Microwave oven + water: dielectric heating or ion drag? When you place a water or food in a microwave oven, it heats.
Which process commits more energy to that: dielectric heating, or ion drag i.e. resistive heating?
AFAIK, in distilled water (which is a dielectric) dielectric heating is close to 100%.
What about regular (non-distilled) water? Mineral water? Sea water? Salty & wet food?
Is dielectric heating still gives more energy to the water/food then the resistive heating i.e. ion drag?
 A: So, this is an old post that I came across when I had a similar question. Here's a paper where they dissolve different amounts of ions in the water and found that the ability for the microwave oven to heat the water actually reduces as more ions are introduced. 
So from this study, you could conclude that ion drag is not source of heating in the water. 
A: Common microwaves always operate on the principle of dielectric heating; resistive heating is not a factor.  In general, microwaves are designed to be non-ionizing, meaning that the individual photons don't have enough energy to knock off electrons. A typical microwave operating at $2.45$ $\text {GHz}$ has photons with an energy $E = hf = 1.01\times10^{-5}\text {eV}$.
Typical electron orbital energies are on the order of $\text {eV}$, so this is far too small to dislodge an electron and generate a current, and thus resistive heating.  Thus, the fact that something is salty and thus has a reduced resistivity doesn't help it transmit a current here because there are no free electrons.  For metals, however, the ionization energy is much lower and ionization is possible, which is why one doesn't put tin foil in the microwave.
