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

Gastric devolution of transglutaminase-induced acid and rennet-induced casein gels using dynamic DIDGI® and static COST action INFOGEST protocols

Limited studies in the literature have compared in vitro dynamic and in vitro static protocols for modelling the gastric digestive process of food systems. This experiment explores the differences between two different in vitro approaches to the devolution of a transglutaminase-induced acid gel (TG, pH 5.1-5.3) and rennet-induced gel (RG, pH 6.5-6.7). Gels were exposed to a simulated oral phase, followed by either the dynamic DIDGI® or static COST action INFOGEST protocol to simulate gastric conditions. Protein hydrolysis was evident from 15 min onwards for TG exposed to the dynamic protocol where levels continued to increase at a steady rate. In contrast, RG exhibited a notable lag-phase before levels increased from around 60 min onwards. Under the static protocol, protein hydrolysis was observed for both TG and RG upon exposure to the gastric environment which continued to increase over time. Despite these differences, similar levels of protein hydrolysis were found for TG and RG at the gastric endpoint using either protocol demonstrating that both the dynamic DIDGI® and static COST action INFOGEST methods provide a suitable and comparable environment for the in vitro digestion of casein protein under simulated gastric conditions.

Investigating casein gel structure during gastric digestion using ultra-small and small-angle neutron scattering

This study aimed to understand the structural devolution of 10% w/w rennet-induced (RG) and transglutaminase-induced acid (TG) gels in H₂O and D₂O under in vitro gastric conditions with and without pepsin. The real-time devolution of structure at a nano- (e.g. colloidal calcium phosphate (CCP) and micelle) and micro- (gel network) level was determined using ultra-small (USANS) and small-angle neutron scattering (SANS) with electron microscopy. Results demonstrate that gel firmness or elasticity determines disintegration behaviour during simulated mastication and consequently the particle size entering the stomach. Shear of mixing in the stomach, pH, and enzyme activity will also affect the digestion process. Our results suggest that shear of mixing primarily results in erosion at the particle surface and governs gel disintegration behaviour during the early stages of digestion. Pepsin diffusivity, and hence action, occur more readily in the latter stages of gastric digestion via access to the particle interior. This occurs via the progressively larger pores of the looser gel network and channels created within the larger, less dense casein micelles of the RG gels. Gel firmness and brittleness were greater in the D₂O samples compared to H₂O, facilitating gel disintegration. Despite the higher strength and elasticity of RG compared to TG, the protein network strands of the RG gels become more compact when exposed to the acidic gastric environment with comparatively larger pores observed through SEM imaging. This led to a higher degree of digestibility in RG gels compared to TG gels. This is the first study to examine casein gel structure during simulated gastric digestion using scattering and highlights the benefits of neutron scattering to monitor structural changes during digestion at multiple length scales.

Structure and shelf stability of milk protein gels created by pressure-assisted enzymatic gelation

In this work, pressure-assisted enzymatic gelation was applied to milk proteins, with the goal of enhancing the structure and stability of pressure-created milk protein gels. High-pressure processing (HPP) at 600 MPa, 3 min, and 5°C was applied to milk protein concentrate (MPC) samples of 12.5% protein concentration, both in the absence and in the presence of calf chymosin [up to 60 IMCU (international milk-clotting units)/kg of milk] or camel chymosin (up to 45 IMCU/kg of milk). Gel hardness, water-holding capacity, and degree of proteolysis were used to assess network strength and shelf stability. The processing trials and all measurements were conducted in triplicate. Statistical analyses of the data were performed by ANOVA, at a 95% confidence level. After HPP treatment, we observed significant structural changes for all samples. Pressurization of MPC, with or without chymosin addition, led to extensive protein aggregation and network formation. The strength of HPP-created milk protein gels without chymosin addition, as measured by the elastic modulus (G'), had a value of 2,242 Pa. The value of G' increased with increasing chymosin concentration, reaching as high as 4,800 Pa for samples with 45 IMCU/kg of camel chymosin. During 4 wk of refrigerated storage, the HPP and chymosin MPC gels maintained higher gel hardness and better structural stability compared with HPP only (no chymosin) MPC gels. The water-holding capacity of the gels without chymosin remained at 100% during 28 d of refrigerated storage. The HPP and chymosin MPC gels had a lower water-holding capacity (91-94%) than the HPP-only counterparts, but their water-holding capacity did not decrease during storage. Overall, these findings demonstrate that controlled, fast structural modification of high-concentration protein systems can be obtained by HPP-assisted enzymatic treatment, and the created gels have a strong, stable network. This study provides insights into the possibility of using HPP for the development of milk-protein-based products with novel structures and textures and long refrigerated shelf life, along with the built-in safety imparted by the HPP treatment.

New insights into food hydrogels with reinforced mechanical properties: A review on innovative strategies

Enhancement on the mechanical properties of hydrogels leads to a wider range of their applications in various fields. Therefore, there has been a great interest recently for developing new strategies to reinforce hydrogels. Moreover, food gels must be edible in terms of both raw materials and production. This paper reviews innovative techniques such as particle/fiber-reinforced hydrogel, double network, dual crosslinking, freeze-thaw cycles, physical conditioning and soaking methods to improve the mechanical properties of hydrogels. Additionally, their fundamental mechanisms, advantages and disadvantages have been discussed. Important biopolymers that have been employed for these strategies and also their potentials in food applications have been summarized. The general mechanism of these strategies is based on increasing the degree of crosslinking between interacting polymers in hydrogels. These links can be formed by adding fillers (oil droplets or fibers in filled gels) or cross-linkers (regarding double network and soaking method) and also by condensation or alignment of the biopolymers (freeze-thaw cycle and physical conditioning) in the gel network. The properties of particle/fiber-reinforced hydrogels extremely depend on the filler, gel matrix and the interaction between them. In freeze-thaw cycles and physical conditioning methods, it is possible to form new links in the gel network without adding any cross-linkers or fillers. It is expected that the utilization of gels will get broader and more varied in food industries by using these strategies.

Enzyme-catalyzed acylation improves gel properties of rapeseed protein isolate

BACKGROUND

Although rapeseed protein isolate (RPI) possessed some good functional properties, the use of RPI as an ingredient in the food industry is restricted mainly due to its inferior gelation. The purpose of this study was to improve the heat-induced gel properties of RPI using double processes of acylation and additional transglutaminase catalysis.

RESULTS

Scanning electron microscopy showed that the gel formed by native RPI exhibited randomly aggregated particulate network structures whereas transglutaminase (TG)-assisted RPI gels significantly improved gelation properties. More importantly, the combined modifications of RPI using TG-assisted acylation can form a gel with unique percolating and small porous structure. Furthermore, TG-catalyzed 5% acylated RPI gel (100 U g^-1 , protein basis) exhibited excellent gel properties in terms of gel strength, thermal stability, surface roughness and apparent viscosity compared to non-treated or single modification of RPI gel as determined by texture analyzer, atomic force microscopy and rheometer. Mechanistically, Fourier-transform infrared spectra and gel dissociation test revealed that TG-catalyzed acylation extensively unfolded the hydrophobic and sulfhydryl residues of RPI, in turn, reinforced re-assembly of protein molecules via hydrophobic interactions and disulfide bonds during gel formation.

CONCLUSION

Combined processes of acylation and additional TG catalysis improved the thermal gelation properties by altering inter- and intra-protein structures. Such sequential processes will provide a promising approach to improve the protein gelation that could be potentially applied in the food industry. © 2020 Society of Chemical Industry.

Review transglutaminases: part II-industrial applications in food, biotechnology, textiles and leather products

Because of their protein cross-linking properties, transglutaminases are widely used in several industrial processes, including the food and pharmaceutical industries. Transglutaminases obtained from animal tissues and organs, the first sources of this enzyme, are being replaced by microbial sources, which are cheaper and easier to produce and purify. Since the discovery of microbial transglutaminase (mTGase), the enzyme has been produced for industrial applications by traditional fermentation process using the bacterium Streptomyces mobaraensis. Several studies have been carried out in this field to increase the enzyme industrial productivity. Researches on gene expression encoding transglutaminase biosynthesis were performed in Streptomyces lividans, Escherichia coli, Corynebacterium glutamicum, Yarrowia lipolytica, and Pichia pastoris. In the first part of this review, we presented an overview of the literature on the origins, types, mediated reactions, and general characterizations of these important enzymes, as well as the studies on recombinant microbial transglutaminases. In this second part, we focus on the application versatility of mTGase in three broad areas: food, pharmacological, and biotechnological industries. The use of mTGase is presented for several food groups, showing possibilities of applications and challenges to further improve the quality of the end-products. Some applications in the textile and leather industries are also reviewed, as well as special applications in the PEGylation reaction, in the production of antibody drug conjugates, and in regenerative medicine.

Effect of transglutaminase on rennet-induced gelation of skim milk and soymilk mixtures

BACKGROUND

Protein gels made from cow milk and soymilk can yield products of exceptional value. Transglutaminase (TG) affect rennet-induced gelation of proteins, and improves the functionality of the final products. In this paper, TG and rennet were added to skim milk and soymilk mixtures simultaneously, and the rennet-induced coagulation was studied. Diffusing wave spectroscopy and rheology measurements were used to access the structural changes of the mixtures during renneting. Syneresis analysis and microscopy can give more information for understanding the system.

RESULTS

Soymilk and TG have synergetic effects and inhibit rennet-induced gelation to a certain degree. With increasing soymilk and TG, elastic index and storage modulus decreased, gelation time was delayed, and curd yield and moisture content increased. At excess soymilk and TG, no curds can be formed. There were significant effects of soymilk and TG on curd microstructure. Soymilk inhibited the aggregation of casein micelles and contributed to more coarse and heterogeneous networks. TG limited reorganization of the proteins, leading to more homogenous networks with small pores.

CONCLUSION

The use of soymilk and TG simultaneously impair rennet-induced gelation and curd syneresis, and consequently lead to a higher yield of high-moisture curd. © 2018 Society of Chemical Industry.

Effect of Enzyme Modified Soymilk on Rennet Induced Gelation of Skim Milk

In this study, soymilk was hydrolyzed to different degrees with flavourzyme, and then soymilk and enzyme modified soymilk at various levels were added to skim milk respectively, to generate a mixed gel using rennet. Rheological properties, scanning electron microscopy imaging, and physical and chemical indexes were examined to reveal the effect of enzyme modified soymilk on rennet induced gelation of skim milk. Results showed that soymilk inhibited the aggregation of skim milk, led to a decrease in storage modulus (G'), significantly increased moisture content and curd yield, and the resulting network was coarse. Enzyme modified soymilk with a molecular weight below 20 kDa led to a more uniform curd distribution, which counteracted the reduction of G' and allowed for the formation of a stronger gel. Both the moisture content and the curd yield increased with the addition of soymilk and enzyme modified soymilk, and overall the effect of adding a high degree of hydrolysis of enzyme modified soymilk was superior. Compared to untreated soymilk, the addition of a certain amount of enzyme modified soymilk resulted in a new protein structure, which would improve the texture of blend cheese.

Current insight and futuristic vistas of microbial transglutaminase in nutraceutical industry

Microbial transglutaminase (MTGase) has become a driving force in the food industry cross-linking the food proteins. MTGase-the nature's molecular glue is recognized to reorient food protein's functional properties without affecting its nutritive value. The scope and approach of this review is to have insight on the action mechanism of MTGase and impact of molecular linkage on functional proteins in various protein moieties in development of innovative features in food production for better consumer's choice and satisfaction. The study covers a wide range of published work across food industries involving innovative use of MTGase, an environment friendly production approach for commercial utilization to get better outcome in terms of culinary delight. The intrinsic biochemical properties and structural information by sequence analysis and clustering validates the mode of reaction mechanism of the biological glue enzyme. The review singles out how the MTGase emerged as a prime choice in ever evolving food industry.

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.