Excessive cross-linking of caseins by microbial transglutaminase and its impact on physical properties of acidified milk gels

Effects of microbial transglutaminase on textural, water distribution, and microstructure of frozen-stored longtail southern cod (Patagonotothen ramsayi) fish mince gel

Frozen-stored fish mince tend to have poor gelling ability due to significant myosin denaturation caused by freezing. In this study, microbial transglutaminase (MTGase) was used to improve the quality of fish mince gel products made from frozen-stored longtail southern cod (LSC). The gel strength of the gel product increased with the addition of MTGase and reached a plateau value of ~19 N mm beyond 300 U/kg of MTGase, at the same condition, T₂₂ was reduced from 57.22 to 49.77 ms, T₂₃ was reduced from 1,273.88 to 1,072.27 ms. As the MTGase addition increased from 0 to 400 U/kg, the hardness of the fish surimi gel increased from 14.52 to 21.36 N, and the microstructure changed from loose to dense, respectively. This study showed that MTGase could promote gelation to improve the quality of frozen-stored LSC fish mince gel, especially at 300 U/kg, which potentially can be utilized to produce good surimi gel products out of frozen-stored fish.

Size Modulation of Enzymatically Cross-Linked Sodium Caseinate Nanoparticles via Ionic Strength Variation Affects the Properties of Acid-Induced Gels

Enzymatic cross-linking by microbial transglutaminase is a prominent approach to modify the structure and techno-functional properties of food proteins such as casein. However, some of the factors that influence structure-function-interrelations are still unknown. In this study, the size of cross-linked sodium caseinate nanoparticles was modulated by varying the ionic milieu during incubation with the enzyme. As was revealed by size exclusion chromatography, cross-linking at higher ionic strength resulted in larger casein particles. These formed acid-induced gels with higher stiffness and lower susceptibility to forced syneresis compared to those where the same number of ions was added after the cross-linking process. The results show that variations of the ionic milieu during enzymatic cross-linking of casein can be helpful to obtain specific modifications of its molecular structure and certain techno-functional properties. Such knowledge is crucial for the design of protein ingredients with targeted structure and techno-functionality.

Acid-Induced Gelation of Enzymatically and Nonenzymatically Cross-Linked Caseins-Texture Properties, and Microstructural Insights

Casein gels consist of a fractal organized network of aggregated casein particles. The gel texture thereby depends on the structure, the spatial distribution, and the interaction forces of the network's elementary building blocks. The aim of this study was to explore the technofunctional consequences of a possible specificity of Maillard reaction-induced cross-linking reactions on casein with respect to texture and microstructure of acid gels. Therefore, sodium caseinate glycated with lactose in the dry state (60 °C, a_w 0.5) was compared with casein samples cross-linked with methylglyoxal, with glutaraldehyde, or via microbial transglutaminase, respectively, at similar levels of protein cross-linking as confirmed by size-exclusion chromatography under denaturing conditions. Casein gels prepared by acidification with glucono-δ-lactone were characterized concerning pH kinetics during gelation, mechanical texture properties under large deformation, and water-holding capacity, while viscometric properties of casein suspensions were obtained prior to gelation. The gel microstructure was captured by confocal laser scanning microscopy and evaluated by means of image texture analysis. All protein cross-linking reactions studied led to an enhanced gel strength which was accompanied by an increased interconnectivity of the gel network and a decrease in apparent pore sizes. Gels with more densely packed strands, as was the case for enzymatically modified casein, exhibited pronounced mechanical stability. The spontaneous destabilization of the gel network upon prolonged glycation reactions, which was not obviously displayed by microstructural features but connected to an increased viscosity and pronounced pseudoplastic flow of the unacidified suspension, suggests a limitation of particle rearrangements and the weakening of interparticle protein-protein interactions by additional structure attributes formed during the early Maillard reaction (glycoconjugation).

Property changes of caseinate in response to its dityrosine formation induced by horseradish peroxidase, glucose oxidase and d-glucose

BACKGROUND

A ternary system containing horseradish peroxidase (HRP), glucose oxidase and d-glucose using one- or two-step treatment was evidently able to cross-link proteins via dityrosine formation and thus was assessed for its possible impact on several properties of a protein ingredient caseinate.

RESULTS

HRP, glucose oxidase and d-glucose were used at 200 U, 6 U and 0.05 mmol g^-1 protein to treat caseinate by one- and two-step methods, producing two cross-linked caseinates named CLCN-I and CLCN-II, respectively. In response to the conducted cross-linking, both CLCN-I and CLCN-II gained slightly reduced dispersibility at pH 5-10, enlarged hydrodynamic radius (particle size distribution, 266.37 and 258.33 versus 226.67 nm) and negative zeta-potential (-26.60 and -22.27 versus -14.30 mV) in dispersions, increased water-binding (3.70 and 3.09 versus 2.68 kg kg^-1 protein), decreased oil-binding (1.75 and 2.74 versus 2.87 kg kg^-1 protein) and emulsifying activity (76.2 and 82.3 versus 94.3 m² g^-1 protein), increased emulsion stability (84.3% and 82.5% versus 78.6%), and enhanced thermal stability with lower mass loss (58.5% and 59.6% versus 64.3%) or higher decomposition temperatures (331.2 °C and 328.7 °C versus 327.6 °C) upon heating at 105-450 °C. In addition, CLCN-I and CLCN-II had decreased gelling temperatures and shortened gelling times when forming acid-induced gels, and the gels were endowed with increased values in four textural indices and finer microstructure. Moreover, CLCN-I with a higher cross-linking extent showed greater property changes than CLCN-II.

CONCLUSION

This ternary system could be used in caseinate cross-linking to improve properties such as aggregation, emulsification, gelation and thermal stability. © 2020 Society of Chemical Industry.

Expression of Zea mays transglutaminase in Pichia pastoris under different promoters and its impact on properties of acidified milk protein concentrate gel

BACKGROUND

Transglutaminase (TGase) catalyzes post-translational modification of proteins by γ-glutamyl-ϵ-lysine chain links, covalent conjugation of polyamines, and deamidation. Zea mays TGase (TGZ) is a plant TGase with potential application prospects in the food industry. In this study, two promoter types, P_FLD1 and P_TEF1 , were compared to improve the expression of TGZ, and the cross-linking effect of recombinant TGZ on the properties of acid-induced milk protein concentrate (MPC) gel was assessed.

RESULTS

A higher expression of TGZ was obtained under the induction of P_FLD1 with a production of 635 U L^-1 . After purification using chromatography, TGZ activity was 0.4 U mg^-1 . The results indicated that TGZ treatment has effectively improved the textural properties of MPC gel at strength level and water-holding capacity. Optimal texture of MPC gel was achieved after TGZ treatment using 2 U g^-1 TGZ for 2 h at 35 °C and pH 7.

CONCLUSION

Comparative analysis of the promoters has greatly contributed to the production of TGZ in the industrial field. Furthermore, the modification of MPC gel texture by TGZ indicated that this recombinant enzyme has a practical value in dairy product, especially in yoghurt industry. © 2019 Society of Chemical Industry.

The effect of the addition of microbial transglutaminase before the fermentation process on the quality characteristics of three types of yogurt

The effect of the addition of microbial TGase to milk on selected physical properties of the final product and the viability of lactic acid bacteria cultures during storage at 6 °C for 56 days was studied. Three types of set-style yogurt were made with varying parameters. Weekly analyses included the determination of syneresis and water-holding capacity, texture, pH, and the lactic acid bacteria population. Our research has confirmed that mTGase may be used to stabilize yogurts, although the syneresis, the water-holding capacity of yogurts, and the textural features of yogurts were dependent on the step in the production process at which mTGase was added to milk. The presence of mTGase had no relevance with regard to the acidity of yogurts stored under refrigerated conditions. The addition of mTGase had no effect on lactobacilli, but had a variable effect on Streptococcus thermophilus, depending on the duration of enzymatic activity.

Acid-Induced Gelation of Caseins Glycated with Lactose: Impact of Maillard Reaction-Based Glycoconjugation and Protein Cross-Linking

During food processing or storage, milk proteins can react with reducing sugars via the Maillard reaction (glycation), which may alter their techno-functional properties. The aim of this study was to investigate the relationship between molecular changes of casein occurring during different stages of the Maillard reaction and its acid-induced gelling properties. Therefore, sodium caseinate was heated in a dry state at 60 °C in the presence of lactose and analyzed for structural modifications by determining Amadori compounds (glycoconjugation) indirectly as furosine, the total lysine modification, and the extent of protein cross-linking. For techno-functional characterization, acid-induced gels were prepared by the addition of glucono-δ-lactone and evaluated by measuring pH kinetics during gel formation, gel strength, and water holding capacity. The time to reach pH 4.6 during the gelation process was significantly delayed with increasing extent of the Maillard reaction. Glycation with lactose also led to a significant increase in gel strength and water holding capacity. The increase in gel stability was rather independent from the amount of sugars covalently bound to the proteins during the early phase of the Maillard reaction but strongly correlated to the degree of protein polymerization. Small- and medium-sized casein oligomers, formed during advanced stages of the Maillard reaction, contributed considerably to the formation of stronger gels with higher water holding capacity, whereas a sharp increase in the relative amount of the polymer fraction observed during prolonged cross-linking processes caused a spontaneous destabilization of the gel network. Knowledge about structure-function relationships on a molecular level can provide useful information to control food texture by raw material quality.

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.

Influence of milk protein cross-linking by transglutaminase on the rennet coagulation time and the gel properties

BACKGROUND

Transglutaminase (TGase) modifies milk proteins by cross-linking of caseins, with increased cheese yield being the main technological benefit. In the present work the influence of TGase addition in different concentrations (0, 1, 2 and 3 U g(-1) protein in the system) and under different incubation conditions (0 h, 40 °C/2 h, 25 °C/4 h and 5 °C/16 h) on the rennet coagulation time (RCT) and the comprehensive rennet gel properties were investigated.

RESULTS

Modification of milk proteins by TGase in a concentration-dependent manner caused longer RCT and lower gel firmness. The highest TGase concentration and incubation at 40 °C for 2 h resulted in the longest RCT and the lowest gel firmness. Rennet gels obtained from TGase modified milk were characterised by significantly lower values of texture parameters, lower syneresis and were composed of smaller casein micelles, thinner chains and smaller clusters than those obtained from the control milk. The content of whey proteins in the gel from modified milk was higher and the content of individual casein fractions in the milk samples and rennet gels decreased upon TGase modification.

CONCLUSION

Rennet cheese with modified textural and nutritional properties and improved yield can be obtained upon TGase modification but simultaneous addition of rennet and TGase is recommended. © 2015 Society of Chemical Industry.

Digestibility of transglutaminase cross-linked caseinate versus native caseinate in an in vitro multicompartmental model simulating young child and adult gastrointestinal conditions

Aim of this study was to investigate the digestion of transglutaminase cross-linked caseinate (XLC) versus native caseinate (NC) in solution and in cheese spread under digestive conditions for adults and children mimicked in a gastrointestinal model. Samples were collected for gel electrophoresis and nitrogen analysis. The results showed no relevant differences between XLC and NC for total and α-amino nitrogen in digested fraction under adult and child conditions. However, the rate of digestion was depending on the food matrix. Gel electrophoresis showed the gastric breakdown of XLC without formation of pepsin resistant peptides larger than 4 kDa. NC was slowly digested in the stomach with formation of pepsin resistant fragments and was still detectable in the stomach after 90 min. In the small intestine the proteins were rapidly digested. XLC was digested to small peptides, while NC was resistant against pepsin digestion under gastric conditions of adults and children.