Observation of delayed coagulation during egg white denaturation by pulsed holmium laser

Summary form only given. The authors report here on the process of rapid denaturation (in the hundreds of microseconds range) of egg white induced by pulsed laser irradiation with heating rates of 105 to 106 K/s. Holmium laser pulses (/spl lambda/=2.12 /spl mu/m) of 130 to 1,000 /spl mu/s were delivered via an optical fiber in a cuvette filled with raw chicken egg white. The dynamics of the induced photothermal denaturation process was observed by monitoring the increase of light scattering by time-resolved microvideography. Two distinct laser-induced processes were observed according to radiant exposure. At radiant exposure above 45 J/cm/sup 2/ the egg white is rapidly heated above the water vapor transition temperature and a cavitation bubble is formed. Coagulated egg white filaments are formed at the bubble boundary. Below 40 J/cm/sup 2/ no bubble was observed, although temperatures of up to 250/spl deg/C are expected to be reached at the tip of the fiber according to a measured absorption coefficient of 25.5 cm/sup -1/. A bullet-like zone of whitened egg-white is formed at the tip of the fiber. The onset of gelation is significantly delayed from the start of the laser. At low fluence, gelation starts even after the end of the laser irradiation. A threshold fluence value of 11 J/cm/sup 2/, which corresponds to a temperature of 90/spl deg/C, is needed to induce a detectable coagulated volume. The final size of the denatured volume appears to be primarily a function of the delivered fluence.