Proteolysis of goat casein by calf rennet

Changes in Milk Protein Functionality at Low Temperatures and Rennet Concentrations

This study aimed to evaluate the influence of low-concentration rennet on the chemical, rheological characteristics, and protein fractions of skim milk (SM) at 4 ± 1 °C. Skimmed milk (SM) was divided into four lots of 500 mL, and diluted rennet (1:10,000) was added at different levels at 4 ± 1 °C. The treatments included control (no rennet), T1 (0.001 mL/rennet), T2 (0.01 mL rennet), and T3 (0.1 mL rennet) treatments, which were incubated for 24 h. The sampling was performed at 0, 1, 2, 6, 12, and 24 h, and the SM after incubation time was heated to 73 °C/16 s to denature the rennet enzyme. Skim milk samples (SMS) (control and rennet-added samples) were evaluated for proximate composition, capillary gel electrophoresis (CGE), hydrodynamic diameter, zeta potential, and rheology at 0, 1, 2, 6, 12, and 24 h. Foaming ability, foaming stability, water-holding capacity (WHC), oil emulsifying activity (OEA), and emulsion stability (ES) were performed at 0, 12, and 24 h of incubation time. There was a significant (p < 0.05) increase in non-proteins by 0.50% and in non-casein nitrogen by 0.81% as incubation progressed. The results showed that aggregation or curd was not formed during storage time. The CGE data indicated that increasing the rennet concentration had a significant (p < 0.05) effect on decreasing κ-CN, and breakdown increased at higher levels of rennet usage. There was a significant (p < 0.05) increase in the hydrodynamic diameter and a decrease in the zeta potential values in rennet-added samples at the end of the incubation time (24 h). The rheological results showed no changes in the storage modulus (G'), loss modulus (G″), or viscosity values. Increasing the rennet amount and storage time led to a significant (p < 0.05) decrease in the foaming ability and foaming stability and a significant (p < 0.05) increase in the oil emulsifying activity and emulsion stability of rennet-added SMS. This study concluded that milk protein functionality can be changed without aggregating or curd formation, and rennet milk can be processed.

Allergen Risk Assessment for Specific Allergy to Small Ruminant's Milk: Development of Sensitive Immunoassays to Detect Goat's and Sheep's Milk Contaminations in Dairy Food Matrices

Despite a high level of sequence identity between cow's, goat's, and sheep's milk (CM, GM, and SM, respectively) proteins, some patients tolerant to CM are allergic to GM and SM. In most cases, this specificity is due to the presence of IgE antibodies that bind only to caprine and ovine caseins. The patients may then develop severe allergic reactions after ingestion of CM products contaminated with low amounts of GM or SM. We thus aimed to develop an assay able to detect traces of caprine/ovine β-caseins in different food matrices, irrespective of the presence of the bovine homolog. We produced monoclonal antibodies (mAbs) specific to caprine caseins in mice tolerized to the bovine whole casein then sensitized to the caprine whole casein. In order to develop a two-site immunometric assay, we selected mAbs that could discriminate the caprine β-casein from its bovine homolog. Characteristics and performances of two tests were determined with various dairy products. Results were analyzed in relation with the IgE-immunoreactivity of the food matrices, thanks to sera from CM, GM/SM allergic patients. Our two-site immunometric assays demonstrated a high sensitivity with a detection limit of 1.6–3.2 ng/mL of caprine and ovine β-caseins. The tests were able to detect contaminations of GM in CM at the ppm level. Heat-treatment, ripening and coagulation processes, usually applied to dairy products that exhibit a very high IgE-immunoreactivity, did not impair the test sensitivity. These quantitative assays could then be useful for the risk assessment of food products potentially contaminated with GM and SM in order to prevent adverse reactions in patients specifically allergic to these milks.

Principal component analysis of proteolytic profiles as markers of authenticity of PDO cheeses

The casein fraction of 13 Portuguese PDO cheeses were analysed using Urea-PAGE and reverse phase-high performance liquid chromatography (RP-HPLC) and then subjected to chemometric evaluation. The chemometric techniques of cluster analysis (CA) and principal component analysis (PCA) were used for the classification studies. Peptide mapping using Urea-PAGE followed by CA revealed two major clusters according to the similarity of the proteolytic profile of the cheeses. PCA results were in accordance with the grouping performed using CA. CA of RP-HPLC results of the matured cheeses revealed the presence of one major cluster comprising samples manufactured with only ovine milk or milk admixtures. When the results of CA technique were compared with the two PCA approaches performed, it was found that the grouping of the samples was similar. Both approaches, revealed the potential of proteolytic profiles (which is an essential aspect of cheese maturation) as markers of authenticity of PDO cheeses in terms of ripening time and milk admixtures not mentioned on the label.

Effect of Frozen Storage on the Proteolytic and Rheological Properties of Soft Caprine Milk Cheese

Freezing and long-term frozen storage had minimal impact on the rheology and proteolysis of soft cheese made from caprine milk. Plain soft cheeses were obtained from a grade A goat dairy in Georgia and received 4 storage treatments: fresh refrigerated control (C), aged at 4 degrees C for 28 d; frozen control (FC), stored at -20 degrees C for 2 d before being thawed and aged in the same way as C cheese; and 3-mo frozen (3MF), or 6-mo frozen (6MF), stored at -20 degrees C for 3 or 6 mo before being thawed and aged. Soft cheeses had fragile textures that showed minimal change after freezing or over 28 d of aging at 4 degrees C. The only exceptions were the FC cheeses, which, after frozen storage and aging for 1 d at 4 degrees C, were significantly softer than the other cheeses, and less chewy than the other frozen cheeses. Moreover, after 28 d of aging at 4 degrees C, the FC cheeses tended to have the lowest viscoelastic values. Slight variation was noted in protein distribution among the storage treatment, although no significant proteolysis occurred during refrigerated aging. The creation and removal of ice crystals in the cheese matrix and the limited proteolysis of the caseins showed only slight impact on cheese texture, suggesting that frozen storage of soft cheeses may be possible for year-round supply with minimal loss of textural quality.

Rheological and proteolytic properties of Monterey Jack goat's milk cheese during aging

To enhance the understanding of the quality traits of goat's milk cheeses, rheological and proteolytic properties of Monterey Jack goat's milk cheese were evaluated during 26 weeks of 4 degrees C storage. As expected with aging, beta-casein levels decreased with concomitant increases in peptide levels and were correlated with changes in rheological properties of the cheese. Hydrolysis of the protein matrix resulted in more flexible (increased viscoelastic properties) and softer (decreased hardness, shear stress, and shear rigidity) cheeses. During the first 4-8 weeks of storage, cheese texture changed significantly (P < 0.05) and then stabilized. Characterization of rheological and proteolytic properties of the goat's milk semihard cheese during aging provided insight into the changes occurring in the protein matrix, the relationship to structure, and a shift in cheese quality.