Effect of variety and drying temperature on physicochemical quality, functional property, and sensory acceptability of dried onion powder

Abstract In Ethiopia, onion is one of the most important vegetables/spices produced as a source of cash income and for flavoring foods. However, postharvest loss during storage remains a major challenge. In this study, the effects of variety (Bombay red, Qellafo and Sweet carolin) and drying temperature (Fresh, 50, 60, 70, 80, and 90°C) on physicochemical quality, functional property, and sensory acceptability of dried onion powder were determined. The results indicated that total color change of Bombay red was not affected by temperature, but Qellafo and Sweet carolin varieties showed an increase in color change as drying temperature increases. Bulk density, shrinkage ratio, and water hydration capacity increased with increasing temperature for all three varieties. The degradation of vitamin C, pyruvic acid, and desired sensory attributes increased with increasing oven drying temperature. All in all, Qellafo dried at 70°C for 5 hr was found to be desirable for production of dehydrated onion powder. The findings of this study will allow the identification of the best of the three commonly grown onion varieties in Ethiopia, and the preferred temperature for production of dried onion with minimal negative effect on physicochemical, functional, nutritional, and sensory properties.

the postharvest loss (PHL) of onion is quite serious in the country due to lack of appropriate handling, storing, and value addition techniques. The bulky nature of fresh onions does also create problems during transportation and storage. At ambient storage environment (20°C, 75%-80% relative humidity), bulbs can be stored for 20 days, but under low temperature storage (0°C, 75%-80% relative humidity), bulbs can be stored for more than 5 months (Thompson, 1996). However, low temperature storage facility is not affordable by small scale farmers in developing countries. According to Daniels and Fors (2015), estimated postharvest loss of onion is 13% due to different factors. Due to fear of spoilage and loss on the field during peak harvesting season, producers are often obliged to sell the produce at uneconomically low price. On the other hand, the limited supply of onions during the off-peak seasons creates shortage of onion in the market that results in high prices. During market glut, the price of onion at the farm gate could be between 0.04 and 0.074 USD (1 USD = 27 ETB) per kg while during the off season and festivals, it could increase to between 0.59 and 0.74 USD (personal observation), which is almost 10 times the price during the peak harvest. To mitigate this problem, producers commonly use sun drying as a traditional method of reducing onion moisture to increase storage life. But, this method results in an erratic and less efficient drying, which eventually leads to less quality product, and could enable the occurrence of mycotoxins due to the growth of mesospheric mycotoxin-producing fungi. Thus, under such circumstances, determining a more efficient and cost-effective drying method that insures better quality finished product with acceptable physicochemical, functional, nutritional, and sensory attributes is important. As stated in Jayathunge and Illeperuma (2001), optimized drying reduces wastage, reduces volume, minimizes transportation and packaging cost and increases the availability of powdered onion throughout the year as an ingredient for food processing industries and home consumption. Therefore, the aim of this work was to study the effect of variety and drying temperature on quality attributes of dried onion powder.

| Description of the experimental site
The experiment was carried out at the Postharvest management laboratory of Jimma University College of Agriculture and Veterinary Medicine (JUCAVM) in Jimma, Ethiopia.

| Sample collection and preparation
Three onion varieties (Bombay red, Qellafo, and Sweet carolin) were procured from local farmers near the town of Zeway, in central rift valley of Ethiopia. Before drying, bulbs were washed; roots were removed, peeled, and sliced to 5-mm thickness as described in Mitra, Shrivastava, and Rao (2011).

| Drying process
The sliced onions were placed inside hot air oven (Model: Leicester, LE67 5FT, UK) at a specific temperature until a constant mass was achieved. As higher temperature needs shorter time, and based on a preliminary study, drying times were set at 2.30, 4, 5, 6, and 7.30 hr for the samples dried at 50, 60, 70, 80, and 90°C, respectively. Then, the dried samples were cooled inside desiccators to avoid condensation of moisture and rehydration. Then, samples cooled to room temperature were ground into powder and packed in high-density polyethylene bags and stored at ambient atmospheric condition during the study period.

| Experimental design
The experiment was carried out as two-factor factorial design with three replications. The first factor was onion variety with three levels (Bombay red, Qellafo, and Sweet carolin), and the second factor was oven drying temperature with six levels (Fresh (control), 50, 60, 70, 80, and 90°C).

| Total color change (ΔE)
The total color change of each sample was measured using color chromameter (Model: Accu-probe HH06M, America). Color measurements were recorded using Hunter L*, a*, and b* scale (Hassani & Sharifi, 2012) after the required white color calibration. Total color change was calculated to estimate the extent of change in color of dried onion samples using Equation 1. where, ΔE = total color change, L o , a o , and b o are color parameters of fresh onion varieties, L*, a*, and b* color parameters of dried onion powder.

| Bulk density (BD)
Bulk density was determined by the method described in Goula and Adamopoulos (2005) and calculated as:

| Shrinkage ratio (SR)
Shrinkage is expressed by the ratio of the volume of onion slice before and after drying. Shrinkage ratio was calculated at each instant Weight of dried sample Volume of dried sample after tapping of the drying according to Equation 3 (Dissa, Desmorieux, Savadogo, Segda, & Koulidiati, 2010).
where, SR (%) is shrinkage ratio, V i (cm 3 ) is the apparent volume of the raw sample before drying, and V f (cm 3 ) is the apparent volume of the sample after drying.

| Water hydration capacity (WHC)
Water hydration (absorption) capacity was determined using the centrifugation method described in Beuchat (1977), which is calculated as:

| Water activity (a w )
The water activity of the samples was determined by LabMaster-a w instrument (Novasina AG, CH-8853 lachen, Switzerland) according to ADOGA (2005). where M i = intial mass before drying and M d = final mass after drying.

| pH
The pH value was determined using digital pH meter according to AOAC (2011) official method 981.12.

| Titratable acidity (TA)
Titratable acidity was determined using the classical method described in Akhtar, Abbasi, and Hussain (2010). TA was calculated using Equation 6.

| Vitamin C
Vitamin C content of samples was determined according to the method described in Sadasivam and Manickam (2005).

| Pyruvic acid content
Onion pungency is caused by several volatile sulfur-containing compounds. These compounds are produced when the onion cell is mechanically disrupted, bringing the enzyme alliinase present in the cell cytoplasm into contact with the flavor precursors, S-alk(en) yl-L-cysteine sulphoxides. In addition to producing volatile sulphurcontaining compounds, the enzymatic breakdown of the Salk (en) yl-L-cysteine sulphoxides also produces stoichiometric amounts of ammonia and pyruvic acid. The amount of pyruvic acid generated enzymatically upon homogenization of onion in a blender is thus a good measure of the action of the alliinase on the flavor precursors and has been shown to be correlated with perceived onion pungency (Schwimmer & Weston, 1961). Therefore, pyruvic acid content, as an index of the flavor strength of onions, was measured in mmol/g with a spectrophotometric method (Schwimmer & Weston, 1961).

| Sensory evaluation
Sensory analysis of the samples was conducted after rehydrating onion powder (1:2 ratio powder: water v/v) in distilled water for one hour. Each rehydrated sample was coded with three-digit number and presented to 50 semi-trained panelists in random order for sensory evaluation of dried onion powder. After tasting each coded sample, the panelists were instructed to rinse their mouth before tasting the next sample. The 5-point hedonic scale (5 = like extremely, 4 = like moderately, 3 = neither like nor dislike, 2 = dislike moderately, and 1 = dislike extremely) was used to determine sensory acceptability of the samples (Kolapo, Popoola, Sanni, & Afolabi, 2007). The sensory acceptability response variables were color, aroma, taste, and overall acceptability.

| Statistical analysis
The main and interaction effects of variety (three levels: Bombay red, Qellafo, and Sweet carolin) and drying temperature (six levels: Fresh, 50, 60, 70, 80, and 90°C) on physical and functional properties (total color change, bulk density, shrinkage ratio, water hydration capacity, and water activity), chemical properties (% moisture, total soluble solids, pH, titratable acidity, vitamin C, and pungency [pyruvic acid]), and sensory characteristics (color, aroma, taste, and overall acceptability) were determined using ANOVA of a 3 × 6 factorial design with three replications. The analysis was completed using the Mixed Procedure of SAS (SAS, 2014), and the validity of model assumptions (normal distribution and constant variance of the error terms) was verified by examining the residuals as described in Montgomery (2017). Independence assumption was assured through randomization of the experiment (Montgomery, 2017). For significant (p-value (4) WHC (%) = Initial volume of water − final volume of water <0.05) and marginally significant (p-value between 0.05 and 0.1) effects, multiple means comparison was completed using Tukey's Multiple Means Comparison method at the 5% level of significance.

| Effect of temperature and variety on physical and functional properties
The interaction effect of variety and temperature was significant on total color change (ΔE) ( Table 1). The change increased with an increase in drying temperature, but the increase was significantly different among onion varieties ( gives an indication of the relative volume of packaging material required. Drying at high temperature makes a product denser, which leads to requiring stronger packaging material. Furthermore, lower bulk density is desired for powder because it enhances greater ease of dispensability (Udensi & Okoronkwo, 2006) when reconstituted in water.
Qellafo variety showed significantly lower shrinkage ratio (40.3) than Sweet carolin, but like that of Bombay red (Table 3). Table 3 indicates that shrinkage ratio gradually increased with increasing drying temperature. The highest ratio (53.5) was obtained at 90°C,  (Table 3). The change in water hydration capacity with an increase in temperature was also reported in Krokida and Maroulis (2000), and in Abasi et al. (2009). As powder absorbs considerably more volume of water, higher water hydration capacity is desirable.
The interaction effect of variety and temperature on water activity was significant ( With an increase in drying temperature, the water activity values decreased in all varieties, but with smaller amount (0.04) for Qellafo (Table 2). Decreasing water activity with increasing temperature was also reported previously (Goula & Adamopoulos, 2005). Storage life of dehydrated agricultural products is highly related with water activity values. Lower value is better because it suppresses microbial growth, biochemical activities, and nonenzymatic browning.

| Effects on chemical properties
The interaction effect of variety and temperature was significant on moisture content (Table 4). For fresh onion, the moisture content of Sweet carolin was significantly higher (90.3%) than that of Bombay red (83.3%) and Qellafo (81.7%; Table 5). However, for dried onion, the change in moisture content of the three varieties was mostly similar; all three varieties had less than 8% moisture at 70°C, and at 90°C, the moisture contents were 6.6%, 5.3%, and 4.9% for Sweet carolin, Bombay red, and Qellafo, respectively (Table 5). These results show that with increasing temperature, water loss and solid content increase. The lowest moisture contents in this study are slightly above those reported (2.721% and 1.148% when dried at 60 and 90°C, respectively) in Abasi et al. (2009), which could be due to differences in variety, hold time in the oven, and thickness of slices during drying.
The main effects of variety and drying temperature on total soluble solid (TSS) content were significant, but not the interaction effect (Table 4). The average TSS value of fresh onions was significantly lower than that of dried product ( The main effects of variety and temperature on titratable acidity of reconstituted dried onion powder were significant, but not the interaction effect (Table 4). Regardless of variety, as temperature increased, titratable acidity also increased from 0.155 (for fresh) to 0.345 when dried at 90°C (Table 3). Titratable acidity of the three varieties was significantly different from one another, with Qellafo giving the highest and Sweet carolin giving the lowest (Table 3). These results indicate that acidity increases as drying temperature increases, which might be due to the conversion of sugar available in onion into organic acids. Ozgur et al. (2011) and Raj et al. (2006) also reported an increase in titratable acidity of onion and leeks after drying, respectively. The results observed for titratable acidity are in line with the changes in pH as drying temperature increases; however, unlike the changes in titratable acidity, the extent of changes in pH was variety specific. That is, there was significant interaction between variety and temperature for pH, but not for titratable acidity.
TA B L E 4 ANOVA p-values showing the significance of the main and interaction effects of variety and temperature (Temp) on chemical (TSS = total soluble solids, pH, TA = titratable acidity, VC = vitamin C, pungency, and % moisture) response variables. Significant effects that require multiple means comparison are shown in bold TA B L E 5 Mean pH, vitamin C, pungency (pyruvic acid content), and % moisture obtained from the combinations of variety and temperature (Temp) Vitamin C is considered as an indicator of food processing quality because of its low stability during thermal treatments (Podsędek, 2007). The interaction effect of variety and drying temperature on vitamin C content was significant (  Table 6 shows that the interaction effect of variety and temperature was significant on color, aroma, and taste, and marginally significant on overall acceptability. Color is one of the most important sensory properties of food products. In general, with an increase in drying temperature, the color score values decreased (Table 2), which might be associated with nonenzymatic browning (NEB) that occurred faster at high temperature (Lopez, Mathers, Ezzati, Jamison, & Murray, 2006). Results of this study are also in agreement with the findings of Olalusi (2014) who reported that color of onion slice decreased when drying temperature increased from 50 to 70°C.

| Effect on sensory acceptability of onion powder
Aroma is imparted by volatile compounds and perceived by the odor receptor sites of the smell organ. Particularly, the pungent taste of onion during preparation, processing, and storage is because of the presence of sulfur-containing volatile oil Allyl-propyl-disulphied in the bulb (Mitra, Shrivastava, & Rao, 2012). This compound significantly contributes to the aroma of fresh and dried onion. As indicated in Table 2, with an increase in drying temperature, aroma of onion powder reduced for all varieties, but less in Bombay red and more in Sweet carolin. The same result was observed in different works (Mitra et al., 2011;Olalusi, 2014). This might be associated with the loss of volatile components of powders (Sacilik & Unal, 2005). At a drying temperature of 70°C, Bombay red and Qellafo had better aroma than Sweet carolin ( Table 2).
As indicated in Table 2, with an increase in drying temperature, taste of onion powder reduced in all varieties. But, the taste of powder dried at 50°C from Sweet carolin variety was the best. Likewise, at a drying temperature of 70°C, Sweet carolin was rated slightly better than Bombay red and Qellafo (Table 2). However, at 90°C, all onion varieties did not have statistically significant difference. Our results agree with those indicated in Olalusi (2014) where taste of onion slice decreased with increment of drying temperature from 50 to 70°C.
The overall acceptability scores of fresh onions (Table 2) for Bombay red and Qellafo were significantly higher than that of Sweet carolin, and then, it decreased as drying temperature increased for all varieties. Although the overall acceptability of fresh onion of Bombay red and Qellafo varieties was higher, when dried at 90°C, all three varieties have statistically the same overall acceptability.

| CON CLUS IONS
This study showed the presence of variability among onion varieties and the level of drying temperature on physical, chemical, and sensory properties of onion powder. In general, an increase in drying temperature results in reduction of most of the desired properties of onion slices. But a reduction in water activity and pH values of onion powders are desired properties in terms of safety and shelf stability of dried powders. Among the investigated temperatures, drying temperature of 70°C for 5 hr of drying time resulted in comparatively acceptable physical, chemical, and sensory properties. At 70°C, Qellafo resulted in superior results for most of the parameters as compared to the other two varieties. Therefore, Qellafo is recommended as a suitable onion variety when production of dehydrated onion product is desired.

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