In this video, you can see at a winery located in Rioja Alta, how they have started the harvest of the white grapes and doing an analysis of the grape must to check the sugar and therefore the alcohol content of the soon to be wine. The sugar content is measured using a handheld refractometer. When held to the light, you can see the approximate Brix of sugar that will give a predictive amount of Alcohol once this must has undergone fermentation. This is also judged and monitored by the Consejo Regulador in order to qualify each and every batch that comes into the winery. They are there measuring the amount of grapes in kilos per hectare and then qualifying it based on the Brix level.
The grapes that mature fastest tend to be the white grapes and therefore, the harvest generally begins with the white grapes and within a few days, the red grapes depending on their maturity and location will be picked next.
These grapes will be pressed first, the first pressing, a light pressing that will release the first juice will be set aside to be made into a higher quality wine and the second pressing will be used for the the next best wine. This is common practice in most all wineries.