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Journal of Food Science and Technology logoLink to Journal of Food Science and Technology
. 2020 May 16;58(1):147–158. doi: 10.1007/s13197-020-04524-0

Influence of drying techniques on bioactive properties, phenolic compounds and fatty acid compositions of dried lemon and orange peel powders

Mehmet Musa Özcan 1,✉, Kashif Ghafoor 2, Fahad Al Juhaimi 2, Nurhan Uslu 1, Elfadıl E Babiker 2,✉, Isam A Mohamed Ahmed 2, Ibrahim A Almusallam 2
PMCID: PMC7813948  PMID: 33505059

Abstract

Lemon peel powder (LPP) obtained after drying (microwave, infrared, and oven) showed the lowest (58.72%) DPPH-radical scavenging activity in oven-dried and the highest (67.84%) in infrared-dried LPP while that of fresh lemon peel remained 63.22%. Orange peel powder (OPP) showed the lowest DSA (61.65) after microwave and the lowest (63.54%) after infrared-drying while that of fresh orange peel was 63.48%. Total phenolics were between 114.58 (fresh) and 179.69 mgGAE/100 g (oven) in LPP and between 158.54 (fresh) and 177.92 mgGAE/100 g (infrared) in OPP. The total flavonoid contents were 380.44 (fresh)–1043.04 mg/100 g (oven) in case of LPP and 296.38 (fresh)–850.54 mg/100 g (oven) in case of OPP. The gallic acid contents were 2.39 (fresh)–14.02 mg/100 g (oven) in LPP. The (+)-catechin contents were 1.10 (fresh)–49.57 mg/100 g (oven) for LPP and 0.82 (fresh)–7.63 mg/100 g (infrared) in case of OPP. The oleic acid content was 22.99 (infrared)–58.85% (fresh) in LPP-oil and 28.59 (microwave)–61.65% (fresh) in OPP-oil. The linoleic acid contents were 13.76 (fresh)–36.90% (oven) in LPP-oil and 14.14 (fresh)–37.08% (infrared) in case of OPP-oil. The drying techniques showed profound but variable effects on radical scavenging activity, total phenolics, flavonoid, carotenoids, phenolic compounds and fatty acid composition of both LPP and OPP and oven-drying (60 °C) was the most effective in improving these bioactive constituents.

Keywords: Lemon peel, Orange peel, Total phenol, Radical scavenging activity, Flavonoid, Carotenoid, Fatty acids, Phenolic compounds, GC, HPLC

Introduction

Citrus plants belong to family Rutaceae and their fruit has global significance, one-third of their total produce is processed into different products, which results in the production of huge quantities of different by-products (Jiang et al. 2014). Citrus fruits and their by-products are rich sources of phytochemicals and various types of bioactive compounds, such as phenolic compounds, which have high antioxidant potential and other biological activities (Fejzić and Ćavar 2014; Abou-Arab et al. 2016). The phenolic constituents of citrus are dependent on the cultivar and the fruit part (peel, seed, and pulp) and may vary in fruit products obtained using different techniques. Peel is generated as the primary by-product representing approximately 50–65% of the fruit weight. Peel and other by-products are usually discarded causing environmental issues due to being prone to microbial spoilage. The citrus fruits’ by-products are important sources of bioactive constituents carrying health potential (Mandalari et al. 2006; Ramful et al. 2011; Nayak et al. 2015), hence, their utilization can, not only yield commercial benefits, but also help in countering environmental pollution problems arising due to their disposal (Kumar et al. 2011).

Lemon (Citrus limon) and orange (Citrus sinensis) are among major citrus fruits and their processing may results in production of peel as main by-product in food and juice extraction industry. Citrus peel can be used for the production of pectin, flavonoids (narirutin), phenolics, antioxidants, natural colorants and nutraceutical products which can be employed in the prevention of oxidation in certain foods and in the development functional foods having additional health benefits (Wang et al. 2008). Due to these potential applications, health benefits and phenolic compounds, citrus peel can serve as important raw material for industrial applications aiming at the production of health products (Sawalha et al. 2009; Chen et al. 2012; Albishi et al. 2013). Citrus processing by-products are also rich in dietary fiber, in addition to phenolic constituents, that can be a potential ingredient for functional foods for consumers with special dietary requirement (Marin et al. 2002; Omoba et al. 2015). Different studies have been carried out on the extraction and utilization of natural antioxidants, phenolic acids, flavonoids from orange peel and it has been found to be rich in various health promoting constituents such as phenolics, flavonoids, essential oils vitamin C, folic acid, potassium, and pectin (Bocco et al. 1998; Manthey and Grohmann 2001; Xu et al. 2007). The predominant phenolic compounds in lemon peel include phenolic acids (naringin, neo hesperidin, ferulic, p-coumaric, hesperidin, narirutin and sinapic acids) (Bocco et al. 1998) and flavonoids (flavanones, flavonols and flavones Swapna and Rekha Bhaskar 2013). These phytochemicals are reported to carry antimicrobial (Dhanavade et al. 2011), cancer preventive (Wang et al. 2014) and antioxidant properties (Park et al. 2014). Drying is one the important processing step for the recovery and utilization of bioactive constituents from citrus fruits wastes as optimum drying can contribute in the maximum extraction and bioavailibity of these constituents (Khoddami et al. 2013; Abou-Arab et al. 2016). Previous reports suggest that the bioactive constituent recovery is significantly affected by the drying temperature, drying method and tissue structure of the plant matrix being dried (Chen et al. 2011). Lou et al. (2015) reported that the drying time and temperature significantly affect phenolic compounds’ recovery from immature kumquat (Citrus japonica var. margarita). They further stated that high temperature heat treatment might result in deleterious effects on bioactive compounds in kumquat. Heat treatment, although reported to affect nutritional value, physical properties and microstructures of plant matrix, is the most common drying method during recovery of bioactive from fruits, vegetables and their by-products (Akter et al. 2010). Higher temperatures (> 100 °C) have been reported to negatively affect the flavonoids and decrease their biological activities (Xu et al. 2007). Similarly, prolonged heating results in reduced free radical scavenging activities in citrus fruit (Ho and Lin 2008). Heat treatment on the other hand may also help in modifying microstructure of citrus peel matrix, thereby helping in release of lower molecular weight bioactive compounds and increase their biological effects (Jeong et al. 2004). Romdhane et al. (2015) study also reveals that during drying of lemon peel at 40–60 °C, a decrease in temperature was related to lower yield of phenolic and flavonoids residual contents. Generally, conventional thermal treatments including, hot-air oven drying and sun drying have applications for enzyme inactivation, microorganisms’ reduction and water activity reduction in plant matrix. However, higher temperatures and long drying periods usually reduce the quality of dried products therefore different innovative studies reported the applications of innovative methods for the reduction of drying time. Deng et al. (2020) used hot air impingement drying for orange peel using variable temperatures. They reported that the drying kinetics could be described using Weibull model and different quality characteristics of dried orange peel were stable while drying temperature decreased from 150 to 75 min with a concurrent increase in temperature (from 50 to 70 °C). Far-infrared heat assisted pulse vacuum drying application (Wang et al. 2018) also showed reduction in heating time (4–5 h) with temperature increment (60 to 75 °C) during drying of lemon slices at various temperatures (60, 65, 70, and 75 °C). Mello et al (2020) reported that a combined technique involving atmospheric freeze-drying (−10 °C) and moderate temperature convective drying (50 °C) can also help in reducing the need of high temperature for drying heat sensitive plant materials. Similarly, applications of microwave heating, including various modifications of this technique, proved to be effective alternate heat source for drying different types of plant matrix while preserving the nutritional quality of dried product (Zielinska et al. 2019). Hence, as discussed above different innovative, modified and conventional drying methods were employed in drying fruit by-products and for studying their effects on the quantities and biological activities of phenolic compounds present therein. However, there is lack of comparative studies involving different modern and conventional drying techniques with reference to certain quality attributes, particularly for the recovery of bioactive constituents from two different citrus fruit by-products. A comparison of drying techniques using oven, microwave and infrared heating on the fatty acids, polyphenols, antioxidant activity, total phenolic and flavonoid contents of orange and lemon peels can yield useful results for the optimization of drying methods in addition to highlighting the distinctive effects of these methods on each quality parameter under study. Hence, the current study was aimed at examining the effects of different drying systems/methods, such as microwave drying (940 W), hot air drying in oven (at 60 °C) and infrared oven drying on total phenol, total carotenoid, total flavonoid, antioxidant activity, individual phenolic constituents and fatty acid compositions of lemon and orange peels.

Material and methods

Material

Lemon and orange fruits (at a commercially ripe stage) were obtained directly from citrus orchards in Büyükeceli-Gülnar (Mersin), Turkey during April 2018 followed by washing with tap water. Peels were then removed manually and edible parts separated followed by drying using different techniques.

Methods

Drying process

Lemon and orange peel contained 82.08 and 74.40% moisture, respectively. The slices of orange and lemon peel were dried at 60 °C in a hot air oven; at 940 W in microwave oven and 60 °C in an infrared oven until the moisture content was below 20%. These experimental conditions were selected based on preliminary trials as drying at higher temperatures (reducing moisture contents lower than 20%) resulted in lower yields of phenolics in dried samples. Each drying treatment was carried out in triplicates and dried peels were chopped further and ground to powder form. Powdered peel samples were passed through a 1.4 mm steel sieve, packaged in sealed containers and stored at − 18 °C until further analyses.

Moisture content

Moisture contents of orange and lemon peel samples were estimated by oven drying (Nüve FN055 Ankara, Turkey) at 105 °C and periodically weighed until the constant weight.

Sample extraction

Extraction from samples was based on Garcia-Salas et al. (2013) method after slight modifications. Powdered samples (2 g) were mixed with methanol (10 ml) followed by 1 min vortex shaking and 30 min sonication. Centrifugation (4500 rpm) of the mixture was carried out for 10 min. All these procedures were repeated before separating the supernatants from the centrifuge tubes and concentrating them at 37 °C in a rotary evaporator connected to a vacuum pump. The concentrated extract was mixed with methanol to make a final volume of 5 ml and this mix was passed through a 0.45 µm nylon filter before injection to chromatographic system.

Total phenolic content

The total phenolic content of extracts was analyzed following Folin-Ciocalteu (FC) reagent method (Yoo et al. 2004). FC (1 ml) was mixed with sample (0.4 ml) for 5 min followed by the addition of 10 ml of 7.5% Na2CO3. Deionized water was then added to the reaction mixture and volume was made 25 ml accompanied with mixing. The samples were incubated at room temperature for 1 h. A spectrophotometer set at wavelength of 750 nm was used to observe absorbance of the reaction mixture and these values were then compared with gallic acid calibration curve (0–200 mg/ml). The results were reported as mg gallic acid equivalent (GAE)/100 g of fresh weight.

Total flavonoid content

The determination of total flavonoid contents of extracts was carried out following a colorimetric method as explained by Hogan et al. (2009) with slight modification. The extract (1 ml) from lemon and orange peel was mixed with 0.3 ml of NaNO2, 0.3 ml of AlCl3 and 2 ml of NaOH. The resulting mixture was shaken well and the absorbance values were recorded at 510 nm using a spectrophotometer. The results for total flavonoids were expressed as mg catechin (CA)/g of fresh weight.

Radical scavenging activity

The radical scavenging activities of extracts were determined using DPPH (1,1-diphenyl-2-picrylhydrazyl) radicals’ solution in methanol according to the method of Lee et al. (1998). An aliquot of the extract (1 ml) was dissolved in 2 ml DPPH solution and shaken vigorously. The reaction mixture was incubated at room temperature for 30 min followed by the measurement of absorbance values at a wavelength of 517 nm using a spectrophotometer. The antioxidant activity was expressed as percentage.

Chromatographic analysis for phenolic constituents

Phenolic compounds analysis was carried out in a HPLC system (Shimadzu, Japan) which consisted of a PDA detector and an Inertsil ODS-3 (5 µm; 4.6 × 250 mm) column. Elution profile was gradient and separation was carried out using a mobile phase consisting of acetic acid (0.05% in water) as A and acetonitrile as B. The mobile phase flow rate was kept at 1 ml/min while temperature was 30 °C. The sample (20 µl) was injected to the system and PDA detector was set at 280 and 330 to measure peaks during a 60 min total run time for each sample. The absorption spectra and peak retention times were used for identification and quantification of phenolic compounds after comparison with their standards.

Total carotenoid content

A method as reported by Silva da Rocha et al. (2015) was used to quantify total carotenoids in citrus peel powders which included mixing 2 g of sample with 25 ml of acetone followed by 10 min mixing on vortex and filtration through Whatman no. 1 filter paper. The separation and fractionation of the mixture was carried out in a separatory funnel using petroleum ether (20 ml) as solvent and distilled water (100 ml) for acetone removal. The procedures were repeated before filtering the mixture through Whatman no. 1 filter paper containing anhydrous Na2SO4 (5 g) to absorb water residues. Petroleum ether fraction was separated and its volume was made 25 ml by adding more of the solvent. The absorbance values of the resulting filtered sample were recorded at 450 nm and total carotenoid contents were expressed as µg/g of the sample.

Oil content

Citrus peel oil content was estimated following AOAC (1990) method. Petroleum benzene was used as solvent in Soxhlet Apparatus and oil was extracted for 5 h followed by solvent removal in a rotary vacuum evaporator at 50ºC.

Fatty acid composition

The oil samples obtained from lemon and orange peel powders were esterificated following ISO-5509 (1978) method. Methyl esters of the fatty acids were then evaluated in a gas chromatography system (Shimadzu GC-2010) which consisted of a flame-ionization detector (FID) and a capillary column (Tecnocroma TR-CN100, 60 m × 0.25 mm, film thickness: 0.20 µm). The temperature at injection block and detector was set at 260ºC. Mobile phase was nitrogen at a flow rate of 1.51 ml/min and 80 ml/min total flow rate at 1/40 split ratio. The temperature of the column remained 120 °C for 5 min initially, however increased to 240 °C subsequently (4 °C/min increase) and held at this temperature for 25 min. Methyl esters of standard fatty acid compounds (Sigma Chemical Co.) were also injected to compare with peaks and relative retention time of the samples containing citrus fatty acids (AOAC 1990). Fatty acid contents were presented as percentage of the oil extracted from fresh and dried lemon and orange peel powders.

Statistical analyses

Experiments were carried out following a complete randomized split plot block design and analysis of variance (ANOVA) technique was used to analyze obtained data in a statistical software (JMP, v 9.0, SAS Inst. Inc., Cary, N.C., U.S.A.). All treatments and analyses were carried out in triplicates and the reported results are means ± standard deviations (MSTAT C) of independent citrus peels and drying methods (Püskülcü and İkiz 1989).

Results and discussion

Total phenolic, total flavonoid, total carotenoid, oil contents and antiradical activity

The chemical and bioactive properties (total carotenoid, antioxidant activity, total phenol and total flavonoid contents) of lemon peel powder (LPP) and orange peel powder (OPP) obtained after drying using oven, microwave and infrared techniques are presented in Table 1. The moisture content was 10.12 (oven)–82.014% (fresh) in LPP and 11.35 (oven)–74.40% (fresh) in OPP. Oil content, extracted by Soxhlet apparatus, was 0.07 (fresh)–1.02% (infrared) in LPP and 0.13 (fresh)–1.20% (oven) in OPP.

Table 1.

The physico-chemical and bioactive properties of fresh and dried lemon and orange peels

Drying process Moisture content (%) Oil content (%) Carotenoid content (μg/g) Antioxidant activity (%) Total phenolic content (mg/100 g) Total flavonoid content (mg/100 g)
Lemon peel
Fresh 82.08 ± 0.04*a 0.07 ± 0.01d 0.85 ± 0.02d 63.22 ± 0.00c 114.58 ± 0.02d 380.44 ± 0.0d3
Oven 10.12 ± 0.06d** 0.95 ± 0.02b 4.58 ± 0.04a 58.72 ± 0.03d 179.69 ± 0.01a 1043.04 ± 0.01a
Microwave 11.89 ± 0.04c 0.83 ± 0.01c 3.55 ± 0.03b 64.65 ± 0.00b 170.52 ± 0.00c 988.88 ± 0.02b
Infrared 15.23 ± 0.02b 1.02 ± 0.04a 3.05 ± 0.05c 67.84 ± 0.00a 178.96 ± 0.01b 918.04 ± 0.01c
Orange peel
Fresh 74.40 ± 0.07a 0.13 ± 0.00c 27.79 ± 0.05d 63.48 ± 0.00b 158.54 ± 0.02d 296.38 ± 0.01d
Oven 11.35 ± 0.04d 1.20 ± 0.02a 62.92 ± 0.02b 62.76 ± 0.00c 173.23 ± 0.01b 850.54 ± 0.02a
Microwave 11.78 ± 0.05c 1.10 ± 0.02b 91.10 ± 0.06a 61.65 ± 0.00d 170.10 ± 0.02c 460.44 ± 0.03c
Infrared 16.43 ± 0.03b 1.07 ± 0.01b 57.28 ± 0.02c 63.54 ± 0.00a 177.92 ± 0.01a 733.04 ± 0.01b
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*Standard deviation, **Values within each column followed by different letters are significantly different at P < 0.05

Total carotenoid content was 0.85 (fresh)–4.58 µg/g (oven) in LPP and 27.79 (fresh)–91.10 µg/g (microwave) in OPP. The antioxidant activity, as determined by DPPH-radical scavenging activity (DRSA), was 58.7 (oven)–67.84% (infrared) in LPP and 61.65 (microwave)–63.54% (infrared) in OPP. The total phenol contents varied between 114.58 (fresh) and 179.69 mgGAE/100 g (oven) in LPP and 158.54 (fresh) and 177.92 mgGAE/100 g (infrared) in OPP. The total flavonoid contents were 380.44 (fresh)–1043.04 mg/100 g (oven) in LPP and 296.38 (fresh)–850.54 mg/100 g (oven) in OPP. Drying methods altered the chemical and bioactive properties of LPP and OPP.

The crude oil, total carotenoid, antioxidant activity (excluding that of OPP), total phenol and total flavonoid values of LPP and OPP obtained using oven; microwave and infrared drying were higher than fresh undried sample (control). The effects of drying was less marked on the DRSA values of LPP and OPP as compared to that of control (fresh). The moisture, crude oil and total carotenoid contents of the OPP dried using different techniques were partially higher than values for LPP. This increase may be due to the increase in concentration of bioactive compounds and their properties per unit weight, low drying temperature, moisture reduction and structural changes in LPP and OPP matrices as result of different drying techniques. DRSA and total phenol contents of both LPP and OPP were partly similar with some minor differences. Statistically significant (P < 0.05) differences were also observed among bioactive properties of LPP and OPP in relation to drying methods. These differences, in addition to drying methods, can also relate to agricultural factors, species, varieties, harvest time and analytical conditions.

Fresh citrus peel of Thompson navel, mandarin, and lemon were characterized by high moisture contents but lower oil contents than microwave-dehydrated samples (Nesrine et al. 2012). Citrus fruit and their products are important sources of essential minerals and bioactive compounds such as ascorbic acid, carotenoids, flavonoids, phenolic compounds and pectin (Ghasemi et al. 2009). Hegazy and Ibrahium (2012) reported that total phenol and flavonoid contents of orange peel extracts were 63.20 (hexane)–169.56 mg/g (ethanol) and 13.89 (hexane)–29.75 µg/g (ethanol), respectively depending on the extraction solvent. The flavonoid contents of orange peels dried by microwave changed between 437.50 (Tangerine) and 453.33 mgQE/100 g (C. valencia and C. balady oranges) (Abou-Arab et al. 2016). In addition, the flavonoid contents of orange peels dried by air oven-drying varied between 150.83 (C. balady) and 327.50 mgQE/100 g (C. valencia) (Abou-Arab et al. 2016). The total flavonoid (22.28 and 33.72 mg/g) and total phenolics (1.51-2.23 mgGAE/g) in sweet OPP varied depending on drying methods (Sankalpa et al. 2017). Drying processes, such as high temperature drying using hot air and prolonged drying using solar energy might destroy flavonoid compounds as they may oxidize at high temperature and exposure to sunlight (Larrauri et al. 1998; Li et al. 2006). Garau et al. (2007) reported the effects of different drying and grinding methods on total flavonoid content of sweet orange peel and observed that their contents in fresh orange peel (506.82 mgQE/g) were decreased in OPPs to 309 and 365.40 mgQE/g after microwave and oven drying, respectively (Abd-El Ghfar et al. 2016). The total flavonoid contents (469.08 mgQE/100 g) of the lemon peel dried by hot-air oven were higher (P < 0.05) than those of microwave dried (442.79 mgQE/100 g) and fresh peel (430.58 mg/100) samples (Abd-El Ghfar et al. 2016). The DRSA values of orange and lemon peels dried by microwave were found to be 68.85–69.83% and 56.69–56.01%, respectively (Abd-El Ghfar et al. 2016). However, the DRSA values of oven-dried orange and lemon peels were 56.29–53.83% and 50.93–52.64%, respectively. Total phenol contents of microwave-dried orange and lemon peels were 1535.94–3026.34 mgGAE/100 g and 1323.31–2632.81 mgGAE/100 g, respectively whereas the total phenolics in oven-dried orange and lemon peels were 1410.73–2453.75 mgGAE/100 g and 1180.78–2505.40 mgGAE/100 g, respectively (Abd-El Ghfar et al. 2016). Another study (Omoba et al. 2015) reported that the major phenolic constituents of unripe orange peel were quercetin (18.77 mg/ml), rutin (18.65 mg/ml), and quercetin (10.39 mg/ml) whereas their contents were 22.61, 17.93, and 14.03 mg/ml, respectively in case of ripe orange peel. The antioxidant activities of ripe orange peel [2.71 ± 0.03 mg/ml EC50 values for 2,2-diphenyl-1-picrylhydrazyl (DPPH) inhibition, 0.67 ± 0.03 mg/ml for hydroxyl radicals (OH*) inhibition, 0.57 ± 0.02 mg/ml for Fe2 + chelation, and 0.63 ± 0.06 mg/ml for malondialdehyde (MDA) inhibition] were higher than those of unripe orange peel The ripe orange peel contained 9.40 mgGAE/g of total phenolics and 4.20 mgQE/g of total flavonoid (Omoba et al. 2015). Rafiq et al. (2019) observed that the total phenol content, flavonoid content and antioxidant activity values of fresh and tray-dried kinnow peels were 24.51–16.84 mgGAE/g, 19.12–11.11 mgQE/g and 86.55–83.28%, respectively and it was stated that changes in the structure of bioactive compounds, during drying, would have resulted in the formation of lower molecular weight compounds, which reduced their antioxidative activities.

Phenolic constituents in citrus peel powders obtained by different drying methods

Contents of phenolic constituents in LPP and OPP, obtained after drying peels in hot air, microwave and infrared ovens, are presented in Table 2. Gallic acid, 3,4-dihydroxybenzoic acid, (+)-catechin and syringic acid were the key phenolic constituents in both LPP and OPP (Fig 1). The gallic acid contents were 2.39 (fresh)–14.02 mg/100 g (oven) in LPP and 1.56 mg/100 g (fresh)–9.54 mg/100 g (oven) in OPP. The 3,4-dihydroxybenzoic acid contents varied between 0.11 (fresh) and 5.76 mg/100 g (microwave) in LPP and between 1.46 (fresh) and 6.64 mg/100 g (oven) in OPP. The (+)-catechin contents were 1.10 (fresh)–49.57 mg/100 g (oven) in LPP and 0.82 (fresh)–7.63 mg/100 g (infrared) in OPP. The syringic acid was 0.34 (oven)–1.36 mg/100 g (infrared) and 0.05 (fresh)–0.46 mg/100 g (microwave) in LPP and OPP, respectively. The 1,2-dihydroxybenzene contents were 0.57 (microwave)–2.50 mg/100 g (oven) in LLP and 0.19 (fresh)–3.65 mg/100 g (oven) in OPP.

Table 2.

Phenolic compounds of fresh and dried lemon and orange peels

Phenolic compounds (mg/100 g) Fresh Oven Microwave Infrared
Lemonpeel
Gallic 2.39 ± 1.25*d 14.02 ± 0.27a 4.05 ± 6.64c 6.25 ± 1.44b
3,4-Dihydroxybenzoic 0.11 ± 0.08d** 4.93 ± 3.97c 5.76 ± 2.73a 5.42 ± 0.91b
(+)-Catechin 1.10 ± 1.72d 49.57 ± 6.35a 48.61 ± 5.96b 8.57 ± 5.03c
1,2-Dihydroxybenzene 2.30 ± 3.44c 52.50 ± 0.00a 0.57 ± 0.34d 4.19 ± 2.05b
Syringic 0.40 ± 0.62c 0.34 ± 0.25d 0.77 ± 0.59b 1.36 ± 0.69a
Caffeic 0.04 ± 0.02d 0.60 ± 0.15b 0.05 ± 0.05c 0.88 ± 0.12a
Rutin trihydrate 0.05 ± 0.01b 0.04 ± 0.01c 0.05 ± 0.02b 8.51 ± 1.09a
p-Coumaric 0.01 ± 0.00c 0.01 ± 0.00c 0.02 ± 0.02b 0.14 ± 0.16a
trans-Ferulic 0.03 ± 0.02d 0.10 ± 0.08b 0.70 ± 0.51a 0.06 ± 0.02c
Apigenin 7 glucoside 0.04 ± 0.00d 1.27 ± 0.45a 0.33 ± 0.16b 0.20 ± 0.24c
Resveratrol 0.01 ± 0.01d 0.03 ± 0.02c 0.05 ± 0.05b 0.79 ± 0.35a
Quercetin 0.11 ± 0.03d 0.53 ± 0.52b 0.49 ± 0.28c 5.28 ± 0.67a
trans-Cinnamic 0.02 ± 0.00c 0.20 ± 0.10b 0.72 ± 0.48b 0.77 ± 0.37a
Naringenin 0.04 ± 0.03d 1.51 ± 0.39b 1.37 ± 0.99c 1.57 ± 1.20a
Kaempferol –*** – 2.97 ± 0.48a 1.81 ± 0.21b
Isorhamnetin 0.15 ± 0.08d 1.69 ± 1.44c 2.62 ± 2.15b 4.05 ± 3.01a
Orange peel
Gallic 1.56 ± 1.11d 9.54 ± 6.15a 4.83 ± 1.38c 6.45 ± 2.03b
3,4-Dihydroxybenzoic 1.46 ± 1.64d 6.64 ± 3.53a 2.65 ± 1.55c 6.32 ± 1.58b
(+)-Catechin 0.82 ± 0.79d 6.09 ± 3.34b 2.27 ± 3.78c 7.63 ± 0.01a
1,2-Dihydroxybenzene 0.19 ± 0.08d 3.65 ± 2.66a 2.10 ± 3.69c 3.45 ± 4.10b
Syringic 0.05 ± 0.02d 0.22 ± 0.20b 0.46 ± 0.56a 0.20 ± 0.22c
Caffeic 0.02 ± 0.00d 0.07 ± 0.03c 0.69 ± 0.77a 0.26 ± 0.18b
Rutin trihydrate 0.07 ± 0.02d 0.50 ± 0.47a 0.41 ± 0.27b 0.34 ± 0.30c
p-Coumaric 0.02 ± 0.01b 0.01 ± 0.01c 0.03 ± 0.02a 0.01 ± 0.01c
trans-Ferulic 0.04 ± 0.02d 0.15 ± 0.10a 0.09 ± 0.07b 0.05 ± 0.01c
Apigenin 7 glucoside 0.04 ± 0.02c 0.21 ± 0.15a 0.02 ± 0.00d 0.05 ± 0.04b
Resveratrol 0.02 ± 0.02d 0.39 ± 0.06b 0.37 ± 0.24c 0.43 ± 0.45a
Quercetin 0.09 ± 0.01d 1.18 ± 1.49a 0.17 ± 0.23c 0.18 ± 0.15b
trans-Cinnamic 0.03 ± 0.02c 0.12 ± 0.09a 0.06 ± 0.01b 0.06 ± 0.01b
Naringenin 0.06 ± 0.00d 0.71 ± 0.50a 0.12 ± 0.03c 0.31 ± 0.05b
Kaempferol 0.23 ± 0.24d 0.86 ± 0.76b 0.62 ± 0.16c 1.38 ± 0.69a
Isorhamnetin 0.16 ± 0.09d 0.98 ± 1.15a 0.45 ± 0.28c 0.87 ± 0.25b
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*Standard deviation; **values within each column followed by different letters are significantly different at P < 0.05; *** − : not dedected

Fig. 1.

Fig. 1

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Chromatograms of phenolic compounds dected in fresh and dried lemon and orange peels

Kaempferol was not detected in fresh lemon peel (control) and dried LPP. In general, the phenolic components’ concentration in LPP and OPP (obtained after oven, microwave and infrared drying) was higher than respective fresh peels. Furthermore, certain phenolic constituents [gallic acid, (+)-catechin, 1,2-dihydroxybenzene, apigenin 7 glucoside] were higher in oven-dried LPP and OPP than those obtained after microwave and infrared drying of respective citrus peel. Similarly, syringic acid (1.36 mg/100 g), rutin-trihydrate (8.51 mg/100 g), quercetin (5.28 mg/100 g) and isorhamnetin (4.05 mg/100 g) contents were higher in infrared-dried LPP in comparison to other samples. The increase in the phenolic components in LPP and OPP during drying (in comparison to fresh peels) may result from reduction in moisture content and changes in intracellular structures of peel matrix. Hot air oven drying process may be preferable over microwave and infrared drying due to significantly (P < 0.05) higher contents of phenolic constituents in LPP and OPP.

Furthermore, the temperature used during oven drying of citrus peel was moderate (60 °C), and higher temperature and/or heat may promote the degradation of phenolic compounds (Ghafoor, et al. 2011). The contents of the identified individual phenolic constituents were significantly influenced by the drying methods (P < 0.05). Xu et al. (2007) demonstrated that phenolic acids’ free fraction increased, whereas their ester, glycoside and ester bound fractions decreased after heat treatment. The p-coumaric acid, ferulic and sinapic acids and chlorogenic acid contents of orange peel extracts heated at 90–150 °C changed between 5.58–54.10 µg/g, 4.56–69.98 µg/g and 17.47–126.58 µg/g and 2.26–10.66 µg/g, respectively (Xu et al. 2007). Appreciable quantities of phenolic compounds, including gallic acid (111.6 µg/g dw), vanillic acid (191.4 µg/g dw), p-coumaric acid (301.4 µg/g dw), ferulic acid (441.7 µg/g dw), p-hydroxybenzoic acid (29.0 µg/g dw), caffeic acid (19.3 µg/g dw), naringin (39.9 µg/g dw), naringinin (512.3 µg/g dw) and rutin (163.4 µg/g dw), were detected in kinnow peel (Rafig et al. 2019). Omoba et al. (2015) reported that ripe orange peel extract contained 12.49 mg/g catechin, 3.57 mg/g caffeic acid, 5.71 mg/g naringin, 6.08 mg/g epicatechin, 17.93 mg/g rutin, 22.61 mg/g quercitrin, 14.03 mg/g quercetin, 3.76 mg/g kaempferol and 5.83 mg/g luteolin. The drying of lemon pomace at 70 °C under vacuum showed p-coumaric acid content (1,69 μg/ml), higher than hot air drying, demonstrating the positive effect of vacuum heating on p-coumaric acid contents. Increase in temperatures during hot air drying (from 90 to 110 °C) and vacuum heating (from 70 to 110 °C) showed sharp decrease in p-coumaric acid (31 and 44%, respectively) and this decrease was attributed to the heat sensitivity of hydroxycinnamic or p-coumaric acid (King and Young 1999). The p-coumaric acid degradation may also be related to the oxidation reactions and high temperature during heat treatment however, Sun et al. (2015) reported that heat treatment might exert positive effects on the availability of this constituent. The p-coumaric acid reduction in samples dried at 60 °C for longer time was related to the oxidation reaction due to longer exposure time (Wojdyło et al. 2014) and varietal differences (Gorinstein et al. 2001). Samples dried in hot air oven are more susceptible to oxidation than vacuum drying due to the presence of oxygen, which was also evident with reference to lower gallic acid contents in hot air oven dried sample. It was also stated that the gallic acid biosynthetic pathway is still less understood (King and Young 1999). Quercitrin and rutin contents, in powders obtained from dried peel from ripe orange, were higher than those in powder from dried peel of unripe lemon. Hence, it was stated that the phenolic composition of citrus peel might also vary with the cultivar type, the climatic conditions, the agronomic practices, and fruit ripening (Omoba et al. 2015).

Fatty acids in citrus peels dried using different methods

The fatty acid composition of lemon peel oil (LPO) and orange peel oil (OPO) obtained after drying peels using three different drying methods are shown in Table 3 and Fig 2. Palmitic, stearic, oleic, linoleic and linolenic acids were the major fatty acids of lemon and orange peel powder oils. The fatty acid contents in oils from LPO and OPO showed differences depending on methods used for drying peels and with respect to those of fresh ones. Among them, the contents of palmitic acid varied between 7.74 (microwave) and 17.37% (fresh) in case of LP), whereas in case of OPO, it changed between 15.04% (oven) and 16.61% (fresh). Stearic acid contents were 1.86 (oven)–5.38% (fresh) in LPO and 2.07 (microwave)–4.17% (fresh) in OPO. The oleic acid contents were 22.99 (infrared)–58.85% (fresh) in LPO and 28.59 (microwave)–61.65% (fresh) in OPO. Elaidic acid contents were 1.70 (microwave)–2.34% (fresh) and 2.08 (oven)–2.30% (fresh) in LPO and OPO, respectively. Linoleic acid contents were 13.76 (fresh)–36.90% (oven) in LPO and 14.14 (fresh)–37.08% (infrared) in OPO. Linolenic acid remained 2.30 (fresh)–15.66% (infrared) in LPO and 1.12 (fresh)–9.87% (microwave) in OPO. Myristic, arachidic, behenic, and arachidonic acids were detected in oils from both fresh lemon and orange peels. Erucic acid was detected in fresh LPO only. In general, fatty acid contents (except for linoleic and linolenic acids) in oils from fresh lemon and orange peels were higher than those obtained after drying peels using different methods (oven, microwave and infrared). Oleic acid content (except for microwave) of OPO was higher than that in oils from fresh and dried lemon peels. It is important that linoleic acid content of OPO were higher than in oils from fresh and dried lemon peels. In addition, erucic acid was found in only in LPO from peel dried using oven, microwave and infrared, and their contents were under 0.25%. Drying was significantly effective on the fatty acid contents of dried lemon and orange peel oils. The reduction of the fatty acid contents of the dried samples may be due to increased oxidation and enzymes activity during heating process.

Table 3.

Fatty acid compositions in oils obtained from lemon and orange peels either fresh (control) or dried using different methods

Fatty acids (% of oil) Fresh Oven Microwave Infrared
Lemon peel
Myristic – 1.34 ± 0.01a 1.12 ± 0.00b 1.34 ± 0.03a
Palmitic 17.37 ± 0.25*a 15.20 ± 0.18c 7.74 ± 6.34d 15.98 ± 0.13b
Stearic 5.38 ± 0.01a** 1.86 ± 0.03d 2.26 ± 0.01c 2.34 ± 0.03b
Oleic 58.85 ± 0.43a 24.76 ± 0.02c 35.42 ± 0.06b 22.99 ± 0.04d
Elaidic 2.34 ± 0.08a 1.88 ± 0.01c 1.70 ± 0.01d 1.97 ± 0.02b
Linolelaidic –*** – 0.17 ± 0.01 –
Linoleic 13.76 ± 0.21d 36.90 ± 0.04a 31.84 ± 0.04c 36.43 ± 0.03b
Arachidic – 0.27 ± 0.00c 0.28 ± 0.00b 0.34 ± 0.01a
Linolenic 2.30 ± 0.04d 14.91 ± 0.04b 11.09 ± 0.00c 15.66 ± 0.05a
Behenic – 0.22 ± 0.01b 0.21 ± 0.00c 0.32 ± 0.00a
Erucic – 0.25 ± 0.01a 0.19 ± 0.00b 0.25 ± 0.01a
Arachidonic – 2.09 ± 0.02b 1.49 ± 0.02c 2.15 ± 0.01a
Orange peel
Myristic – 2.01 ± 0.10c 2.47 ± 0.07a 2.27 ± 0.04b
Palmitic 16.61 ± 0.28a 15.04 ± 0.48d 16.02 ± 0.14b 15.33 ± 0.05c
Stearic 4.17 ± 0.04a 2.14 ± 0.02c 2.07 ± 0.05d 2.32 ± 0.02b
Oleic 61.65 ± 0.01a 34.32 ± 0.27b 28.59 ± 0.11d 29.73 ± 0.01c
Elaidic 2.30 ± 0.01a 2.08 ± 0.02d 2.15 ± 0.05b 2.10 ± 0.01c
Linoleic 14.14 ± 0.26d 33.92 ± 0.30c 36.39 ± 0.16b 37.08 ± 0.03a
Arachidic – 0.32 ± 0.02a 0.28 ± 0.02b 0.28 ± 0.03b
Linolenic 1.12 ± 0.05d 7.31 ± 0.05c 9.87 ± 0.03a 8.33 ± 0.23b
Behenic – 0.37 ± 0.02b 0.36 ± 0.01c 0.43 ± 0.01a
Arachidonic – 2.01 ± 0.04a 1.79 ± 0.06c 1.88 ± 0.10b
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*Standard deviation;**values within each column followed by different letters are significantly different at P < 0.05; ***–: not dedected

Fig. 2.

Fig. 2

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Fatty acid chromatograms of oils from fresh and dried lemon and orange peels

Conclusion

Drying methods significantly affected the total phenol contents, antioxidant capacity, total flavonoid, total carotenoids, phenolic compounds and fatty acids in lemon and orange peel powders. In order to utilize these bioactive ingredients for nutraceuticals and functional foods development, the selection of appropriate drying method is imperative, which seems to be hot air (60 °C) oven-drying in the current study. The antioxidant activities of the extracts of lemon and orange peel powders as determined by DPPH-radical scavenging activity are strongly associated with the presence of phenolics and their derivatives, thereby supporting the relevance of these by-products as important and potential source of antioxidant compounds and nutraceutical products. Orange and lemon peel powders can be exploited as natural antioxidant materials in food applications. The can be incorporated in health supplements or functional foods formulations and further studies may be carried out to support this idea including determination of more biological activities using different in vitro and in vivo models and drying using other modified and innovative techniques.

Acknowledgements

The authors would like to extend their sincere appreciation to the Deanship of Scientific Research at King Saud University for its funding the Research Group No. (RG-1441-325).

Footnotes

Publisher's Note

Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.

Contributor Information

Mehmet Musa Özcan, Email: mozcan@selcuk.edu.tr.

Elfadıl E. Babiker, Email: elfadilbabiker@yahoo.com

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