Equipment and chemicals The equipment used in this study included an electric balance (ML204, Mettler Toledo, OH, USA), a hot plate (SB162–3, Stuart, NY, USA), a vortex mixture (SA8, Camlab, Cambridge, UK), a hot-air oven dryer (MOV-112, Sanyo, Osaka. Japan), a muffle furnace (SYD-508, Westtune, Zhejiang, China), and a spectrophotometer (UV-1800 UV/ Vis, Shimadzu, Kyoto, Japan). Chemicals, including sulfuric acid, sodium hydroxide, ethanol, methanol, sodium carbonate, sodium thiosulfate, boric acid, petroleum ether, and carboxymethyl cellulose (CMC), were obtained from Merck (Merck India Ltd., Mumbai, India). Gallic acid and 2,2- diphenyl-1-picrylhydrazyl (DPPH) were purchased from Sigma–Aldrich (St. Louis, MO, USA).
Sample collection and preparation of WRP Fresh watermelons with yellow, dark green, and pale green rind were purchased from a local market in Dhaka, Bangladesh. The watermelons were thoroughly washed using tap water to remove adhered dirt particles and sand; subsequently, they were sliced, and the pulp was scraped off to obtain the watermelon rind. The watermelon rind was further sliced into thin pieces using a slicer and spread onto trays for hot-air oven drying. The drying parameters—60 °C for 24 h— were selected based on published literature to obtain WRP for product development (Al-Sayed and Ahmed 2013; Ho and Che Dahri 2016; Naknaen et al. 2016). The dehydrated rind slices were ground to a fine powder for physicochemical analyses and the preparation of value-added noodles.
Preparation of noodles: The noodles were prepared using commercial wheat flour, whole egg, salt, CMC, and water, whereas wheat flour was partially substituted by WRP at 0% (control), 10%, 15%, and 20%. Initially, wheat flour was mixed with rind powder and then salt and CMC were added to the mixed flour. After thorough mixing, a suspension of egg and water was added to prepare a dough. To obtain the desired noodle thickness, the dough was stored at room temperature for 10 min before being sheeted on a noodle machine. The wet noodles were dried at 60°C for 8 h using a hot-air oven dryer.
Proximate compositions The proximate compositions of the WRP, including moisture, ash, crude protein, and fat, were determined using standard methods described by the association of official analytical chemists (AOAC) (1995).
Total phenol content and free radical–scavenging activity Briefly, 40 mg of each dried rind powder sample was mixed with 20 mL of methanol in a conical flask and transferred to a shaking water bath at 40°C for 3 h. The solution was filtered to separate the methanol extract and solid residues. The total phenol content of the WRP was determined using the Folin–Ciocalteu method described by Alhakmani et al. (2013). The methanol extract (0.5 mL) was gently mixed with 2.5 mL of Folin–Ciocalteu reagent (diluted 10 times with water) and 2.0 mL of Na2CO3 (7.5% w/v) solution, and the mixture was then incubated for 20 min at room temperature (25°C). The absorbance was measured at 760 nm using a UV spectrophotometer with methanol as the blank. A standard curve was prepared using gallic acid solution at concentrations of 0.2 to 1 mg/L. The free radical-scavenging activity of WRP in terms of DPPH was determined according to the method described by Brand-Williams et al. (1995). The methanol extract (2.0 mL) was mixed with 3.0 mL of a DPPH solution (200 μg/mL). After a 30-min reaction period at room temperature, the absorbance was measured at 515 nm against pure methanol (blank) using a UV spectrophotometer.
Color analysis Color analysis of the WRP in terms of L*, a*, and b* values was performed using a chroma meter (Minolta, CR-300, Osaka, Japan), where L* indicates the lightness value (0 = black, 100 = white), a* the red/green value (+ value = redness, − value = greenness), and b* the yellow/blue value (+ value = yellowness, −value = blueness). The hue angle and chroma (C) were calculated as hab = tan−1 − (b/a) and C = (a2 + b2 ), respectively.