Impact of Different Drying Techniques on the Nutritional Components of Plum Tomatoes ^†

Abstract

Tomatoes are currently regarded as one of the world’s major vegetable crops. Tomatoes have a significant economic impact on the earnings of many growers worldwide. The tomato is one of the most widely grown vegetables in the world and is ranked second in many nations. The majority of agricultural products have higher moisture content, ranging from 25 to 90%. This moisture content value is significantly higher than what is needed for extended preservation. The effects of bacteria, enzymes, and yeast are slowed down in crops when their moisture content is reduced to a certain degree. This study’s goal is to determine the quality of dried plum tomatoes using various methods. Samples were dried in the sun, on a heated plate, and in a solar dryer. Using accepted techniques, the dried tomato samples were assessed for total lycopene, ascorbic acid, pH, and titratable acidity. The findings demonstrated that, in comparison to the control sample, the sun-dried and solar-dried sample had higher levels of pH. Compared to the control, the ascorbic acid and lycopene levels in the sun-dried and solar-dried tomato were lower. Comparing the heated plate-dried sample to the control sample, a notable rise in lycopene and ascorbic acid content was observed. The results showed that the optimum method for maintaining the quality attributes of dried tomatoes was to use a heated plate drying method.

1. Introduction

Tomatoes are regarded as an essential component of the human diet and a significant agricultural crop globally. Even though fresh tomatoes are frequently eaten, processed tomato products like sauce, ketchup, and tomato juice account for more than 80% of tomato consumption. Tomato-rich diets may have health benefits, according to recent studies [1].

Dried fruits and vegetables improve shelf life, require less packing, and weigh less during transportation [2]. Dehydration is a key preservation method, extending shelf life and reducing transportation and storage costs, while preserving high-added-value components. Foods such as dried fruits can greatly contribute to overcoming hunger during winter and drought seasons [3]. It is reported that in order to extend shelf life and support food security, more than 20% of the world’s perishable crops are dried. Tomatoes must be eaten right away or stored for later use, just like other agricultural products, because they expire. Since not all fresh tomatoes can be eaten right away after they are harvested, preservation creates a bigger market and enables customers to purchase the product all year round. Sun-dried tomatoes are now widely accepted as an ingredient in the food industry and in the food service sector. However, according to [4], the sun-drying industry currently has trouble reliably producing dried tomatoes of high quality. Sun-drying food is one of the most affordable food preservation techniques. Reducing postharvest losses is one of the primary goals of sun-drying tomatoes in developing nations [5]. In contrast, sun-dried tomatoes are regarded as a “gourmet” ingredient in developed nations. Dehydration significantly reduces weight and bulk for perishable goods with extremely high moisture contents, which saves money on storage and distribution expenses [6]. The manipulation of the storage environment (temperature and relative humidity) [7]; the addition of chemical preservatives, waxing, or edible coatings [8]; and the use of modified atmosphere packaging [9], drying and product formulations are just a few of the techniques that different researchers have reported for extending the shelf life of tomatoes. However, the effectiveness of these techniques relies on the specifications of the consumable product quality. However, the majority of methods that have been reported are unable to prolong shelf life. In Mediterranean countries, food was traditionally preserved by drying, an age-old method. Of all the food preservation and processing techniques, drying is the most effective and practical because it significantly reduces the moisture content of the final product, thereby preventing microbial deterioration [10]. Furthermore, after drying, the volume and weight of the dried goods are significantly reduced, which can reduce the expense of packaging, storing, and shipping [11].

This work aimed to assess the impact of various drying methods, including sun, solar, and heated plate methods, on the nutritional characteristics of tomatoes.

2. Materials and Methods

2.1. Preparation of Raw Materials

Roma VF plum tomatoes were gathered in Zaria, Kaduna, at Nigeria’s Samara market in June 2023. They were picked when they reached the table-ripe maturity stage, which guarantees their color, freshness, consistency in size, and lack of flaws. The tomatoes were dried with absorbent paper after being washed under running tap water. Using a stainless-steel knife, the cleaned fruits were sliced into 5 mm thick pieces and then exposed to a heated plate, sunlight, and sun drying. The temperatures for the heated plate were 40 °C and the sun drying and solar drying took place at temperatures of 36 °C.

2.2. Experimental Setup and Procedure

The heat lost through the thickness of the plate to the tomato slices was measured using a thermometer to calibrate the water bath. The purpose of this was to ignore the heat lost during the drying process. The samples were appropriately sprayed onto an aluminum plate with an average diameter of 1 mm that was set over a water bath. Weight balances were used to measure the sample weights. Sample moisture contents were measured every 60 min until a constant moisture content was reached using all drying techniques. The dried samples were packed in an air-tight container for analysis.

2.3. Qualitative Analysis

The following information was collected for the tomato samples in both their fresh and dried states: color, moisture content, pH, lycopene, ascorbic acid, and titratable acidity.

2.3.1. Color Evaluation

The dried tomato samples were rehydrated and subjected to color evaluation in order to assess consumers’ reaction with regard to the color of the dried tomato samples. Thirty untrained panelists were selected from groups of students, laboratory technicians, and academic staff members of the chemical engineering department. The samples were presented and participants were asked to evaluate the color.

2.3.2. Moisture Content Determination

First, 5 g sample of tomato was weighed in a metal dish. The sample had been previously weighed, heated to 105 °C for 15 min, and then cooled in a desiccator. The sample and metal dish were placed in the oven and heated to 105 °C for one and a half hours. After being allowed to cool in the desiccator, the sample was weighed as soon as it reached room temperature [12]. After cooling, it was heated again for 30 min at the same temperature. The process was repeated until a constant weight was maintained and the % moisture was calculated as follows:

$$\% \mathrm{M}\mathrm{o}\mathrm{i}\mathrm{s}\mathrm{t}\mathrm{u}\mathrm{r}\mathrm{e}={\displaystyle \frac{\mathrm{w}\mathrm{e}\mathrm{i}\mathrm{g}\mathrm{h}\mathrm{t} \mathrm{b}\mathrm{e}\mathrm{f}\mathrm{o}\mathrm{r}\mathrm{e} \mathrm{d}\mathrm{r}\mathrm{y}\mathrm{i}\mathrm{n}\mathrm{g}-\mathrm{w}\mathrm{e}\mathrm{i}\mathrm{g}\mathrm{h}\mathrm{t} \mathrm{a}\mathrm{f}\mathrm{t}\mathrm{e}\mathrm{r} \mathrm{d}\mathrm{r}\mathrm{y}\mathrm{i}\mathrm{n}\mathrm{g}}{\mathrm{w}\mathrm{e}\mathrm{i}\mathrm{g}\mathrm{h}\mathrm{t} \mathrm{o}\mathrm{f} \mathrm{s}\mathrm{a}\mathrm{m}\mathrm{p}\mathrm{l}\mathrm{e}}}\times 100$$

2.3.3. To Determine the pH

To determine the pH, 10 g of tomato sample was combined with 20 mL of distilled water and thoroughly mixed. The pH value was recorded after the pH meter dielectrode was dipped into the sample and the reading stabilized [12].

2.3.4. Titratable Acidity Determination

In a 250 conical flask, 10 g of the sample was weighed and 200 mL of distilled water was added. Four to five drops of phenolphthalein were added and titrated against 0.1 M NaOH [12]. After noting the titer value, the formula was created as follows:

TA = titer value × 0.09

2.3.5. Ascorbic Acid Determination

A 20 g sample was weighed and ground in a mortar with a small amount of glacial acetic acid. The extract was quantitatively transferred into a 50 mL volumetric flask with distilled water, then quickly filtered, and additional water was added until the solution reached the measurement mark. Ten milliliters of filtrate and one drop of diluted acetic acid were added to a conical flask. In the burette, the mixture was titrated against a solution of the redox dye 2, 6-dichlorophenol. It was noted how much dye was needed to decolorize the 10-milliliter sample. The titration process was repeated by substituting the tomato extract with a standard ascorbic acid solution (1 mg of pure vitamin C per 100 mL) [2]. The ascorbic acid content per 100 g of tomato was calculated as follows:

$$\mathrm{m}\mathrm{g}/100 \mathrm{g}={\displaystyle \frac{W1+W2\times VI\times 100}{W1\times W3\times VI}}\times 100 (v\times f)$$

where

$W1$ = weight of sample (g).

$W2$ = weight of extracting acid (g).

$W3$ = weight of slurry taken for analysis (g).

$VI$ = volume to which slurry sample is diluted (ml).

v = volume of dye solution used for titration.

f = ascorbic acid equivalent of dye (mg/mL) [12].

2.3.6. Lycopene Determination

One gram of lycopene powder was weighed into one hundred milliliters of hexane–acetone mixture to create lycopene standard. This was left to stand for one hour prior to filtration. Subsequently, test tubes containing 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, and 0.9 mL of the stock were measured and filled to a capacity of 10 mL each using a hexane–acetone mixture. The blank was an acetonic hexane mixture (about 10 mL). After this mixture was allowed to stand for an additional half-hour, the absorbance at 475 wavelengths was measured using a UV spectrophotometer, yielding a standard curve. Next, 1 g of each sample was added to 100 mL of hexane and acetone for an hour while being vigorously shaken. Approximately 3 g of tomato sample was ground with a pestle and mortar. Each sample was taken from the stock at a volume of 1 mL up to 10 mL with a hexane and acetone mixture, and the absorbance was measured at 475 on the spectrophotometer [13].

3. Results and Discussion

3.1. The Result of the Color, Moisture, PH, and Titratable Acid

Color is one of the most significant quality-limiting attributes in dehydrated fruits and vegetables. Certainly, possible color changes would dictate the organoleptic characteristics of dried tomato samples and would limit their intended uses. Consumers opt for dried samples which are red in color, not burnt or browned. The results showed that the color obtained from the heated plate samples has higher acceptability compared to solar and sun-dried samples.

Table 1. Evaluation of chemical, antioxidant and physical properties of heated plate-dried, sun-dried, and solar-dried plum tomato samples.

Attributes	Fresh Plum Tomato	Heated Plate Sample	Sun Dry Sample	Solar Dry Sample
Moisture content (%)	93.94	13.25	14.21	14.97
Lycopene (mg/100 g)	49.26	52.17	42.37	37.01
Ascorbic acid (Vitamin C) (g/L)	0.34	0.41	2.9	2.48
pH	4.8	5	5.4	5.7
Titratable acid	5.97	5.84	5.701	5.74

shows the results of the titratable acid, PH, and moisture of the fresh and dried tomato samples dried on a heated plate, in the sun, and using a solar dryer. The fresh tomatoes’ moisture content percentage was 93.94%, which was higher than that required for tomato preservation. The moisture content percentage of tomato samples that were dried using a heated plate, the sun, and a solar dryer varied from 13.25 to 14.97, respectively. These findings are in line with the study of [14]. The dried tomato sample showed that the titratable acid ranged from 5.84 to 5.71 and the pH from 5.4 to 5.7. This is in line with the study of [14], who found that tomatoes oven-dried at 60 °C had a significantly lower TA than fresh tomato samples. The results of [14] support the hypothesis that the pH of the fresh tomato sample was lower than that of the drying sample.

3.2. Lycopene and Vitamin C

Table 1 displays the results of the lycopene and vitamin C tests for the dried samples. The fresh tomato samples’ lycopene and vitamin C values were 49.26 and 0.34. The lycopene values of the heated plates and sun-dried and solar-dried samples were 52.17, 42.37, and 37.01, and the vitamin C values were 0.41, 2.9, and 2.48, respectively. In comparison to solar and sun drying, heated plate drying had a higher lycopene content in our experiments. Because of the short drying time and low drying temperature, samples dried using a heated plate have higher lycopene content than the fresh ones. The study’s findings are consistent with those of [15], who found that processed tomato paste contained more lycopene than fresh tomato paste. Tomato samples exposed to the sun and solar dryer had lycopene contents that were lower than those of the control sample. This might be because of the low initial drying temperature, which promotes the growth of microorganisms and enzymatic activities that break down lycopene, in accordance with research by [14]. It can be observed that the vitamin C content was higher in the sample dried using a heated plate compared to that of the fresh sample; this is also in line with the work of [16] who reported a 2.2 times increase in the vitamin C content of dried tomato compared to fresh.

3.3. Conclusions

The study looked into how the heated plate, sun, and solar drier affected the dried tomato’s quality. The parameters of the tomato drying process had a major impact on the sample’s quality. This research offers a useful method for creating dehydrated tomatoes with noticeably improved quality. In summary, the sun-dried and solar-dried samples had higher pH levels than the control sample. The ascorbic acid and lycopene levels in the sun-dried and solar-dried tomato were lower than in the control. A significant increase in ascorbic acid and lycopene content was seen when the heated plate-dried sample was compared to the control sample. The findings indicated that using a heated plate drying method was the best way to preserve the quality characteristics of dried tomatoes.