Effects of pretreatment during drying on the antioxidant properties and color of selected tomato varieties

Abstract Drying is essential in lowering the water activity and increasing the shelf stability of perishables. Thus, this study investigated the effect of pretreatment on the retention of the antioxidant properties and color of four tomato varieties; that is, Anna F1, Kilele, Prostar F1, and Riogrande during drying. Prepared quarters were treated by spraying with 0.5% sodium metabisulfate, 0.5% calcium chloride, and distilled water. The quarters were oven dried at 50°C, 60°C, and 70°C to 13% moisture content. Lycopene, β carotene, total phenolics, color, and moisture content were determined in both the fresh and dried samples. Initial moisture content among the four varieties did not differ significantly and ranged between 94.2 and 94.6%. Results showed that the main effects were significant (p < .05) on all measurable variables. Significantly (p < .05) higher retention levels in lycopene, β carotene, total phenolics, and lightness was observed in chemically pretreated samples compared to the control during drying.


| INTRODUCTION
Tomato (Lycopersicon esculentum mill) is among the most highly consumed and popular vegetable in the world (Hanson et al., 2004).
Nutritionally, tomatoes are rich sources of antioxidant compounds such as β carotene, lycopene, ascorbic acid, and phenolic compounds (Georgé et al., 2011). Lycopene and β carotenes are carotenoids responsible for the red, yellow, and orange colors of most plants which cannot be synthesized by animals in vivo and require consumption in the diet (Eldahshan et al., 2013). Scientific evidence shows that consumption of these phytonutrients on a regular basis contribute to significant health benefits such as prevention against diseases such as prostate cancer, age degenerative diseases, and cataracts (Gümüsay, Zoran, Ercal, & Demirkol, 2015). This is attributed to their ability to quench singlet oxygen and trap peroxyl radicals (Gümüsay et al., 2015).
Tomato production in Kenya has increased in the recent past with the adoption of greenhouses. Production increased from 20,985 Ha in 074 Ha in 2014, representing a 15% increase in area under production (MOA, 2015). However, being climacteric crops, tomatoes are inherently perishable with a shelf life of 8-12 days in their fresh state after harvest (Ahmed, Islam, Sarker, Hasan, & Mizan, 2016). This has led to extensive postharvest losses in the product that have been estimated to be as high as 50%. These losses translate to a subsequent imbalance in supply and demand and consequential losses in income to both small-and large-scale farmers. In order to sustain surplus harvest, appropriate postharvest preservation methods are needed to extend the commodity's shelf life. One such technology is drying which lowers the moisture content and consequently the water activity of food to a level that does not support bacterial and mold growth (Joshi, Orsat, & Raghavan, 2009). However, during drying some nutrients are degraded by heat thus affecting the quality and acceptance of the final product (Taylor, Goula, Adamopoulos, Goula, & Adamopoulos, 2010). The degree and extend of oxidation as well as isomerization is directly related to the duration and intensity of heating (Eldahshan et al., 2013). As a result, there is need to enhance the rate of drying to ensure maximum retention of antioxidant molecules in tomatoes as well as reduce oxidative, nonenzymatic, and isomerization reactions thus protect these molecules from degradation. Since color, lycopene, and total phenolic content in tomato are regarded as good quality indicators of the dehydration process (Santos-Sánchez, Valadez-Blanco, Gómez-Gómez, Pérez-Herrera, & Salas-Coronado, 2012), minimization of quality degradation in these indicators is paramount. In this regard, osmotic assisted dehydration with compounds such as calcium chloride and sodium chloride has been shown to increase the drying rate in plant tissues by enhancing water mobility. It has also been reported that osmotic assisted dehydration improves general product quality (Azoubel & Oliveira, 2008) by preserving the nutritional, sensorial, and functional properties of the food matrix (Kennedy, 2007). However, to ensure water activity below 0.9 is achieved, convective drying methods should be carried out after osmotic dehydration (Kennedy, 2007).
Therefore, this study investigated the effects of pretreatment on lycopene, β carotene, total phenolics, and color during oven drying of four tomato varieties.

| Tomato growing, harvesting, and sample preparation
The four tomato varieties under study were grown in a greenhouse followed commercial practices of tomato production. Pruning was carried out to remove undesirable side branches after every 2 weeks.
Tomato varieties of uniform maturity (red ripe maturity stage), color, size, and shape were randomly selected for the study. Color selection was based on the USDA color scale (1997) where at least 90% of the surface of the tomato was red.
Nine kilograms of each tomato variety was harvested after reaching red ripe maturity stage, weighed using a digital balance and subdivided into three equal batches of three kilograms each. The harvested tomatoes were washed in running tap water to remove debris and dried using a dry cloth. Each tomato was longitudinally sliced into four equal quarters using a manual slicer. The first batch was further subdivided into three equal batches each one kilogram and treated by spraying with 0.5% w/v sodium metabisulfate (0.5% N.M). The second and third batches were similarly subdivided and sprayed with 0.5% w/v calcium chloride (0.5% C.C) and distilled water (control), respectively. The treated samples were allowed to stand for 20 min to drain away excess spray.

| Drying procedure
Drying was carried out in an oven drier (memmert UF 110 model; Germany).

| Oven drying
The oven drier was operated for 1.5 hr before drying to achieve steady-state conditions. The batches were separately placed in a single layer on 2 mm aperture 60 cm × 30 cm removable aluminum meshed trays and dried at 50°C, 60°C, and 70°C. Drying was done to final moisture content of ~13%.
At the end of each drying procedure, moisture content of the samples was determined and the dried samples were put into zip lock bags and stored at −20°C away from light until further analysis. All the experiments were carried out in triplicates and the results expressed on dry weight basis (db) except moisture content which was expressed on fresh weight basis.

| Moisture content determination
Moisture content in the fresh and dried tomato samples was analyzed according to the (A.O.A.C, 1999) and the % moisture content calculated based on the formula:

| Color determination
The color of both the fresh and dried samples was determined following the method detailed by Dadalı, Kılıç Apar, and Özbek (2007) with some modifications using a hunter lab color difference meter (Minolta, Tokyo, Japan). The instrument was standardized with a black and white ceramic plate before use. The color of the samples was measured at three regions along the blossom end, the stem end, and around the equatorial region. Reflected colors L*, a*, and b* were determined directly as displayed on the color meter screen. L* values were used as an indicator of lightness in the samples analyzed.

| Determination of total phenolic compounds
Folin ciocalteu method was used to determine the amount of total phenolic compounds as described by Ainsworth and Gillespie (2007) with slight modifications. Gallic acid was used as the standard. 2 g of each tomato sample was crushed in a pestle and mortar and put in a vial after which 50 ml of cold methanol was added. The sample was vortexed for 3 hr and incubated for 72 hr at 25°C away from light. The extract was filtered to remove the debris and centrifuged at 13,000g for 10 min at 25°C and the supernatant collected.
A 1 ml of the extract was passed through a 0.45 μl membrane filter.
A 2 ml of 10% (V/V) Folin ciocalteu reagent was added and vortexed after which 4 ml of 0.7 mol/L Na 2 CO 3 solutions was added.
Folin ciocalteu was added before Na 2 CO 3 to prevent air oxidation of the phenols in the extract. The mixture was allowed to stand for 2 hr at 25°C and the absorbance measured at 765 nm using UVvis spectrometer (Shimadzu UV Vis 1800, Tokyo, Japan model). A standard curve was plotted from the blank corrected absorbance of the gallic acid standard. The amount of total phenolic content was expressed as gallic acid equivalents GAE) per 100 g of the sample.
% moisture content = change in weight∕sample weight × 100

| β carotene and lycopene content determination
The method suggested by Chen (2005) was employed with some modification for the determination of lycopene and β carotene.
About 5 g of crushed tomato sample was weighed using a digital balance and put into amber bottles after which 50 ml of hexaneacetone-ethanol solution (2:1:1 v/v/v) containing 1% BHT (w/v) was added to solubilize lycopene. The content was then agitated for 20 min after which 15 ml of distilled water was added to the mixture and mixed for 10 min. The solution was separated into polar and a nonpolar phase using a separating funnel. A 50 ml of the upper hexane layer was collected and 1.5 ml of it was micro- whereas the detection wavelength for lycopene was 470 nm and that of β carotene was 445 nm. The temperature of the oven was maintained at 30°C. Lycopene and β carotene standard concentrations were prepared in hexane. Quantification was done using chromatographic peak areas generated to determine the lycopene content and β carotene content. Lycopene and β carotene in the sample was identified by comparing the retention time of pure lycopene and β carotene.

| Statistical analysis
The experiment was carried out in triplicate and data subjected to analysis of variance (ANOVA) using Stata SE version 12 (Stata Corp LP, TX, USA). ANCOVA which combines features of both ANOVA and regression was applied to test effects of pretreatments, variety, and temperatures during drying. When the coefficient of the interaction term was significant (p < .05), it was concluded that there was a significant difference between treatments. One-way ANOVA was performed where treatment outcomes needed to be compared.
Means were separated using Bonferroni adjustment at 95% level of significance.

| Drying time required to attain stable moisture content
The initial moisture content (m. c) in the four varieties was not  (Alibas, 2009). This was attributed to increased mass transfer associated with increase in temperature.

| Color changes during drying
Color is an important quality indicator in most food products and plays a key role in consumer preference during purchase (Ringeisen, Barrett, & Stroeve, 2014). Thus, a change in the color of a product during processing is generally associated with decrease in the quality and marketability of that product. L* indices were used to characterize the coloration of both fresh and dried tomato samples in this study as shown in Table 2. Fresh tomato varieties under this study had an L* in the range of 41.12-42.33. Significant decrease in lightness relative to the fresh samples occurred in all the samples upon drying as follows: 0.5% N.M < 0.5% C.C< control as shown in Table 2.
Generally, it was observed that the lowest degree of darkening was found in samples pretreated with 0.5% N.M and the highest degree of darkening was in the control samples after oven drying.
This indicated that chemical pretreatment was preventive against oxidation that is characterized by formation of darkened products.
The low luminosity in the control samples was indicative of possible darkening that is characteristic of enzymatic and nonenzymatic reactions that occur during heat processing Luterotti, Bicanic, and Markovi (2014)

| Effect of pretreatment on the total phenolic content of tomato varieties during oven drying
The effect of pretreatment on the total phenolic content (TPC) content in oven-dried tomato samples is shown in Figure 1.  Total phnolic content (mg GAE/100 g db) sample was compromised thus facilitating higher extractability of TPC as compared to lower drying temperatures. Dadalı et al.(2007) Figure 2 shows the effect of pretreatment on the β carotene content in tomatoes during oven drying. The β carotene content in the fresh tomato varieties was found to be significantly different (p = .0001).  Georgé et al. (2011) who found β carotene content to occur in the range of 0.6 ± 0.1-1.0 ± 0.1 mg/100 g f. w in tomatoes.

| Effect of pretreatment on the β carotene content of tomato during oven drying
Statistically, there was an interaction effect between drying temperatures, pretreatment, and the variety on the content of β carotene in the dried samples (df = 35; F = 427.34; p = .0001). The effect of the main effects including temperature (p = .0001) and pretreatment (p = .0001) were found to be significant on the β carotene content retained after oven drying. Higher β carotene retention values were observed in the pretreated-dried samples as compared to the control.
The percentage retention values were 4. 30-22.56, 24.41-46.85, and 33.81-65.58 in the control, 0.5% C.C, and 0.5% N.M, respectively. On the other hand, 7. 28-47.16, 8.97-54.20, 8.33-44.21, and 4.30-65.58% of β carotene relative to the fresh was retained in Anna F1, Prostar F1, and Riogrande varieties, respectively. Based on temperature, 16.26-65.58, 11.05-55.21, and 4.30-41.52% of the initial β carotene was retained after drying at 50, 60, and 70°C in all the varieties studies. A key observation, however, was that raising the drying temperature during oven drying from 50°C to 70°C resulted in a higher β carotene loss despite shorter exposure to drying air required to attain stable moisture content. This shows that β carotene retention was highest in samples dried at 50°C as compared to those dried at 60°C and 70°C despite increased drying time. This was attributed to possible higher isomerization and oxidation rate at 70°C as compared to drying at 50°C (Eldahshan & Singab, 2013). This therefore suggests that for maximum β carotene retention the recommended

| Effect of pretreatment on the lycopene content of tomato during oven drying
The effect of pretreatment on the lycopene content in tomatoes during oven drying is shown in Figure 3. The lycopene content among the fresh tomato varieties studied differed significantly (p = .001 The % retention of lycopene was 33. 05-58.11, 52.44-74.13, and 59.38-88.33% in the control, 0.5% C.C, and 0.5% N.M pretreated samples, respectively. Sodium metabisulfate has been reported to retard and inhibit oxidation that is mainly linked to adverse degradation of carotenoids (Sahin et al., 2011). This is attributed to their ability to act as reducing agents during oxidative reactions (Sra & Sandhu, 2011). With regard to temperature, lycopene content was found to be best retained after drying at 60 C whereby

| CONCLUSION
Attempt to lower moisture content in Anna F1, Kilele, Prostar F1, and Riogrande tomato varieties to ensure shelf stability resulted in overall reduction in lycopene, β carotene, total phenolic compounds, and color compared to the fresh samples. However, pretreatment with 0.5% N.M and 0.5% C.C significantly preserved the overall quality of dried tomato samples during oven drying as compared to the control.
The overall quality of the dried tomato was highly influenced by drying temperature, tomato variety, and chemical pretreatment. Lycopene, total phenolic compounds and β carotene was best retained by drying at 60°C (0.5% N.M), 70°C (0.5% N.M), and 50°C (0.5% N.M), respectively, in all varieties under study. The degree of darkening was least in the dried samples as follows: 0.5% N.M < 0.5% C.C < control. The study therefore showed that pretreatment is one of the techniques that can be used in controlling undesirable quality changes that occur during tomato drying.