## 1 Introduction  
Bread is considered one of the most important foodstuffs worldwide [1]. In Portugal, whole maize flour is used to produce a traditional bread known as _broa_[2], considered one of the 50 world's best breads by CNN Travel in 2019 [3]. Over the last centuries, several traditional open-pollinated maize varieties have been developed to produce high-quality _broas_ with improved flavors and aromas that are not possible to achieve using the presently available more productive commercial maize hybrids [2, 4]. The aroma of bread plays a key role in its acceptance by consumers [1, 5, 6]. The nature of aroma is very complex [7, 8], and the contribution of an individual compound to the overall aroma depends on several factors, such as its odor description and threshold (the intensity perceived by olfaction), adsorption to the food matrices, content, and interactions with other volatiles [5; 7; 9; 10; 11]. In addition, the sensory description of a volatile may change depending on its concentration, therefore influencing, positively or negatively, the overall food aroma [12]. Only a small portion of the volatile compounds contribute to the overall aroma of bread [5; 7; 8; 10; 11; 12]. These volatiles are often referred to as \"key\" or \"character-impact\" compounds [13; 14; 11] and are generally present at trace levels [5; 15], while other compounds present in high concentrations are only scarcely perceived by the human nose [5]. Frequently, many potent aroma compounds must blend to give the integrated aroma and flavor perception of a certain food [16]. The origin of bread volatiles is difficult to determine [17] once they result from lipid oxidation, fermentation and baking reactions [1; 11]. However, at the baking stage, some of the most valuable aroma impact compounds are generated since they generally have low odor thresholds [18] and desirable sensory characteristics [7; 9; 15; 19; 20; 21]. These reactions are often quoted as nonenzymatic browning reactions [21] and include mainly the Maillard reaction and, to a lower extent, the caramelization reactions [7; 11]. Around 2 to 3% of sugars present in the dough undergo caramelization [22], giving rise to carbonyl compounds, furans, and brown-colored complex polymers [5; 7; 11; 14; 21; 22]. The more relevant Maillard reaction occurs between carbonyls (most often reducing sugars) and free amino groups of amino acids, peptides, or proteins [5; 7; 9; 18; 20; 23]. The main steps of this reaction are summarized in Figure 1. In the initial stage, free amino groups and reducing sugars condense and originate the Amadori or Heyns products [20; 24]. Then, at the intermediate stage, furans, furanones, pyrans and pyranones are formed [20; 24], following by amino acid degradation (Strecker reaction), when volatile compounds are formed without the need for sugars [25]. Ultimately, condensation and polymerization occur, leading to highly colored melanoidins and several aroma compounds, including pyrazines, pyrroles, pyridines, furans, oxazoles, thiazoles and thiophenes [7; 11; 15; 20; 21; 26]. A previous study on _proas_ has suggested that the volatiles produced at the baking stage, special pyranones, may positively contribute to the taste and aroma of this ethnical bread [27]. The volatile composition of bread has been usually characterized by HS-SPMS-GC-MS techniques (1D-GC) [27]. However, taking into account the complexity of bread aroma and the presence of very low amounts of important key volatiles [5; 7; 8; 10; 11], an analysis with a highly sensitive technique, such as comprehensive two-dimensional gas chromatography (GC\(\times\)GC) [13; 28], may allow the detection of important Maillard volatiles, which would be otherwise difficult or even impossible to identify. GC\(\times\)GC employs two independent columns to separate sample analytes, and, therefore, the separation potential is greatly enhanced. ToFMS (time-of-flight mass spectrometry) detects unambiguous identification and ensures high selectivity throughout the chromatogram [28].