Importance To Food Quality And Utilization In Food Processing

Heating of milk causes protein denaturation resulting in exposure of the sulfhydryl group, Cys121, in ft-lactoglobulin and other chemical reactions of the sul-fhydryl and disulfide residues of proteins yielding volatile sulfur compounds resulting in formation of a "cooked" flavor. Ultra-high-temperature (UHT) sterilized and aseptically packaged milk, typically processed at 140-150°C for 2-4 sec, initially has strong cooked flavor that slowly dissipates with storage (11). The intensity of the flavor is objectionable to most North Americans and northern Europeans who are accustomed to high-quality refrigerated pasteurized milk. UHT treatment causes extensive denaturation of the globular whey proteins and complete loss of sulfhydryl oxidase activity. Dissipation of the cooked flavor occurs concomitantly with the oxidation of sulfhydryl groups. Furthermore, sulfhydryl oxidase-catalyzed oxidation of the exposed sulfhydryls also results in elimination of cooked flavor (11-16).

A number of studies using various preparations and forms of sulfhydryl oxidase have shown that the cooked flavor of UHT milk can be immediately eliminated by treatment with the enzyme (12-16). For example, bioreactors for treatment of UHT milk have been prepared by covalent immobilization of the enzyme on succinamidopropyl porous glass beads (12-15) or by direct adsorption from whey on Spherosil QA (16). The concentration of free sulfhy-dryl groups in freshly processed UHT milk was rapidly depleted by treatment with sulfhydryl oxidase bioreac-tors operating in either a fixed-bed or a fluidized-bed configuration. Activities of the immobilized enzyme were routinely assayed using reduced glutathione (GSH) as the substrate. These data allowed a correlation to be established between the percent oxidation of GSH and the "normalized residence time'' which is the ratio of units of activity/the flow rate through the bio-reactor (Fig. 1) (12, 15, 16). The flavor of the enzyme-treated UHT milk was evaluated by a trained taste panel. Most significantly, a direct correlation was observed between the percent of the judges that

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NORMALIZED RESIDENCE TIME E/F (UNITS-MIN/L)

Figure 1 Conversion of GSH to GSSG as a function of normalized residence time. Column assays (*); recirculation assays (B); batch assays (~). (From Ref. 12.)

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NORMALIZED RESIDENCE TIME E/F (UNITS-MIN/L)

Figure 1 Conversion of GSH to GSSG as a function of normalized residence time. Column assays (*); recirculation assays (B); batch assays (~). (From Ref. 12.)

could detect the cooked flavor and the normalized residence time in the bioreactor (Fig. 2) (16). Two patents have been issued describing preparation of the enzyme and treatment of UHT milk with the enzyme for removal of cooked flavor (17, 18).

Some evidence suggests that enzymatic oxidation of heat-liberated sulfhydryl groups may provide longer flavor stability. A major defect of UHT milk that develops during the long-term storage arises from flavors derived from oxidative reactions (19). Autoxidation of sulfhydryls produce superoxide anion that can lead to other activated oxygen species that have a pro-oxidative effect on milk lipids (20). However, sulfhydryl oxidase-catalyzed oxidation of sulfhydryls does not produce superoxide anions (5). Comparison of untreated UHT milk with UHT milk treated with soluble filter-sterilized sulfhydryl oxidase after a 10-month storage at ambient temperature indicated that the untreated milk had undergone extensive browning and was undrinkable because of its strong oxidized flavors, whereas the treated milk retained its original color and the flavor was acceptable (unpublished observations).

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