  The hard exclusive electroproduction of $\pi^0,\eta,\eta'$ mesons by the
longitudinal photons is considered. We calculate the ratios of the $\pi^0 :
\eta : \eta'$ yields for the electroproduction processes off nucleons and
deuterons based on the QCD factorization theorem, the low energy QCD chiral
dynamics, and the QCD anomaly. Thus investigation of these ratios gives a new
way to probe the physics of the spontaneous chiral symmetry breaking.
