Retracted: Experimentally investigated the asparagus (Asparagus officinalis L.) drying with flat‐plate collector under the natural convection indirect solar dryer

The above article, published online on 21 February 2018 in Wiley Online Library (wileyonlinelibrary.com), has been retracted by agreement between the authors, the journal Editor in Chief, Y. Martin Lo, and Wiley Periodicals, Inc. The retraction has been agreed at the authors’ request due to unattributed overlap between this article and the following article published in the Journal of Mechanical Engineering and Sciences, “Analysis of ginger drying inside a natural convection indirect solar dryer: An experimental study” by S. K. Sansaniwal and M. Kumar, Volume 9, pp. 1671–1685.

it was observed that convective heat transfer coefficient decreased with the increasing mass of the samples. Similarly, the progression of drying months with variation from 0.59 to 5.42 W/m 2 °C for the different mass of samples was noted. Therefore, from the results of the experiment, it was reported that moisture removing rate increased with the increase in mass of asparagus samples and significantly decreased with progression of drying months. Similarly, during experiments, the average collector efficiency was noted to vary from 14.97% to 16.14% under the increasing and decreasing trends of solar irradiations from morning to noon and noon to evening, respectively. For describing the drying behavior of the different mass of Asparagus samples, modified Henderson and Pabis were reported. During experiments, experimental error in terms of percent uncertainty was observed in the range from 29.19% to 46.25%.  Karim & Hawlader, 2005). The basic ingredients of Asparagus were energy, starch, proteins, mineral matters, vitamins, fats, and carbohydrates. Asparagus is an important ingredient of the food with high nutritional value and has become a compulsory item in the kitchen (Bhagat & Lawankar, 2012).

K E Y W O R D S
It is not only used to add food palatability, but it is also widely used in medicines, bakery products, wine and meat products, a toiletry product, etc. (Denis & Mikulic-Petkovsek, 2017). Asparagus is the most important cash crop of the world, cultivated in China, Pakistan, Indian, Afghanistan, Uzbekistan, Japan, and Indonesia. The china is the largest producer of Asparagus, contributing about 45.48% of the total world's Asparagus production with a total production of 17 million tons a year (Deshmukh, Varma, Yoo, & Wasewar, 2013).
About half of the total production of Asparagus is being consumed as white and red Asparagus, whereas the remaining 30% is converted into dry Asparagus for medicinal purposes, and 20% is used as seed material (Deshmukh, Varma, Yoo, & Wasewar, 2014).
Agricultural product drying has a vital role in the preservation and shelf life improvement of the product after harvesting (Eze & Agbo, 2011). In developing countries, sun-drying is a popular, effective, and economical method for drying of food and herbal products.
Sun-drying is a common food preservation technique used to control the moisture content of the agricultural products (Gürlek, Özbalta, & Güngör, 2009 (Kumar, Khatak, Sahdev, & Prakash, 2011;Kumar, Sansaniwal, & Khatak, 2015;Kumar & Tiwari, 2009) have been reported that drying rate under the hybrid dryer was greater than sun-drying with the efficiency of 15% during the summer season. Modified Henderson and Pabis were reported to be best suited to describe the drying behavior of Asparagus (Lu, Yu, & Ding, 2003). The drying characteristics of Asparagus undertray and heat pump-assisted dehumidified drying were also incorporated by single and two stages drying, which reduced the drying time by 59.32% at 40°C (Kumar, 2013a,b). Peeled and unpeeled Asparagus drying under sun-drying and solar cabinet dryer have been compared, and better drying rate was observed in solar drying against sun-drying (Hoque et al., 2013). Other researchers (Kumar, 2013a(Kumar, , 2014Norm, 2003)  reported that solar dryer to be better than a sun-drying method for asparagus samples drying in the aspects of quality. In this study, an indirect natural convection solar dryer has been fabricated to study the drying kinetics of Asparagus shrubs in the meteorological conditions of Nanjing, China. Furthermore, solar flat-plate collector efficiency has also been evaluated for the given drying time interval.
The schematic view of flat-plate collector under the natural convection indirect solar dryer; T(i.e.) is the fresh air inlet temperature in (°C), PVC means poly vinyl chloride pipe, T s is the absorber surface temperature in (°C), T(i, d) is the inlet pipe diameter temperature in (°C), T e is the product surrounding temperature in (°C), T c is the product temperature in (°C), and RH is the relative humidity of the product in (%) F I G U R E 2 Asparagus samples before drying (a) and after drying (b)

| Description of experimental setup
The    T (i,c) is the temperature at collector inlet in (°C), T (o,c) is the temperature at collector outlet in (°C), T c is the product temperature in (°C), T e is the product surrounding temperature in (°C), M evp is the moisture evaporation in (g), and M removing rat is the moisture removing rat in the products with the unit of (%db)  (2).

Time (h) T s (°C) T i,c (°C) T o,c (°C) T c (°C) T e (°C) M evp (g)
From the literature, it was observed that the Asparagus should be dried from its average initial moisture content of 89% to the final moisture content of 8% Xiong et al., 2012). For the determination of the suitability of best thin layer drying model, the following R 2 , X 2 , and RMSE were considered to be the primary criteria as given in Equations (3) and (4).
The model suitability was determined by considering the higher value of the coefficient of determination and least value of chi-square and root-mean-square error (Phoungchandang & Saentaweesuk, 2011;Rahman et al., 2009). Statistical parameters obtained from selected thin layer drying models are given in

| Theoretical considerations
The convective heat transfer coefficient for evaporation was determined using the following relations (Neiton et al., 2017;Yuezhao & Jiang, 2007). Equation 6 represents the rate of heat utilized to evaporate moisture. While substituting h c from equations (5) and (6) becomes equation (7). The moisture evaporated is determined by dividing equation 7 by latent heat of vaporization (⋌) and multiplying the area of the tray (A t ) and drying time interval (t). (1)

Time (h) T s (°C) T i,c (°C) T o,c (°C) T c (°C) T e (°C) M evp (g)
Applying log to equation (9) on both side and we get equation (10), Similarly, applying the linear equation y = mx + c on equation (10) and then we get Values of "m" and "C" are obtained with the using of simple linear regression methods with the following formula.
The total heat of the collector outlet can be determined from the equation 19.
The total amount of heat received by the solar flat-plate collector is given by equation (20).
The variation of solar irradiation and mass of the product with respect to time for 78 and 48 no. of asparagus samples; MSR shows the mean solar irradiance in watt/m 2 for the 3 months, that is, Jun, July, and August; Mass (Jun, July, and August) is the mass of the product of both 78 and 48 No. samples of asparagus F I G U R E 4 The variations in convective heat transfer coefficients and efficiency with respect to time for 78 and 48 no. of asparagus samples; Mean Eff is the mean efficiency in (%) for all the 3 months; hc is the convective heat transfer coefficients in W/ m 2 °C for the 78 and 48 No. of samples dried in the 3 months, that is, June, July, and August

R E T R A C T E D
The efficiency of solar flat-plate collector can be determined by dividing equations (19) and (20).

| Experimental errors
The experimental errors were evaluated in terms of percentage of uncertainty using equation (21) for the mass of moisture evaporated during drying of Asparagus samples (Tesfamariam et al., 2015;Tiago & Maria, 2014).

| RE SULTS AND DISCUSS ION
The hand-peeled cylindrically shaped (diameter 1.7 cm, length 3 cm)  The maximum collector efficiency was reported between 12:00 and 14:00 as solar irradiation intensity was observed higher during the same time interval. So, the collector efficiency is observed to be a strong function of solar irradiation data (Fahim, Kang, et al., 2016;Gang et al., 2010;Kalogirou, 2009). The results of the study were in agreement with the findings by Jayashree et al., 2014;Karna & Koo, 2017;Kong et al., 2013. Similarly, the researchers (Kumar, 2014;Maskan et al., 2002;Norm, 2003) reported results were similar with our results; they studied that efficiency of the collector increased with increasing of solar irradiance.
The Table 3 shows the moisture removing rate is observed to be dependent on the total moisture present in the product mass, and hence, it has been observed that the moisture removing rate increases with increase in ginger samples mass and decreases significantly with the progression of drying days (El-Shobaki, El-Bahay, Esmail, Abd El-Megeid, & Esmail, 2010;Gang et al., 2010;Jamil, Osama, & Ahmed, 2014). However, the moisture removing rate is also dependent on the ease of heat transfer. Forced convection drying system has been reported to be best suitable for faster drying as the value of the coefficient of convective heat transfer associated with them is more than the natural convection drying (Azharul Karim & Hawlader, 2005;Deshmukh et al., 2013Deshmukh et al., , 2014. From Figure 4, it has been observed that the values of convective heat transfer coefficient decrease with the progression of drying months, that is, June, July,

| CON CLUS IONS
The research reported in this study includes the evaluation of convec- • Moisture removing rate on a dry basis was observed to be increased with increase in asparagus samples mass and decreases significantly with the progression of drying months.
• Average collector efficiency during the drying process was observed to vary from 14.97 to 16.14%.
• Modified Henderson and Pabis were reported to be best suited for describing the drying behavior for both masses of asparagus samples.
• The experimental errors were evaluated in terms of percent uncertainty ranging from 29.19% to 46.25%.

| RECOMMENDATIONS
• The experimental errors occurred during the drying process further reduced using certain countermeasures such as sophisticated monitoring devices, design accuracy.
• The collector efficiency can be further improved using high conductive absorber material.
• The overall system efficiency can also be enhanced using phase change materials.
• The computer-based simulation tool was also an important method to study the design optimization and scalability of the system. The present research work could be considered for the optimum design of a solar dryer for quality drying of various products.

E TH I C A L S TATE M E NT
The submitting a paper do so on the understanding that the manuscript has been read and approved by all authors and agree to the submission of the manuscript to the Journal. All the authors have made an active contribution to the conception and design and/or analysis and interpretation of the data and/or the drafting of the manuscript, and all have critically reviewed its content and have approved the final version submitted for publication.

ACK N OWLED G M ENTS
We thank Min Kang for providing laboratory 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.

CO N FLI C T S O F I NTE R E S T
The authors declare no conflict of interest.

E TH I C A L R E V I E W
This study does not involve any human or animal testing, and also this study was approved by the Institutional Review Board of Nanjing Agricultural University, China.

I N FO R M ED CO N S ENT
Written informed consent was obtained from all study participants.