Hi Eduardo et al,
Most of us have probably encountered certain lots of shells, or at
least individual specimens, that were memorably malodorous.  I have
not noticed that any shells smell much if cleaned promptly after
collecting.  So, optimal handling will usually prevent odor problems.
 However, travel and vacation plans often preclude ideal handling of
collected materials; and I assume you are speaking of shells in this
category, which have hung around unrefrigerated for several days or
more before cleaning, a situation that many of us have likely
encountered.
As long as the animals remain alive, no unpleasant odors usually
develop.  Some species may survive a couple of days in a collection
bucket, if kept in a relatively cool area.  Some may survive a week
or more if refrigerated.  But sooner or later the critters are going
to expire.  In the trunk of a car on a hot summer day this will occur
sooner - much sooner!  In any case, as soon as they give up the ghost,
two processes start to occur.  First, enzymes and other substances
that were kept under control in the living molluscan cells are
suddenly released, and begin to digest the tissues from within, a
process known as autolysis.  Secondly, microorganisms (primarily
bacteria) which were also kept at bay by the various chemical
defenses of the living animal, start to proliferate, feeding on and
invading the dead tissues, a process known as putrefaction.  Both
these processes result in the breakdown of cellular components into
simpler chemicals.  The properties of a substance often differ
markedly from the properties of its component substances  (for
example, the salt we sprinkle on our food is composed of an
explosively reactive metal and a poisonous gas).  Therefore, when
non-odoriferous substances decompose, some of the resulting breakdown
products may be extraordinary malodorous.
The intensity of the odor depends on several factors, including the
volume of tissue involved, the rate and extent of decomposition, the
specific organic compounds present in the tissue, and the identity of
the microorganisms responsible.  Of these four, we have no influence
over the last two, so our odor-control efforts have to be focused on
the first two.  The first point is, I think, self-evident.  All other
factors being equal, a big snail rotting in a bucket will smell more
than a small snail rotting in a bucket.  Also, a shell with half the
soft parts left inside will smell more than an identical shell with
90% of the tissue removed.  However, the present discussion involves
those times when it not possible to remove the soft parts from the
shell promptly.  In such cases, preventing the production of
odoriferous substances means preventing autolysis and putrefaction.
Both the enzymes of autolysis and the organisms of putrefaction
require certain physical conditions in order to be active, most
notably the presence of water and a suitable temperature; therefore
both processes can be prevented or arrested by removing one of these
necessary elements.  If the tissue is thoroughly desiccated before
odorous compounds have a chance to develop, then such substances will
not develop as long as desiccation is maintained.  This method is
often used with very small gastropods.  Water is removed either by
alcohol extraction or simply by drying them in the sun or in a low
temperature oven, and the resulting specimens are virtually odorless.
 However, if such specimens are exposed to moisture, then bacteria and
fungi (as well as other organisms such as mites) may eventually break
down the tissues.  Many collectors are familiar with old specimens
containing a sawdust-like residue of slowly decomposed tissue.  Due
to the very gradual rate of decomposition, this does not produce any
noticeable odor.  Also, collectors working with old shells have
probably found specimens which "rattle" when you shake them, due to a
chunk of shrunken, "mummified" tissue which has not decomposed in
fifty years or more, because the shell was stored in dry conditions.
 
Alcohol retards autolysis and putrefaction not only by extracting
water from tissues, but also by denaturing and inactivating enzymes,
and by killing many kinds of microorganisms on contact.  However, if
I may state the obvious, it produces these effects only in tissues
that are in direct contact with it.  Unfortunately, the tissues of a
gastropod mollusc are encased in a long, impenetrable tube of hard
calcium salts (a gastropod shell is essentially a tube of gradually
increasing diameter, wound around a central core, and a 4 inch Turbo
shell, if "unwound", might be a foot in length).  Alcohol can enter
the shell only through the aperture; however, the soft parts of the
animal extend all the way to the apex.  Therefore, when a gastropod
shell more than a few centimeters in length is placed into alcohol,
the fluid usually doesn't reach the tissue in the upper whorls.
After a few weeks' storage, when you remove the shell for cleaning,
what you find is a well-preserved, rock-hard foot (alcohol hardens
muscle greatly) which may be difficult to remove from the shell,
followed by a slimy, foul smelling mass of decomposed organ tissues.
Except for very small specimens, all you can say about alcohol as a
temporary storage medium is - better than nothing!
A more reliable approach, when possible, is the use of temperature
extremes.  Boiling the shell with its enclosed soft parts, as someone
already mentioned, greatly retards the development of odors.  This is
because a temperature of 100' C (212' F) permanently deactivates
enzymes, and also kills most microorganisms.  Once boiled, shells can
be refrigerated for a couple of weeks without odor, or placed in
alcohol indefinitely without additional tissue changes taking place.
(However, the soft parts are easier to remove immediately after
boiling, while they are still warm.)
Freezing the fresh specimens is also a good method of long-term
storage for eventual cleaning.  However, this method does not
permanently denature enzymes, nor does it kill many microorganisms -
it only inactivates them while they are frozen.  In addition, the
freezing/thawing does extensive microscopic damage to the tissue
cells, and causes a certain amount of tissue breakdown in and of
itself.  As a result, such specimens, once thawed, should be cleaned
promptly, as autolysis and putrefaction can set in quickly.
Personally, I have not noticed that particular species or genera are
more prone to disagreeable odors than others.  I believe it has more
to do with methods of handling than with taxonomic relationships.
However, some groups of molluscs tend to be smelly because their
shell shape or structure makes it difficult to remove all the soft
parts.  And, it is not inconceivable that some families of molluscs
might possess specific proteins with particularly odoriferous
breakdown products, not present in other families.  There is one
other good reason to avoid tissue decomposition before cleaning -
some breakdown products of organic compounds are acidic, and may
dissolve the shell, especially the relatively thin spire, from the
inside.  Or, if they trickle out of the aperture onto a glossy
columella, they may cause permanent dull, discolored, or pitted areas
on that part of the shell.
Regards,
Paul M.