Microwave drying vs hot-steam-air drying of seedless noni fruit: drying performance and post-drying quality

Noni fruit contains several beneficial compounds for health, including scopoletin as its active component. The fruits contain a high moisture content, generally above 80%, rendering them to rot quickly. The drying process can potentially increase the fruit’s shelf life as it reduces the moisture content, stop the enzymatic reactions/microorganism activities and inhibit the quality degradation. This study compared the drying performance and post-process quality of noni fruit slices from the microwave and hot-steam-air drying at 50°C and 60°C. The drying performance parameters were time and rate. Several post-drying quality parameters stipulated in Indonesia pharmacopoea standard, such as scopoletin, total ash, and insoluble-acid ash contents, were observed. The results show the drying rate of noni fruit ranged from -3.51 x 10−3 % min−1 (for microwave-drying) to -2.45 x 10-5 % min−1 (for hot-steam-air drying at 50°C). Noni fruits from hot-steam-air drying at 60°C had the highest scopoletin content (22.47 mg kg−1) and the lowest total ash content (4.78%). Microwave-dried noni fruit had the lowest insoluble-acid ash content (0.02%). Drying methods significantly affected the drying rate, total ash content, and insoluble-acid ash content of noni fruit. Treatment scoring shows microwave- and hot-steam-air drying at 60°C are potential options for drying noni fruit.


Introduction
There are approximately 30,000 plant species growing in Indonesia and 940 of them are potential as herbal medicines [1].One of the herbal plants is noni fruit (Morinda citrifolia L).This fruit grows not only in Indonesia but also in India, Malaysia, the Hawaiian and Tahitian islands [2,3].Almost all parts of noni plants, such as leaves, fruit, seed, flowers, skin, and roots, are efficacious.Those parts can be used to reduce high blood pressure, anti-tumor, anti-cancer, anti-inflammatory and anti-bacterial [2].Other pharmacological activities of noni fruit, seed, leaf, and root include antimicrobial, antiseptic, antifungal, antioxidant, anti-arthritic, anti-diabetic, wound healing activity, memory-enhancer activity, anxiolytic and sedative activity, analgesic activity, gastric ulcer healing activity, antiemetic, gout and hyperuricemia healing activity, anti-psoriasis, immunity enhancer activity, antiviral, anti-parasitic, antituberculosis, osteoporotic and otoscopic enhance activity [4].
Noni fruit is considered the most beneficial part of this herbal species due to its enormous contents of flavonoid, alkaloid, saponin, and triterpenoid compounds [5].Its active component, scopoletin, is known to have a powerful antioxidant activity which can prevent cell oxidation and deterioration [6].This coumarin compound also exhibits potential anticancer properties [7].1309 (2024) 012004 IOP Publishing doi:10.1088/1755-1315/1309/1/012004 2 Nevertheless, noni fruits contain a high moisture content, reaching up to 86% [8].Due to its high moisture content, noni fruits have a short shelf life and rot quickly.Fruits with high moisture content do tend to soften quickly due to the fast fermentation process [9].On the other hand, consuming noni fruit in liquid juice form is often limited by its unpleasant odors [10].
To increase the shelf life of noni fruit, the drying process is a potential method to apply.Drying removes some water from a biomaterial by evaporating the water molecules using heat energy.Generally, the advantage of drying is improving the material's durability.Since the drying process reduces the moisture content of noni fruits, the enzymatic reactions and microorganism activities that can cause decay in any herbal plant's parts will stop and further inhibits the herbal quality degradation [11].In addition, dried noni fruit slices also have commercial value and are sought by many customers, especially in South Korea [12,13].Its domestic price at farmers' level in 2018 reached IDR 18.000 per kilogram [13].
Direct sun drying is often opted by the farmers to dry noni fruit slices due to the technique's simplicity and no specific equipment required.However, air drying of any herbal plants in open fields depends on the weather heavily.The technique also puts the herbal plants at risk of getting attacked by other living organisms during the process which could affect the herbal's hygiene and requirement as medicinal ingredients.Adopting artificial drying in a dryer is more promising to obtain dried noni fruit slices in a short period and covered from any living organism's attack.
Several artificial drying techniques can be utilized for drying noni fruit slices.Two methods investigated in this study were hot-steam-air and microwave-drying processes.Hot-steam-air drying uses hot water vapour resulting from the water boiling process to its boiling point.The hot water vapour is further distributed to heat the drying chamber up to the targeted temperature.This technique has been tried for silkworm cocoons with satisfactory results [14].
Microwave drying uses a specific electromagnetic wave operating at the frequency range of 300 MHz (3 x 10 8 Hz) -300 GHz (3 x 10 11 Hz) and a corresponding wavelength range of 1 mm -1000 mm [15].This technology has been used widely for not only the development of science and technology but also for industrial purposes, particularly food industries [16].However, similar to the hot-steam-air drying process, microwave technology for drying herbal plants, such as noni fruit, has not been applied elsewhere.No data is yet available providing information on the effectiveness of both hot-steam-air and microwave drying for noni fruit slices.In addition to that, it is acknowledged that the drying process itself could affect the quality of herbal plants being exposed to.Previous studies on drying of several types of herbal plants, such as sedge grass (Cyperus rotundus L), brown seaweed (Padina sp), sage leaves, thyme, mint, lemon, Dryopteris erythrosora leaves and mulberry leaves (Morus alba) show that each type of plant has differences in sensitivity to temperature, drying time and the drying technique used [17][18][19][20][21].
Considering the aforementioned knowledge gap as above, this study aimed to investigate the drying performance and post-drying quality of noni fruit slices exposed to microwave drying and hot-steam-air drying processes.The post-drying quality examined were scopoletin content, total ash content, and ash content in insoluble acid.Those parameters are recommended in determining the quality parameter of noni fruit as medicinal ingredients and mentioned in Indonesia pharmacopoea standard [22].The standard itself becomes a reference for the Regulation of BPOM Head no 32 year 2019 on The Requirements for Safety and Quality of Traditional Medicines.

Materials and equipment
The primary material used was seedless, semi-ripe noni fruits (figure 1) which were collected from one of the germplasm collections from Sukamulya Experiment Garden, Indonesian Center for Testing Instrument Standard for Spices, Medicinal, and Aromatic Plants (previously Indonesian Research Institute for Spice and Medicinal Crops), Sukabumi, West Java.The noni fruit trees were introduced in 2013 as one of the germplasm plants developed in the location.The types of equipment used for the 1309 (2024) 012004 IOP Publishing doi:10.1088/1755-1315/1309/1/0120043 experiment were 900 watt-microwave oven (home type, Krisbow brand), a hot-steam-air dryer (figure 2), fruit cutting/slicing tools, glass plates (for microwave-drying), aluminum tray (for hot-steam-air drying), laboratory oven (Memmert brand) and digital laboratory scale.

Research procedure 2.2.1. Sample preparation and initial moisture content determination.
All noni fruits were washed with running water and further dried with paper/cloth.Fruits were thinly sliced at 10-mm thick for each cut.Each drying sample used approximately 1000 g of noni fruit slices.For initial moisture content determination, approximately 2 g of noni fruit slices were taken from each drying sample, and oven dried at + 103°C.After 4 hours of oven drying, the fruit slices were reweighed and further returned to the oven.This step was repeated for one more time.After the 2 nd repetition, the fruit slices were returned to the oven, dried for up to 16 hours, and reweighed for the last time.

Drying trials with microwave and hot-steam-air dryer.
This study applied 3 drying treatments, which were: microwave drying in a 900-watt home-type microwave oven (Krisbow brand); hot-steamair drying at 50 °C and 60 °C.The hot-steam-air drying process was carried out in a self-engineered hotsteam-air dryer.For each drying treatment, approximately 3 sample units were used, each consisted of a minimum of 998 g of noni fruit slices (final weight after 2 g was taken for determining the initial moisture content).
The microwave-drying process was carried out at maximum power (900 watts) throughout the whole heating process.At first, the noni fruit slices were microwave-heated for 15 minutes.The heating was stopped and each sample was reweighed and returned to the microwave oven.The drying process, still at maximum power, was then continued.After 10 minutes, the drying process was stopped again, each sample was reweighed and then returned to the microwave oven.This second step was repeated until the color of noni fruit slices turned dark brownish, but its texture was still soft.After this fruit condition was reached, the drying process was continued, stopped every 5 minutes, and each sample was reweighed.This last step was repeated until each sample reached a constant weight for 2 subsequent weighing processes and the fruit texture got dry and easy to break.
The hot-steam-air drying process was carried out at selected temperatures (50 °C and 60 °C).The drying chamber was heated at a selected temperature prior to sample loading.During the drying process, each sample was weighed 2 times daily (morning at 8 am and afternoon at 4 pm).The drying process was carried out until the samples reached a weight range of 100-150 g, the fruit texture got dry and easy to break.

Observation parameters and data analysis.
Two parameter groups were observed, which were: (i) drying performance parameters, consisting of drying rate and drying period; and (ii) post-drying quality parameters, consisting of scopoletin content (the active component of noni fruit), total ash content, and ash content in insoluble-acid.The active component of herbal plants and ash contents are some of the general parameters used to determine the herbal quality according to the Regulation of Head of BPOM No. 32/2019 on The Requirements for Safety and Quality of Traditional Medicines.Scopoletin content was analysed at Qlab of the University of Pancasila in Jakarta.Total ash content and ash content in insoluble acid (in this study, the acid used was HCl) were determined at the testing laboratory of the Center for Standardization and Agro-Industry Services in Bogor.The testing procedures for each quality parameter followed the standard of Indonesia's pharmacopoea for domestic herbal plants [22].
To determine the drying rate of noni fruit slices from each drying method, a specific procedure was followed: (i) the moisture content at t-observation time (Mt) was determined according to equation (1); (ii) all moisture content observed from each sample were plotted in a graph, and their linear equation was extracted; (iii) the intercept from each linear equation was further converted into its logarithm value, and (iv) the drying rate was determined by dividing the log value of each intercept with the drying period of each sample, resulting in a final value with % min -1 as the unit.The importance of steps (ii) and (iii) was to minimize the influence of the sample's initial moisture content variation.

𝑀𝑡 = (
with: Mt : sample moisture content (%) at t (minutes) Wt : sample weight (g) at t (minutes) Wf : final sample weight (g) Wg : green/initial sample weight (g) t : observation time (minutes) All parameter data were tabulated and analyzed descriptively.To assess the effect of drying methods on drying rate and each quality parameter of noni fruit, a 1-way analysis of variance (ANOVA) was conducted by using Minitab 16 software.Tukey-test followed when the ANOVA results showed a significant effect.The experimental design used was determined according to equation (2): with: Yijk : observation value of sample-j from drying method-i µ : mean value αi : effect of drying method-i εi : error of drying method-i i : (1) microwave, (2) hot-steam-air drying at 50°C, and (3) hot-steam-air drying at 60°C j : sample number 1, 2, 3

Drying performance
Table 1 shows the average values of initial moisture content, drying period, and drying rate of noni fruit slices from the 3 observed drying methods.The initial or green moisture contents of noni fruit slices used for this study were in the range of 88.94-89.63%.This result is a bit higher than the average moisture content of seeded-noni fruit (86%) investigated in a past study [8].
The results show that the average drying rate of noni fruit slices exposed to microwave-drying was approximately -3.51 x 10 -3 % min -1 with an average drying time of 89 minutes.These values are higher than those observed for noni fruit slices exposed to hot-steam-air drying at 50℃ or 60℃.Hot-steamair dried noni fruit slices from 60℃ had faster drying rate (-7.52 x 10 -5 % min -1 ) and shorter drying time (6,160 minutes) than those from 50℃ (their drying rate was -2.45 x 10 -5 % min -1 with average drying time of 10,200 minutes).Further statistical tests confirm the significant mean differences between microwave drying and hot-steam-air drying at both 50℃ and 60℃.
The superiority of microwaves in drying an object faster than other drying techniques have also been reported in previous studies although with different objects.Intermittent microwave drying of fresh and isomaltulose-impregnated strawberry was faster and consumed less energy than heated-air drying [23].Microwave drying of hawthorn slice at various power levels (180 watt, 360 watt, and 600 watt) gave shorter drying time and higher drying rate than conventional drying at 60℃.The drying rate also increased as the microwave power increased [24].Similar results were also obtained from a previous study comparing the effectiveness of microwave drying at various power levels (from 200 W to 800 W) and convection drying at various temperatures (from 40℃ to 120℃) for potato peels [25].
Microwave drying works in a different mechanism from any conventional drying process, including the hot-steam-air drying process.In conventional drying, the drying chamber is heated up.The heat produced is further transferred to the biomass inside the chamber through a convection mechanism.On the other hand, the heat transfer in a microwave dryer/oven works through a radiation mechanism directly into the object.Due to the dielectric properties possessed by a biomass, the microwave energy is absorbed and causes friction and collision between the internal polar molecules inside the biomass.As a further result, the heat generates inside the biomass, the internal temperature increases and the drying process starts to initiate from the internal part of the biomass being exposed to microwave heating [26].
To confirm the above assumption and obtain a big picture of how the temperature changes inside the chamber and noni fruits during a microwave heating process, another additional experiment was set up at the same power (900 watts) and heating phases were carried out as the original experiment.A manual thermohygrometer and pin-type cooking thermometer were used to measure the temperature inside the chamber and fruit.The fruit temperatures were measured on 3 points (middle part and 2 side parts of fruit slices piles) and further averaged.
The results show that the internal fruit temperature was higher, at the range of 67-88℃, than the chamber temperature (approximately 40-42℃) throughout the whole microwave heating process (figure 3).This result indicates that the fruits did absorb microwave energy vastly.The drying chamber temperature tended to remain the same with a small change (only 1-2°C) throughout the whole IOP Publishing doi:10.1088/1755-1315/1309/1/0120046 microwave heating process (figure 3).On the other hand, the fruit temperature was higher at the beginning of the drying phase, then decreased slowly as the heating process approached its end.This declining trend of internal fruit temperature is in line with the gradual decrease in fruit weight during the drying process.It indicates the noni fruit's ability to absorb microwave energy decreased as the fruits became dry.

Post-drying quality
Table 2 shows the average values of scopoletin content, total ash content, and ash content in insoluble acid of dried noni fruit slices from microwave and hot-steam-air-drying processes.Noni fruit slices from the hot-steam-air-drying at 60℃ had the highest scopoletin content (22.47 mg kg -1 ).The lowest scopoletin content, 22.23 mg kg -1 , was observed in dried noni fruit slice from the hot-steam-air drying at 50℃.However, further statistical test shows that the scopoletin content of noni fruit exposed to microwave drying and hot-steam-air drying was not significantly different from each other.The scopoletin contents of dried noni fruit slices in this study are below the minimum standard required for their purpose as herbal ingredients.This result is presumably due to the naturally low content of scopoletin in seedless noni fruit from the beginning.Scopoletin, also known as 6-methoxy-7 hydroxycoumarin, is an active component of noni fruit that has antioxidant activity and medicinal properties such as blood dilution.It is one of the natural coumarin compounds found in several types of plants usually consumed for health [27].The coumarin compound has been reported to start to degrade after 60 minutes of heating at 200℃ or higher temperature and remain stable at 100-150℃ or lower temperature [28].
Total ash content shows a mixture of inorganic components or minerals found in a material.It indicates the purity, total mineral content, and cleanliness of the material [29].Table 2 shows the total ash content of noni fruit slices from microwave-drying and hot-steam-air drying at 60℃ have met the Indonesia pharmacopoea standard (lower than 7%).Noni fruit slices from the hot-steam-air-drying at 60℃ had the lowest total ash content (4.78%), followed by microwave-dried noni fruit slices in the 2 nd lowest position.On the other hand, the ash content of noni fruit from the hot-steam-air drying at 50℃ was the highest one among all, and higher than the allowed standard value.This result indicated that 50℃ from hot-steam-air-drying was not powerful to eliminate majority of inorganic components within noni fruit.Further statistical test confirms the significant effect of different drying methods on the total ash content of dried noni fruit slices.The results of this study are in line with claims from several previous studies that drying temperature could affect the ash content of several biomass such as Tanjung fruit and Trichogaster pectoralis fish [30,31].
Acid-insoluble-ash content indicates the content of external contaminants, such as sand from the soil and/or dust, adhering to the biomass being dried.The determination of acid-insoluble ash content is closely related to the mineral content contained in a material, the purity and cleanliness of the material [32].Table 2 shows the acid-insoluble ash contents of dried noni fruit slices from all drying methods were below the maximum standard allowed in the Indonesia pharmacopoea standard (lower than 2%).Microwave-dried noni fruit slices showed the lowest insoluble-acid ash content (0.02%).On the other hand, similar to the case of total ash content above, the hot-steam-air dried noni fruit slices from 50℃ had the highest insoluble-acid ash content (0.13%).This result indicated that microwave energy was more powerful in reducing the mineral content or eliminating any impurities from the noni fruit slice than the conventional heated-air supplied from boiling water.Further statistical test confirms the significant effect of different drying methods on the insoluble-acid ash content of dried noni fruit slices.The result obtained indicates that the heating process with microwave energy is more powerful than hotsteam-air drying in eliminating unwanted external contaminants, such as sand, dirt, and dust adhering to noni fruit slices.

Drying method matrix
In order to obtain recommendations on effective or potential drying technique for noni fruit slices, the 3 compared drying techniques in this study were ranked based on the value for each parameter being assesed.Rank 1 was given for a particular drying technique with the highest drying rate, the highest scopoletin content, the lowest total ash content or insoluble-acid ash content.Rank 3 was given for a particular drying technique with the lowest drying rate, the lowest scopoletin content, the highest total ash content or insoluble-acid ash content.Total scores for each drying technique were further divided by the number of parameters being compared.The ranking results were shown in table 3.
The matrix shows that microwave-drying and hot-steam-air drying at 60 °C share the same average score (1.5).This result indicates that one of the 2 drying techniques can be opted for drying seedless noni fruit slices because the results given will not be much different.Nevertheless, if scopoletin content is considered as the most important parameter for consideration due to its important bioactivities, then hot-steam-air drying is the most suitable option.

Conclusion
The study investigated the drying rate, time, and several quality parameters of noni fruit slices exposed to hot-steam-air and microwave drying.Hot-steam-air drying was carried out at 2 different temperatures, 50℃ and 60℃.The noni fruit quality parameters being assessed were scopoletin, total ash, and insoluble-acid ash content.
The results showed the noni fruit dried faster in a microwave oven (drying rate of -3.51 x 10 -3 % min - 1 and drying period of 89 minutes) than that in a hot-steam-air dryer at either 50℃ or 60℃.On the other hand, the hot-steam-air drying at 60℃ provided better quality as it resulted in the highest scopoletin content (22.47 mg kg -1 ) and the lowest total ash content (4.78%) of dried noni fruit.Nevertheless, statistical tests showed that different drying methods significantly affected drying rate, total ash content and insoluble-acid ash content, but not the scopoletin content of noni fruit.Further scoring of drying methods showed that microwave-drying or hot-steam-air drying at 60℃ can be opted for drying noni fruit slices.

Figure 2 .
Figure 2. Hot-steam-air dryer used for the study (A= drying racks; B= chamber door; C= water storage; D= connecting pipe to distribute water from the storage to water tub (E); E= water tub (behind the chamber wall); F= gas burner; G= exhaust fan).

Figure 3 .
Figure 3. Temperature changes in noni fruit slices and drying chamber throughout the microwave-heating process.

Table 1 .
Drying time (minutes) and rate (% min -1 ) of noni fruit from microwave and hot-steamair drying.

Table 2 .
Scopoletin content, total ash content and ash content in insoluble-acid (HCl) of dried noni fruits from microwave and hot-steam-air drying (average and standar deviation values).

Table 3 .
Drying method and parameter ranking matrix.