Bovine milk procathepsin D: Presence and activity in heated milk and in extracts of rennet-free UF-Feta cheese

The behavior of cathepsin D during milk processing and its contribution to bitterness in a model fresh cheese

The bovine endopeptidase cathepsin D was investigated regarding its temperature-dependent inactivation and ability to form bitter peptides within a spiked model fresh cheese. Cathepsin D was found to be more susceptible than other milk endogenous peptidases to temperature treatments in skim milk. Inactivation kinetics revealed decimal reduction times of 5.6 min to 10 s in a temperature range from 60 to 80°C. High temperature and ultra-high temperature (UHT) treatments from 90 to 140°C completely inactivated cathepsin D within 5 s. A residual cathepsin D activity of around 20% was detected under pasteurization conditions (72°C for 20 s). Therefore, investigations were done to estimate the effect of residual cathepsin D activity on taste in a model fresh cheese. The UHT-treated skim milk was spiked with cathepsin D and acidified with glucono-δ-lactone to produce a model fresh cheese. A trained bitter-sensitive panel was not able to distinguish cathepsin D-spiked model fresh cheeses from the control model fresh cheeses in a triangle test. Model fresh cheese samples were also analyzed for known bitter peptides derived from casein fractions using a HPLC-tandem mass spectrometry (MS) approach. In accordance with the sensory evaluation, the MS analyses revealed that the bitter peptides investigated within the cathepsin D-spiked model fresh cheese were not found or were below the limit of detection. Even though cathepsin D may be present during the fermentation of pasteurized milk, it does not seem to be responsible for bitter peptide formation from milk proteins on its own.

Identification of peptides reflecting the storage of UHT milk by MALDI-TOF-MS peptide profiling

Proteolysis during the storage of UHT milk is associated with major technological problems, particularly bitter off-flavors and age gelation limiting the shelf life of milk. In this study, untargeted peptide profiling by MALDI-TOF-MS identified peptides that were formed by proteolysis and reflected the storage of UHT milk. Analysis of nine different commercial UHT samples recorded peptide profiles during and at the end of their shelf life. Relative quantification and sequencing of the peptides revealed that the concentrations of 22 peptides increased significantly during the storage of UHT milk due to the activity of endogenous milk proteases and microbial proteases as well as other unidentified proteolytic mechanisms. Based on highly discriminative AUC values from receiver operator characteristic (ROC) curve analysis, we selected ten peptides as marker candidates. Among those, the peptide β-casein_192-206 (m/z 1668.9) was the most suitable marker differentiating expired-UHT from regular-UHT samples with 100% accuracy. Additionally, β-casein_191-206 (m/z 1782.0) showed 100% specificity and β-casein_139-161 (m/z 2696.4) 100% sensitivity. Thus, β-casein_192-206, either by itself or in combination with β-casein_191-206 and β-casein_139-161, presents a reliable marker to monitor the storage of UHT milk based on proteolytic mechanisms. SIGNIFICANCE: Enzymatic hydrolysis is the main reason why processed milk spoils during storage. The present study recorded peptide profiles to monitor the release or degradation of peptides in stored UHT milk. Among the detected peptides, statistical analysis revealed that the relative concentration of β-casein_192-206 reflected those proteolytic processes most precisely. Food authorities can now refer to β-casein_192-206 as a reliable marker to differentiate between freshly processed milk and products at the end of their shelf life. Furthermore, the food industry can use this marker peptide to improve production processes by monitoring the proteolysis during storage. The recorded peptide profile helps to explain the basic mechanisms leading to storage-induced proteolysis.

Flow Cytometry Approach to Quantify the Viability of Milk Somatic Cell Counts after Various Physico-Chemical Treatments

Flow cytometry has been used as a routine method to count somatic cells in milk, and to ascertain udder health and milk quality. However, few studies investigate the viability of somatic cells and even fewer at a subpopulation level to follow up how the cells can resist to various stresses that can be encountered during technological processes. To address this issue, a flow cytometry approach was used to simultaneously identify cell types of bovine milk using cell-specific antibodies and to measure the cell viability among the identified subpopulations by using a live/dead cell viability kit. Confirmation of the cell viability was performed by using conventional microscopy. Different physico-chemical treatments were carried out on standardized cell samples, such as heat treatment, various centrifugation rates and storage in milk or in PBS pH 7.4 for three days. Cytometry gating strategy was developed by using blood cell samples stored at 4°C in PBS and milk cell samples heat-treated at 80°C for 30 min as a control for the maximum (95.9%) and minimum (0.7%) values of cell viability respectively. Cell viability in the initial samples was 39.5% for all cells and varied for each cell population from 26.7% for PMNs, to 32.6% for macrophages, and 58.3% for lymphocytes. Regarding the physico-chemical treatments applied, somatic cells did not sustain heat treatment at 60°C and 80°C in contrast to changes in centrifugation rates, for which only the higher level, i.e. 5000×g led to a cell viability decrease, down to 9.4%, but no significant changes within the cell subpopulation distribution were observed. Finally, the somatic cells were better preserved in milk after 72h storage, in particular PMNs, that maintained a viability of 34.0 ± 2.9% compared to 4.9±1.9% in PBS, while there was almost no changes for macrophages (41.7 ± 5.7% in milk vs 31.2 ± 2.4% in PBS) and lymphocytes (25.3 ± 3.0% in milk vs 11.4 ± 3.1% in PBS). This study provides a new array to better understand milk cell biology and to establish the relationship between the cell viability and the release of their endogenous enzymes in dairy matrix.

Role of somatic cells on dairy processes and products: a review

Somatic cells are an important component naturally present in milk, and somatic cell count is used as an indicator of udder health and milk quality. The role of somatic cells in dairy processes and products is ill-defined in most studies because the role of these cells combines also the concomitance of physicochemical modifications of milk, bacterial count, and the udder inflammation in the presence of high somatic cell count. The aim of this review is to focus on the role of somatic cells themselves and of endogenous enzymes from somatic cells in milk, in dairy transformation processes, and in characteristics of final products overcoming biases due to other factors. The immune function of somatic cells in the udder defense and their protective role in milk will be primarily considered. Different characteristics of milk induced by various somatic cell counts, types, and their endogenous enzymes influencing directly the technological properties of milk and the final quality of dairy products will be discussed as well. By comparing methods used in other studies and eliminating biases due to other factors not considered in these studies, a new approach has been suggested to evaluate the effective role of somatic cells on dairy processes and products. In addition, this new approach allows the characterization of somatic cells and their endogenous enzymes and, in future research, will allow the clarification of mechanisms involved in the release of these components from somatic cells during dairy processes, particularly in cheese technologies.

Effect of high-pressure treatment at various temperatures on indigenous proteolytic enzymes and whey protein denaturation in bovine milk

The objective of the present study was to determine the effect of high pressure (HP) processing (200, 450 and 650 MPa) at various temperatures (20, 40 and 55°C) on the total plasmin plus plasminogen-derived activity (PL), plasminogen activator(s) (PA) and cathepsin D activities and on denaturation of major whey proteins in bovine milk. Data indicated that transfer of both PL and PA from the casein micelles to milk serum occurred at all pressures utilized at room temperature (20°C). In addition to the transfer of PL and PA from micelles, there were reductions in activities of PL (16–18%) and PA (38–62%) for the pressures 450 and 650 MPa, at room temperature. There were synergistic negative effects between pressure and temperature on residual PL activity at 450 and 650 MPa and on residual PA activity only at 450 MPa. Cathepsin D activity in the acid whey from HP-treated milk was in general baroresistant at room temperature. The residual activity of cathepsin D decreased significantly at 650 MPa and 40°C and at the pressures 450 and 650 MPa at 55°C. Synergistic negative effects on the amount of native β-lactoglobulin were observed at 450 and 650 MPa and on the amount of native α-lactalbumin at 650 MPa. There were significant correlations between enzymatic activities (PL, PA and cathepsin D) and the residual native β-lactoglobulin and α-lactalbumin in bovine milk. In conclusion, HP significantly affected the activity of indigenous proteolytic enzymes and whey protein denaturation in bovine milk. Reduction in activity of indigenous enzymes (PL, PA and cathepsin D) and transfer of PL and PA from the casein to milk serum induced by HP is expected to have a profound effect on cheese yield, proteolysis during cheese ripening and quality of UHT milk during storage.

Exploitation of endogenous protease activity in raw mastitic milk by MALDI-TOF/TOF

The proteic profiling of bovine milk produced by cows with subclinical mastitis was obtained by MALDI mass spectrometry. A simple procedure of chemical fractionation of raw milk was developed, whereby less complex fractions of proteins were obtained prior to mass spectrometric and SDS-PAGE analysis. The profiles of milk proteins thus obtained could allow the identification of either early markers of the acute phase of mastitis or endogenous peptide of innate immune response. The activity of the endogenous proteases in raw milk produced from each quarter of healthy and mastic cows was therefore assayed over 24-, 48-, 96-, and 216-h incubation at 37 degrees C at both physiological and acid pH. Sequence-specific peptides were identified for each fraction by MS/MS experiments, and all tandem mass spectra were evaluated using MASCOT database searching. The results show a specific proteolytic activity of endogenous enzyme toward beta-casein precursor (P02666), alpha-S2-casein (P02663), alpha-S1-casein (P02662), and kappa-casein (P02668).

Influence of residual milk-clotting enzyme on alpha(s1) casein hydrolysis during ripening of Reggianito Argentino cheese

Milk-clotting enzyme is considered largely denatured after the cooking step in hard cheeses. Nevertheless, typical hydrolysis products derived from rennet action on alpha(s1)-casein have been detected during the ripening of hard cheeses. The aim of the present work was to investigate the influence of residual milk-clotting enzyme on alpha(s1)-casein hydrolysis in Reggianito cheeses. For that purpose, we studied the influence of cooking temperature (45, 52, and 60 degrees C) on milk-clotting enzyme residual activity and alpha(s1)-casein hydrolysis during ripening. Milk-clotting enzyme residual activity in cheeses was assessed using a chromatographic method, and the hydrolysis of alpha(s1)-casein was determined by electrophoresis and high performance liquid chromatography. Milk-clotting enzyme activity was very low or undetectable in 60 degrees C- and 52 degrees C-cooked cheeses at the beginning of the ripening, but it increased afterwards, particularly in 52 degrees C-cooked cheeses. Cheese curds that were cooked at 45 degrees C had higher initial milk clotting activity, but also in this case, there was a later increase. Hydrolysis of alpha(s1)-casein was detected early in cheeses made at 45 degrees C, and later in those made at higher temperatures. The peptide alpha(s1)-I was not detected in 60 degrees C-cooked cheeses. The results suggest that residual milk-clotting enzyme can contribute to proteolysis during ripening of hard cheeses, because it probably renatures partially after the cooking step. Consequently, the production of peptides derived from alpha(s1)-casein in hard cheeses may be at least, partially due to this proteolytic agent.