*2.2. By-Product Materials*

The raw materials for the research were by-products of selected fruits and vegetables including oranges *(Citrus sinensis)*, apples *(Malus domestica)*, tomatoes (*Solanum lycopersicum)*, peppers (*Capsicum annum*), and prickly pears (*Opuntia ficus-indica*). These by-products were sourced from Algerian food industries.

The orange pomace (peel, a small amount of pulp and seeds) was obtained after juice extraction from N'Gaous-Conserves (SPA, Batna, Algeria).

Apple, tomato, and pepper pomace were collected from Maison Latina (Chelghoum Laid- Mila, Algeria). Apple pomace (peel, a small amount of pulp, and seeds) came from the processing of apple jam. Tomato pomace (peels and seeds) was derived from the production of tomato paste. The paprika pomace from green sweet pepper and red-hot pepper (peels and seeds) was obtained after the preparation of *Hmiss* (a local Algerian product).

The dried peel of prickly pear seeds was formed during a process of separating the prickly pear seeds from the pulp (Figure 1), which was supplied by Nopaltec (Souk Ahras, Algeria). Fresh, ripe red-yellow prickly pear fruits were harvested from private farms (Aïn beïda Ahriche, Mila, Algeria).

**Figure 1.** Seeds and seed peels of prickly pear during the processing of separation.

2.2.1. Preparation of By-Product Samples

The orange and apple by-products were dried at 40 ◦C in a ventilated dryer (MAXEI, S.A. ARRAS MAXEI, Type MC 100, Arras, France). Then, the dried pomace was manually separated from the seeds. The tomato and pepper by-products were dried at 40 ◦C in a fluid bed dryer (Retsch. TG 200, Haan, Germany). Then, the seeds were separated from the pomace by sieving (1100 μm).

The prickly pear fruits were washed several times using distilled water to remove the thorns, placed on a sieve to drain out surface water, and manually peeled with a knife. The obtained peel (46.60 ± 2.40% of peel) was cut into small pieces, and then the samples were dried in the ventilated dryer at 35 ◦C. The peel of prickly pear seeds donated by the food industry was already dried. The weight percent of each by-product fraction is shown in Table 1.


**Table 1.** The proportion of pulp, peel, and seeds in selected fruit and vegetable by-products.

\* Dried pomace. \*\* Fresh pomace.

All dried seedless by-product samples (Figure 2) including dried orange pomace (DOP), dried apple pomace (DAP), dried pepper peel (DPP), dried tomato peel (DTP), dried prickly pear peel (DPPP), and dried prickly pear seed peel (DPPSP) were milled to a particle size of 500 μm. The by-product powders were stored at room temperature (25 ± 5 ◦C) until further use.

#### 2.2.2. Physico-Chemical Analysis of By-Product Samples

#### Proximate Composition

The moisture, ash, fat, and protein contents of the by-product powders (pomace and peels) were determined by the standard AOAC method [24]. The moisture content was determined by AOAC method 926.12, the ash content by the AOAC method 942.05, and the protein content by the AOAC method 960.52. The fiber content was determined according to the Weende method [25] using a raw fiber extractor. The total carbohydrates were calculated by subtracting: 100 − (%water + %ash + % total fat + % total protein + % total fiber). The chemical composition was determined 3 times.

**Figure 2.** Dried fruit and vegetable by-product samples. DOP: dried orange pomace; DAP: dried apple pomace; DPP: dried pepper peel; DTP: dried tomato peel; DPPP: dried prickly pear peel; DPPSP: dried prickly pear seed peel.

Extraction of Pectins from By-Product Samples

Extraction was carried out on powders from dried pomace and peels. Each sample (2 g) was mixed with 40 mL of hydrochloric acid (0.1 N), heated at 90 ◦C for 45 min, and then cooled to room temperature. The insoluble material was then removed by filtration through a nylon strainer. The filtrate was dispersed in 2 volumes of ethanol 95% (*V*/*V*); pectin was precipitated overnight at room temperature away from light. The precipitate was collected by filtration through a nylon strainer, washed twice with 70% ethanol, and centrifuged (20 min; 10,000 tr/min, 10 ◦C). The supernatant was then discarded and the obtained wet pectin was dried at 65 ◦C until constant weight [26–28]. The percentage yield of pectin samples was calculated as follows [29]:

$$\text{Pectin yield} \left( \% \right) = \frac{\text{Obtained product}}{\text{Initial by product powder}} \times 100$$

Water-Holding Capacity

The water-holding capacity was calculated as the water weight which is retained by 1 g of dry by-product sample after soaking and centrifugation [30]. The water-holding capacity (WHC) of samples was determined by mixing 1 g of by-product powder with 15 mL of distilled water in a centrifuge tube. The mixture was vortexed (VELP Scientifica, ZX3) for 30 s and allowed to hydrate overnight. The suspension was then centrifuged (Sigma 3-30K, Osterode am Harz, Germany) at 15,000× *g* for 20 min. The supernatant was discarded, and WHC was expressed as gram water retention per gram dry powder [31,32].

#### *2.3. Characteriscics of the Bread Dough*

The wheat control dough was made from wheat flour and without by-products. The gluten-free control dough was made with corn flour and chickpea flour, also without by-products. The fortified gluten-free doughs were prepared by adding by-products at different levels (2.5, 5, and 7, 5% *w*/*w*). Doughs were prepared according the same process of breadmaking. Then, the properties of the dough were examined with the help of Chopin's rheofermentometer and a rheoviscosimeter.

#### 2.3.1. Rheofermentometric Analysis

A Chopin rheofermentometer (F3 Chopin, Villeneuve La Garenne Cedex, France) was used to measure the gas production and dough development parameters during the fermentation process. A piece of dough (200 g), prepared as described in the breadmaking process, was placed in the rheofermentometer basket, and the piston was placed on dough without the 2 kg of cylindrical weight for gluten-free dough [33–35]. The dough was then fermented for 90 min at 37 ◦C. The recorded parameters were: Maximum height of dough (Hm), time at a maximum height of dough (T1), maximum height of gaseous release (H'm), time of maximum gas formation (T´1), height of dough at the end of the test (h), weakening coefficient (W) = (Hm-h)/Hm, total CO2 production (Vt), volume of CO2 loss (Vl), volume of CO2 retained (Vr), and CO2 retention coefficient (R). The analyses were performed in duplicate.

Moreover, to isolate the effect of yeast activity, the adjusted maximum height (*Hmadj*) was calculated according to the method of Altuna and Ribotta [36].

$$H\_m^{adj} = \left(\frac{H\_m}{V\_t}\right) \times V\_{t0}$$

where *Vt0* is the total volume of the gas obtained from the control dough.
