## 1 Introduction  
In the coastal Croatian region Dalmatia, off-vine dried grapes characterized by their high sugar content are used for making the traditional naturally sweet wine Prosek. The most common technique to increase the sugar content in grapes is their exposure to sun for partial dehydration, such as in the production of naturally sweet Passito wines [1] or in Mediterranean dessert wines, such as 'Pedro Ximenez' [2]. Dehydration of the grapes by sun drying is a natural process strongly dependent on the environmental conditions; several factors can affect the quality of the grapes, such as insect attack, intense solar radiation, and the growth of fungi that can produce toxins, such as ochratoxin A [3]. Chamber-drying techniques in controlled conditions as an alternative to traditional sun drying revealed a significant impact of the dehydration rate and temperature on the modification of volatile compounds from the grapes [4; 5; 6]. Grape dehydration causes substantial water loss and a gradual increase in sugar concentration, which promotes the synthesis of metabolites that lead to the formation of the typical flavor of the sweet wines [4; 7; 8; 9; 10]. Therefore, grape dehydration is one of the most important steps in the production of naturally sweet wine Prosek that influences the production of key volatile compounds in the grape variety used.  
Several chemical classes of compounds arising from grapevine metabolism, such as terpen, norisoprenoids, benzenoids, and C6 alcohols, are responsible for the aroma and flavors of the grapes. These compounds can be found in either free volatile or odorless glycosidically bound form [11; 12]. Glycosidically bound compounds have been identified in many grape varieties as grape precursors responsible for some of the specific varietal aroma attributes of the wines because they can be hydrolyzed to volatile aroma compounds by the action of glycosidase enzymes or due to acidic conditions in grape juice/wine [8; 11; 12]. Despite the components belonging to the varietal aromas being typically found in low concentrations in wines compared to fermentation aromas, they are powerful odorants due to their very low sensory perception, especially free monoterenes, which are responsible for the floral and fruity aromas [12; 13].  
_Vitis vinifera_ grape varieties belonging to the Muscat group and other terpenol-related varieties, such as 'Riesling' or 'Gewurztraminer', are characterized by the highest concentration of terpenols at the level above the sensory threshold, namely linalool geraniol, nerol, and \(\alpha\)-terpineol, whereas non-Muscat or \"neutral\" varieties are mainly characterized by the presence of alcohols and aldehydes although terpen are present in their juice [12; 14; 15]. The biosynthesis of aroma compounds in grapes is a highly complex process that includes many pathways and chemical reactions [16; 17]. In addition, the grape metabolism is changed after harvest from aerobic to anaerobic, which impacts on the aroma compounds from a grape variety [9; 18].  
Croatian white native grapes of 'Marastina' (_Vitis vinifera_ L.) have been used for making the high-quality, sweet, and traditional Prosek wine in Dalmatia for a long time. 'Marastina' is considered to have an excellent oenological potential for drying, but no data on the grape volatile composition of this variety exist in the literature. The aim of this research was to identify and characterize free and bound volatile aroma compounds in the 'Marastina' grapes and to determine the influence of grape dehydration carried out in the greenhouse and chamber conditions on the volatiles. This is the first detailed research study on the free and glycosidically bound volatile compounds in the grape of cv. 'Marastina'. This work contributes to the understanding of the varietal grape aroma and to improving the quality of naturally sweet wine Prosek.