Asymmetric flow field flow fractionation for the investigation of caseins cross-linked by microbial transglutaminase

Field-Flow Fractionation in Molecular Biology and Biotechnology

Field-flow fractionation (FFF) is a family of single-phase separative techniques exploited to gently separate and characterize nano- and microsystems in suspension. These techniques cover an extremely wide dynamic range and are able to separate analytes in an interval between a few nm to 100 µm size-wise (over 15 orders of magnitude mass-wise). They are flexible in terms of mobile phase and can separate the analytes in native conditions, preserving their original structures/properties as much as possible. Molecular biology is the branch of biology that studies the molecular basis of biological activity, while biotechnology deals with the technological applications of biology. The areas where biotechnologies are required include industrial, agri-food, environmental, and pharmaceutical. Many species of biological interest belong to the operational range of FFF techniques, and their application to the analysis of such samples has steadily grown in the last 30 years. This work aims to summarize the main features, milestones, and results provided by the application of FFF in the field of molecular biology and biotechnology, with a focus on the years from 2000 to 2022. After a theoretical background overview of FFF and its methodologies, the results are reported based on the nature of the samples analyzed.

Dextrans of Weissella cibaria DSM14295: Microbial production, structure and functionality

Lactic acid bacteria of the genus Weissella contribute to spontaneous fermentation in, e.g., sourdough or sauerkraut, but are not registered as starter cultures because of their pending safety assessment. Some strains are able to produce high amounts of exopolysaccharides. This study aims to demonstrate the techno-functionality of five dextrans from W. cibaria DSM14295, produced under varying cultivation conditions, with respect to structural and macromolecular properties. A maximum of 23.1 g/L dextran was achieved by applying the "cold shift" temperature regime. The dextrans differed in molecular mass (9-22∙108 Da, determined by HPSEC-RI/MALLS), intrinsic viscosity (52-73 mL/g), degree of branching (3.8-5.7 % at position O3, determined by methylation analysis) and their side chain length and architecture, determined by HPAEC-PAD after enzymatic hydrolysis. Stiffness of acid gels from milk spiked with these dextrans increased linearly with dextran concentration. Principal component analysis showed that dextrans produced in a semi-defined medium are primarily described by moisture sorption and branching properties, whereas dextrans produced in whey permeate were similar because of their functional and macromolecular properties. Overall, dextrans from W. cibaria DSM14295 have a high potential because of the high production yield and their functionality which can be tailored by the conditions during fermentation.

Soy protein nanoparticles prepared by enzymatic cross-linking with enhanced emulsion stability

Particle-stabilized emulsions have shown increasing potential application in food emulsion systems. Here, soy protein, an abundant and inexpensive plant-based protein, was used to develop nanoparticles for emulsion stabilizer applications. An enzymatic cross-linking method based on microbial transglutaminase (mTG) was developed for the fabrication of soy protein nanoparticles (SPNPs). The emulsion stability was compared between soy protein isolate (SPI) and three different nanoparticles. The size of SPNPs ranged from 10 nm to 40 nm, depending on the production conditions. The emulsions stabilized by SPNPs were stable for at least 20 days at room temperature, whereas the emulsion that was stabilized by SPI showed a significant creaming and phase separation phenomenon. The SPNPs also showed a higher antioxidant and reducing effect compared to SPI. The use of mTG induced cross-linking resulted in the formation of covalent bonding between protein molecules, and led to the formation of nanoparticles with higher stability. The approaches support the utilization of inexpensive and abundant plant-based resources as emulsion stabilizers in food applications.

Unaided efficient transglutaminase cross-linking of whey proteins strongly impacts the formation and structure of protein alginate particles

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Microbial transglutaminase (MTG) cross-linked >70% β-lactoglobulin (β-Lg) at pH 8.5.

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Initial MTG catalyzed isopeptide bond formation caused partial unfolding of β-Lg.

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>75% of whey protein cross-linked, forming hetero-polymers containing β-Lg.

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50% less alginate is needed to form particles with cross-linked than with native β-Lg.

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Cross-linked β-Lg and alginate formed suspendable hydrophobically driven particles.

There is a dogma within whey protein modification, which dictates the necessity of pretreatment to enzymatic cross-linking of β-lactoglobulin (β-Lg). Here microbial transglutaminase (MTG) cross-linked whey proteins and β-Lg effectively in 50 mM NaHCO₃, pH 8.5, without pretreatment. Cross-linked β-Lg spanned 18 to >240 kDa, where 6 of 9 glutamines reacted with 8 of 15 lysines. The initial isopeptide bond formation caused loss of β-Lg native structure with t_1/2 = 3 h, while the polymerization occurred with t_1/2 = 10 h. Further, cross-linking effects on protein carbohydrate interaction have been overlooked, leaving a gap in understanding of these complex food matrices. Complexation with alginate showed that β-Lg cross-linking decreased onset of particle formation, hydrodynamic diameter, stoichiometry (β-Lg/alginate) and dissociation constant. The complexation was favored at higher temperatures (40 °C), suggesting that hydrophobic interactions were important. Thus, β-Lg was cross-linked without pretreatment and the resulting polymers gave rise to altered complexation with alginate.

A Green Analytical Method Combined with Chemometrics for Traceability of Tomato Sauce Based on Colloidal and Volatile Fingerprinting

Tomato sauce is a world famous food product. Despite standards regulating the production of tomato derivatives, the market suffers frpm fraud such as product adulteration, origin mislabelling and counterfeiting. Methods suitable to discriminate the geographical origin of food samples and identify counterfeits are required. Chemometric approaches offer valuable information: data on tomato sauce is usually obtained through chromatography (HPLC and GC) coupled to mass spectrometry, which requires chemical pretreatment and the use of organic solvents. In this paper, a faster, cheaper, and greener analytical procedure has been developed for the analysis of volatile organic compounds (VOCs) and the colloidal fraction via multivariate statistical analysis. Tomato sauce VOCs were analysed by GC coupled to flame ionisation (GC-FID) and to ion mobility spectrometry (GC-IMS). Instead of using HPLC, the colloidal fraction was analysed by asymmetric flow field-fractionation (AF4), which was applied to this kind of sample for the first time. The GC and AF4 data showed promising perspectives in food-quality control: the AF4 method yielded comparable or better results than GC-IMS and offered complementary information. The ability to work in saline conditions with easy pretreatment and no chemical waste is a significant advantage compared to environmentally heavy techniques. The method presented here should therefore be taken into consideration when designing chemometric approaches which encompass a large number of samples.

Tracking Heme-Protein Interactions in Healthy and Pathological Human Serum in Native Conditions by Miniaturized FFF-Multidetection

The interaction of heme with blood serum proteins plays an important role in many physiological and pathological processes involving enzyme activity, gene expression and cell proliferation. The mechanisms underlying these interactions are; however, not yet fully understood. New analytical methods able to investigate protein-heme binding in native, biologically representative conditions are thus required. In this work, we present a method based on miniaturized, hollow-fiber flow field-flow fractionation with multiple spectrophotometric and light-scattering detection for size separation of high-abundance serum proteins and selective detection of heme-bound subpopulations. Heme is found to mainly interact with serum albumin, whereas a low amount also binds to other proteins such as IgM. The ability to bind heme in physiological conditions is also investigated for individual serum proteins. IgG is found unable to bind heme at clinically relevant concentrations. The proposed method allows separation, quantitation, and mass/size characterization of serum high-abundance proteins, providing information of heme-protein complex stability and preferred heme-clearing pathways. The same approach could be in perspective extended to the investigation of specific heme-antibody binding, and to further studies involving other molecules of pharmaceutical/clinical interest.

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.

Association of Enzymatically and Nonenzymatically Functionalized Caseins Analyzed by Size-Exclusion Chromatography and Light-Scattering Techniques

The influence of covalent protein modifications resulting from the Maillard reaction (glycation) of casein and lactose on the noncovalent association behavior of the protein was studied. Nonenzymatic cross-linking with methylglyoxal (MGO) and glutaraldehyde (GTA) as well as enzymatic cross-linking with microbial transglutaminase (mTG) was investigated in comparison. Molar mass, particle size, and conformational characteristics of nonmicellar casein associates as well as the extent of intraparticle protein cross-linking were examined utilizing size-exclusion chromatography (SEC) combined with UV detection and static and dynamic light scattering. Cross-linking resulted in the stabilization of a certain fraction of casein associates, with particle sizes of approximately 30 nm in radius of gyration (R_g), and promoted an incorporation of further casein molecules into those particles, yielding molar masses (M_w) of 1.0-1.2 × 10⁶ g/mol. When caseins were additionally conjugated with lactose during the early Maillard reaction, a further growth of the associates up to approximately 50 nm in R_g with a M_w of 2.1 × 10⁶ g/mol was observed. Furthermore, glycation reactions induced a transition from slightly elongated, random-coil structures toward more anisotropic conformations. Associates consisting of caseins cross-linked with GTA appeared to preserve the original particle conformation.