Plasmin in milk and dairy products: an update

Advances in separation and identification of biologically important milk proteins and peptides

Milk is a rich source of biologically important proteins and peptides. In addition, milk contains a variety of extracellular vesicles (EVs), including exosomes, that carry their own proteome cargo. EVs are essential for cell-cell communication and modulation of biological processes. They act as nature carriers of bioactive proteins/peptides in targeted delivery during various physiological and pathological conditions. Identification of the proteins and protein-derived peptides in milk and EVs and recognition of their biological activities and functions had a tremendous impact on food industry, medicine research, and clinical applications. Advanced separation methods, mass spectrometry (MS)-based proteomic approaches and innovative biostatistical procedures allowed for characterization of milk protein isoforms, genetic/splice variants, posttranslational modifications and their key roles, and contributed to novel discoveries. This review article discusses recently published developments in separation and identification of bioactive proteins/peptides from milk and milk EVs, including MS-based proteomic approaches.

Effect of temperature and protein concentration on the protein types within the ultracentrifugation supernatant of liquid micellar casein concentrate

Liquid micellar casein concentrate (MCC) is an ideal milk-based protein ingredient for neutral-pH ready-to-drink beverages. The texture and mouthfeel of liquid MCC-based beverages depend on the beverage protein content, as well as the composition of soluble proteins in the aqueous phase around the casein micelle. The objective of this study was to determine the composition of soluble proteins in the aqueous phase around the casein micelles in skim milk and liquid MCC containing 7.0% and 11.6% protein content. Skim milk was pasteurized and concentrated to 7% protein content by microfiltration and then to 18% protein content by ultrafiltration. The 18% MCC was then serially diluted with distilled water to produce 11.6% and 7.0% protein MCC. Skim milk, 7.0% MCC, and 11.6% MCC representing starting materials with different protein concentrations were each ultracentrifuged at 100,605 × g for 2 h. The ultracentrifugation for each of the starting materials was performed at 3 different temperatures: 4°C, 20°C, and 37°C. The ultracentrifugation supernatants were collected to represent the aqueous phase around the casein micelle in MCC solutions. The supernatants were analyzed by Kjeldahl to determine the crude protein, casein, and casein as a percentage of crude protein content, and by sodium dodecyl sulfate PAGE to determine the composition of the individual proteins. Most of the proteins in MCC supernatant (about 45%) were casein proteolysis products. The remaining proteins in the MCC supernatant consisted of a combination of intact αS-, β-, and κ-caseins (about 40%) and serum proteins (14-18%). Concentrations of αS-casein and β-casein in the supernatant increased with decreasing temperature, especially at higher protein concentrations. Temperature and interaction between temperature and protein explained about 80% of the variation in concentration of supernatant αS- and β-caseins. Concentration of supernatant κ-casein, casein proteolysis products, and serum protein increased with increasing MCC protein concentration, and MCC protein concentration explained most of the variation in supernatant κ-casein, casein proteolysis products, and serum protein concentrations. Predicted MCC apparent viscosity was positively associated with the dissociation of αS- and β-caseins. Optimal beverage viscosity could be achieved by controlling the dissociation of these proteins in MCC.

Bioluminescence resonance energy transfer biosensor for measuring activity of a protease secreted by Pseudomonas fluorescens growing in milk

Bacterial proteases are sporadic contributors to milk spoilage, reducing the quality of ultra-heat treated (UHT) milk and other dairy products. Current methods for measuring bacterial protease activity in milk are insensitive and too slow to be used in routine testing in dairy processing plants. We have designed a novel bioluminescence resonance energy transfer (BRET)-based biosensor to measure the activity of proteases secreted by bacteria in milk. The BRET-based biosensor is highly selective for bacterial protease activity compared with other proteases tested, notably including plasmin, which is abundant in milk. It incorporates a novel peptide linker that is selectively cleaved by P. fluorescens AprX proteases. The peptide linker is flanked by green fluorescent protein (GFP²) at the N-terminus and a variant Renilla luciferase (RLuc2) at the C-terminus. Complete cleavage of the linker by bacterial proteases from Pseudomonas fluorescens strain 65, leads to a 95% decrease in the BRET ratio. We applied an azocasein-based calibration method to the AprX biosensor using standard international enzyme activity units. In a 10-min assay, the detection limit for AprX protease activity in buffer was equivalent to 40 pg/mL (≈0.8 pM, 22 μU/mL) and 100 pg/mL (≈2pM, 54 μU/mL) in 50% (v/v) full fat milk. The EC₅₀ values were 1.1 ± 0.3 ng/mL (87 μU/mL) and 6.8 ± 0.2 ng/mL (540 μU/mL), respectively. The biosensor was approximately 800x more sensitive than the established FITC-Casein method in a 2-h assay, the shortest feasible time for the latter method. The protease biosensor is sensitive and fast enough to be used in production settings. It is suitable for measuring bacterial protease activity in raw and processed milk, to inform efforts to mitigate the effects of heat-stable bacterial proteases and maximise the shelf-life of dairy products.

Comparative Label-Free Liquid Chromatography-Mass Spectrometry Milk Proteomic Profiles Highlight Putative Differences between the Autochthon Teramana and Saanen Goat Breeds

Goat's milk is an excellent source of nutrients, with greater benefits compared to cow's milk. Limited information is available on autochthon goat breeds, which are important for biodiversity preservation. In this study, the aim of using label-free quantification was to investigate the milk proteome of two goat breeds, the autochthon Teramana and Saanen breeds, which are commonly used by the industry. Utilising label-free proteomic analysis, 749 and 666 proteins, respectively were identified and quantified from the Teramana and Saanen goat milk. Moreover, utilising statistical analysis, 29 proteins were able to discriminate the two goat breeds, with many of the identified proteins involved in complement and coagulation cascades. This work enhances our understanding of the goat milk proteome and shows differences between the two breeds, leading to an important contribution toward a more detailed molecular-view of this unique substrate. Additionally, charactersation of the milk proteins can help in guiding genetic improvements in the goat herds, and thus increasing its use in human nutrition.

Effect of plasmin on casein hydrolysis and textural properties of rennet-induced model cheeses

Plasmin plays an important role in casein hydrolysis during cheese ripening, which may influence the properties of the casein network and the texture of the final product. In view of this, the relation between plasmin-induced casein hydrolysis and textural changes of cheese during ripening was investigated in this study. Four batches of model cheese with different concentrations of added plasmin (0, 0.4, 0.6 and 1.0 μL/g milk) were prepared, and were stored for 12 weeks at 16 °C. During this period, plasmin activity, casein hydrolysis, textural properties and other compositional characteristics (pH, dry matter) were determined. Our results show that the addition of plasmin had significant effect on both the degree and the pattern of proteolysis. As a result, cheeses with different plasmin content showed different textural properties. With increased plasmin concentration, Young's modulus, hardness, resilience and cohesion decreased, while brittleness increased. All textural properties showed linear relations with the degree of casein hydrolysis, and logarithmic relations with the percentage of intact casein fractions. At the beginning of ripening, only slight changes in textural properties were found, although a substantial part (40-60 %) of the casein fractions was already been broken down. When ripening progressed, ongoing proteolysis significantly weakened the protein network and consequently led to noticeable textural changes. Model cheeses became softer, more brittle and less elastic. The knowledge gained from this study provide new insights in the changes of different textural parameters of model cheese. This will help to optimize the existing products and create new ones.

Influence of storage time on protein composition and simulated digestion of UHT milk and centrifugation presterilized UHT milk in vitro

The centrifugation presterilizing UHT (C-UHT) sterilization method removes 90% of the microorganism and somatic cells from raw milk using high-speed centrifugation following UHT treatment. This study aimed to study the changes in protein composition and plasmin in the UHT and C-UHT milk. The digestive characteristics, composition, and peptide spectrum of milk protein sterilized with the 2 technologies were studied using a dynamic digestive system of a simulated human stomach. The Pierce bicinchoninic acid assay, laser scanning confocal microscope, liquid chromatography-tandem mass spectrometry, and AA analysis were used to study the digestive fluid at different time points of gastric digestion in vitro. The results demonstrated that C-UHT milk had considerably higher protein degradation than UHT milk. Different processes resulted during the cleavage of milk proteins at different sites during digestion, resulting in different derived peptides. The results showed there was no significant effect of UHT and C-UHT on the peptide spectrum of milk proteins, but C-UHT could release relatively more bioactive peptides and free AA.

Expression profile of different classes of proteases in milk derived somatic cells across different lactation stages of indigenous cows (Bos indicus) and riverine buffaloes (Bubalus bubalis)

Proteases play a significant role in milk and its products by affecting flavor, texture and longevity. The expression of endogenous proteases varies across different stages of lactation. The study was conducted to understand the transcriptional pattern of different classes of protease-pathways associated genes (CTSB, CTSD, CTSH, CTSL, CTSK, CTSS, CTSZ, PLAU, PLAT) and potential protease inhibitors (SERPIN E2 and SERPIN F2) in 40 milk somatic cells (MSC) samples isolated during early, peak, mid and late lactation stages of Sahiwal cows and Murrah buffaloes - the two most important dairy breeds of India. In Sahiwal cows, except CTSK and PLAU, the expression of other proteases class was not affected significantly (p > 0.05) across lactation stages. However, in Murrah buffaloes, the expression of different proteases increased as the lactation progressed. Most of the proteases showed lower expression during early and peak lactation stages while their expression tends to increase during mid to late lactation stages. The overall trend was somewhat similar in both the dairy species albeit the level of expression was higher in buffalo MSC as compared to cow MSC. The study has provided valuable information on expression kinetics of different proteases in milk somatic cells of two major dairy breeds of India.

Identification of the initial reactive sites of micellar and non-micellar casein exposed to microbial transglutaminase

To investigate the influence of the internal micellar structure on the course of enzymatic cross-linking especially in the initial phase of the reaction, casein micelles isolated from raw milk via ultracentrifugation were incubated with microbial transglutaminase (mTG) in comparison with non-micellar sodium caseinate. Reactive lysine and glutamine residues were identified using a label-free approach, based on the identification of isopeptides within tryptic hydrolysates by targeted HRMS as well as manual monitoring of fragmentation spectra. Identified reactive sites were furthermore weighted by tracking the formation of isopeptides over an incubation time of 15, 30, 45 and 60 min, respectively. Fifteen isopeptides formed in the early stage of mTG cross-linking of caseins were identified and further specified concerning the position of lysine and glutamine residues involved in the reaction. The results revealed lysine K176 and glutamine Q175 of β-casein as the most reactive residues, which might be located in a highly flexible region of the molecule based on different possible reaction partners identified in this study. Except for the isopeptide αs1 K34–αs2 Q101 in sodium caseinate (SC), all reactive sites were detected in micellar and in non-micellar casein, indicating that the initial phase of enzymatic cross-linking is not affected by micellar aggregation of caseins.

Graphical abstract

Differentiating ultra-high temperature milk and reconstituted milk using an untargeted peptidomic approach with chemometrics

An untargeted peptide profiling based on ultra-performance liquid chromatography quadrupole time-of flight mass spectrometry with chemometrics was performed to differentiate ultra-high temperature processed milk and reconstituted milk. Thirty-three marker peptides were identified, primarily released from the C- or N-terminal of β-casein and α_s1-casein. These peptides were produced by heating and protease hydrolysis. Additional heating and storage time experiments showed that the level of 18 marker peptides increased with heat load and storage time, whereas 15 peptides were solely influenced by heat load. The peptides from β-casein showed higher sensitivity to thermal stress compared to those from α_s1-casein. Additionally, eight modified peptides of casein were identified as indicators of milk thermal processing. The identified marker peptides can distinguish ultra-high temperature processed milk and reconstituted milk, and are suitable for monitoring heating processes and storage of milk.

Surface-enhanced Raman scattering-based detection of plasmin activity by specific peptide substrate

In this study, a new surface-enhanced Raman scattering (SERS)-based method has been developed for the detection of plasmin activity. Firstly, different peptide sequences, which are specific to plasmin, were examined. Then, SERS substrates were prepared by chosen peptide substrate. Enzyme activity was determined by pursuing the reduction of DTNB band at 1331 cm^-1 with Raman spectroscopy. The reduction in SERS intensity was related to the plasmin activity, and changes in SERS intensity vs. plasmin concentration graph was obtained. Limit of detection (LOD) and limit of quantification (LOQ) values were calculated as 2.14 U/mL and 6.42 U/mL, respectively. Intra- and inter-day repeatability results were determined as 1.45% and 1.47% relative standard deviation (RSD). Also, recovery of the method was determined for the plasmin spiked milk samples. The results demonstrated that the proposed method could be successfully used to detect the plasmin activity in milk samples.

Acoustic Biosensors and Microfluidic Devices in the Decennium: Principles and Applications

Lab-on-a-chip (LOC) technology has gained primary attention in the past decade, where label-free biosensors and microfluidic actuation platforms are integrated to realize such LOC devices. Among the multitude of technologies that enables the successful integration of these two features, the piezoelectric acoustic wave method is best suited for handling biological samples due to biocompatibility, label-free and non-invasive properties. In this review paper, we present a study on the use of acoustic waves generated by piezoelectric materials in the area of label-free biosensors and microfluidic actuation towards the realization of LOC and POC devices. The categorization of acoustic wave technology into the bulk acoustic wave and surface acoustic wave has been considered with the inclusion of biological sample sensing and manipulation applications. This paper presents an approach with a comprehensive study on the fundamental operating principles of acoustic waves in biosensing and microfluidic actuation, acoustic wave modes suitable for sensing and actuation, piezoelectric materials used for acoustic wave generation, fabrication methods, and challenges in the use of acoustic wave modes in biosensing. Recent developments in the past decade, in various sensing potentialities of acoustic waves in a myriad of applications, including sensing of proteins, disease biomarkers, DNA, pathogenic microorganisms, acoustofluidic manipulation, and the sorting of biological samples such as cells, have been given primary focus. An insight into the future perspectives of real-time, label-free, and portable LOC devices utilizing acoustic waves is also presented. The developments in the field of thin-film piezoelectric materials, with the possibility of integrating sensing and actuation on a single platform utilizing the reversible property of smart piezoelectric materials, provide a step forward in the realization of monolithic integrated LOC and POC devices. Finally, the present paper highlights the key benefits and challenges in terms of commercialization, in the field of acoustic wave-based biosensors and actuation platforms.

An evaluation of the in vitro antioxidant and antidiabetic potentials of camel and donkey milk peptides released from casein and whey proteins

In this study, some biological activities including antioxidant activity (DPPH radical scavenging activity, ABTS radical scavenging activity, and CUPRAC assay), DPP-IV enzyme inhibitory activity, and α-glucosidase enzyme inhibitory activity of peptides released from in vitro gastrointestinal digested casein and the whey proteins of camel and donkey milk were evaluated. While the highest antioxidant activity was determined to be in the digested camel casein fraction using the ABTS and CUPRAC methods, the digested donkey casein fraction was determined to have the highest radical scavenging activity using the DPPH method. The highest DPP-IV inhibitory activity was detected in digested camel and donkey milk casein fractions. Digested whey fractions of camel and donkey milk had a lower DPP-IV inhibitory activity compared to the digested casein fractions. However, digested whey fractions of camel and donkey milk did not show α-glucosidase inhibitory activity, and digested donkey casein fraction showed the highest α-glucosidase inhibitory activity with a 12.5 µg/mL IC₅₀ value. It was concluded that peptides released from digested casein fraction of camel and donkey milk have potent antioxidant and particularly antidiabetic properties.

Changes in Particle Size, Sedimentation, and Protein Microstructure of Ultra-High-Temperature Skim Milk Considering Plasmin Concentration and Storage Temperature

Age gelation is a major quality defect in ultra-high-temperature (UHT) pasteurized milk during extended storage. Changes in plasmin (PL)-induced sedimentation were investigated during storage (23 °C and 37 °C, four weeks) of UHT skim milk treated with PL (2.5, 10, and 15 U/L). The increase in particle size and broadening of the particle size distribution of samples during storage were dependent on the PL concentration, storage period, and storage temperature. Sediment analysis indicated that elevated storage temperature accelerated protein sedimentation. The initial PL concentration was positively correlated with the amount of protein sediment in samples stored at 23 °C for four weeks (r = 0.615; p < 0.01), whereas this correlation was negative in samples stored at 37 °C for the same time (r = −0.358; p < 0.01) due to extensive proteolysis. SDS-PAGE revealed that whey proteins remained soluble over storage at 23 °C for four weeks, but they mostly disappeared from the soluble phase of PL-added samples after two weeks’ storage at 37 °C. Transmission electron micrographs of PL-containing UHT skim milk during storage at different temperatures supported the trend of sediment analysis well. Based on the Fourier transform infrared spectra of UHT skim milk stored at 23 °C for three weeks, PL-induced particle size enlargement was due to protein aggregation and the formation of intermolecular β-sheet structures, which contributed to casein destabilization, leading to sediment formation.

Application of high-resolution ultrasonic spectroscopy for detection of the plasmin activity toward β-casein

High-resolution ultrasonic spectroscopy (HR-US) was applied for precise detection of plasmin activity towards β-casein in buffer at pH 7.8 and 37 °C. The evolution of ultrasonic velocity and ultrasonic attenuation measured at 15.5 MHz is related to the concentration of peptide bonds hydrolyzed and loss of β-casein aggregates, respectively. The ultrasonic assay presents sensitive and direct activity-based quantification of plasmin levels in milk. The variation in plasmin concentration between HR-US and ELISA method owed to the differing detection principles. The real-time ultrasonic profiles of hydrolysis were utilized to describe the kinetic aspect of plasmin activity. The non-linear activity curve was fitted with classic and inverse Michaelis-Menten type models. Within 1-8.6 mg·mL^-1 β-casein, the V_max and K_M obtained were (6.30 ± 2.21) × 10^-5 mol.kg^-1·min^-1 and 10.33 ± 3.50 mg·mL^-1, respectively. The maximum peptide bond cleaved was 5-6 (2.7% degree of hydrolysis) achieved at 1 mg·mL^-1 β-casein.

Can cheese mites, maggots and molds enhance bioactivity? Peptidomic investigation of functional peptides in four traditional cheeses

Aside from their amino acid content, dairy proteins are valuable for their ability to carry encrypted bioactive peptides whose activities are latent until released by digestive enzymes or endogenous enzymes within the food. Peptides can possess a wide variety of functionalities, such as antibacterial, antihypertensive, and antioxidative properties, as demonstrated by in vitro and in vivo studies. This phenomenon raises the question as to what impact various traditional cheese-making processes have on the formation of bioactive peptides in the resulting products. In this study, we have profiled the naturally-occurring peptides in two hard and two soft traditional cheeses and have identified their known bioactive sequences. While past studies have typically identified fewer than 100 peptide sequences in a single cheese, we have used modern instrumentation to identify between 2900 and 4700 sequences per cheese, an increase by a factor of about 50. We demonstrated substantial variations in proteolysis and peptide formation between the interior and rind of each cheese, which we ascribed to the differences in microbial composition between these regions. We identified a total of 111 bioactive sequences among the four cheeses, with the greatest number of sequences, 89, originating from Mimolette. The most common bioactivities identified were antimicrobial and inhibition of the angiotensin-converting enzyme. This work revealed that cheese proteolysis and the resulting peptidomes are more complex than originally thought in terms of the number of peptides released, variation in peptidome across sites within a single cheese, and variation in bioactive peptides among cheese-making techniques.

Structural changes in cow, goat, and sheep skim milk during dynamic in vitro gastric digestion

Coagulation of milk in the stomach is the first crucial step in its digestion. Using a human gastric simulator, the dynamic gastric digestion of goat and sheep skim milk were compared with that of cow skim milk, focusing particularly on their physical characteristics. The gastric contents were analyzed for changes in dry matter and microstructure, and the extent of protein digestion. The study revealed that the skim milk from all species formed a curd within the first 15 min of gastric digestion, at which time the pH was ~6.1 to 6.3. Compared with cow skim milk, the dry matter contents of the clots formed from goat and sheep skim milk were lower and higher, respectively, which was due to the differences in their total solids and protein contents. Microstructural analysis showed that, as digestion progressed, the clot structure became more cohesive, along with a decrease in moisture content, which in turn affected the breakdown and hydrolysis of caseins by pepsin; this phenomenon was similar for milk from all species. However, the extent of moisture retained in the sheep skim milk clot appeared to be lower than those of the cow and goat skim milk clots. In addition, the relative firmness of the sheep milk clot was higher than those of the cow and goat milk clots at the end of gastric digestion. The pattern of protein hydrolysis by pepsin was similar for the milk of all species, despite the differences in the proportions of different proteins. The study provided insight into the coagulation kinetics of goat and sheep skim milk under in vitro gastric digestion conditions.

Comprehensive Profiling of the Native and Modified Peptidomes of Raw Bovine Milk and Processed Milk Products

Bovine milk contains a variety of endogenous peptides, partially formed by milk proteases that may exert diverse bioactive functions. Milk storage allows further protease activities altering the milk peptidome, while processing, e.g., heat treatment can trigger diverse chemical reactions, such as Maillard reactions and oxidations, leading to different posttranslational modifications (PTMs). The influence of processing on the native and modified peptidome was studied by analyzing peptides extracted from raw milk (RM), ultra-high temperature (UHT) milk, and powdered infant formula (IF) by nano reversed-phase liquid chromatography coupled online to electrospray ionization (ESI) tandem mass spectrometry. Only unmodified peptides proposed by two independent software tools were considered as identified. Thus, 801 identified peptides mainly originated from α_S- and β-caseins, but also from milk fat globular membrane proteins, such as glycosylation-dependent cell adhesion molecule 1. RM and UHT milk showed comparable unmodified peptide profiles, whereas IF differed mainly due to a higher number of β-casein peptides. When 26 non-enzymatic posttranslational modifications (PTMs) were targeted in the milk peptidomes, 175 modified peptides were identified, i.e., mostly lactosylated and a few hexosylated or oxidized peptides. Most modified peptides originated from α_S-caseins. The numbers of lactosylated peptides increased with harsher processing.

Advances in Analysis of Milk Proteases Activity at Surfaces and in a Volume by Acoustic Methods

This review is focused on the application of surface and volume-sensitive acoustic methods for the detection of milk proteases such as trypsin and plasmin. While trypsin is an important protein of human milk, plasmin is a protease that plays an important role in the quality of bovine, sheep and goat milks. The increased activity of plasmin can cause an extensive cleavage of β-casein and, thus, affect the milk gelation and taste. The basic principles of surface-sensitive acoustic methods, as well as high-resolution ultrasonic spectroscopy (HR-US), are presented. The current state-of-the-art examples of the application of acoustic sensors for protease detection in real time are discussed. The application of the HR-US method for studying the kinetics of the enzyme reaction is demonstrated. The sensitivity of the acoustics biosensors and HR-US methods for protease detection are compared.

DIA proteomics reveals hypotensive and immune-enhancing constituents in buffalo whey from different altitudes

The selection of raw milk with high levels of functional components that have health-promoting activities is very important for the exploitation and production of functional milk, but the differences in the functional components of whey from buffalo raised at different altitudes have not been thoroughly investigated. Here, we detected the effects of altitudes on the functional components in whey from dairy buffalo farms situated at low altitude (LA), medium altitude (MA), and high altitude (HA) sites with data-independent acquisition proteomic approaches. In 33 samples, 9331 peptides corresponding to 1008 high-confidence proteins were detected. HA-whey had a lower level of angiotensinogen than that of the LA- and MA-whey, and conversely contained higher levels of immune-enhancing components than for the latter two groups. Differential proteins were involved in vascular smooth muscle contraction, complement and coagulation cascades, and the secretion, production and regulation pathways in immune components. LA-whey showed higher levels of lymphocyte antigen and selenoprotein F than that of the HA-whey. Owing to the biological functions of their most abundant components, HA- and LA-whey are suitable for the processing of functional milk for lowering blood pressure, and the production of immune milk, respectively.

Pulsed electric field and mild heating for milk processing: a review on recent advances

Pulsed electric field (PEF) treatment consists of exposing food to electrical fields between electrodes within a treatment chamber, which can improve the preservation of fresh-like products such as milk. Although several studies support the use of PEF technology to process milk at low temperature, these studies reported microbial reductions of around 3 log₁₀ cycles and also indicated a limited impact of PEF on some endogenous and microbial enzymes. This scenario indicates that increasing the impact of PEF on both enzymes and microorganisms remains a major challenge for this technology in milk processing. More recently, combining PEF with mild heating (below pasteurization condition) has been explored as an alternative processing technology to enhance the safety and to preserve the quality of fresh milk and milk products. Mild heating with PEF enhanced the safety of milk and derived products (3 log₁₀ -6 log₁₀ cycles reduction on microbial load and drastic impact on the activity enzymes related to quality decay). Moreover, with this approach, there was minimal impact on enzymes of technological and safety relevance, proteins, milk fat globules, and nutrients (particularly for vitamins) and improvements in the shelf-life of milk and selected derived products were obtained. Finally, further experiments should consider the use of milk processed by PEF with mild heating on cheese-making. The combined approach of PEF with mild heating to process milk and derived products is very promising. The characteristics of current PEF systems (which is being used at an industrial level in several countries) and their use in the liquid food industry, particularly for milk and some milk products, could advance towards this strategy. © 2019 Society of Chemical Industry.

Binding characteristics of polyphenols as milk plasmin inhibitors

BACKGROUND

The potential use of polyphenols to improve the functional characteristics of dairy products has gained much attention. However, the effects of the polyphenols on naturally occurring enzymes in milk have not been studied extensively. Excess plasmin activity in dairy products might result in several quality defects. The objective of this study was to assess the ability of polyphenols to inhibit plasmin in milk using a molecular and kinetic approach.

RESULTS

Epicatechin gallate (ECG), epigallocatechin gallate (EGCG), quercetin (QUER), and myricetin (MYR) caused a significant decrease in plasmin activity by 60, 86, 65, and 90%, respectively. The inhibition rates were alleviated in the presence of milk proteins. EGCG, QUER, and MYR, exhibited noncompetitive inhibition against plasmin, whereas ECG caused a mixed-type inhibition. A decrease in the random structure of plasmin upon the complex formation with ECG, EGCG, QUER, and MYR was found. The other phenolics that were evaluated did not cause any significant changes in plasmin conformation. The observed inhibitory phenolic-plasmin interactions were dominated by H-bonds and electrostatic attractions. Green tea extract (GTE) rich in catechins also inhibited plasmin activity in the milk.

CONCLUSION

Significant changes in the secondary structure of plasmin upon binding of ECG, EGCG, QUER, and MYR led to diminished plasmin activity both in the absence and presence of milk proteins. These flavonoids with promising plasmin inhibitory potential could be used in new dairy formulations leading to controlled undesired consequences of plasmin activity. © 2019 Society of Chemical Industry.

BIOPEP-UWM Database of Bioactive Peptides: Current Opportunities

The BIOPEP-UWM™ database of bioactive peptides (formerly BIOPEP) has recently become a popular tool in the research on bioactive peptides, especially on these derived from foods and being constituents of diets that prevent development of chronic diseases. The database is continuously updated and modified. The addition of new peptides and the introduction of new information about the existing ones (e.g., chemical codes and references to other databases) is in progress. New opportunities include the possibility of annotating peptides containing D-enantiomers of amino acids, batch processing option, converting amino acid sequences into SMILES code, new quantitative parameters characterizing the presence of bioactive fragments in protein sequences, and finding proteinases that release particular peptides.

Peptide profile of Camembert-type cheese: Effect of heat treatment and adjunct culture Lactobacillus rhamnosus GG

Several factors might impact the proteolysis during cheese manufacture and ripening and, therefore, the release of bioactive peptides. These factors include the heat treatment of the milk, the type of starter and secondary culture used and the ripening time. Thus, the objective of this study was to evaluate the effect of the milk heat treatment and the use of adjunct culture in the development of the peptide profile of Camembert-type cheese during ripening. The cheeses were made from raw and heat-treated milk, with and without the addition of Lactobacillus rhamnosus GG. The results obtained by mass spectrometry (MALDI ToF/MS) and analyzed by chemometrics (PLS-DA) revealed a complex hydrolysis profile of the caseins with 103 peaks found, of which 70 peptides were identified and 15 presented bioactive potential. The potential bioactive peptides important for the separation of cheeses were all derived from β-casein. The heat treatment of the milk, the addition of the adjunct culture and the ripening time affected the peptide profile of the cheeses. At the beginning of ripening the cheeses presented a very similar peptide profile, which differed over time, and this differentiation is clearer for cheeses obtained from raw milk.

Age Gelation, Sedimentation, and Creaming in UHT Milk: A Review

Demand for ultra-high-temperature (UHT) milk and milk protein-based beverages is growing. UHT milk is microbiologically stable. However, on storage, a number of chemical and physical changes occur and these can reduce the quality of the milk. These changes can be sufficiently undesirable so as to limit acceptance or shelf life of the milk. The most severe changes in UHT milk during storage are age gelation, with an irreversible three-dimensional protein network forming throughout, excessive sedimentation with a compact layer of protein-enriched material forming rapidly at the bottom of the pack, and creaming with excessive fat accumulating at the top. For age gelation, it is known that at least two mechanisms can lead to gelation during storage. One mechanism involves proteolytic degradation of the proteins through heat-stable indigenous or exogenous enzymes, destabilizing milk and ultimately forming a gel. The other mechanism is referred to as a physico-chemical mechanism. Several factors are known to affect the physico-chemical age gelation, such as milk/protein concentration, heat load during processing (direct compared with indirect UHT processes), and milk composition. Similar factors to age gelation are known to affect sedimentation. There are relatively few studies on the creaming of UHT milk during storage, suggesting that this defect is less common or less detrimental compared with gelation and sedimentation. This review focuses on the current state of knowledge of age gelation, sedimentation, and creaming of UHT milks during storage, providing a critical evaluation of the available literature and, based on this, mechanisms for age gelation and sedimentation are proposed.

Role of Microbial Cultures and Enzymes During Cheese Production and Ripening

Many different kinds of cultures, enzymes, and methods are used during the production and ripening of a variety of cheese types. In this chapter, the importance, types, and applications of microbial cultures during cheese production are discussed. Moreover, an overview of the important role of enzymatic systems, either derived from these cultures or directly added to the milk fermentation, is presented. The main biochemical events including glycolysis, lipolysis, and proteolysis during cheese ripening are explained, focusing on their end products, which contribute to the development of the overall aroma of cheese.

Identification of the Peptide PyroQ-βCasein194-209 as a Highly Specific and Sensitive Marker to Differentiate between Ultrahigh-Temperature Processed (UHT) Milk and Mildly Heated Milk

In this study, a new approach was introduced to identify marker peptides that reflect the thermal treatment of commercial milk samples and differentiate ultrahigh-temperature processed (UHT) milk from mildly heated milk. Peptide profiles of training set samples, pasteurized (n = 20), extended shelf life (n = 29), and UHT (n = 29) milk, were recorded by MALDI-TOF-MS after StageTip microextraction. As marker candidates, 13 peptides were selected, and their cutoff levels were defined. The quality of the cutoff levels was then tested with a blind test set. Thus, the peptide m/z 1701.0, which was identified as pyroQ-βcasein_194-209, could ideally differentiate UHT milk from mildly heated milk with an accuracy of 100%. Due to its high reliability and sensitivity, this peptide may be applied in routine analysis to monitor thermal processing of milk. An additional heating experiment showed that the marker peptide candidates are formed during milk processing by endogenous enzymes and selective thermal cleavage.