## 1 Introduction
In particular, many studies are available in the literature about the use of flaxseed and its by-products, mainly seed cake, to improve bakery product compositional attributes, especially fatty acid profiles (\(\alpha\)-linolenic acid, n-6 to n-3 ratio, etc. ), soluble and insoluble dietary fiber (mucilage gums, cellulose and lignans, i.e., seciosolariciresinol diglucoside), plant protein and micronutrients (vitamins, i.e., tocopherols, and minerals as calcium, magnesium, and phosphorus) content [5; 31; 32; 33; 34; 35]. Namely, flax lignans, which are known for their antioxidant and healthy properties [34], are proved to be stable to baking conditions without losing their functionality [35]. Moreover, the fortification with ground flaxseed hulls at levels lower than 5% was sufficient to significantly boost the antioxidant power in breads [23]. Furthermore, although the addition of oilseeds can deeply affect the organoleptic profile as well as the technological features of the product [36], it has been demonstrated that if compared with other oilseed used for bread fortification such as chia, sunflower, sesame, etc., the incorporation of flaxseed does not significantly affect the consumption preferences [6]. Together with the scientific evidence supporting the consumption of flaxseed for its peculiar nutraceutical profile [37], the recovery of its by-products, such as the flaxseed cake obtained after the industrial oil extraction process, is a topic of growing importance. After screw-pressed oil extraction, a huge amount of bioactive compounds remains in the pressed flaxseed cake, making it a valuable by-product. Thus, flaxseed cake, can be proficiently valorized through novel food product development, in accordance with a circular economy perspective [38]. Thereafter, recently our research group investigated the effect of fortification by means of flaxseed cake in combination with the use of sourdough as a leavening agent [39]. The choice of the leavening agent actually affects a number of technological factors (e.g., costs and time of processing) as well as the bread sensory, texture, nutritional, and shelf life features [12; 40; 41; 42; 43]. Nevertheless, to the best of our knowledge, there are no studies available in the literature aimed at evaluating the effect of flaxseed cake fortification as a function of the leavening agents adopted. Thus, we investigated how and if the choice between sourdough fermentation and the use of baker's yeast could exploit the potential of flaxseed cake to fortify bread, focusing on its compositional and sensory attributes. So, in the present study, fortified bread at increasing percentages of flaxseed cake were produced using baker's yeast or sourdough fermentation to assess the effect of leavening strategy on bread fortification efficacy as well as on bread quality. Breads obtained by baker's yeast or sourdough fermentation were subjected to a compositional and sensory characterization and compared to assess the effect of fermentation on bread fortification by flaxseed cake. Finally, the best formulation as a function of both the leavening agent and the percentage of flaxseed cake used will be individuated to produce new products with nutritional advantages and precise sensory characterization.