Pumping-over at the time of vatting homogenizes the must and consequently enables a more precise analysis of the grape harvest. This is effective for a destemmed and/or crushed harvest.
However, in the case of whole bunches, there is very little juice, so pumping over is not possible.
For white wines, since there is only juice, it is unnecessary.
A peristaltic pump is often the most practical option. However, it is possible to use a system that uses gas to push from the bottom of the vat, for example. A few minutes are sufficient depending on the vat's volume.
At this stage, it is advisable to protect against oxygenation in most cases. The only case where it is inappropriate is when hyperoxygenation is desired for a white must in order to eliminate phenols prior to fermentation.
See the chapter on white wine vinification. Even in this case, it can be worth controlling must oxygenation, as excessive oxygenation can cause certain aromatic molecules to break down.
An alternative that is just as effective, if not more so, is to take grape samples from the sorting line throughout the sorting of the plot in order to obtain a representative batch. Measurements of the grape input are often more precise, probably because the juice from the pumping-over homogenization is filtered by the harvest bed. The juices from this pumping-over are often less acidic compared to the analysis of the grape input.
Different fractions of the juice have varying levels of acidity, mineral elements, etc. The must in the vat at the beginning of the harvest does not have the same proportions because there are still a lot of intact or only lightly crushed berries.