is first excited by absorbing a photon of light
from its ground electronic state to one of the various vibrational
states in the excited electronic state. Collisions with other molecules
cause the excited molecule to lose vibrational energy until it reaches
the lowest vibrational state of the excited electronic state. The molecule
then drops back down to one of the various vibrational levels of
its ground electronic state, emitting a photon in the process. As molecules
can drop down into any of several vibrational levels in the
ground state, the photons emitted will have different energies and
thus frequencies. Therefore analyzing the different frequencies of
light emitted in fluorescent spectroscopy, along with their relative intensities,
makes it possible to determine the structure of the different
vibrational levels. Tryptophan is an important intrinsic fluorescent
probe that can be used to assess the nature of the tryptophan microenvironment
in muscle. Proteins that lack tryptophan can be attached
to an extrinsic fluorophore probe. For opaque samples such as meat
products, front face fluorescence is used, and since these products
contain tryptophan, this technique has been used in meat and muscle
science to investigate sample structure without using extrinsic
fluorophore probes. A recent paper has provided an overview of fluorescence
spectroscopy measurement for the quality assessment of
food systems (Karoui & Blecker, 2011). Lastly, it is worthwhile noting
that fluorescence is a selective method, as the species excited depend
on the excitation wavelength