Wine Chemistry and Biochemistry

5 Use of Enological Additives for Colloid and Tartrate Salt Stabilization 139

must presents a high turbidity (1000–1500 NTU). During alcoholic fermentation,

the must is bottled when the densityis around 1040–1050 g/L; the fermentation

continues in the bottle until the pressure reaches generally 6 bar, then stops sponta-

neously and the sediment is removed after 6–8 months bottle-aging. Riddling and

disgorging are all the more easy that the turbidity of the must is low. Centrifugation

after pressing or before bottling gives bad clarifications; filtration is very difficult

to achieve and expensive. For these reasons, organic and mineral fining agents

are commonly used to clarify white musts using the flotation technique (Davin

and Sahraoui 1993; D ́echaudat 1995; Ferrarini et al. 1992; Marchal et al. 2003;

Sahraoui 1991).

5.1.4.1 Principle of the Flotation

Flotation is a solid-liquid separation process used when the density of the particles is

lower than that of the liquid containing them. In a must, the greatest part of particles

have a density close to that of the must. This density can be artificially reduced
using gas bubbles that catch to insoluble particles from the grapeberry (Fig. 5.2a,b).

When air bubbles are directly injected into the must to improve the separation of the

particles, the process is called assisted flotation (Fig. 5.2a,ba). When air bubbles are

injected into a must treated with a fining agent, the association between air bubbles

and must floculates give insoluble complexes with very low density compared with

that of the liquid. Then, these complexes reach easily the surface of the tank; the

process is called induced flotation (Fig. 5.2a,bb). When the diameter of air bubbles

is between 40 m and 70 m, the process is called dissolved-air flotation. The speed

at which the gas-particle complexes rise through the liquid to reach the liquid/air

surface largely depends on the diameter of microbubbles, the mass of particles, the

Air micro-

bubble

insoluble particle

from the grape

ρ particle = ρ must

Floculating agent

A

B

ρ aggregate < ρ must

ρ aggregate << ρmust

superficial Adhesion

of micro-bubbles

Micro-bubbles

imprisonned in an

aggregate

Fig. 5.2a,bPrinciple of the flotation