**1. Introduction**

The industrial processing of fruits and vegetables leads to large quantities of byproducts, such as peels, seeds, and residual pulp, which are usually used as animal feed or/and discarded in the environment. Nowadays, enormous efforts, from both scientific and application points of view, are made to explore the functional potential of these byproducts as sources of important chemical compounds with significant added value for human and animal health. In general, these compounds are represented by polyphenols, carotenoids, and triterpenes [1], which display several biological activities, namely, antioxidant [2], anti-inflammatory [3], antimicrobial [4], and antidiabetic effects [5], among many others [6].

The genus *Actinidia* contains about 60 species, with kiwi probably being the most consumed fruit in natura due to its well-described health benefits [7]. Kiwifruits can be preserved in the fresh state for a prolonged time (for many months) without any decrease in quality, requiring controlled temperature (0 ◦C). At the industrial level, innovative kiwi products are processed and commercialized, such as juice, frozen juice, sweets, and ice creams, among other products [8]. Industrial processing of kiwifruits causes significant

**Citation:** Ilie, G.-I.; Milea, S, .-A.; Râpeanu, G.; Cîrciumaru, A.; St ˘anciuc, N. Sustainable Design of Innovative Kiwi Byproducts-Based Ingredients Containing Probiotics. *Foods* **2022**, *11*, 2334. https:// doi.org/10.3390/foods11152334

Academic Editors: Marco Poiana, Francesco Caponio and Antonio Piga

Received: 13 July 2022 Accepted: 2 August 2022 Published: 5 August 2022

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**Copyright:** © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

amounts of peels and pomace, which are still under-explored and have aroused grea<sup>t</sup> interest due to their high contents of bioactive molecules, such as phenolic compounds [9]. The development of kiwi-derived products is based on their nutritional and biological properties due to their rich contents of dietary fiber and bioactive compounds, such as vitamins (C, E, and A), phenolic compounds, and minerals [6].

Significant data from the literature have shown that one of the most recommended ways to prevent and treat gastrointestinal alterations is to alter the commensal microbiota [10–12]. It is well known that microorganisms coexist with eukaryotic cells at the mucosal surfaces of vertebrates in a complex and harmonious symbiosis [11]. However, the administration of antibiotics at a large scale causes various side effects, such as disrupting normal microflora in the human body by destroying normal gu<sup>t</sup> and genital tract bacteria [13]. These imbalances can be adjusted by probiotic supplementation in the diet, providing a load of live bacterial cells sufficient for beneficial effects on human health at both metabolic and immune levels [11]. The engineering of probiotic foods is challenging due to their low acid-bile tolerance and requirements for their non-pathogenicity, nontoxicity, ability to survive and metabolize in the gu<sup>t</sup> environment, resistance to low pH and organic acid, and potential to remain viable for a long period under storage and other conditions [14]. An optimal population of probiotic bacteria is critical for the preservation and proper functioning of the digestive system [15].

Prebiotics are defined as non-fermentable components that are transferred into the colon to be selectively used by host microorganisms [16]. Multiple benefits have been suggested for prebiotics, such as the mediation of host health (such as improved intestinal function), the regulation of glucose and lipid metabolism, immune response, bone health, and the regulation of satiety [17]. It has been suggested that prebiotics can be classified into three categories: oligosaccharides (fructooligosaccharide, xylooligosaccharide, galactooligosaccharide, isomaltose, inulin, etc.), fiber (β-glucan, pectin, cellulose, dextrin, etc.), and polyols (xylitol, mannitol, lactulose, etc.) [17]. Other compounds have been proposed as candidates for prebiotics, such as linoleic acid, polyunsaturated fatty acids, phenols, and polyphenols, such as anthocyanins [18].

Given the need to use prebiotics in the engineering of probiotic foods, different complex food matrices should be considered, including fibers, polyphenolics, and oligosaccharides. For example, black rice (*Oryza sativa* L.) flour is a valuable source of protein, fat, carbohydrates, phenols, flavonoids, and anthocyanins [19] and could be a potent candidate for complex prebiotic activity. Additionally, buckwheat is a type of underutilized pseudocereal belonging to the genus *Fagopyrum*, and it could be regarded as a potential source for food and nutritional applications [20]. According to FoodData Central [21], the proximate composition of buckwheat includes starch as the major component (~70%), followed by protein (~12%), dietary fiber (~10%), lipids (~3%), and ash (2.5%); minor components with biological significance, such as polyphenols, D-chiro-inositol, and vitamins, were also reported [22].

Therefore, our study aims to contribute to the innovative development of kiwi byproduct-based products containing probiotics. Although the kiwi phytochemical profile is already advanced in the scientific literature, based on our knowledge, no studies have explored the potential of transforming kiwi byproducts into foods and ingredients containing probiotics, consequently reducing food waste. Kiwi pomace was enhanced with two prebiotic sources, namely, black rice and buckwheat flour, and inoculated with *Lacticaseibacillus casei* (*L. casei*) 431®, whereas kiwi peels were inoculated to the same extent with *L. casei*. Four powders containing probiotics were obtained by freeze-drying and characterized for phytochemicals (anthocyanins, polyphenols, and carotenoids), antioxidant activity, color, cell viability, and the bioaccessibility of polyphenols and probiotics. The powders were tested for food applications by introducing them into a protein bar formulation. The foods were tested for phytochemicals, antioxidant activity, and cell viability to assess their potential as functional foods.
