Or the effect of haki fused with his fire, since the lightning effects appeared when he talked about his haki
Haki is depicted with black lighting
this is my take:
THE SCIENCE BEHIND SANJI'S NEW IFRIT JAMBE AND WHY IT PRODUCES LIGHTNING STREAKS.
Glossary :
electric current : quantity of any charge that flows or moves through any point in a conductive medium
electrical discharge :when electric current flows through a gaseous medium due to ionization of the gas.
__
this is the process..
.
Corona discharges, which are caused by an electrical discharge produced by the ionization of the surrounding atmosphere ...
that and given that sanji says that he reinforce his leg to substantially increase his flame strength.
what i hypothesize is going on is that.. sanji's leg is so hot that its rapidly converting air molecules into ions (charged either positive or negative) which makes it possible for electric current to exist in the air around Frit jambe.. (aka ionization of the atmosphere around his heated leg)
Hence enabling producing the huge white/purple Lightning (which is an electric current) strikes rapidly which is what corona discharge is and, are no different than big mom's Indra ( design wise).