Watermelon was purchased from the local fruit and vegetable market in Riyadh City, Saudi Arabia. The samples were rinsed several times with tap water to remove dust and dirt. They were then cut into small pieces using a stainless-steel knife and dried at 50°C until reaching a constant weight (24 h). The dried pieces were milled to pass through a 40 mm sieve, sealed in an air-tight container, and stored at -20°C for further use. Other ingredients for making biscuits, including ammonium bicarbonate, buttermilk, egg, skimmed milk powder, sodium bicarbonate, sugar, and vanilla, were obtained from the local market. High-quality chemicals were procured from Sigma-Aldrich (Sigma, MO, USA).

The standard procedure for preparing semi-hard biscuits (peti-pier type, Al-Kuraieef, 2021) was used to make biscuits using a composite flour of wheat and watermelon at different levels of watermelon substitution (0, 10, 15, and 20%). The concentration of watermelon flour was selected based on preliminary pilot-scale baking trials, and the ingredients listed in Table 1 were used for the formulation of the developed biscuits. The biscuit dough was prepared according to AACC Standard Method 10-54 (AACC International, 2001) for creaming, mixing, and cutting the dough into biscuits of various sizes (5 mm thickness × 20 mm diameter). The biscuits were baked at 180°C for 12–15 minutes until they reached an acceptable appearance (Figure 1).

The proximate composition of watermelon flour, wheat flour, and biscuits made from the wheat–watermelon composite flour was determined using official standard methods. The AOAC (1995) method numbers 977.11 (oven drying), 923.03 (dry ashing), 955.04 (Kjeldahl), 960.39 (Soxhlet), and 991.43 (gravimetric) were used to determine the moisture, ash, protein, fat, and fiber contents, respectively, of watermelon flour, wheat flour, and biscuit samples in triplicate analyses.

Carbohydrate content (%) was calculated using BeMiller and Low’s (1998) difference method as follows: Carbohydrate (%) = 100% – [ash (%) + moisture (%) + protein (%) + fat (%).

The Commission Internationale de L‘Eclairage (CIE) surface color attributes (L*, lightness; a*, redness; and b*, yellowness) of the developed biscuits were assessed using a Minolta CR-400 colorimeter (Konica, Tokyo, Japan). After calibrating the colorimeter with a white plate, triplicate analyses were conducted for each biscuit sample.

The atomic absorption method, as described by Shahidi et al. (1999), was used to determine minerals (namely calcium, iron, sodium, and magnesium) in composite flour and biscuit samples. Briefly, the samples were subjected to ashing at 550°C in a muffle furnace, followed by digestion using a mixture of distilled water, HCl, and HNO₃ (3:2:1 v/v), and the mineral concentrations were expressed as parts per million (ppm). Phosphorus was measured using a colorimetric method (Fila et al., 2013). After mixing 0.5 mL of digested sample with 4.0 mL of double-distilled water (ddH₂,O), 3.0 mL of 750 mM H₂,SO₄, 0.4 mL of 10% (w/v) ammonium heptamolybdate tetrahydrate [(NH₄)₆Mo₇O₂₄•4H₂O], and 0.4 mL of 2% (w/v) ascorbic acid, the mixture was allowed to stand at room temperature for 20 min before measuring absorbance at 660 nm, following the molybdenum–blue method of Murphy and Riley (1962).

The bulk density of wheat–watermelon composite flour and biscuits with different levels of watermelon in the composite flour was measured using the measuring cylinder packing method, as described by Onabanjo and Ighere (2014). In brief, an empty 10 mL measuring cylinder was initially weighed and then filled with the sample to a 10 mL volume, followed by gentle tapping on the bench to achieve a constant volume. The mass of the measuring cylinder containing the sample was then recorded. The bulk density (BD) was calculated as the mass per volume of the sample (g/mL).

The water absorption capacity (WAC) of wheat–watermelon composite flour and biscuits with different levels of watermelon in the composite flour was measured as described by Oyeyinka et al. (2014). Briefly, a graduated centrifugal tube was weighed, and 1 g of the sample was added to the tube, mixed well with 10 mL of distilled water, allowed to stand for 1 min, and then centrifuged at 5000 rpm for 30 min. After discarding the supernatant, the tube was weighed, and the mass gained was considered as the WAC of the sample, expressed as g absorbed water/g sample.

The method described by Oyeyinka et al. (2014) was used to measure the oil absorption capacity (OAC) using a graduated centrifugal tube of known weight. A 1 g sample was added to the tube, mixed with 10 mL of refined soybean oil, allowed to stand for 30 min at room temperature, and then centrifuged at 2000 rpm for 30 min. The supernatant was discarded, and the oil absorbed by the flour was weighed. The OAC was calculated as g oil absorbed/g sample.

A panel of 20 trained members was used to analyze the sensory quality attributes (flavor, color, taste, crispness, and overall acceptability) of the biscuits. The analysis was conducted under appropriate conditions in a Sensory Evaluation Laboratory. A 10-point hedonic scale was used for sensory evaluation, where 10 indicates “like very much” and 1 indicates “dislike very much” (Watts et al., 1989).

The data from triplicate samples were analyzed statistically using the Statistical Package for the Social Sciences (SPSS) 20.0 software (SPSS Inc., Chicago, IL, USA). One-way analysis of variance (ANOVA) and Duncan’s Multiple Range Test were used to determine statistical significance at p < 0.05. The results were presented as mean values of three individual replicates ± standard deviation.

The results in Table 2 show the percentages of moisture, protein, fat, ash, fiber, and carbohydrate in wheat and watermelon flour samples. Higher (P < 0.05) protein, fat, and carbohydrate contents were observed in wheat flour compared to watermelon flour, whereas watermelon flour showed higher (P < 0.05) moisture, ash, and fiber contents than wheat flour. These findings indicate a difference in the proximate composition between the raw materials used to prepare the biscuits. The variations are likely due to differences in genotypes, environmental conditions, and growing practices. Comparable levels of moisture, ash, protein, and fiber have been reported by Nisar et al. (2020). In addition, Abyssinian purple wheat flour contains 10.76% moisture, 8.5% protein, 3.03% fat, 7.3% fiber, 5.5% ash, and 64.85% carbohydrates (Kassegn, 2018), which is partially comparable to our findings. It has also been reported that melon peel from different melon types is a good source of ash, fiber, and carbohydrate content (Hussain et al., 2024), which aligns partially with the findings of this study. Furthermore, varying amounts of protein, ash, fiber, fat, and carbohydrates have been reported in the rind of different types of watermelon (Al-Sayed and Ahmed, 2013; Ashoka et al., 2021; Sadiq et al., 2022).

The results of the bulk density (BD), water absorption capacity (WAC), and oil absorption capacity (OAC) are presented in Table 3. The highest (P < 0.05) BD was observed in biscuits with 10% watermelon flour, followed by those made from wheat flour only. BD is a crucial parameter in food manufacturing, as it reflects the capacity of packaging materials and is influenced by various factors, including surface properties, particle size, geometry, solid density, starch content, composition, and the measurement method (Awuchi et al., 2019). High BD indicates the suitability of the flour for food preparations, whereas low BD indicates its suitability for complementary foods (Suresh and Samsher, 2013). The high BD of biscuits made from wheat flour only or wheat flour with 10% melon flour could be attributed to the type and composition of starch in wheat flour and the size of the flour particles (Awuchi et al., 2019). Increasing the levels of watermelon flour in the biscuit formulations concurrently (P < 0.05) increased the WAC and OAC, reaching maximum levels in biscuits containing 20% watermelon flour, followed by those containing 15% and 10% watermelon flour. The lowest (P < 0.05) WAC and OAC were observed in biscuits made using wheat flour only, without the addition of watermelon flour. The protein and starch in food affect the WAC and OAC of the final product (Bello and Oladeji, 2024); therefore, the variation in the WAC and OAC of biscuits made with composite flour of wheat and watermelon is likely due to differences in starch and protein composition between wheat and watermelon flour (Awuchi et al., 2019; Suresh and Samsher, 2013). In agreement with our results, Imoisi et al. (2020) reported that as the concentration of watermelon rind flour increases in bread formulations, the WAC and OAC also increase concurrently. In addition, the WAC and OAC of a composite flour made from cocoyam and watermelon rind flour increased concurrently with the increase in watermelon rind extract in the blend (Bello and Oladeji, 2024). The increase in WAC is generally linked to enhanced solubility and leaching of amylose and starch, as well as losses in the crystalline structure (Imoisi et al., 2020). The increase in OAC is typically associated with increased protein hydrophobicity and non-polar side chains that bind to oil molecules, thereby entrapping the oil in the flour (Imoisi et al., 2020).

The results on the color attributes of wheat-watermelon composite flour biscuits are presented in Table 4. The visual appearance of the developed biscuits is shown in Figure 1, indicating that the biscuit formulations have an acceptable appearance. Biscuits made with 100% wheat flour exhibited the highest L* (lightness) values (P < 0.05), and L* decreased significantly (P < 0.05) as the watermelon flour concentration increased, reaching its lowest point at 20% watermelon inclusion. In contrast, a* (redness) and b* (yellowness) values increased progressively with the increase in watermelon concentration, reaching the highest levels in biscuits fortified with 20% watermelon flour (P < 0.05). The lowest a* and b* values were observed in biscuits made with wheat flour only. These findings suggest that incorporating watermelon into biscuits had a marked negative effect on lightness and a positive effect on redness and yellowness. Bolaji et al. (2022) reported that the incorporation of watermelon seed flour into white bread formulations increased all the color attributes of the product. Wójcik et al. (2023) observed that watermelon seed flour increased the lightness and redness of low-carbohydrate, high-protein bread and did not affect the yellowness of the product. Al-Sayed and Ahmed (2013) stated that increasing the level of watermelon rind flour in cake formulations increased redness values while reducing the lightness and yellowness values of the crust and crumb. The incorporation of watermelon flour likely altered the product’s color both because of the flour’s natural pigments and the formation of Maillard reaction products between proteins and carbohydrates during baking (Al-Sayed and Ahmed, 2013; Wani et al., 2012).

The proximate composition of biscuits as affected by the incorporation of different levels of watermelon flour is presented in Table 5. The highest (P < 0.05) levels of moisture, ash, fiber, and fat were observed in biscuits containing 20% watermelon flour, followed by those with 15% watermelon flour, while the lowest (P < 0.05) values were recorded in biscuits made with wheat flour only. The highest levels of protein and carbohydrate were observed in biscuits made from wheat flour only, whereas the lowest levels were found in biscuits made using composite flour containing 20% watermelon flour. These findings indicate that incorporating watermelon flour into biscuit formulations concurrently increased moisture, ash, fiber, and fat, while reducing protein and carbohydrate contents. This increase is likely due to the higher levels of moisture, ash, and fiber, and lower levels of protein in watermelon flour compared to wheat flour, as shown in Table 2. Previous reports have indicated that the incorporation of watermelon seed or peel flour has varied effects on the proximate composition of the final products. Peter-Ikechukwu et al. (2018) reported that increasing the level of watermelon seed flour in biscuit formulations increased the protein, ash, fiber, and fat content, while reducing the carbohydrate content of both the blend and the biscuits. Hussain et al. (2024) reported that increasing the levels of watermelon peel flour in biscuit formulations concomitantly increased the moisture, ash, fiber, fat, and carbohydrate content, while reducing the protein content of biscuits fortified with 5% and 10% watermelon peel powder. Imoisi et al. (2020) stated that increasing the levels of watermelon rind flour in wheat bread formulations significantly affected the proximate composition, as it increased the ash, protein, and carbohydrate content while reducing the moisture and fat content of the bread. Variations in the developed food products may explain these discrepancies, including the genotypes employed, cultivation and environmental conditions, and postharvest processing methods of the fruit. Overall, the increased levels of moisture, ash, fiber, and fat in the developed watermelon-based biscuits in the current study are of high importance from both nutritional and technological perspectives. Fradinho et al. (2015) found that increased moisture retention in biscuits enriched with melon peel powder enhanced both their textural properties and shelf stability. In the developed biscuits, higher ash content indicates increased mineral levels, highlighting their nutritional significance given the vital roles minerals play in physiological processes (Hussain et al., 2024). Fiber is known for its beneficial effects on digestive system health; therefore, increased levels of fiber in biscuits incorporated with watermelon flour could have significant nutritional and health potential. It has been reported that watermelon peel flour influences the gut microbiota and leads to increased production of short-chain fatty acids with high health-promoting potential. Gómez-García et al. (2022) demonstrated that watermelon peel flour modulates gut microbiota and enhances short-chain fatty acid production. Accordingly, the elevated fat content in the biscuits developed in this study—when derived from heart-healthy unsaturated oils such as extra-virgin olive oil (rich in oleic acid, 70–85% of total fatty acids; Di Mattia et al., 2020), safflower oil (rich in linoleic acid, mean 70.66%; Matthäus et al., 2015), and flaxseed oil (rich in α-linolenic acid, 39.35–60.11%; Koçak, 2024)—could confer additional health benefits.

The mineral contents of biscuits as affected by the incorporation of watermelon flour into the biscuit formulations are presented in Table 6. The addition of watermelon flour influenced the mineral composition in various ways. Increasing the concentration of watermelon flour in the biscuit formulations concomitantly increased the levels of all minerals, except magnesium, which decreased as the amount of watermelon flour added increased (P < 0.05). The highest (P < 0.05) amounts of calcium, phosphorus, and iron were observed in biscuits made from composite flour containing ≥15% watermelon flour, whereas the lowest (P < 0.05) values were recorded in biscuits made from wheat flour only. Sodium was higher (P < 0.05) in biscuits containing watermelon flour than in those made with wheat flour only. Magnesium was highest (P < 0.05) in wheat flour biscuits, and its concentration decreased concurrently as the level of watermelon in the biscuits increased. Increasing the levels of melon peel powder in biscuits increased the quantities of calcium, magnesium, iron, potassium, and zinc in the developed products (Hussain et al., 2024), which is partially consistent with the findings of this study. In addition, the amounts of calcium, magnesium, potassium, phosphorus, and iron in wheat-watermelon flour cookies significantly increased as the levels of watermelon peel flour increased in the blend (Olaitan et al., 2017). Moreover, the incorporation of watermelon rind flour into sponge cake resulted in higher levels of calcium, phosphorus, and iron compared to the control cake without watermelon peel flour (Ashoka et al., 2023), which aligns with the findings of this study. Calcium contributes to bone health and structure, phosphorus is essential for energy production, iron supports the quality and quantity of hemoglobin and myoglobin, and sodium is required for normal bodily function and overall health (Chakrabarty et al., 2019; Mente et al., 2021; Ali, 2023). Therefore, the increase in calcium, sodium, phosphorus, and iron content in biscuits following the addition of watermelon flour indicates high health potential, highlighting the use of watermelon flour in food products to enhance their mineral content and nutritional quality.