Influence of transglutaminase treatment of skim milk on the formation of ε-(γ-glutamyl)lysine and the susceptibility of individual proteins towards crosslinking

Effect of transglutaminase on gelation and functional proteins of mung bean protein isolate

This study aimed to improve mung bean protein's gelation qualities via microbial transglutaminase (mTGase) cross-linking. The mTGase treatment significantly improved gel hardness and storage modulus (G') at higher enzyme levels (2 IU/g), peaking hardness at 3 h. The scanning electron microscopy imaging demonstrated more cross-linked structures at 2 IU/g, evolving into a dense network by 3 h. The water-holding capacity for mTGase-treated samples (2 IU/g, 3 h, 55 °C) tripled to 3.77 ± 0.06 g/g versus control (1.24 ± 0.02 g/g), alongside a 15 % decrease in zeta potential (-30.84 ± 0.901 mV versus control's -26.63 ± 0.497 mV) and an increase in emulsifying activity index to 4.519 ± 0.004 m2/g from 3.79 ± 0.01 m2/g (control). The confocal images showed a more uniform lipid droplet distribution in mTGase-treated samples, suggesting enhanced emulsifying activity. Thus, mTGase treatment significantly improved gel strength and emulsifying properties, making it ideal for plant-based seafood products.

Identification of the initial reactive sites of micellar and non-micellar casein exposed to microbial transglutaminase

To investigate the influence of the internal micellar structure on the course of enzymatic cross-linking especially in the initial phase of the reaction, casein micelles isolated from raw milk via ultracentrifugation were incubated with microbial transglutaminase (mTG) in comparison with non-micellar sodium caseinate. Reactive lysine and glutamine residues were identified using a label-free approach, based on the identification of isopeptides within tryptic hydrolysates by targeted HRMS as well as manual monitoring of fragmentation spectra. Identified reactive sites were furthermore weighted by tracking the formation of isopeptides over an incubation time of 15, 30, 45 and 60 min, respectively. Fifteen isopeptides formed in the early stage of mTG cross-linking of caseins were identified and further specified concerning the position of lysine and glutamine residues involved in the reaction. The results revealed lysine K176 and glutamine Q175 of β-casein as the most reactive residues, which might be located in a highly flexible region of the molecule based on different possible reaction partners identified in this study. Except for the isopeptide αs1 K34–αs2 Q101 in sodium caseinate (SC), all reactive sites were detected in micellar and in non-micellar casein, indicating that the initial phase of enzymatic cross-linking is not affected by micellar aggregation of caseins.

Graphical abstract

Influence of Transglutaminase Crosslinking on Casein Protein Fractionation during Low Temperature Microfiltration

Low temperature microfiltration (MF) is applied in dairy processing to achieve higher protein and microbiological quality ingredients and to support ingredient innovation; however, low temperature reduces hydrophobic interactions between casein proteins and increases the solubility of colloidal calcium phosphate, promoting reversible dissociation of micellar β-casein into the serum phase, and thus into permeate, during MF. Crosslinking of casein proteins using transglutaminase was studied as an approach to reduce the permeation of casein monomers, which typically results in reduced yield of protein in the retentate fraction. Two treatments (a) 5 °C/24 h (TA) and (b) 40 °C/90 min (TB), were applied to the feed before filtration at 5 °C, with a 0.1 µm membrane. Flux was high for TA treatment possibly due to the stabilising effect of transglutaminase on casein micelles. It is likely that formation of isopeptide bonds within and on the surface of micelles results in the micelles being less readily available for protein-protein and protein–membrane interactions, resulting in less resistance to membrane pores and flow passage, thereby conferring higher permeate flux. The results also showed that permeation of casein monomers into the permeate was significantly reduced after both enzymatic treatments as compared to control feed due to the reduced molecular mobility of soluble casein, mainly β-casein, caused by transglutaminase crosslinking.

Study on β-Casein Depleted Casein Micelles: Micellar Stability, Enzymatic Cross-Linking, and Suitability as Nanocarriers

β-Casein is an amphiphilic protein and thus considered as multilaterally bound in casein micelles. Its polar molecule part, in particular the phosphoserine residues, can interact electrostatically with colloidal calcium phosphate (CCP) to form nanoclusters and its nonpolar molecule part enhances micellar stability by forming hydrophobic bonds to other caseins. Because cooling weakens hydrophobic interactions, a substantial portion of β-casein can be irreversibly removed from the casein micelle by repeated depletion steps, including cooling and subsequent ultracentrifugation. Although this effect of cooling on the micellar β-casein concentration has been well known for decades, the influence of depletion on the main characteristics of casein micelles has been less investigated yet. Therefore, we aimed to analyze the consequences of β-casein depletion on the stability as well as the functionality of casein micelles to evaluate the suitability of depleted compared to native casein micelles as nanocarriers. Up to 43.2% of the total β-casein was irreversibly sequestered from native casein micelles by repeated cooling and ultracentrifugation steps. Depletion showed no effect on size distribution as well as polydispersity and particle concentration of micelle suspensions as measured via dynamic light scattering (DLS) and nanoparticle tracking analysis (NTA), respectively. Furthermore, the stability of the micelles against ethanol or the chelating agent ethylene glycol-bis(β-aminoethyl ether)-N,N,N',N'-tetraacetic acid (EGTA) was not influenced by β-casein depletion. Notwithstanding, depleted micelles were less susceptible to enzymatic cross-linking by microbial transglutaminase (mTG), indicating narrowed water channels due to depletion. Additionally, loading experiments showed that depleted micelles could be loaded with linoleic acid (LA) as intensively as native micelles, whereupon LA displaces up to 81.3% of β-casein from native micelles. Our results confirm that depletion does not enhance the ability of the casein micelle to act as a nanocarrier for hydrophobic substances but could support the understanding of the casein micelle structure. Based on the observed unchanged stability against EGTA, the hindered enzymatical cross-linking, and the efficient displacing of β-casein by LA, we suggest that the major portion of micellar β-casein is hydrophobically incorporated into the micelle structure without impact on the formation of calcium phosphate nanoclusters. The main role of β-casein for the casein micelle structure, therefore, might be to facilitate the high hydration of the interior and thus the high permeability of casein micelles.

Reassembling of Alkali-Treated Casein Micelles by Microbial Transglutaminase

In milk, caseins interact to form nanoparticles called casein micelles. Under weak alkaline conditions, casein micelles swell reversibly and are disrupted at pH values above 8.5. The enzyme microbial transglutaminase (mTG) is widely used in food industry to modify the functional properties of proteins. Here, we evaluated the potential of mTG as a stabilizer for alkaline disrupted casein micelles. Hence, enzymatic cross-linking of casein micelles as well as sodium caseinate was studied at the natural milk pH 6.8 and under alkaline conditions at pH 7.9 by analyzing oligomerization via size exclusion chromatography, monomeric caseins via RP-HPLC-UV, and extra-micellar protein via Bradford assay. Additionally, alkaline swelling as well as enzymatic reconstruction of casein micelles was observed via scanning electron microscopy and dynamic light scattering. The results showed that the extent of cross-linking is mainly influenced by protein conformation and not by pH value. However, micellar α_s2-casein was much more cross-linked at pH 7.9 compared to pH 6.8, whereas an opposite tendency was determined for micellar κ-casein. This leads to the conclusion that α_s2-casein is mainly located in the inner center of casein micelles and is only accessible for enzymatic cross-linking after alkaline swelling of the micelle. Alkaline disrupted casein micelles are reassembled due to intramicellar cross-linking by mTG. On the basis of the results, an enhanced model of the structure of casein micelles was developed.

The Use of Trisodium Citrate to Improve the Textural Properties of Acid-Induced, Transglutaminase-Treated Micellar Casein Gels

In this study, the effect of trisodium citrate on the textural properties and microstructure of acid-induced, transglutaminase-treated micellar casein gels was investigated. Various concentrations of trisodium citrate (0 mmol/L, 10 mmol/L, 20 mmol/L, and 30 mmol/L) were added to micellar casein dispersions. After being treated with microbial transglutaminase (mTGase), all dispersions were acidified with 1.3% (w/v) gluconodelta-lactone (GDL) to pH 4.4–4.6. As the concentration of trisodium citrate increased from 0 mmol/L to 30 mmol/L, the firmness and water-holding capacity increased significantly. The final storage modulus (G′) of casein gels was positively related to the concentration of trisodium citrate prior to mTGase treatment of micellar casein dispersions. Cryo-scanning electron microscopy images indicated that more interconnected networks and smaller pores were present in the gels with higher concentrations of trisodium citrate. Overall, when micellar casein dispersions are treated with trisodium citrate prior to mTGase crosslinking, the resulted acid-induced gels are firmer and the syneresis is reduced.

Transglutaminase Applications in Dairy Technology

Consumers' expectations from a dairy product have changed dramatically during the last two decades. People are now more eager to purchase more nutritious dairy foods with improved sensory characteristics. Dairy industry has made many efforts to meet such expectations and numerious production strategies and alternatives have been developed over the years including non-thermal processing, membrane applications, enzymatic modifications of milk components, and so on. Among these novel approaches, transglutaminase (TG)-mediated modifications of milk proteins have become fairly popular and such modifications in dairy proteins offer many advantages to the dairy industry. Since late 1980s, a great number of researches have been done on TG applications in milk and dairy products. Especially, milk proteins-based edible films and gels from milk treated with TG have found many application fields at industrial level. This chapter reviews the characteristics of microbial-origin TG as well as its mode of action and recent developments in TG applications in dairy technology.

Physico-chemical, microstructural and rheological properties of camel-milk yogurt as enhanced by microbial transglutaminase

Camel milk produces watery texture when it is processed to yogurt. Despite the extensive studies on microbial transglutaminase (MTGase) in dairy research, the effect of this enzyme on the properties of yogurt made from camel milk has not been studied. This study aims to investigate the impact of MTGase with and without bovine skimmed milk powder (SMP), whey protein concentrate (WPC),or β-lactoglobulin (β-lg) on physico-chemical, rheological, microstructural, and sensory properties of camel-milk yogurt during 15 days of storage period. MTGase treatment markedly reduced the fermentation time of unfortified and SMP-fortified camel milk. The fortification of camel milk without MTGase failed to give set-type yogurt. The treatment of unfortified milk with MTGase enormously improved the viscosity and the body of yogurt samples. Fortification of MTGase-treated milk impacted positively on the viscosity, the water holding capacity, and the density of the protein matrix in the gel microstructure, which were influenced by the type of dairy ingredients. All MTGase-treated yogurts differed from each other in hardness and adhesiveness values. Electrophoresis results showed that the susceptibility of the individual milk proteins to MTGase varied, and there were differences among the treatments toward the enzyme. SMP-fortified yogurt was the most accepted product. Generally, the addition of MTGase preparation at a concentration of 0.4%, simultaneously with starter culture, to fortified camel milk was considered an effective tool to solve the challenges of producing set-type yogurt from this milk.

Microwave-assisted cross-linking of milk proteins induced by microbial transglutaminase

We investigated the combined effects of microbial transglutaminase (MTGase, 7.0 units/mL) and microwave irradiation (MI) on the polymerization of milk proteins at 30 °C for 3 h. The addition of MTGase caused the milk proteins to become polymerized, which resulted in the formation of components with a higher molecular-weight (>130 kDa). SDS-PAGE analysis revealed reductions in the protein content of β-lactoglobulin (β-LG), α_S-casein (α_S-CN), κ-casein (κ-CN) and β-casein (β-CN) to 50.4 ± 2.9, 33.5 ± 3.0, 4.2 ± 0.5 and 1.2 ± 0.1%, respectively. The use of MTGase in conjunction MI with led to a 3-fold increase in the rate of milk protein polymerization, compared to a sample that contained MTGase but did not undergo MI. Results of two-dimensional gel electrophoresis (2-DE) indicated that κ-CN, β-CN, a fraction of serum albumin (SA), β-LG, α-lactalbumin (α-LA), α_s1-casein (α_s1-CN), and α_s2-casein (α_s2-CN) were polymerized in the milk, following incubation with MTGase and MI at 30 °C for 1 h. Based on this result, the combined use of MTGase and MI appears to be a better way to polymerize milk proteins.

Transglutaminase cross-linking effect on sensory characteristics and antioxidant activities of Maillard reaction products from soybean protein hydrolysates

To improve the yield of Maillard peptides, a microbial transglutaminase (MTGase) was used to increase the content of 1000-5000Da peptides in soybean protein hydrolysates by using a cross-linking reaction. The sensory characteristics and antioxidant activities of corresponding Maillard Reaction Products (MSPC) was then evaluated. After cross-linking treatment the content of 1000-5000Da peptides in protein hydrolysates and the yield of Maillard peptides increased by 21.19% and 8.71%, respectively, which contributed to the improved mouthfulness of MSPC. The bitter amino acids were significantly decreased and the umami acids were markedly increased in MSPC. Volatile compounds identified by GC-MS analysis showed that the content of the important meaty flavour compounds (such as 2-methyl-3-furanthiol, bis(2-methyl-3-furyl)disulfide) of MSPC were dramatically higher than that of MRPs from uncross-linking peptides. Combined with sensory evaluation, it was confirmed that MTGase cross-linking improved the flavour Characteristics and did not affect the antioxidant activity of MSPC.

Enzyme-catalyzed protein crosslinking

The process of protein crosslinking comprises the chemical, enzymatic, or chemoenzymatic formation of new covalent bonds between polypeptides. This allows (1) the site-directed coupling of proteins with distinct properties and (2) the de novo assembly of polymeric protein networks. Transferases, hydrolases, and oxidoreductases can be employed as catalysts for the synthesis of crosslinked proteins, thereby complementing chemical crosslinking strategies. Here, we review enzymatic approaches that are used for protein crosslinking at the industrial level or have shown promising potential in investigations on the lab-scale. We illustrate the underlying mechanisms of crosslink formation and point out the roles of the enzymes in their natural environments. Additionally, we discuss advantages and drawbacks of the enzyme-based crosslinking strategies and their potential for different applications.

Susceptibility of the individual caseins in reconstituted skim milk to cross-linking by transglutaminase: influence of temperature, pH and mineral equilibria

The susceptibility of total casein and the individual caseins in reconstituted skim milk to transglutaminase (TGase)-induced cross-linking was studied as a function of incubation temperature (5–40 °C), pH (5·0–7·0) and mineral addition. Within the ranges studied, the level of total casein cross-linked increased with increasing temperature, pH and concentration of added trisodium citrate, whereas adding calcium chloride had the opposite effect. These effects can be largely related to the effects of these parameters on TGase activity. In addition, the parameters were also found to influence the susceptibility of κ-casein, and to a lesser extent β-casein, to cross-linking, whereas the susceptibility of αs1-casein was not affected. The susceptibility of κ-casein to cross-linking increased with increasing temperature and calcium chloride addition, but decreased with increasing pH and citrate content, whereas the susceptibility of β-casein to TGase-induced cross-linking decreased with increasing temperature, but was not affected by other parameters. These findings highlight the fact that selection of environmental conditions during cross-linking can be applied to tailor the surface, and hence possibly colloidal stability, of casein micelles in TGase-treated milk.