Effects of Different Drying Methods on Some Physical and Chemical Properties of Loquat (Eriobotrya japonica) Fruits

ABSTRACT

In this communication, we have discussed and compared the different drying processes i.e. flat plate solar collector, open sun and mechanical dryer with the preservative solution of Sodium Benzoate. Loquats were pretreated with preservative solution and subject to the drying process. Drying kinetics in terms of moisture lost per hour and drying rate, texture and antioxidants retention were determined before and after drying. Statistical analysis of the data showed that the different drying process has significantly affected on the kinetics of loquat fruits with the confidence interval of (α ≤ 0.05), whereas the preservative solution showed a non-significant effect on texture and antioxidants retention. High moisture lost per hour (5.4%) was recorded in mechanical dryer followed by flat plate solar collector dryer (3.7 %) while minimum (1.8%) was recorded in open sun drying. Good textured dried loquat with optimum hardness (313 g) and stiffness (141 g mm⁻¹) was found in flat plate solar collector dried samples. Similarly more antioxidants (52%) retained in samples dried with hot air using flat plate solar collector followed by the mechanical dryer (47%) while minimum (41%) was recorded in open sun drying. Although mechanical drying is fast, it does not help to maintain a good texture and more antioxidants. From the study, it was concluded that hot air drying using a flat plate solar collector is better for drying process to get valuable dried loquat with good texture and more antioxidants.

KEYWORDS:

• Drying process
• preservative solution
• loquat
• texture
• antioxidants

Introduction

Loquat (Eriobotrya japonica) (Chinese “pipa”) is a species of flowering plant which belongs to the family of Rosaceae, cylindrical, edible fruit in the cooler hill region of China (Coşkun et al., 2013). It was also quite common fruits in the Northern areas of Pakistan, Korea, Japan, and could be found in the Northern part of Philippines and Sri Lanka, while Spain, Italy, and Turkey produce much more than other cited countries (Ali et al., 2011). Other studies by Balagat et al. (2013) reported that this fruit was grown commercially for its yellow fruits and also it’s evergreen shrub, which was cultivated as an ornamental plant. They are a rich resource of carbohydrates, proteins, potassium, minerals, vitamins, phenolics, flavonoids, anthocyanins, and important antioxidants (Bige et al., 2016). They have a good aroma and sweet agreeable taste and are consumed as a fresh or dried form. However, they have a very short shelf life and degrade quickly due to their high moisture content reported by Carranza-Concha et al. (2012).

To preserve these fruits, they are dried in open air and consumed in the extreme winters. This method is very old and unhygienic, uncontrolled, and not safe from the attack of microbes and pests. Almost 40% of the fruit is lost due to contamination and pest attack (Cetinkaya et al., 2006). The food industry is at present looking for new advanced, alternative of open sun drying, inexpensive, hygienic and controlled preservation technology that yielding in good quality of dried fruits have a minimum alteration in the quality attributes of dried fruits (Morrison, 1981). Additionally, the development of alternative technologies for dehydration and processing of fruits is necessary to reduce losses and obtain best quality value-added fruits. Thus the development of new drying technologies which was in energy efficient and providing base for quality drying is the goal of modern researchers (Elif et al., 2008).

To develop new dehydrators and overcome the conventional open sun drying of fruits, numerous researchers had analyzed the dehydration of different fruits. Ullah et al. (2016b) studied the drying kinetics and quality attributes of mulberry dried by a solar air heater and open sun, pretreated with different preservatives. They reported that hot air drying using solar air heater is quick and the quality of mulberries was very good as compared to sun drying. Carranza-Concha et al. (2012) conducted experiments on mulberry dried in sun and a hot air provided by a 2.04 m² solar air heater. They reported good kinetics and quality of samples dried by a solar collector. Drogoudi et al. (2008) studied the impact of drying process on quality attributes of mulberry. They reported that there was a significant difference found in between solar dried and oven dried samples. Solar dried give much better results in antioxidants content as compared to oven drying. Ali et al. (2011) studied that hot air drying of mulberries at a temperature lower than 50°C was the best practice to maintain phenolic, flavonoids, and antioxidants.

There are different drying methods applied to different varieties of fruits and vegetables under different preservative solution and their effect on the drying kinetics and quality parameters i.e. texture and antioxidants retention (AAR). Thus the goal of the study was to find and compare different drying methods i.e. flat plate solar collector, open sun and mechanical dryer with the preservative solution of sodium benzoate (SB) in loquat.

Materials and methods

Description of drying process

For the drying of loquat fruits, three different drying methods were applied with the preservative solution of SB. Flat Plate Solar Collector was tilted at the angle of 35° with the facing of North-South with the horizontal axis for getting maximum solar radiation for the purposes drying fruits. The schematic diagram of Flat Plate Solar Collector is shown in Figure 1 with the dimension of 0.9 m × 1.8 m and 1.62 m² was the total area of the collector. The drying chamber is the part of the flat plate solar collector, consisted of a steel box fully insulated inside with the help of polystyrene foam with the dimension of 1 m × 0.55 m × 0.57 m. There was an exhaust fan in the drying box which was used for the sucking hot air from the absorber pipe and the outlet pipe (0.02 m diameter) at the toper of the drying chamber, to remove humid air from the drying unit. The second drying method was mechanical dryer, which was fully insulated and mechanically controlled environment during the drying process of fruits. Whereas, the third drying method was the open sun, in which the preserved fruits were put open space for drying.

Figure 1. Schematic diagram of flat plate solar collector with drying chamber.

Experimental site location

The experiment was performed in the College of Engineering, Nanjing Agricultural University, China with the 32.134°N latitude and 118.685°E longitude. Fresh loquat fruits were selected from the market and stored at 1°C. Prior to drying each fruit was selected on the basis of size and more than 90% maturity and these fruits were washed for 30 s with water at room temperature and dried with the help of tissue paper and the initial characteristics of the fruit was determined before drying up to 86%, 11.50° Brix, 0.98, and 118 mg/g for moisture content, total soluble solids, water activity, and AAR of the fruit, respectively.

Experimental procedure

Before drying loquat fruit, fruits were pretreated with 1 % solution of SB for achieving high quality and less degradation. After pretreatment with preservative, Loquat was subjected to dehydration until moisture content becomes less than 20%. Table 1 shows the drying conditions of the three drying methods. In sun drying, loquat was put on clean trays and covered with a polyethylene sheet, while in flat plate solar collector drying chamber, fruits were placed in a drying chamber connected with a flat plate solar collector that provided hot air for drying. Whereas, in a mechanical dryer, samples were subjected in a polyethylene tray inside the mechanical dryer which was fully controlled mechanically. Weight loss data was recorded after each hour with the help of electronic balance.

Table 1. Shows the different drying process condition of loquat.

S. no	Drying method	Temperature	Humidity	Total time
1	Flat plate solar collector drying	50 ± 5	10–15%	23 h
2	Open sun drying	28 ± 5	45–78%	48 h
3	Mechanical dryer	52 ± 5	>10%	16 h

Experimental data analysis

Solar radiation intensity is the radiation which falls on the collector per unit area from the sun during daytime in the form of electromagnetic radiation and can be calculated with the digital solar meter (Model: SM206). The radiations fall from the sun on the earth were recorded monthly basis during the experiment, which could determined by the using of equation [1] studied by (Akwasi, 1996).

Therefore, moisture content (wet basis) for each hour of drying of Loquat was calculated with the using of equation 1, from the derivation of the previous study by Ullah et al. (2016a). Where in the equation, M_c represents the moisture content (% wb), mt denoted the total mass in g and mw represents the water mass in g. whereas, during the experiment, moisture ratio (MR) and drying rate were calculated with the using of equation [2 and 3] derived from the previous study by Atul et al. (2009). Similarly, the drying rate (kg_water kg_{dry solid}⁻¹) was derived from the MR calculated over time (t, h) using equation 3. Where “a” and “k” were constants to be fitted in the graph of DR and MR.

(1)

(2)

(3)

For texture analysis, two-bite compression tests were performed using texture profile analyzer (Model, TPA-XT2 Stable Micro Sys.UK) having a 5 mm cylindrical probe with 1.5 mm thickness. Samples were subject to a constant compression speed of 0.45 mms⁻¹ at room temperature and the parameters were analyzed before and after drying with the hardness (g) and stiffness (gmm⁻¹). Stiffness and hardness are more or less the same things; it refers to the ability of a material to resist deformation. Hardness tester series (Model No: HFH80) was used for determination of hardness (g) of fruits. The tester measures the force required to push a plunger tip of specified size into the fruit pulp. The force reading assists in determining the appropriate picking time or monitoring fruit softening during storage. Similarly, stiffness is the tendency of a material to react with a small deformation when the material is stressed. It is measured with Young’s Modulus, which is the angular coefficient, or slope, of the linear stress–strain curve. This property depends directly on the bond type between the atoms. The stronger the bond, the higher the modulus (or the stiffness). Therefore for antioxidant analysis, the antioxidant before drying and retained in the samples after drying was determined using 2,2-diphenyl-1-picrylhydrazyl (DPPH) radical scavenging activity assay of ZZ. Measurements were done with 1.5 ml of DPHH added to different aliquots of ethanol and reacted for 2 h at 25°C. The absorbance spectrum was measured at 500 nm with a spectrometer (Model, Hitachi – U-1900, UK). The sample mass to inhibit 50% DPPH was determined. The determinations were expressed as the percentage of AAR and calculated using equation 4 given by (Baigvand et al., 2015). The symbol used in the equation “AAo” represents the initial and “AAt” denoted the final antioxidant retention capacity.

(4)

Statistical analysis

Analysis of variance (ANOVA) was determined using two factorial completely randomized designs. Drying methods and preservative were considered as two different factors. Results were expressed as mean ± SD, while means were compared using the LSD test at α = 5 % and effect on quality parameters was considered at 5 % significance level.

Results and discussions

Moisture loss and drying rate

Percent moisture lost per hour from the loquat is given in Table 2. The ANOVA showed that drying method significantly (α ≤ 0.05) affected the moisture loss per hour while the preservative solution had a non-significant effect on Loquat. The maximum loss of moisture of 5.40% was found in samples dried by mechanical dryer followed by flat plate solar collector drying with 3.75% while minimum moisture loss of 1.81% was recorded for open sun drying method. The reason for high moisture lost at mechanical dryer had a uniform drying environment and minimum relative humidity due to which the air had more capacity to extract moisture from the Loquat. On the other hand, hot air provided by solar collector had temperature and humidity fluctuations causing relatively slow moisture loss. Samples dried in open sun showed minimum moisture loss due to uncontrolled drying environment. The air was humid causing it to take less moisture uptake from the fruits of loquat. The results were in accordance with the findings of Belessiotis and Delyannis (2011), who reported their results that hot air drying was 45% quicker than sun drying. (Carranza-Concha et al., 2012) also reported a decrease in drying time of 73% by loquat dried by oven as compared to sun drying. The finding results are in contradictory with the results by (Ehiem et al., 2009) that black mulberries (Morus nigra L.) lost more moisture.

Table 2. Shows the moisture lost and drying rate from different drying methods.

S. no	Drying method	Moisture content	Drying rate
1	Flat plate solar collector drying	3.75% b	0.041 (kgwater kgdry solid^−1) ab
2	Open sun drying	1.81% c	0.028 (kgwater kgdry solid^−1) c
3	Mechanical dryer	5.40% a	0.046 (kgwater kgdry solid^−1) a

Note: Means followed by different alphabets are significantly (α ≤ 0.05) different from each other.

Drying rate of the loquat was given in Table 2. The ANOVA showed that drying method significantly (α ≤ 0.05) affected the drying rate of Loquat while the preservative showed a non-significant effect. Maximum drying rate of 0.046 kg_water kg_{dry solid}⁻¹ was recorded for samples dried by mechanical dryer followed by flat plate solar collector drying with 0.041 kg_water kg_{dry solid}⁻¹ while minimum drying rate of 0.028 kg_water kg_{dry solid}⁻¹ was recorded for open sun drying method. The results of drying rate were related to the moisture lost per hour. These results were in accordance with the findings of (Elif et al., 2008) who reported their results that hot air drying was given higher drying rates than open sun drying. These results were also in accordance with the findings of El-Sebaii and Shalaby (2012).

Texture

Statistical analysis of the data showed that the values of hardness (g) and stiffness (gmm⁻¹) were only affected by drying methods while there was a significant effect in the drying methods on the overall texture of the dried loquat. The means of hardness (g) of the dried loquats as compared with fresh and stiffness is shown in Figure 2. The results showed that the loquat becomes harder and stiffness increases if we dry them in a mechanical dryer. Less hard loquats were obtained during open sun drying while leathery and gently hard dried loquats were obtained from hot air drying (flat plate solar collector). The reason was slow and controlled drying with the help of hot air having minimum enthalpy and optimum capacity of gaining moisture from the samples. The results were in accordance with the finds of Ullah et al. (2016a) they reported that the samples dried by oven have more hardness as compared to hot air drying.

Figure 2. Comparison of hardness and stiffness of loquats under different drying methods.

Antioxidants retention

Statistical analysis of the data showed that the values of AAR (%) were only affected by the drying methods. The means of data as compared with fresh is shown in Figure 3. More antioxidants of 52% retained in loquats dried by hot air drying using flat plate solar collector followed by 47% in mechanical drying, while a minimum of 41% antioxidants was recorded the samples dried in open sun drying. The fact of the minimum antioxidant retained in open sun drying was the ultraviolet rays of the sunlight. These ultraviolet rays degraded the antioxidants and reduce their amount in dried loquats. The results were in accordance with the findings of (Carranza-Concha et al., 2012) who reported the reduction in the percent of antioxidants in open sun-dried Loquats as compared to fresh. (Balagat et al., 2013) also studied the composition of solar dried, oven dried and open sun-dried mulberries and reported that more antioxidants and flavonoids lost occurred in open sun drying system. Another study by (Bige et al., 2016) was also reported the reduction in antioxidants percentage as compared with hot air drying. They concluded from the results that hot air drying method was better for getting more valuable antioxidant retention in dried fruits.

Figure 3. Antioxidants retention in loquat fruits using different drying methods.

Conclusion

In this study, different drying process showed a significant effect on drying kinetics, texture and AAR in loquats. From the experimental results, it was noted that mechanical dryer was fast in drying and it does not help to maintain a good texture and more valuable AAR in the dried fruits. From the results of the study, it was noted that good textured of dried loquat with optimum hardness and stiffness of 313 g and 141 gmm⁻¹, respectively was found in flat plate solar collector dried samples. Similarly, more antioxidants of 52% retained in samples dried with hot air using flat plate solar collector. Therefore, it was concluded from the results of the experiment, that hot air drying using a flat plate solar collector was better for drying process of different fruits and vegetables to get valuable dried fruits and vegetable with good texture and more AAR.

Author’s contribution

Fahim Ullah designed the research and performed all the lab works. Min Kang provided the financial and technical support for designing and conducting research as well as supervised the whole research process. Mansoor Khan Khattak and Muhammad Sohail Memon performed the review, statistical analysis and Fahim Ullah wrote the manuscript with comments from all authors and finalized the manuscript under the supervision and guidelines of Min Kang.

Declaration of interest

Declare conflicts of interest or state “The authors declare no conflict of interest.

Acknowledgments

We thank Min Kang for providing lab facilities and extend our thanks to the China Scholarship Council and the College of Engineering, Nanjing Agricultural University, Nanjing, for supporting and providing research facilities for this study.