Collection, preparation, and processing of experimental samples

Freshly harvested purple-fleshed sweet potatoes (60 kg) were collected from farmers’ fields in Kapasia, Gazipur, Bangladesh (24°6′0″ N, 90°34′14.88″ E) and transported to the packhouse of the Postharvest Technology Division, Bangladesh Agricultural Research Institute (BARI), Gazipur. Upon arrival, the tubers were carefully sorted to eliminate any that were damaged, diseased, or rotten. The selected sweet potatoes (variety: BARI Mistialu-17) (Figure 1a) were then washed thoroughly under running tap water to remove soil, sand, and other extraneous materials. Peeling was carried out manually using a stainless-steel vegetable peeler (AYT0008, China), followed by a second wash in clean water to ensure cleanliness. The peeled BARI Mistialu-17 was sliced into small, uniform pieces (Figure 1b) (3 mm thickness) using a handy food slicer, and immersed in a freshly prepared solution containing 0.1% potassium metabisulfite (KMS) and 5 mL of lemon juice to prevent enzymatic browning. The pretreated slices were then blanched by steam using a Boiler free Steamer (Figure 1c) (Model: E64005D10000200; Accutemp Products Inc., Fort Wayne, IN, USA) at 100°C for 15 min to inactivate enzymes and to conserve their characteristic color. Subsequently, the blanched sweet potato pieces were dried using a laboratory freeze dryer (Alpha 1-2 LSC basic; Martin Christ, Germany). The sweet potato slices were frozen at –18°C for 24 h in a deep freezer (WCG-3J0-DDGE-XX; Walton, Bangladesh) prior to freeze-drying. Then frozen slices were immediately placed in the drying chamber (–56° temperature; 1.8 Pa vacuum) of the freeze-dryer and covered with aluminum foil paper for light shielding for 24 h. Next, freeze-dried sweet potato pieces were milled into a fine flour (Figure 1d) using a laboratory grinder (Jaipan CM/L-7360065; Japan) to achieve a particle size of approximately 0.2 mm. The resulting flour was sieved to ensure uniformity, packaged in airtight high-density polyethylene (HDPE) bags, and stored in a desiccator at ambient temperature (25 ± 2°C) until further analysis and food product development.

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Figure 1. Photographic representation of purple-fleshed sweet potato biscuits. a) Selected sweet potato (BARI Mistialu-17), b) small potato pieces, c) blanching using a steam blancher, d) freeze-dried sweet potato flour, e) raw biscuit dough, f) raw biscuit for baking, g) baking at 170 ± 2°C, 13 min, h) manufactured PFSPF incorporated biscuits and i) textural hardness analysis.

Laboratory chemicals

The experiments were executed at the Post-harvest Food Processing and Analytical Laboratory of BARI, Gazipur, Bangladesh. Analytical-grade reagents were obtained from Merck (Germany) and Thermo Fisher Scientific (USA).

Sources of other experimental materials for biscuit formulations

Wheat flour (WF, Sunshine Atta, 1 kg pack, Akij Flour Mills Ltd., 304, Wilson Road, Bandor, Narayangonj, Dhaka, Bangladesh), granulated sugar (Fresh Sugar, 1 kg pack), and table salt (Molla Super Salt, Molla Salt (Triple Refined) Industry Ltd., Fatullah, Narayanganj, Bangladesh) were procured for biscuit preparation. Other ingredients included unsalted butter (Milk Vita, 200 g pack, Gazipur, Bangladesh), refined vegetable oil (Fresh soyabean oil, 2 L bottle, Sonargaon Seeds Crushing Mills Ltd., Sonargaon, Bangladesh), fresh eggs (Paragon Brown Egg, Paragon Agro Ltd., Dhaka, Bangladesh), baking powder (AgroPure baking powder, 160 g, AgroPure Trading Corporation, Dobadia, Uttarkhan, Dhaka-1230, Bangladesh), and milk powder (Diploma, 1 L pack, New Zealand Dairy Products Bangladesh Ltd., Bhulta, Rupganj, Narayanganj, Bangladesh). Additional flavoring ingredients such as almonds and cardamom were also sourced. All materials were procured from Shawpno, Joydebpur Bus Stand Express Outlet, Gazipur Sadar, Gazipur, Bangladesh.

Formulation of composite flour for nutritious biscuit development

The formulations were selected based on a literature review regarding sweet potato incorporation (0 to 50%) and a preliminary trial for biscuit manufacture. Refined WF was substituted by purple-fleshed sweet potato flour (PFSPF) in different percentages (10%, 20%, 30%, 40%, and 50%) to formulate nutritious biscuits, with 100% WF serving as a control. WF and PFSPF were mixed in different amounts (%, w/w) to get the following ratios: 100:0 (T₁: control), 90:10 (T₂), 80:20 (T₃), 70:30 (T₄), 60:40 (T₅), and 50:50 (T₆). To ensure the mixes were evenly mixed, they were blended using a Kenwood mixer for 3 min at a speed of 8 rpm. Before being used again, the resultant flours were placed in airtight high-density polyethylene (HDPE) bags and kept at room temperature (25 ± 2°C). These flours were stored (3–15 days) for quality analysis and subsequently utilized to manufacture biscuits (Figure 1e, 1f, 1g, 1h) within 5 days (all the treatments were replicated 3 times) and were examined for quality parameters (Figure 1i).

Formulation of PFSPF incorporated biscuits

The formulation of PFSPF incorporated biscuits is depicted in Table 1. The developed biscuits were prepared by supplementing the WF with PFSPF in different percentages (10%, 20%, 30%, 40%, and 50%), whereas the control included only WF. The standard procedure outlined by Dey et al. [29] was used to make the biscuits (Figure 1), with a few minor modifications. The recipes included about 200 g of composite flour (WF:PFSPF) for each formulation, as shown in Table 1, and 60 g of butter, 35 g vegetable oil, 45 g sugar, 1.2 g salt, 100 g egg, 1.74 g cashew powder, 0.80 g cardamom, 10 g milk powder, and 2.78 g baking powder.

Composite flours were used in accordance with the treatments to form biscuit dough, which was then kneaded and rolled out into a sheet with a thickness of 7.8 mm. In order to make raw biscuits, the sheet was then cut into a circle with a diameter of 6 cm using a biscuit cutter and placed on a tray to bake. After 13 min of baking at 170 ± 2°C in a free-standing baking oven, the baked biscuits were allowed to cool at ambient temperature. Finally, baked biscuits were properly sealed in airtight containers or foil pouch packets and stored at room temperature (25 ± 1°C) for further quality assessment. The color parameters, textural hardness, sensorial evaluation, and physical analyses were conducted on the day of biscuit manufacturing and the next days, but the physicochemical, nutritional parameters, bioactive phytochemicals, and mineral composition of PFSPF incorporated biscuits were analyzed within 25 days.

Physicochemical analysis of the PFSPF and formulated biscuits

Bioactive phytochemicals analysis

Mineral profile analysis

The concentrations of Na, K, Ca, Mg, P, S, B, Cu, Mn, Fe, and Zn were determined using the method described by Molla et al. [40]. To digest 1 g of dry biscuit powder, a solution of nitric acid (HNO₃) and perchloric acid (HClO₄) was used at 320°C. After cooling, the digest was diluted with 30 mL of deionized water and filtered using Whatman filter paper. The resultant filtrate served as the stock solution for mineral analysis. An atomic absorption spectrophotometer (AAS; Shimadzu AA-6800, Japan) was used to detect the concentrations of Na, Ca, Fe, K, Zn, Mg, Mn, and Cu, while phosphorus content was assessed using UV-visible spectrophotometry (Shimadzu UV-3600). Boron was measured using the CaCl₂ extraction method, whereas sulfur content was calculated using a turbidimetric process with BaCl₂. Individual mineral levels were measured in relation to standard reference minerals received from Sigma Aldrich Chemical Co., USA.

Functional properties of the PFSPF

The oil and water absorption capacities (OAC/WAC) of PFSPF were assessed by Dey et al. [29]. After being pre-weighed and thoroughly combined with 25 mL of either distilled water or culinary soybean oil, the sample (3.0 g) was placed in a 50 mL oil/water capacity centrifuge tube, allowed to sit at room temperature for 30 min, and then centrifuged for 20 min at 3,000 rpm. The extra water or oil was thrown away, leaving the sediment of the sample that was discovered at the tube’s bottom. After that, the centrifuge tube with sediment was reweighted. OAC/WAC was measured by the following equation.

where W₀ stands for weight of sample, W₁ for weight of centrifuge tube + sample and W₂ for weight of centrifuge tube + sediment.

Physical profile analyses of PFSPF incorporated biscuits

In accordance with Dey et al. [29], physical profile analysis of PFSPF-incorporated biscuits was conducted. The biscuits were weighed with an analytical scale as soon as they cooled. A ruler was used to measure the diameter of six biscuits; each biscuit was measured twice perpendicularly, and the mean diameter (mm) was computed from three repetitions. A digital Vernier caliper with an accuracy of ± 0.01 mm was used to measure the thickness. To assess biscuits’ capacity to expand and the quality of the flour, the spread ratio (diameter:thickness) was calculated. The area of the biscuit multiplied by its thickness (cm²) was used to determine its volume. By dividing the observed mass by the volume (g/cm³), the biscuit density was calculated.

Textural profile analysis of PFSPF incorporated biscuits

The textural profile of the control and developed biscuits was evaluated using a texture analyzer (TA. XT Plus; Stable Micro Systems, Godalming, Surrey, UK) equipped with tensile grips (A/TG) mounted on a heavy-duty platform with a 5 kg load cell, following the method described by Kesselly et al. [42] with slight modifications. Three biscuit samples from each treatment were cut at the center using a biscuit-cutting probe (3PB). The biscuit strips were positioned in the texture analyzer such that the center of the strips was clamped between the stationary platform and the moving arm of the instrument at a fixed distance of 38 mm. Tensile tests were performed under the following conditions: pre-test speed of 1.5 mm/s, test speed of 5 mm/s, return speed of 10 mm/s, post-test speed of 10 mm/s, and a trigger force of 0.049 N. The clamps pulled the biscuit strips until rupture occurred. The rupture force (FR) for textural hardness was measured and expressed as force (kg), which was further converted to Newton (N) for convenience (1 kg force is equal to 9.80665 N).

Color parameters of PFSPF incorporated biscuits

A versatile chromameter (Model CR-400; Konica Minolta, Japan) was used for analyzing the color profile of the prepared biscuits in accordance with the methods outlined by Medhe et al. [43]. A standard white calibration plate was used to calibrate the apparatus before the measurement to verify accuracy. The CIE Lab* color space, where L* stands for brightness (0 = black, 100 = white), a* for the red-green axis (positive = red, negative = green), and b* for the yellow-blue axis (positive = yellow, negative = blue), was used to measure color parameters. The following formulas were used to figure out the hue angle (H°) and chroma (C) from the observed a* and b* values.

Energy value of PFSPF incorporated biscuits

The energy profile of the control and BARI Mistialu-17 flour-incorporated nutritious biscuits was evaluated using a bomb calorimeter (Parr 6100 Calorimeter; Parr Instrument Company, Moline, IL, USA) using the technique stated by Molla et al. [40]. Approximately 1 g of each sample was placed into the crucible and combusted in pure oxygen following electrical ignition. The energy content of the biscuits was calculated as kilocalories per 100 g (kcal/100 g).

Sensory evaluation of PFSPF incorporated biscuits

The sensory properties of the biscuits were assessed using the procedure established by Dey et al. [29], with minor changes. Sensory evaluation took place over three days at the Postharvest Technology Division’s Sensory Laboratory, which was outfitted with consistent fluorescent lighting and began on the day of biscuit production. There were 54 participants, including 27 men and 27 women, aged 21 to 59, who were diverse researchers, scientists, and technical experts from the Bangladesh Agricultural Research Institute (BARI) in Gazipur, Bangladesh. To assess the replication, they were divided into three groups at random, each consisting of eighteen (18) participants. Every member of the panel had some baking experience and was semi-trained. Smokers were not allowed to take part in this study. In compliance with ethical guidelines, the sensory evaluation of the formulated biscuits made with PFSPF was carried out without endangering any people or animals. The study complies with ethical principles outlined by the Helsinki Declaration (2024), where the exclusion criteria were applied to eliminate any potential risks associated with the study. Ethical approval for the sensory evaluation was obtained from the Research Committee (Expert Panel) at BARI (Experiment No. 5.2.2.14). Participants were told about the study methodology and provided their informed consent before testing. The biscuits were arranged on translucent plastic plates and coded with random three-digit numbers. The study did not explicitly isolate or quantify the halo effect (e.g., through blindfolded testing or separate evaluation of visual and non-visual attributes). A nine-point hedonic scale was used to rate sensory attributes (1 being “extremely unacceptable” and 9 being “extremely acceptable”). Panelists were given tissue paper, drinkable water for palate cleansing in between samples, a pen, and an assessment sheet. Ethical guidelines for participant privacy and rights were strictly followed.

Polynomial Regression Models

Each quality attribute was fitted to a second-order (quadratic) polynomial of the form:

All models were statistically significant at p < 0.05, and most achieved very high coefficients of determination, supporting their use for prediction and optimization.

Multi-response optimization via desirability function

A simultaneous optimization approach was applied by normalizing each response to a [0, 1] desirability scale—maximizing fiber, TPC, TFC, DPPH scavenging activity, and Ca, while minimizing hardness. Individual desirabilities were averaged to compute an overall desirability score.

Mantel tests and correlation analysis

The Mantel test assessed the multivariate association between six groups of biscuit quality attributes (nutritional, physicochemical, bioactive, mineral, color, and texture) and eight key compositional/bioactive variables (fat, protein, fiber, carbohydrate, β-carotene, anthocyanin, TPC, TCC).

Experimental design and statistical analysis

The experiment was laid out in a Completely Randomized Design (CRD) with three replications.

All analyses, with the exception of energy content and color parameters (conducted five times), were executed in triplicate, and the results are presented as mean ± standard deviation. One-way analysis of variance (ANOVA) was used for normally distributed data, and the Kruskal-Wallis (KW) test was used for non-normally distributed data to identify significant differences among the means. Post hoc comparisons were conducted with Tukey’s multiple comparison test in the case of ANOVA at a 95% confidence level (p < 0.05).

Quality characteristics of PFSPF

The physicochemical, nutritional, antioxidant, and bioactive constituents of PFSPF, including moisture, ash, crude fat, crude protein, crude fiber, carbohydrate concentrations, TS, pH, acidity, TSS, β-carotene, anthocyanin, ascorbic acid, total phenolic, flavonoid, and carotenoid content, are represented in Table 2. As per the findings, PFSPF had the following contents: total carbohydrate (81.30%), TS (27.50%), ash (1.92%), moisture (4.89%), crude fat (3.59%), crude protein (5.02%), crude fiber (3.28%), pH (6.43), acidity (0.68%) and TSS (2.57 °Bx).

The antioxidant and bioactive phytochemical profile from Table 2 showed that TPC, TFC, TCC, β-carotene, anthocyanin, and ascorbic acid content of PFSPF were 25.07 mg GAE/100 g, 13.71 mg QE/100 g, 27.90 mg/100 g, 4.96 mg/100 g, 0.63 mg/100 g, and 17.47 mg/100 g, respectively. Besides, the antioxidant activity (DPPH radical scavenging) was 43.51%. The color parameters, including L*, a*, b*, C, and H° values, were found to be 63.48, 8.80, 10.15, 13.43, and 49.06, respectively. Table 2 displays the mineral composition and functional properties of PFSPF. For calcium, sodium, magnesium, potassium, zinc, iron, and manganese, the average concentrations were 45.60 mg/100 g, 0.49%, 1.13%, 1.90%, 9.00 ppm, 61.33 ppm, and 44.67 ppm, respectively. Additionally, the results demonstrated that PFSPF had an oil absorption capacity of 162.67% and a water absorption capacity of 181.33%.

Physicochemical, nutritional, and bioactive phytochemicals of PFSPF incorporated biscuits

Table 3 represents the physicochemical, nutritional, and bioactive profiles of the control and PFSPF-incorporated biscuits. The control biscuits and PFSPF developed biscuits with various formulations (0–50% substitution) are depicted in Figure 2. The moisture content of the formulated biscuits ranged from 4.26% to 5.92%. PFSPF contained less protein, as seen by the substantial decrease in protein content (from 11.02% in T₁ to 6.18% in T₆). From 0.46% (T₁) to 6.38% (T₆), the fiber content increased substantially, highlighting the function of PFSPF as a dietary fiber source. The levels of carbohydrates varied slightly, from 60.26% to 67.41%. The fat content gradually dropped from 22.83% to 12.36%. TS of the PFSPF incorporated biscuits ranged from 4.50% to 12.84%. The TSS went from 0.60 °Bx in the control (T₁) to 2.40 °Bx in T₆. The pH readings remained relatively constant, rising from 7.14 to 7.27, whereas the acidity moved slightly from 0.26% to 0.48%. The TPC increased markedly from 1.44 mg GAE/100 g to 6.91 mg GAE/100 g between T₁ and T₆. In the same way, the TFC went up from 3.41 mg QE/100 g in T₁ to 9.06 mg QE/100 g in T₆. The TCC went up from 4.42 mg/100 g in the control (T₁) to 6.32 mg/100 g in T₆. From 0.28 mg/100 g in T₁ to 2.64 mg/100 g in T₆, anthocyanin levels rose significantly, indicating that PFSPF is crucial in increasing antioxidants associated with pigments. β-carotene content of the PFSPF incorporated biscuits ranged between 1.05 and 3.29 mg/100 g. Because vitamin C is sensitive to heat during baking, the ascorbic acid content peaked in T₄ at 8.43 mg/100 g and then slightly decreased in T₅ and T₆. The results, as shown in Table 3, demonstrate a significant increase in antioxidant activity (DPPH radical scavenging) from 5.30% in the control (T₁) to 39.51% in T₆.

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Figure 2. The control biscuits and PFSPF incorporated biscuits with various formulations (0–50% substitution). T₁: WF (100%) or control; T₂: WF (90%) + PFSPF (10%); T₃: WF (80%) + PFSPF (20%); T₄: WF (70%) + PFSPF (30%); T₅: WF (60%) + PFSPF (40%); T₆: WF (50%) + PFSPF (50%). WF: wheat flour; PFSPF: purple-fleshed sweet potato flour.

Mineral and energy profile of PFSPF incorporated biscuits

Table 4 illustrates the mineral and energy profile of the control biscuits (T₁) and biscuits incorporated with PFSPF (T₂–T₆). The components that had been studied include calcium (Ca), phosphorus (P), sodium (Na), magnesium (Mg), potassium (K), sulfur (S), zinc (Zn), iron (Fe), manganese (Mn), copper (Cu), and boron (B). Results revealed that the mineral content of the flours increased with the substitution of PFSPF. The energy values of biscuits T₁, T₂, T₃, T₄, T₅, and T₆ were 7.65, 8.31, 8.51, 8.68, 8.88, and 9.08 kcal/100 g, respectively (Table 4).

Physical profile analysis of PFSPF incorporated biscuits

The physical properties of the control biscuits alongside those containing PFSPF are shown in Table 5. The weights of the formulated biscuits ranged from 11.28 g to 11.85 g, with the highest weight recorded in formulation T₆. The diameter of the biscuit samples T₃ (6.71 cm) was slightly higher than that of the control (T₁: 6.25 cm), whereas the other formulations exhibited smaller diameters. Biscuits exhibited thickness values ranging from 0.79 cm to 0.85 cm. The spread ratio of the control biscuit (T₁) was 7.91, while the values for T₂, T₃, T₄, T₅, and T₆ were slightly lower at 7.76, 7.71, 7.46, 7.31, and 7.08, respectively. The volume of the control and PFSPF-enriched biscuits ranged from 23.60 cm³ to 23.95 cm³. Biscuit volume did not differ significantly (p > 0.05) among the formulations, except for T₆. Similarly, biscuit density remained statistically non-significant (p > 0.05), ranging from 0.48 to 0.49 g/cm³.

The textural hardness of PFSPF incorporated biscuits is illustrated in Figure 3 and Table 5. The control biscuits (T₁) had a hardness of 21.97 N (2.24 kg force), which progressively increased to 23.71 N (2.45 kg force) in T₂, 24.68 N (2.52 kg force) in T₃, 26.17 N (2.67 kg force) in T₄, 27.09 N (2.76 kg force) in T₅, and peaked at 34.25 N (3.49 kg force) in T₆. The substitution of PFSPF significantly (p < 0.05) increased the hardness of the formulated biscuits.

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Figure 3. Textural profile analysis (TPA) curve of PFSPF incorporated biscuits. T₁: WF (100%) or control; T₂: WF (90%) + PFSPF (10%); T₃: WF (80%) + PFSPF (20%); T₄: WF (70%) + PFSPF (30%); T₅: WF (60%) + PFSPF (40%); T₆: WF (50%) + PFSPF (50%). WF: wheat flour; PFSPF: purple-fleshed sweet potato flour.

Color profile of PFSPF incorporated biscuits

Figure 4 summarizes the color profile of the control biscuits and those formulated with PFSPF. The control biscuits (T₁) exhibited the highest lightness (L*) value (48.24), which progressively decreased with increasing levels of PFSPF substitution. Both the redness (a*: 16.16 to 4.13) and yellowness (b*: 33.17 to 16.12) values also declined gradually, indicating a reduction in these color components as the proportion of PFSPF increased. Adding this flour also altered the chroma (C) and hue angle (H°), which affected the overall perception of color. Statistical analysis revealed that the color parameter differences among formulations were significant (p < 0.05).

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Figure 4. Color profile (mean ± standard deviation) of PFSPF incorporated biscuits. T₁: WF (100%) or control; T₂: WF (90%) + PFSPF (10%); T₃: WF (80%) + PFSPF (20%); T₄: WF (70%) + PFSPF (30%); T₅: WF (60%) + PFSPF (40%); T₆: WF (50%) + PFSPF (50%). WF: wheat flour; PFSPF: purple-fleshed sweet potato flour.

Sensory profile of PFSPF incorporated biscuits

As illustrated in Figure 5, the sensory scores of the biscuits indicated noticeable changes with increasing levels of PFSPF. The sensory scores showed that the overall acceptability of the developed biscuits was highest for T₆, followed by T₅, T₄, T₃, and T₂, compared to the control (T₁). Significant differences (p < 0.05) were observed among the formulations in terms of color, flavor, taste, texture (crispiness), and overall acceptability.

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Figure 5. Sensory profile of PFSPF incorporated biscuits. T₁: WF (100%) or control; T₂: WF (90%) + PFSPF (10%); T₃: WF (80%) + PFSPF (20%); T₄: WF (70%) + PFSPF (30%); T₅: WF (60%) + PFSPF (40%); T₆: WF (50%) + PFSPF (50%). WF: wheat flour; PFSPF: purple-fleshed sweet potato flour.

Effects of sweet potato flour substitution on the properties of the biscuits

The study investigated the effect of substituting PFSPF at six levels (0%, 10%, 20%, 30%, 40%, and 50%) on biscuit quality. Three analytical approaches were applied: Polynomial Regression Modeling of individual responses (Figure 6), desirability-function-based multi-response optimization (Table 6 and Figure 7), and Mantel tests with a Pearson correlation heatmap to assess inter-variable associations (Figure 8).

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Figure 6. Polynomial Regression Models for PFSPF incorporated biscuits at different substitution levels (0–50%). CHO: carbohydrate; TA: titratable acidity.

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Figure 7. Multi-response optimization of PFSPF substitution.

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Figure 8. Mantel tests and correlation analysis among biscuit quality attributes (nutritional, physicochemical, bioactive, mineral, color, and texture).

Quality characteristics of PFSPF

The nutritional composition of sweet potato flour is highly variable and depends on cultivar, agro-ecological circumstances, post-harvest management, and processing methods [6, 9, 10]. The physicochemical properties represent the prime quality of the raw material, which will be incorporated in the final food products [44]. From Table 2, the value of moisture content below 10% attained by freeze drying techniques indicates that these flours have a longer shelf life or a lower possibility of microbial growth [45]. The ash content indicates the presence of total mineral concentrations of the sample, and a higher value may indicate a greater presence of essential minerals such as potassium, calcium, magnesium, and trace elements, which can enhance the nutritional value of the flour [7]. The relatively high ash content (1.92%) may be attributed to several factors, including the intrinsic mineral composition of the BARI Mistialu-17 variety, the soil characteristics where the sweet potatoes were cultivated, and the presence of fibrous material during processing. PFSPF is valued for its rich nutritional profile, being an excellent source of protein, dietary fiber, ascorbic acid, β-carotene, essential vitamins, along with biologically active phytochemicals like carotenoids, phenols, and anthocyanins (Table 2). These findings corroborate those obtained by Alam [7] and Ivane et al. [46]. Our findings in terms of anthocyanin content are similar to those of the previous study by Ekaputra and Pramitasari [47]. The flour’s richness in phenolic compounds, anthocyanins, flavonoids, and carotenoids contributed to significant enhancements in antioxidant and bioactive potential in the formulated biscuits [46]. Previous studies have revealed noteworthy differences in macronutrients, mineral composition, and bioactive phytochemicals across varieties, postharvest handling or pretreatment and drying techniques [6–9]. Results showed that postharvest pretreatment with potassium metabisulfite and lemon juice is an excellent choice for better color retention and antioxidant qualities in freeze-dried flours. The findings for color attributes were consistent with a former study by Bakar et al. [45]. The mineral contents are noticeably found in this flour. The findings interpreted that PFSPF contained valuable minerals, namely iron, potassium, calcium, zinc, and magnesium. The results also indicated that PFSPF has good water and oil absorption capacities. Our findings are corroborated with the previous study described by Badiora et al. [8] who found that yellow fleshed sweet potato contained 106.5 to 531% WAC and 83.5 to 168.5% OAC. High PFSPF absorption of water and oil results in competitive hydration, where water is bound by fiber and damaged starch, preventing the production of gluten. As a result, the gluten network becomes weaker, less elastic, and more viscous. Consequently, the dough becomes less extensible and firmer, which reduces spread and increases the hardness of the biscuits [48]. Moreover, these functional properties are useful in structure interaction in food, especially in flavor retention, palatability improvement, and shelf-life extension, particularly in bakery products [49].

Physicochemical, nutritional, and bioactive phytochemicals of PFSPF incorporated biscuits

Table 3 presents the physicochemical, nutritional, and bioactive profiles of the control and PFSPF-incorporated biscuits. Better moisture retention was demonstrated by PFSPF at higher substitution levels (T₄–T₆), most likely as a result of the fiber content's stronger water-holding capacity over refined WF. Asrat et al. [50] noted comparable patterns and credited the hydrophilic qualities of dietary fibers with the higher moisture content of sorghum-enriched biscuits. Besides, there was minimal risk of microbiological spoiling because all moisture levels were within the recommended range for biscuit shelf stability (< 10%) [45]. As shown in Table 3, the protein content of the formulated biscuits (11.02% to 6.18%) notably decreased with higher levels of PFSPF substitution because they contained lower amounts of protein. Sweet potato proteins interact with the wheat-based protein matrix, which is mostly made up of gluten-forming proteins (glutenins and gliadins), in the biscuit formulation. Sweet potato proteins don't have the ability to make gluten, so adding them dilutes the gluten network. The dough matrix becomes less cohesive and elastic as a result [51]. These proteins can, however, still engage in non-covalent interactions with gluten proteins and starch components, such as hydrogen bonding and hydrophobic interactions, which affect the viscosity and moisture retention of dough [52]. The fiber content of the PFSPF incorporated biscuits significantly increased with higher substitutions of that flour. This improvement is comparable to the previous study documented by Tortoe et al. [53]. Moreover, researchers stated that fiber enrichment not only increases the nutritional content but also helps individuals feel fuller and regulate blood sugar levels [54]. The intrinsic fiber composition of PFSPF, which includes both soluble dietary fiber and insoluble dietary fiber fractions, is responsible for the significant increase in dietary fiber content from 0.46% (T₁) to 6.38% (T₆). The main components of sweet potato dietary fiber are cellulose, hemicellulose, lignin (an insoluble component), and pectin (a soluble component). Based on the proportions described by the previous study of Mei et al. [55], insoluble fiber predominates, but soluble fiber is also found in significant amounts. Soluble fiber may partially offset these effects by enhancing water retention and lubricity within the dough system, but increased fiber incorporation, especially insoluble fiber, can result in denser texture, decreased spread ratio, and increased hardness from a sensory standpoint [55]. Sweet potato dietary fibers have been demonstrated to have prebiotic effects, supporting good gut bacteria like Lactobacillus and Bifidobacterium while inhibiting harmful species [56]. Fiber-rich formulations can prolong feelings of fullness and potentially help with weight management by delaying stomach emptying and modulating the postprandial glucose response. Consequently, adding PFSPF to biscuits improves their functional and health-promoting qualities in addition to changing their physicochemical and sensory characteristics [48, 57]. Due to the greater fiber contents of PFSPF, which largely replace digestible carbohydrates, the amounts of carbohydrates varied considerably; however, no discernible trends were seen [58]. From Table 3, the slight increase in carbohydrate levels at higher PFSPF substitution levels emphasizes the possible advantages of such formulations for developing functional food products. The carbohydrate amount remained nutrient-sufficient to provide energy [59]. The combination of starch, dietary fiber, and the protein matrix probably affects the biscuits’ glycemic response, even if the variations in overall carbohydrate content were marginal. By increasing digesta viscosity and limiting enzymatic accessibility, increased fiber content, especially soluble fractions, may decrease starch digestibility and glucose release, lowering the expected glycemic index [60, 61]. In order to determine these associations, more research utilizing in vitro starch digestibility tests or an in vivo glycemic index study of the developed biscuits via PFSPF inclusion is recommended. PFSPF has less lipid than WF, hence the fat level decreased over time. The findings of Khrisanapant et al. [62], who found that legume fortification enhances the fatty acid profile and decreases fat levels, confirm this tendency. The majority of the lipids in sweet potatoes are unsaturated, which could improve flavor but make them more prone to oxidation, which could shorten their shelf life [63]. The texture of PFSPF incorporated biscuits may also be affected by lower fat content. People who are health-conscious and looking for lower-calorie options can find these formulations more enticing because of the noted reduction in fat content. Results found that adding composite flour instead of refined WF to baked items made them have higher levels of minerals. It additionally suggests that PFSPF might be utilized to supplement food with additional minerals [64]. A higher TSS content in formulated biscuits may result in a sweeter taste, requiring less sugar during initial formulation. Since biscuits are frequently categorized as low-moisture items, it is important to note that an increase in TSS is usually linked to a decrease in free water availability, which may improve shelf stability. Furthermore, additional research will be carried out to investigate microbial growth patterns, water activity, and storage stability in order to thoroughly evaluate the safety and quality consequences of elevated TSS. The acidity changed slightly, but the pH values stayed mostly the same for all the formulations. Nutritive and physicochemical components such as proteins, minerals, and phosphates in composite flour can contribute to buffering action, assisting to maintain a relatively stable pH during processing. Ionic strength also influences water binding and protein interactions, which are important factors that determine the final biscuit texture and dough rheology. Similar findings have been documented by Tanger et al. [65], who demonstrated that pH stability and structural characteristics in food products are influenced by buffering effects and ionic strength.

The results indicated that replacing PFSPF in formulated biscuits enhanced the TPC and TFC (Table 3). The results align with the Dinu et al. [66] publication. By reducing oxidative stress, these substances are recognized to play a crucial role in their anti-inflammatory and antioxidant properties. This increase is consistent with the presence of total phenolic and flavonoid compounds, which are well known for their health benefits, including heart protection and inflammation reduction [37]. The enrichment of the carotenoid profile makes the developed biscuits more nutritious and able to combat free radicals [67]. Significant increases in anthocyanin levels in the developed biscuits suggest that PFSPF plays a critical role in boosting pigment-related antioxidants. The higher anthocyanin content points to potential health benefits, especially in reducing inflammation and oxidative stress. This study supports the findings of Guclu et al. [68]. Results revealed that the amount of β-carotene in the formulated biscuits was more than tripled than control biscuits. In line with previous research, the rise in β-carotene emphasizes the nutritional advantages of using PFSPF as a precursor of vitamin A [69]. Results demonstrated that ascorbic acid levels continued to be nutritionally significant, which helped to boost the biscuit’s antioxidant capacity [70]. As shown in Table 3, results also demonstrated a significant rise in antioxidant activity, indicating that the prepared biscuits are healthier when additional PFSPF is added. Eid et al. [64] have shown comparable improvements in antioxidant capacity. Antioxidant chemicals can interact with macromolecules, including proteins, lipids, and carbohydrates, to increase their stability during processing by decreasing their accessibility to heat and oxygen [71]. Due to the high moisture content and latent heat of water evaporation, the internal temperature of the biscuit matrix stays much lower for the majority of the baking period, even though the oven temperature during baking exceeds the typical 170°C. This phenomenon has been extensively documented in baked systems [72]. The properly stored biscuits in airtight containers or foil pouch packets may be consumed for 2 months.

Mineral and energy profile of PFSPF incorporated biscuits

As shown in Table 4, the calcium content of the developed biscuits progressively increases across all the treatments indicates a dose-dependent effect of PFSPF substitution. The enhancement in calcium aligns with the previous study of Roger et al. [73]. Phosphorus levels have no clear trend across treatments. The minimal variation and absence of significant differences (p > 0.05) indicate that the phosphorus concentration is not significantly affected by the substitution of this flour. The prior study, as reported by Eid et al. [64], supports these findings. The sodium level of the manufactured biscuits increased because of the inherent sodium content of the substituted flour and the retention of added salt during baking. Similar trends were reported by Ozgolet et al. [74]. The increase in magnesium content can be attributed to the naturally high magnesium content of PFSPF, which aligns with the research conducted by Mengeneh and Chukwuma Ariahu [75]. The comparable trend for potassium is consistent with the findings of the Garg and Sharma [76] investigation. The initial loss of sulfur during high-temperature baking may be attributed to sulfur loss, whereas the subsequent increase is probably caused by an elevated concentration of sulfur-containing amino acids [50]. According to studies by Ozgolet et al. [74], which emphasized the significance of composite flour in zinc enrichment, zinc deficits in diets based on cereals can be reduced. The research findings, as shown in Table 4, indicate that PFSPF serve as a potential iron-rich fortifier. Moreover, this notable rise aligns with the findings of Ozgolet et al. [74], who highlighted the effectiveness of composite flour in enhancing the iron content of baked goods. The differences in the manganese content of the formulated biscuits may be the reason for the substitution level of the composite flour, as supported by the previous findings of Roger et al. [73]. Roger et al. [73] also assert that this consistent increase validates the role of composite flour in copper enrichment. The presence of boron in the formulated biscuits varied among different substitution levels of PFSPF, which supports its potential to improve bone health and metabolic processes [64]. Therefore, instead of refined WF, the incorporation of PFSPF can be used as a functional ingredient to add valuable macro- and micronutrients to baked biscuits to make them healthier and fill in mineral gaps.

Food energy is released during the oxidation of proteins, lipids, carbohydrates, and other organic compounds [44]. When the primary calorigenic macronutrients (carbohydrates, fats, and proteins) in a food portion are completely burned in the presence of enough oxygen, they release energy in the form of calories [77]. The heat of combustion, which is essential for sustaining fundamental metabolic processes, physical activity, growth, and general health, is frequently measured using a bomb calorimeter [78]. Results showed that the energy content of T₆ biscuits (50% PFSPF incorporated) was significantly higher (p < 0.05) than that of the other formulations (Table 4). This increase is likely attributable to the higher concentrations of calorigenic nutrients present in PFSPF, suggesting its potential as a richer energy source compared to refined WF (T₁).

Physical profile analysis of PFSPF biscuits

The weights and thickness of PFSPF incorporated biscuits were slightly increased (Table 5). Variations in biscuit weight were primarily attributed to differences in dough piece weights, which are influenced by spatial inconsistencies in the thickness of the dough sheet during cutting. A gradual increase in both weight and thickness was observed with the incorporation of PFSPF as a partial replacement for WF; however, the test samples showed a notable reduction in diameter. The diameter of the biscuit sample T₃ (6.71 cm) was slightly higher than that of the control (T₁: 6.25 cm), whereas the other formulations exhibited smaller diameters. These alterations in diameter and thickness were reflected in the spread ratio. The decrease in diameter, coupled with an increase in thickness, resulted in lower spread ratios in the developed biscuits, which appeared to be directly influenced by the flour composition [29, 79]. The reduced diameter and spread ratio in the enriched formulations may be attributed to the higher content of hydrophilic sites and water-soluble proteins in PFSPF, which tend to compete for the scarce free water available within the dough matrix. Consequently, reduced water availability during dough mixing increases viscosity, ultimately leading to smaller diameters and lower spread ratios [80]. The volume of the control and PFSPF-enriched biscuits did not differ significantly (p > 0.05) among the formulations, except for T₆. Similarly, biscuit density remained statistically non-significant (p > 0.05).

Texture is the most noticeable physical attribute of biscuits to attract the consumer after color. Adding BARI Mistialu-17 flour significantly (p < 0.05) increased the hardness of the formulated biscuits (Figure 3). The results indicated that the hardness of composite biscuits was comparable to that reported by Dey et al. [29]. The lower gluten concentration resulting from partial substitution with PFSPF most likely contributed to the formation of a weaker gluten network, which affected the textural hardness of the biscuit samples. The PFSPF has a higher dietary fiber content, which contributes to the significant (p < 0.05) improvement in biscuit hardness. These results indicate that differences in fat and fiber content markedly affect the texture of the produced biscuits. Saeed et al. [80] have reported similar trends.

Color profile of PFSPF incorporated biscuits

Color is one of the main physical characteristics that influences consumer perception of food [59]. Anthocyanins are primarily responsible for the distinctive purple color of PFSPF; however, these pigments are heat-sensitive and may break down or change during baking, resulting in a decrease in color intensity and a shift toward brown hues. This is in line with the biscuits’ observed darker hue, which may be explained by both the breakdown of anthocyanins and the production of brown pigments via non-enzymatic browning reactions [81]. The higher PFSPF content resulted in darker colors, and the lightness decreased in all formulations [82]. The decrease in lightness (L*) is probably due to the presence of bioactive phytochemicals in PFSPF, particularly anthocyanin pigments. These components impart the manufactured biscuits’ dark and purple color. As the a* and b* values decrease, the formulated biscuits gradually lose their redness and yellowness (Figure 4). Fortifying PFSPF also altered the chroma and hue angle, which influenced the overall perception of color quality. The Maillard reaction during baking, which generates colored compounds, is the primary cause of the color development in the manufactured biscuits [83]. Moreover, anthocyanin or purple pigments, water activity, dough composition, baking temperature, baking time, and the presence of sugars, starches, and proteins are some of the variables that affect the color intensity of the PFSPF incorporated biscuits [45].

Sensory profile of PFSPF incorporated biscuits

From the questionnaires, Figure 5 depicted that T₆ (50% PFSPF) had the highest overall acceptability score, while all biscuit samples received scores above the minimum acceptable threshold of 5 on a 9-point hedonic scale, indicating that the formulations remained within the range of consumer acceptability. Sensory evaluation results interpreted that a slight improvement in visual appearance was noted at higher substitution levels, possibly attributable to anthocyanin pigments and uniform porosity of PFSPF, which may have contributed to optimum appearance scores. These findings are comparable to those documented by Amani et al. [84]. Moreover, fortifying PFSPF also changed the flavor of the formulated biscuits. As substitute levels increased, sensory panelists observed a progressive loss of the unique biscuit flavor, most likely as a result of reduced gluten network development and increased fiber content. Furthermore, taste and texture (crispiness) showed comparable trends. Texture and taste evaluations increased marginally as the amount of PFSPF increased. This might be the result of the optimized biscuits being less cohesive and harder [85]. According to the sensory scores, panelists’ opinions of the various treatments were strongly correlated with important sensory characteristics, especially color, texture, and flavor. Samples with more desirable natural color (dark color for PFSPF), suitable textural qualities (like optimal crispness), and balanced flavor profiles received higher acceptable scores. The sensory assessment ratings showed that T₆ biscuits were the most popular overall (7.60), which means that test panelists mostly preferred PFSPF incorporated biscuits. These findings indicated that PFSPF is a promising enhancement for the nutritional quality of bakery food items while maintaining consumer appeal.

Effects of PFSPF substitution on the properties of the biscuits

Conclusions

This study illustrates that PFSPF could be used as a functional ingredient to make the biscuits more nutritious. The abundance of phenolic compounds, anthocyanins, and carotenoids in the sweet potato flour greatly increased the antioxidant and bioactive potential of the PFSPF incorporated biscuits. However, the substitution of more PFSPF altered the color parameters, leading to a decrease in whiteness and an increase in brownness. The ideal substitution level was found to be 50%, which strikes a balance between desired sensory and physical qualities and functional benefits. Lastly, it can be said that T₆ biscuits with PFSPF (50%) demonstrated favorable overall acceptability, a unique color, and a desirable texture, and gave people more energy than control biscuits. Our future research direction will mainly focus on assessing the glycemic index in vitro or in vivo of the PFSPF developed biscuits to substantiate the potential health benefits.