Nonthermal Resonant Effects of 42 GHz Microwaves on the Growth of Yeast Cultures

Abstract

Repetition of our earlier experiment has confirmed that the growth rate of aqueous yeast cultures is affected by weak microwave radiation in a frequency-selective manner. The extensions of the experimental procedure included a refined frequency stabilization, refined power recording, impedance matching elements, two geometrically different antenna structures and two recording photometers. Laser thermometry was employed to locate any hot spots.

The result is that depending on the frequency (near 42 GHz) both increases and decreases of the growth rate occur, within resonance bands of only 8 MHz full width at half maximum. The effects are reproducible over long periods (years) and with different irradiation geometries. A rather flat intensity dependence is found, e.g. constancy of the effect at 41782 MHz for nearly an order of magnitude variation of applied power.

Microwave-induced heating was measured to amount to 0.6°C for the highest power applied, and to be spatially homogeneous within 0.02°C throughout the stirred liquid culture. This calls for a nonthermal origin of the effect. On the other hand single-quantum effects seem unlikely since the microwave photon energy is much smaller than the thermal energy kT. These findings therefore support theoretical models which suggest the existence of specialized many-quanta receivers, e.g. coherent molecular oscillations.