# Shedding Light on the Volatile Composition of _Broa_, a Traditional Portuguese Maize Bread
## 1 Introduction  
The culture of maize in Portugal started during the sixteenth century and rapidly spread across the country. For centuries, natural and human selection adapted varieties to different environments, creating a diverse maize germplasm [1]. Portuguese maize traditional open pollinated varieties (OPVs) are commonly used for the production of _broa_, a traditional Portuguese maize bread [2], which plays an important economic and social role in Central and Northern Portuguese rural communities, where it is still widely consumed [1]. This type of bread is traditionally prepared with maize flour (50-100%) and rye and/or wheat flours (0-50%) that are mixed with hot water, and leavened dough fromlate _broa_, acting as sourdough [2; 3]. After preparation, dough is baked in a wood-fired oven [3]. Due to the progressive adoption of more productive hybrids, Portuguese traditional maize varieties are at risk of disappearing [1]. Nevertheless, traditional varieties have persisted due to their better technological capacity and aroma characteristics highly valued for the production of _broa_, in addition to their better resilience to pests, diseases and abiotic stresses, qualities exploited by the Portuguese long-term participatory maize breeding VASO program [4]. In fact, previously reported sensory evaluation results of _broas_ have demonstrated a preference for traditional or participatory improved OPVs in detriment of commercial hybrid maize varieties for _broa_ production [3]. Additionally, traditional maize varieties still under production have an important role in the conservation and evolution of the available genetic maize variation, meeting the future demands for increasing yields and quality in high-stress environments in the context of climate changes [4]. Quality criteria important to consumers' acceptance of breads are related to both their rheological (texture) and organoleptic (color, volume and flavor) properties [3; 5; 6]. Flavor is often considered the most important attribute [7; 8; 9] and results, in part, from the perception of volatile compounds that can interact with olfactory receptors [6]. The volatile composition of breads depends on the ingredients and processing techniques employed, and mainly results from oxidation of lipids and carotenoids, fermentation by yeast and lactic acid bacteria (LAB) and browning reactions (Maillard and caramelization reactions) during baking [10; 11]. Aldehydes, alcohols, ketones, esters, acids, pyrazines, pyrrolines, hydrocarbons, furans and lactones [12] have been described as the main volatiles present in cereal breads, but the exact origin of each volatile is difficult to determine [13]. For instance, aldehydes, alcohols and furans may all be derived from lipid oxidation, yeast fermentation or Maillard and caramelization reactions [14; 15; 16]. In general, lipid oxidation products are associated with off-flavors [17; 18] and are produced by plants through the action of lipoxygenases in response to wounding, thus playing an important role in plants' defense strategies and signaling [7; 19]. Flavor development during bread making is influenced by lipoxygenase activity [11] and by the type of sugars and amino acids present in the flour [19]. Lipoxygenases act on unsaturated fatty acids and produce unstable peroxide derivatives that can be transformed into carbonyl compounds, such as aldehydes [11; 16], which can be enzymatically reduced to the corresponding alcohols [7] or be oxidized into acids and esters [16; 17; 20]. 4- or 5-Hydroxy carboxylic acids may further be converted to lactones [7; 17], and hydrocarbons can be formed through the oxidation of the radical to the carbonium ion and decarboxylation [21]. The activity of endogenous lipoxygenases increases during grinding of the grain, after the addition of water to the flour and during kneading [15; 16]. Additionally, lipid oxidation reactions may also occur by the action of enzymes associated with the metabolic activity of yeasts and LAB during fermentation [6].