## 1 Introduction  
Many individuals rely on bread as a valuable source of energy and an indispensable stable in their diet. It has been claimed that bread is the basic key element of human nutrition [1; 2; 3]. Wheat bread is regarded as a basis of human nutrition in many countries. Nevertheless, because of the multiple benefits of rye, there is a range of studies focused on bread prepared with rye sourdough [4]. Bakery products of this type have longer shelf-life, higher nutritional value, and usually better organoleptic properties, including smell and taste [4; 5; 6; 7]. Utilizing sourdough enables the creation of products that possess a more robust aroma, attributed to the metabolic action of lactic acid bacteria, which, in addition to lactic acid, also produce acetic acid, succinic acid, ethyl alcohol, acetoin, diacetyl, acetone, 2,3-butylenediycl, carbon dioxide, and hydrogen. All these compounds influence the formation of the appropriate structure of the dough, control the enzymatic processes, and enhance the desired flavor and aroma of the bread [8]. The last decade has seen a global trend to use natural substances that are rich in antioxidants to fortify bread. Enriching bread with bioactive ingredients contributes to the pro-health properties of the bread. The most commonly used bioactive ingredients in the production of bread are blackcurrant extract, grape seed extract, ground onion (_Allium cepa_), ground green coffee beans, red/purple potato flour, sugar beet molasses, pomegranate peel powder, banana pseudo stem flour, yam flour, and apple pectin [3; 9; 10; 11; 12; 13; 14; 15; 16]. In recent times, there has been an increasing utilization of by-products generated from the processing of fruits and vegetables [17; 18; 19; 20; 21], because they are rich in bioactive compounds, and their recycling fits well into the zero-waste trend, occurring recently throughout the world. Among such fruit and vegetable by-products, special attention should be paid to colored potato pulp, remaining after starch processing. Such pulp contains phenolic acids (chlorogenic acid, gallic acid, ferulic acid, and protocatechuic and caffe acids), flavonoids (catechin, epicatechin, quercetin, rutin, myricetin, and kaempfer), and the anthocyanins acylated with phenolic acids mentioned above, which are more stable in comparison with anthocyanins present in colored fruit pomace [22; 23]. Pulp derived from purple potatoes contains petunidin- and malvidin-3-rutinoside-5-glucosidase acided with p-coumaric and ferulic acid, whereas pulp from red potatoes is rich in pelargonidin and peonidin-3-rutinoside-5-glycos acylated with p-coumaric and ferulic acid [24; 25]. The mentioned ingredients encompass a variety of polyphenols, which serve as antioxidants and exhibit numerous health benefits. These include regulating blood sugar levels, lowering cholesterol, preventing cancer, reducing post-meal blood sugar spikes and hypertension, possessing anti-inflammatory, antiviral, antimicrobial, and anti-allergenic properties, as well as acting as anticoagulants. Additionally, they help lower the risk of conditions such as atherosclerosis, cardiovascular diseases, diabetes, genetic damage, bone degeneration, and neurodegenerative diseases such as Alzheimer's disease [26; 27]. These properties may contribute to the health-promoting features of bread. It should be noted, however, that bread manufacture is a set of processes in which the dough (consisting of flour, water, salt, and yeast) is mixed, kneaded, proofed, and baked, and at each of these stages there may be quantitative changes and variation in the bioavailability of phenolic compounds added in the form of plant materials. During baking, an increase in temperature contributes to the formation of crosslinks in this group of compounds. Furthermore, the gelatinized starch granules and the gluten network interact, leading to a dissipation of kinetic energy and a consequent rise in firmness.