**1. Introduction**

Global market data have shown 10.4% compound annual growth rate in gluten-free sales between 2015 and 2020 because of the incidence of celiac disease and other glutenassociated allergies [1,2]. The gluten-free market today provides a variety of foods that can be eaten safely by patients suffering from celiac disease [3]. Bread is one of the major staple foods [4,5]. In general, breads formulated with gluten-free raw materials have low nutritional properties, poor taste, and are of inferior quality. In the absence of gluten, dough presents poor rheological properties (viscoelasticity) and is unable to develop a protein network, affecting the final quality of gluten-free bread [6,7].

The replacement of gluten is one of the most challenging problems in food technology [8]. The use of alternative ingredients including starch, hydrocolloids, protein, enzymes, emulsifiers, and fibers in the preparation of gluten-free bread can improve the texture, mouthfeel, acceptability, shelf life, and nutritional properties of the products. In

**Citation:** Djeghim, F.; Bourekoua, H.; Rózyło, R.; Bie ´ ˙ nczak, A.; Tana´s, W.; Zidoune, M.N. Effect of By-Products from Selected Fruits and Vegetables on Gluten-Free Dough Rheology and Bread Properties. *Appl. Sci.* **2021**, *11*, 4605. https://doi.org/10.3390/ app11104605

Academic Editor: Silvia Mironeasa

Received: 18 April 2021 Accepted: 12 May 2021 Published: 18 May 2021

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**Copyright:** © 2021 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/).

addition, the viscoelastic properties of gluten can be imitated [9–12]. Similar effects can be expected when adding fruit and vegetable by-products to bread [13].

Some by-products from fruit and vegetable processing are appropriate sources of nutrients and functional ingredients for gluten-free products, and could be used as lowcost ingredients. Fruit and vegetable by-product processing includes pomace, peel, and seed fractions, which are good sources of functional substances. These products offer beneficial bioactive compounds such as carotenoids, enzymes, polyphenols, oils, vitamins, dietary fibers, amino acids, and proteins for gluten-free products [14–16]. Dried fruit pomace can be used in bakery products to substitute flour, sugar, or fat, increasing the amount of fiber and antioxidants and reducing the energy consumption [8]. Each fruit and vegetable contains approximately 5–50% of peel/skin by-products, which are rich in cellulose, hemicellulose, and lignin as major constituents and may also contain other functional groups of lignin, including aldehydes, ketones, alcohols, carboxyl, hydroxide, and phenols [17]. In addition, antioxidant components, flavonoids, minerals, and vitamins are also available in such by-products [18–20].

Many studies are in progress to evaluate the effects of fruit and vegetable by-products (such as orange peel, pomegranate peel, mango peel, grape peel, and potato peel) on wheat bread to improve the texture and quality of the final products [21–23], but only a few studies have been conducted on the utilization of fruit and vegetable peels for gluten-free bread products.

Thus, in this study we evaluated the rheological and gluten-free properties of bread prepared with corn and chickpea flours and enriched with two different types of pomace (from orange and apple) and four different types of peels (from tomato, pepper, prickly pear, and prickly pear seeds). These breads were compared with the non-enriched gluten-free bread and wheat bread.

#### **2. Materials and Methods**

#### *2.1. Breadmaking Materials*

Soft white wheat flour was obtained from Kenza (Constantine, Algeria), and contained 12.01% moisture, 0.52% ash, 2.08% fat, 7.9% protein content, 77.04% carbohydrates, and 0.45% fiber. Corn flour (Bio Aglut, Constantine, Algeria) had 9.96% moisture, 0.83% ash, 1.8% fat, 7.9% protein content, 76.31% carbohydrates, and 3.2% fiber. Chickpea flour (9.22% moisture, 2.67% ash, 4.35% fat, 22% protein content, 58.33% carbohydrates and 3.43% fiber) was obtained after grinding dried chickpea grains provided by CCLS (Constantine, Algeria). Salt (ENAsel, Setif, Algeria) and instant dry yeast (Saf-instant, Maisons-Alfort, France) were purchased from a local market.
