128 R. Marchal and P. Jeandet
sedimentation, centrifugation and clarification with continuous alluviation filters
do not protect wines against colloidal haze. For these reasons, both organic and
inorganic fining agents are still commonly used to clarify and stabilize white wines,
thus avoiding natural haze. Fining involvesthe formation of a floccular precipitate in
wine which will absorb the natural haze-forming constituents while settling. After
a settling period, the supernatant can be withdrawn and receive a polishing filtra-
tion prior to sale. It is important thatthe settling process should be efficient in
the removal of the natural haze. More rarely, juice particles are eliminated using
a flotation technique, but the objective is the same. The fining process must also be
reasonably rapid, and the loss of saleable products in the sediment or lees should
be minimal. Finally, the clarified beverage should remain clear and neither the clar-
ification nor the fining process should have any undesirable effects, like removal of
desired flavorants or addition of undesired flavor components.
At this time, the chemistry of the fining process is poorly understood, and hence
it is not surprising that this process is often considered as an art rather than a sci-
ence. For this reason, the fining process sometimes remains inefficient. It is thus our
intention here to review the chemistry of fining and present some results of our own
investigations.
Fining is the addition to wine of a reactive or an adsorptive substance (in some
cases, two and even three fining agents are used simultaneously) to remove or
reduce the concentration of one or more undesirable constituents. Fining agents are
added for the purposes of achieving clarity, color, flavor and/or stability modification
in juices and wines. Fining agents are grouped according to their general nature
(Zoecklein 1988a):
- Earths: montmorillonite, bentonite, kaolinite (commonly named kaolin)
- Animal proteins: gelatin, isinglass, caseins
- Plant proteins: wheat gluten, soya, lupin, (garden) pea
- Wood charcoal (carbons)
- Synthetic polymers: PVPP
- Silicon dioxide (kieselsols)
- Metal chelators and enzymes (pectinases)
The most commonly used fining agents in the wine industry are “bentonites”
(mainly containing montmorillonite) and proteins associated to tannins or a mineral
agent. Gelatin has also been used in enology since the nineteenth century, but due to
the crisis of the “mad cow” disease in 1999, plant proteins have widely been experi-
mented and are now more and more used on an industrial scale. For this reason, this
chapter will essentially focus on bentonite as well as plant proteins.
Many fining agents contain an electric charge. If this charge is the opposite of
the particles in suspension, then neutralization and adsorption may occur. During
fining, small particles of suspended solids coalesce so that they form larger particles
which settle. In most cases, the fining agent adsorbs the suspended material and
exerts some clarifying action by virtue of formation of particles of high density,
thus increasing filterability.