## 1 Introduction  
Plant-based or non-dairy milk alternatives is a fast-growing food product category of functional and specialty beverages that are, nowadays, widely consumed across the world. The origin of the increasing popularity of these products includes cow milk allergies, lactose intolerance, calorie concern, prevalence of hypercholesterolemia as well as increasing preference for functional beverages and vegan diets. [1] Therefore, various alternatives to cow milk products have been developed. Soy and oat-based drinks were, until recently, the most popular milk alternatives. In recent years, several nut-based beverages such as peanut, almond, hazelnut, tiger nut or walnut beverages have become commercially available. Health benefits of such products include the high content of dietary fibers, phytochemicals and vitamins in combination with their relatively low caloric content. Therefore, such products are widely consumed nowadays [1; 2; 3; 4].  
The flavor of these drinks can be mainly attributed both to the nuts used for their production and to the production process that was implemented [5]. Flavor is the combination of taste and olfaction, and it is a very critical factor that can influence the consumer's acceptance of foods [6; 7]. The profile of volatile compounds is directly associated with food flavor. Ketones, aldehydes, alcohols,alkanes and terpenes are the most common chemical classes of volatiles that can be found in nuts. Moreover, pyrazines, furans and furfurals can be generated during heat treatment processes such as nut roasting [5]. Therefore, it is important to know the typical chromatographic pattern of various food products to identify changes during processing and storage and to relate the food flavor with its volatile composition [6; 7].  
Solid-phase microextraction (SPME) is a well-established technique for the determination of volatile compounds in nuts [7; 8; 9] and nut products [5; 10; 11]. This technique was developed by [PERSON] and co-workers in 1990 [12; 13]. With this sample preparation technique, the analytes are directly extracted and preconcentrated to the outer coating of a fused-silica fiber [14]. As a result, reduction of matrix interference and analyte preconcentration can take place in only one solvent-free stage followed by thermal desorption of the analytes into the Gas Chromatograph injection port [15].  
The headspace-SPME (HS-SPME) technique has been the most popular approach for volatile compounds profiling in various food matrices over previous years [6; 16; 17; 18; 19; 20]. The critical factors that affect the HS-SPME technique depend on analyte properties as well as on sample matrix and should be optimized. These factors are the fiber coating, the extraction temperature, the stirring speed, the equilibration and extraction time and the ionic strength of the solution [6; 21].  
A few research articles regarding the determination of volatile compounds of nut-based beverages are reported in the literature [5; 11]. However, these articles are focused on beverages derived by certain nuts (i.e., almond and tiger nut) and they were not applied to a wide variety of nut-derived beverages. Moreover, the quantification of volatiles of these methods are based on only one chemical compound. Thus, the aim of this research was the development, optimization and validation of a reliable analytical HS-SPME-GC-MS method for the determination of volatile components of a wide range of nut-based beverages such as almond, peanut and walnut beverages. Representative chemical compounds of most classes of volatiles were used for the first time for volatiles' quantification in nut-based milk's alternative beverages. To the best of our knowledge, the volatile profile of peanut and walnut milk alternatives beverages is examined for the first time.