Introduction

Ziziphus spina-christi L., known as Sidr, is widely distributed in Asia, Africa, and Tropical America1. This species is acknowledged for its uses in traditional medicine. Sidr is identified as a multifunctional tree, with its fruit juices providing significant bioactive components, including phenols, flavonoids, alkaloids, vitamin C, botulinic acid, glycosides, steroid tannins, and bioactive polysaccharides2. Moreover, the pulp of Sidr is rich in minerals, fiber, vitamins, and polyphenols3. The fruit pulp has elevated vitamin A and C levels relative to other plant components. In modern civilization, there is an increasing tendency in the use of natural plant-based medicines for treating various disorders and for their preventive characteristics4.

The various components of the Sidr plant are abundant in total phenolic and flavonoid content, with catechin recognized as the primary ingredient in both the powdered and aqueous extracts of the pulp. Rutin, a flavonoid, was recognized as the primary constituent in the aqueous extract obtained from Sidr pulp4. According to Regulation (EC) No 1924/2006 of the European Parliament and Council of December 20, 2006, on nutrition and health claims connected to food items, Sidr’s high fiber content allows the statement “high fiber content” to be used on the packaging. Due to its many advantageous functions, fiber must be included in the human diet. While systemic effects affect metabolic processes, localized effects are linked to fibre availability in the gastrointestinal tract5,6. highlight that important effects of dietary fiber include its viscosity, fermentability, water-binding ability, bile acid binding, interaction with metal ions, and stool volume augmentation. Fiber plays a critical role in lowering the risks of obesity, diabetes, and cardiovascular diseases7, and adding foods high in fiber to the diet can increase satiety, decrease hunger, and provide essential nutrients as needed daily8.

The present study is novel in its comprehensive comparison of Egyptian and Saudi Ziziphus spina-christi L. (Sidr) pulp as functional ingredients in protein bar formulations, an application that has not been previously explored. Unlike earlier research that focused primarily on the medicinal or basic nutritional properties of Sidr, this work investigates the impact of partially substituting glucose syrup with varying levels (5%, 10%, and 15%) of Sidr pulp from two distinct regions on the nutritional, physicochemical, textural, and sensory characteristics of protein bars designed for athletes. The study uniquely evaluates how the geographical origin of Sidr pulp influences the enhancement of fiber, mineral content, antioxidant activity, sensory acceptability, and shelf-life of the bars. The central hypothesis is that incorporating Egyptian and Saudi Sidr pulp into protein bar formulations will significantly improve their nutritional value, sensory qualities, and storage stability, with measurable differences based on the source of the pulp.

Materials and methods

Materials

Plant materials

Egyptian fruits of Ziziphus spina-christi L. were sourced from Al-Azhar University, Egypt, and their identification was confirmed by Prof. ElSayed M. Desoky, a plant taxonomist at Zagazig University. A voucher specimen was deposited under the code CAIRC-M-127. Saudi Arabian Z. spina-christi fruits were collected from Riyadh, Saudi Arabia. The identity of these samples was verified using established botanical references9,10,11,12,13. All required permits and collection licenses for Ziziphus spina-christi L. (Sidr) were secured from the Ministry of Environment, Water and Agriculture in Saudi Arabia and the Egyptian Environmental Affairs Agency (EEAA) prior to sampling. All collections adhered to national regulations and institutional guidelines. Whey protein isolate (Foodcom, Warsaw, Poland) and psyllium husk powder (Imtenan, Cairo, Egypt) were also used in this study.

Chemicals

Solvents were purchased from Sigma-Aldrich (Steinheim, Germany), and 2,2-diphenyl-1-picrylhydrazyl (DPPH) and Folin-Ciocalteu reagent were also supplied by the same company. Sodium carbonate, potassium persulfate, and sodium dihydrogen phosphate dihydrate were obtained from Lachema (Brno, Czech Republic). Double-distilled water was prepared using a Heraeus apparatus (Heraeus Quarzschmelze, Hanau, Germany). Epicatechin and catechin standards were acquired from Fluka (North Carolina, USA), while p-coumaric acid, gallic acid, rutin, quercitrin, caffeic acid, and ferulic acid were procured from Sigma-Aldrich.

Methods

Preparation of Sidr powder

The seeds were carefully extracted to procure the fruit’s pulp. The pulp fruits were meticulously cleaned and washed before the drying process (40 °C for 48 h). The dry pulp was milled and sieved using a 40-mesh screen, and thereafter kept at 4 °C until analysis14.

Preparation of protein bars

The bar material was designed to have a weight of 70 g, and the protein bars were manufactured using different formulae as specified in Table 115. The ingredients were mixed manually using a spatula. Glucose syrup, warmed to 80 °C, was mixed with glycerol for 1 min. Thereafter, 50% of the protein component was integrated into the mixture. Subsequent to a 30-second mixing interval, the remaining half of the protein was included, followed by a further 30 s of mixing, during which the container’s edges were scraped to guarantee consistency. A concluding mixing phase of 30 s was executed. The resultant high-protein bar mass was further sliced transversely into individual bars measuring 8.00 × 3.00 × 1.50 mm. All samples were permitted to equilibrate at room temperature for 24 h prior to storage at 37 ºC with ambient humidity to promote thermodynamic alterations in the bars.

Table 1 Formulations of protein bars.
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Formulation of the protein bars

The formulation presented in Table 1 was developed based on a combination of previous literature on protein bar composition and preliminary trials conducted to optimize texture, nutritional value, and sensory acceptability15. Standard protein bar formulations typically utilize glucose syrup as a primary binder and sweetener; however, recent studies have highlighted the potential of incorporating fruit pulps to enhance the nutritional profile and functional properties of snack bars. The decision to partially replace glucose syrup with Sidr pulp powder at 5%, 10%, and 15% was informed by the high fiber, mineral, and polyphenol content of Sidr, as reported in earlier research2,3. The aim was to reduce the overall sugar content while increasing dietary fiber and bioactive compounds, thereby creating a more health-promoting product suitable for athletes and health-conscious consumers. Psyllium husk was included as a natural binding agent to compensate for any changes in texture due to the reduction of glucose syrup. This approach allowed to systematically evaluate the impact of Sidr pulp incorporation on the nutritional, functional, and sensory characteristics of the protein bars, distinguishing our formulation strategy from conventional recipes.

Analytical techniques

Chemical analysis

The determination of mineral content, such as iron and zinc, along with the overall chemical composition analyses (moisture, protein, ash, fat, crude fiber, and total carbohydrates), were carried out according to AOAC methods 2011.14, 925.10, 940.26, 920.39, 962.09, 979.06, and 2020.07 respectively16. The quantification of reducing and non-reducing sugars followed the procedures established by Sri’s study17.

Bio-active constituents

A sample weighing 250 mg was combined with 25 mL of 70% methanol and subjected to ultrasonic extraction for 30 min at 30 °C using an ultrasonic cleaner. After extraction, the mixture was centrifuged at 13,000 rpm for 2 min to separate the solid residues. The resulting supernatant was filtered through a 0.22-µm nylon filter to obtain a clean extract suitable for further analysis18.

Total phenolic content (TPC)

To determine TPC, 100 µL of the filtered extract was mixed with 900 µL of distilled water and 100 µL of Folin-Ciocalteu reagent. After a 5-minute wait, 300 µL of a 20% sodium carbonate solution was added, and the mixture was incubated in the dark for one hour at room temperature. The absorbance was measured at a wavelength of 765 nm using a spectrophotometer. TPC was expressed in milligrams of gallic acid equivalents per gram of sample (mg GAE/g sample), following the methods outlined in Najafi’s study19.

Total flavonoid content (TFC)

TFC was assessed using the aluminium chloride colorimetric method of Criste’s study20. In a 10 mL test tube, 1.5 mL of the extract was mixed with 4.5 mL of distilled water. Following this, 0.15 mL of a 5% sodium nitrite solution was added and incubated at room temperature for six minutes. Afterwards, 0.15 mL of a 10% aluminium chloride solution was introduced, and the mixture continued to react for another six minutes. Finally, 1 mL of 1 M sodium hydroxide was added to adjust the total volume to 10 mL with distilled water. The absorbance was measured at 510 nm using a spectrophotometer, with quercetin serving as the standard for calibration; TFC was reported as milligrams of quercetin equivalents (QE) per gram of sample.

Antioxidant efficacy by DPPH assay

The antioxidant capacity of the extracts was evaluated using the DPPH assay19. A DPPH solution was prepared by dissolving 24 mg of DPPH in 100 mL of methanol to achieve a concentration of 0.1 mM. In this test, 100 µL of the filtered extract was combined with 3.9 mL of the DPPH solution and allowed to incubate in the dark for half an hour at room temperature. The absorbance at 517 nm was then measured using a spectrophotometer to assess the reduction in absorbance, with antioxidant activity expressed as a percentage of DPPH scavenging calculated with the formula:

DPPH Scavenging activity (%) = [A0 -A1] / A0.

A0 represents the absorbance of the blank (DPPH solution devoid of extract), whereas A1 denotes the absorbance of the DPPH solution containing the extract.

Identification of phenolics and flavonoids using HPLC

The chemical profiling of polyphenolic fractions employed high-performance liquid chromatography (HPLC)21. An Agilent 1260 series HPLC instrument with a Zorbax Eclipse Plus C8 column (4.6 mm x 250 mm i.d., 5 μm) facilitated separation. The mobile phase for HPLC analysis of phenolic and flavonoid compounds consisted of solvent A (water) and solvent B (acetonitrile with 0.05% trifluoroacetic acid), delivered at a flow rate of 0.9 mL/min using a linear gradient. Detection was performed using a UV-Vis detector at 280 nm.

Amino acid profiling

The amino acid composition of the experimental sample was determined using HPLC Pico-Tag method22. Phenylisothiocyanate (PITC, or Edman’s reagent) was used for pre-column derivatization, while reversed-phase gradient elution high-performance liquid chromatography (HPLC) separates the phenylthiocarbamy (PTC) derivatives, which were detected by their UV absorbance. The chromatographic analysis using HPLC was carried out using the following gradient of Pico-Tag solvent A and B (P/N 88108 and 88112). Sample was injected and loaded on a Pico-Tag amino acids column (150 × 3.9 mm) stainless steel. Detection of the PTC derivatives is by ultraviolet absorption measurements using a fixed wavelength (254 nm) Waters detector. Before injecting the sample, the illustrated was calibrated by two injections of the standards. Chemical scores were calculated based on essential amino acid composition compared to reference values from optimal sources like breast milk according to FAO/WHO/UNU23.

Fatty acid profiling

Fatty acid analysis utilized gas chromatography with flame ionization detection (GC-FID). Oil samples were converted into fatty acid methyl esters (FAMEs) through a process involving sodium methoxide and hexane, followed by phase separation via centrifugation24, followed by separation and quantification an HP 6890 plus gas chromatography (Hewlett Packard, USA), equipped with a flame ionization detector (FID). The GC system was operated with a capillary column (HP-88, 100 m × 0.25 mm × 0.20 μm film thickness), with helium as the carrier gas at a flow rate of 1.0 mL/min. The injector and detector temperatures were set at 250 °C, and the oven temperature was programmed from 140 °C (held for 5 min) to 240 °C at 4 °C/min. Fatty acid methyl esters were identified by comparing retention times with those of authentic standards.

Color instrumental analysis

The color characteristics of protein bars were assessed using a Chroma Meter, (Model: CR-400 Konica Minolta, Japan). Calibration with a white reference ensured accuracy in measuring color parameters based on CIE lab color system values: L*, a*, and b*. The overall color difference between samples was calculated using:

$$\:\:E=\sqrt{{\left(a1-a2\right)}^{2}+{(b1-b2)}^{2}+{(L1-L2)}^{2}}$$

The Chroma Meter CR-400 is esteemed for its mobility and accuracy, rendering it appropriate for many applications, including food quality evaluation. It possesses an 8 mm measuring aperture and can internally store up to 1000 measurements or 2,000 with an optional data processor. This gadget enables user-defined evaluation algorithms and delivers quick pass/fail evaluations based on established criteria, hence augmenting its value in quality control operations across several sectors25.

Water activity measurement

Water activity (aw​) was measured using an AWMD-10 water activity meter with an infrared detector (NAGY, Gäufelden, Germany) ± 0.001 accuracy in aw units, under controlled temperatures (25 °C ± 2)26. The measurement relies on equilibrium relative humidity principles.

Textural analysis

Cutting force evaluation

The cutting force required to slice protein bars was measured using a Texture Analyzer equipped with Warner Bratzler blade assembly. Each formulation underwent five trials to determine the maximum force required for complete severance27.

Hardness assessment

Protein bar hardness was measured at a testing velocity of 1 mm/s using the TA-XT2i Texture Analyzer in conjunction with Texture Expert Software. A 36 mm diameter probe (SMS P/36R) was used to compress the protein bars twice in order to achieve 70% deformation, with a 5-second gap between probe movements. By measuring the force required to compress the sample, this method provides a reliable assessment of protein bar hardness and provides important information about their textural qualities and consumer appeal27. The TA-XT2i is recognized for its versatility and precision in evaluating diverse physical properties of materials, making it an indispensable instrument in food technology and product development.

Evaluation of sensory attributes

A sensory review of protein bars was performed by 10 nutrition experts (approved by National Research Centre) utilizing a 9-point hedonic rating system to evaluate aspects including flavor, texture, appearance, and general acceptability. Before the assessment, the panelists were given comprehensive instructions on the sensory evaluation procedure and were supplied with samples of the protein bars, as well as essential items such as silverware, tissues, and filtered water. Each expert sampled the protein bars and subsequently completed a sensory rating form to document their impressions. To avert flavor contamination and guarantee precise evaluations, panellists were directed to cleanse their palates with water following the tasting of each bar. The stringent technique guarantees that the sensory data obtained is dependable and indicative of the product’s attributes, hence facilitating the enhancement and evolution of the protein bars to align with customer preferences28,29.

Statistical analysis

All statistical analyses were performed in triplicate. Data were analyzed using IBM SPSS Statistics, version 29.0 (IBM Corp., Armonk, NY, USA; https://www.ibm.com/products/spss-statistics). One-way analysis of variance (ANOVA) was applied to determine significant differences among means, followed by Duncan’s multiple range test at a significance level of p < 0.05. Microsoft Excel for Microsoft 365 (Microsoft Corp., Redmond, WA, USA; https://www.microsoft.com/en-us/microsoft-365/excel) was used for data entry, preliminary calculations, and graphical representation. All results are presented as mean ± standard deviation (SD).

Results and discussion

Approximate analysis of Sidr pulp from Egypt and Saudi Arabia on a dry-weight basis

Based on dry weight, the estimated chemical composition of Saudi Sidr (SS) and Egyptian Sidr (ES) was examined (Table 2). Compared to the 83.63% total carbs in SS powder, the ES powder had higher amounts of ash, protein, fiber, and lipids (4.32%, 5.11%, 11.58%, and 1.05%, respectively), but a lower amount of total carbohydrates (77.94%). Consequently, SS exhibited the greatest levels of both reducing and non-reducing sugars. The mineral content analysis revealed that ES powder exhibited elevated levels of iron and zinc (31.65 and 12.94 mg/100 DM, respectively) in comparison to SS powder (21.66 and 9.03 mg/100 DM, respectively). These results align with Osman and Ahmed who reported that Sidr pulp comprised 4.56% protein, 1.17% lipids, 4.49% ash, 85.69% total carbohydrates, 37.35% reducing sugar, and 21.94% non-reducing sugar, while noting a lower crude fiber content of 4.15% compared to our findings of 9.55–11.58% crude fiber1. The proximate analysis of five cultivars of Chinese jujube revealed significant (p < 0.05) variations in moisture (17.38 to 22.52%), proteins (4.75 to 6.86%), carbohydrates (80.86 to 85.63%), lipids (0.37 to 1.02%), ash (2.26 to 3.01%), fiber (5.24 to 7.18%), fructose (18.60 to 42.90%), glucose syrup (19.20 to 27.20%), and sucrose (0.21 to 17.40%)5. Lower levels of iron (4.68 to 7.90 mg/100 g) and zinc (0.35 to 0.63 mg/100 g) were observed comparing to this study findings, which also indicated reduced iron and zinc levels relative to other study1, which reported iron at 71.99 mg/100 g and zinc at 65.23 mg/100 g.

Table 2 Chemical composition of Egyptian and Saudi Sidr pulp.
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Phytochemical examination

Quantification of total phenolics and flavonoids in Sidr powder

Significant (p < 0.05) differences in the phytochemical compositions of Saudi Sidr (SS) and Egyptian Sidr (ES) powders are shown by comparing their total phenolic content (TPC) and total flavonoid content (TFC). (Fig. 1) shows that the total phenolic content (TPC) of the Saudi Sidr powder (SS) was higher than that of the Egyptian version (ES). This discovery indicates that fruits from Saudi Arabia may exhibit enhanced antioxidant capability owing to their elevated phenolic content. In Criste’s study of Turkish Sidr genotypes and found that their total polyphenolic content ranges from 213 to 554 mg GAE per 100 g, are consistent with this conclusion20. In a study of 187 plants from 17 wild populations of Sidr in the trans-Himalaya region, it was known that phenolic compounds have antibacterial, anti-inflammatory, and antioxidant properties that can enhance overall health and well-being30. The phytochemical profile of plants can be significantly (p < 0.05) changed by a variety of factors, including soil composition, environmental conditions, and cultivation methods, all of which can have an impact on the variation in total polyphenolic content31.

Fig. 1
figure 1

Total phenolics and flavonoids content of Sidr powder.

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The Saudi Sidr powder (SS) had a greater total flavonoid content (TFC) compared to the Egyptian Sidr powder (ES). The acquired result aligns with the documented TFC of sea Sidr, around 23 mg quercetin equivalent per gram of dry extract31. Flavonoids are a subclass of phenolic chemicals recognized for their strong antioxidant properties and several health advantages, including anti-cancer actions and cardiovascular protection. The elevated flavonoid concentration in SS indicates potential superior health advantages over ES, rendering it a more attractive choice for functional food applications, especially in products designed to boost athletic performance and recuperation. These results demonstrate how important environmental and regional factors are in determining the phytochemical makeup of Sidr fruits. Both powders may be effective natural antioxidant sources in dietary supplements and functional meals, according to the increased TPC in ES and TFC in SS. This information is crucial for furthering food industry research and development, particularly when developing goods for athletes and health-conscious consumers who want to increase their intake of natural nutrients.

Radical scavenging activity (% RSA) of Sidr powder

The radical scavenging activity (RSA%) assessed by the DPPH test for both Saudi (SS) and Egyptian (ES) Sidr (Zizyphus spina-christi) fruit powders demonstrates significant (p < 0.05) disparities in their antioxidant capabilities at different doses. (Fig. 2) displays the RSA% results for both extracts at varying concentrations. The findings demonstrate that Saudi Sidr powder (SS) consistently showed superior radical scavenging performance relative to Egyptian powder (ES) at all evaluated concentrations. At the maximum concentration (0.5 mL), SS attained an RSA% of 74.19, whereas ES exhibited an RSA% of 55.35. This tendency indicates that the Saudi Sidr may possess a higher concentration of antioxidant chemicals capable of efficiently neutralizing free radicals.

Fig. 2
figure 2

Radical scavenging activity (% RSA) of Sidr powder.

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The elevated RSA% in SS corresponds with other research that underscores the substantial antioxidant capabilities of sea Sidr. Research indicates that Sidr berries are abundant in phenolic chemicals and flavonoids, which are recognized for their antioxidant properties. Phenolic substances, such as flavonoids, have demonstrated significant (p < 0.05) radical scavenging capabilities, therefore safeguarding cells from oxidative damage30,32. The discrepancies in RSA% between the two sources may be ascribed to several reasons, including genetic variety, environmental circumstances, and growing procedures. Prior research demonstrates that environmental stresses, including soil composition and temperature, can markedly affect the phytochemical profiles of plants, resulting in differences in their antioxidant capabilities30. The ramifications of these findings are substantial for the food and nutraceutical sectors, as the elevated antioxidant activity of SS indicates its possible application in functional meals and dietary supplements designed to enhance health and mitigate oxidative stress-related ailments. The antioxidant capabilities of Sidr render it a beneficial component for goods aimed at health-conscious customers and athletes wanting to improve their nutritional intake via natural sources. Phenolic and flavonoid components in the Saudi (SS) and Egyptian (ES) Sidr (Zizyphus spina-christi) fruit extracts were identified and quantified using HPLC analysis. The acquired data are presented in Table 3 and shown in Fig. 3.

Table 3 Identification of phenolic and flavonoid compounds of Sidr pulp by HPLC.
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Fig. 3
figure 3

Phenolic and flavonoid chromatogram of Sidr pulp by HPLC for both SS “A” and ES “B”.

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In comparison to the Egyptian powder (ES), the Saudi Sidr powder (SS) contained higher concentrations of the majority of phenolic components and flavonoids. This finding is consistent with Ercisli’s study32, which found significant (p < 0.05) variations in the phenolic profiles of various Sidr genotypes. With a concentration of 65.43 µg/g, rutin was the most abundant flavonoid in SS; however, its level in ES was much lower, at 3.65 µg/g. Also, the results of Ciesarová’s study33 are consistent with this result; it was found that rutin is a major flavonoid in Sidr leaves. Compared to ES, SS has higher levels of gallic acid and chlorogenic acid. Phenolic acids have been associated with a number of health benefits, such as antibacterial, anti-inflammatory, and antioxidant properties. Interestingly, quercetin was only detected in ES, while a number of compounds, such as hesperetin, kaempferol, and ellagic acid, were only identified in SS. The differences could be explained by the plants’ genetic diversity and the influence of the environment on the synthesis of specific phenolic chemicals30. Saudi Sidr powder may offer more antioxidant and health-promoting qualities than Egyptian powder, according to the variations in phenolic profiles between SS and ES. The development of functional foods and dietary supplements targeted at athletes and health-conscious customers looking for natural antioxidant sources depends on this understanding.

Fatty acid derived from Sidr pulp

The analysis of fatty acids from Saudi (SS) and Egyptian (ES) Sidr (Zizyphus spina-christi) fruit extracts, conducted via gas chromatography (flame ionization detection), indicates significant (p < 0.05) disparities in their fatty acid compositions, potentially affecting their nutritional and health-promoting attributes. The fatty acid profiles of SS and ES exhibit differences in saturated and unsaturated fatty acids, as outlined in Table 4. The pulp of Saudi sidr had a greater total amount of saturated fatty acids (35.04%) in comparison to ES (29.53%). Palmitic acid (C16:0) was the most saturated fatty acid in both extracts, comprising 22.54% of SS and 18.74% of ES. Palmitic acid is recognized for its involvement in energy metabolism and cellular architecture; yet, excessive consumption correlates with elevated LDL cholesterol levels and heightened cardiovascular risk34. The elevated concentrations of saturated fatty acids in SS may enhance its stability and shelf life, rendering it appropriate for many culinary uses. Conversely, ES had a greater overall proportion of unsaturated fatty acids (68.60%) than SS (59.90%). Oleic acid (C18:1n9c) was the main unsaturated fatty acid in ES, constituting 40.85% of the total fatty acids, whereas SS contained 30.78%. Oleic acid is associated with several health benefits, including improved cardiovascular health and reduced inflammation35. The elevated oleic acid concentration in ES indicates potential advantages for cardiovascular health, corroborating other research that underscores the significance (p < 0.05) of monounsaturated fatty acids in dietary intake.

Table 4 Fatty acid profiling of Sidr fruits.
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The omega fatty acid compositions differed between the two extracts. ES had a greater quantity of omega-9 fatty acids (46.5%) attributable to its increased oleic acid content. Omega-9 fatty acids are recognized for their capacity to enhance lipid profiles and diminish the risk of cardiovascular disease34. Both extracts exhibited comparable concentrations of omega-6 fatty acids, with ES somewhat elevated at 15.64% relative to 15.04% in SS. SS had a greater content of omega-3 fatty acids (7.60%) in comparison to ES (5.88%). Omega-3 fatty acids are vital for promoting cardiovascular health and reducing inflammation36. The elevated omega-3 levels in SS may confer supplementary health advantages, especially for chronic disease prevention. The analysis of fatty acids from SS and ES reveals notable disparities in their nutritional makeup. SS has elevated quantities of saturated and omega-3 fatty acids, but ES is more abundant in unsaturated fatty acids, especially oleic acid. These changes indicate that both extracts provide distinct health advantages, rendering them advantageous for diverse uses in functional foods and nutritional supplements. Subsequent studies should investigate the implications of these disparities on health outcomes and the feasibility of using these extracts to improve cardiovascular health and overall well-being.

The protein bar examination

The pattern of amino acids in the protein bar

Whey protein isolate (WPI) serves as the primary protein source in the formulated protein bars, prompting an amino acid analysis to assess the protein quality. Essential amino acids (EAAs) and non-essential amino acids (NEAAs) for the protein bars are illustrated in Figs. 4 and 5, respectively, while Tables 5 and 6 present the protein quality and chemical scores of the protein bars. The analysis of EAAs indicates that leucine is the predominant amino acid in the protein bars, with a concentration of 6.56 g per 100 g of protein. In contrast, the sulfur-containing amino acids, methionine and cysteine, have lower concentrations at 2.65 g and 1.23 g per 100 g of protein, respectively. Additionally, WPI contains high levels of branched-chain amino acids (BCAAs)—leucine, isoleucine, and valine—totalling 19.05 g per 100 g of protein, compared to 10.58 g per 100 g for aromatic amino acids like phenylalanine and tyrosine. The Fisher ratio was calculated at 1.80%, indicating a strong biological value of 82.66%. A high Fisher ratio reflects a composition rich in oligopeptides with elevated BCAA levels and lower aromatic amino acid content, which are thought to provide significant (p < 0.05) physiological benefits such as antioxidant properties, liver repair, anti-fatigue effects, and energy support37. Consequently, WPI contains 48.32 g of total essential amino acids per 100 g of protein versus 42.89 g of non-essential amino acids. Histidine stands out with a notable chemical score of 212.11%, followed by isoleucine and valine at scores of 183.43% and 180.71%, respectively. Thus, whey protein is recognized as a crucial source of essential amino acids, valued for its physiological advantages38. Whey protein supplements are acknowledged for their superior nutritional quality, rapid absorption rates, and high levels of essential amino acids, particularly BCAAs such as isoleucine, leucine, and valine39.

Fig. 4
figure 4

Essential amino acids of the protein bar (g/100 g protein).

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Fig. 5
figure 5

Non-essential amino acids of the protein bar (g/100 g protein).

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Table 5 Protein quality of Whey protein isolate.
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Table 6 The chemical scores of the protein bar compared with the recommended pattern (FAO/WHO/UNU) 18.
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The estimated assessment of prepared protein bars

Chemical analysis is essential for evaluating the quality of protein bars. This procedure involves the assessment of many factors, including moisture, protein, fat, and total carbohydrate content, to ensure compliance with regulatory standards and fulfil designated quality criteria. The impact of varying replacement ratios (5.0%, 10%, and 15.0%) of Egyptian Sidr (ES) and Saudi Sidr (SS) powders, as a partial substitute for glucose syrup, with 1.0% psyllium husk utilized as a binding agent instead of glucose syrup. In Table 7, the control protein bar (CPB) exhibited the lowest moisture content at 9.72%. Moisture content progressively increased with higher levels of ES and SS powders, with only the 10% and 20% replacement levels of ES and SS powders showing statistically significant (p < 0.05) increases in moisture content compared to that of the CPB. All treated samples had a considerable increase in ash content relative to the control bar, whereas no substantial differences in protein and fat content were observed across the samples. Furthermore, no crude fiber was identified in CBP. The substitution procedure (utilizing either ES or SS powders at different replacement levels) led to an augmentation of the product’s fiber content, which was closely associated with the rise in the replacement rate. Statistical analysis revealed a substantial disparity in crude fiber concentration among all treated samples. Total carbohydrates exhibited an inverse relationship with the escalating replacement amounts of ES or SS powders (5, 10, and 15%), with the control protein bar demonstrating the highest total carbohydrate content at 61.43%. Data demonstrated that bars added with ES (5%, 10%, and 15%) exhibited elevated levels of iron and zinc in comparison to bars supplemented with SS. The greatest concentrations of iron and zinc (6.19 and 2.39, respectively) were achieved by the ES-supplemented bar at a 15% replacement level, exhibiting significant (p < 0.05) differences from other samples. The main ingredient of high-protein nutrition bars is protein. As of right now, there is no set standard for these products; nonetheless, protein bars typically provide 15–45% protein by weight, 10–50% carbohydrates, and 10–15% fat40. High-protein nutrition bars frequently contain lipids in the form of cocoa butter15,41 or vegetable shortening42. Additionally, a range of oils, including peanut, soybean, and canola, may be used43.

Table 7 Approximate chemical composition of protein bars (on d.w.b.)
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Color characteristics of a processed bar

The color of food products is essential in assessing their quality, akin to the importance of flavor. Marketers claim that color significantly (p < 0.05) impacts customer purchase decisions. A significant (p < 0.05) proportion of customers primarily assess a food product’s flavor based on its color44. (Table 8) indicates that the control protein bar exhibited the highest L* value (65.30), while substituting glucose syrup with varying ratios (5%, 10%, and 15%) of ES or SS powder adversely affected the lightness parameter, resulting in significant (p < 0.05) reductions in lightness values for all treated samples compared to the control bar.

Table 8 Mean values of color parameters (L*, a*, b*) of protein bars.
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Regarding a* values (redness-greenness), ES-supplemented bars (5%, 10%, and 15%) exhibited elevated redness values relative to the control protein bar (3.94). Conversely, SS-supplemented protein bars (5%, 10%, and 15%) demonstrated diminished redness, with the 5% SS-supplemented bar recording the lowest value (1.07), while the 10% and 15% SS-supplemented bars displayed negative a* values, indicating a shift towards a green hue. The increase in replacement levels of SS powders resulted in a notable reduction in yellowness values (b*) compared to the control protein bar, whereas ES powder caused an elevation in b* values, exhibiting significant (p < 0.05) differences when compared to the control protein bars and those substituted with 10% and 15% of ES powder.

Water activity (aw) of formulated bars

The impact of varying replacement ratios (5.0%, 10%, and 15.0%) of Egyptian Sidr (ES) and Saudi Sidr (SS) powders, as partial substitutes for glucose syrup, combined with 1.0% psyllium husk as a binding agent, on water activity at zero time, after 15 days, and after 30 days in the manufactured protein bars. In Table 9, at zero storage time, the control protein bar (CPB) exhibited the highest water activity (aw) value of 0.734, significantly (p < 0.05) differing from other treated samples. The pronounced hydrophilic properties of whey protein-based nutrition bars lead to quality deterioration during storage, thereby limiting the shelf life. In contrast, reduced replacement ratios of ES or SS powder yielded low aw values, with the minimum aw values (0.663 and 0.681) seen in samples containing 5% ES powder and 5% SS powder, respectively, showing no significant differences between them. Thus, elevated aw values (0.710 and 0.715) were seen in protein bars containing 15% ES powder and 15% SS powder, respectively, with no significant (p > 0.05) differences noted between the two. High-protein bars are thought to contain water in two forms: bound water and free water. The interaction between the protein surface and free water is often minimal, but bound water demonstrates a considerable attraction for both protein and saccharide molecules45. The results align with other studies demonstrating that dietary fiber decreases water activity46.

Table 9 Mean values of water activity of protein bars.
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Increased fiber inclusion correlated with a decrease in expressible moisture, possibly due to the differing water-holding capabilities of various fiber types47. Psyllium husks have a significant (p < 0.05) capacity to bind free water and diminish water activity, hence extending the product’s shelf life, making them suitable as a food additive to enhance longevity and palatability. Throughout various storage durations, the water activity (aw) of CPB exhibited a progressive rise after 15 days (0.762) and 30 days (0.792). In contrast, the other treated samples demonstrated diminished aw values relative to the initial measurement, corroborating prior research that indicated the incorporation of psyllium husk initially elevated the water activity of the cake, followed by a decline during storage, thereby extending its shelf life in comparison to the control cake48.

Hardness, tensile strength, and cut resistance of fabricated bars

Texture profile analysis (TPA) is a quantitative analytical tool that evaluates texture qualities, which are crucial for assessing the acceptance of food items49. The impact of substituting glucose syrup with 5%, 10%, and 15% of Egyptian Sidr (ES) or Saudi Sidr (SS) on hardness force and cutting force was examined (Fig. 6). The cutting test functions as a measure of a product’s hardness. The top front teeth can be envisioned as transitioning from a curved formation to a linear alignment, like a knife edge. The use of a knife blade closely simulates the action of cutting or biting50. The control protein bar (CPB) had the greatest resistance to cutting force, at 13.00 N. Conversely, replacing glucose syrup with different concentrations (5%, 10%, and 15%) of either Egyptian Sidr (ES) or Saudi Sidr (SS) led to reduced cut resistance values relative to the CPB. Furthermore, it is important that a rise in the replacement % is associated with a considerable decrease in hardness, as shown in both Egyptian and Saudi Sidr varieties. This behavior may be ascribed to increased fiber content, which presumably reduces the textural cohesiveness of the protein bars.

Fig. 6
figure 6

Hardness force (N) and cutting resistance force (N) of prepared protein bars.

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The control protein bar (CPB) demonstrated the highest hardness measurement at 64.60 N, confirming the hardness data obtained and establishing a relationship between bar hardness and the force needed for cutting. This supports the notion that the force required to cut a bar is influenced by its hardness. The inclusion of fiber appears to enhance textural softness by increasing moisture levels and decreasing the density of the protein matrix. Additionally, certain flavors added to the formulation may impact the texture of the protein-polyphenol dietary fiber bars; for example, flavors with high moisture content, such as those derived from fruits, could introduce additional moisture, resulting in a softer texture51. The hardness observed in the bars can be attributed to several mechanisms, including protein aggregation through intermolecular disulfide bonds and noncovalent interactions, as well as Maillard reactions, which are a form of non-enzymatic browning52. Furthermore, factors like moisture migration and phase separation53, along with lipid oxidation54, also play a role in contributing to this hardness. The hardening of high protein bars (HPB) results from a combination of various physical, chemical, and thermodynamic factors rather than a single process53.

Sensory characteristics of the formulated bars

All samples underwent sensory evaluation for flavor, texture, appearance, and overall acceptability (Table 10). Data have shown that substituting glucose syrup with varying amounts of Egyptian Sidr powder (ES) or Saudi Sidr powder (SS) resulted in enhanced sensory attributes relative to the control protein bar (CPB). A little rise was noted between the CPB and other samples, substituted with 5% ES or SS powders for all sensory measures, with no significant (p > 0.05) variations reported among them. No notable variations in taste were observed between CPB and the others with 10% SS powder. Simultaneously, the highest taste scores (8.48 and 8.42) were obtained from a bar added with 15% ES and a bar supplemented with 15% SS, respectively, with no significant (p > 0.05) differences seen between the two. The texture parameter exhibited a notable enhancement at a 10% replacement level of ES or SS powder, achieving the optimal texture score, followed by a considerable decline at a 15% replacement level of either powder compared to the 10% replacement level across all samples. The 15% SS-supplemented protein bar had the highest acceptability score of 8.22, followed by the 15% ES-supplemented bar with a score of 8.05, with no significant (p > 0.05) differences seen between the two. The data demonstrated a gradual improvement in the overall acceptability of treated samples, corresponding with increased levels of replacement (ES or SS). The enhancement can be ascribed to the distinctive flavor imparted by the inclusion of Sidr. The results are consistent with those of Atwaa’s study, which included that to unfortified camel milk, the addition of Sidr powder greatly enhanced taste, flavor, and general acceptance55.

Table 10 Mean values of sensory parameters of protein bars.
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Constraints and prospective directions

This study emphasizes the nutritional advantages of integrating Egyptian Sidr pulp (ES) and Saudi Sidr pulp (SS) into protein bars, although many limitations must be recognized. The sensory evaluation was first performed with a restricted sample size, potentially failing to adequately reflect wider consumer preferences. The study concentrated mostly on immediate nutritional and sensory results, neglecting to evaluate the long-term health effects of ingesting these enhanced protein bars. Future studies must encompass bigger, more heterogeneous participant cohorts to substantiate sensory preferences and undertake clinical studies to assess the health advantages linked to the habitual intake of these goods. Additionally, issues like as fluctuations in the quality and content of Sidr pulp, influenced by regional variations, harvesting methods, and processing procedures, may impact the consistency of results. Resolving these problems necessitates established processes for procuring and processing Sidr pulp to guarantee consistency in nutritional value. To enhance the nutritional profile and marketability of protein bars, future studies should examine the viability of combining Sidr pulp with other beneficial ingredients, including nuts or seeds. In the end, more research in this area might result in innovative formulations that cater to health-conscious consumers and promote the use of underutilized natural ingredients like Sidr pulp in the food industry.

Conclusion

This study demonstrated that incorporating Egyptian and Saudi Sidr (Ziziphus spina-christi L.) pulp into protein bar formulations, at varying levels, can significantly (p < 0.05) enhance the nutritional and functional properties of the bars. Among all tested formulations, the protein bar containing 15% Egyptian Sidr pulp demonstrated the most favorable balance of enhanced nutritional content, antioxidant activity, and overall sensory acceptability, making it the optimal formulation in this study. Specifically, the partial replacement of glucose syrup with Sidr pulp resulted in increased dietary fiber, essential minerals (such as iron and zinc), and antioxidant activity, while maintaining desirable sensory qualities and texture. The study also highlighted differences between Egyptian and Saudi Sidr pulp in terms of their fatty acid profiles and nutrient contributions. These findings support the potential of Sidr pulp as a valuable functional ingredient for developing protein bars tailored to athletes and health-conscious consumers. Further research is recommended to evaluate the long-term stability and consumer acceptance of Sidr-enriched protein bars under real storage and market conditions. Future research is recommended to explore the long-term health effects of regular Sidr-enriched protein bar consumption, investigate consumer preferences in broader populations, and assess the scalability of production for commercial applications.