## 1 Introduction  
A diet rich in fruit and vegetables plays an important role in the prevention of oxidative stress-related diseases. Such disorders are most often caused by a poor diet consisting of processed foods, alcohol consumption, or smoking. This leads to an overproduction of free radicals in the body, resulting in accelerated ageing processes and the development of cancer, atherosclerosis, stroke, and Parkinson's or Alzheimer's disease. The likelihood of these diseases can be significantly reduced by consuming fruit containing antioxidant compounds, such as polyphenols, carotenoids, or ascorbic acid. Introducing fresh fruit into the diet may be the most effective way of supplying the aforementioned components to the body [1, 2]. However, today's lifestyle of the typical consumer makes the use of unprocessed foods quite problematic. Thus, there is a growing interest in foods that are convenient, yet healthy and minimally processed. Consumers are increasingly turning to natural juices (NFC--Not From Concentrate). The scale of their production is also increasing, which in turn results in an increasing amount of post-production waste, i.e., pomace [3, 4]. Fruit residues contain large amounts of valuable nutrients. For example, grape residues have been shown to contain 55.6 g of total dietary fibre per 100 g of dry mass and almost 13% of protein in dry mass [5]. They are also an important source of polyphenols [6]. The main phenolic constituents of grape pomace are anthocyanins, flavands, and stilbenes. The total polyphenol content ranges from 0.68 to 0.75 mg GAE/100 g of dry mass and the anthocyanin content varies from 84.4 to 131 mg/100 g of dry mass [7]. Similarly, apple pomace has high content of antioxidant substances. [PERSON] et al. (2003) [8] showed that freeze-dried apple pomace extract contained 117.6 mg/kg phenolic compounds. In addition, apple residue is a source of pectin (0.66 g per 100 g fresh weight) and dietary fibre (2.33 g per 100 g fresh weight) [9; 10]. Besides the high level of active ingredients, the fruit pomace by-product still contains a high amount of water and is a perishable material. Therefore, it must be processed to be protected from microorganism growth and prepared for storage and subsequent consumption. Properly processed fruit residues can serve as a raw material for the extraction of specific active substances [6] or can be used as an additive in various types of products. The addition of by-products to food can increase its nutritional value. However, unfavourable changes in the physical and organoleptic properties of the products may pose a problem. Numerous studies on the extraction of active ingredients from expeller or fortification of food with residues from the food industry have recently been conducted [11; 12; 13; 14; 15; 16]. So far, the research has mainly been focused on the possibility of using plant residues for the fortification of basic products, such as pasta or bread. However, there is not enough information on the possibility of using by-products to produce convenience foods, including cereal snacks. Cereal bars are an interesting alternative for the management of fruit pomace. They are valued by consumers for their simple natural composition. Among cereals, oats are the most widely used to prepare cereal bars due to the high quality and content of protein, predominance of unsaturated fatty acids, and dietary fibre composition. Oat grain also contains significant amounts of soluble phenolic compounds, \(\beta\)-glucans, minerals, and vitamins, including fat-soluble vitamin E. The high dietary value of oat grain is reflected in its suitability for the production of functional foods [17; 18]. However, the use of oats increases the water absorption of the dough and increases its softening. Additional incorporation of by-products of the fruit industry may result in a significant change in the structure and strength of the products obtained.