Enzymatic Modification Processes

Rermeting is the most used enzymatic process in the dairy industry. When 

On cleavage, K-casein is split into two polypeptides with very different 

The rate of the enzymatic cleavage can proceed at temperatures as low as 4 °C, although increasing the temperature increases the rate of the reaction (Dalgleish, 1979). Another way of affecting the enzymatic process is to decrease the pH. The pH of maximum velocity is pH 6.0 (Van Hooydonk et al., 1986b). 

Heat treatment of milk above 60 °C, which promotes whey protein denaturation and its complexation with K-casein at normal milk pH (6.6), also affects renneting properties. An increase in rennet coagulation time and a decrease in gel firmness were observed with increased heat treatment of milk (Menard and Gamier, 2005). Ultra-high temperature (UHT) treated milk failed to coagulate completely but the coagulation properties were restored by threefold concentration of the UHT milk (McMahon ef al., 1993). 

Milk gels can be made by the combined action of rennet and acid. With the combined action of acid and rennet, gels can be made over a broader pH and temperature range than by acidification alone, with both the pH and rermet action influencing the resulting gel properties (Roefs et al., 1990). 

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Noncovalent forces have also been reported to play an important role in these enzyme-catalyzed reactions [50,56]. Moreover, Hofsten and Lalasidis [50] were of the opinion that covalent forces did not play a role in these reactions. Several investigators have shown that the product produced during the plastein reaction is composed of aggregates held together by hydrophobic and ionic bonds [57,58]. Others [59] emphasized an entropy-driven aggregation process. Trans-peptidation has been considered by a number of authors [46,60,61] as the mechanism of enzymatic modification processes (resynthesis, plastein reaction, EPM). That means that a great number of peptide bonds are split and new covalent bonds formed in the course of the enzymatic process. 

D. Effects of Additives in the Reaction Media on the Enzymatic Modification Process... 

Applications of peroxide formation are underrepresented in chiral synthetic chemistry, most likely owing to the limited stability of such intermediates. Lipoxygenases, as prototype biocatalysts for such reactions, display rather limited substrate specificity. However, interesting functionalizations at allylic positions of unsaturated fatty acids can be realized in high regio- and stereoselectivity, when the enzymatic oxidation is coupled to a chemical or enzymatic reduction process. While early work focused on derivatives of arachidonic acid chemical modifications to the carboxylate moiety are possible, provided that a sufficiently hydrophilic functionality remained. By means of this strategy, chiral diendiols are accessible after hydroperoxide reduction (Scheme 9.12) [103,104]. 

Some form of chemical labeling process must be used regardless of whether the final oligo conjugate is created by enzymatic or strictly chemical means. If enzymatic modification is to be done, the initial label still must be incorporated into an individual nucleoside triphosphate, which then is polymerized into an existing oligonucleotide strand (Section 1, this chapter). Fortunately, many useful modified nucleoside triphosphates are now available from commercial sources, often eliminating the need for custom derivatization of individual nucleotides. 

Classic solid phase substrates used in biotesting, such as microtiter plates, membrane filters or microscope slides, have been the first supports used for NA immobilization in array fabrication [27]. Desired attributes of any DNA array substrate include (i) chemical homogeneity (ii) thermal and chemical stability (iii) ability to control surface chemical properties such as polarity or hydrophobicity (iv) ability to be activated with a wide range of chemical functionalities (v) reproducibihty of the surface modification processes involved (vi) inert with respect to enzymatic activity especially ones involved in DNA manipulation and (vii) ultra-low intrinsic fluorescence. 

The main current potential application of lipase-catalyzed fat-modification processes is in the production of valuable confectionery fats for instance, alternative methods of obtaining cocoa-butter equivalents by converting cheap palm-oil fats and stearic acid to cocoa-butter-like fats. The reaction is executed in a water-poor medium, such as hexane, to prevent hydrolysis. At least one commercial apphcation exists. Loders Croklaan (Unilever) has an enzymatic interesterification plant in Wormerveer, the Netherlands. Many other new potential applications of lipases have been proposed of which some will certainly be economically feasible. Examples and details can be found in chapter 9 of this book. 

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). 

The earliest commercial milk protein enzymatic modification dates back to the 1940s, when the first formulas for allergenic infants were made. The aims of this process were to reduce allergenicity as well as to change the functional properties of proteins while preserving their nutritional value for clinical use. Unfortunately the hydrolysates thus obtained were characterized by bitter taste, and for mainly this reason proteolysis, as a technological process, enjoyed very little popularity. 

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 effects of pH, temperature, ionic environment, heat denatura-tion, enzymatic modification and processing are also considered in this regard. 

The enhanced solubility of all natural polyhydroxylated compounds tested in ionic liquids as compared to traditional organic solvents used for the enzymatic modification of these compounds can be highly useful for the production of great amounts of their acylated derivatives in a single-step biocatalytic process. It is noteworthy that, when the concentration of the above phenoHc substrates was near their solubility limit, the amount of their monoacylated derivatives formed in [bmim]BF4 reached values up to 30.0g/l (in the case of naringin), which were considerably higher than those reported for organic media [10, 30]. 

Enterohepatic circulation provides an example of a special case of intestinal absorption. Certain chemicals, like methyl mercury, after undergoing biotransformation in the liver, are excreted into the intestine via the bile. They then can be reabsorbed in the intestine, sometimes after enzymatic modification by intestinal bacteria. This process can markedly prolong the stay of chemicals in the body. It can be... 

A significant portion of the more water-soluble metabolites secreted in bile is ultimately excreted in the feces. Some metabolites, however, may undergo further enzymatic modification by the intestinal bacterial flora to a state of greater lipid solubility. This metabolic step facilitates reabsorption of such chemicals and extends their life in the body. The process is known as the enterohepatic cycle. 

Enzymatic modification of milk fats with lipolytic enzymes has already been mentioned above. Besides this it is possible to manufacture complex cheese flavours today also by fermentation of raw materials of cheese processing with defined microorganisms. Roquefort and other blue cheese flavours fermented by the mould Penicil-lium Roqueforti are currently in commercial production. 

The effect and action of enzymes seems to be very limited because ol the stronger conditions of alkali of mercerizing strength. Enzymatic hydrolysis is accelerated when mercerization is carried out without tension [44]. The greater accessibility and lower crystallinity of cellulose mercerized without tension is a decisive factor in the enzymatic hydrolysis process. Mercerized cotton is generally more prone to enzymatic modification than untreated cotton. 

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