The authors have declared that no competing interests exist.
The use of conventional drying methods for dehydrating vegetables and fruits with high sugar, amino acid, and moisture content is not feasible. Under extreme conditions, drying reduces the amount of functional ingredients present in these vegetables and fruits. Recently, there has been an increasing demand for powdered tomatoes because of their nutritional and functional components. This study aimed to evaluate tomato drying under normal pressure and low-temperature conditions, to efficiently reduce water content without compromising on product quality. A convection dryer, most widely used in food factories, was used in this study. This equipment uses a low temperature zone not normally used for drying, and processes the raw material to increase the drying area for rapid drying. The proposed method was validated by comparing the moisture and functional component contents, and the antioxidant activity of the dried product with those of the dried product obtained via freeze-drying. The results suggest that the proposed low-temperature drying method can produce functional dried food at food processing sites faster than using freeze-drying, with a residual rate of functional ingredients exceeding 90 %. Thus, low-temperature drying can be used as a simple and cost-effective method for the production of uniform dry tomato powder.
Kumamoto is an agricultural prefecture located in the central part of Kyushu, Japan. It ranks fifth in agricultural output across the country with cauliflower, tomatoes, watermelon, eggplants, and ginger among its high-yielding crops
Drying is a crucial step in food processing because the reduction in the water content of food prevents spoilage and degradation caused by microorganisms, enzymes, and chemical reactions. This process allows for the prolonged storage of food and its subsequent distribution to distant locations. In Japan, as early as the Edo period (1600–1868), drying has been employed to ginger and perilla for medicinal use. Currently, dried food, or kanbutsu in Japanese, has gained popularity because it offers the benefit of being ready-to-eat or ready-to-cook while retaining its nutrient content. Dried vegetables, in particular, can be used to address the low consumption of vegetables in Japan, which is currently at 20 % of the target value. Advancing from the simple technique of sun-drying, food dehydration has become increasingly sophisticated over the years. The innovations made in drying technology are usually driven by the increasing demand for processed products.
Owing to its simple operation, low cost, and high versatility
Other food drying techniques include vacuum drying, microwave drying
In agricultural processing, the commercialization of dried processed goods has become increasingly popular because of their convenience and long shelf-life. Notably, Kumamoto records the highest production volume of tomatoes in Japan. Recently, there has been an increasing demand for powdered tomatoes because of their nutritional and functional components. In this study, we also investigated the effect of the ripening level of tomatoes on the nutritional content of the powdered product. Since large quantities are often discarded as a consequence of fruit thinning, there has been a significant demand for the effective utilization of unripe tomatoes; therefore, the nutritional content of powdered green tomatoes was also evaluated in this study.
In this study, the following three types of mature fruits (red tomatoes) were considered Animo TY12, Furinkazan, and Housakukigan 1103. In addition, the following four types of immature fruits (green tomatoes) were included: Animo TY12, Furinkazan, Housakukigan 1103, and Momotaro (Takii & Co., Ltd., Kyoto, Japan). The tomatoes were cultivated in an agricultural plastic greenhouse at the Kumamoto Prefectural Agricultural Research Center (Koshi City, Kumamoto, Japan).
Variety & Maturity | Parameter | |||
Sugar(Brix |
Citric acid(mg% |
Glutamic acid(mg% |
||
Red | Animo TY12 | 4.6 ± 0.24 | 346.0 ± 0.13 | 137.9 ± 5.41 |
Furinkazan | 4.3 ± 0.27 | 317.0 ± 6.21 | 108.7 ± 4.26 | |
Housakukigan1103 | 4.5 ± 0.36 | 353.6 ± 0.15 | 126.6 ± 3.27 | |
Green | Animo TY12 | 4.6 ± 0.23 | 502.9 ± 1.79 | 18.3 ± 0.26 |
Furinkazan | 4.5 ± 0.20 | 476.6 ± 3.37 | 15.3 ± 0.41 | |
Housakukigan1103 | 4.5 ± 0.18 | 412.4 ± 3.62 | 14.9 ± 0.22 | |
Momotaro | 5.6 ± 0.31 | 421.7 ± 22.24 | 25.7 ± 1.46 |
Concentration (wt%) of solid matter in liquid solution as measured using a refractometer
mg per 100g fresh fruit weight
LTD was performed using a compact-type food dehydrator SM10S-EH-DPC (Kihara Works Co., Ltd., Yamaguchi, Japan). Approximately 200 g (0.2 g cm-2) of each crushed tomato was loaded into a container made of parchment paper (25 cm × 40 cm) and dried at 40, 50, and 60 °C. The samples were weighed every hour until no weight change was observed at the appropriate drying temperature. For the residual test of the functional ingredients, each tomato sample was dried at 40 °C for 10 h, based on the high lycopene content obtained at this temperature. Subsequently, each dried tomato sample was crushed and used for analyses. For the FD method, approximately 200 g of each crushed tomato was dried using a VirTis Genesis Pilot Lyophilizer (SP Scientific, Pennsylvania, US), which was set for 70 h, and then crushed prior to analysis. Each drying test was performed in triplicates using both methods, and the mean and standard deviation values obtained were used for further estimations.
Lycopene content was measured using the method proposed by Ito
The Folin–Ciocalteu method
The total polyphenol content was quantified by adding Folin-Ciocalteu’s reagent solution and saturated sodium carbonate, developing colors, which were measured at an absorbance of 760 nm, where the total polyphenol content was calculated as the gallic acid equivalent per 100 g of dry weight (mg GAE/100 g-dry weight), as presented in the method by Sawai
A mixture of 0.2 M 2-Molphorinoethanesulfonic acid, monohydrate buffer and 400 mM 1,1-Diphenyl-2-pycrilhydrazyl (DPPH) Free Radical ethanol solution was added to the sample to develop color, and the absorbance at 520 nm was measured and quantified. The measured value was represented by an amount equivalent to Trolox, used as the standard substance as described in the work of Oki
The organic acid concentrations in each sample were measured. Approximately 100 mg of dried sample was mixed with 2 mL of 80 % aqueous methanol solution, stirred for at least 20–30 s until the mixture was homogenous, and sonicated for 10 min in an ultrasonic bath at 40 °C. The resulting solution was centrifuged at 1690 × g for 10 min, and the supernatant was collected. This procedure was repeated twice. The collected supernatant was adjusted to 10 mL and filtered using a 0.2 µm pretreatment filter. The filtrate was analyzed using high-performance liquid chromatography. The organic acid concentration in each extract was measured using a Prominence series Organic Acid Analysis System (Shimadzu Corporation, Kyoto, Japan) with two Shim-pak SCR-102H columns connected in series (Shimadzu GLC Ltd., Tokyo, Japan, 7 µm, 8.0 mm × 300 mm).
The free amino acid concentration of each extract was measured using a Nexera X2 automatic precolumn derivatized amino acid analysis system (Shimadzu, Kyoto) equipped with a Kinetex EVO C18 column (Phenomenex, Inc., Torrance, California, USA; 2.6 µm, 3.0 mm × 100 mm, pore size (A): 100). Amino acid mixed standard solution type H (FUJIFILM Wako Pure Chemical Corporation, Osaka, Japan) was used to prepare the calibration curve.
Two statistical tests were used in this study to determine if the results obtained were significantly different after variations in the following three factors: drying method, ripeness level, and temperature. Initially, the
Furthermore, analysis of variance (ANOVA) was performed to assess if temperature had a significant effect on the concentration of polyphenols in tomatoes after drying. The one-way ANOVA was used because only a single independent variable (
In, both, the
To determine the ideal temperature at which the components were less likely to depreciate, the drying test was conducted at an agricultural processing plant at temperatures of 40, 50, and 60 oC. Furinkazan was used as the test tomato sample.
Drying temperature (°C) | Amount (mg/100 g dried tomatoes) | |
Polyphenol | Lycopene | |
40 | 428.1 ± 5.3 | 115.6 ± 19.8 |
50 | 479.2 ± 1.7 | 105.8 ± 17.6 |
60 | 578.2 ± 7.5 | 92.1 ± 13.8 |
As shown in
Differences in the amounts of functional components at each drying temperature were measured using polyphenols and lycopene as representative nutrients. The polyphenol content increased with temperature. The
Source of Variation | SSa | dfb | MSc |
|
e
|
|
Between Groups | 34,907.23 | 2 | 17,453.62 | 401.29 | 4.09×10–7 | 5.14 |
Within Groups | 260.96 | 6 | 43.49 | |||
Total | 35,168.19 | 8 |
aSums of Squares, bDegrees of Freedom, cMean Squares, dF value: Variation between sample means/variation within the samples, eF critical value, also called F Statistic
Unlike polyphenols, the lycopene content exhibited the highest residual content when the samples were dried at 40 °C and tended to decrease with increasing drying temperature. The relationship between the drying temperature of agricultural products and their antioxidant capacity is known. According to Vega-Glavez
The amount of water lost using LTD was 92.4–94.4 %, whereas that lost through FD was 92.1–94.3 %. The largest difference between them was 0.9 % for Furinkazan (red tomato). The LTD product was also frozen. The water content reduction using both these drying processes were not significantly different. Thus, as seen in
Ripeness | Variety | Moisture removal efficiency (%) | ||
Low |
Freeze-drying | |||
Red | Animo TY12 | 94.22±0.99 | 94.24±0.53 | 0.9807 |
Furinkazan | 93.82±0.54 | 94.28±0.35 | 0.2967 | |
Housakukigan 1103 | 94.26±0.55 | 94.32±0.33 | 0.8923 | |
Green | Animo TY12 | 94.44±0.49 | 94.19±0.27 | 0.4888 |
Furinkazan | 93.31±0.70 | 93.66±0.35 | 0.5077 | |
Housakukigan 1103 | 92.64±0.54 | 93.54±0.12 | 0.0971 | |
Momotaro | 92.41±0.41 | 92.10±0.34 | 0.3568 |
Lycopene, which is a type of carotenoid, is a major functional component in tomatoes with strong antioxidant properties compared to those of other carotenoids
Variety | Lycopene (mg/100 g fresh fruit weight) | ||
Raw fruit | LTDa | Freeze-drying | |
Animo TY12 | 6.69 ± 0.885 | 15.17 ± 0.301 | 18.60 ± 2.013 |
Furinkazan | 10.03 ± 1.711 | 15.68 ± 0.675 | 17.89 ± 1.447 |
Housakukigan 1103 | 9.00 ± 0.645 | 15.67 ± 0.621 | 17.57 ± 1.212 |
aLow-temperature drying
Several studies have reported on the lycopene content after drying. In some studies, it was found that lycopene content after FD was significantly reduced, whereas other studies reported a significant increase in the lycopene content after oven-drying
In this study, there was no significant difference in the lycopene content using the LTD and FD methods. Furthermore, the lycopene content of the dried powder obtained in this study did not differ significantly from that of fresh tomato fruits.
Unique polyphenols, with strong antiallergic activity, are known to be present on the tomato skin
Ripeness | Variety | Polyphenol (mg GAE/100 g fresh fruit weight) | ||
Raw fruit | Low |
Freeze-drying | ||
Red | Animo TY12 | 24.52 ± 0.249 | 24.12 ± 0.541 | 24.18 ± 0.887 |
Furinkazan | 24.33 ± 0.406 | 25.00 ± 0.829 | 24.93 ± 0.887 | |
Housakukigan 1103 | 25.97 ± 0.190 | 24.18 ± 0.336 | 24.33 ± 0.465 | |
Green | Animo TY12 | 17.34 ± 0.129 | 15.64 ± 0.558 | 19.19 ± 0.970 |
Furinkazan | 19.36 ± 0.210 | 18.68 ± 0.313 | 20.06 ± 0.837 | |
Housakukigan 1103 | 17.75 ± 0.407 | 19.40 ± 0.396 | 16.86 ± 0.318 | |
Momotaro | 34.05 ± 1.578 | 33.70 ±0.774 | 33.70 ± 1.040 |
Variety | ||
Low |
Freeze-drying | |
Animo TY12 | 6.10E×10–5 | 2.73×10–3 |
Furinkazan | 1.60×10–3 | 4.67×10–4 |
Housakukigan 1103 | 4.06×10–6 | 8.59×10–5 |
Momotaro | --- | --- |
Wojdylo
Hot air drying and convection drying have been reported as drying methods for crops other than omatoes
Various anti-acid components in foods exhibit inhibitory effects on the generation and action of active oxygen and free radicals
Ripeness | Variety | DPPH (equivalent µmol Trolox/100 g fresh weight) | ||
Raw fruit | Low |
Freeze-drying | ||
Red | Animo TY12 | 63.93 ± 1.267 | 79.76 ± 0.769 | 86.00 ± 2.472 |
Furinkazan | 65.83 ± 1.232 | 86.27 ± 0.374 | 89.76 ± 2.990 | |
Housakukigan 1103 | 84.94 ± 0.603 | 86.61 ± 0.472 | 92.64 ± 1.444 | |
Green | Animo TY12 | 24.54 ± 0.765 | 41.30 ± 1.116 | 43.66 ± 2.252 |
Furinkazan | 18.68 ± 0.0781 | 42.96 ± 0.164 | 42.09 ± 2.715 | |
Housakukigan 1103 | 8.161 ± 0.533 | 33.60 ± 1.735 | 37.08 ± 1.425 | |
Momotaro | 37.37 ± 4.662 | 63.86 ± 2.293 | 74.81 ± 1.607 |
The DPPH activity of the LTD tomato was approximately 4-7% lower than that of the FD dried mature tomatoes, and 0-15% lower than that of untreated immature tomatoes. For instance, DPPH concentration was shown to be affected by the type of drying method when using red varieties of tomatoes, except for Momotaro, the concentration of DPPH in the green varieties of tomatoes was not affected by the type of drying method. Therefore, it is clear that ripeness affects the amount of DPPH remaining after LTD and FD. Consequently, we analyzed the effect of ripeness on the amount of DPPH remaining after each drying method in the red and green varieties. The obtained p-values (<0.05) in all cases, presented in
Variety | ||
Low |
Freeze-drying | |
Animo TY12 | 1.36×10–4 | 4.88×10–5 |
Furinkazan | 1.05×10–3 | 7.61×10–4 |
Housakukigan 1103 | 5.87×10–3 | 4.91×10–3 |
Momotaro | --- | --- |
Organic and amino acids influence the taste of tomatoes, and the organic acid content is a crucial sourness factor. Therefore, the organic acid content in each sample of the two drying methods was compared. Since tomato contains more than 10 types of organic acids
Glutamic acid is a typical amino acid present in tomatoes. The umami (savor) components contain glutamic acid, which is unevenly distributed in the fruit part, and the pulp contains approximately four times as much glutamic acid as the flesh part
Subsequently, the relationship between the amino acid content and maturity of tomatoes was studied. In mature fruits, drying did not result in a decrease in any amino acid. Additionally, in some immature green fruit varieties, amino acid content decreased to less than 50 % of those in fresh fruits. From this result, it is speculated that the amino acids present during the maturation process have unstable chemical structures, which can be degraded using any treatment, such as drying. The magnitude of change in amino acid concentrations during LTD was compared to that of FD, and it was found to be similar to that found in fresh fruits. Therefore, the results suggest that LTD does not reduce the amino acid components, thus validating the efficiency of this drying method and its ability to result in good-quality products.
To investigate the advantages of LTD using physical methods, the rate of moisture removal in each drying process was calculated by dividing the water removal rate by the drying time and compared, as shown in
Ripeness | Variety | Rate of moisture removal (%/hr) | ||
Low |
Freeze-drying | |||
Red | Animo TY12 | 9.422 ± 0.081 | 1.346 ± 0.006 | 4.532 × 10–5 |
Furinkazan | 9.382 ± 0.044 | 1.347 ± 0.004 | 1.275 × 10–5 | |
Housakukigan 1103 | 9.426 ± 0.045 | 1.347 ± 0.004 | 1.368 × 10–5 | |
Green | Animo TY12 | 9.444 ± 0.04 | 1.346 ± 0.003 | 1.083 × 10–5 |
Furinkazan | 9.331 ± 0.058 | 1.338 ± 0.004 | 2.369 × 10–5 | |
Housakukigan 1103 | 9.264 ± 0.044 | 1.336 ± 0.001 | 1.541 × 10–5 | |
Momotaro | 9.241 ± 0.033 | 1.316 ± 0.004 | 6.674 × 10–5 |
A kinetic analysis for tomato drying using LTD was performed. So far, there have been various reaction kinetic models that have been proposed. In this study, the pseudo-first-order kinetic model (Eq. 2) which is the most used, and the Page model (Eqs. 3 and 4) were employed to calculate the drying kinetic parameters for tomato using the low-temperature drying method
where
The -ln(
|
|
|
|
|
|
|
°C | h-1 |
|
kJ mol-1 | |||
First-order | 40 | 0.0063 | _ | 0.9472 | -35.50 | 0.9632 |
50 | 0.0110 | _ | 0.9934 | |||
60 | 0.0152 | _ | 0.9686 | |||
Page model | 40 | 0.2859 | 1.6453 | 0.9796 | -36.39 | 0.9618 |
50 | 0.5084 | 1.9386 | 0.9847 | |||
60 | 0.6596 | 0.4161 | 1.0000 |
Next, we investigated whether the relationship between the drying constants calculated at each temperature and the drying temperature could be expressed by the Arrhenius-type rate equation (Eq. 5) or not.
where ln
In summary, the preservation characteristics of polyphenols, DPPH antioxidant activity, organic acids, amino acids, and taste components obtained using the LTD process were similar to those obtained using FD. Moreover, LTD is the preferred drying method for developing functional dried products at food processing sites in a shorter time and at a lower cost than those with FD.
Using conventional drying methods, it is difficult to obtain dried powder products of vegetables and fruits with high-concentrations of sugars, amino acids, and water, such as tomatoes. Additionally, these drying methods reduce the concentration of functional components of the raw materials. Consequently, we carried out drying in a temperature range that does not reduce the concentration of functional components (LTD), and compared the results to those of FD, which preserves the raw characteristics and causes minimum changes in ingredients. We further examined the effectiveness of both methods, and arrived at the following conclusions:
After drying, the moisture content derived from both the methods was 5–8 %. In comparison, LTD was conducted for 10 h, whereas FD lasted for 70 h, which was at least seven times higher than the time spent on FD.
The total polyphenol content obtained after LTD was 84–104 % (including both red and green varieties) of that for FD; the difference in the residual content using the two methods was not statistically significant.
The DPPH radical scavenging activity, an indicator of antioxidative activity, was approximately 4–8 % for red tomatoes, 2–17 % for green tomatoes, and the activity was lower using LTD than that with FD.
he residual ratio of organic acids and free amino acids in LTD was ≥ 90 %, which was similar to that obtained with FD.
The rate of moisture removal during LTD was approximately seven times faster than that during FD.
The kinetics analysis with the pseudo-first-order reaction rate model and Page model done in this study has not been reported in other works for this type of drying and this type of fruit. Furthermore, the drying behavior at 40℃ followed both of the models, especially the Page model.
In conclusion, LTD can produce a uniform dry powder with a small decrease in the concentration of functional components even in a small food processing factory where it is difficult to introduce FD. Thus, it is possible to easily produce dry powder with a high concentration of functional ingredients equivalent to FD at a comparatively lower cost. In addition, it is also possible to handle various raw materials with high sugar or moisture content. We believe that this study on LTD produces evidence for a new drying method that can easily produce high-quality powder in a short time.
LTD, Low-Temperature Drying; FD, Freeze Drying; SEM, Scanning Electron Microscope; WHC, water holding capacity; WSI, water solubility index; DPPH, 1,1-diphenyl-2-picrylhydrazyl.
The authors report that there are no competing interests to declare.
The authors would like to thank Editage (www.editage.com) for English language editing.
The authors did not receive support from any organization for the submitted work.
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All data generated or analyzed during this study are included in this published article.
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Ms. Masayo Nishizono designed the experimental and analytical systems for this study and also discussed with the authors so that it could be summarized for the manuscript.
Ms. Cinthya Soreli Castro Issasi edited the draft of Ms. Nishizono and Dr. Mizukami had prepared on the basis of the experimental and analytical findings.
Dr. Jonas Karl Christopher N. Agutaya worked on the statistical analysis of the obtained results and the edition of this manuscript.
Prof. Mitsuru Sasaki supervised this study and discussed with the co-authors so that the results could be effectively summarized for the manuscript.
Dr. Hiroyuki Mizukami investigated related research on freeze-drying and high-temperature drying of natural plants, vegetables, and fruits, Furthermore, discussed the findings with the co-authors so that the findings could be summarized for the manuscript.
A low-temperature drying method produces high-quality tomato powder.
Tomato powder made via LTD has minimal functional and nutritional component loss.
LTD uses a convection dryer, resulting in a shorter operating time than FD.