Formation of di-Tyrosine in pasteurized milk during shelf storage

Formation of the protein crosslink di-Tyrosine was studied in PET-bottled pasteurized milk exposed to fluorescent light in a commercial display cabinet. An HPLC method with fluorescence detection was developed and intra-laboratory validated using pure di-Tyrosine synthesized on purpose. Di-Tyrosine was detected after 1-day lightening and increased up to 7 days, reaching around 250 and 320 µg/g protein in whole and partly skimmed milk, respectively. Afterward, a progressive decrease occurred. By transmission electron microscopy with specific immune gold labelling, presence of di-Tyrosine was observed for the first time on the surface of casein micelles of lightened milk. The crosslink formation, however, did not bring to protein aggregation phenomena detectable by laser light scattering measurements. Exposure to light also induced degradation of riboflavin and decrease of yellowness index. Di-Tyrosine proved to be a suitable indicator to evaluate the progress of protein oxidation in pasteurized milk during storage on the market.

Evaluation of milk photooxidation based on peptidomics

Photooxidation is one of the main causes of the deterioration of milk quality during processing and marketing. This study aimed to investigate the variation in peptides after photooxidation using peptidomic techniques, and how cow species, oxygen content, and light intensity affect photooxidation. The different peptides were identified and quantified using ultraperformance liquid chromatography coupled with quadrupole time-of-flight mass spectrometry (UPLC-Q-TOF-MS). Eighteen milk samples were subjected to light treatment. Seven types of peptides were identified as photooxidation markers. Subsequently, the effects of milk variety, oxygen content, and light intensity on photooxidation were studied, and sensory evaluations were performed. Dairy cow breed, oxygen content, and light intensity all affect photooxidation. Sensory evaluation verified that light and oxygen are necessary for the photooxidation of milk. The peptide m/z+ 529.2783 (LLDEIKEVV), both in different varieties of milk and in different brands of commercially available milk, showed a large variation in multiplicity, and its content was closely related to oxygen and light. This peptide was not produced in the absence of oxygen and light, and its relative content increased with the duration of light exposure. These results suggest that the peptidomics method is an effective tool for distinguishing between normal and photooxidized milk.

Application of Multiharmonic QCM-D for Detection of Plasmin at Hydrophobic Surfaces Modified by β-Casein

Plasmin protease plays an important role in many processes in living systems, including milk. Monitoring plasmin activity is important for control of the nutritional quality of milk and other dairy products. We designed a biosensor to detect the proteolytic activity of plasmin, using multiharmonic quartz crystal microbalance with dissipation (QCM-D). The β-casein immobilized on the hydrophobic surface of 1-dodecanethiol on the AT-cut quartz crystal was used to monitor plasmin activity. We demonstrated detection of plasmin in a concentration range of 0.1–20 nM, with the limit of detection about 0.13 ± 0.01 nM. The analysis of viscoelastic properties of the β-casein layer showed rapid changes of shear elasticity modulus, μ, and coefficient of viscosity, η, at plasmin sub-nanomolar concentrations, followed by modest changes at nanomolar concentrations, indicating multilayer architecture β-casein. A comparative analysis of viscoelastic properties of β-casein layers following plasmin and trypsin cleavage showed that the higher effect of trypsin was due to larger potential cleavage sites of β-casein.

Effect of ripening time on peptide dynamics and bioactive peptide composition in Tulum cheese

Skin bag Tulum cheeses traditionally produced in the Central Taurus region of Turkey were studied to identify peptide profiles by liquid chromatography-tandem mass spectrometry over 180 d of ripening. After mass spectrometry analysis, 203 peptides were identified: 59 from α_S1-casein (CN), 11 from α_S2-CN, 129 from β-CN, and 4 from κ-CN. Numbers of α_S1- and β-CN-derived peptides increased with increasing number of ripening days due to the dependence of newly formed peptides on proteolysis. However, similar increases were not observed for α_S2- and κ-CN-derived peptides. Most identified peptides consisted of β-CN-derived peptides, followed by α_S1-, α_S2-, and κ-CN-derived peptides. Among these, bioactive peptides were found, including antihypertensive, antibacterial, antioxidant, dipeptidyl peptidase-4 inhibitory, metal chelating, skin regenerating, glucagon-like peptide-1 secretion enhancing, opioid, cathepsin B inhibitory, prolyl endopeptidase inhibitory, immunomodulatory, brain function improving, antiamnesic, antihypercholesterolemic, anti-inflammatory, and anticarcinogenic peptides.

Okara residue as source of antioxidants against lipid oxidation in milk enriched with omega-3 and bioavailability of bioactive compounds after in vitro gastrointestinal digestion

The residue from soy processing (okara) was evaluated for phenolic compounds, flavonoids, radical scavenging activity and reducing power ability. The ability of okara to decelerate the oxidation of omega-3 fatty acids caused by light exposure in a milk matrix, as well as its bioavailability after in vitro digestion, were investigated. Okara contained phenolic compounds (106.7 mg gallic acid equivalents (GAE)/100 g) and flavonoids (32.7 mg quercetin equivalents/100 g) and showed antioxidant activity. The addition of okara to omega-3 fatty acids-enriched milk inhibited the production of conjugated dienes and malonaldehyde during 8 days of storage compared to the control (without okara), particularly at the higher (3 g/L) than lower inclusion concentrations studied (1 and 2 g/L). The bioavailability assay demonstrated that polyphenols (41 mg GAE/100 g) were still present after in vitro digestion and had antioxidant activity. Okara, which is considered a residue and is discarded, can potentially be used as an antioxidant ingredient for the enrichment of foods.

Discordance between in silico & in vitro analyses of ACE inhibitory & antioxidative peptides from mixed milk tryptic whey protein hydrolysate

ACE inhibitory and antioxidative peptides identified by LCMS/MS, from mixed milk (Bubalus bubalis and Bos taurus) tryptic whey protein hydrolysate, were compared with the in silico predictions. α la and ß lg sequences, both from Bubalus bubalis and Bos taurus, were used for in silico study. SWISS-PROT and BIOPEP protein libraries were accessed for prediction of peptide generation. Study observed gaps in the prediction versus actual results, which remain unaddressed in the literature. Many peptides obtained in vitro, were not reflected in in silico predictions. Differences in identified peptides in separate libraries were observed too. In in silico prediction, peptides with known biological activities were also not reflected. Predictions, towards generation of bioactive peptides, based upon in silico release of proteins and amino acid sequences from different sources and thereupon validation in relation to actual results has often been reported in research literature. Given that computer aided simulation for prediction purposes is an effective research direction, regular updating of protein libraries and an effectual integration, for more precise results, is critical. The gaps addressed between these two techniques of research, have not found any address in literature. Inclusion of more flexibility with the variables, within the tools being used for prediction, and a hierarchy based database with search options for various peptides, will further enhance the scope and strength of research.

Plasmin digest of κ-casein as a source of antibacterial peptides

This study investigated the antibacterial properties of plasmin, the plasmin hydrolysis of bovine κ-casein and the fractions (named κC1, κC2, κC3, κC4, and κC5) liberated from it using RP-HPLC. The target bacteria wereEscherichia coli, Staphylococcus aureus(pathogenic),Lactobacillus caseiandLactobacillus acidophilus(probiotic). Three peptides (kC1, kC3, and kC4) were found to have antibacterial activity, with κC3 peptide being the most active. The plasmin digest of bovine κ-casein proved to be stronger than any of its fractions in terms of antibacterial potential. Measurement of the minimum inhibitory concentration (MIC) showed that Gram-positive bacteria are generally more sensitive to antibacterial activity than Gram-negative bacteria. The MIC of nisin, as a bacteriocin peptide, was also measured. The three antibacterial peptides were identified using LC-Mass. The molecular mass of kC1, kC3, and kC4 corresponded to the f(17–21), f(22–24), and f(1–3) of bovine κ-casein, respectively. It was also found that the positive charge and hydrophobicity of a peptide are not key factors in antibacterial activity. On the whole, the present study demonstrated that the plasmin digest of κ-casein has a high antibacterial potential and can be considered as a natural antibacterial agent in the food chain.

Identification of modified lysozyme peptides upon photo-oxidation by LC-TOF-MS

Protein oxidation can have major implications on the quality and safety of foods, but the majority of methods to evaluate oxidative damage lack specificity. Therefore, this study aimed to identify specific markers for protein oxidation. A well-characterized protein, lysozyme, was modified by photo-oxidation and subsequently hydrolyzed prior to peptide analysis by LC-TOF-MS. A semiquantitative analysis of the peptides indicated that from the seven peptides containing sensitive amino acids, two peptides (HGLDNYR and WWCNDGR) were highly affected upon photo-oxidation and have the potential to serve as markers for protein oxidation. Site-specific modifications enabled the description of the degradation pathway of several lysozyme peptides but also indicated that the surrounding amino acids and the 3D structure of the protein have an impact on the induced modifications. It is therefore advisable to evaluate protein oxidation on the intact protein.

The impact of photo-induced molecular changes of dairy proteins on their ACE-inhibitory peptides and activity

Among all dietary proteins, dairy proteins are the most important source of bio-active peptides which can, however, be affected by modifications upon processing and storage. Since it is still unknown to which extent the biological activity of dairy proteins is altered by chemical reactions, this study focuses on the effect of photo-induced molecular changes on the angiotensin I converting enzyme (ACE) inhibitory activity. Milk proteins were dissolved in phosphate buffer containing riboflavin and stored under light at 4 °C for one month during which the molecular changes and the ACE-inhibitory activity were analysed. An increase in the total protein carbonyls and the N-formylkynurenine content was observed, besides a decrease in the free thiol, tryptophan, tyrosine and histidine content. These changes were more severe in caseins compared with whey proteins and resulted moreover in the aggregation of caseins. Due to these photo-induced molecular changes, a significant loss of the ACE-inhibitory activity was observed for casein peptides. A peptide analysis moreover illustrated that the decreased activity was not attributed to a reduced digestibility but to losses of specific ACE-inhibitory peptides. The observed molecular changes, more specifically the degradation of specific amino acids and the casein aggregation, could be assigned as the cause of the altered peptide pattern and as such of the loss in ACE-inhibitory activity.