On-Line Sterilization of Cheese Whey Using Ultraviolet Radiation

Photophysics, photochemistry and energetics of UV light induced disulphide bridge disruption in apo-α-lactalbumin

Continuous 295 nm excitation of whey protein bovine apo-α-lactalbumin (apo-bLA) results in an increase of tryptophan fluorescence emission intensity, in a progressive red-shift of tryptophan fluorescence emission, and breakage of disulphide bridges (SS), yielding free thiol groups. The increase in fluorescence emission intensity upon continuous UV-excitation is correlated with the increase in concentration of free thiol groups in apo-bLA. UV-excitation and consequent SS breakage induce conformational changes on apo-bLA molecules, which after prolonged illumination display molten globule spectral features. The rate of tryptophan fluorescence emission intensity increase at 340 nm with excitation time increases with temperature in the interval 9.3-29.9°C. The temperature-dependent 340 nm emission kinetic traces were fitted by a 1st order reaction model. Native apo-bLA molecules with intact SS bonds and low tryptophan emission intensity are gradually converted upon excitation into apo-bLA molecules with disrupted SS, molten-globule-like conformation, high tryptophan emission intensity and red-shifted tryptophan emission. Experimental Ahrrenius activation energy was 21.8 ± 2.3 kJ x mol(-1). Data suggests that tryptophan photoionization from the S(1) state is the likely pathway leading to photolysis of SS in apo-bLA. Photoionization mechanism(s) of tryptophan in proteins and in solution and the activation energy of tryptophan photoionization from S(1) leading to SS disruption in proteins are discussed. The observations present in this paper raise concern regarding UV-light pasteurization of milk products. Though UV-light pasteurization is a faster and cheaper method than traditional thermal denaturation, it may also lead to loss of structure and functionality of milk proteins.

Reduced fouling and enhanced microbial inactivation during online sterilization of cheese whey using UV coil reactors in series

The effectiveness of coil UV reactor series for the online sterilization of cheese whey was compared to those of the single conventional and coil reactors at various flow rates (5-70 mL/min). The residence time varied from 168 to 12 min and from 48 to 24 min for the single and the series reactors, respectively. Hundred percent destruction efficiency could not be achieved in the single reactors whereas in the coil reactor series the destruction efficiency reached 100% at the flow rates of 35 and 40 mL/min. The rate of microbial destruction was described by polynomial equation for the single coil reactor and by exponential equations for the single conventional reactor and the coil reactor series. The temperature of the effluent decreased with the increase in flow rate in all the reactors. The maximum effluent temperatures in the single conventional reactor, single coil reactor and coil reactor series were 45.8, 46.1, and 36.4 degrees C (Deltat = 20.8, 21.1, 11.4 degrees C), respectively. The flow in all the reactors was laminar (R ( e ) = 1.39-20.10) and the Dean number was in the range of 1.09-15.41 in the coil reactors. Visual observation revealed less fouling on the UV lamps of coil reactors than on that of the conventional reactor due to the impact of Dean flow. The total operating time during which 100% destruction efficiency is achieved prior to the advent of fouling was 240 min in the coil reactor series compared to only 45 min in the conventional reactor.