Study of Four Onion Varieties Drying Kinetics in an Oven and a Solar Greenhouse

Onion production (Allium cepa L.) in Senegal reached 390 000 tons in 2016. Due to post-harvest losses, annual demand (150 000 and 250 000 tons) is being met through imports. This work consists in proposing a drying process at a lower cost to overcome this dependence and preserve the quality of the product. The optimization of local onion varieties drying in an oven and in solar greenhouse, as well as the physicochemical characterization of the products were carried out. The moisture of fresh onion bulb varies between 85.56 ± 0.60 and 89.13 ± 0.69 (%). To obtain a moisture  8.89 ± 0.16 (%) ensuring stability, the optimal drying conditions in the oven are 60° C / 6H (Galmi Violet) and 7H (Safari, Gandiol F1 and Orient F1). Under these conditions, the content of polyphenols in g equivalent of gallic acid / 100 g db increases (0.111 ± 0.0040 to 0.312 ± 0.0041 before drying, 0.546 g ± 0.0117 to 0.837 ± 0.0091 after drying). Optimum solar drying in a greenhouse is obtained between temperatures of 35 to 65° C / 8H-9H. From a perspective of sustainable development, the perspective is the modeling of drying kinetics in a solar greenhouse.


Introduction
Senegalese agriculture, particularly rainy and seasonal agriculture, contributed about 18% of GDP in 2015 (National Agency for Statistics and Demography ANSD) and is one of the key levers for ensuring food security. Horticultural is the most dynamic sub-sector of this Senegalese agriculture, with a growth of between 5% and 10% since 2004. (ANSD, 2014;Direction de l'Horticulture DH, 2015). This performance is mainly driven by the growth of the onion sub-sector which represents 60% of horticultural production (DH, 2016).
In fact, onion (Allium cepa L.), a very popular vegetable in Senegal, with an annual consumption of between 150,000 and 250,000 tons (ARM, 2016) represents 25% of household expenditure. The most common use of onion in households across the country is fragmenting the bulb into pieces that are incorporated into recipes for flavor development. However, despite a record production of 390,000 tons in 2016, meeting household demand over the year remains dependent on imports due to significant post-harvest losses due to the high moisture content of the onion. Thus, to reduce post-harvest losses, developing the dehydration process seems to be an excellent opportunity but the process requires a lot of energy. (Lhendup, 2005;Tom, 2015).
On the other hand, the use of solar drying, a major lever to overcome the energy constraint, preserves the quality of products despite the variability of climatic conditions (Boughali, 2010;Jannot, 2006;Mendez Lagunas, 2007).
These findings and the absence of data in the literature on local varieties justify the initiation of this research.
The objective of this study is to optimize the drying of four onion varieties in an oven and in solar greenhouse and to compare solar drying, which reduces the energy bill, to electric convective drying. Optimal drying conditions are determined by studying the impact of this process on some major biochemical and physicochemical parameters.

Plant Material
The local onion is collected in the cooperative of the locality of RAO in Saint-Louis (Senegal). The varieties studied are Galmi violet, Safari, Gandiol F1 (Gandiolais) and Orient F1 (Orient) with a maturity of 85% of leaves falling at harvest.

Analysis Equipment
Analysis equipment: an oven with ventilation (Memmert brand), a solar greenhouse equipped with a ventilation system to regulate the ambient air temperature and humidity, sensors for temperature and humidity readings, capsules in pyrex, drying racks, a scale (Denver instrument brand with a reliability rate of 0.0001g), a thermohygrometer (Voltcraft brand with a precision of 1°C and 3.5%), a water activity meter (Rotronic HP 23 brand), a pH meter (HI 23 brand), a burette, a spectrophotometer (Specord brand), a mineralizer, a distiller and laboratory glassware.

Graphical and Statistical Representation Tools
Data exploitation is carried out with both the R version 3.4.0 (Team R Core, 2017) software for the comparison test between the two methods of drying, the analysis of variance and the concordance of the measurements, and the Excel version 2016 software as a tool for scientific calculations for graphic representations.

Methods
The peeled onions are washed in chlorinated water 100 ppm (0.1 mg / L water), rinsed three times with clean water, dewatered and finely chopped with a chopper to neglect the deformation of the product during the drying process.
The thickness of the samples is in the range of 1.7 mm.

Kinetics Study
The tests are conducted in an oven in the temperature range of 50° C to 70° C with a step of 5° C to determine the optimum temperature / time to obtain stable products.
Ten grams are taken from the chopped onions of three different bulbs, and spread in pyrex cups. The experiments are carried out with ventilation at a fixed air velocity of 2.4 m/s and a relative humidity between 10 and 15% (Babalis & Belessiotis, 2004;Clemente, Frí as, Sanjuan, Benedito, & Mulet, 2011;Kiranoudis et al., 1992;Krokida et al., 2003;Sarsavadia et al., 1999). Regarding solar greenhouse drying, the four varieties are dried simultaneously (one variety per drying rack). Each rack is squared in four parts of equal size (0.74 X 0.71 m) on which three kg of onion are spread in monolayer. Inside the solar greenhouse, removable room sensors make it possible to follow the evolution of the temperature and the relative humidity, two determining parameters for the drying. During solar greenhouse drying, the relative humidity varies between 10-60% and the temperature varies between 35° C and 65° C.

Physico-Chemical Analyses
The various biochemical and physicochemical analyses performed on raw materials as well as onion powders obtained by dehydration in an oven and in a solar greenhouse are the pH according to NF V76-122: 1994, NF EN 1132, titratable acidity according to NF V 05-101 January 1974, European Standard EN 12147 December 1996, the moisture content with reference to standard NF ISO 712: 2009), the activity of water according to standard NF EN ISO 17025). In addition, the polyphenols, the most important functional elements to be preserved during the drying of the four onion varieties, are evaluated by the Folin-Ciocalteu reagent spectrophotometric assay method in a basic medium at 760 nm. The total polyphenol content is expressed in gallic acid equivalent.
All moisture measurements and characterization analyses are performed in triplicate to ensure repeatability.

Statistical Analyses
The evaluation of the reproducibility and the repeatability of the measurements is made by the numerical method which is the LIN coefficient (Lawrence & Lin, 1989).
The Lin's concordance coefficient varies between -1 and 1, where the values -1, 0 and +1 respectively mean perfect discordance, zero concordance and perfect match.
The Student's parametric test is used for the comparison of the:  characteristics of onion varieties (water activity, titratable acidity, pH and polyphenol content) before drying;  stability moisture of the oven-dried samples and those dried in a solar greenhouse.
All statistical analyses are performed with a significant threshold of p <0.05.

Characteristics of Onion Varieties before Drying
The physical and chemical characteristics of the samples before drying are presented in table 1.
All varieties are marked by a high moisture content and Aw, pH and polyphenols values are almost identical. Only the acidity of the Safari variety seems to stand out (9.23 mEq / 100g db) from that of the other varieties (between 4 and 6 mEq / 100g db).

Optimal Drying Conditions
The Lin coefficients obtained for the measurement concordance test for all oven drying and solar greenhouse drying kinetics vary between 0.9993555 and 0.9999317 with a confidence interval of 0.9991869; 0,9999431. This indicates that there is a perfect match between the three measurements made for each test.
The results of the statistical test for the comparison of the oven drying kinetic data to that in solar greenhouse are between -0.44906 -0.73362 for Student's parameter (t), 0.4697 -0.9572 for the pvalue and 24 -26 for the degree of freedom (df). Therefore, regardless of the oven drying temperature, p> 5% values show that there is no significant difference between oven drying and solar greenhouse drying kinetics.
The stability of dried fruits and vegetables is guaranteed with a moisture content of 8 ± 2% or less and an Aw between 0.5 and 0.6 to avoid any microbial activity (Bernard & Carlier, 1992;ESA, 2004;Faiveley, 2003;Le Meste & Chiotelli, 2002). These values serve as a reference to determine the optimal drying conditions taking into account the evolution of the physicochemical characteristics after drying. Figure 3 shows the evolution of the moisture content of Galmi Violet, Safari, Gandiol F1 and Orient F1 varieties dried in an oven at different temperatures.

Figure 3. Evolution of moisture content of oven-dried onion varieties at different temperatures
The optimal drying time in the oven, which makes it possible to reach the stability moisture, changes inversely with the increase in temperature. Over the temperature range between 50° C and 70° C, the results ( Figure 3) obtained for the four varieties are as follows:  the initial moisture content of Galmi Violet (85.56 ± 0.60%) decreases to a moisture stability of between 8.89 ± 0.16 and 5.23 ± 0.34 (%) for an optimal time between 8H and 4H depending on the drying temperature. At each increase in temperature (+5° C), the drying time decreases (-1H);  with an initial moisture content of 88.11 ± 0.61 (%), the stability moisture of the Safari variety, between 8.14 ± 0.52 and 6.30 ± 0.26 (%) according to the drying temperature; is reached for an optimal time between 10h and 5h. The drying time decreases (-1H) at each increase in temperature (+ 5 ° C) except from 55° C to 60° C where the time step is (-2H);  the Gandiol F1 variety with an initial moisture content of 86.99 ± 0.10 (%) has a stability moisture of between 8.68 ± 0.33 and 7.70 ± 0.39 (%) depending on the drying temperature. The optimal drying time varies between 11H and 5H with a pitch of (-2H) for each increase of a step of 5° C in the temperature range of 50° C to 60° C and (-1H) for that ranging from 60° C to 70° C;  for the Orient F1 variety with an initial moisture content of 89.13 ± 0.69 (%), the stability moisture is between 8.54 ± 0.41 and 5.02 ± 0.24 (% db) depending on the drying temperature. The optimal drying time is between 9H and 5H with a variation of (-1H) for each temperature increase of a step of 5° C in the range 50° C to 60° C, of (-2H) in the range of 60° C to 65° C and no variation in that of 65° C to 70° C.

Figure 4. Evolution of the moisture content of the four sun-dried onion varieties at varying temperatures during drying
The drying of onions in a solar greenhouse unlike the oven depends on the sun. Thus during the tests the temperature and humidity in the solar greenhouse varied in the respective ranges of 35-65° C and 10-60%. The drying kinetics of the four varieties are essentially identical. Figure 4 shows that the stability moisture values are reached from 8H solar greenhouse drying and that from 10H the values are almost stable. Table 2 displays the different moisture contents for drying times from 8H to 10H. The moisture content is inversely proportional to the drying time in the solar greenhouse and the elimination of water is different depending on the variety. These results are compared with Aw and polyphenol values to determine the optimal drying time.

Physico-chemical Characterization of the Samples after Drying
Water activity (Aw), titratable acidity, and pH are characteristics of the environment as important in the stabilization of food products as the moisture content. To avoid any microbial activity, an Aw between 0.5 and 0.6 is necessary. Moreover, the more acidic the medium (pH less than 4.5), the more it is unfavorable to chemical and biochemical degradation reactions (Bernard & Carlier, 1992;Charreau & Cavaille, 1991;Faiveley, 2012).

Evolution of Water Activity
The evolution of water activity for the four varieties ( Figure 5) shows that the initial Aw values between 0.940 ± 0.01 and 0.950 ± 0.001 decreases with increasing drying temperature. The initial Aw values are divided by a factor between 1.70 and 2.78 for each temperature step of + 5° C in the oven. Aw reaches values between 0.362 ± 0.003 and 0.447 ± 0.069 at 60 ° C. On the other hand, the Aw values of the samples for the 65° C and 70° C temperatures remain relatively stable in this range, except for the Orient F1 variety at 65° C (0.497 ± 0.002). Figure 5. Water activity of the samples after optimal drying

Evolution of Titratable Acidity and pH
The monitoring of titratable acidity and pH at 10% of oven-dried samples ( Figure ) indicates that with increasing drying temperature, the initial values of titratable acidity and pH at 10% ranging respectively between 4.51 ± 0.02 and 9.23 ± 0.00 mEq / 100g (db) and 6.29 ± 0.06 and 6.42 ± 0.03, change inversely for the four varieties. The multiplicative factor for titratable acidity is between 0.84 and 2.53 while the pH is divided by a factor in the range of 1.12 to 1.28. However, over the temperature range of 50° C to 70° C, the difference is not significant for both titratable acidity and pH (all p values for Student's test are greater than 0.05). The initial levels of total polyphenols (Table 1) of the four varieties, ranging from 0.111 ± 0.0040 to 0.312 ± 0.0041 g EAG / 100g (db), increase with drying temperature (Figure 7Figure ). The increase in total polyphenol content is greatest at a temperature of 60° C with 0.546 ± 0.0117 g EAG / 100 g (db) for Galmi Violet; it is 0.837 ± 0.0091 g EAG / 100g (db) for Safari, 0.694 ± 0.0173 g EAG / 100 g (db) for Gandiol F1 and 0.691 ± 0.0162 g EAG / 100 g (db) for Orient F1. This effect of temperature on polyphenol content is similar to those found in the literature (Ali et al., 1999;Lombard et al., 2005;Yang et al., 2004).
Nevertheless, from 65° C, a decrease of about 0.3% to 37% is observed. This decrease is accentuated at 70° C, showing the negative impact of temperature on polyphenols. The temperature range 50° C to 65° C with an optimal drying time between 6H and 11H makes it possible to obtain characteristics (Aw and moisture content) ensuring the stability of the product. However, the concern to maintain the functional properties of the polyphenols and to reduce the energy consumption makes it possible to determine the best drying time-optimal temperature pair.
Thus, the 60° C temperature with an optimal time of 6H for Galmi Violet and 7H for the other three varieties, is the best time/temperature pair because the polyphenol content is at its maximum. The products dried under these optimal conditions in the oven have a water activity of about 0.4 and a moisture content respectively for Galmi Violet, Orient F1, Safari and Gandiol F1 of 7.96% ± 0.42; 7.17 ± 0.63; 8.42% ± 0.05 and 8.67 ± 0.15%.
With these moisture and Aw values, the biochemical and physicochemical reactions and the development of the microorganisms responsible for the perishability of the products are then inhibited (Bernard & Carlier, 1992;Faiveley, 2003).

Characterization of Samples after Drying in a Solar Greenhouse
In the solar greenhouse drying conditions with the respective temperature and humidity ranges in the greenhouse of 35-65 ° C and 10-60%, the initial water activity of the samples decreases after 8h drying (Table 1). The water activity values of the four dried onion varieties ranged from 0.577 ± 0.007 to 0.675 ± 0.041 (Table 3). These results are in the range to avoid any microbial activity (Bernard & Carlier, 1992;Charreau & Cavaille, 1991;Faiveley, 2012).
This decrease in the water activity of varieties continues with the increase in drying time in the solar greenhouse. Thus, for a drying time of 9H, the values of the water activity of the samples are between 0.505 ± 0.005 and 0.550 ± 0.018 while at the end of 10H of drying, they are between 0.415 ± 0.012 and 0.491 ± 0.006. The stability moisture of samples dried in a solar greenhouse at different times (Table 2) shows that the optimal drying time is 9H for Galmi Violet and Orient F1 varieties and 8H for Safari and Gandiol F1 varieties. At these optimal times, the values of the water activity (Table 3) between 0.536 ± 0.005 and 0.675 ± 0.041 ensure the absence of any microbial activity. The characteristics of the products dried under these optimal conditions are presented in Table 4. Photos of onion powders obtained after drying in an oven at temperatures of 60° C, 65° C and 70° C and those obtained after drying in a solar greenhouse in 8H, 9H and 10H time are shown in the Figure 8. The higher the temperature and the drying time increase, the darker the color of the powders obtained is.
The impact of temperature on polyphenols (Ali et al., 1999;Lombard et al., 2005;Yang et al., 2004), constituents of therapeutic interest (Ali et al., 2000;Griffiths et al., 2002;Zohri et al., 1995), associated with the phenomenon of crusting when the removal of water is done too quickly and browning dried products (Figure 8), allow to avoid temperatures above 65° C and times greater than 10H for drying onions. On the other hand, too long exposure times consume not only a lot of energy, but can also affect the quality of the product. Therefore, the best temperature / time pairs are 55 to 65° C / 6H to 8H with an optimum at 60° C.
As for the results obtained by drying in the greenhouse presented in Table 2, Table 3 and Table 4, they show that moisture and Aw stability are obtained without browning the products after 8 hours to 9 hours of drying with a temperature in the greenhouse varying from 35 to 65° C during the day. Moreover, in this solar greenhouse temperature range, the polyphenol contents of the four onion varieties increase after drying. These results are comparable to those obtained in the oven.
The results of the parametric test of Student confirm that there is no significant difference between oven drying and solar greenhouse drying kinetics under the study conditions because all p values are > 5%.

Conclusion
The research carried out in the framework of this study made it possible to optimize the dehydration process of onion bulbs using two different energy sources. The ideal drying ranges are 55° C to 65° C / 6H to 8H in an oven and 35 to 65° C / 8H to 9H in a solar greenhouse to obtain products with low moisture content ( 8%).
Reducing high moisture content and water activity in the onions by drying in the oven as well as in solar greenhouse thus ensures the stability of the dried products. In addition, although the drying time in solar greenhouse is greater than that in the oven, the impact of drying on the evolution of the polyphenol content is substantially identical regardless of the energy source used. These results guide the choice towards the solar source for the management of post-harvest losses through the dehydration of onions.
However, lack of control of solar greenhouse drying temperatures can affect the nutritional and organoleptic quality of dried onions. The Establishment of the desorption isotherms and the modeling of the drying kinetics is thus necessary to control the parameters and ensure the regularity of the quality of the finished product. A study of the stability of onion powders including the monitoring of the re-humidification and color changes during storage should be considered. The reconstitution of dried onions and the sensory analysis by the consumers will be the next stages to be explored for a possible vulgarization of the products.