Effect of osmotic treatments and drying methods on bioactive compounds in papaya and tomato

Abstract

We determined the retention of bioactive compounds, including phenolic acids, flavonoids and carotenoids, in papaya and tomato as affected by osmotic treatment and drying methods. Two drying methods, namely, combined far-infrared radiation and air convection (FIR-HA) drying and hot air (HA) drying, were used for drying the untreated and osmotically treated samples. Five treatments groups were studied, including untreated sample dried with FIR, untreated sample dried with HA, osmotically treated sample, osmotically treated sample dried with FIR, and osmotically treated sample dried with HA, compared with a fresh sample. The results showed that non-osmotically treated samples dried with FIR had the highest values of total phenolic content, DPPH and FRAP among all samples, including fresh papaya and tomato. Chlorogenic acid was increased by FIR and HA drying in an untreated sample, while sinapic and ferulic acids were most preserved by osmotic treatment. It was found that the lycopene and lutein contents were significantly increased by both FIR and HA methods in papaya without osmotic treatment. However, the contents of beta-carotene and total flavonoids were decreased by all treatments.

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1. Introduction

Fruits contain many kinds of bioactive compounds including flavonoids, phenolics, carotenoids and vitamins, which are all considered beneficial to human health for decreasing the risk of non-communicable diseases^1,2 such as cardiovascular diseases³ and certain cancers.^3,4 In recent years, studies of bioactive compounds in fruit species have been popular for intensive investigations.⁵ However, the bioactive compounds and antioxidant properties of fruits could be affected by processing. In this study, we selected two popular fruits, papaya and tomato, which are considered to contain high antioxidants, to be investigated. Papaya (Carica papaya L.) is a popular and economically important fruit of tropical and subtropical countries. It can be consumed fresh, dried, as juice and as other processed products. Papaya has been reported to exhibit antioxidant activity because it contains high levels of phenolic compounds and carotenoids.^6,7 Tomato is one of the most widely used and versatile vegetable crops. They are consumed fresh and are also used to manufacture a wide range of processed products.⁸ Tomatoes and tomato products are rich in health-related food components as they are good sources of carotenoids (in particular, lycopene), ascorbic acid (vitamin C), vitamin E, folate, flavonoids and potassium.^9,10 Drying is an important process for preserving biomaterials in order to extend shelf life, because the drying process inhibits enzymatic degradation and limits microbial growth. Furthermore, drying reduces the weight of raw materials thus reducing the cost of transportation.¹¹ Among many drying techniques, hot-air drying (HA) is the most commonly employed commercial technique for drying vegetables and fruits. Heated air is driven from various directions, depending on the nature of the products being dried.¹² The major disadvantage associated with HA drying is that the long drying time needed causes degradation of food quality¹² and nutritional losses.^13,14 Far-infrared radiation (FIR) has been reported to be successfully applied in the drying of fruit, vegetable and agricultural products, since it can preserve the color and retain bioactive compounds in plant preparations such as potato,¹⁵ onion,¹⁶ apple,¹⁷ rice¹⁸ and mulberry tea.¹⁹ In addition to drying, the osmotic process has received considerable attention as a pre-drying treatment for reducing energy consumption and improving food quality.²⁰ Although dried papaya and tomato products have long been consumed and available in the markets either with or without osmotic treatment, so far, there have been limited published reports on the effects of drying on bioactive compounds and on the antioxidant properties of papaya. Therefore, the main aim of this study was to investigate the effect of two different drying methods, namely FIR-HA and HA drying, on changes in the antioxidant properties and bioactive compounds in untreated and osmotically treated papayas. We expect the results to lead to the establishment of an appropriate method of dried papaya and tomato with respect to bioactive compounds and antioxidant activity.

2. Materials and methods

2.1 Chemicals and reagents

Folin–Ciocalteu reagent; phenolic acids standards, namely gallic, protocatechuic, p-hydroxybenzoic, vanilic, chorogenic, caffeic, syringic, p-coumaric, ferulic and sinapic acids; standards flavonoids such as catechin, rutin, myricetin, quercetin, apigenin and kaempferol; 2,4,6-tripyridyl-S-triazine (TPTZ); 2,2-diphenyl-1-picrylhydrazyl (DPPH); lycopene; beta-carotene; and lutein were purchased from Sigma-Aldrich Co. (St. Louis, MO, USA). Methanol, acetonitrile and other reagents used in the HPLC analysis were purchased from Merck (Darmstadt, Germany). All other solvents were purchased from Fisher Scientific (Leicester, UK) and were of analytical grade.

2.2 Sample preparation

Samples of papaya (Carica papaya L.), cultivar Khaek Dam and tomato (plum tomato) were purchased from a local market in Maha Sarakham Province, Thailand. At each market, approximately 2 kg of samples were sampled from three representative outlets. Single composite samples for each representative market were prepared by combining about 500 g of sample. The ripe fresh papaya samples were peeled manually, and the seeds removed before processing. Fresh plum tomatoes were cleaned. Then, all samples were cut into cubes of 1.5 cm³ and divided into two groups. The first was pretreated by soaking in 60% sucrose as an osmotic agent (see below) prior to being dried, while the latter was directly dried by FIR-HA and HA methods without pretreatment. The samples were stored in a refrigerator (4 ± 1 °C) before use.

2.3 Osmotic dehydration

Sucrose (food grade) dissolved in distilled water was used as the osmotic agent. The sucrose concentrations used were 40%, 50% and 60% (w/w), containing appropriate amounts of 0.1 M calcium chloride and 0.1 M lactic acid. These salt and acid concentrations were selected in previous tests of 30 min of osmotic dehydration. The sample cubes, previously weighed and identified, were placed into 250 mL beakers containing the osmotic solution. A fruit–solution ratio of 1 : 10 was used. The samples were immersed for 24 h in each of the following of sucrose solutions in succession: 40%, 50% and 60%. After 72 h of dehydration in sucrose solutions, the samples pieces were drained, rinsed with distilled water and placed on absorbent paper to remove excess solution. Afterwards, the papaya pieces were dried with HA and FIR-HA.

2.4 Drying processes

2.4.1 Hot air drying. Hot air (HA) drying was done using a laboratory-scale dryer. The sample tray (25.4 × 37 cm²) was placed midway between and parallel to the top and bottom heaters, and the distance between each set of heaters and the tray was fixed at 15 cm. The sample tray was supported on a balance, which enabled continuous recording of the mass the product throughout the test.¹⁹ The drying temperature was set at 60 °C, and the air velocity at 1.5 m s⁻¹ for 18 h (untreated) and for 32 h (osmotic treated) to achieve a moisture content of 17% on a dry basis. The moisture content of samples was determined according to the AOAC method in a vacuum oven (Shellab, model 1410) at 103 ± 1 °C, and the dry weight of samples was calculated from the % moisture.²¹

2.4.2 Combined far-infrared radiation and air convection (FIR-HA) drying. The laboratory-scale dryer used in this study was developed in the Research Unit of Drying Technology for Agricultural Products, Faculty of Engineering, Mahasarakham University, Thailand. We used the FIR drying method of Wanyo et al.¹⁹ Briefly, the papaya and tomato samples were placed onto a mesh tray and irradiated with a combination of far-infrared radiation with hot air convection at FIR intensities of 5 kW m⁻², a HA temperature of 40 °C, a HA velocity of 1 m s⁻¹ and a drying time of 4 h to achieve the moisture content of 17% on a dry basis.

2.5 Sample extraction

The sample extraction for determination of total phenolic content, total flavonoid content and antioxidant activity was performed using a method described previously.⁵ Fresh and dried samples (1 g, on a dry weight basis) were extracted three times with 10 mL of 80% methanol at room temperature for 2 h on an orbital shaker at 180 rpm. Then, the mixture was centrifuged at 1400 × g for 20 min, and the supernatant was transferred to a 30 mL of vial and stored at −20 °C until analysis.

2.6 Determination of total phenolic content

Total phenolic content (TPC) was determined using a Folin–Ciocalteu reagent as described by Kubola and Siriamornpun²² and as adapted from Velioglu et al.²³ Briefly, 300 μL of the extract was mixed with 2.25 mL of Folin–Ciocalteu reagent (previously diluted 10-fold with distilled water) and allowed to stand at room temperature for 5 min; 2.25 mL of sodium carbonate (60 g L⁻¹) solution was added to the mixture. After 90 min at room temperature, absorbance was read at 725 nm using a spectrophotometer. The TPC in samples was calculated based on the linear regression equation of the gallic acid standard curve (y = 0.002x + 0.008; R² = 0.998). Results were expressed as mg gallic acid equivalents per g of dry weight (mg GAE g⁻¹ DW).

2.7 Determination of total flavonoid content

Total flavonoid content (TFC) was determined using the colorimetric method described by Bakar et al.⁵ and as adapted from Dewanto et al.²⁴ Briefly, 0.5 mL of the extract was mixed with 2.25 mL of distilled water in a test tube followed by the addition of 0.15 mL of 5% NaNO₂ solution. After 6 min, 0.3 mL of a 10% AlCl₃·6H₂O solution was added and allowed to stand for another 5 min before 1.0 mL of 1 M NaOH was added. The mixture was mixed well by vortexing. The absorbance was measured immediately at 510 nm using a spectrophotometer. The TFC in samples was calculated using the linear regression equation of the rutin standard curve (y = 0.001x; R² = 0.999) and expressed as mg rutin equivalents per g dry weight (mg RE/g DW).

2.8 Determination of antioxidant activity

2.8.1 DPPH˙ scavenging activity. The antioxidant activity of each sample was measured in terms of radical scavenging ability or hydrogen donating ability using the DPPH method.²⁵ The sample was diluted in methanol, and then 0.1 mL of diluted sample was added to 3 mL of 0.1 mM DPPH solution dissolved in methanol. The mixture was shaken and placed in the dark at room temperature for 30 min. The absorbance of the resulting solution was measured at 517 nm using a spectrophotometer against a control. DPPH˙ scavenging activity was calculated using the following equation:

DPPH˙ scavenging activity (%) = [1 − (A_(sample) − A_(control))] × 100

2.8.2 Ferric reducing antioxidant power (FRAP). The FRAP assay is based on the reduction of Fe³⁺–TPTZ to a blue colored Fe²⁺–TPTZ using the method of Benzie and Strain with slight modification.²⁶ The antioxidant potential of the extract was determined against a standard curve for ferrous sulphate (Fe(II), 0, 0.5, 1.0, 1.5, 2.0, 2.5 and 3.0 mM) in distilled with 0.1% (v/v) HCl. The FRAP reagent was freshly prepared by mixing 100 mL of 300 mM acetate buffer (pH 3.6), 10 mL of 10 mM TPTZ solution in 40 mM HCl, 10 mL of 20 mM FeCl₃ at a ratio of 10 : 1 : 1 (v/v/v) and 12 mL distilled water, at 37 °C. To perform the assay, 1.8 mL of FRAP reagent, 180 μL of distilled water and 60 μL of sample were added to the same test tube and then incubated at 37 °C for 4 min. The absorbance of the mixture was read at 593 nm, using the FRAP working solution as a blank. Data were calculated according to the following linear regression equation of the FeSO₄ standard curve (y = 0.874x + 0.092; R² = 0.995) and then expressed as μmol Fe(II) per g dry weight (μmol Fe(II)/g DW).

2.9 Determination of phenolic compounds by HPLC

2.9.1 Phenolic compounds extraction. The phenolic compounds in samples were extracted using the method described previously by Uzelac et al.²⁷ A sample (5 g) was mixed with 50 mL methanol–HCl (100 : 1, v/v), which contained 2% tert-butyl hydroquinone, in an inert atmosphere (N₂) over 12 h at 35 °C in the dark. After that, the extract was centrifuged at 1400 × g, and the supernatant was evaporated to dryness using a rotary evaporator under vacuum at 40 °C. The residue was redissolved in 25 mL of water–ethanol (80 : 20, v/v) and extracted three times with 25 mL of ethyl acetate. The organic fractions were combined, dried for 30–40 min with anhydrous sodium sulphate, filtered through a Whatman-40 filter, and evaporated to dryness as described earlier. The residue was redissolved in 5 mL of methanol–water (50 : 50, v/v) and filtered through a 0.45 μm filter before injection (20 μL) into the HPLC instrument.

2.9.2 Analysis of phenolic acids and flavonoids using RP-HPLC. The content and composition of phenolic acids and flavonoids were determined using RP-HPLC as described previously.²⁸ The RP-HPLC instrument consisted of Shimadzu LC-20AC pumps, an SPD-M20A diode array detection (DAD) and an Inetsil ODS-3, C18 (4.6 mm × 250 mm, 5 μm) column (Hichrom Limited, Berks, UK). The mobile phase consisted of 1% acetic acid in water (solvent A) and acetonitrile (solvent B) at a flow rate of 0.8 mL min⁻¹. Gradient elution was performed as follows: from 0 to 5 min, linear gradient from 5% to 9% solvent B; from 5 to 15 min, 9% solvent B; from 15 to 22 min, linear gradient from 9% to 11% solvent B; from 22 to 38 min, linear gradient from 11% to 18% solvent B; from 38 to 43 min, linear gradient from 18% to 23% solvent B; from 43 to 44 min, linear gradient from 23 to 90% solvent B; from 44 to 45 min, linear gradient from 90 to 80% solvent B; from 45 to 55 min, isocratic at 80% solvent B; from 55 to 60 min, linear gradient from 80% to 5% solvent B and a re-equilibration period of 5 min with 5% solvent B used between individual runs. The operating conditions were as follows: column temperature, 38 °C, injection volume, 20 μL and UV-diode array detection at 280 nm for phenolic acids and at 370 nm for flavonoids. Phenolic acids and flavonoids in the samples were identified by comparing their relative retention times and UV spectra with those of authentic compounds and were detected using an external standard method.

2.10 Extraction and determination of carotenoids

Carotenoid (lycopene, beta-carotene and lutein) contents in samples were extracted and quantified according to a method described previously.^29,30 For extraction, each dried sample (5 g) was extracted three times with 50 mL of methanol and stored at room temperature and evaporated under reduced pressure at 25 °C. The contents of lycopene, beta-carotene and lutein were determined using RP-HPLC (LC-20AC, Shimadzu, Japan), SPD-M20A diode array detection and chromatographic separation on an Inetsil ODS-3, C18 column (4.6 mm × 250 mm, 5 μm, Hichrom Limited, Berks, UK). The mobile phase used was acetonitrile–dichlorometane–methanol (70 : 20 : 10) at a flow rate of 1.3 mL min⁻¹, and the isocratic elution conditions were described previously by Siriamornpun et al.³⁰ The operating conditions were as follows: column temperature 40 °C, injection volume 20 μL and UV-diode array detection at 454 nm. The carotenoid content in the samples was calculated using the linear equation obtained from a calibration curve of the external standard.

2.11 Statistical analysis

All experiments were performed in triplicate, and the results were expressed as mean ± standard deviation (SD). Analysis of variance (ANOVA) was carried out to determine any significant differences in measurements using the SPSS statistical software (SPSS 11.5 for Windows; SPSS Inc., Chicago, IL, USA) and considering the confidence level of 95%. The significance of the difference between the means was determined using the Duncan test, and the differences were considered to be significant at p < 0.05.

3. Results and discussion

We investigated the effects of pretreatment with and without the osmotic process followed by drying with two different methods, hot air (HA) and combined far-infrared radiation and air convection drying (FIR-HA), on the retention of bioactive compounds in papaya and tomato. Five treatments of the two fruits were studied, and the details with abbreviations are provided in Table 1.

Table 1 Description of samples

Sample codes	Description of treatments
Fresh	Fresh papaya (half ripen, green and yellow (peel), orange (pulp), 11–12 °Brix, 150–180 days after blooming), fresh ripe tomato (ripe, red colour (peel) pink (pulp), 7–8 °Brix, 35–45 days after blooming)
U-FIR-HA	Untreated and dried with FIR-HA
U-HA	Untreated and dried with HA
OT	Osmotic treated
OT-FIR-HA	Osmotic treated and dried with FIR-HA
OT-HA	Osmotic treated and dried with HA

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3.1 Effect of drying methods and osmotic treatments on TPC, TFC and antioxidant activity

The TPC of samples after use of these different methods ranged from 63 to 551 μg GAE g^–1 DW in papaya and 43 to 341 μg GAE g^–1 DW in tomato. The highest value of TPC was found in U-FIR-HA, followed by U-HA and fresh papaya (FP), while OT-HA contained the lowest TPC compared to other samples for both papaya and tomato. Similar trends were found for FRAP and DPPH, and the results showed U-FIR-HA resulted in the highest values compared to other treatments and those in fresh samples. Unlike others, TFC was found to be highest in fresh sample for papaya and decreased after processing (Table 2). The level of the TFC in tomato varied significantly between 7 μg RE g^–1 DW in OT-HA and 36 μg RE g^–1 DW in U-FIR-HA. It was observed that the osmotically treated samples contained significantly (p < 0.05) lower contents of phenolic compounds and antioxidant activities than did the samples without osmotic treatment; of these, osmotically treated and HA dried papaya and tomato had the lowest values for all parameters tested. Our findings were in agreement with those of previous work by Bchir et al. who reported that the total phenolic content and antioxidant activity of pomegranate seeds were significantly decreased during the osmotic and osmotic-drying processes.³¹ Our results indicated that osmotic treatment may involve the degradation or decomposition of some phenolic compounds depending on their chemical structures. Degradation of certain bioactive compounds in fruit tissues might lead to a decrease in the biological activity of the dried products. As during osmotic treatment, a cell placed in a hypertonic solution, which possesses a higher osmotic pressure than that of the cell, causes the loss of water within the cell and could provoke changes in the biochemical properties of the fruits.³² Additionally, a previous study has reported that losses of phenolic compounds during the osmotic process could partially happen from enzymatic oxidation of polyphenoloxidase (PPO).³³ Previous works showed that dehydration or drying of plants stimulates changes in chemical compositions, bioactive compounds and functional properties as well as physical characteristics.^19,22,30,34 In addition, the rehydration process is also important for the evaluation of sensory properties.³⁵ The difference in rehydration characteristics could be caused by differences in surface hardening, the degree of structural damage, and cell shrinkage induced by dehydration.^36–38 The rates of rehydration of dehydrated materials using rotating tray drying were highest with the values of the rehydration ratio (RR) ranging from 3.7–4.8,³⁹ followed by hot-air drying (RR < 4.5)⁴⁰ and sun drying (RR 2.7–3.2).⁴¹ In our present study, it was observed that the samples dried using FIR maintained a higher rehydration capacity than HA dried materials (data not shown). For FIR, the rehydration ratio was decreased as the FIR intensity was increased.⁴²

Table 2 Changes of TPC, TFC, FRAP and DPPH in samples as affected by different treatments^a

Samples	TPC (μg GAE g^–1 DW)	TFC (μg RE g^–1 DW)	FRAP (μmol FeSO4 g^–1 DW)	DPPH (% inhibition)
a Results are expressed as mean ± SD (n = 3). Values with different letters in the same column represent significant differences at p < 0.05, TPC, total phenolic content; TFC, total flavonoid content; FRAP, ferric reducing antioxidant power and DPPH, 2,2-difenyl-1-picrylhydrazyl radical scavenging activity.
Papaya
Fresh	443.23 ± 24.32^c	92.15 ± 2.00^a	190 ± 4.08^b	42.51 ± 0.61^b
U-FIR-HA	551.21 ± 10.31^a	76.21 ± 3.34^b	230 ± 10.11^a	47.21 ± 2.25^a
U-HA	512.91 ± 20.62^b	57.91 ± 1.82^c	180 ± 4.21^c	42.55 ± 1.52^b
OT	94.42 ± 5.21^e	52.35 ± 2.3^d	110 ± 6.78^e	22.79 ± 0.15^d
OT-FIR-HA	122.32 ± 12.11^d	49.44 ± 0.64^e	140 ± 6.88^d	26.73 ± 0.52^c
OT-HA	63.22 ± 9.12^f	47.41 ± 0.59^f	90 ± 9.98^f	22.11 ± 1.76^d
Tomato
Fresh	231.14 ± 4.04^b	15.75 ± 0.36^d	290 ± 4.08^c	52.54 ± 2.15^c
U-FIR-HA	341.34 ± 10.23^a	35.72 ± 2.11^a	350 ± 12.36^a	62.91 ± 2.06^a
U-HA	330.11 ± 10.80^a	33.36 ± 4.90^b	302 ± 1.12^b	57.45 ± 2.12^b
OT	54.56 ± 3.11^d	10.32 ± 1.3^e	130 ± 6.78^e	32.79 ± 0.11^e
OT-FIR-HA	62.34 ± 7.01^c	20.44 ± 0.64^c	160 ± 6.88^d	36.73 ± 0.52^d
OT-HA	43.32 ± 2.19^e	7.41 ± 0.59^f	110 ± 9.98^f	25.11 ± 1.76^f

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In the case of HA, with longer drying times, HA drying causes damage to sensory characteristics and nutritional properties of foods, oxidation of pigments, destruction of vitamins, and solute migration from the interior of the food to the surface.⁴³ Apart from losses of phenolic compounds, degradation of vitamin C (ascorbic acid) should be considered with respect to decreases in antioxidant activities as reported by Demarchi et al. who studied apple leather.⁴⁴ Demarchi et al. suggested that less-severe drying technology should be studied to replace HA drying, because the functional compounds in the dried products may not be preserved by this method.⁴⁴ Conversely, an increase in antioxidant activities after FIR drying may be explained by the fact that FIR creates internal heating with molecular vibrations of materials; thus, it may break down complex covalent molecular structures and release some antioxidant compounds such as flavonoids, carotene, lycopene, tannin, ascorbate, flavoprotein or polyphenols from repeating polymers, hence increasing antioxidant activities.^30,45 Many antioxidant phenolic compounds in plants are most frequently present in a covalently bonded form with insoluble polymers.⁴⁵ FIR treatment could liberate and activate low-molecular-weight natural antioxidants in plants if this bonding is weak.⁴⁶ Previous studies found that antioxidant activities and total phenolic contents increased after exposure of rice hulls,⁴⁶ peanut hull⁴⁷ and mulberry tea to FIR radiation.¹⁹ Since a cell is placed in a hypertonic solution during the osmotic process and osmotic dehydration, it will lose water, and this may lead to decreases in phenolic compounds and in a subsequent antioxidant activity.⁴⁸ Nunez-Mancilla et al. reported that total antioxidant activity was decreased in all osmotically treated strawberries compared with fresh samples.⁴⁹ This is also supported by a previous study in which anthocyanin content and antioxidant activity decreased in osmo-dehydrated dried blueberries.⁵⁰ According our results (Table 2) in this study, the TPC seemed to be responsible for the antioxidant activities assessed by FRAP and DPPH assays as antioxidant activities increased with an increase in TPC for both papaya and tomato.

3.2 Effect of drying methods and osmotic treatments on phenolic acids

The phenolic acid composition and content in papaya and tomato were detected and quantified using HPLC-DAD and are shown in Table 3. According to our available 10 authentic standards, namely gallic acid, protocatechuic acid, p-hydroxybenzoic acid, chlorogenic acid, vanillic acid, caffeic acid, syringic acid, p-coumaric acid, ferulic acid and sinapic acid, it was possible to identify five phenolic acids, namely chlorogenic acid, caffeic acid, p-coumaric acid, ferulic acid and sinapic acid, in fresh papaya and all untreated dried papaya and tomato samples. On the other hand, p-coumaric acid, caffeic acid and chlorogenic acid had disappeared from all osmotically treated samples. Nevertheless, the levels of ferulic acid and sinapic acid could be preserved by osmotic treatment, which did not produce any significant difference (p < 0.05) compared to the two fresh samples. The results showed that p-coumaric acid, caffeic acid and chlorogenic acid all increased as a result of FIR-HA in the untreated samples, while these compounds were not detected in any of the osmotically treated papayas and tomatoes. We observed that caffeic acid was found in tomato samples dried with U-FIR-HA and U-HA, while this compound was not detected in fresh and osmotically treated tomato. U-HA drying also caused a significant increase in the level of chlorogenic acid compared to that in fresh ripe papaya and tomato. It could be said that caffeic acid, p-coumaric acid and chlorogenic could be enhanced by heat treatment. Changes of individual phenolic acid levels, as affected by different drying processes, have been reported in mulberry leaf tea¹⁹ and marigold flower.³⁰ However, phenolic acids may differ in regards to chemical structures, including their linkages or bindings. Therefore, the responses to various processes may be different. For example, there were greater amounts of all phenolic acids in mulberry leaf dried by HA and FIR compared to fresh samples. Of those, nine out of eleven phenolic acids were found to be higher in FIR dried samples, and only chlorogenic and syringic were found to be higher in HA dried mulberry leaf.¹⁹ For marigold flowers, FIR and HA drying were shown to enhance the release of phenolic acids but freeze drying did not.³⁰ Thermal processing disrupts the cell wall of fruits and vegetables, resulting in the release of oxidative and hydrolytic enzymes such as PPO that can damage some antioxidants especially phenolic compounds.^51,52 However, thermal processing can break down the cellular constituents, thus releasing more bound and small molecules of phenolic acids.⁵¹

Table 3 Concentration of phenolic acids in samples as affected by different treatment^a

Samples	Phenolic acids (mg 100 g^–1 DW)
	Chlorogenic acid	Caffeic acid	p-Coumaric acid	Ferulic acid	Sinapic acid
a Results are expressed as mean ± SD (n = 3). Values with different letters in the same column represent significant differences at p < 0.05, nd: not detected.
Papaya
Fresh	2.19 ± 0.07^b	2.59 ± 0.01^c	3.16 ± 0.05^b	65.34 ± 4.11^a	15.44 ± 2.90^a
U-FIR-HA	3.03 ± 0.31^a	2.63 ± 0.01^a	5.68 ± 0.09^a	33.53 ± 1.79^b	3.75 ± 0.08^c
U-HA	3.18 ± 0.14^a	2.60 ± 0.01^b	2.33 ± 0.01^c	28.92 ± 2.65^d	3.64 ± 0.41^c
OT	nd	nd	nd	64.56 ± 3.28^a	15.83 ± 1.72^a
OT-FIR-HA	nd	nd	nd	31.87 ± 1.23^c	14.12 ± 1.18^b
OT-HA	nd	nd	nd	35.23 ± 1.13^b	14.92 ± 1.34^b
Tomato
Fresh	3.35 ± 0.11^b	nd	2.50 ± 0.13^b	63.44 ± 2.46^a	16.50 ± 1.73^b
U-FIR-HA	13.53 ± 1.65^a	3.52 ± 0.07^a	3.02 ± 0.12^a	61.36 ± 1.29^a	31.84 ± 1.36^a
U-HA	14.59 ± 2.09^a	2.65 ± 0.03^b	3.03 ± 0.05^a	34.38 ± 3.01^c	16.53 ± 1.91^b
OT	nd	nd	nd	62.93 ± 2.94^a	15.91 ± 1.21^b
OT-FIR-HA	nd	nd	nd	41.65 ± 2.11^b	15.12 ± 1.04^b
OT-HA	nd	nd	nd	34.19 ± 1.61^c	14.87 ± 1.12^b

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According to the literature, changes in phenolic acids as a result of osmotic treatment have not been previously reported. Rózek et al. demonstrated that the contents of phenolic compounds such as gallic acid, protocatechuic acid and catechin in grape seed extract were significantly lost upon osmotic and osmotic-air drying treatments.⁵³ Although most phenolic acids were destroyed by osmotic treatment, ferulic acid and especially sinapic acid could even be preserved by osmotic treatment, as these compounds were not significantly altered (p < 0.05) from the respective levels for fresh or dried samples. Although the five phenolic acids identified in the samples are hydroxybenzoic acids, the difference between ferulic and sinapic acids on the one hand, and the remainder on the other hand is the presence of a methoxyl group as indicated in Fig. 1. Sinapic acid contains two methoxyl groups, and ferulic has one, while the others do not. The plausible explanation for how these two phenolic acids could be preserved by osmotic treatment may involve the linkages or bindings of the osmotic solution (sucrose) and the methoxyl groups or may be caused by the hydrophobicity of methoxyl groups against water solubility. However, this must be studied further.

Fig. 1 Chemical structures of standard phenolic acids.

3.3 Effect of drying methods and osmotic treatments on flavonoid compounds

The flavonoid content of papaya and tomato with different drying methods and osmotic treatments were quantified by comparing their HPLC-DAD retention times with available authentic standards, namely those for rutin, myricetin, quercetin, apigenin and kaempferol. The flavonoid contents of the evaluated samples are presented in Table 4. It was possible to identify all flavonoids in both fresh samples, except for apigenin, which was not detected in fresh tomato. The results showed that rutin, quercetin and kaempferrol were the most predominant flavonoids in all samples. It was found that U-FIR-HA dried tomato had the remarkably significantly highest content of rutin and quercetin with values of 621 and 263 μg g⁻¹ DW, respectively. On the other hand, OT-HA dried papaya contained the highest rutin content compared to other treated samples including fresh papaya. Myricetin was found to be highest in fresh and untreated dried papayas, while this compound was not detected in osmotically treated and dried papayas. This may be caused by a higher number (six) of hydroxyl groups in the molecular structure compared with other flavonoids, leading to greater water solubility of myricetin in fresh and untreated dried papayas than in osmotically treated samples. Apigenin was increased in dried untreated osmotic samples (U-FIR-HA, U-HA), while this compound was not detected in all the osmotically treated papayas and tomatoes except for OT-FIR dried papaya. In our present study, it was observed that kaempferol was the most stable flavonoid during processing of these two fruits. Thermal processing can have positive and negative effects on phenolic compound contents and antioxidant activity. For example, the cell walls of fruits and vegetables were disrupted by thermal processing, resulting in the release of oxidative and hydrolytic enzymes⁵¹ such as PPO (polyphenoloxidase) that can damage some antioxidants, especially phenolic compounds.⁵² On the other hand, thermal processing can break down the cellular constituents, thus releasing more bound and small molecules of phenolic acids, resulting in an increase of active molecules and consequently antioxidant activities.⁵¹ Unlike the trends for phenolic acids in Table 3, different trends were observed for flavonoids in papaya and tomato samples as affected by treatments. Therefore, apart from treatments or processing methods, the retention of flavonoids or other bioactive compounds may also be dependent on the nature of the plant matrix and chemistry of bioactive compounds.

Table 4 Concentration of flavonoid compounds in samples as affected by different treatments^a

Samples	Flavonoid compounds (μg g^−1 DW)
	Rutin	Myricetin	Quercetin	Apigenin	Keampferol
a Results are expressed as mean ± SD (n = 3). Values with different letters in the same column represent significant differences at p < 0.05, nd: not detected.
Papaya
Fresh	5.0 ± 0.01^b	19.72 ± 0.04^b	26.46 ± 0.05^a	5.14 ± 0.11^d	12.44 ± 1.00^a
U-FIR-HA	4.2 ± 0.20^c	23.96 ± 0.30^a	12.75 ± 0.20^c	12.3 ± 0.19^b	10.32 ± 0.90^b
U-HA	3.18 ± 0.14^d	12.40 ± 0.01^c	8.33 ± 0.01^d	28.92 ± 2.05^a	10.64 ± 0.41^b
OT	4.48 ± 0.30^c	nd	21.35 ± 2.40^b	nd	12.01 ± 1.02^a
OT-FIR-HA	4.31 ± 0.21^c	nd	12.05 ± 0.67^c	7.32 ± 0.60^c	10.26 ± 1.18^a
OT-HA	7.24 ± 0.63^a	nd	22.33 ± 2.96^b	nd	12.01 ± 1.34^a
Tomato
Fresh	97.61 ± 5.21^c	21.25 ± 4.00^a	12.94 ± 0.13^e	nd	12.65 ± 1.22^a
U-FIR-HA	620.61 ± 12.40^a	nd	262.99 ± 10.38^a	10.80 ± 1.03^b	11.56 ± 1.22^a
U-HA	12.94 ± 5.10^f	nd	81.79 ± 9.40^b	34.38 ± 3.01^a	11.79 ± 1.01^a
OT	28.52 ± 2.08^e	nd	14.21 ± 1.93^e	nd	10.18 ± 1.21^b
OT-FIR-HA	69.02 ± 3.65^d	nd	30.21 ± 4.02^d	nd	11.39 ± 1.00^b
OT-HA	121.75 ± 9.32^b	20.29 ± 2.10^a	52.97 ± 5.22^c	nd	10.61 ± 1.12^b

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3.4 Effect of drying methods and osmotic treatments on carotenoid content

Changes in the carotenoid content of samples after treatment are shown in Table 5. Among the different drying methods, HA drying was found to provide the highest content of lycopene (507 μg g⁻¹ DW) in tomato, whereas FIR-HA drying gave the highest value (256 μg g⁻¹ DW) in papaya. The highest lutein content was found in U-FIR-HA dried samples, followed by U-HA dried and fresh samples for both papaya and tomato. Beta-carotene contents were decreased in all treated and dried samples. Obviously, all osmotically treated samples, including those with and without drying, had comparatively low concentrations of all carotenoids tested. Our previous studies reported on changes in lutein, lycopene and beta-carotene in marigold flower resulting from different drying methods, namely freeze drying, HA drying and FIR drying. We found that all carotenoids tested were enhanced by all means of drying.³⁰ Lutein was found to be highest in freeze dried and FIR dried samples, while beta-carotene and lycopene contents were highest in FIR and HA dried marigold petals. In contrast, HA drying gave the highest lycopene content in gac arils among the three drying methods used, namely HA drying, FIR drying and low relative humidity air drying (LRH).³⁴ In addition, they found that the beta-carotene content was reduced by all means of drying, with the greatest loss being due to FIR drying.³⁴ Accordingly, it is obvious that individual carotenoids react differently in their susceptibility to heat and other treatments. It has been reported that lycopene is relatively stable during thermal processes.⁵⁴ On the other hand, beta-carotene seemed to be sensitive to thermal processes, as demonstrated by the results of our present study, and a non-thermal process such as freeze drying, as reported by Kubola et al.³⁴

Table 5 The contents of lycopene, beta-carotene and lutein in fresh and treated samples^a

Samples	Carotenoid contents (μg g^−1 DW)
	Lycopene	Beta-carotene	Lutein
a Results are expressed as mean ± SD (n = 3). Values with different letters in the same column represent significant differences at p < 0.05.
Papaya
Fresh	126.16 ± 2.01^c	9.36 ± 0.65^a	14.4 ± 0.41^c
U-FIR-HA	256.13 ± 1.87^a	5.45 ± 0.35^c	37.11 ± 3.24^a
U-HA	208.30 ± 1.55^b	6.80 ± 0.48^b	18.90 ± 2.08^b
OT	39.11 ± 2.88^f	4.59 ± 0.22^d	7.4 ± 0.41^f
OT-FIR-HA	49.38 ± 1.02^d	4.73 ± 0.19^d	13.11 ± 0.29^d
OT-HA	46.81 ± 0.86^e	3.87 ± 0.19^e	10.17 ± 1.03^e
Tomato
Fresh	301.11 ± 1.42^c	54.4 ± 0.14^a	41.35 ± 0.07^c
U-FIR-HA	435.55 ± 1.58^b	38.5 ± 0.20^b	100.71 ± 1.91^a
U-HA	506.60 ± 8.74^a	19.2 ± 0.15^c	52.2 ± 0.09^b
OT	63.11 ± 3.63^f	10.44 ± 0.22^d	22.4 ± 0.41^d
OT-FIR-HA	70.38 ± 1.45^e	7.18 ± 0.19^e	17.11 ± 0.29^e
OT-HA	80.81 ± 4.56^d	5.72 ± 0.19^f	13.17 ± 1.03^f

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4. Conclusion

Drying and osmotic processes have varying effects on the contents of bioactive compounds, including phenolics, flavonoids and carotenoids, leading to the degradation of phytochemicals, and thereby affecting the total antioxidant activity of papaya and tomato. In addition to treatments or processing methods, we also found that the retention of bioactive compounds may also be dependent on the nature of the plant matrix and chemistry of bioactive compounds. Interestingly, ferulic acid, sinapic acid and kaempferol contents in both papaya and tomato during osmotic treatments were similar to or even higher than those after all treatments tested, whereas the amounts of other compounds were significantly decreased, indicating that the osmotic process can protect these compounds from degradation during further drying. The drying process using FIR enhanced the content of some bioactive compounds such as phenolic compounds along with antioxidant properties. According to our present results, we suggest that FIR drying should be considered as a good drying method for papaya and tomato based on the consideration of preserving bioactive compounds and antioxidant properties. However, combination with an appropriate process or pretreatment is needed for food manufacturing with respect to maintaining not only bioactive compounds but also sensory properties.

Acknowledgements

This research was funded by the Office of the Higher Education Commission and Mahasarakham University Development Fund. We also wish to thank Dr Colin Wrigley, Adjunct Professor at University of Queensland, Australia, for language revision. The authors also wish to thank the laboratory equipment center of Mahasarakham University for providing access to the HPLC instrument.

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