Introduction

Grapefruit (Citrus paradisi) belongs to the Rutaceae family, and it is a hybrid between two citruses such as sweet orange (Citrus sinensis) and pummelo (Citrus maxima or Citrus grandis)1,2. The global grapefruit market was valued at approximately US$7,939.7 million in 2018 and is projected to grow at a compound annual growth rate (CAGR) of 3.8% from 2019 to 20273. Grapefruits are rich in dietary fibre, vitamin C, and polyphenols, which contribute to their health-promoting properties. Industrial grapefruit processing results in wastes and byproducts that make up over 50% of the fruit, with the peel component accounting for between 35% and 41% of them4. These wastes are usually discarded, which causes harm to the environment5,6. However, commercial exploitation of grapefruit bio-waste is essential to creating valuable added economic products7. Many studies show that grapefruit peel is rich in natural active ingredients such as essential oils, flavonoids, dietary fiber, polysaccharides etc. and provide protection against diseases8,9. But grapefruit skin can be utilized as a potential and high-quality source of natural components, such as dietary fiber10. In recent year, dietary fiber attracts a great deal of attention from researchers, the food industry, and consumers, due to its health benefits11. Dietary fiber is generally classified into two types: soluble dietary fiber, which includes pectin and gums, and insoluble dietary fiber, which comprises cellulose, hemicellulose, and lignin12. These materials possess many health-beneficial properties, such as antioxidant, antitumor, hypoglycemic, anti-diabetic and immune-modulatory action13. The usable properties of dietary fiber include the hydration, hydrocolloidal and rheological properties, the bulk volume, which provide execution in grain formula design and food production. Grapefruit dietary fiber also known as wall material is primarily found in the cell wall and intracellular layers of grapefruit peel, with the wall material serving as the main structural component14. These structural polysaccharides play a critical role in food application due to its stabilizing, coagulating, combining and thickening properties15. Moreover, grapefruit polyphenols are bound to cell wall polysaccharides via hydrophobic interactions, hydrogen bonding, or covalent glycosidic linkages, enhancing functional properties16. Thus, grapefruit wall material (GFWM) utilized in the grain industry as an effective part to enhance functional and nutritional attributes, hence, to increase shelf life of the goods17. The incorporation of citrus peels and their fibers into wheat flour-based foods has been widely studied due to their health benefits18. Diet rich in dietary fibers and polyphenols are linked to a lower risk of diseases like diabetes, cardiovascular issues, obesity, hypertension, and gastrointestinal disorders19. Worldwide, there is a huge demand for bakery products. Bread, as a staple food, offers an excellent medium for functional ingredient enrichment to address insufficient dietary fiber intake in nutritional plan20. The rising need for wheat-based products fortified with effective parts, such as fibers and antioxidants, has driven the food industry to innovate bread-making technologies to enhance nutritional value, taste, and quality. While bread is rich in carbohydrates, it lacks essential nutrients like minerals and dietary fibers, emphasizing the need for enriched variants to meet nutritional needs21. The objective of this study was to extract and characterize GFWM obtained from grapefruit peels using an alkali extraction method and to evaluate its nutritional composition. Additionally, the study aimed to assess the effect of incorporating GFWM into wheat bread on its sensory, color and textural properties. It was hypothesized that the incorporation of GFWM, rich in non-starch polysaccharides and phenolic compounds, would improve the functional and organoleptic properties of bread, enhancing its texture, color, and overall acceptability while contributing to value-added utilization of grapefruit peel by-products.

Materials and methods

Procurement of raw material

Grapefruits were obtained from the local market of Faisalabad. Chemicals (sodium hydroxide, ethanol, H2SO4) were obtained from Sigma-Aldrich and Merck Pvt Ltd. The reagents were of analytical grade. The bread material was obtained from the Food Processing Hall Department of Food Science, Government College University Faisalabad, (GCUF) Pakistan. The current study was conducted in the Food Analysis Lab Department of Food Science, GCUF. All the analysis was performed in triplicates.

Sample preparation

Grapefruit peels were collected, cut into small pieces, and dried at 60℃ for 36 h22. After drying, the peels were grounded using a laboratory-scale grinder and passed through a 100-mesh sieve to obtain a uniform grapefruit peel powder23. The grapefruit peel powder was stored in vacuum-packed bags for one week and then utilized for further analysis and extraction.

Chemical composition of grapefruit peel powder

The chemical composition of grapefruit peel powder was evaluated following the methods described by AACC1, including moisture content (4415 A) determined with a hot air oven at 105 5 °C, ash content (0801) measured using a muffle furnace (DAIHAN), crude protein (4610) assessed via the Kjeldahl method, crude fat (3025) extracted with a Soxhlet apparatus, and crude fibre (3210). The nitrogen-free extract (NFE) was calculated using the following equation:

$${\text{NFE}}\% {\text{ }}={\text{ 1}}00{\text{ }} - {\text{ }}\left( {{\text{Moisture }}\% {\text{ }}+{\text{ ash}}\% {\text{ }}+{\text{ protein}}\% {\text{ }}+{\text{ fat}}\% {\text{ }}+{\text{ fiber }}\% } \right).$$

Extraction of wall material from grapefruit peel

Extraction of wall material was done by using the alkali extraction technique as described by Ye et al.24 with some amendments. 100 g of grapefruit peel powder was suspended in 500 ml of ethanol to eliminate lipophilic substances. The suspension was then passed through a Millipore filter with a pore size of 2.7 μm. The sample was then heated at 100 °C for 30 min in 1000 ml water with continuous stirring intended for enzyme inactivation and starch gelatinization. The sludge was then passed through a filter bag of 300-mesh. Moreover, the filtered sample was added to 1000 ml of NaOH (0.5 mol/L) solution and then incubated in a shaker (100 rpm, 10℃) for 2 h. NaOH loses the strength of phenolic compounds. After alkali treatment, the leftover solid materials were removed by centrifugation (9000 RPM, 4 °C, and 20 min). The supernatant was exposed to cooling at atmospheric condition before being precipitated for 12 h with 95% ethanol in a ratio of 1:3 (v/v). In order to produce dried wall material, the resulting precipitates were collected through vacuum filtration and dried for 8 h in a hot air oven at 50 °C. The yield of GFWM was calculated on a dry matter basis according to the given formula:

$$\text{Y}\text{i}\text{e}\text{l}\text{d}\text{\%}\left(\text{d}\text{r}\text{y}\text{b}\text{a}\text{s}\text{i}\text{s}\right)=\frac{\text{M}\text{a}\text{s}\text{s}\text{o}\text{f}\text{d}\text{r}\text{i}\text{e}\text{d}\text{G}\text{F}\text{W}\text{M}\left(\text{g}\right)}{\text{D}\text{r}\text{y}\text{w}\text{e}\text{i}\text{g}\text{h}\text{t}\text{i}\text{n}\text{i}\text{t}\text{i}\text{a}\text{l}\text{s}\text{a}\text{m}\text{p}\text{l}\text{e}\left(\text{g}\right)}\times100$$

Structural characterization of grapefruit wall material

Fourier transforms infrared spectroscopy

The functional groups of the GFWM were analyzed using a Fourier transforms infrared spectrophotometer (FTIR, Shimadzu-8400, Japan) following the method described by Gan et al.25. For FTIR analysis, one milligram of wall material powder was mixed with 150 mg of KBr (Sigma-Aldrich, USA) and compressed into a transparent pellet using a hydraulic press. The spectra was recorded in the wavelength range of 650–4000 cm-1 at a resolution of 4 cm-1 over 64 scans.

Scanning electron microscope

The morphological characterization of wall material samples was performed using a Scanning Electron Microscope (SEM) (Emcraft CubeSeries, south Korea) at the Department of Physics, (GCUF), following the methodology described by Karaman et al.26. For SEM imaging, the samples were finely mounted on aluminum stubs using double-sided adhesive tape, coated with a thin layer of gold to enhance conductivity, and examined at an accelerating voltage of 20KV. High-resolution micrographs were captured at 5000x magnification to analyze and interpret the surface morphology and structural characteristics of the GFWM.

Functional properties of grapefruit wall material

Total phenolic content

The Total Phenolic Content (TPC) of the dried wall material extract was determined through Folin-Ciocalteu method according to the method given in27,28 and expressed as milligrams of gallic acid equivalent per gram of sample (mg GAE/g). For the analysis, 1 mL of the ethanolic extract was mixed with 5 mL of 10% (v/v) Folin-Ciocalteu reagent and allowed to react for 5 min. Then, 4 mL of 7.5% (w/v) sodium carbonate solution was added, and the mixture was adjusted to a final volume of 10 mL with distilled water. The samples were incubated for 2 h at room temperature in darkness, and absorbance was measured at 760 nm using a spectrophotometer (IRMECO, U2020). Gallic acid was used as a standard, and results were expressed as mg GAE/g of dry matter. All analysis were conducted in triplicate, and results are presented as mean ± standard deviation (SD).

Total flavonoid content

The total flavonoid content (TFC) of the dried wall material was determined following the method described by Islam et al.29. Briefly, 100 mg of the sample was mixed with 100 mL of ethanol and shaken thoroughly, followed by filtration. The filtrate was then mixed with 0.3 mL of sodium nitrite solution (5% NaNO2) and diluted six times with distilled water (DW). Subsequently, 0.3 mL of aluminum chloride solution (10%) was added, and the mixture was incubated for 5 min. Finally, 2 mL of sodium hydroxide (1 M NaOH) was then added to the solution. The absorbance of the resulting solution was measured at 517 nm using a double-beam spectrophotometer (UV-1900i, Shimadzu, Japan). Results were expressed as mg QE/g DM, and all measurements were performed in triplicate, with data presented as mean ± SD.

Antioxidant activity

DPPH (2,2-diphenyl-1-picrylhydrazyl)

The antioxidant activity of the wall material was evaluated using the DPPH radical scavenging assay as described by Peng et al.30. A total of 100 mg of dried wall material was dissolved in 100 mL of 99% ethanol and shaken for 120 min at 300 RPM using an IKA-WERKE shaker (Germany). An aliquot of 0.1 mL of the ethanolic extract was mixed with 3.9 mL of 0.025 g/L DPPH methanolic solution, previously prepared fresh. The mixture was incubated in the dark for 30 min at room temperature, and the absorbance was measured at 517 nm using a spectrophotometer (IRMECO, U2020). The blank was prepared by mixing 0.1 mL of ethanol with 3.9 mL of the DPPH solution under the same conditions. The scavenging activity (%) was calculated using the following equation:

$$\text{S}\text{c}\text{a}\text{v}\text{e}\text{n}\text{g}\text{i}\text{n}\text{g}\text{a}\text{c}\text{t}\text{i}\text{v}\text{i}\text{t}\text{y}\left(\text{\%}\right)=\frac{\text{A}0-\text{A}\text{s}}{\text{A}0}\times100$$

Where A0 is the absorbance of the blank and As is the absorbance of the sample.

FRAP (ferric reducing antioxidant power assay)

The Ferric Reducing Antioxidant Power (FRAP) of the wall material extract was determined following the method described by Ahmed et al.31, with slight modifications. A 100 mg portion of dried wall material was extracted in 100 mL of 99% ethanol, and the mixture was shaken for 120 min at 300 RPM using an IKA-WERKE shaker (Germany). For the assay, 100 µL of the ethanolic extract was added to 3 mL of freshly prepared FRAP reagent, which consisted of 300 mmol/L acetate buffer (pH 3.6), 20 mmol/L FeCl₃·6 H₂O, and 10 mmol/L TPTZ (2,4,6-tripyridyl-s-triazine) in 40 mmol/L HCl, mixed a 10:1:1 (v: v:v) ratio. The reaction mixture was incubated at 37 °C for 30 min in the dark, after which the absorbance was measured at 593 nm using a spectrophotometer (IRMECO, U2020). A blank was prepared using ethanol instead of extract, while the control contained the FRAP reagent without FeCl3 to correct baseline absorbance. A standard calibration curve was constructed using FeSO₄·7 H₂O (1–1000 µmol/L), and results were expressed as µmol Trolox Equivalent per gram (µmol TE/g) of dry matter. All analysis were conducted in triplicate, and results are presented as mean ± standard deviation (SD).

Bread preparation

GFWM was directly mixed with wheat flour and fortified bread was prepared according to the straight dough method (AACC Method 10-10B, AACC, 2000)1, which involves mixing all ingredients in a single step, followed by fermentation, molding, proofing, and baking. Briefly, the formulation included wheat flour, GFWM, shortening, instant yeast, sugar, salt, water. However. Three bread formulations (T0 = control, T1 = 2% GFWM, and T2 = 4% GFWM) were developed according to the treatment plan described in Table ST1 to evaluate the effect of grapefruit peel wall material (GFWM) enrichment on bread quality.

Texture analysis

A texture analyzer was employed in accordance with a technique described by Bagdat et al.32; Kutlu et al.21 to determine the texture. A 25 mm diameter cylindrical probe was provided with the analyzer. For analyzing the hardness, chewiness, cohesiveness, and springiness characteristics of bread the following criteria were employed: The trigger force was 5 g, the side-center rate was 3.0 mm/s, the down-pressure variable was 50%, the pre-test rate was 1.0 mm/s and the interval period was 5 s. The bread slice used for this investigation was cut from the center at a thickness of around 20 mm.

Color analysis

Color characteristics of bread crust and crumb were analyzed using a colorimeter (ST-CP60, Stalwart, China) as described by Mohamed33, . Color parameters were expressed in the CIE Lab* system, where L* represents lightness (0 = black, 100 = white), a* denotes the green (-) to red (+) axis, and b* denotes the blue (-) yellow (+) axis. Measurements were performed at three random points on each bread slice at room temperature, and results were expressed as mean ± standard deviation (SD).

Sensory analysis

The sensory evaluation of bread was carried out by a trained panel from the Department of Food Science using the nine-point hedonic scale, following the method described by Yalinkilic and Cigdem34, . Bread samples were presented to faculty members and postgraduate (MS & PhD) students aged 22–35 years under controlled sensory conditions. The trained panel consists of 12 members evaluated the samples for color, flavor, taste, texture, and overall acceptability. Each attribute was rated on a nine-point hedonic scale, where 9 = liked extremely and 1 = disliked extremely.

Statistical analysis

All experimental data were statistically analyzed to determine the level of significance using SPSS software (version 2024). Results were expressed as mean ± SD following the method of Deng et al.35. Tukey’s post hoc test was applied to evaluate the effects of GRWM supplementation on bread quality parameters, with p ≤ 0.05 considered statistically significant.

Ethical approval

This study did not directly involve human participants to check the therapeutic \effect of the product and was therefore exempt from ethical approval, as confirmed by the Office of Research, Innovation and Commercialization (ORIC) by following all the guidelines of National Biosafety Rules 2005, Punjab Biosafety Rules 2014, Punjab Animal Health Act 2019 and Bioethics Protocol Government College University, Faisalabad, Pakistan and in accordance with the Declaration of Helsinki. The sensory evaluation was conducted with trained PhD scholars. Informed consent was obtained from all participants prior to sensory evaluation.

Results and discussion

Chemical composition of grapefruit peels

The chemical composition of grapefruit peel has been mentioned below in Table 1, and the outcomes showed that grapefruit peels contain moisture, ash, crude fat, protein, fibre and nitrogen-free extract with 76.0 ± 0.75%, 0.81 ± 0.76%, 2.56 ± 0.30%, 6.04 ± 0.36%, 6.96 ± 0.50% and 8.17 ± 0.30% respectively on dry matter basis. Current results were closely related to Ahmad et al.36, who examined that moisture, ash, crude fat, fibre, protein and nitrogen-free extract in grapefruit peels were 71.35 ± 1.33%, 3.45 ± 0.09%, 1.68 ± 0.05%, 6.11 ± 0.19%, 6.24 ± 0.12% and 11.06 ± 1.65%. Another study by Ali et al.37. reported that ash and nitrogen-free extract in grapefruit peels were 2.16 ± 0.01 and 10.56 ± 1.97, respectively. Grapefruit peels contain low fat and ash content, while the value of ash content in previous studies was higher than our results. However, the minor deviations in the moisture, ash, crude protein, fat, fiber, and nitrogen-free extract content were due to differences in soil type, climate, season, cultural practices, and fruit maturity and ripening conditions.

Table 1 Chemical composition, yield and antioxidant potential of grapefruit peels.
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Yield (%) of grapefruit wall material

In the current research, GFWM was extracted by using an alkali extraction technique. From 100 g of grapefruit peels sample, 7.0 ± 0.03% of wall material was finally recovered as shown in Table 1. The alkaline extraction technique enhances the functional properties of grapefruit wall material. Our current result of GFWM were in line with the finding of Cui et al.38. and Gan et al. (2020), who reported that extraction of wall material from grapefruit peels was 17.93–24.92% by alkali extraction and 8.31 ± 0.07% by microwave and enzymatic extraction respectively. By dissolving the cellulose and hemicellulose’s cross-linked polymeric network framework, the alkaline conditions served to release wall material, giving the highest yield.

Structural characterization of grapefruit wall material

Fourier transform infrared spectroscopy

Infrared spectroscopy is an efficient approach to evaluate the functional groups found within the sample. In the current research, the extracted sample of wall material from grapefruit peels was analyzed for their functional characterization, and the peaks showed multiple stretches, as shown in (Fig. 1). The FTIR spectra of the GFWM sample were recorded from 650 cm− 1 to 4000 cm− 1. The results showed that the peak at 3244 cm− 1 gives a medium and sharp appearance with O˗H stretching vibration, indicating the presence of an alcoholic group. A strong absorption peak was obtained at 2381 cm− 1 with O = C=O stretching, which indicates the presence of the carbon dioxide group. The medium peak at 1375 cm− 1 forms the CH3 bending, indicating the presence of an alkanes group. Moreover, the peak absorbance at 1047 cm− 1 represented the flexing vibrations of CO˗O˗CO, which indicates the anhydride group and the glycosidic bond (C-O), which is crucial in the functional properties of wall material. Our results were similar to the findings of Xie et al.39., who examined the FTIR spectrum of grapefruit wall material, which showed peaks at similar wavelengths and represents the presence of glycosidic bond (C-O-C) and carboxylic group (COOH) that had been methyl esterified. However, the configuration of wall material contains beta glycosidic linkages and forms branched structures.

Fig. 1
Fig. 1The alternative text for this image may have been generated using AI.
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FTIR spectra of GFWM.

Scanning electron microscope

Scanning electron microscope (SEM) is a powerful analytical technique that provides high resolution images (500,000x magnification) and detailed information on the surface morphology and microstructure of organic and inorganic materials40. In the current study, the dried wall material extracted from grapefruit peels was analyzed using SEM to characterize the microstructural features of the isolated Non-Starch Polysaccharides (NSP). The SEM images (Fig. 2) revealed numerous small, granular, and spherical-shaped micro particles dispersed across the surface of the dried NSP. The GFWM showed a rough, wrinkled and porous morphology with visible cavities and hollow regions. This distinct structure increases the relative surface area, improving the Water Holding Capacity (WHC), Oil Holding Capacity (OHC), Cation Adsorption Capacity (CAC), and Glucose Adsorption Capacity (GAC) of Non-Starch Polysaccharides (NSP) (Gan et al., 2020). A study was conducted by Deng et al.35, who reported that the GFWM sample showed granulated microstructure and large hollow and clear cavities within the grapefruit peel cell wall. These features are closely related to our current study. Wall materials with porous and furrowed morphology displayed better solubility, which may improve their functional properties.

Fig. 2
Fig. 2The alternative text for this image may have been generated using AI.
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Morphological view of GFWM.

Total polyphenol contents

The vast class of phytochemicals known as polyphenol compounds, including phenolic and flavonoid content, is present throughout the plant kingdom and has oxidative potential due to the hydroxyl group41. The current study evaluated the total phenolic and flavonoid contents of Grapefruit Peel Wall Material (GFWM). The results showed that GFWM contained 11.83 ± 1.66 mg GAE/g DM of total phenolic content (TPC) and 0.92 ± 0.33 mg QE/g DM of total flavonoid content (TFC).Previously, Peng et al.30 reported a total phenolic content of 12.50 ± 0.27 mg GAE/g DM in GFWM, which is consistent with the results of the present study. Similarly, flavonoid content in grapefruit wall material was also probed by Stabrauskiene et al.42, which supported the results of this study. (GFWM) contains a high concentration of phenolic compounds, which help prevent oxidation and offer health-promoting effects against oxidative stress-related diseases. The flavonoids present in GFWM have been reported to inhibit the progression of various diseases, including liver, breast, and prostate cancers, as well as tumors of the spinal cord, melanoma, and glioblastoma cells43.

Antioxidant activity

In the current study, the antioxidant potential of GFWM was assessed by using two commonly used methods FRAP and DPPH and their results were presented in Table 1. The DPPH was found to be 31.81 ± 0.20% indicating strong free radical inhibition capacity. Similarly, at 34.60 ± 0.26 µmol TE/g DW, the FRAP value was also found to be significant, indicating the material’s strong antioxidant and reducing power. These findings align with previous studies by Islam et al.29 and Chen et al.44, who reported DPPH activities of 29.05 ± 0.06 and 36.3 ± 0.90 mmol TE/g, respectively, in grapefruit peel wall material. The nutritional and functional qualities of value-added food products, like baked goods, can also be improved by adding GFWM. GFWM fortification has been shown in prior studies to improve shelf life, increase product FRAP values, and inhibit microbial growth, especially that of mould and yeast45,46,47.

Similarly, Chen et al.44 found FRAP values that were similar to ours, 39.77 and 40.72 mmol TE/g DW. The rich phenolic and flavonoid content of GFWM is responsible for its high antioxidant activity, as these compounds are essential for scavenging free radicals.

Product development

Bread enriched with GFWM (T1 and T2) was successfully developed and compared with the control (T0). The incorporation of GFWM influenced the product’s physical and sensory characteristics, leading to noticeable changes in texture, color, and overall acceptability as shown in (Figure SF 1 & SF 2).

Texture analysis

Hardness is the force required for deforming bakery items, while chewiness measures the effort required to masticate a solid product into a swallowable form. Springiness refers to the recovery period between the initial bite and the onset of these conditions in the mouth, while adhesiveness measures the effort needed to overcome the attraction between a product and the touch surface48. Bread texture is a crucial indicator of consumer acceptance and accounts for bread quality. In the current research, GFWM were incorporated in bread dough by following treatments: T0 (0gWM), T1 (2gWM) and T2 (4gWM). Incorporating GFWM affects bread’s internal structure, as shown in SF 2. Moreover, Control (T0) bread contained a fine porous structure; T1 bread contained pores larger than control, while T2 bread contained pores larger than T0 and T1. The bread hardness, chewiness, cohesiveness, adhesiveness, and springiness values after two days of storage have been stated in Table 2. The hardness values of T0, T1, and T2 were 2.80, 2.26, and 1.39 N, respectively, indicating that increasing levels of grapefruit wall material (GFWM) reduced bread hardness. Similarly, chewiness decreased from 2.42 (T0) to 1.8 (T2) N ×mm, which may be attributed to gluten dilution and the enhanced moisture retention capacity of GFWM, leading to a softer crumb structure. In contrast, cohesiveness increased from 0.42 to 0.54 N /m, while springiness and adhesiveness rose from 11.9 to 13.19 mm and 0.40 to 0.49 N, respectively, with higher GFWM incorporation. These increases can be linked to the water-binding and gel-forming abilities of dietary fibers and polysaccharides present in GFWM, which strengthen the crumb network and improve elasticity. Our results followed Wang et al.49., who examined that significant correlations were found between chewiness and hardness, and the chewiness of bread followed a similar pattern as hardness. Our results also correlate with the previous study of Ma et al.50 reported that adding 1.0% fermented soluble dietary fiber (F-SDF) improved bread quality by reducing hardness. The differences among treatments were statistically significant (p < 0.05), confirming that GFWM concentration had a measurable impact on bread texture after two days of storage.

Table 2 Effect of addition of grapefruit peels wall material on texture characteristics of bread.
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Color analysis

The color of bread crust is a significant factor affecting consumer acceptance. This research assessed the impact of grapefruit peel wall material (GFWM) on the coloration of both bread crust and crumb, utilizing a colorimeter (Minolta CR-400, Japan) in accordance with the CIE Lab color system after a storage period of two days (refer to Table 3). The L* (lightness) values for both the crust and crumb were found to be greater in the control group (T0) compared to the GFWM-enriched samples (T1 and T2), suggesting that higher levels of GFWM resulted in darker bread. Conversely, the a* (redness) and b* (yellowness) values exhibited an increase with the incorporation of more GFWM, indicating a rise in browning intensity and yellowish hues. This alteration can be linked to the Maillard reaction occurring between amino acids and reducing sugars during the baking process, alongside the presence of natural pigments and phenolic compounds in GFWM that facilitate color enhancement. The total color difference (ΔE) indicated a significant (p < 0.05) variation in the color of the crust and crumb between the control and the enriched samples, with L* demonstrating the most pronounced change. Similar observations were made by Bouaziz et al.51, who noted that increased fiber content diminished lightness in bread, and by Suri et al. (2022), who discovered that the addition of citrus peel led to a decrease in L* and an increase in a* and b* values. In summary, these results suggest that the incorporation of GFWM enhances the coloration of both crust and crumb through mechanisms related to both Maillard reactions and pigment interactions.

Table 3 Effect of addition of grapefruit peels wall material on the color characteristics of bread.
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Sensory analysis

In this analysis, the sensory evaluation of GFWM-enriched bread was performed in order to know about bread’s acceptability, quality and consumer expectations. The values of appearance, taste, flavor, color, texture, and general acceptance after two days of storage have been presented in Table 4. The results indicated that T2 bread received the highest ratings for its appearance and texture, whereas its taste and flavor were rated slightly lower, primarily due to the distinct citrus flavor contributed by GFWM. The reduction in flavor acceptability could be linked to the presence of bioactive compounds such as flavonoids (naringin, hesperidin), which may induce a mild bitterness52. Statistical analysis (p < 0.05) revealed significant differences in taste, flavor, and texture across the various treatments. In general, T2 bread was still deemed acceptable by the panelists, consistent with the findings of Gan et al.25 and Suri et al.53, who noted that citrus fiber improves bread texture and quality, albeit with a slight impact on flavor perception.

Table 4 Effect of addition of grapefruit peels wall material on sensory properties of bread.
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Conclusion

This study investigated the nutritional composition, structural, and functional properties of GFWM and its impact on bread quality. The chemical analysis showed significant nutritional value of GFWM, while SEM and FTIR analysis accentuated its structural and functional groups. Furthermore, the antioxidant activities of GFWM were assessed by DPPH and FRAP, revealed its significant potential to provide health benefits and protection against chronic diseases. The incorporation of varying concentration of GFWM into bread formulations showed improved textural and sensory attributes. Thus, the incorporation of GFWM into bread formulations improve quality attributes and demonstrates its potential as a functional ingredient for healthier cereal-based products.