Nutritional Composition, Bioactive Compounds and Antioxidant Potential of Pineapple Rind Flour as Functional Food Ingredient
Abstract
The global drive toward transformative food systems has intensified focus on value addition and the development of functional foods from agricultural by-products. Pineapple processing significantly contributes to the food industry yet generates substantial waste, primarily in the form of peels. These by-products are often discarded, representing an untapped resource for potential functional food ingredients. This study aimed to comprehensively evaluate the nutritional and chemical composition of pineapple rind flour (PRF) as a novel functional ingredient. PRF was prepared by dehydrating fresh pineapple rinds at 50°C for 12 h using a 21S01 DOOSAT air fryer oven. The flour was subjected to various analyses including proximate composition, vitamin and mineral profiling, essential amino acid quantification, phytochemical characterization, physicochemical property assessment and antioxidant capacity evaluation using standard analytical procedures. The results revealed that PRF contains significant nutritional components: 5.99% protein, 3.92% crude fibre and 3.81% ash. Furthermore, mineral analysis showed high concentrations of potassium (941 mg/100 g), zinc (16.04 mg/100 g) and iron (36.22 mg/100 g). Vitamins A (6.52 mg/100 g) and C (28.82 mg/100 g) were present in appreciable quantities, with B-vitamins ranging from 2.33 mg/100 g to 4.16 mg/100 g. Also, PRF showed significant essential fatty acid including oleic (65.77 mg/100 g), linoleic (172.00 mg/100 g) and arachidonic acids (169.04 mg/100 g). Additionally, phytochemical analysis revealed substantial amounts of total phenols (131.32 mgGAE/g), flavonoids (97.08 mgQE/g) and cardiac glycosides (64.26 mg/g). The antioxidant capacity of PRF as measured by FRAP, DPPH and ABTS was 181.31 μmol FeSO4/g), 96.47% and 88.22 μmol TE/g DW, respectively. Importantly, antinutritional factors were present in low concentrations. These findings demonstrate the potential of PRF as a nutrient-dense functional food ingredient, suggesting that its incorporation into food products could contribute to improved their nutritional profiles while concurrently offering a sustainable solution to agricultural waste management.
1. Introduction
Pineapple (Ananas comosus) belonging to the Bromeliaceae family is a tropical fruit with significant economic importance cultivated extensively in tropical and subtropical regions of the world. Pineapple ranks third in the world after citrus and banana as the most exported food product [1]. Available statistics indicate that the global production of pineapple has witnessed an impressive increase to approximately 29.36 million metric tons in 2021 [2]. Nigeria being the leading producer in Africa occupying the eight position worldwide accounts for 1.61 million metric tons. Pineapple’s popularity is attributed to its unique flavour profile and versatile applications in the food industry; ranging from fresh consumption to processed products such as juices, canned fruits, jams and confectioneries. Intriguingly, the surge in pineapple production and processing has led to a concomitant increase in agricultural waste, primarily in the form of rinds, stems, cores, leaves and crowns, which constitute up to 50% of the total pineapple mass [3]. The disposal of these by-products (waste materials) presents significant environmental challenges and represents a loss of potentially valuable resources. Hitherto, these by-products largely remain underutilized and end up in landfills, thereby causing negative environmental and economic impacts, which include contributing to the emission of greenhouse gases during decomposition. However, in recent years, there has been considerable interest in the concept of value addition and sustainable food systems, which emphasizes the valorization of agricultural by-products. This approach not only addresses waste management issues but also offers potential economic benefits through the development of value-added products [4–6]. Pineapple by-products, particularly the rinds, have garnered attention due to their potential as sources of bioactive compounds [7], which could be converted to functional food ingredients with health and economic benefits [4–6]. Specifically, flours from agricultural products have been employed in the production of crackers with improved nutritional contents for humans [8], cakes [9] and animal feeds [10] as well as land fertilizers and bioethanol productions [11]. Unlike developed climes where valorization has long become a strategy for food fortification [12], developing countries are still less optimal in this regard owing to inadequate technological and infrastructural advancement. While previous studies have documented various applications of pineapple by-products, a comprehensive nutritional and functional characterization of pineapple rind flour (PRF) is lacking but will be crucial in guiding its incorporation into food products. Consequently, this study was aimed at carrying out a comprehensive evaluation of the nutritional and phytochemical profiles as well as antioxidant potential of PRF.
2. Materials and Methods
2.1. Sample Collection and Preparation
Ripe pineapple fruits were purchased from a local railway fruit market in Makurdi metropolis of Benue State, Nigeria, and were thoroughly washed under running water and manually peeled with a stainless steel knife to obtain the rinds. The rinds were cut into smaller pieces and spread on a tray in a 21S01 DOOSAT air fryer oven (Ningbo, China) to dehydrate at 50°C for 12 h. The dry samples were then milled using an HS-202 DOOSAT super machine blender into flour. It was then sieved using a 0.5-μm sieve and stored in a laminated pouch at room temperature for further use.
2.2. Proximate Analysis
The proximate analysis of samples was determined using standard procedures [13]. The moisture content was determined by oven drying method, the total ash content was determined by dry the ashing method using a muffle furnace operated at 500°C for 12 h, the protein content was determined using the Kjeldahl method, while the crude fat content was determined by the solvent extraction method. The crude fibre content was determined by the refluxing method, while the carbohydrate content was measured by the difference approach.
2.3. Determination of Mineral Content
The mineral content was evaluated according to the method described in Ref. [13], using an Agilent 2000 series atomic absorption spectrophotometer (Thermo Scientific Solar Wizard). Two grams of the sample was weighed and prepared into ash dissolved in 5 mL of 0.1 N HCl before being filtered with Whatman No. 42 filter paper. Exactly 1 mL of the filtrate was measured into labelled cuvettes and inserted in the spectrophotometer. The instrument was loaded with appropriate hollow cathode lamps for the required metals and was turned on. The lamps were allowed to warm up for a minimum of 15 min. Then, the monochromator was positioned to a wavelength corresponding to the desired minerals to be analysed, and the hollow cathode current was adjusted to 2A. The burner and nebulizer flow rate were appropriately adjusted before the flame was lit up. The concentration of the metals was read directly from the calibration curve at wavelengths of 766.5 nm for potassium, 394 nm for zinc, 213.9 nm for iron, 324.7 nm for copper, 530 nm for manganese, 422.7 nm for calcium, 285.2 nm for magnesium and 470 nm for phosphorous.
2.4. Determination of Vitamin Content
The vitamin content was determined by the double-injection method as described in Ref. [13] using a Thermo Scientific UltiMate 3000 Dual Gradient Standard HPLC system (ACD Spectrus Processor).
2.5. Determination of Water-Soluble Vitamins (B1, B2, B3, B6, C)
The water-soluble vitamins were determined using the double-injection method of HPLC as described in Ref. [13]. Exactly 1 g of the sample was weighed into a 100-mL volumetric flask, while 80 mL of deionized water was added to the sample and allowed to stand for 15 min. The 100-mL mark was made up with the addition of deionized water. The solution was thereafter filtered through a 0.2-μm filter paper. Ten microlitre (10 μL) of the sample was then measured into the cuvette on the HPLC system and appropriate wavelength selected. Numerical readings were copied as displayed on the system monitor.
2.6. Determination of Fat-Soluble Vitamin A
The Vitamin A content was determined using HPLC according to the method described in Ref. [13]. About 1.5 g of the sample was weighed into a 10-mL volumetric flask, and 8 mL of CH3OH–CH2Cl2 (1:1 v/v) was then added to it and allowed to stand for 15 min. The CH3OH–CH2Cl2 (1:1 v/v) was further added to reach the 10-mL mark. The solution was then filtered through a 0.2-μm filter paper. Ten microlitre (10 μL) of the sample was measured into the cuvette on the HPLC system, and the appropriate wavelength was selected. Numerical values displayed on the system monitor were recorded.
2.7. Determination of Essential Amino Acid (EAA) Content
The amino acid profile was carried out by the retention time method described in Ref. [13] using a Thermo Scientific UltiMate 3000 Dual Gradient Standard HPLC system (Chromeleon 7). The sample was first lyophilized for 90 min. Ten grams (10 g) of the sample was then weighed into a test tube and hydrolysed with 200 μL of constant 6 N HCl and 40 μL of phenol through vapour-phase hydrolysis. The sample was dried in an oven operated at 112–116°C for 20–24 h to remove excess HCl. The tube was vacuum-dried for 90 min after which the sample was reconstituted with 100 μL of 20 mM boiling HCl. Twenty microlitre (20 μL) of the reconstituted sample was then measured and added to 20 μL borate buffer and vortexed for homogeneity. The solution was transferred into a vial, which was heated in a water bath at 55°C for 10 min. Ten microlitres (10 μL) was injected into the HPLC system. The amino acids were detected based on the retention time, and the respective concentrations were displayed on the system monitor for recording.
2.8. Determination of Fatty Acid Content
The fatty acid content was determined according to the method described in Ref. [13] using a mass spectrometer coupled with an Agilent 1260 infinity HPLC system (ACD Spectrus Processor). Ten grams (10 g) of the sample was added into 10 mL of methanol in a volumetric flask and heated for 15 min in a water bath at 65°C. Exactly 2 mL each of oleic, linoleic, and arachidonic acid standards was added to the mixture in addition to 5 mL dichloromethane and 2 mL HCl before being warmed in a preset water bath at 85°C for 1 min. The mixture was filtered with Whatman No. 5 filter paper and cooled. Exactly 1 μL of the prepared sample was injected into the HPLC-coupled mass spectrophotometer. The reading for oleic acid was taken at 380 nm, that for linoleic acid at 1320 nm and that for arachidonic acid at 580 nm as produced by ACD Spectrus Processor Quantitative Analysis software (version B.05.00)
2.9. Phytochemical Analysis
2.9.1. Determination of Total Phenolic Content
The total phenolic content was determined as described in Ref. [13] method using a UV-1900i UV–vis spectrophotometer (LabX, USA). Three grams of the sample was dissolved in 10 mL of methanol using a clean beaker. Exactly 1 mL of Folin–Ciocalteu phenol reagent was added to the beaker and briefly vortexed. The mixture was incubated for 5 min before the addition of 10 mL of 7% Na2CO3 solution to stop the reaction. Exactly 20, 40, 40, 60, 80 and 100 μg/mL standards of gallic acid solutions were then prepared and incubated for 90 min at 30°C. The sample and standards were then loaded in the UV–visible spectrophotometer and read at the wavelength of 550 nm. The total phenolic content of the sample was read and recorded as mg of mgGAE/g per sample.
2.9.2. Determination of Total Flavonoid Content
The flavonoid content was determined as described in Ref. [13] method using a UV-1900i UV–vis spectrophotometer (LabX, USA). Two grams of the sample was dissolved in 5 mL of water in a 20-mL volumetric flask. About 0.30 mL of 5% sodium nitrite was added to the flask and allowed to react for 5 min. Also, 0.3 mL of 10% aluminium chloride was added to the flask and incubated for another 5 min after which 2 mL of 1M NaOH was then added. The solution was thereafter diluted with 10 mL distilled water. Quercetin standard solutions of 20, 40, 60, 80 and 100 μg/mL were then prepared, and 1 mL of both sample and the standards was then injected into the UV–visible spectrophotometer and read at a wavelength of 510 nm. The total flavonoid content was read and recorded as mg of mg QE/g of sample.
2.9.3. Determination of Alkaloids
The alkaloid content was determined as described in Ref. [13] using a UV-1900i UV–vis spectrophotometer (LabX, USA). Three milligram (3 mg) of the sample was dissolved in 5 mL of dimethylsulphoxide. Also, 1 mL of 2N HCl was added to the solution and filtered. The solution was then transferred to a separating funnel and 5 mL of bromocresol green solution, 5 mL of phosphate buffer, and 4 mL of chloroform were added and vigorously shaken. The extract was collected and separated in a 10-mL volumetric flask before being diluted to 10 mL volume with chloroform. Reference standard solutions of atropine were prepared in 0, 40, 60, 80 and 100 μg/mL and 1 mL of both sample and standards were injected into the UV/visible spectrophotometer and read at the wavelength of 470 nm. The total alkaloid content was recorded and calculated as mg of mgAE/g of sample.
2.9.4. Determination of Cardiac Glycosides
The cardiac glycoside content was determined as described in Ref. [13] using a UV-1900i UV–vis spectrophotometer (LabX, USA). Similarly, 3 g of the sample was dissolved in 8 mL of methanol in a beaker and the solution was filtered at room temperature and made up to 10-mL mark with methanol. The sample solution was then mixed with 10 mL of freshly prepared Baljet’s reagent and allowed to stand for 60 min. The mixture was then diluted with 20 mL distilled water, and then, 1 mL of the sample was injected into the UV/visible spectrophotometer and read at the wavelength of 495 nm. The cardiac glycoside content was recorded as mg/g of sample.
2.9.5. Determination of β-Carotene Content
The beta-carotene content was determined using the method described in Ref. [13] using a UV-1900i UV–vis spectrophotometer (LabX, USA). Two grams of sample was dissolved in 10 mL of ethanol, and the solution was placed in a water bath at 70–80°C for 20 min with periodic shaking. The supernatant (extract) was decanted and allowed to cool. Also, 10 mL of deionized water was added to the solution and allowed to cool in a container of ice water for 5 min. Then, 1 mL of the extract was injected into the UV/visible spectrophotometer and read at the wavelength of 436 nm. The β-carotene content was recorded as mg/100 g of sample.
2.10. Determination of Physiochemical Properties
2.10.1. Determination of Bulk Density
2.10.2. Determination of Dispersibility
2.10.3. Determination of Water Absorption Capacity
2.11. Determination of Antioxidant Activity
2.11.1. Determination of Ferric Reducing Property
The ferric reducing property was determined according to the method in Ref. [13] using a UV-1900i UV–vis spectrophotometer (LabX, USA). Two grams of the sample was dissolved in 5 mL of 0.2 M sodium phosphate buffer at pH 6.6. The mixture was heated to 50°C and incubated for 20 min. Then, 5 mL of 10% aqueous trichloro acetic acid, 0.1% ferric chloride and 5 mL distilled water were added to the mixture and allowed to stand at room temperature for 10 min. The mixture was centrifuged at 1000 rpm for 15 min. Following this, 1 mL of the supernatant was injected into the UV–vis spectrophotometer and read at 700 nm wavelength.
2.11.2. Determination of DPPH Free Radical Scavenging Ability
2.11.3. ABTS 2,2′-Azino-Bis (3-Ethylbenthiazoline-6-Sulphonic Acid) Scavenging Ability
The ABTS scavenging ability was determined according to the method in [13] using a UV-1900i UV–vis spectrophotometer (LabX, USA). Measured 88 μL of 140 mM potassium persulfate solution was added to 5 mL 7 Mm ABTS stock solution. The mixture was incubated at room temperature for 16–18 h in amber-coloured bottles. Then, 5 g of the sample was dissolved in 10 mL distilled water and the mixture was filtered. Exactly 5 mL of the sample mixture was then pipetted into 96-well microplate where 50 μL of the incubated standard solution was then added dropwise on the coated walls of the plate. Then, 1 mL of the mixture was injected into the spectrophotometer and read at 750 nm wavelength.
2.11.4. Determination of Phytate Content
The phytic acid composition was determined according to the method described in Ref. [13] using a UV-1900i UV–vis spectrophotometer (LabX, USA). One gram of the sample was weighed into a 30-mL Erlenmeyer flask, and 50 mL of 0.5 M NaHCO3 solution was added to it. The mixture was vortexed at 6000 rpm for 30 min and filtered through Whatman No. 5 filter paper. Thereafter, 1-mL aliquot of the mixture was pipetted into a test tube where 8 mL distilled water and 0.5 mL ascorbic acid were also added. The mixture was then vortexed at 100 rpm and allowed to stand for 15 min. Exactly 1 mL of the mixture was then injected into a quartz cuvette inserted in the UV–vis spectrophotometer, while absorbance reading was taken at 720 nm wavelength.
2.11.5. Determination of Tannin Content
The tannin content was determined according to the method described in Ref. [13] using a UV-1900i UV–vis spectrophotometer (LabX, USA). Here, 1 g of the sample was weighed into a 30 mL Erlenmeyer flask with 8 mL of 1% HCl solution added into the flask, which was then vortexed at 1000 rpm for 10 min and filtered through Whatman No. 5 filter paper. Thereafter, 1 mL of the extract was pipetted into a test tube with 5 mL of vanillin reagent added into the tube. The tube was then incubated in a water bath at 30°C for 20 min. Then, 1 mL of the mixture was injected into the UV–vis spectrophotometer and the reading for the tannin content was taken at 500 nm.
2.11.6. Determination of Oxalate Content
2.11.7. Determination of Saponin Content
The saponin content was determined according to the method in Ref. [13] using a UV-1900i UV–vis spectrophotometer (LabX, USA). Two grams of the sample was weighed and dissolved in 70% ethanol. It was then vortexed at 1000 rpm and allowed to stand for 30 min before being filtered through Whatman No. 5 filter paper. The sample was injected in the UV–vis spectrophotometer, and the reading for total saponin content was taken at 325 nm.
2.12. Statistical Analysis
Measurements of each parameter were carried out in triplicate for all determinations, and the results were subjected to a one-way analysis of variance and expressed as mean with standard deviation. The differences between means were separated by Duncan’s multiple range test (DMTR) using SPSS Statistics Programme, Version 22.0. Significant differences were expressed at a 5% level.
3. Results
Table 1 shows the result of the proximate composition of pineapple rind flour (PRF). While Tables 2 and 3 present the results of the mineral and vitamin content of PRF. The results for the EAA and essential fatty acids (EFAs) are shown in Tables 4 and 5, respectively. Furthermore, the results of the phytochemical content and physicochemical properties of PRF are shown in Tables 6 and 7, while those of the antioxidant capacity and antinutrient content are represented in Tables 8 and 9, respectively.
| Parameter | Content |
|---|---|
| Moisture (%) | 9.68 ± 0.01 |
| Protein (%) | 5.99 ± 0.02 |
| Fat (%) | 5.87 ± 0.01 |
| Crude fibre (%) | 3.92 ± 0.06 |
| Ash (%) | 3.81 ± 0.01 |
| Carbohydrate (%) | 70.73 ± 0.03 |
- Note: Values are means ± standard deviation of three replicate determinations.
| Mineral (mg/100 g) | Content |
|---|---|
| K | 941.05 ± 0.01 |
| Zn | 16.04 ± 0.01 |
| Fe | 36.22 ± 0.01 |
| Cu | 4.80 ± 0.01 |
| Mn | 5.36 ± 0.01 |
| Ca | 261.07 ± 0.02 |
| Mg | 107.48 ± 0.01 |
| P | 128.50 ± 0.01 |
- Note: Values are means ± standard deviation of three replicate determinations.
| Vitamins (mg/100 g) | Content |
|---|---|
| Vitamin A | 6.52 ± 0.01 |
| Vitamins B1 | 2.33 ± 0.01 |
| Vitamins B2 | 1.94 ± 0.02 |
| Vitamins B3 | 4.16 ± 0.01 |
| Vitamins B6 | 3.73 ± 0.01 |
| Vitamins C | 28.82 ± 0.04 |
- Note: Values are means ± standard deviation of three replicate determinations.
| Essential amino acids (EAAs) (mg/100 g) | Content |
|---|---|
| Valine | 0.52 ± 0.01 |
| Histidine | 2.94 ± 0.01 |
| Leucine | 3.96 ± 0.01 |
| Iso-leucine | 3.42 ± 0.01 |
| Lysine | 4.21 ± 0.01 |
| Methionine | 2.04 ± 0.01 |
| Phenylalanine | 2.88 ± 0.01 |
| Threonine | 3.50 ± 0.01 |
- Note: Values are means ± standard deviation of three replicate determinations.
| Essential fatty acid (mg/100 g) | Content |
|---|---|
| Oleic acid | 65.77 ± 0.01 |
| Linoleic acid | 172.00 ± 0.01 |
| Arachidonic acid | 169.04 ± 0.01 |
- Note: Values are means ± standard deviation of three replicate determinations.
| Phytochemicals | Content |
|---|---|
| Total phenols (mgGAE/g) | 131.32 ± 0.01 |
| Total flavonoids (mgQE/g) | 97.08 ± 0.01 |
| Total alkaloids (mgAE/g) | 1.39 ± 0.01 |
| Cardiac glycoside (mg/g) | 64.26 ± 0.01 |
| β-Carotene (mg/100 g) | 58.83 ± 0.01 |
- Note: Values are means ± standard deviation of three replicate determinations.
| Properties | Content |
|---|---|
| Bulk density (BD) (g/mL) | 0.29 ± 0.01 |
| Water absorption capacity (WAC) (g/g) | 2.82 ± 0.01 |
| Dispersibility (%) | 65.76 ± 0.01 |
- Note: Values are means ± standard deviation of three replicate determinations.
| Antioxidant capacity | Content |
|---|---|
| FRAP (μmol FeSO4/g) | 181.31 ± 0.01 |
| DPPH (%) | 96.47 ± 0.01 |
| ABTS (μmol TE/g DW) | 88.22 ± 0.01 |
- Note: Values are means ± standard deviation of three replicate determinations.
| Antinutrient | Amount (mg/100 g) |
|---|---|
| Phytate | 1.31 ± 0.01 |
| Tannins | 13.43 ± 0.01 |
| Oxalate | 1.39 ± 0.01 |
| Saponins | 5.33 ± 0.01 |
- Note: Values are means ± standard deviation of three replicate determinations.
4. Discussion
4.1. Proximate Composition of PRF
The result of the proximate composition of PRF presented in Table 1 shows that pineapple rinds are rich in nutrients. The moisture content, which is an important parameter for shelf stability during the storage of flours, was observed to be 9.68% in the PRF sample; although it was slightly higher than in some literature studies, the difference may be attributed to differences in the drying method, duration, and temperature used, as reported in available literature studies [8, 14]. The report however agreed with that of Ramelle, Rani and Manohar [5]. The result of the protein content 5.99%, crude fibre content 3.92%, ash content 3.81%, fat content 5.87% and carbohydrate content 70.73%, which were all observed in significant amount in the PRF sample, agreed with the study by Owoeye et al. [14] but was in contradiction to that of Ref. [8]. The difference may be attributed to the difference in the drying temperature and duration. Since the moisture content is below the 11% maximum moisture content recommended for the safe storage of dried food products, PRF flour can be safely stored for future use.
4.2. Mineral Content of PRF
Table 2 represents the results of the mineral content of the PRF sample. Pineapple rinds are a rich source of many health-related components and in most cases can supply up to half of the recommended daily allowance (RDA) of most minerals required for disease prevention. Potassium neutralizes large concentration of sodium hence, stabilizing blood pressure [15]. The potassium content of PRF was revealed to be 941.05 mg/100 g, which agrees with the findings of Safriani, Dewi and Lestari [16], who reported a far greater potassium content in the peels of pineapple (938,48 mg/kg) than in the flesh (485.28 mg/kg) and core (12.98 mg/kg). Zinc and iron categorized as trace minerals needed in small quantities for metabolic processes. The role of zinc in cell-mediated immunity, tissue growth, bone formation, alcohol metabolism, sexual development, and reproduction, as well as in brain function, has been established (Bagherani and Smoller [17]; Adebanjo et al. [18]) while iron reportedly helps in haemoglobin and myoglobin synthesis, which assist in carrying oxygen in the blood and muscles, respectively [19]. The findings in our study revealed 36.22 mg/100g of iron and 16.04 mg/100 g of zinc, which is sufficient to meet the RDA stipulated for men and 18 mg/100 g for iron and 8 mg/100 g for zinc for nonmenstruating and pregnant women [20]. The result showed that PRF is potentially a better source of zinc and iron than reported in pawpaw, orange, banana, mango and watermelon peel flour [5, 21]. Although both results did not agree with the finding of Romelle, Rani and Manohar [5], who reported a lower zinc and iron content in pineapple peel flour. This variation may be attributed to the differences in the nutritional composition of pineapple, which may be dependent on factors such as variety used, growing conditions and degree of ripeness [22]. Calcium is required for the development and maintenance of healthy bones and teeth with a recommended daily requirement of 150 mg/day. The calcium content of the PRF sample was 261 mg/100g, which was significantly higher than the 29.15 mg/100 g reported in watermelon rinds flour [21] This is an indication that consuming value-added products made from PRF flour can contribute to providing the required amount of calcium required by the body.
Magnesium (Mg), which is known to help the maintenance of normal nerve and muscle function, bone growth and integrity, support a healthy immune system, reduce migraine attacks and keep the heartbeat steady [23], was reported in PRF to be 107.48 mg/100 g, which is more than half the amount required to cater for the daily magnesium needs of the bodies [24]. Phosphorus is an important component of energy precursor adenosine triphosphate (ATP) and is also a component of DNA and RNA [19, 25], reported in PRF to be 128.50 mg/100 g, which is below the RDA of 1200 mg/day for children under 18 years and 800 mg/day for adults [26]. This indicates that pineapple peel consumption alone may not provide adequate phosphorus required by the body. Although copper and manganese are required in trace amount, copper assists in iron absorption and the regulation of heart rate and blood pressure, while manganese is required for succinate dehydrogenase and the synthesis of chondroitin sulphate necessary for the formation of cartilage [19, 27]. The copper and manganese contents of PRF were 4.80 mg/100 g and 5.36 mg/100 g, respectively, which is significantly higher than the 0.45 mg/100 g and 1.42 mg/100 g reported in watermelon rinds flour [21]. The result in our study was however similar to that of Ramelle, Rani and Manohar [5], who also reported the manganese content of pineapple peel powder to be 5.32 mg/100 g.
4.3. Vitamin Content of PRF
The result of the vitamin content of the PRF sample is presented in Table 3. Vitamins help in the growth and optimal function of the body as they are known to act as coenzymes to several metabolic processes. Pineapple peels are a good source of bioactive compounds such as Vitamins C and A [28]. Although a low vitamin content was generally observed in PRF, which may have been attributed to the heat applied during the drying process, the B-complex vitamins (thiamine B1, riboflavin B2, niacin B3 and pyridoxine B6) were still reportedly higher in the PRF sample than in the pulp and juice of pineapple [1, 29]. The Vitamin C content of PRF in our study was 28.82 mg/100 g, which did not corroborate the report of Avneet et al. [30], who reported lower Vitamin C content in the peels of pineapple. This may be attributed to the differences in variety, drying temperature and technique used. Although the Vitamin A content in PRF could not meet up with the stipulated RDA of 15 mg/day, it was reported in appreciable amount. Our finding is an indication that incorporating PRF as a value-added ingredient for the formulation of health-promoting food may offer valuable contributions to nutrient deficiencies.
4.4. EAA Content of PRF
Table 4 shows the result of EAAs of the PRF sample. Amino acids are an important class of nutrients the body needs for protein synthesis. The result of the EAAs in PRF in this study revealed that lysine, which is a limiting amino acid in commonly consumed cereals such as wheat, had the highest value, while the lowest was reported in valine. The values reportedly ranged from 0.52 mg/100 g to 4.21 mg/100 g. The results however did not concur with the report of Ref. [31] who observed a slightly lower value for most of the EAAs in the pineapple peel sample; this difference may also be attributed to the processing technique used in sample preparations as well as variety used.
4.5. Essential Fatty Acid Content of PRF
The essential fatty acids are required by the body but cannot be synthesized by it but obtained through dietary sources. The result revealed significant quantities of oleic, linoleic and arachidonic fatty acids in the PRF sample. The results in our study were comparable to that reported in some seeds and nuts as presented in Table 5. This shows that PRF when used as a value-added ingredient in food formulation is capable of contributing to the prevention of large body of diseases.
4.6. Phytochemical Content of PRF
Table 6 presents the result of the phytochemical content of the PRF sample. Phytochemicals are bioactive components of plants responsible for the beneficial health effects derived from the consumption of the plants. The total phenolic and flavonoid contents of PRF were 131.32 mgGAE/g and 97.08 mgQE/g, respectively. The result did not agree with that of a previous study [32] that reported the total phenolic and flavonoid contents in pineapple peels to increase with an increase in drying temperatures. Cardiac glycosides, which are secondary metabolites in several plants, were reported to be present in the PRF sample (64.26 mg/g), which is below 100 mg/100g considered a safe range useful for the control of congestive heart failure by inhibiting the amount of Na+ and K+ pump while increasing Ca2+ levels that may be available for the contraction of heart muscles. However, the cardiac glycoside content above 100 mg/100 g has been reported to be capable of creating health concerns such as activating the nerve centre in the brain that causes vomiting. Our findings did not agree with those of Owoeye et al. [14] and Sharma et al. [33], who reported the below detectable limit for cardiac glycoside in pineapple peel. This may be attributed to the analytical technique used by previous researchers in comparison with what was used in our study. Although the β-carotene content in PRF was below the 300 mg/100g RDA, the 58.83 mg/100g observed is still significant enough to improve human vision [34]. Furthermore, the alkaloid content in our study agrees with the previous research by Dewi and Simamora [32] but contradicts that of Fitryanti, Hendrawan and Astuti [35], who reported the absence of alkaloid in pineapple peels dried at 40°C. This may be attributed to the difference in drying temperatures. Several literature studies such as Refs. [14] and [32] reported the presence of these phytochemicals in pineapple peels. These findings are a pointer to the potentials of PRF as a functional food ingredient for the development of new food products or the fortification of the existing ones.
4.7. Physicochemical Properties of PRF
The physicochemical properties of the PRF sample are presented in Table 7. The bulk density measures the heaviness of a flour sample, which determines its packaging requirements. According to Mane et al. [36], it was observed that low bulk density of a flour favours its use for formulation of weaning foods. The WAC of the PRF sample was within the range of 2.45 g/g, reported for commonly consumed wheat flour [37], which suggests PRF usefulness in combination with other flours for the production of novel food products. Dispersibility, a factor that determines effective reconstitution of a flour in water, was recorded to be 65.76%. This value is reasonably sufficient to prevent lumps when the flour is consituted, thus making the PRF suitable for food processing.
4.8. Antioxidant Capacity of PRF
The antioxidant capacity of the PRF sample as measured by FRAP, DPPH and ABTS is presented in Table 8. The results revealed that the antioxidant capacity measured by DPPH was 96.47%, which agrees with Lasunon et al. [38]. While the FRAP and ABTS activity of 181.31 μmol FeSO4/g and 88.22 μmol TE/g DW were observed in our PRF sample. This however did not agree with Refs. [8, 38]. The variations may be attributed to drying at 60°C for 48 h done by the former and the utilization of pineapple with about 20%–40% maturity levels by the latter. Other authors such as Refs. [4, 11, 32] all reported varying quantities of antioxidant capacity in pineapple peel waste. According to Compos et al. [39], differences in the antioxidant capacity of fruits and their by-products may also be the result of factors such as the Vitamin C and E content, flavonoid, carotenoid and other polyphenol contents present, as previously reported for the orange peel extract [40].
4.9. Antinutritional Content of PRF
Table 9 represents the result of the antinutritional content of the PRF sample. Although phytates, tannins, oxalates and saponins are phytochemicals, their antinutrient ability seems to overshadow their health benefits; hence, they are generally considered to be antinutrients [41]. The phytate and oxalate content of the PRF sample was reported to be 1.31 mg/100 g and 1.39 mg/100 g, respectively. The values in our study were lower than those reported by Romelle, Rani and Manohar [5] and Bakri et al. [42]. This variation may be a result of differences in varieties used as well as drying of peels of pineapple that was done at room temperature in comparison with the oven dry method utilized in our study. The low quantity of oxalate and phytate observed in PRF is a good indication of the low limiting effects of antinutrients on minerals (iron, zinc and calcium).
5. Conclusion
This study has provided robust evidence to support the potential use of PRF as a valuable functional food ingredient. The analysis revealed that PRF is a rich source of essential nutrients, bioactive compounds and antioxidants, positioning it as a promising candidate for value-added food applications. The significant levels of protein, fibre and minerals—particularly potassium, zinc and iron—underscore PRF’s nutritional value. The presence of Vitamins A and C, along with B-complex vitamins and essential fatty acids further, enhances its nutritional profile. Similarly, the high concentrations of phytochemicals and demonstration of significant antioxidant capacity highlight PRF’s potential role in promoting health and preventing oxidative stress-related diseases, thus suitable for use as the functional food ingredient. The low levels of antinutritional factors further support its suitability for food applications. Put together, these findings have far-reaching implications in terms of incorporation of PRF into food products, which could significantly improve their nutritional quality, potentially addressing micronutrient deficiencies, particularly in resource-limited settings. Furthermore, the transformation of waste materials into value-added products presents economic opportunities for the food industry and pineapple producers and could incentivize more sustainable agricultural practices and reduce the environmental impact of pineapple processing. Future research would focus on optimizing PRF incorporation into various food products, assessing its impact on organoleptic properties, evaluating consumer acceptability and conducting clinical studies to confirm its health benefits. Additionally, the scalability and cost-effectiveness of PRF production should be investigated to facilitate its widespread adoption in the food industry.
Conflicts of Interest
The authors declare no conflicts of interest.
Funding
This research work was part of Bai Emmanuella’s PhD programme and was self-sponsored by her family. The authors however acknowledge the Centre for Food Technology and Research Benue State University, Makurdi, for funding the publication.
Open Research
Data Availability Statement
The data that support the findings of this study are available from the corresponding author upon reasonable request.
