Effect of protein-glutaminase on the texture, rheology, microstructure and sensory properties of skimmed set-type yoghurt

Engineering of protein glutaminase for highly efficient modification of fish myofibrillar protein through structure-based and computational-aided strategy

Protein glutaminase (PG; EC 3.5.1.44) is a class of food-grade enzyme with the potential to significantly improve protein functionality. However, its low catalytic activity and stability greatly hindered industrial application. In this study, we employed structural-based engineering and computational-aided design strategies to target the engineering of protein glutaminase PG5, which led to the development of a combinatorial mutant, MT8, exhibiting a specific activity of 31.1 U/mg and a half-life of 216.2 min at 55 °C. The results indicated that the flexible region in MT8 shifted from the C-terminus to the N-terminus, with increased N-terminal flexibility positively correlating with its catalytic activity. Additionally, MT8 notably boosted fish myofibrillar proteins (MPs) solubility under the absence of NaCl conditions and enhanced their foaming and emulsifying properties. Key residues like Asp31, Ser72, Asn121, Asp471, and Glu485 were crucial for maintaining PG5-myosin interaction, with Ser72 and Asn121 making significant energy contributions.

Discovery and mechanistic analysis of a novel source protein glutaminase PG5 and its potential application

In this study, we successfully obtained a novel source protein glutaminase PG5 with specific activity of 10.4 U/mg, good tolerance and broad substrate profile through big data retrieval. Structural analysis and site-directed mutagenesis revealed that the catalytic pocket of Mature-PG5 contained a large number of aromatic amino acids and hydrophobic amino acids, and that Ser72 greatly affects the properties of the catalytic pocket and the affinity of PG5 for the substrate. In addition, molecular dynamics analysis revealed that the opening and closing between amino acid residues Gly65 and Thr66 with Cys164 at the catalytic cleft could affect substrate binding and product release. In addition, PG5 effectively improved the solubility of fish myofibrillar proteins under low-salt conditions while enhancing their foaming and emulsification properties. This study offers valuable insights into the catalytic mechanism of PG5, which will contribute to its future directed evolution and application in the food industry.

Improvement of rheological and sensory properties of Lactobacillus helveticus fermented milk by prebiotics

The fermentation characteristics and aroma-producing properties of Lactobacilli could influence the flavour quality of fermented milk, an important influencing factor of consumers' preference. In this study, fermented milk was prepared using Lactobacillus helveticus and the dynamic changes in the sensory quality of fermented milk throught fermentation were to assess the dynamic changes in sensory quality of fermented milks throughout the fermentation process, including rheological properties and flavour profiles. Styrene, linalool, octanoic acid, and 1-nonanol were considered as the key flavour components during fermentation. The quality of the fermented milk tends to be stabilized after 24-h, showing the minimal off-flavour at 48-h and optimal fermented aroma at 72-h. Three prebiotics (inulin, Galactooligosaccharides and inulin mixed with Galactooligosaccharides) were added to Lactobacillus spiralis fermented milk separately, and the results showed that inulin mixed with Galactooligosaccharides was the most effective group in improving the organoleptic quality of the fermented milk. Overall, the experimental results provide deeper insights into the release and retention of aroma compounds during fermentation and scientific reference for broadening the application of prebiotics and flavour-producing Lactobacilli in fermented milk processing.

Functional and Mechanistic Dissection of Protein Glutaminase PG3 and Its Rational Engineering for Enhanced Modification of Myofibrillar Proteins

Protein glutaminases (PG; EC = 3.5.1.44) are enzymes known for enhancing protein functionality. In this study, we cloned and expressed the gene chryb3 encoding protein glutaminase PG3, exhibiting 39.4 U/mg specific activity. Mature-PG3 featured a substrate channel surrounded by aromatic and hydrophobic amino acids at positions 38-45 and 78-84, with Val81 playing a pivotal role in substrate affinity. The dynamic opening and closing motions between Gly65, Thr66, and Cys164 at the catalytic cleft greatly influence substrate binding and product release. Redesigning catalytic pocket and cocatalytic region produced combinatorial mutant MT6 showing a 2.69-fold increase in specific activity and a 2.99-fold increase at t65 °C1/2. Furthermore, MT6 boosted fish myofibrillar protein (MP) solubility without NaCl. Key residues such as Thr3, Asn54, Val81, Tyr82, Asn107, and Ser108 were vital for PG3-myosin interaction, particularly Asn54 and Asn107. This study sheds light on the catalytic mechanism of PG3 and guided its rational engineering and utilization in low-salt fish MP product production.

Effect of prebiotics on rheological properties and flavor characteristics of Streptococcus thermophilus fermented milk

The fermentation characteristics and aroma production properties of lactic acid bacteria can influence the flavor quality of fermented milk, which is one of the important factors influencing the consumer preference. In this study, fermented milk was prepared using Streptococcus thermophilus, and dynamic changes in its quality, including rheological properties and flavor characteristics, were evaluated throughout the fermentation process. The results showed that benzaldehyde, 2-undecanone, octanoic acid, n-hexanol and 2-nonanol were the key flavor components during the fermentation process. The quality of the fermented milk tends to be stabilized after 24-h, showing the minimal off-flavor and optimal fermented aroma at 48-h. Three prebiotics (inulin, Galactooligosaccharides and inulin mixed with Galactooligosaccharides) were added to Streptococcus thermophilus fermented milk separately, and the results showed that inulin was the most effective group in improving the organoleptic quality of the fermented milk. These findings contribute to our understanding of the release and retention of flavor compounds during fermentation and can be used as a scientific reference for the application of probiotics and flavor-producing lactic acid bacteria in fermented milk processing.

Optimizing Skim Milk Yogurt Properties: Combined Impact of Trans-glutaminase and Protein-Glutaminase

The lack of fat in yogurt can lead to alterations in taste and whey separation, reducing consumer acceptance. In this study, the feasibility of enhancing the quality of skim milk yogurt through a combination of transglutaminase (TG) and protein-glutaminase (PG) was investigated. The combination of TG and PG resulted in simultaneous cross-linking and deamidated of casein micelles, with PG deamidation taking priority over TG cross-linking, leading to higher solubility and lower turbidity of milk proteins compared with TG alone. When 0.06 U/mL TG and 0.03 U/mL PG were added, firmness and viscosity indexes significantly increased by 38.26 and 78.59%, respectively as compared with the control. Microscopic images revealed increased cross-linking with casein and filling of cavities by smaller sub-micelles in the combination of TG and PG treatment. Furthermore, the combination of TG and PG resolved issues of rough taste and whey separation, leading to improved overall liking. This study highlights the benefits of using both enzymes in dairy production and has important implication for future research.

Physicochemical, microbiological and metabolomics changes in yogurt supplemented with lactosucrose

Lactosucrose (LS) is a known prebiotic that has gained recognition for its low caloric content and various health benefits. However, its potential in food applications remains largely unexplored. In this study the effects of adding LS to milk at concentrations (0 %, 2 %, 5 % and 8 % w/v) for yogurt production, and the relevant changes in yogurt texture, microbial composition and metabolomics were investigated. Our findings revealed that LS played a role in promoting the formation of a structured gel during fermentation, resulting in increased elasticity and viscosity while reducing fluidity. Additionally incorporating high doses of LS into yogurt led to reduced post-acidification, enhanced survival of starter bacteria, improved water retention capacity and overall texture throughout a refrigerated storage period of 21 days. Notably higher concentrations of LS (8 % w/v) exhibited effects on enhancing yogurt quality. Furthermore, untargeted metabolomics analysis using UPLC Q TOF MS/MS revealed 45 differentially expressed metabolites, including up-regulated L-arginine, L-proline and L-glutamic acid along with the down-regulated glutathione, L-tyrosine, L-phenylalanyl and L-proline. These differential metabolites were primarily associated with amino acid metabolism such as thiamine metabolism, nicotinic acid salt and nicotinamide metabolism, and pyrimidine metabolism. As a result, the inclusion of LS in yogurt had an impact on the production of various beneficial metabolites in yogurt, highlighting the importance of combining prebiotic LS with probiotics to obtain desired physiological benefits of yogurt.