AR and glycated peptides are early-stage MRPs with notable antibacterial activity (53,54). AR has been identified in thermally processed milk and has been shown to inhibit the proliferation of foodborne bacteria, including S. aureus, Bacillus subtilis, Enterococcus faecalis, Listeria innocua, Listeria monocytogenes, E. coli and Salmonella typhimurium (55,56). AR, formed in a lactose-butylamine model system, has also been reported to inhibit the proliferation of Pseudomonas aeruginosa and antibiotic-resistant bacteria, including multidrug-resistant Pseudomonas aeruginosa, methicillin-susceptible S. aureus and methicillin-resistant S. aureus (48,49). Moreover, AR has demonstrated significant antibacterial activity against H. pylori under acidic conditions, both in vitro and in a mouse infection model, highlighting its potential for the treatment of infectious gastric diseases (47). The antibacterial mechanism of AR is considered to involve interference with bacterial cell wall integrity and membrane permeability, with its efficacy influenced by factors, such as cell wall thickness, lipid composition and surface charge (48,49). Additionally, AR has been shown to enhance the effectiveness of conventional antibiotics, such as amikacin and levofloxacin, although the underlying mechanism of this synergy remains unclear (49).

Glycopeptides, a significant class of antibacterial agents, serve as a critical platform for antibiotic discovery (57). Several glycopeptides, such as vancomycin and teicoplanin, are widely used in clinical settings. Additionally, numerous glycopeptides synthesized from sugar-free amino acids have been evaluated for their antibacterial activity against various microorganisms (54). Notably, glycopeptides formed during the early stages of the Maillard reaction, such as histidine-asparagine-lactose glycopeptide and gluten-hydrolysate-glucosamine glycopeptide, have demonstrated enhanced antibacterial activity compared with their native counterparts (35,36,54,58).

α-dicarbonyl compounds are intermediate-stage products of the Maillard reaction and are widely present in both processed and unprocessed foods (59-61). These highly electrophilic compounds readily interact with proteins, potentially altering their bioactivity (50). Among the most extensively studied α-dicarbonyls are diacetyl, glyoxal and methylglyoxal (MGO), which have demonstrated significant antibacterial properties against various bacterial strains, including E. coli, Pseudomonas fluorescens, S. aureus, Salmonella typhimurium and Salmonella enteritidis (45,51,62,63). Furthermore, glyoxal, MGO and diacetyl have shown a strong negative correlation with bacterial viability, suggesting that α-dicarbonyl compounds contribute significantly to the antibacterial activity of MRPs (50). Notably, MGO has been reported to impair biofilm formation by disrupting fimbriae and flagella at lower concentrations, while at higher concentrations, it compromises microbial membrane integrity (64,65). However, the antibacterial effects of MRPs cannot be solely attributed to α-dicarbonyl compounds. Other MRP components, such as melanoidins, also play a role in the overall antibacterial activity. It is likely that these components act synergistically, enhancing antibacterial efficacy through complementary mechanisms such as membrane disruption and metal chelation (Fig. 1A) (66).

Additionally, some α-dicarbonyl compounds undergo cyclization and condensation to form pyrazoles, pyrazines, furans and pyrroles, which have been demonstrated to exhibit antibacterial activity in their purified form rather than as components of MRPs (67-71). Although direct evidence linking these compounds to the antibacterial effects of MRPs is limited, their structural similarity to known antibacterial agents suggests that they may contribute to MRP-mediated antimicrobial activity (31). Further research is needed to elucidate their precise mechanisms of action.

The formation of HMW compounds, particularly melanoidins, marks the late stage of the Maillard reaction (23). Melanoidins are primarily responsible for the characteristic brown coloration of heat-treated foods and have gained attention for their multiple bioactivities, including antioxidant, antihypertensive, prebiotic and antibacterial properties (32). Numerous studies have demonstrated their broad-spectrum antibacterial activity against both foodborne and pathogenic bacteria. Moreover, HMW melanoidins exhibit stronger antibacterial activity than their low-molecular-weight (LMW) counterparts (11,20,32).

Structurally, melanoidins derived from model systems primarily consist of furans, carbonyl compounds, pyrroles, pyrazines and pyridines (23). Additionally, melanoidins are negatively charged, with HMW fractions exhibiting a greater negative charge than LMW fractions. These structural characteristics contribute to their bioactivity, particularly in antibacterial mechanisms (23,72).

Melanoidins, as anionic hydrophilic polymers, exhibit strong metal-binding properties, effectively chelating essential metals such as iron, copper and magnesium (Fig. 1A) (39,66,73). Since iron is crucial for bacterial proliferation and magnesium serves as a key activator of numerous enzymes, the sequestration of these metals by melanoidins may contribute to inhibition of bacterial proliferation (29,39,66,73-75). By depriving bacteria of these essential nutrients, melanoidins interfere with their metabolic functions and survival. Additionally, MRPs have been reported to exert bactericidal effects on Gram-negative bacteria by causing irreversible damage to both the inner and outer membranes (23). Notably, magnesium chelation has been reported as a potential inducer of membrane disruption, suggesting that melanoidins may contribute to bacterial membrane damage by sequestering magnesium and disrupting membrane integrity (66). This dual action, nutrient depletion and membrane disruption, enhances their antibacterial properties.

In addition to nutrient deprivation, melanoidins may exert bactericidal effects on Gram-negative bacteria by disrupting both the inner and outer membranes (20). Studies have shown that magnesium chelation can destabilize bacterial membranes, suggesting that melanoidins may enhance this effect by sequestering magnesium and interfering with membrane integrity (66). Moreover, melanoidin treatment has been observed to increase dye uptake, promote pore formation and induce intracellular component leakage, ultimately leading to bacterial cell death (20,23,39,76,77). This suggests that melanoidins not only act as metabolic inhibitors but also function as membrane disruptors, further contributing to their antibacterial efficacy.

In addition to the well-characterized antibacterial compounds found in MRPs, several other bioactive molecules contribute to their antimicrobial properties (Fig. 1A). Phenols, quinones, nitrogen-containing heterocycles (such as 1,4-dihydropyridines) and hydrogen peroxide (H₂O₂) have all been reported to exhibit antibacterial activity. These compounds likely exert their effects by interfering with microbial metabolic pathways, disrupting bacterial cell walls, proteins and nucleic acids, and ultimately impairing bacterial survival (31,42,78-80). For instance, alginetin (3,8-dihydroxy-2-methylchromone), a flavonoid-derived polyphenol, has been isolated from a xylose-phenylalanine model system and identified as a contributor to the inhibitory effects on Bacillus cereus and Salmonella typhimurium (31).

Although these compounds have been identified as potential contributors to the antibacterial properties of MRPs, their specific roles within complex Maillard reaction systems remain to be fully elucidated. Future research should focus on determining their precise mechanisms of action and evaluating their synergistic effects with other bioactive MRPs.

The pH of the reaction system plays a critical role in determining the kinetics, composition and bioactivity of MRPs (22,82). Studies have revealed that under alkaline conditions, amino acids and sugars react more efficiently, leading to increased browning intensity and enhanced antibacterial activity (19,37,38,82). Alkaline conditions enhance the nucleophilicity of amino groups, accelerating their interaction with carbonyl compounds and facilitating the formation of Schiff bases and Amadori products (83-85). However, extreme alkalinity (pH>12) has been reported to reduce the formation of melanoidins, likely due to degradation reactions that disrupt polymerization (86). The impact of pH also varies across different reaction stages. In the intermediate and advanced stages, pH influences the breakdown of Amadori products, thereby modulating the production of α-dicarbonyl compounds and metal-chelating melanoidins (22,73,87). Notably, MRPs derived from glucosamine self-reaction exhibit antibacterial activity at pH 5 but not at pH 7, suggesting that pH not only affects the reaction pathway but also alters the functional properties of the final MRPs (88). Further research is needed to determine whether dynamically adjusting pH during the Maillard reaction can be used as a strategy to fine-tune the antibacterial activity of MRPs.

The method and intensity of heating significantly influence the antibacterial properties of MRPs. Prolonged reaction times and elevated temperatures generally enhance the formation of HMW compounds, which have been associated with stronger antibacterial activity (19,23,36,50). This effect is primarily attributed to the greater extent of Maillard reaction progression, leading to increased production of bioactive melanoidins and α-dicarbonyl intermediates (73,89). However, excessive heating can have detrimental effects. Overly complex molecular structures may form, reducing the metal-chelating abilities and bioactivity of MRPs. Moreover, prolonged thermal processing may generate unwanted byproducts, some of which could be cytotoxic or detrimental. To address these challenges, alternative processing techniques have been explored. Mild-temperature Maillard reactions (for example, ~40˚C) have been investigated for their ability to generate bioactive MRPs while minimizing undesirable byproducts (35). Additionally, emerging technologies such as ultrasound-assisted Maillard reactions and γ-irradiation have been explored as innovative, non-toxic strategies to enhance the antibacterial properties of MRPs (46,90).

The choice of sugar reactants critically impacts the composition and bioactivity of MRPs. Most studies have focused on reducing sugars, such as glucose and fructose, due to their high reactivity and prevalence in food systems (9,19,32,37). However, pentose sugars, including xylose and ribose, have been reported to produce MRPs with superior antibacterial activity compared with hexoses such as glucose (41). This difference is likely due to the stronger reducing properties of pentoses, which accelerate Maillard reaction kinetics and promote the formation of highly reactive intermediates (32,91). In addition to monosaccharides, the use of oligosaccharides and polysaccharides can further modulate the antibacterial properties of MRPs (34,36,76,92). Polysaccharides not only serve as reactants but also provide carbohydrate backbones that contribute to the structural diversity of HMW melanoidins (25).

Amino acid composition is another crucial determinant of the antibacterial activity of MRPs. Different amino acids lead to the formation of distinct reaction products, resulting in variations in bioactivity (23,70). For instance, MRPs derived from glucose-tryptophan systems have shown stronger antibacterial effects against E. coli than those produced from other amino acids (29). Similarly, xylose-phenylalanine MRPs exhibit potent antibacterial activity against Bacillus cereus and Brevibacillus brevis (32). These findings suggest that the antibacterial potential of MRPs is highly dependent on the specific amino acid involved, though variations in experimental conditions and microbial targets may contribute to differences in observed efficacy. One possible explanation for these differences is the variation in metal-chelating properties and π-electron interactions among different amino acid-derived MRPs, which may influence their ability to disrupt bacterial membranes (23). Additionally, amino acids with nucleophilic side chains, such as lysine, react more readily with reducing sugars, accelerating the formation of Schiff bases and altering the overall reaction kinetics (85).

In more complex Maillard reaction systems, antimicrobial peptides (AMPs) may also form as a result of amino acid modifications. AMPs, such as human LL-37, exert antibacterial effects through membrane disruption, facilitated by their cationic secondary structures, hydrophobic domains and specific amino acid residues (for example, proline, histidine and tryptophan) (93). The Maillard reaction can simultaneously induce peptide degradation and cross-linking, leading to the formation of novel bioactive peptides with enhanced antibacterial properties (37,94). Notably, research has identified several novel AMPs, including those with specific amino acid sequences such as RVAPEEHPTL and WLPVVR, from MRPs of half-fin anchovy hydrolysates and glucose. These peptides demonstrate stronger inhibitory effects against E. coli (42). This finding suggests a potential link between the Maillard reaction and antimicrobial peptide generation. These include overlapping amino acid compositions (for example, proline and cysteine), a net cationic charge and antibacterial activity mediated by bacterial membrane disruption (42,95-98).

Notably, MRPs formed in glycolaldehyde and bovine serum albumin model systems have been reported to inhibit lipopolysaccharide (LPS) uptake by macrophages, a property shared by certain AMPs that neutralize LPS toxicity by direct binding (97,99). However, whether these MRPs interact with LPS through the same mechanism as AMPs remains unclear, warranting further investigation.

These findings suggest that the antibacterial potential of MRPs is highly dependent on the specific amino acid involved. While certain amino acids enhance antimicrobial activity through metal chelation and membrane disruption, variations in experimental conditions and microbial targets may also contribute to differences in observed efficacy. Further studies are needed to systematically compare the effects of different amino acids on MRPs bioactivity under controlled conditions.

The ratio of sugar to amino acids significantly affects the composition, MW distribution and antibacterial efficacy of MRPs (100). Higher sugar concentrations tend to promote the formation of HMW melanoidins, which exhibit strong metal-chelating and antioxidant properties that contribute to bacterial inhibition (91). However, excessive sugar levels can lead to over-polymerization, resulting in insoluble compounds with diminished bioactivity (101). Conversely, increasing the amino acid concentration shifts the reaction toward the production of nitrogen-containing LMW compounds, such as Strecker aldehydes and pyrazines, which are known for their direct antibacterial effects (32). Therefore, optimizing the sugar-to-amino acid ratio is crucial for enhancing the antibacterial activity of MRPs. Additionally, the sugar-to-amino acid ratio influences the pH of the reaction system, further modulating the composition and bioactivity of MRPs (101). A high sugar proportion typically lowers the pH, enhancing inhibition against acid-sensitive bacterial strains, whereas a higher amino acid content can promote the formation of nitrogenous heterocyclic compounds with distinct antibacterial mechanisms (32).

Several additional factors, including metal ions, water activity and external additives, further influence the formation and antibacterial properties of MRPs (9). Increased phosphate buffer concentrations have been shown to accelerate glucose consumption, indicating enhanced Maillard reaction progression (83). Similarly, the presence of iron and copper ions has been reported to catalyze Maillard reactions and promote the formation of reactive oxygen species and dicarbonyl compounds, which could enhance MRP antibacterial efficacy (102). Moreover, certain additives, such as sulfites, act as reducing agents that scavenge reactive carbonyl groups and inhibit formation of MRPs (103). Other inhibitors, including polyphenols, may also interfere with the antibacterial activity of MRPs, although their precise roles remain underexplored (104). These intricate mechanisms highlight the importance of optimizing reaction parameters to maximize the antibacterial potential of MRPs. Further research is needed to determine how carbonyl scavengers and reaction modifiers affect different stages of the Maillard reaction and their impact on the bioactivity of MRPs.

The presence of electrophilic α-dicarbonyl compounds has been associated with direct antibacterial effects, likely due to their reactivity with nucleophilic residues in bacterial biomolecules (62,105). Additionally, the anionic and hydrophilic nature of melanoidin contributes to metal ion chelation and facilitates interactions with bacterial membranes (23). Other reactive groups, such as phenolic hydroxyls and quinones, may also participate in redox-based mechanisms that disrupt microbial cellular function (106,107). The MW of MRPs also influences their antibacterial potency. HMW compounds, such as melanoidins, generally exhibit stronger antibacterial activity, potentially due to their enhanced metal-chelating capacity and membrane-disruptive properties (75,81).

Beyond specific functional groups, several integral structural features of MRPs have been linked to enhanced antibacterial activity. These include negative surface charge, electrophilic carbonyl groups, aromatic rings and nitrogen-containing heterocycles, all of which may interfere with bacterial metabolism, enzyme function or membrane integrity (108-110). The choice of reaction precursors also plays a critical role. MRPs derived from pentose sugars and aromatic amino acids (for example, xylose-phenylalanine systems) tend to exhibit stronger antibacterial activity than those formed from hexoses and aliphatic amino acids, possibly due to the formation of more reactive or structurally diverse intermediates (32).