of short-chain fatty acids. Additionally, mannan enhances the expression of genes associated with the electron transport chain and ATP synthesis in mitochondria, supplying energy to intestinal cells and thereby contributing to the maintenance of proper intestinal morphology and function [14].

Table 3. Prebiotic properties of preparations rich in mannoproteins isolated from yeast.

Action	Preparation	Dosage of the Preparation	Structure and Chemical Composition	Literature
Higher FROM 600 Lactobacillus paracasei ssp. ZY-1 tolerance towards the control group and inulin	Three fractions of α-mannan (LZ-MPS, MC-MPS, G-MPS) isolated from Kluyveromyces marxianus	2% (20 mg/mL) in MRS for growth; 100 mg/9 mL in fecal fermentation; 80 mg/10 mL in digestion test	LZ-MPS: yield 71.1 mg/g, sugars—mannose, sugar content: 97.1%, protein: –; MC-MPS: yield 84.7 mg/g, sugars—mannose, sugar content: 96.88%, protein: –; G-MPS: yield 77.3 mg/g, sugars—mannose, sugar content: 91.13%, protein: –	[13]
Stimulation of Lactobacillus arabinosus ATCC 8014 growth by ~159% and Bifidobacterium animalis subsp. lactis B12 by ~135% compared to control group (at 1–2% concentration)	Mannoproteins isolated from cell biomass of the yeast Saccharomyces cerevisiae ATCC 7090	0.5–2%	Molecular weight: 65, 14, 1.9 (Mw/kDa), nitrogen: 6.96%, carbon: 31.9%, hydrogen: 4.42%, protein: 42.3%, total sugars: 55.7% Characterization of preparations using NMR and FT-IR	[48]
Stimulation of Lactobacillus arabinosus ATCC 8014 growth by ~214% at 2% concentration	Mannoproteins isolated from the cell biomass of the yeast Metschnikowia reukaufii WLP 4650	0.5–2%	Molecular weight: 150, 13, 1.9 (Mw/kDa), nitrogen: 6.21%, carbon: 30.38%, hydrogen: 4.12%, protein: 37.8%, total sugars: 60.0% Characterization of preparations using NMR and FT-IR	[48]
Stimulation of Lactobacillus arabinosus ATCC 8014 growth by ~140–160%, depending on the dose (0.5–2%)	Mannoproteins isolated from cell biomass of the yeast Wickerhamomyces anomalus CCY 38-1-13	0.5–2%	Molecular weight: 84, 20, 1.9 (Mw/kDa), nitrogen: 5.41%, carbon: 28.77%, hydrogen: 3.93%, protein: 32.9%, total sugars: 52.8% Characterization of preparations using NMR and FT-IR	[48]
Increased cecal counts of Lactobacillus (7.80 log CFU/g) and Bifidobacterium (8.60 log CFU/g) in Cobb broilers compared to control group (7.50 and 8.37 log CFU/g, respectively)	Wall cellular yeast from Luoyang Hongxiang Biological Feed Laboratory of Henan University of Science and Technology (Henan, China)	0.5, 1.0, and 1.5 g/kg feed	Yeast cell wall (48.3% D-glucose; 32.3% D-mannose)	[59]
Improving adhesion to the intestinal wall: Enterococcus faecium, Lactobacillus plantarum, and Lactobacillus salivarius in an artificial intestine	Mannoprotein extracts isolated from the yeast S. cerevisiae (Laffort, Guipuzcoa, Spain)	15 mg/mL	Protein: 31.3%, polysaccharides 66.1%, mannose 92.7% of individual sugars from polysaccharides	[60]
Increased concentrations of formic (1.05 vs. 0.73 mmol/g), acetic (76.92 vs. 57.81 mmol/g), propionic (30.08 vs. 21.66 mmol/g), isoamyl (1.02 vs. 0.86 mmol/g), and butyric (18.73 vs. 11.71 mmol/g) acids in cecal content of broilers vs. control group	S. cerevisiae yeast cell wall extract (Actigen, Alltech, Nicholasville, KY, USA)	0.5 g/kg feed	No detailed characteristics	[61]

presents data collected based on an analysis of literature regarding the prebiotic activity of mannoprotein preparations isolated from the yeast cell wall. Three fractions of α-mannan isolated from Kluyveromyces marxianus yeast demonstrated a stronger stimulation of bacteria belonging to the Lactobacillus paracasei ssp. tolerans ZY-1 genus compared to the control group (lack of carbon source in the culture medium) and inulin. It is noteworthy that both the bacterium and the mannan fractions were isolated from Tibetan kefir, which may have contributed to the observed stronger stimulation compared to other tested bacteria in this trial. Depending on the fraction of α-mannan, a greater increase in selected Lactobacillus bacteria was observed compared to the control group (lack of carbon source in the culture medium) or inulin. Notably, since both the bacterium and the α-mannan fractions were isolated from Tibetan kefir, the observed stimulation might be partly attributed to their shared origin. Depending on the specific α-mannan fraction, researchers observed an increased abundance of selected Lactobacillus bacteria relative to the control or inulin. In parallel, the addition of these α-mannan fractions resulted in a higher population of Bacteroides ovatus and Phascolarctobacterium faecium, while E. coli numbers decreased, as determined by 16S rRNA gene analysis [13]. Mannoproteins isolated from the cellular biomass of Saccharomyces cerevisiae ATCC 7090 yeast demonstrated a stimulatory effect on the growth of bacteria. In particular, the growth of Lactobacillus arabinosus ATCC 8014 increased by approximately 159% and that of Bifidobacterium animalis subsp. lactis B12 by roughly 135% compared to a control group receiving a 1–2% addition of the mannoprotein preparation. In the case of mannoproteins isolated from Metschnikowia reukaufii WLP 4650 yeast, a concentration of 2% mannoproteins was observed to stimulate the growth of Lactobacillus arabinosus ATCC 8014 by approximately 214%. In contrast, the mannoprotein preparation derived from the yeast biomass of Wickerhamomyces anomalus CCY 38-1-13 stimulated the growth of Lactobacillus arabinosus ATCC 8014 by approx. 140–160% depending on the preparation dosage [50]. In other studies investigating the impact of yeast cell wall preparations rich in mannoproteins (see Table 3) on male Cobb broilers, a higher level of Lactobacillus (7.80 log CFU/g) and Bifidobacterium (8.60 log CFU/g) was observed in the cecal contents compared to the control group (7.50 and 8.37 log CFU/g, respectively). A higher level of Bifidobacterium (8.60 log CFU/g) bacteria was observed in the cecal contents compared to the control group (7.50 and 8.37 log CFU/g, respectively) [59].

In the studies conducted in artificial intestinal juice, extracts of mannoproteins isolated from S. cerevisiae, obtained by the thermal method, were used. These were then subjected to ultrafiltration and purification by chromatography using concanavalin A. The preparations stimulated growth and influenced the greater survivability of five strains of lactic acid bacteria. Notably, the extracts improved the viability of strains such as Enterococcus faecium, Lactobacillus plantarum, and Lactobacillus salivarius [60].

The impact of yeast S. cerevisiae cell wall extract on the concentration of short-chain fatty acids in the cecal digesta of male broilers was investigated in experimental models. The study demonstrated that broilers supplemented with the yeast cell wall extract exhibited higher concentrations of various SCFAs—namely formic, acetic, propionic, isoamyl, and butyric acids—compared to the control group. Specifically, the SCFA concentrations in the supplemented group were 1.05, 76.92, 30.08, 1.02, and 18.73 mmol/g, respectively, versus 0.73, 57.81, 21.66, 0.86, and 11.71 mmol/g in the control group [61]. With regard to α-mannan isolated from yeast, which is consumed by humans, the results indicated that it may facilitate the growth of intestinal bacteria, including Bacteroides spp. [14,62].

3.4. Immunostymulating Effect

Mannoproteins serve as inducers of humoral and cellular immunity in both humans and animals. They can activate the effectors of human innate immunity, such as macrophages and natural killer (NK) cells, and induce the secretion of lactoferrin. Moreover, mannoproteins interact with lectins, which are mannose-binding proteins that participate in the opsonization process and facilitate immunophagocytosis [63]. This binding enhances phagocytosis and lectin interaction, ultimately activating the complement system and initiating innate immune responses [64]. This constitutes a mechanism of non-specific immunity [63]. In addition, mannan regulates the immune response by stimulating macrophages to produce nitric oxide and promoting their mitosis [65].

Table 4. Immunostimulatory properties of selected preparations isolated from yeast.

Action	Preparation	Dosage of the Preparation	Structure and Chemical Composition	Literature
Producing more H2O2 during bacterial lipopolysaccharide stimulation in Beagle dogs	Preparation of mannoproteins isolated from the cell wall of the yeast S. cerevisiae (Actigen, Alltech, Lexington, KY, USA)	400 mg/kg feed	No detailed characteristics	[66]
Stimulation of nitric oxide-induced angiogenesis in umbilical vein endothelial cells ex vivo -eNOS signaling pathway in umbilical vein endothelial cells ex vivo	Mannoproteins isolated from the yeast cell wall of S. cerevisiae K48L3 (mutant)	100, 500, and 1000 ng/mL	No detailed characteristics	[67]
Increased in vitro production of SBD-1 (increased 5.8-fold compared to control) protein in rumen epithelial cells	Mannan isolated from the cell wall of the yeast S. cerevisiae (Sigma, Munich, Germany)	50 µg/mL	No detailed characteristics	[68]
Increased expression of cytokines in blood serum after challenge with E. coli bacterial lipopolysaccharide	Yeast cell wall (Canadian Bio-System Inc., Calgary, AB, Canada)	0.25%	Polysaccharides 43.3%, including sugars: mannose 22.9%, glucose 20.0%; protein 17.2%	[69]
E. coli bacteria cells by 31% Salmonella cells by 20% Increased production of γ interferon (IFN-γ mRNA) in the intestine during coccidial infection Depletion of regulatory T cells (Tregs)	Yeast cell wall of Pichia guilliermondii; CitriStim, ADM, Quincy, IL, USA	0.1 or 0.2%	No detailed characteristics	[70]
Reducing the number of E. coli and Salmonella cells in the intestinal content of broilers Increased humoral memory of the immune response of broilers to Newcastle disease	Cell wall of the yeast S. cerevisiae (AB Vista, Marlborough, Wiltshire, UK)	1000 mg/kg feed	Weight: 975 g/kg DM; crude protein: 610.2 g/kg, including 97.6 g/kg total N; raw fat: 39 g/kg; total ash: 37 g/kg; glucans and mannan in total approximately 40% of the total sample	[71]
Increase in the height of intestinal villi in the jejunum of pigs Increase in the content of dry fecal matter in pigs Increasing the duration of intestinal transit of pig digestive contents	Biomass yeast Candida utilis LYCC 7549 (Lallemand Inc., Salutaguse, Estonia)	40% of the protein content in the diet came from the yeast biomass C. utilis	Dry weight: 970 g/kg; protein: 470 g/kg; raw fat 16 g/kg; ash 78 g/kg	[72]

presents data from a literature review on the immunostimulatory properties of preparations rich in mannoproteins derived from yeast cell walls.

Beagle dogs receiving a diet supplemented with mannoproteins from S. cerevisiae (400 mg/kg) exhibited significant modulation of both specific and non-specific immune responses. Neutrophils from these animals, when stimulated by bacterial lipopolysaccharides (LPSs), produced increased amounts of hydrogen peroxide (H₂O₂) compared to those in the control group. Ref. [66] demonstrated in their study that the addition of mannoproteins to the diet of Beagle dogs significantly affected the specific and non-specific immune responses of these animals. In another study, mannoproteins isolated from the cell wall of mutant strain S. cerevisiae K48L3 were shown to stimulate nitric oxide-induced angiogenesis and activate the Akt-eNOS signaling pathway in human umbilical vein endothelial cells (HUVECs) ex vivo [67]. These findings further confirmed the immunostimulatory properties of mannan on ovine rumen epithelial cells (ORECs). The impact of mannan (at a dose of 50 μg/mL) on the gastrointestinal environment of sheep was investigated in vitro by analyzing the expression of SBD-1 protein (a peptide secreted by rumen epithelial cells with antimicrobial activity) and further signaling pathways stimulated by ORECs. Mannan was found to significantly increase the expression of SBD-1 protein (5.8-fold compared to the control group), thereby confirming the immunostimulatory properties of mannan on ORECs [68]. In broiler chickens challenged with bacterial LPSs from E. coli, supplementation with yeast cell wall preparations resulted in a notable modulation of cytokine expression in blood serum [69]. A yeast cell wall preparation from Pichia guilliermondii (at doses of 0.1% or 0.2%) was employed in the diet of broilers infected with coccidia, resulting in an enhancement in the immunity of the broilers. This was evidenced by a reduction in the number of regulatory T cells (Tregs) and an increase in the level of interferon γ (IFN-γ) in the intestine [70]. A study by Ref. [71] demonstrated that the inclusion of S. cerevisiae yeast cell wall at a concentration of 1000 mg/kg in the diet of Ross 308 broilers led to improvements in feed conversion ratio, a reduction in the number of E. coli and Salmonella cells in the gut content, and an enhancement of the humoral immune memory response of broilers to Newcastle disease virus.

The effects of three yeast species, namely Cyberlindnera jadinii, Blastobotrys adeninivorans, and Wickehamomyces anomalus, on the mitigation of intestinal inflammation were investigated in Atlantic salmon. The research findings indicated that the autolysate of W. anomalus was effective in alleviating intestinal inflammation, whereas the other two strains demonstrated only limited effects. The efficacy of the yeast preparations was found to depend on the type and production method employed [73]. The replacement of up to 40% of dietary protein with yeast biomass from Candida utilis LYCC 7549 was found to positively influence intestinal transit time and stool structure in pigs. This indicates that deactivated yeast cells of C. utilis enhance digestive system function and promote the growth of probiotic bacteria [72].

3.5. Antioxidant Activity

Mannan also exhibits antioxidant properties (

Table 5. Antioxidant properties of selected preparations isolated from yeast.

Action	Preparation	Dosage of the Preparation	Structure and Chemical Composition	Literature
Delay in the rate of lipid oxidation in pork sausages	Mannoproteins isolated from the cell wall of S. cerevisiae	25, 50, 75, 100%	Ratio of mannan to protein: 14.5	[76]
51.8% DPPH free radical scavenging	S. cerevisiae yeast extract MYN04	0.5 mg/mL emulsifier addition	Carbohydrates: 27.1%, protein: 72.9%	[77]
Neutralization of free hydroxyl radicals Chelating effect on copper and iron The ability to scavenge peroxide radicals by the KMM-1 fraction	Mannan fractions isolated from Kluyveromyces marxianus yeast	0.05, 0.1, 0.25, 0.5, 1.0 mg/mL	KMM-1 (glucose–mannose sugars: 85.9%, protein: 0.11%, molecular mass: 7.6 kDa) KMM-2 (glucose–mannose sugars: 95.2%, protein: 0.51%, molecular mass: 26.1 kDa) KMM-3 (mannose sugars: 96.1%, protein: –, molecular weight: 41.3 kDa) KMM-4 (mannose sugars: 93.4%, protein: 1.21%, molecular weight: 75.1 kDa)	[63]
Radical scavenging ability (PM and CMP-M) Antioxidant capacity towards lipids (all tested preparations) (PM and CMP-M: +15% hydroxyl radical scavenging vs. unmodified mannan)	Mannans extracted from the yeast cell wall and subjected to chemical modification: mannan phosphorylated (PM), sulfated mannan (SM), mannan carboxymethylated (CM-M), mannan carboxymethylated–phosphorylated (CMP-M) and mannan carboxymethylated–sulfated (CMS-M)	0.1, 0.2, 0.4, 0.8, 1.6, and 3.2 mg/mL	Mannan: sugars 96.15%, PM: sugars 76.33%, SM: sugars 64.08%, CM-M: sugars 57.28%, CMP-M: sugars 38.03%, CMS-M: sugars 23.25%	[75]
Increase in the concentration of reduced glutathione, glutathione reductase, and glutathione S-transfer in the blood of broilers after 21 days of supplementation with a yeast cell wall preparation (increased the rate of r-GSH (13.8–16.6%), GR (9.7–24.6%) at 0.5–1.5 g/kg feed)	Wall cellular yeast from Luoyang Hongxiang Biological Feed Laboratory of Henan University of Science and Technology (Henan, China)	0.5, 1.0, 1.5 g/kg feed	D-glucose: 46.2%, D-mannose: 29.8%	[59]

), which contribute to the maintenance of optimal functioning in animal organisms by enhancing the activity of antioxidant enzymes, including superoxide dismutase, catalase, and glutathione peroxidase. Moreover, the modified mannans exhibited an enhanced capacity to protect lipids from oxidation compared to their unmodified counterpart [74,75]. The inhibitory effect of mannans extracted from yeast cell walls and subjected to chemical modifications on lipid peroxidation and their capacity to scavenge hydroxyl radicals were investigated. The modifications yielded phosphorylated mannan (P-M), sulfated mannan (S-M), carboxymethylated mannan (CM-M), carboxymethylated–phosphorylated mannan (CMP-M), and carboxymethylated–sulfated mannan (CMS-M). The scavenging ability of hydroxyl radicals was observed to be 15% higher in the case of P-M and CMP-M compared to unmodified mannan. Moreover, the antioxidant capacity against lipids was found to be higher than that of unmodified mannan [75]. Mannoproteins extracted from the cell wall of Saccharomyces cerevisiae have also demonstrated practical antioxidant applications. For instance, their incorporation into pork sausages delayed the oxidation rate of lipids, irrespective of the percentage of pork fat replaced by the mannoprotein preparation [76]. Furthermore, the extract from S. cerevisiae MYN04 demonstrated DPPH free radical scavenging activity at a level of 51.8% (with a preparation addition of 0.5 mL/mL) [77]. Studies utilizing the unconventional yeast strain Kluyveromyces marxianus, from which four mannan fractions differing in composition and molecular weight were isolated (refer to Table 5), demonstrated that all fractions possessed antioxidant activity and neutralized hydroxyl free radicals in a dose-dependent manner. Notably, only the KMM-1 fraction also showed the ability to neutralize superoxide ions [63].

It is crucial to emphasize that the antioxidant activity of mannoproteins containing protein fragments may also be associated with the reducing potential of the aromatic amino acid tryptophan [78]. A series of studies conducted on broilers, whose diets were supplemented with a cell wall-based supplement (at doses of 0.5, 1.0, and 1.5 g/kg), demonstrated an increase in the levels of reduced glutathione and glutathione reductase in the intestine. These findings corroborate the notion that yeast cell wall components possess the capacity to regulate glutathione pathways, which are of paramount importance for antioxidant defense [79].

3.6. Emulsifying Properties

Yeast-derived glycoproteins are distinguished by their non-toxicity, low production cost, and biodegradability, rendering them an appropriate choice for use as emulsifying and stabilizing agents. This property is attributed to their amphiphilic structure, in which proteins are bonded to hydrophilic mannose polymers. Furthermore, these glycoproteins display beneficial emulsifying properties in vitro, which are influenced by pH levels and the concentrations of selected salts used in the food industry [80,81]. Under appropriate conditions—such as during homogenization—they can form microemulsions, nanoemulsions, and macroemulsions. Furthermore, the emulsifying performance of these glycoproteins is closely tied to their molecular weight; higher molecular weights, which reflect an increased protein content in mannoprotein preparations, correlate with improved emulsifying capacity [82].

Table 6. Emulsifying properties of selected preparations isolated from yeast.

Action	Preparation	O/W Phase Ratio	Dosage of the Preparation	Structure and Chemical Composition	Literature
Maintaining emulsion stability at a dose of 6 and 8% mannoproteins after 7 days of storage (100% emulsion phase preserved at 8%)	Mannoproteins isolated from the cell wall of S. cerevisiae	50%/5%	2, 4, 6, 8%	Ratio of mannan to protein: 14.5	[83]
Improved emulsion stability observed at 1.25% and 1.5% mannoprotein concentrations; associated with higher negative zeta potential, smaller oil droplets, and increased viscosity	Mannoproteins isolated from the cell wall of Kluyveromyces marxianus IBRC-M 30114	20%/80% and 80%/20%	0.5, 0.75, 1, 1.25, and 1.5%	Molecular weight: 107.2 kDa, protein: 28.8%, carbohydrates: 68.22%	[84]
Improved textural properties of pork sausages with 25–100% replacement of animal fat by emulsions containing mannoproteins	Mannoproteins isolated from the cell wall of S. cerevisiae	50% oil/5% sodium caseinate/6% mannoprotein preparation (w/v)	Oil replacement in doses: 25, 50, 75, 100%	Ratio of mannan to protein: 14.5	[76]
Maintenance of emulsion phase after 48 h at pH 3 with 25 mg/7.5 mL extract addition; better stability at acidic pH values	Saccharomyces cerevisiae EC 1118 yeast extract (Lalle—mand Inc., Montreal, QC, Canada)	2:1	25 mg of added extract, additionally: corn oil 5 mL, Mcllvine buffer 2.5 mL	Sugars: 564.4–980.5 mg/g s.s. (depending on the isolation method), including mannose: 69.4–93.5% (depending on the isolation method), protein: 10.6–48.3 mg/g DS (depending on the insulation method)	[85]
Maintaining a stable emulsion for 90 days at 4 °C (76% of the emulsion phase)	Mannoproteins isolated from the yeast K. marxianus FII 510700 (FRR 1586)		0.12 g of mannoprotein, 4 mL of water, 6 mL of corn oil	Carbohydrates: 90% (mannan), protein: 4–6%	[86]
Emulsification index of 80% for wheat germ oil, corn oil, and olive oil after 24 h with 0.5 mg/mL extract addition	S. cerevisiae yeast extract MYN04	-	0.5 mg/mL emulsifier addition	Carbohydrates: 27.1%, protein: 72.9%	[77]
Maintained mayonnaise stability after 28 days at refrigeration with 0.6–1.0 g/100 g mannoprotein addition; stable pH and improved color	Mannoproteins derived from the yeast cell wall after Saccharomyces beer production uvarum	-	0.6, 0.8, 1.0 g/100 g	Fractions of 58 kDa and 64 kDa	[80]
Emulsion stability after 30 days at 4:6 oil-to-water ratio; emulsifying capacity at 72% with 1% mannoprotein addition (ultraturrax method)	Mannoproteins derived from Saccharomyces yeast pastorianus (BSY) Super Bock Group, S.A. (Leça do Balio, Portugal)	4:6	1%	Alkaline insulation: yield: 3–17%; sugars: 16–79%, including glucose: 23–90 mol%, mannose: 10–77 mol%; protein: 32% Subcritical fluid insulation with high temperature: yield: 8–24%; sugars: 15–79%, including glucose: 50–71 mol%, mannose: 29–50 mol%; protein: 32%	[87]

presents a review of the literature on the emulsifying properties of mannoproteins and yeast extracts.

Mannoproteins isolated from the cell wall of S. cerevisiae demonstrated a significant effect on maintaining emulsion stability after seven days of storage (at mannoprotein concentrations of 6% and 8%) compared to the control group (without mannoprotein addition) [83]. In other studies, using extracts from S. cerevisiae MYN04 (containing 27.1% carbohydrates and 72.9% protein), an emulsification index of 80% was achieved in emulsions of wheat germ oil, corn oil, and olive oil after 24 h of storage (with an extract addition of 0.5 mg/mL) [77]. The phase of the emulsion was maintained after 48 h of storage in emulsions at pH 3, where an extract from Saccharomyces cerevisiae EC 1118, isolated using physical and Sur Lies methods (25 mg/7.5 mL), was used. The extracts contained sugars ranging from 564.4 to 980.5 mg/g dry weight, with mannose accounting for 69.4–93.5% and protein content ranging from 10.6 to 48.3 mg/g dry weight (variability depending on the isolation method). Furthermore, it was demonstrated that the extracts were more efficacious in maintaining the emulsion phase at lower pH levels [85]. A mannoprotein preparation derived from the cell wall of Kluyveromyces marxianus IBRC-M 30114 (at concentrations of 1.25% and 1.5%) was found to result in enhanced emulsion stability, a more negative zeta potential, smaller oil droplets, and an increased emulsion viscosity [84]. Emulsion stability at a level of 76% of the emulsion phase after 90 days of storage at 4 °C was observed in the presence of mannoprotein preparations isolated from K. marxianus FII 510700 (FRR 1586) with an addition of 0.12 g/10 mL [86]. Furthermore, emulsions (4:6) with the addition of mannoprotein preparations from S. pastorianus, isolated using subcritical fluid at high temperature, demonstrated emulsifying capacity and stability at 72% after 30 days of storage [87].

Mannoproteins isolated from the cell wall of S. cerevisiae have been demonstrated to enhance the network structure and textural properties of pork sausages without compromising the sensory characteristics of the final product. The replacement of animal fats in food products with emulsions containing mannoproteins can beneficially alter the lipid profile of the product by reducing the total fat content, as well as the saturated and trans-fat fractions, while increasing the polyunsaturated fatty acids present in vegetable oils (e.g., canola oil, sunflower oil, olive oil) [76]. Mannoprotein preparations derived from the cell wall of Saccharomyces uvarum, which are produced as a by-product of beer production, were employed as a substitute for xanthan gum (an emulsifier) in the production of mayonnaise. Mayonnaise samples formulated with mannoprotein additions (0.6, 0.8, or 1.0 g/100 g) maintained stable pH values after 28 days of refrigerated storage and exhibited superior stability compared to control samples containing xanthan gum. In addition, these mayonnaise samples displayed a bright color with reduced yellowing during storage, and they received high scores in organoleptic evaluation [80].

4. Conclusions

This review article examines the potential applications of mannoproteins isolated from the yeast cell wall, considering their diverse functional properties. These include prebiotic, immunostimulatory, antimicrobial, antibiofilm, emulsifying, and antioxidant activities. A critical aspect discussed is the synergistic effect observed when mannoproteins are combined with antibiotics, a strategy that may allow for reduced drug dosages without sacrificing therapeutic efficacy. These findings have significant implications for both animal husbandry and therapeutic prospects in humans. The absence of precise dose–response data—together with incomplete chemical characterization—limits our understanding of the mechanisms underlying their biological effects. Many studies do not include detailed structural analysis of these compounds, hindering the ability to effectively correlate their functionality with specific chemical features. Future research efforts should prioritize comprehensive structural and chemical analyses of mannoproteins to establish a robust correlation between their functional properties and molecular composition. Moreover, while mannoproteins from Saccharomyces cerevisiae have been extensively studied, mannoproteins derived from nonconventional yeast remain largely unexplored, representing a valuable avenue for future investigation. It is therefore of interest to continue research in this area, which may provide new insights into the potential applications of these glycoproteins in the food, medical, and cosmetic industries, as well as in animal and human nutrition and prophylaxis.

Nonetheless, certain research gaps remain unaddressed. These include the lack of harmonized methodologies for mannoprotein isolation and quantification, insufficient structure–activity data, and limited exploration of strain-specific biofunctional properties. Future studies should aim to fill these gaps by applying advanced analytical techniques, conducting comparative assessments across yeast species, and integrating functional assays with molecular profiling. Such efforts will be essential to expand the scientific foundation for the targeted application of mannoproteins in health and industry.