Effects of pulsed electric fields on ovalbumin solutions and dialyzed egg white

Ovalbumin solutions (2%, pH 7.0, 200 ohm . cm) and dialyzed fresh egg white (pH 9.2, 200-250 ohm cm) were subjected to 50-400 exponential decay pulses with an electric field strength of 27-33 kV/cm. The pulse width was ca. 0.3 mu s (at a capacitance of 20 nF) or 0.9 mu s (at 80 nF), and the corresponding dissipated energy was 0.7 or 2.3 J/(pulse . mL) of solution. The sample temperature was maintained below 29 degrees C. While the four sulfhydryl groups of native ovalbumin did not react with DTNB, they became reactive immediately after pulse processing, indicating either partial protein unfolding or enhanced SH ionization. The extent of SH reactivity increased with dissipated energy, 3.7 SH groups becoming reactive after 100 or 200 pulses at 31.5 kV/cm and 80 nF. However, SH reactivity was reversible, since only 0.79 or 0.2 SH group was found to remain reactive 30 min or 8 h after pulse processing. The fourth derivatives of UV spectra of ovalbumin were determined, before and 15-30 min after pulse processing, to assess possible polarity and conformation changes in the environment of tyrosine and tryptophan. No differences were observed. Thermal gels prepared from fresh or dialyzed egg white had markedly different mechanical and water retention characteristics. Pulse processing of dialyzed egg white (200 pulses, 30 kV/cm, 80 nF) only slightly reduced its gelling properties. Thus electric pulses known to induce significant microbial inactivation did not cause notable changes in the proteins investigated.