Food loss encompasses the total quantity of food produced that is not ultimately utilized or is discarded throughout various stages of the production process, whereas food waste refers specifically to the share of this loss consisting of food intended for human consumption that is discarded [1].

According to United Nations Environment Programme (2024), approximately 13.2% of food is lost along the supply chain, while an additional 19% is wasted at the retail, food service, and household levels [2].

Food waste represents a major global challenge with significant environmental, economic, and social implications. Its strong connection to natural resource use, environmental sustainability, and food security highlights the urgent need for more efficient food systems and improved waste management practices [3].

In this context, fruit by-products, particularly peels, represent valuable functional ingredients due to their high content of dietary fiber and bioactive compounds. These compounds, including polyphenols and carotenoids, are associated with health benefits and a reduced risk of chronic diseases, highlighting the potential of peels in functional food development [4][5].

Therefore, this review focuses on the valorization of fruit peels as functional ingredients in bakery products, including biscuits, muffins, crackers and gluten-free formulations.

In many countries around the world, increasing attention is being given to improving the nutritional quality of food and promoting healthier eating habits, which has become an important part of public health policies. This growing focus on health preservation and improvement has led to a noticeable expansion of the market for food products that are promoted as beneficial to health or linked to the concept of overall well-being [6].

In this context, the concept of functional foods has gained increasing attention as scientific research continues to improve our understanding of the link between diet and human health [7].

The Functional Food Center (FFC) describes functional foods as a broad category that includes natural foods, processed products, and foods enriched with bioactive compounds. For a product to be classified as functional, these components must be present in safe and effective amounts, and their beneficial effects on human health should be supported by clinical evidence, often verified using specific biomarkers. The concept of functional foods therefore goes beyond simply satisfying hunger, as such foods are intended to provide essential nutrients and biologically active substances that may support health maintenance and contribute to physiological functions beyond basic nutrition [8].

There has been increasing interest in foods rich in dietary fiber and antioxidants. As a result, the market for products and ingredients containing these compounds has expanded significantly. The intake of dietary fiber and phytochemicals, such as polyphenols, carotenoids, tocopherols and ascorbic acid, has been associated with beneficial health outcomes and a lower risk of certain chronic diseases in observational and experimental studies. Fruit by-products are important sources of these bioactive compounds. They contain phenolic substances such as anthocyanins and flavonoids, as well as carotenoids and other phytochemicals. These compounds exhibit antioxidant and bioactive properties that have been investigated for their potential role in reducing oxidative stress and supporting health. Some studies have suggested possible associations with reduced risk of certain chronic and neurodegenerative diseases. However, direct clinical evidence remains limited [5][9]. For this reason, fruit by-products have considerable potential for use in the enrichment of existing food products, as well as in the development of new food formulations [10]. Fruit processing by-products represent a valuable source of bioactive compounds and offer significant potential for the development of low-cost functional foods [11]. In the context of this review, fruit peels are considered functional ingredients because they provide dietary fiber, phenolic compounds and other bioactive constituents that can improve the nutritional and technological properties of bakery products. However, the presence of these compounds could not be interpreted as direct evidence of specific health benefits, which require confirmation of well-designed human clinical studies.

Although rich in valuable compounds, banana peels and other plant residues are traditionally discarded or composted in most parts of the world, except in some producing regions in Asia and Latin America. However, banana peel is a valuable source of bioactive compounds, including anthocyanins (delphinidin), cyanidins, β-carotene, gallocatechin, and vitamins A, C, and E [12]. Additionally, it contains high levels of dietary fiber, particularly hemicellulose and pectin polysaccharides [13]. According to Borges et al. [14], banana peel contains higher levels of phenolic compounds and minerals than the pulp. Such a composition likely contributes to its reported bioactive properties, supporting its traditional use in wound healing (especially burns) as well as in the prevention or mitigation of certain conditions, including depression [12].

Similarly, citrus peels represent the primary by-product of citrus fruit processing and are rich in dietary fibers (cellulose, hemicellulose, and pectin), as well as bioactive compounds such as phenolics, flavonoids, carotenoids, and vitamin C [12]. In line with this, Yaqoob et al. [15] reported that kinnow peel contains a diverse range of bioactive compounds, including anthocyanins, limonoids, phenolic acids, flavones, flavan-3-ols, flavanones. Other studies further highlight citrus peels as promising sources of pectin. β-carotene, naringin, hesperidin, neohesperidin. Overall, citrus peels have attracted considerable attention due to their high content of essential oils, primarily composed of volatile compounds, particularly oxygenated derivatives, including aldehydes (e.g., citral), alcohols, and esters, as well as terpenes (e.g., limonene) [16][17]. For instance, tangerines grown in Madeira are characterized by a particularly intense and distinctive aroma, mainly attributed to high levels of volatile bioactive compounds in their peels, such as thymol and dimethyl anthranilate [18].

Passion fruit peel accounts for approximately 52% of the total fruit weight and is characterized by a high content of fiber [19], pectin [20], and phenolic compounds, predominantly isoorientin, along with lower levels of other flavonoids such as vicenin, isovitexin, vitexin, orientin, and schaftoside. It has also been identified as an excellent source of essential minerals, particularly potassium (K) and calcium (Ca) [21]. The incorporation of orange passion fruit peel flour into food products (e.g., bread, cakes, biscuits, and cereal bars) has been shown to enhance their nutritional value by increasing the content of dietary fiber, minerals, and bioactive compounds [22].

Mango peel has attracted considerable attention due to its high content of bioactive compounds, including polyphenols, carotenoids, enzymes, and dietary fiber, highlighting its potential for value-added food applications. Nevertheless, it is predominantly produced as by-product of mango processing (e.g., pulp and amchur production) and it remains largely underutilized, often treated as waste, which represents both an environmental burden and a missed opportunity for sustainable use. In addition to its bioactive components, mango peel is rich source of vitamins C and E, as well as minerals such as K, Ca, Mg, Fe, Na Mn, Cu and Zn [23]. It additionally contains pectins and anthocyanins [24] and has been shown to possess higher concentration of polyphenols than mango pulp [23]. Mango peel contains a wide range of phytochemicals, including fatty and phenolic acids, mangiferin, xanthones, benzophenone derivates and procyanidins as well as compounds such as methyl gallate, penta-O-galloyl-glucoside, tetra-O-galloyl-glucoside, maclurin di-O-galloyl glucoside and isoquercitrin [25].

Pineapple is a widely appreciated tropical fruit due to its nutritional and organoleptic properties. However, the rapid growth of the pineapple market in recent years has led to the generation of large amounts of by-products, mainly peel and core, which pose economic and environmental challenges [26]. Pineapple peel contains carbohydrates, proteins, bromelain, pectin and phenolic compounds, including catechin, epicatechin, gallic acid and ferulic acid. acid [27].

The chemical components in pear peel are present at levels approximately 6 - 20 times higher than in the flesh. Among the monomeric compounds, arbutin, oleanolic acid, ursolic acid, chlorogenic acid, epicatechin, and rutin are predominant in both peel and flesh across different cultivars. Overall, these findings indicate that pear peel represents an excellent source of polyphenols and triterpenes [28].

Apple peel is generated as a by-product during the manufacture of dried apple products and contains higher levels of phenolic compounds, dietary fiber, and minerals compared to other edible parts of the fruit. Apple peel powder (APP) is therefore considered a valuable source of polyphenols and flavonoids, particularly quercetin, with demonstrated antioxidant and antiproliferative properties [29]. Moreover, its high dietary fiber content may contribute to metabolic health by slowing carbohydrate absorption and reducing postprandial blood glucose spikes, thereby offering protective effects against metabolic syndrome and type 2 diabetes [30][31].

Despite their promising nutritional and functional properties, several limitations should be considered when evaluating the use of fruit peels in bakery products. The bioavailability of many bioactive compounds after consumption remains insufficiently understood and requires further investigation.

In addition, the composition of peels may vary depending on factors such as cultivar, maturity stage, growing conditions and processing methods.

Technological treatments, including drying, milling and baking may also affect the stability and retention of bioactive compounds.

Furthermore, safety aspects such as pesticide residues, environmental contaminants and microbial quality should be carefully assessed before the large-scale use of peel-derived ingredients.

Future studies should address these limitations to support the safe and effective application of fruit peels in functional food development.

Fruit peels represent an abundant and underutilized source of dietary fiber, phenolic compounds, flavonoids, and other bioactive substances with significant potential for application in functional bakery products. Their incorporation has shown significant potential for improving nutritional value and antioxidant capacity. Studies have demonstrated successful applications of peel-derived flours and powders from prickly pear [33], banana [35], jackfruit [37], hog plum [38], and pomegranate [40] in biscuits, cookies, crackers, muffins, and bread products.

Peel incorporation can affect texture, color, hardness, and other technological properties. However, appropriate substitution levels generally preserve sensory quality and consumer acceptability. While many studies reported favorable nutritional, technological, and sensory acceptance at incorporation levels of approximately 5% - 10%, the optimal substitution level remains dependent on the characteristics of the peel and the specific bakery product formulations.

The use of fruit peels supports food waste valorization and contributes to a circular economy. Therefore, peel-derived ingredients represent a promising sustainable approach for the development of functional bakery products.

The authors thank Aspira University of Applied Science, Croatia for giving them funding for publication of paper.