







































 
 

 

145 
© 2019 by the authors; licensee Asian Online Journal Publishing Group 
 

Agriculture and Food Sciences Research 
Vol. 6, No. 1, 145-154, 2019 

ISSN(E) 2411-6653/ ISSN(P) 2518-0193 
DOI: 10.20448/journal.512.2019.61.145.154 

© 2019 by the authors; licensee Asian Online Journal Publishing Group 

    
 

 
 
Effect of Pretreatments on the Drying Characteristics and Quality of African Star 
Apple (Chrysophyllum albidum) 

 
Komolafe, O.M.1    

Hussein, J. B.2    

Adebayo, Q.3     

Abiona, O.O.4    

Oke, M.O.5     

 
 

( Corresponding Author) 
 
1Department of Food Science Technology, Federal Polytechnics, Ilaro, Nigeria. 

 
2Department of Food Science and Technology, Modibbo Adama University of Technology, Yola, Adamawa State, 
Nigeria.  

 
3Department of Food Science and Technology, Federal University Dutsinma, Katsina State, Nigeria. 

 
4Department of Chemical Sciences, Osun State University, Osogbo, Osun State, Nigeria. 

 
5Department of Food Science and Engineering, Ladoke Akintola University of Technology, Ogbomoso, Oyo State, 
Nigeria. 

 

 
Abstract 

Fruits, due to high moisture content, deteriorate quickly when not adequately preserved. Drying, 
a common preservation method, will affect quality of final product if not properly controlled. 
Pretreatments prior to drying have been established as a way of retaining product qualities. This 
study investigated the effect of pretreatments on the qualities of dried African star apple flesh. 
African star apple fleshes were sliced (2, 4, 6 and 8 mm) and subjected to pretreatments 
[blanching (80 ºC for 3 min), lime juice (100%), ascorbic acid and salt solution (1:25 w/v)]. 
Untreated samples served as control. Samples were dried in cabinet dryer (50, 55 and 60 ºC) at 2 
m/s constant air flow-rate, monitored at intervals, until constant weight was obtained. Ascorbic 
acid and colour measurement of the fresh and dried sample were determined using standard 
method.  The moisture contents of the African star apple flesh were observed to reduce from a 
mean value of 70.44%, 71.55% and 73.24% to 2.38%, 1.91% and 3.23% at temperatures of 50, 55 
and 60 °C, respectively. The total drying time ranged between 7 to 8 h. Colour of the dried 
African star apple was significantly preserved by the pretreatments used, and low overall color 

change (ΔE) was obtained at the lowest drying temperature (50oC). The ascorbic acid and lime 
pretreatments however had better colour overall. Lime pretreatment for 2 mm thick at 50ºC gave 
the best result in terms of ascorbic acid retention. Hence, lime juice pre-treatment has potentials 
of retaining quality of dried fruits. 

 
Keywords: Pretreatments, Drying characteristics, African star apple, Drying qualities. 

 
Citation | Komolafe, O.M.; Hussein, J. B.; Adebayo, Q.; Abiona, 
O.O.; Oke, M.O. (2019). Effect of Pretreatments on the Drying 
Characteristics and Quality of African Star Apple (Chrysophyllum 
albidum). Agriculture and Food Sciences Research, 6(1): 145-154. 
History:  
Received: 21 March 2019 
Revised: 30 April 2019 
Accepted: 4 June 2019 
Published: 15 July 2019 
Licensed: This work is licensed under a Creative Commons 

Attribution 3.0 License  
Publisher:  Asian Online Journal Publishing Group 
 

Contribution/Acknowledgement: All authors contributed to the conception 
and design of the study. 
Funding: This study received no specific financial support. 
Competing Interests: The authors declare that they have no conflict of 
interests. 
Transparency: The authors confirm that the manuscript is an honest, 
accurate, and transparent account of the study was reported; that no vital 
features of the study have been omitted; and that any discrepancies from the 
study as planned have been explained. 
Ethical: This study follows all ethical practices during writing.   

 

 

Contents 
1. Introduction .................................................................................................................................................................................... 146 
2. Materials and Methods ................................................................................................................................................................. 146 
3. Results and Discussion ................................................................................................................................................................. 147 
4. Conclusion ....................................................................................................................................................................................... 153 
References ............................................................................................................................................................................................ 153 
 

http://crossmark.crossref.org/dialog/?doi=10.20448/journal.512.2019.61.145.154&domain=pdf&date_stamp=2017-01-14
http://crossmark.crossref.org/dialog/?doi=10.20448/journal.512.2019.61.145.154&domain=pdf&date_stamp=2017-01-14
http://creativecommons.org/licenses/by/3.0/
http://creativecommons.org/licenses/by/3.0/
http://www.asianonlinejournals.com/index.php/AESR/article/view/807
https://orcid.org/0000-0002-3537-1878
https://orcid.org/0000-0002-9029-5468
https://orcid.org/0000-0001-6490-1797
https://orcid.org/0000-0003-4928-8546
https://orcid.org/0000-0003-1175-0301


Agriculture and Food Sciences Research, 2019, 6(1): 145-154 

146 
© 2019 by the authors; licensee Asian Online Journal Publishing Group 

 

 

Contribution of this paper to the literature 
This study contributes to existing literature by investigating the effect of pretreatments on the qualities 
of dried African star apple flesh. 

 
1. Introduction 

The preservation of fruits and vegetables are essential for extending the shelf life and quality of the product. 
Preservation technologies include drying, canning, dehydration and others. Among these, drying (dehydration) is 
especially suited for developing countries with inadequate facilities. It offers a highly effective and practical means 
of preservation which reduces post-harvest losses and off-set shortages in supply [1]. African star apple 
(Chrysophyllum albidum) is a forest fruit tree commonly found throughout tropical Africa. It has a fleshy flesh of the 
fruits which is popularly consumed. The flesh can taste either very sweet or sour. The variation of the fruit colour 
is said to be correlated with the flesh taste.  

The covering of the sweet fruits are seen to be yellow while that of the sour ones are usually a mix of yellow 
and green colours when mature [2]. It is of great economic value due to its diverse industrial, medicinal and food 
uses. The fruits are not only consumed fresh but also used to produce jam, jellies, stewed fruit, marmalade, syrup 
and several types of soft drinks. It is also used for medical purposes due to properties of stalk and fruits. The leaves 
and seed of some of these fruits and vegetables are used in pharmaceuticals [3]. 

Pretreatment of fruits prior to drying is highly recommended because it helps to keep light-coloured fruits 
from enzymatic browning during drying and storage. It also speeds the drying of fruits with tough skins, such as 
grapes and cherries [4]. Different pretreatment methods have been developed for fruit drying, amongst which are 
lemon juice, salt solution, honey dip, ascorbic acid, sulfuring, osmotic pre-treatment and blanching [5]. The 
objectives of this study were to investigate the effects of pretreatments on the qualities (colour and ascorbic acid 
contents) of dried African star apple flesh. 
 

2. Materials and Methods 
2.1. Experimental Procedure 

The African star apples were sorted to remove over-ripe and damaged ones. The fruits were thoroughly 
washed and peeled before cutting into slices for drying. The flesh of the fruits was cut, using a simple improvised 
grater, into thin slices of 2, 4, 6 and 8 mm thicknesses. The average initial moisture content of the tropical almonds 
was determined using AOAC [6] methods as 71.74 %, wet basis. 
 
2.2. Pretreatment of the Samples 

The samples were pre-treated according to the methods of Tunde-Akintunde [7] and Doymaz [8]. The 
Completely Randomized Design (CRD) was used for the drying experiments with the four pretreatments and one 
control replicated three times to ensure the consistency of the results. The pretreatments used on the fruit samples 
were on a weight per volume basis with 1 part of the solute in 25 parts of distilled water (1:25 w/v for 10 min) 
except for hot water blanching where no solute was added to the distilled water. The pretreatments were coded as 
shown below: 

• CON: No pre-treatment solution. 

• BLA: blanched in water bath (80 ºC for 3 min), removed and cooled immediately by placing in a large 
quantity of tap water for another 3 min to remove excess heat. It is drained in a metal sieve and the excess 
water blotted off before further processing. 

• ASC: 1 part of ascorbic acid in 25 parts of distilled water (1:25 w/v for 10 min). 

• LIM: 1 part of lime juice in 25 parts of distilled water (1:25 w/v for 10 min). 

• SAL: 1 part of salt in 25 parts of distilled water (1:25 w/v for 10 min). 
 

2.3. Drying Procedures  
For the drying experiments, 0.2 kg of fresh pretreated and untreated African star apple fleshes was spared on a 

single layer inside a perforated tray and dried at temperatures of 50 ºC, 55 ºC and 60 ºC respectively.  The drying 
was accomplished using an air-convective cabinet dryer of Rhong Machinery Manufacturing Limited series cabinet 
dryer (model RCD-5) designed to dry food materials under controlled conditions. The temperature of the air in the 
drying chamber was controllable to suit the drying temperature desired. The air velocity in the drying chamber 
was controlled by a fan speed controller (adjustable air valve at air inlet and exhaust) constant at 2 m/s flow-rate. 
Also, there was a time controller to monitor drying time. The moisture loss of the flesh during drying was 

measured at 5 minutes intervals for the first hour, 10 minutes for second hour and at 15 minutes intervals 
thereafter during the drying process. This was measured using a Gallenkamp electronic balance (model MP 
10001), which has 0 - 1,000 g measurement range with a reading sensitivity of ±0.01 g. The drying processes were 
stopped when the weight loss became negligible. The experiments were replicated three times and the average 
results were recorded. After drying, samples were packed and sealed in black polythene to prevent exposure to 
light and stored until further analyses. 
 

2.4. Determination of Ascorbic Acid 
Ascorbic acid was determined using the AOAC [6] method. An aliquot (10 g) of the sample was diluted to a 

fixed volume (100 ml) with 3% HPO3 and then titrated with 2, 6-dichlorophenolindophenol.  A standard ascorbic 
acid solution of 5 mL was added to 5 mL of 3% HPO3 and titrated with dye solution to a pink colour, which 
persisted for 15 s. Ascorbic acid (mg/100g) of reconstituted juice was calculated using the formula:   

𝐴𝑠𝑐𝑜𝑟𝑏𝑖𝑐 𝑎𝑐𝑖𝑑 (𝑚𝑔 100𝑚𝑙) =
𝑇×𝐷𝐹×𝑉1

𝑉2×𝑉3
⁄       (1) 

where, T = titre; DF = Dye factor; V1 = volume made up (100 ml); V2 = aliquot of extract taken for estimation (10 
g) and V3= volume of sample taken for estimation (10 ml). 



Agriculture and Food Sciences Research, 2019, 6(1): 145-154 

147 
© 2019 by the authors; licensee Asian Online Journal Publishing Group 

 

 

2.5. Colour Measurements  
The fresh and dried fruit colour was determined by direct reading using a colorimeter (MINOLTA model CR-

200; Minolta Camera Co., Ltd., Osaka, Japan) to obtain the colour values: L* (brightness/darkness), a* 
(redness/greenness) and b* (yellowness/blueness) as described by Ashebir, et al. [9]. The instrument was 
calibrated using standard white and black plates before each color measurement. The measurements were taken 
from randomly selected fresh fruits at three different parts (around halfway from the blossom end) of a fruit and the 
mean of three readings at these random locations on the sample was used. It should be noted that these same fruit 
parts were dried and subject to colour measurement after drying. 

The fresh African star apple was taken as the ideal sample. The total colour change (∆E) parameter is 
calculated as follows: 

∆𝐸 = [(𝐿∗ − 𝐿∗∗)2 + (𝑎∗ − 𝑎∗∗)2 + (𝑏∗ − 𝑏∗∗)2]0.5     (2) 
where; 
∆E = indicates the total colour change of a sample in comparison to colour values of an ideal sample 

having colour values of L*, a* and b*.  
L* = the lightness of fresh samples; L** = the lightness of dried samples 
a* = the redness of fresh samples; a** = the redness of dried samples 
b* = the yellowness of fresh samples and b** = the yellowness of dried samples. 

 

2.6. Statistical Analysis 
All experiments were performed in triplicate, and the results were expressed as means ± standard error (SE). 

Analysis of variance (ANOVA) was carried out to determine any significant differences in measurements using the 
SPSS statistical software (SPSS 20.0 for Windows; SPSS Inc., Chicago, IL, USA) and considering the confidence 
level of 95%. The significance of the difference between the means was determined using the Duncan Multiple 
range test, and the differences were considered to be significant at p< 0.05. 
 

3. Results and Discussion 
3.1. Drying Characteristics 

The drying experiments gave different values for the moisture contents of the African star apple fruits at 
different temperatures and thicknesses. Figures 1 – 3 show the drying curves for drying the flesh of African star 
apple of 2, 4, 6 and 8 mm thicknesses at temperatures of 50, 55 and 60 ºC. For drying carried out at 50 ºC, the 
equilibrium moisture content was attained in 8 h. Also, for drying carried out at 55 ºC, the equilibrium moisture 
content was also attained in 8 h. On the other hand, for drying carried out at 60 ºC, the equilibrium moisture 
content was attained in 7 h. The moisture contents of the African star apple flesh were observed to reduce from a 
mean value of 70.44%, 71.55% and 73.24% to 2.38%, 1.91% and 3.23% for temperatures of 50, 55 and 60 °C, 
respectively. In this drying operation, a relative reduction in moisture content with increase in drying time was 
observed. This also resulted in the drying curve obtained showing a downward curve.  
 

 
Figure-1. Drying Curves for African Star Apple Flesh of 2 mm, 4 mm, 6 mm and 8 mm  Thicknesses Dried at 50 °C. 

              Source: Authors’ Findings. 
 

 



Agriculture and Food Sciences Research, 2019, 6(1): 145-154 

148 
© 2019 by the authors; licensee Asian Online Journal Publishing Group 

 

 

 
Figure-2. Drying Curves for African Star Apple Flesh of 2 mm, 4 mm, 6 mm and 8 mm Thicknesses Dried at 55 °C. 

            Source: Authors’ Findings. 

 

 
Figure-3. Drying Curves for African Star Apple Flesh of 2 mm, 4 mm, 6 mm and 8 mm Thicknesses Dried at 60 °C. 

               Source: Authors’ Findings. 

 
The downward trends of the drying curves show that moisture content reduces with increasing drying time. 

The highest moisture loss was observed at the early period of drying due to the removal of free water present at 
the surface of the fruits. Over the course of the drying period, the moisture removal was observed to reduce; the 



Agriculture and Food Sciences Research, 2019, 6(1): 145-154 

149 
© 2019 by the authors; licensee Asian Online Journal Publishing Group 

 

 

temperature of drying not commensurate to the amount of moisture being removed. At no point in the drying 
operations was the moisture removal at a constant rate. This confirms that the whole drying operation of the fruit 
samples largely took place in the falling rate period. This is in line with the results of Taiwo and Adeyemi [10] 
and Tunde-Akintunde [11] on banana slices and chilli peppers, respectively.  

The slice thicknesses of the samples and higher drying temperatures were also observed to have effects on the 
rate of moisture removal; which invariably affected the overall drying process. Faster drying rates were observed at 
2 mm and 4 mm thicknesses compared to the 6 mm and 8 mm thicknesses for drying of the African star apple. This 
result corroborated with Hussein, et al. [12] findings in which the drying time reduced significantly as the 
thickness of slices decreases, because the resistance to moisture movement is relatively higher in thicker slices than 
in thinner ones. Also, the rate of drying at 60 ºC was faster as shown in Figure 3. This was due to high temperature 
gradients created by higher temperature which ease the migration of water from the inner part to the surface to get 
evaporated. The ease of this migration depends on the porosity of the samples, drying temperatures and the surface 
area available as reported by Hussein, et al. [12]. The study also confirmed the reports of Tunde-Akintunde and 
Oke [13] and Sobukola [14] on plantain and apple thin layer drying, respectively. This was explained as higher 
temperatures leading to faster removal of moisture from food materials. 
 
3.2. Effects of Temperature and Pretreatments on Drying Rate 

The moisture contents of the fruits were observed to reduce with increase in the drying time. The drying rates 
of the pretreated samples were higher than the drying rates of the control sample in most of the drying conditions. 
However, these were not in all cases as the pretreatments were not significant in some drying conditions. Figure 4 
– 6 show the drying rate curves for the African star apple flesh. From the figures, the drying rates of the pretreated 
African star apple flesh were observed to consistently having higher drying rates; with the blanching pretreatment 
being more predominant. This may be attributed to the softer and looser structure of the flesh of the fruit, an 
observation reported by Doymaz [15] on carrot pomace. The effect of blanching increasing moisture loss and 
drying rates of agricultural materials has also been reported by Taiwo and Adeyemi [10]. Kaymak-Ertekin [16] 
reported that blanching improves the drying rates of carrots because blanching increases the porosity of cell walls, 
leading to faster movement of water away from the food. This observation is also in agreement with the work of 
Akintunde, et al. [17] on the blanching of green pepper; that blanching operation generally increases the drying 
rate. 
 

 
Figure-4. Drying Rate Curves for 2 mm, 4 mm, 6 mm and 8 mm African Star Apple Flesh Dried at 50 °C. 

                 Source: Authors’ Findings. 

 
 



Agriculture and Food Sciences Research, 2019, 6(1): 145-154 

150 
© 2019 by the authors; licensee Asian Online Journal Publishing Group 

 

 

 
Figure-5. Drying Rate Curves for 2 mm, 4 mm, 6 mm and 8 mm African Star Apple Flesh Dried at 55 °C. 

               Source: Authors’ Findings. 

 

 
Figure-6. Drying Rate Curves for 2 mm, 4 mm, 6 mm and 8 mm African Star Apple Flesh Dried at 60 °C. 

                   Source: Authors’ Findings. 

 
The other pretreatment methods had slightly higher rates than the control samples, but not as predominant as 

the blanched samples. A similar observation was also made by Abano [4] on drying of pineapples with different 
pretreatments. It was reported that there was an initial faster rate of drying with ascorbic acid pretreatment which 
was later reduced due to the leaching effects of ascorbic acid. Salt pretreatment was also found to have a high 
drying rate in the study. The high drying rate was observed to be as a result of common salt made of sodium 
chloride in an acid-base reaction thus, heating caused the loss of moisture. The high drying rate of the salt 
pretreatment was attributed to this reaction. 



Agriculture and Food Sciences Research, 2019, 6(1): 145-154 

151 
© 2019 by the authors; licensee Asian Online Journal Publishing Group 

 

 

3.3. Effect of Temperature and Pretreatment on Ascorbic Acid Retention 
In the drying experiments, the fruits were dried by the reduction of their moisture contents; however, this was 

in tandem with the loss of valuable nutrients such as ascorbic acid content of the fruits. An important indicator of 
the efficient drying of the fruits is the ascorbic acid retention of the drying process. The results for the ascorbic acid 
retention of the drying experiments are shown in Table 1. The total ascorbic acid content of fresh flesh was found 
to be 24.37 mg/100 g. The total ascorbic acid retention showed a consistent trend of samples pretreated with lime 
having highest values, followed by ascorbic acid, control and salt solution respectively. The values ranged from 
14.75 mg/100 g to 19.17 mg/100 g at 50 ºC, 14.02 mg/100 g to 18.48 mg/100 g at 55 ºC and 13.72 mg/100 g to 
18.15 mg/100 g at 60 ºC. The results also showed that there was higher ascorbic acid retention at lower 
temperatures and smaller thicknesses compared to higher temperatures and larger thicknesses. This was 
highlighted by the highest ascorbic acid retention at 50 ºC and 2 mm thickness compared to the lowest at 60 ºC and 
8 mm thickness. 
 

Table-1. Ascorbic Acid Content of Dried African Star Apple Flesh. 

  Ascorbic acid content (mg/100 g) 

Temperature Pretreatment 2 mm 4 mm 6 mm 8 mm 

50 °C Control 17.29 ± 0.11c 16.89 ± 0.08c 16.61 ± 0.08c 15.89 ± 0.07b 

 Blanched 16.79 ± 0.24b 16.38 ± 0.32b 16.39 ± 0.08b 15.49 ± 0.29b 

 Ascorbic 18.10 ± 0.07d 17.68 ± 0.08d 17.12 ± 0.08d 16.59 ± 0.07c 

 Lime 19.17 ± 0.17e 18.83 ± 0.08e 17.58 ± 0.07e 17.17 ± 0.08d 

 Salt 16.21 ± 0.08a 15.81 ± 0.12a 14.74 ± 0.08a 14.75 ± 0.78a 
55 °C Control 16.94 ± 0.04c 16.51 ± 0.03c 16.18 ± 0.25c 15.58 ± 0.08c 

 Blanched 16.76 ± 0.03b 16.33 ± 0.06b 15.49 ± 0.07b 15.31 ± 0.16b 

 Ascorbic 17.83 ± 0.09d 17.44 ± 0.07d 16.84 ± 0.08d 16.46 ± 0.07d 

 Lime 18.48 ± 0.08e 18.07 ± 0.09e 17.32 ± 0.06e 16.81 ± 0.09e 

 Salt 15.84 ± 0.21a 15.49 ± 0.08a 14.37 ± 0.08a 14.02 ± 0.11a 
60 °C Control 17.64 ± 0.02c 16.39 ± 0.09c 15.89 ± 0.63c 15.48 ± 0.07c 

 Blanched 16.67 ± 0.08b 16.19 ± 0.05b 15.25 ± 0.09b 15.25 ± 0.09b 

 Ascorbic 17.64 ± 0.02c 17.19 ± 0.07d 16.74 ± 0.07d 16.21 ± 0.07d 

 Lime 18.15 ± 0.07d 17.73 ± 0.08d 17.19 ± 0.07e 16.63 ± 0.07e 

 Salt 15.25 ± 0.03a 14.79 ± 0.06a 14.15 ± 0.09a 13.72 ± 0.73a 
                       Note: Means within the same column at the same temperature with different alphabet(s) are significantly different at p ˂ 0.05. 

     Source: Authors’ Findings.’ 

 
The results showed a significant reduction in the ascorbic acid content of the dried samples, with increasing 

temperature and thickness also having great significance. The heating foods at higher temperatures cause losses of 
ascorbic acid because it is heat labile. The findings of Gupta, et al. [18] corroborates this claim, where drying at 
temperatures above 60 ºC resulted in substantial ascorbic acid losses in the drying of cauliflower. It was also 
reported that prolonged exposure to higher temperatures destroys compounds in fruits. An explanation on the 
variations in ascorbic acid was given by Garba and Kaur [19] where the variations were attributed to the different 
pre-treatments used. Joshi, et al. [20] also proposed that the ascorbic acid losses during most drying methods 
could be as a result of oxidation of ascorbic acid at high temperatures, which also supports the results of these 
drying operations. 

The ascorbic acid reduction of the fruit samples was consistent across the samples with pretreatments and 
control samples. This showed that hot air drying resulted in ascorbic acid degradation in all the samples. The 
higher values of ascorbic acid retention recorded for the lime and ascorbic acid pretreatment could be due to them 
being high sources of ascorbic acid. This could reduce the degradation kinetics of the ascorbic acid in the fruits 
during the drying operation. The control samples showed higher ascorbic acid retention than the blanched and salt 
pretreatments. This may be due to enzymatic and chemical degradation as well as leaching and heating operations, 
a similar observation noted by Wawire, et al. [21] and Toor and Savage [22]. The results of these experiments 
were higher than the results reported by Ruvini, et al. [23] on similar drying operations with similar 
pretreatments. The observations made by Demiray and Tulek [24] on ascorbic acid retention in fruits at 
temperatures below 70 ºC also support the ascorbic acid retention values for these experiments. The lower values 
for the salt pretreated samples across the fruits may be due to the inhibiting effect salt has on ascorbic acid [25, 
26]. 
 

3.4. Effect of Temperature and Pretreatment on Colour Retention 
The final colour of the dried fruits is of great importance to the consumers. Colour is one of the subjective 

quality characteristics which indicate the level of effects of the drying methods, the pretreatment effects or 
conditions. Hence, colour is of great importance in deciding the preference of consumers [27]. Tables 2 – 5 show 

the colour changes in the dried African star apple. The tables display the values for L*, a*, b* and ΔE which 

indicate the lightness, redness, yellowness and change in colour values respectively. The L*, a*, b* and ΔE values 
for African star apple were 40.29, 15.69, 18.54 and 37.78. The dried fruits all showed considerably lower values for 
all the parameters compared to the fresh samples. This is an indication that the slice thickness and pre-treatments 
were not significant because the colour changes were random and no specific sequence could be ascertained. The 
best colour retention for the African star apple were salt pre-treatment at 50 ºC for 2 mm, salt at 50 ºC for 4 mm, 
ascorbic at 50 ºC for 6 mm and salt at 50 ºC for 8 mm. 
 
 
 
 
 
 



Agriculture and Food Sciences Research, 2019, 6(1): 145-154 

152 
© 2019 by the authors; licensee Asian Online Journal Publishing Group 

 

 

Table-2. Colour Changes for Dried African Star Apple Flesh of 2 mm Thickness. 

Temperature Pre-treatment L* a* b* ΔE 

50 °C Control 28.29 ± 0.62b 11.44 ± 0.90c 5.95 ± 0.87b 18.52 ± 1.34bc 

 Blanched 24.93 ± 0.97a 9.63 ± 0.64b 4.17 ±  1.38b 16.33 ± 1.77ab 

 Ascorbic 28.28 ± 3.60b 7.12 ± 0.17a 1.90 ± 1.43a 16.22 ± 3.56ab 

 Lime 24.33 ± 0.66a 6.64  ± 1.01a 1.90 ± 1.84a 13.14 ± 1.98a 

 Salt 32.31 ± 1.61c 7.70 ± 0.52a 5.74 ± 0.07b 21.57 ± 1.03c 
55 °C Control 27.66 ± 2.77b 11.29 ± 0.76b 5.32 ± 1.94d 17.71 ± 3.16b 

 Blanched 24.37 ± 0.60a 7.32 ± 0.53a 2.04 ± 0.48ab 13.54 ± 0.58a 

 Ascorbic 26.62 ± 0.98ab 6.59 ± 0.47a 1.45 ± 0.77a 14.48 ± 1.04a 

 Lime 26.61 ± 1.65ab 6.96 ± 0.44a 3.23 ± 0.49bc 15.65 ± 1.28ab 

 Salt 28.36 ± 0.81b 6.99 ± 1.04a 3.65 ± 0.27c 17.17 ± 0.49b 
60 °C Control 28.62 ± 1.73c 7.96 ± 0.61b 4.31 ± 0.70c 16.45 ± 13.59d 

 Blanched 26.54 ± 0.26ab 6.79 ± 0.06a 2.84 ± 0.60b 13.57 ± 0.53b 

 Ascorbic 24.97 ± 0.62a 6.77 ± 0.15a 1.88 ± 0.30a 11.84 ± 0.66a 

 Lime 26.82 ± 1.76abc 8.27 ± 0.52b 3.86 ± 0.13c 14.95 ± 1.11c 

 Salt 28.39 ± 1.13bc 7.90 ± 0.44b 4.51 ± 0.09c 16.30 ± 0.82d 

                      Note: Means within the same column at the same temperature with different alphabet(s) are significantly different at p ˂ 0.05. 
     Source: Authors’ Findings. 

 
Table-3. Colour Changes for Dried African Star Apple Flesh of 4 mm Thickness. 

Temperature Pre-treatment L* a* b* ΔE 

50 °C Control 27.01 ± 0.62ab 10.16 ± 0.90b 4.67 ± 0.87ab 17.24 ± 1.34a 

 Blanched 26.21 ± 0.97a 10.91 ± 0.64b 5.45 ± 1.38ab 17.61 ± 1.77a 

 Ascorbic 29.56 ± 3.60b 8.40 ± 0.17a 3.18 ± 1.43a 17.50 ± 3.56a 

 Lime 25.61 ± 0.66a 7.92 ± 1.01a 3.18 ± 1.84a 14.42 ± 1.98a 

 Salt 33.59 ± 1.61c 8.98 ± 0.52a 7.02 ± 0.07c 22.85 ± 1.03b 
55 °C Control 26.38 ± 2.77a 10.01 ± 0.76b 4.04 ± 1.94ab 16.43 ± 3.16ab 

 Blanched 25.65 ± 0.60a 8.60 ± 0.53a 3.32 ± 0.48ab 14.82 ± 0.58a 

 Ascorbic 27.90 ± 0.98ab 7.87 ± 0.47a 2.73 ± 0.79a 15.76 ± 1.04a 

 Lime 27.89 ± 1.65ab 8.24 ± 0.44a 4.51 ± 0.49b 16.93 ± 1.28ab 

 Salt 29.64 ± 0.81b 8.27 ± 1.04a 4.93 ± 0.27b 18.45 ± 0.49b 
60 °C Control 27.34 ± 1.73c 6.68 ± 0.61b 3.03 ± 0.70c 15.17 ± 0.97d 

 Blanched 25.26 ± 0.26ab 5.51 ± 0.06a 1.56 ± 0.60b 12.30 ± 0.53b 

 Ascorbic 23.69 ± 0.62a 5.50 ± 0.15a 0.59 ± 0.30a 10.56 ± 0.66a 

 Lime 25.54 ± 1.76abc 6.99 ± 0.52b 2.58 ± 0.13c 13.67 ± 1.11c 

 Salt 27.11 ± 1.13bc 6.62 ± 0.44b 3.23 ± 0.09c 15.02 ± 0.82d 

                      Note: Means within the same column at the same temperature with different alphabet(s) are significantly different at p ˂ 0.05. 
                     Source: Authors’ Findings. 

 
Table-4. Colour Changes for Dried African Star Apple Flesh of 6 mm Thickness. 

Temperature Pre-treatment L* a* b* ΔE 

50 °C Control 28.73 ± 1.24a 8.94 ± 0.40b 5.74 ± 1.97a 18.51 ± 2.18a 

 Blanched 30.00 ± 3.69a 8.16 ± 0.18a 4.66 ± 1.74a 18.55 ± 3.75a 

 Ascorbic 29.53 ± 2.08a 7.80 ± 0.79a 5.52 ± 0.13a 18.60 ± 1.32a 

 Lime 26.52 ± 0.23a 8.15 ± 0.25a 3.59 ± 0.51a 15.36 ± 0.30a 

 Salt 26.88 ± 1.82a 7.58 ± 0.19a 3.33 ± 2.21a 15.25 ± 2.65a 
55 °C Control 24.66 ± 0.76a 6.49 ± 0.68a 1.68 ± 1.46a 12.28 ± 1.65a 

 Blanched 27.75 ± 1.06b 5.72 ± 0.44a 2.24 ± 0.71ab 14.66 ± 1.34ab 

 Ascorbic 24.22 ± 0.17a 7.22 ± 0.21ab 2.95 ± 0.27ab 13.07 ± 0.27ab 

 Lime 27.79 ± 1.86b 8.62 ± 0.27b 4.73 ± 0.78b 17.14 ± 1.71b 

 Salt 27.68 ± 3.32b 6.86 ± 1.91a 4.02 ± 3.48ab 15.98 ± 5.12ab 
60 °C Control 27.97 ± 0.54c 4.72 ± 0.69a 0.64 ± 0.49a 13.73 ± 0.36a 

 Blanched 24.90 ± 0.71a 6.08 ± 1.78ab 1.76 ± 1.81a 12.44 ± 2.16a 

 Ascorbic 26.39 ± 0.37ab 5.09 ± 0.38ab 1.66 ± 0.43a 13.11 ± 0.52a 

 Lime 26.68 ± 1.70bc 4.88 ± 0.31ab 4.09 ± 4.62a 13.26 ± 1.54a 

 Salt 25.30 ± 1.07ab 6.28 ± 0.57b 1.82 ± 0.47a 12.59 ± 1.17a 

                       Note: Means within the same column at the same temperature with different alphabet(s) are significantly different at p ˂ 0.05. 
                     Source: Authors’ Findings. 

 
Table-5. Colour Changes for Dried African Star Apple Flesh of 8 mm Thickness. 

Temperature Pre-treatment L* a* b* ΔE 
      

50 °C Control 24.42 ± 0.37a 7.82 ± 0.60b 1.71 ± 0.81a 12.70 ± 0.82a 

 Blanched 29.16 ± 4.47b 7.82 ± 0.24b 4.41 ± 1.99b 17.70 ± 4.42b 

 Ascorbic 24.88 ± 0.54a 7.79 ± 0.60b 1.79 ± 0.60a 13.02 ± 0.35a 

 Lime 28.58 ± 1.82b 5.48 ± 0.51a 2.66 ± 0.59a 15.53 ± 1.67ab 

 Salt 29.24 ± 1.95b 7.70 ± 1.37b 5.15 ± 0.69b 18.14 ± 1.54b 
55 °C Control 28.50 ± 0.36a 7.54 ± 0.30a 3.54 ± 0.75b 16.53 ± 0.46a 

 Blanched 27.15 ± 1.28a 9.21 ± 0.59b 4.92 ± 1.58b 17.04 ± 2.08a 

 Ascorbic 27.12 ± 1.44a 6.82 ± 0.64a 1.87 ± 0.22a 14.34 ± 1.02a 

 Lime 27.12 ± 1.45a 7.06 ± 0.68a 4.22 ± 1.48b 16.23 ± 3.13a 

 Salt 27.12 ± 1.46a 7.36 ± 0.29a 4.53 ± 0.58b 16.32 ± 1.93a 
60 °C Control 27.74 ± 2.09a 7.34 ± 1.16b 4.26 ± 1.98b 16.31 ± 3.03b 

 Blanched 25.66 ± 3.34a 6.42 ± 0.86ab 1.08 ± 0.40a 12.84 ± 2.13a 

 Ascorbic 25.04 ± 0.89a 5.30 ± 0.14a 0.80 ± 0.86a 11.59 ± 1.18a 

 Lime 25.46 ± 0.35a 5.45 ± 0.66a 1.81 ± 0.90a 12.59 ± 0.87a 

 Salt 26.39 ± 1.34a 6.17 ± 0.71ab 2.49 ± 1.91ab 13.90 ± 2.34ab 

                     Note: Means within the same column at the same temperature with different alphabet(s) are significantly different at p ˂ 0.05. 
                     Source: Authors’ Findings. 

 
The values recorded were lower compared to values available in literature for drying of similar foods, although 

their values for fresh samples were higher. Also, comparable data on the drying of African star apple were not 



Agriculture and Food Sciences Research, 2019, 6(1): 145-154 

153 
© 2019 by the authors; licensee Asian Online Journal Publishing Group 

 

 

available. However, Doymaz, et al. [28] reported that higher L* and lower a*/b* values are required in dried food 
materials. The results obtained for the drying in this study are in line with that observation. An explanation for the 
colour changes experienced is given by Fellows [29] where it was reported that colour changes in fruits and 
vegetables are caused by heat, oxidation during drying and residual enzyme activity which may cause browning 
during storage. A similar observation was reported by De-Corcuera, et al. [30] on how blanching pretreatments 
made samples darker with loss of yellowness which gave lower L* and b* values. This was associated to the 
destruction of pigments by blanching. Ali, et al. [31] reported a similar observation in the drying of guava slices. It 
was reported that the slices became darker during the drying process and it was attributed to prolonged drying 
time. It was also reported that temperature increase destroys colour quality; therefore, it was suggested that drying 
operations should be carried out at lower temperatures to better retain the colour of dried fruits. 
 

4. Conclusion  
The tropical African star apple fleshes were dried in the cabinet dryer. The drying rate of the African star apple 

flesh samples decreased as drying rate progressed while it increased with increase in drying temperature. The slice 
thickness and pretreatments were observed to affect the drying rates, with smaller thicknesses drying faster and 
blanching pretreatment having the highest drying rate of all. The increase in slice thickness was observed to 
reduce the drying rate and vice-versa.  

The drying time of the fruits ranged between 7 to 8 h. The structures of the samples were however observed to 
be differ compared to the control samples, despite significant ascorbic acid retention as a result of the 
pretreatments; especially the lime and ascorbic acid. The pretreatments were observed to help reduce the effect of 
ascorbic acid degradation by heat. The colour changes of the fruits were also significant as there were significant 
difference between the values of the fresh samples and the dried products. The ascorbic acid and lime pretreatments 
however had better colour overall. Drying at 50 ºC with lime pretreatment gave the best result in terms of ascorbic 
acid retention. 
 

References 
[1] I. Pardeshi, S. Arora, and P. Borker, "Thin-layer drying of green peas and selection of a suitable thin-layer drying model," Drying 

Technology, vol. 27, pp. 288-295, 2009. Available at: https://doi.org/10.1080/07373930802606451. 
[2] L. G. Houessou, T. O. Lougbegnon, F. G. Gbesso, L. E. Anagonou, and B. Sinsin, "Ethno-botanical study of the African star apple 

(Chrysophyllum albidum G. Don) in the Southern Benin (West Africa)," Journal of Ethnobiology and Ethnomedicine, vol. 8, pp. 1-10, 
2012. Available at: https://doi.org/10.1186/1746-4269-8-40. 

[3] F. A. Bello and A. A. Henry, "Storage effects and the postharvest quality of African star apple fruits (Chrysophyllum africanum) 
under ambient conditions," African Journal of Food Science and Technology, vol. 6, pp. 35-43, 2015. Available at: 
https://doi.org/10.14303/ajfst.2015.011. 

[4] E. Abano, "Assessments of drying characteristics and physio-organoleptic properties of dried pineapple slices under different pre-
treatments," Asian Journal of Agricultural Research, vol. 4, pp. 155-161, 2010. Available at: 
https://doi.org/10.3923/ajar.2010.155.161. 

[5] O. R. Karim, "Effect of Pre-Treatment on Drying Kinetics and Quality Attributes of Air-Dehydrated Pineapple Slices," Ph.D, 
University of Agriculture, Abeokuta, Ogun State, Nigeria, 2005. 

[6] AOAC, Official method of analysis. Washington D.C: Association of Official Analytical Chemist, 2010. 
[7] T. Tunde-Akintunde, "Mathematical modeling of sun and solar drying of chilli pepper," Renewable Energy, vol. 36, pp. 2139-2145, 

2011. Available at: https://doi.org/10.1016/j.renene.2011.01.017. 
[8] I. Doymaz, "Drying of potato slices: Effect of pre-treatments and mathematical modeling," Journal of Food Processing and 

Preservation, vol. 36, pp. 310-319, 2011. 
[9] D. Ashebir, K. Jezik, H. Weingartemann, and R. Gretzmacher, "Change in color and other fruit quality characteristics of tomato 

cultivars after hot-air drying at low final-moisture content," International Journal of Food Sciences and Nutrition, vol. 60, pp. 308-315, 
2009. Available at: https://doi.org/10.1080/09637480903114128. 

[10] K. A. Taiwo and O. Adeyemi, "Influence of blanching on the drying and rehydration of banana slices," African Journal of Food 
Science, vol. 3, pp. 307-315, 2009. 

[11] T. Tunde-Akintunde, "Effect of pretreatment on drying time and quality of chilli pepper," Journal of Food Processing and 
Preservation, vol. 34, pp. 595-608, 2010. 

[12] J. Hussein, K. Filli, and M. Oke, "Thin layer modelling of hybrid, solar and open sun drying of tomato slices," Research Journal of 
Food Science and Nutrition, vol. 1, pp. 15-27, 2016. 

[13] T. Tunde-Akintunde and M. Oke, "Thin-layer drying characteristics of tiger nut (Cyperus esculentus) seeds," Journal of Food 
Processing and Preservation, vol. 36, pp. 457-464, 2012. Available at: https://doi.org/10.1111/j.1745-4549.2011.00604.x. 

[14] O. Sobukola, "Effect of pre-treatment on the drying characteristics and kinetics of Okra (Abelmoschus esculetus (L.) Moench) 
slices," International Journal of Food Engineering, vol. 5, pp. 1-22, 2009. Available at: https://doi.org/10.2202/1556-3758.1191. 

[15] İ. Doymaz, "Determination of infrared drying characteristics and modelling of drying behaviour of carrot pomace," Journal of 
Agricultural Sciences, vol. 19, pp. 44-53, 2013. Available at: https://doi.org/10.1501/tarimbil_0000001227. 

[16] F. Kaymak-Ertekin, "Drying and rehydrating kinetics of green and red peppers," Journal of Food Science, vol. 67, pp. 168-175, 2002. 
Available at: https://doi.org/10.1111/j.1365-2621.2002.tb11378.x. 

[17] T. Akintunde, B. Akintunde, and A. Fagbeja, "Effect of blanching methods on drying kinetics of bell paper," African Journal of Food, 
Agriculture, Nutrition and Development, vol. 11, pp. 5457-5474, 2011. 

[18] M. Gupta, V. K. Sehgal, and S. Arora, "Optimization of drying process parameters for cauliflower drying," Journal of Food Science 
and Technology, vol. 50, pp. 62-69, 2013. Available at: https://doi.org/10.1007/s13197-011-0231-5. 

[19] U. Garba and S. Kaur, "Effect of drying and pre-treatment on anthocyanins, flavonoids and ascorbic acid content of black carrot 
(Daucus carrota L.)," Journal of Global Biosciences, vol. 3, pp. 772 – 777, 2014. 

[20] M. Joshi, B. Adhikari, P. Aldred, J. Panozzo, and S. Kasapis, "Physicochemical and functional properties of lentil protein isolates 
prepared by different drying methods," Food Chemistry, vol. 129, pp. 1513-1522, 2011. Available at: 
https://doi.org/10.1016/j.foodchem.2011.05.131  

[21] M. Wawire, I. Oey, F. Mathooko, C. Njoroge, D. Shitanda, and M. Hendrickx, "Thermal stability of ascorbic acid and ascorbic acid 
oxidase in African cowpea leaves (Vigna unguiculata) of different maturities," Journal of Agricultural and Food Chemistry, vol. 59, pp. 
1774-1783, 2011. Available at: https://doi.org/10.1021/jf103469n. 

[22] R. K. Toor and G. P. Savage, "Effect of semi-drying on the antioxidant components of tomatoes," Food Chemistry, vol. 94, pp. 90-97, 
2006. Available at: https://doi.org/10.1016/j.foodchem.2004.10.054. 

[23] L. Ruvini, D. WMMMK, J. Chathuni, V. Rizliya, W. Swarna, and C. Barana, "Effect of different drying methods on antioxidant 
activity of star fruits (Averrhoa carambola L.)," Journal of Nutrition and Diet Supplements, vol. 1, pp. 1-6, 2017. 

[24] E. Demiray and Y. Tulek, "Color degradation kinetics of carrot (Daucus carota) slices during hot air drying," Journal of Food 
Processing and Preservation, vol. 39, pp. 1745-4549, 2013. 



Agriculture and Food Sciences Research, 2019, 6(1): 145-154 

154 
© 2019 by the authors; licensee Asian Online Journal Publishing Group 

 

 

[25] A. Shalata and P. M. Neumann, "Exogenous ascorbic acid (vitamin C) increases resistance to salt stress and reduces lipid 
peroxidation," Journal of Experimental Botany, vol. 52, pp. 2207-2211, 2001. Available at: 
https://doi.org/10.1093/jexbot/52.364.2207. 

[26] M. S. Beltagi, "Exogenous ascorbic acid (vitamin C) induced anabolic changes for salt tolerance in chick pea (Cicer arietinum L.) 
plants," African Journal of Plant Science, vol. 2, pp. 118-123, 2008. 

[27] T. S. Workneh and M. O. Oke, "Thin layer modelling of microwave-convective drying of tomato slices," International Journal of 
food Engineering, vol. 9, pp. 75-90, 2013. Available at: https://doi.org/10.1515/ijfe-2012-0205. 

[28] I. Doymaz, N. Tugrul, and M. Pala, "Drying characteristics of dill and parsley leaves," Journal of Food Engineering, vol. 77, pp. 559-
565, 2006. Available at: https://doi.org/10.1016/j.jfoodeng.2005.06.070. 

[29] P. J. Fellows, Food processing technology - principles and practice, 2nd ed. Cambridge, England, Boca Raton, Boston, New York, 
Washington, DC: CRC Press, Woodhead Publishing Limited, 2000. 

[30] J. I. De-Corcuera, J. R. Cavalieri, and J. R. Powers, Blanching of foods. In D.R. Heldman (Ed.), Encyclopedia of Agricultural, Food, and 
Biological Engineering. New York: Marcel Dekker, Inc, 2004. 

[31] A. Ali, Y. A. Yusof, N. Chin, and M. N. Ibrahim, "Effect of different drying treatments on colour, quality and ascorbic acid 
concentration of guava fruit," presented at the 2nd International Conference on Agricultural and Food Engineering, CAFE 2014, 
Kuala Lumpur, Malaysia, 2016. 

 

 
   

 
 
 
 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

  

Asian Online Journal Publishing Group is not responsible or answerable for any loss, damage or liability, etc. caused in relation to/arising out of the use of the content. 
Any queries should be directed to the corresponding author of the article. 
 


