It is well known that some wine varieties give their best in making passito wines. It is also well known that the culturing techniques, the terroir, the climate and the maturation dynamics have a central role in passito wine making.
There are conditions that can be easily controlled by the wine maker, such as the culturing techniques on the field and the enological strategies in the cellar.
As for the conditions in the vineyard, it is important to gather the grapes when they are still healthy. Damaged or moldy grapes can impair the quality of the final product during the delicate process of withering. As a matter of fact, during the withering phase it is important not to hamper the action of noble molds that contribute to the primary flavor and bouquet of wine. In order to maximize this process it is important to gather healthy grapes at the perfect maturation stage and to grant the maximal homogeneity in the water content of the grapes. Moreover it is important to be sure that the dehydration takes place evenly on all the grapes on the drying-rack.
This is the most challenging condition to be respected. As a matter of fact, in the passito production areas the work force able to choose the proper grapes directly on the field, making the grape stock homogeneous, is disappearing. In small farms is still possible to select the grapes after gathering, at the cost of wasting some grapes, but this operation is quite complex and tends to damage the grapes, that require to be handles as less as possible. Hence, it is necessary to have some rough information about the maturation process to be transferred to the gatherers in order to perform at least a very basic selection during gathering. For instance, it is important to know that the grapes located in shady zones dry slower than the others, getting to the 60% reduction in weight stage (considered as the optimal condition for passito wine making) many days later than other grapes. Apparently this is particularly true for strong vine varieties.
Besides the gathering process, the cellar processing and the enological techniques are pivotal, as well. The alcoholic fermentation has to be very slow for passito wines. It normally takes two to three weeks, under a close temperature control. It is also known, even if sometimes not clear to everybody, why some yeasts contribute more then others in providing the characteristic sweet flavor of passito wines. Typically, the yeast selection is performed empirically, starting from the comparison of previous enological results. Recently, an interesting study performed by the UniversitÓ della Tuscia and the UniversitÓ di Reggio Emilia gives some hints based on scientific evidences. Yeasts containing a protein called Hsp12p are less able to provide the wines for the characteristic sweet flavor than wild yeasts lacking the protein. This is particularly true in the stage of yeast autolysis when in contact with dregs. Moreover, the addition of glycerol and ethanol to these wines is not able to obtain a sweetness level comparable to the wine obtained by Hsp12p-lacking yeasts. This information, which will be deepen in the future, will open the possibility of a more rational choice of yeasts for the alcoholic fermentation of passito and, in general, sweet wines.