Casein enzymatic modification

It is essential to consider the physico-chemical properties of each WPC and casein product in order to effectively evaluate their emulsification properties. Otherwise, results merely indicate the previous processing conditions rather than the inherent functional properties for these various products. Those processing treatments that promote protein denaturatlon, protein-protein Interaction via disulfide interchange, enzymatic modification and other basic alterations in the physico-chemical properties of the proteins will often result in protein products with unsatisfactory emulsification properties, since they would lack the ability to unfold at the emulsion interface and thus would be unable to function. It is recommended that those factors normally considered for production of protein products to be used in foam formation and foam stabilization be considered also, since both phenomena possess similar physico-chemical and functionality requirements (30,31). 

Although whey protein concentrates possess excellent nutritional and organoleptic properties, they often exhibit only partial solubility and do not function as well as the caseinates for stabilizing aqueous foams and emulsions (19). A number of compositional and processing factors are involved which alter the ability of whey protein concentrates to function in such food formulations. These include pH, redox potential, Ca concentration, heat denaturation, enzymatic modification, residual polyphosphate or other polyvalent ion precipitating agents, residual milk lipids/phospholipids and chemical emulsifiers (22). 

The location of methionine incorporation into peptide chains by enzymatic modification was also investigated using L-methionine-S-methyl14C methyl ester hydrochloride and L-3Hmethionine ethyl ester hydrochloride [104]. Substrates of the enzymatic modification used were a tryptic hydrolysate of serum albumin and an a-chymotryptic hydrolysate of casein, a-chymotrypsin was used as catalyst during the EPM reactions. Part of the L-methionine-S-methyl 14C methyl ester was incorporated as Met into peptide chains. A maximum curve... 

Figure 7 Allergenic character of products obtained from cow s milk proteins by food processing or enzymatic modifications. (1) Cow s milk (2) Na-caseinate (3) kefir (4) yogurt (5) cheese (6,7,8) a-chymotryptic, tryptic, and peptic hydrolysates of casein, respectively (9,10,11) a-chymotryptic tryptic, and peptic EPM products of casein, respectively (12,13) a-chymotryptic and tryptic EPM products of casein, respectively, with methionine enrichment (14,15) fractions of a-chymotryptic EPM products of casein (16,17) fractions of peptic EPM products of casein (18) commercial hypoallergenic formula.

Prodnction of yogurt and cheese involves destabilization of the milk emulsion with the assistance of microbial activity. In yogurt, the milk is coagulated and sonred by lactic acid produced by bacteria. Cheese making starts with an enzymatic modification of the casein micelles allowing them to co-precipitate with fat droplets, thus forming the cheese curd. 

It is beyond the scope of this chapter to analyze in detail the various surface interactions and forces that proteins can provide. The number of amphiphilic proteins in the world of proteins is limited, which means that the proteins in use are mainly caseins, whey proteins, P-lactoglobulins (BLGs), egg albumin, bovine semm albumin (BSA), lysozyme, and soy proteins. All other plant proteins have very limited ability to strongly adsorb onto interfaces and reduce interfacial tension to only a minor extent. However, chemical and enzymatic modifications will improve the performance of these proteins (pea, cotton, and cereal proteins), and as a result a few modified proteins can be found in the marketplace that have relatively improved performance. 

Soy protein is a low-cost food protein with good nutritional value, but its uses in foods are limited because of inferior functional properties as compared to those of commonly used animal proteins such as casein and albumin (1.2). Therefore, modifications are often required to make soy protein more suitable for food use. Improved functional properties, particularly in the pH range of 3 to 7 where most food systems belong, have been achieved by non-enzymatic methods, including succinylation (3-5), deamidation (6.7), and phosphorylation (8.9). 

Enzymatic coagulation of milk. The enzymatic coagulation of milk involves modification of the casein micelles via limited proteolysis by selected proteinases, called rennets, followed by calcium-induced aggregation of the rennet-altered micelles ... 

Proteolytic activity was assayed as described by Kembhavi et al. [9], with modification. The reaction mixture was made up of 0.4 mL of casein (Sigma) 0.5% (w/v) in distilled water and 0.4 mL 0.2 M acetate buffer, pH 5.0, to whieh 0.2 mL of the crude enzyme solution was added. The reaction was carried out at 60°C and stopped after 30 min with 1 mL of 10% trichloroacetic acid (TCA). Test tubes were centrifuged at 5,000 rpm/5 min, and the absorbance of the supernatant was measured at 280 nm. An appropriate control was prepared in which the TCA was added before the enzymatic solution. One unit of enzyme activity (U) was arbitrarily defined as the amount of enzyme required to cause an increase of 0.01 in absorbance at 280 nm under the assay conditions. 

Besides being important building blocks in the construction of proteins, some peptides possess their own biological activity. Milk, in particular, is a source of many biologically active peptides. The enzymatic hydrolysis of the milk protein casein releases opioid peptides, which have pharmacological activities such as analgesia and sleep-inducing effects. Other peptides derived from casein are involved in calcium flow in tissues and modification of the immune system response. Other milk peptides... 

The casein hydrolysis was determined essentially according to Berg-meyer, with minor modifications to overcome problems encountered with the insoluble conjugates. The absorbance of the solution or the supernatant at 280 nm was plotted against the enzyme weight to evaluate the enzymatic activity. The hydrolytic activity of papain was determined in a similar manner, except that the assay medium contained 2mH EDTA and SmH cystein. 

Phosphoamino acids that are part of proteins known to bind metal ions are posttranslational modifications introduced by specific protein kinases (Meggio et al, 1981 Vogel and Biidger, 1982c). The bovine milk protein casein and the hen egg-white protein ovalbumin, as well as possibly the human saliva acidic proline-iich proteins share sequence homology of their phosphorylated sites. Dephosphorylation of such sites by enzymatic phosphatase treatment usually reduces the affinity of such proteins for metal-ion binding (Bennick et al., 1981). Hence it is likely that dianionic phosphoryl moieties are directly involved in the complexation of metal ions. This seems particularly important for the two polyelectrolyte proteins that contain large amounts of phosphoserine residues, phosvitin purified from egg yolk (Ta-borsky, 1974), and the phosphoprotein purified from dentine (Linde et al, 1980). 

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