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Pol. J. Environ. Stud. Vol. XX, No. X (XXXX), 1-8
DOI: 10.15244/pjoes/190011 ONLINE PUBLICATION DATE:

Original Research
Physicochemical Properties of Ethiopian
Orange (Citrus Sinensis) Peels
and Extracted Pectin by Varieties

Abebe Teshome Ayele1*#, Sisay Awoke Endalew2#, Charles Odilichukwu R. Okpala3**

Department of Food Engineering, Kombolcha Institute of Technology, Wollo University, Ethiopia

1

2
Department of Chemistry, Wollo University, Ethiopia
3
UGA Cooperative Extension, University of Georgia, Athens, GA 30602, USA

Received: 10 January 2024

Accepted: 12 June 2024

Abstract

The physicochemical properties of Ethiopian orange (Citrus sinensis) peels and their extracted
pectin by varieties were investigated. Specifically, orange (peel) varieties included Valencia, Mandarin,
and Gunda Gundo, and their extracted pectin was physicochemically characterized in terms of yield,
moisture, ash, viscosity, acetyl, equivalent weight, methoxyl, degree of esterification, and anhydrouronic
acid aspects. In many instances, the physicochemical properties of extracted pectin differed significantly
(p<0.05). For instance, the extracted pectin from the Mandarin peel variety showed greater yield,
viscosity, equivalent weight, methoxyl, anhydrouronic acid, and degree of esterification, but less
acetyl, moisture, pH, and ash. Contrarily, the extracted pectin from Valencia showed greater acetyl
concentration, moisture, and ash content but less yield, viscosity, equivalent weight, anhydrouronic
acid, and esterification. Feasibly, the Mandarin variety appears to be a rich pectin resource with high
promise as a significant raw material for the food processing industry.

Keywords: orange peel, varieties, pectin, pectin quality, yield

Introduction besides effectively helping protect the environment,

may contribute to the production of essential ingredients
Food waste is considered unfit for use by consumers needed for food preparation/medicinal purposes [2].
because it comes after a given food produce/product Although pectin remains a well-known product of
has already been processed, distributed, and served/ fruit and vegetable peels, the recovery of non-starch
consumed/utilized [1]. About 30-45% of global polysaccharides is among the vital techniques for
food waste comes from fruit and vegetable sources. producing natural biopolymers. Moreover, different
Therefore, waste utilization (of fruit and vegetables), researchers focused on the extraction, process
conditions optimization, and characterization of pectin
from different plant materials, such as banana peels
[3], lemon and mango peels [4], and citrus peels [5].
Commercially, pectin can be realized when acid at
#These authors had equal contribution
in the preparation of this article
high temperatures is applied to different raw materials,
*e-mail: abebeteshome70@gmail.com
**e-mail: charlesokpala@gmail.com
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2 Abebe Teshome, et al.

for example, citrus fruit peels. The physiochemical would better suit yield by physiochemical quality.
properties of extracted pectin might be influenced Therefore, the choice of an appropriate orange type
by different factors, particularly variety, pH, particle for the manufacture of value-added goods like pectin
size interval, temperature, extraction time, and types would be a prospective field for research. To supplement
of acid used in the extraction [6]. Besides, orange existing information, therefore, the physicochemical
peels are an excellent source of soluble sugar, pectin, properties of Ethiopian orange (Citrus sinensis) peels
phenolic acids, and flavonoids, even containing higher and their extracted pectin by varieties were investigated.
concentrations than in the edible portion. meaning that
several compounds will be retained in the peel [7]. The
soluble sugars that can be found in orange peel include Materials and Methods
glucose, fructose, and sucrose. Pectin, cellulose, and
hemicelluloses make up the insoluble polysaccharides Sample Collection, Chemicals,
that make up the cellular wall of the orange peel. Xylose, and Experimental Facilities
rhamnose, and glucose are in relatively tiny proportions
in pectin and hemicelluloses. Most plants’ cell walls Three different orange varieties, namely Valencia,
contain around one-third of the dry material as pectin, Mandarin, and Gunda Gundo, were bought from
a complex mixture of polysaccharides. Pectin is an a local market in Addis Ababa, South Wollo (hayk town),
important component of the primary cell wall, along and Tigray region, Ethiopia, respectively. Fig. 1 shows
with cellulose and hemicellulose. Because it is abundant the map of Ethiopia, particularly the individual regions,
in the middle lamellae, its principal function was once situating the location of Addis Ababa as well as Tigray.
thought to be intercellular adhesion [8]. In addition Further, plastic net bags were used to transport 10
to dissolving in either a hot diluted acid solution or kilograms of each species to the Chemical Engineering
a hot aqueous chelating agent solution, the amount of Department at the Addis Ababa Science and Technology
pectin, structure, and chemical composition could vary University in Ethiopia, where they were stored at room
depending on the cultivar, maturity time, and various temperature until needed. Chemicals and equipment,
plant parts. Most pectin is extracted and utilized in food including litmus paper, cloth filter, measuring cylinder,
preparations as an emulsifier, gelling agent, glazing milling machine, pH meter, temperature sensor,
agent, stabilizer, and thickening ingredient in fruit juice, shaker, beakers, oven, spoon, refrigerator, stirrer, mass
jellies, jams, marmalades, confections, etc. [9]. Despite balance, water bath heating, precipitator, and dryer,
these benefits, pectin’s exceptional endurance at low pH were purchased from a chemical shop in Addis Ababa.
levels keeps it in use in the pharmaceutical, cosmetic, All chemicals and reagents used in this study were
and acidic food sectors. It can be difficult for processors of analytical grade. Additionally, all experimental
to preserve the quality of pectin because different activities (extraction and various physicochemical
orange peel varieties have distinctly different chemical analyses of pectin) were performed at the Chemical and
compositions. However, given its ability to emulsify, Food Engineering Laboratory, Addis Ababa Science and
pectin from various plant sources must be adequately Technology University, Ethiopia.
extracted in order to produce food emulsions.
Different varieties of orange (Citrus sinensis) are Sample Preparation
cultivated in Ethiopia. Among them, three varieties,
namely: Valencia, Mandarin, and Gunda Gundo, The orange was divided into four equal pieces, each
appear to be the most common and are particularly of which was peeled and cleaned to eliminate dirt, dust,
available in the community/research centers. According and pesticide residues. The peels were then cut into
to research reports, pectin extracted from some smaller pieces for straightforward drying. They were
varieties of orange peels may not be suitable for food milled after being dried in the sun until the moisture
processing due to the low methoxyl content, equivalent level was minimal. Finally, the peel powder was stored
weights, and anhydrouronic acid content. As per earlier for later use at room temperature.
studies, orange peel pectin thickens when jam, jelly,
and marmalade are prepared; hence, higher equivalent Extraction Procedure
weights are required for strong gel formation [10]. In
2011, for example, in Ethiopia, it was well-reported by A citric acid solution was prepared with a pH of 2.
the Ethiopian Horticultural Producers and Exporters In a prepared citric acid solution, 5 g of dried orange
Association (EHPEA) that over 40,000 tons of oranges peel powder is cooked for 30 min at 70ºC with continual
were produced from over 60,000 hectares of fruit- stirring. After the solution had cooled, it was filtered
covered land. However, there is a paucity of relevant using the filter cloth. Equal parts of ethanol and alcohol
information on the specific varieties that suit pectin were added to the filtrate solution and allowed to
production better with maximum quality and yield. precipitate after being added. A cloth filter was used to
Further, there appears to be an increased focus on pectin filter the precipitated gel after the mixed solution was
production from alternative sources such as orange peel, regularly precipitated. The pectin was milled after being
and it would be of interest to evaluate which varieties dried for 20 min in a hot air oven at 40ºC.
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Physicochemical Properties of Ethiopian... 3

Fig. 1. Map of Ethiopia particularly the individual regions, and situating the location of Addis Ababa, as well as Tigray.

Characterization of Orange Peels temperature, each dish with ash was reweighed (M3).
The ash content was expressed as a percentage using the
Determination of Moisture Content following formula:

The moisture content of the orange peel sample was

assessed using the oven drying method. Orange peel
was dried to a consistent weight for three hours at 105ºC.
Using the weight difference between the initial and final
dried orange peel samples, the moisture content of the Determination of Total Soluble Solid (TSS)
orange peel slice was computed and expressed as a
percentage using the formula below. Triplicates of each A refractometer was used to measure the amount of
sample were used for testing. Total Soluble Solids (TSS). The thermostat setting for
the refractometer (A. Kruss Optronic, Germany) was
Moisture content (%) 25ºC, and the scale range was 0.00-95.00% Brix.

Determination of Reducing Sugar Content

Reducing the sugar content of orange peels was

determined by using the Lane and Eynon method [11].
Determination of Ash Content This required 30 g of crushed orange peels added in a
250 ml conical flask containing 5ml each of neutral lead
According to the procedure used by an association acetate and potassium oxalate, and the total volume was
of official analytical chemists and reported by [11], the made up to the mark with distilled water and allowed
ash content of orange peels was described as follows: to seep for 5 min. The mixture thus obtained is filtered
Porcelain dishes were heated to 550ºC in a muffle into another 250 ml conical flask using a muslin cloth.
furnace (Nabertherm, Germany, Temp. 30-3000ºC) for After the sample solution’s filtration, 5 mL of each
30 min prior to cooling in desiccators (with granular Fehling solution A and B were taken in a conical flask
silica gel) and being weighed with an analytical scale and heated until boiling before being added to a drop
(M1). Then orange peel samples weighing approximately of methylene blue indicator 1% w/v solution. Using
5 g were placed in the dish in triplicate (M2). The dishes the sample solution filtrate as the burette solution, this
containing the sample were put inside a muffle furnace was titrated until brick-red precipitates appeared as the
(Nabertherm, Germany, 30-3000ºC) and heated for endpoint. Finally, the percentage of reducing sugar in
three hours at 550ºC until a whitish color appeared and the samples was determined as follows:
cooled for one hour in desiccators. When cooled to room
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4 Abebe Teshome, et al.

Reducing sugar (%) g of pectin sample with 5 ml of ethanol, then mixed

against 0.1 N NaOH, one gram of sodium chloride, 100
milliliters of distilled water, and six drops of phenol red
indicator. The titration point was designated by the color
purple. The following equation was used to calculate the
Dilution factor equivalent weight (EW).
For invert sugar, the following calculations and Equivalent weight (EW)
standardization of Fehling solution were carried out
using about 9.5 g of AR-grade sucrose dissolved in 100
ml of distilled water. After the sucrose was dissolved,
5 ml of HCl was added, and the mixture was allowed
to remain for three days at room temperature in order
to cause inversion. In order to calculate the Fehling Determination of Methoxyl, Degree
solution factors, 25 ml of an inverted sugar solution of Esterification, and Anhydrouronic Acid
was transferred into a 100ml volumetric flask, diluted
to volume (1ml is equal to 2.5 mg of inverted sugar), The degree of esterification (DE) is defined as
and then transferred to a burette with an offset tip. The the ratio of the esterified galacturonic acid group
endpoint was identified by the complete discoloration of to the galacturonic acid group present. The degree
the methylene blue indicator, which was used to titrate of esterification was determined through titration,
against the Fehling solution. The mg of inverted sugar connecting the methoxyl content (MeO) with the
was calculated using the formula below: equivalent weight method [13]. First, 0.50 g of orange
peel pectin was dissolved in a 1:20 v/v solution of
Factor for Fehling Solution (g of inverted sugar) ethanol and water. Next, the mixture was titrated with
0.1 N sodium hydroxide (V1, ml) until the indicator
changed, after which five drops of the phenol red
indicator were added. The sample, heated briefly, was
vigorously stirred after adding 25 ml of 0.25 N NaOH.
Five drops of phenol red and 25 ml of 0.25 N HCl were
Determination of pH Value
subsequently added. After that, 0.1 N NaOH (V2, ml)
was added to the mixture in titrations until the endpoint
A digital pH meter (Model 744, Metrohm, color changed from yellow to pale pink. The following
Switzerland) was used to measure the pH of orange peel equation was then used to calculate the MeO and the
[12]. DE.
Characterization of Extracted Pectin
MeO (%)
Determination of Yield

The extraction yields of pectin from three varieties DE (%)

of orange peels on a dry weight basis were determined
as follows:
The anhydrouronic acid (AUA) content of pectin was
Pectin yield (%) also calculated as follows:

AUA (%)

Determination of Viscosity Where V1 and V2 are the volumes used for the first
and second titrations, respectively, 31 is the molecular
The resultant pectin was dissolved in distilled weight of methoxyl and 176 is the molecular weight of
water at a concentration of 1.0 percent (weight per anhydrouronic acid expressed in mg/meq.
volume). A viscometer (Model HTD13145, Haitongda,
China) was used to measure the mixture’s viscosity at Determination of Acetyl Content
100 revolutions per minute at about 22ºC.
The acetyl content was measured [13] using 0.5 g
Determination of Equivalent Weight (EW) of pectin mixed with 25 ml of 0.1 N NaOH, followed
by rapid agitation to ensure thorough mixing,
The equivalent weight of pectin obtained from then diluted with 50 ml of diluent after being left for 1 h.
orange peels was determined [13]. This required 0.5 The 20 ml of liquor was put into a steam distillation device
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Physicochemical Properties of Ethiopian... 5

together with 20 ml of a magnesium sulfate-sulfuric this disparity between varieties and researchers [6, 17].
acid solution. The solution was steam-distilled, and In fact, microbial growth and unfavorable responses
a small amount of distillate (about 100ml) was collected in food might be significantly influenced by moisture
in the distillation flask. The obtained acetic acid is and water activity. However, low moisture and water
titrated with 0.05 N NaOH to the endpoint of phenol red. activity values remain crucial for long-term storage,
Following a black titration on distilled water, the acetyl with the latter causing the least degradation of pectin.
concentration was calculated using the equation: Indeed, microbial proliferation by the pectinase enzyme
is detrimental to pectin’s quality.
Acetyl content (%) Ash contents resembled those between the three
varieties of orange peels (p>0.05) (Table 1) with 3.25%
(Valencia), 3.13% (Mandarin), and 3.3% (Gunda Gundo),
which seemed not too far from those reported elsewhere
[14, 18]. The reasons for the ash differences across
Statistical Analysis varieties could include the cultivar types, maturation,
fertilizer applied during growing, and meteorological
Using the SAS software package (version 9.4), the conditions [6]. The total soluble solid (TSS) content
experiment data were evaluated by analysis of variance of orange peels was significantly different (p<0.05)
(ANOVA) and significantly expressed at the P<0.05 by variety, between 13.05 and 15.78 Brix, which was
level. likened to that of prickly pear fruit peels (13.02-14.47
Brix) [19]. Here, peels from the Mandarin variety had
a low TSS content (13.05 brix) with a high pectin yield
Results and Discussion (26.3%), whereas those of the Valencia variety had a
high TSS (15.78 brix) with a low pectin yield (20.4%).
Physicochemical Characteristics Feasibly, the TSS of the peel could determine the pectin
of Variety Orange Peels yield during extraction. Despite this, reducing the sugar
resembled (p>0.05) between varieties of orange peels
The results of the physicochemical study of three (Table 1). Reducing sugar content of the orange peel was
types of orange peels (Valencia, Mandarin, and 2.43% for Valencia, 3.02% for Mandarin, and 2.71% for
Gunda Gundo) are presented in Table 1. Specifically, Gunda Gundo varieties.
the physicochemical components included moisture The pH value between varieties of orange peels
content, ash content, total soluble solids (Brix), reducing resembled (p>0.05) (Table 1), given by 5.81 for
sugar content (%), and pH value. Orange peels of Valencia, 4.17 for Mandarin, and 5.19 for Gunda Gundo.
different types were employed in this investigation, Elsewhere, orange peel (OP), citrus peel (CP), lemon
and the moisture content ranged from 8.51% to 11.01%. peel (LP), and jackfruit peel (JFP) showed pH ranges
These differences were significant (p<0.05). Orange from 3 to 4 [20]. Orange peels that have a high pH
peels from the Valencia variety obtained a higher (greater than 4.5) value or low acidity may produce
moisture content (11.01%), but those of the Mandarin a high yield of pectin because of the resistance to
orange peels appeared lower (18.51%), which seems damage (loss of texture, decreases in viscosity of food
above those reported elsewhere for lime orange (10%) products) during extractions of pectin [19]. Thus, it
[14]. It is possible for orange peel moisture levels to is inferred that orange peels of the Mandarin variety,
vary from 2.8 to 7.2% [15], like those of the “Maltease” having a low pH, might offer extracted pectin of a high
variety of orange peels reported at about 3% (dry yield and gel-forming ability compared to the other
basis) [16]. The genetic makeup of the cultivars, their varieties in the current study.
maturity stage, and agricultural techniques, including
harvesting timing and analysis, could all contribute to

Table 1. Physiochemical properties of three varieties of orange peels.

Compostions Valencia Mandarin Gunda Gundo
Moisture Content(%) 11.01 ± 0.45
a
8.51 0.81
b±
9.05bc±1.02
Ash content (%) 3.25a±0.03 3.13a±0.09 3.34a±0.12
Total Soluble Solid (brix) 15.78a±1.02 13.05bc±0.52 14.85ab±0.93
Reducing sugar content(%) 2.43a±0.31 3.02a±0.47 2.71a±0.56
pH value 5.81a±0.92 4.17b±0.59 5.19a±0.56
Values are mean±SD. Means with the same letters in a row are not significantly different (p>0.05).
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6 Abebe Teshome, et al.

Physicochemical Characteristics of Pectin to a lower pectin purity. Therefore, the ash content of
Extracted from a Variety of Orange Peels the pectin extracted from all three varieties of orange
peel was lower than the maximum limit indicated by
The pectin yields significantly differed (p<0.05) IPPA. During food preparation, the ash content of pectin
between Valencia, Mandarin, and Gunda Gundo should reflect the quality of gel formation.
varieties, with values of 20.4, 26.3, and 24.03%, The viscosity of pectin extracted from three different
respectively. These observed pectin yield differences varieties of orange peels is also presented in Table 2.
might be associated with the inherent genetics of the Pectin extracted from Mandarin orange peel had the
varieties [6]. The pectin yield obtained from those three highest viscosity (0.95 Pa. s) and was clearly different
varieties of orange peels was high compared to Elephant (P<0.05) from the Valencia (0.71 Pa. s) and Gunda
Apple peel and Pomelo peels (2.97-10.84%) reported by Gundo (0.69 Pa. s) varieties. The acetyl content of
[21], yet it resembled the yield of pectin extracted from pectin obtained from Valencia, Mandarin, and Gunda
grapefruit peel (23.5%) [22], and ponkan peel (25.6%) Gundo was 1.97, 1.31, and 1.52%, respectively, which
[23]. Elsewhere, the yield of pectin extracted would seemed close to those of white grapefruit peel (1.63%)
widely vary, with ranges from 10.9 to 24.08% in banana and citrus sinensis peel (1.20-1.34%) [25]. Indeed, the
peels (24.08%) over apple pomace (10.91%) [10]. acetyl content of pectin extracted from apple peels
Moisture contents of extracted pectin significantly could be even lower (0.68%) [25], which could affect the
differed (p<0.05) across three varieties of orange peel gelling capacity and degree of acetylation. The pectin
and ranged from 8.02 to 10.64% (Table 2). These results containing 3.5-4.0% of acetyl content should, therefore,
compete favorably with extracted pectin from papaya form weak gels.
peel, mango peel, watermelon peel, and chayote peel, The equivalent weight in milligrams per milliliter
where moisture content ranges from 9.44 to 15.03% [24]. of the pectin extracted from Valencia, Mandarin, and
The excessive moisture levels of pectins may promote Gunda Gundo varieties of orange peels was 544.2,
the development of microorganisms and result in the 672.6, and 603.7%, respectively. The equivalent weight
production of pectinase enzymes, which may have an of the pectin extracted from lemon, orange, and
impact on the pectin’s quality. The ash content of tangerine was 150.4, 116.78, and 97.15%, respectively
pectin extracted from Valencia orange peels was higher [26]. Elsewhere, the equivalent weight of pectin
than that of pectin derived from Mandarin and Gunda extracted from papaya peel via hydrochloric and citric
Gundo varieties of orange peels. According to different acids was 912.2 and 415.1, respectively [27]. Higher
literature reports, the quality of pectin yield decreased amounts of the equivalent weight of pectin would occur
as the ash content increased. More so, the International with gel-forming effects, whereas the lower equivalent
Pectin Producers Association (IPPA) suggested that less weight would occur with higher pectin degradation
than 10% ash in pectin could be a useful criterion for [26]. The highest amount of equivalent weight of pectin
gel formation and pectin quality. Lower levels of ash is used in jam, jellies, and marmalade preparations as
are associated with decreased levels of mineral residues, a gelling agent [28], indicating that pectin extracted
such as calcium and magnesium, which cohydrolysate from Mandarin varieties of orange peel could likely be
with protopectines. Pectin purity was also demonstrated employed in food applications relative to the other two
by ash contents, where a higher ash content corresponded varieties of orange peels in this current study.

Table 2. Physiochemical properties of pectin obtained from three varieties of orange peels.
Compositions Valencia Mandarin Gunda Gundo
Yield (%) 20.4 ±1.01
c
26.3 0.87
a±
24.03b±1.12
Moisture Content (%) 10.64a±0.8 8.02c±0.72 9.81b±0.59
Ash Content (%) 1.42a±0.23 1.25a±0.40 1.35a±0.31
Protin Content (%) 2.45b±0.30 3.80a±0.37 3.41a±0.21
Viscosity 0.69b±0.12 0.95a±0.27 0.71b± 0.21
Acetyl Content 1.97a±0.03 1.31bc±0.1 1.52ab±0.08
Equivalent Weight 544.2bc±4.31 672.6a±3.7 603.7ab±2.26
Methoxyl Content (Meo) (%) 6.25b±0.91 7.44a±0.74 6.31b±0.56
Anhydrouronic Acid (AUA)
70.45c±1.89 79.40a±2.54 74.21b±1.91
(%)
Degree of Esterification (%) 62.9c±3.47 70.2a±4.89 68.8ab±2.98
Values are expressed as mean±SD. The means with the same letters in a row do not differ significantly (p>0.05).
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Physicochemical Properties of Ethiopian... 7

The methoxyl content of the extracted pectin studies could seek the influence of soil contents such as
significantly differed (p<0.05) across the Valencia, organic matter on the orange peel, which might reflect
Mandarin, and Gunda Gundo varieties of orange on the efficacy/quality of the extracted pectin. Also, the
peels (Table 2). Elsewhere, the methoxyl content of short-to-long-term implications of pectin extracted from
pectin extracted from papaya peel using HCl and citric citrus products for the pharmaceutical industry require
acid was 7.5 and 6.2%, respectively [27]. Whereas further exploration. Another future study could be a
some characteristics of pectin, like gel strength and meta-analysis of how the pectin quality of Ethiopian
susceptibility to metal ions, might help determine citrus compares with others from different continents,
the functional characteristics, pectin’s gel texture providing climatic, soil, and geographical perspectives.
would depend on its methoxyl concentration [27].
Besides, gel formation mechanisms should differ in
the methoxyl content of pectin. For instance, a higher Acknowledgments
methoxyl content would form a gel with an increased
sugar concentration. Methoxyl in pectin might affect Authors are grateful for Department of Chemical
how quickly it dissolves in water. More so, pectin with Engineering and Food Engineering at the Addis Ababa
a higher methoxyl content should dissolve more readily. Science and Technology University for permitting the
Besides, low methoxyl pectin appears widely popular in use of their laboratory facilities. Wollo University in
making baked goods. Ethiopia is appreciated for some funding support.
The gelling nature of pectin reflects the degree of
esterification (DE), which typically differs per fraction,
dependent on plant species, followed by physiological Conflict of Interest
changes. For instance, the DE in the pectin obtained
from the various regional varieties of orange peels The authors declare there are no competing interests.
ranged from 62.9 to 70.2% (Table 2). Pectin from
Mandarin peel showed a significantly higher (p<0.05)
DE value compared to Gunda Gundo and Valencia. References
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