Effect of drying methods on the drying time and moisture content of the COR slices with different thickness .... Effect of different drying methods on total phenolic content of the dried
INTRODUCTION
Research rationale
Curculigo orchioides is a medicinal plant commonly grown in the mountainous provinces in the North of Vietnam According to the traditional experiences recorded in the book “Vietnamese medicinal plants and remedies” [1] Curculigo orchioides (COR), also known as Sam Cau and Tien Mao, is a kidney tonic, a physiological enhancer, which can be used as a decoction to drink or soak in wine Additionally, it also has a number of other benificial effects such as: anti-inflammatory activity, antiasthmatic activity, hepatoprotective activity, anti-chronic fatigue syndrome… [2] Curculigo orchioides has been reported to be rich in bioactive ingredients through the research works [3], [4], [5] The most bioactive group of COR is polyphenols, especially the phenolic glycosides, which act as antioxidant agents to prevent the pro-oxidation process or biological oxidative damages in the living cells [6]
Drying is one of the most convenient methods for preserving products that has appeared for a long time, to prolong the shelf-life and retain the bioactive compositions and beneficial nutrients of the raw materials for use in a long period [7] Many drying methods have been investigated and applied to preserve plant materials with different structures, shapes and sizes in order to make sure the color, chemical compounds and nutrients of the food products The drying system for agricultural products is currently being favored by many customers, playing an important role in the industries of food processing, agricultural goods production, biological products, medicines, and even exportation
Although COR has been reported to be rich in valuable bioactive ingredients, it is currently sun dried has been reported to be rich in bioactive ingredients for the long-term preservation However, sun drying may cause significant loss of its bioactive ingredients and medicinal quality Therefore in this study, the effects of different drying methods including sun drying, convective drying, infrared drying, vacuum drying and heat pump drying on drying time, moisture content, total phenolic content and antioxidant activities of COR were investigated to determine the most suitable drying method for this medicinal material In addition, the influence of the thickness of the COR slices on the drying time, physicochemical properties and antioxidant activities of the dried products was also investigated
Research objectives
- Investigate the effect of thickness of the drying material on the drying time, physicochemical properties and antioxidant activities of the dried products
- Determine the most suitable drying method for drying of Curculigo orchioides rhizome for maximizing the retaining of bioactive ingredients and antioxidant activities of the dried COR.
Research contents
- Analysis of the moisture content, physicochemical properties and antioxidant activities of the fresh COR.
- Investigate effects sun drying and the thickness of the COR slices on the drying time, moisture content, total phenolic content and antioxidant activities of the dried COR
- Investigate effects convective drying and the thickness of the COR slices on the drying time, moisture content, total phenolic content and antioxidant activities of the dried COR
- Investigate effects vacuum drying and the thickness of the COR slices on the drying time, moisture content, total phenolic content and antioxidant activities of the dried COR
- Investigate effects infrared drying and the thickness of the COR slices on the drying time, moisture content, total phenolic content and antioxidant activities of the dried COR
- Investigate effects heat pump drying and the thickness of the COR slices on the drying time, moisture content, total phenolic content and antioxidant activities of the dried COR.
Significance of the research results
- The experimental results from the study of drying processes on the COR may be used as the scientific basis for further studies on the drying processes at an industrial scale for COR and other parts of this herbal plant as well as other herbs
- The results from drying methods for COR on retaining beneficial bioactive characteristics are useful knowledge for the development of functional foods and healthy pharmaceutical products.
LITERATURE REVIEW
Overview of Curculigo orchioides
- Curculigo orchioides rhizome has skin in black or brown color, white flesh and has a distinctive aroma when be exposed to the sunlight Ethnic minorities at the Northern mountainous areas of Vietnam usually use this rootstock as a useful medicine so that it called Sam in Vietnam, and the leaves are the same as Cau leaves in Vietnam and this plant is named Sam Cau In addition, it is also known as Tien Mao, Ngai Cau, Co Noc Lan [8]
- The scientific name of this plant is Curculigo orchioides, which belongs to
Amaryllidaceae family, with the English name Golden eye grass or Black Musale, the Hindi name is Kali Musli [2], [9]
Table 2.1 Table of Taxonomical hierarchy is presented in [2]
Order Liliales Family Amaryllidacae Genus Curculigo Species Orchioides
Figure 2.1 Curculigo orchioides plant as illustrated in [10]
- Parts used as medicine: Tuberous root or rhizome
- Curculigo orchioides, is a plant which stays a long time, the height of 20 – 30 cm The rhizome is cylindrical and long, straight, slender at the ends, with accessory roots which are short or long, coarse blackish brown bark and white flesh [8]
- The leaves grow in clusters from the rhizome, folded like Cau leaves
- There is a flower cluster growing on the leaves, the leaf sheath is large and long
- Fruit capsule, oblong, about 1.2 - 1.5 cm long Each fruit contains from 1 to 4 capsules
Figure 2.2 Curculigo orchioides tuberous root [11]
Table 2.2 Quality of Curculigo orchioides crude drug samples collected from different places
Curculigo orchioides distributed mainly in tropical and subtropical Asia such as some southern provinces of China, Laos, Vietnam and a few other countries in Southeast Asia area
This is a plant that likes light and moisture, especially can tolerate shade, so they are often found in the fertile soils of the valley, along the fields or at the foot of limestone mountains in the northern provinces of Vietnam such as Lai Chau, Tuyen Quang, Cao Bang
This species was listed less likely to be threatened by IUCN (The International Union for Conservation of Nature and Natural Resources) in 1999, which was due to overexploitation and destroy the habitat; moreover, it has also low quality of seed and germination is significantly declined in wild plant density [5]
- According to traditional medicine, this herb is spicy and warm
+ Treatment of cold in the back, hands and feet
+ Additionally, Curculigo o also cures hemorrhoids, abdominal pain, cough, jaundice
- According to modern medicine, some studies have indicated that Curculigo o has many health benefits:
+ Be able to replace testosterone – male sex hormone because it contains steroids + Support the body's ability to adapt to the environment without oxygen or in high temperature condition
+ Control heart activity, dilate coronary vessels, protect liver
+ Anti-inflammatory, anti-fungal, against blood-related disorders
+ Enhance muscle contraction, treat convulsions, relieve pain
- The dosage of the herb:
+ The usage level of adults: 5 - 10 gram
+ From the ages of 5 and 16: one half of adult usage
+ From 1 to 5 years old: a quarter of adult usage
Overview of Polyphenol
Polyphenol compounds are organic benzene rings binding one or more hydroxyl functional groups [12] and were heat-labile substances [13]
There are over 8000 phenolic compounds found in most species of flora These compounds in plants originated in an intermediate substrate, phenylalanine, or a precursor and shikimic acid They manifest in a conjugated shape, have the linkage of one or more sugar radicals to hydroxyl functional groups, in spite of direct bonds of sugars (polysaccharide or monosaccharide) to an arene occurs [14]
It acts as preservatives, take n-propyl gallate as an example, it was approved by the FDA [15].
Polyphenols can be composed of different groups according to the number of rings that make up a phenol and the structural principles that bind them together The main categories of polyphenol below are phenolic acids, flavonoids, stilbenes and lignans
The molecular structure of this group generally consists of only one carboxylic acid group [16]
Phenolic acids usually exhibit colour, organoleptic qualities, nutrients and antioxidant characteristics in foods [17]
Additionally, the food factories have conducted on the analysis on profile and dosage of phenolic acids as well as their role on fruit repening, enzymatic control and have used them as preservatives [18] [19] [20]
Currently, citations have pointed to high antioxidant content in fruits and vegetables with inhibitory mechanism on diseases caused by oxidative damage such as cancer, cerebral accident, coronary heart disease [21] [22]
Phenolic acids displace two derivatives: cinnamic and benzoic acid [23]
They are discovered in many species of plants: Cinnamic and benzoic acid derivatives appear in most of edible plants (examples in fruits, vegetables, and grains) and are spread in kernels, leaves, stems and roots of the plant [24] [18]
Phenolic acids are relatively resistant biochemical species; they undergo metabolism in the soil for some microorganisms can consume them as carbon sources [25]
Besides, the natural presence of phenolic acids usually occurs in dried fruit, horse gram plants, mushroom Basidiomycetes and soil humus [26]
Figure 2.3 Cinamic acid and Benzoic acid derivatives 2.2.2 Flavonoids
Chemically, flavonoids are formed on 15 carbon frames, containing 2 aromatic rings,
A and B, connected through a pyran called C as an illustration below [27]
Figure 2.4 Chemical structure of Flavonoids
Normally, flavonoids are synthesized at specific parts in plants and contribute to the color and fragrance of flowers For fruis, flavonoids play a role to attract pollinators and thus helping with dispersal during seed and spore germination and seedling growth and development [28] Flavonoids include many subgroups such as: flavones, flavonols, flavanones, flavanonols, flavanols or catechins, anthocyanins and chalcones [29]
Stilbenes have the structure of a 1,2-diphenylethylene nucleus with hydroxyls attached to the aromatic hydrocarbons and appear as monomers or oligomers; beside that, most of the compounds studied are trans-resveratrol [30]
Stilbenesare phytoalexins generated by plants in response to disease, injury or stress symptoms [31]
Figure 2.5.Chemical structure of Stilbenes Stilbene compounds were formed in many crops including sorghum (Sorghum bicolor), peanut (Arachis hypogaea), grape wine (Vitis vinifera) and many trees such as
Commercially, sources of stilbenes include a variety of flora grown in parts of Asia as folk drugs, are Polygonum cuspidatum, Rhodomyrtus tomentosa, Rheum undulatum, Melaleuca leucadendron, and Euphorbia lagascae For pterostilbene, which is mostly discovered in blueberries, bilberries and other kinds in grapes and juice pulp [33]
The lignans are constituted of two or three mono-lignols at the C position of 6-3 and the catalyst of the reaction is an oxidizing enzyme At units of 6-3, a lignan is formed by the dimerization, which is derived from cynamyl groups [34]
Neolignans, another kind of lignans, are quite widespread in the plant kingdom Neolignans are also dimers of cinnamyl units, but their structures are assembled by coupling of mesomeric radicals other than the ꞵ–ꞵ link typical of lignans [35]
Figure 2.7 Examples of various lignan skeletal units
Overview of drying methods
The quantification of the moisture content in medicinal herbs or food is the determination of the percentage of water to check whether these products meet the moisture standard and more importantly, effects of moisture on the stability and quality of foods In addition, moisture determination is an essential for quantitative calculation or extraction efficiency of bioactive compounds in the medicinal herb [36]
The moisture content of harvested medicinal herbs is often very high, about 60 – 80%
[37] The final moisture content of the medicinal herbs are then stored usually from 10 to 14% [38] Particularly, the moisture content of the fresh gingsen roots is about 72% (wet basis) [39] and the dried ones with good quality need have between 5.5 and 7.5% of the final moisture content, which is dependent on the size of the roots as mentioned in the analysis[40]
Determination of moisture content by drying method:
Drying is the process of using thermal energy to take water out of a solid or liquid material For the purpose of reducing the volume of materials, enhancing the durability and helping preserve foods for a long time [41], [42] Drying can be divided into two types: natural drying and equipment drying (or artificial drying)
Natural drying that means the process of drying materials outdoors, without the use of a drying device
Artificial drying, based on the method of heat supply that can be classified as convective drying, radiation drying, rotary drying, drying with sublimation, microwave drying, fluidized bed drying
This is a type of traditional drying method that exposes drying materials to sunlight and receives mainly radiant energy from the sun Drying only by placing the drying object on a tray and exposing it to direct sunlight (without the use of a drying device) and is most popularly used in agricultural product processing [43]
This is the drying technique to let materials directly contact with hot air or smoke to act as a drying agent at the suitable temperature, humidity and velocity, the flow movement covers the drying material, causing the moisture in this drying object to evaporate and then follow the drying agent out of the drying chamber The air agent can move in the same direction, in the opposite direction of the product flow Convection drying can be done in batches or continuously [7]
There are two processes occurring at the same time in convective drying of the solid material One is the evaporation of the surrounding environment, the other is the transfer of moisture inside the solid [44]
The infrared drying operate by under exposing the raw product to electromagnetic radiation with the wavelength between 0.8 and 1000 àm The energy of infrared radiant is transmitted from the heating power to the surface of the product and no heated air surrounding the product simultaneously In addition, it is supposed to generate heat quickly and efficiently to yield better products in terms of sensory and nutritional qualities with uniform heat and lower final costs [45]
The infrared drying method are designed to be aimed at utilizing radiant heat as the primary source [46].
This process is applied in the reduction of the atmosphere pressure around the product When the boiling point of water or other solvents is near and the moisture begins to evaporate parallel to the temperature reduced [47]
2.3.5 Heat pump drying (or Cold drying)
This drying method uses the sensible heat recovered from the latent heat of the air stream passing through the condenser to dehydrate [48]
At the heat pump evaporator, the airflow is condensed in the purpose of using the latent heat again for reheating the drying gas [45]
The drying temperature and the humidity can be modified [49]; thereby, helping the heat pumpers can be widely applied in agricultural and pharmaceutical products [50]
Another finding mentioned that the heat pump dryer usually uses low energy [51], this dryer can be operated at low temperature and still retain intact of colour and bioactive substances [52], [53].
Literature review of of the studies on Curculigo orchioides
Currently, there are many articles about the composition, biological compounds as well as the uses of this precious medicinal herb, such as: A survey of M Asif on Phytochemical and Ethnopharmacological Activities of Curculigo orchioides in 2012 [3], Jitendra Mehta and Krishnendra Singh Nama (2014) mentioned a report of ethnic medicine originating from COR [2], or the experiments by Praveen Kumar and SK Shukla (2019) on the functions of COR in liver toxicity in roosters [54] There are also some domestic studies such as A research on the genetic diversity of the endangered herb in Vietnam, CORs by Quang and his companers [55], Ngoc and her associates (2015) were sucessful in the isolation of three phenolic compounds from the rootstock of Curculigo orchioides [4], or antioxidant activity and the content of bioactive compounds were carried out on in vitro
Curculigo orchioides rhizomes by Hieu et al (2020) [56]
Moreover, many research have reported the impacts of drying technology on the components and biological activities of many plants such as Results of drying method on total polyphenolic content and antioxidant capacity of mint carried out by Orphanides et.al
(2013) [57], Investigations on drying temperature to physico-chemical features and antioxidant capacity of yellow berries of Jessica et al (2013) [58], or an effect of
13 dehydration of roselle calyxes in total phenolic and its antioxidant activities by Nguyen Quang Vinh et al (2014) [59]
Because of the characteristics and roles of this plant should be produced the dried products long-term and its wide application, this research is considered the contribution of thermal processing and phytonutrient content of Curculigo orchioides were determined at different temperatures we have conducted a research project about “Influence of drying methods on the content of bioactive compounds and antioxidant activities of Curculigo orchioides”
MATERIALS AND METHODS
Materials, Chemicals and Equipments
The fresh Curculigo orchioides rhizomes were brought from Lao Cai province and then transported to the laboratory of the Department of Chemistry and Food Technology, Ho Chi Minh University of Technology and Education within 3 days after being harvested The rhizomes were then washed with tap water and the hairy roots are removed to obtain the central tuberous rhizomes The tuberous rhizomes were then cut into slices for drying
The chemicals used for the study and their corresponding suppliers/manufactures are shown below:
- CH3OH (Methanol, Xilong Scientific Co., Ltd, Shantou City, China)
- Folin-Ciocalteu’s phenol reagent (Merck KgaA, Germany)
- Na2CO3 (Sodium carbonate, Xilong Scientific Co., Ltd, Shantou City, China)
- Gallic acid (3,4,5-trihydroxybenzoic acid, Sigma-Aldrich, U.S.A)
- DPPH (2,2-diphenyl-1-picrylhydrazyl, Thermo Scientific™, Alfa Aesar, U.S.A)
- ABTS (2,2'-azino-bis (3-ethylbenzothiazoline-6-sulfonic acid, Sigma-Aldrich Pty Ltd, Castle Hill, NSW, Australia)
- K2S2O8 (Potassium persulfate, Xilong Scientific Co., Ltd, Shantou City, China)
- C2H9NaO5.3H2O (Sodium acetate trihydrate, Xilong Scientific Co., Ltd, Shantou City, China)
- CH3COOH (Glacial acetic acid, Merck KgaA, Germany)
- TPTZ (2,4,6-Tris(2-pyridyl)-s-triazine, Sigma-Aldrich Pty Ltd, Castle Hill, NSW, Australia)
- FeCl3 (Ferric chloride, Sigma-Aldrich Pty Ltd, Castle Hill, NSW, Australia)
- HCl (Hydrochloric acid, Xilong Scientific Co., Ltd, Shantou City, China)
- Trolox (6-hydroxy-2,5,7,8-tetramethylchroman-2-carboxylic acid, Sigma-Aldrich Pty Ltd, Castle Hill, NSW, Australia)
The dryers are used in this project: including convective drying, vacuum drying, infrared drying, heat pump drying (cold drying system) and other equipment are shown in the figures below
Figure 3.1 Convective dryer (Memmert, Germany)
Figure 3.2 Vacuum dryer (Memmert, Germany)
Figure 3.3 Infrared drying (DS.IR – 03, HCMC University of Technology and Education,
Figure 3.4 Heat pump drying (cold drying system) (DSL – P – L – T – 02, HCMC University of
Figure 3.5 Analytical balance 10mg (LT
(LS3200C and LS320, Precisa, Switzerland)
Figure 3.7 Powder grinder machine (China)
Figure 3.8 Spectrophotometers UV-Vis (UH 5300, Hitachi, Japan)
Experimental design
Figure 3.9 Diagram of the experimental design
Drying procedure a/ Preparation of samples
COR were washed to remove impurities and then drained out of water After that COR were sliced into two different sample thicknesses of 2 mm and 5 mmbefore being dried
Tubers were put into sealed, clean plastic bags and kept in a freezer at 4 o C before the commencement of the tests
Determination of drying time and moisture content
Determination of total phenolic content Determination of antioxidant activities
Figure 3.10 Curculigo orchioides after cutting hairy roots
Figure 3.11 Curculigo orchioides in size 2mm and size 5mm b/ The process of drying [60] :
- For drying methods (convective, infrared, heat pump dryers):
+ Slice samples were weighed about 30 g and were put in the drying tray and then placed it at the dryer The drying experiments were carried out similarly to the samples dried over time and were weighed to unchanged mass
+ The drying process was observed and sample slices were weighed every 1 hour + The tables of moisture variation were made to build the drying curve
+ The drying experiments were finished until the weight was unchanged and each one was done in at least three replicates
Convective dryer: set temperatures at 60 o C, 80 o C, 100 o C
Vacuum dryer: set temperatures at 40 o C, 50 o C, 60 o C and vacuum pressure at 100 mbar
Infrared dryer: set temperature at 80 o C and air velocity at 6m/s
Heat pump dryer: set temperater at 40 o C and air velocity at 8m/s
+ Slice samples were spread on a tray under daylight Until the evening, put these samples in a place out of the wind
+ Slices were weighed in the afternoon and evening of the day
- The variations of the moisture content over time determined by the following formula: G o (100 – W o ) = G j (100 – W j )
After each one hour: = j , the sample has a weight of Gj (g)
W o : the first moisture value of the drying sample (%)
W j : the moisture value of the drying sample after time at j (%)
G o : the first weight of the drying sample (gram)
G j : the weight of the drying sample after time at j (gram)
Extraction of samples
Step 1, the herbal samples were ground with a coarse grinder, then 0.2 g of the crushed sample was put into the beaker, dissolved in 10 mL of Methanol, sealed the beaker with foil and stored in a place protected from light
Step 2, the sample was decanted after soaking for 1 hour, the liquid is poured into a new container, sealed with foil, the sediment in the beaker will be added 5 mL of Methanol and left to soak for 1 hour
Step 3, the process carried out the same as step 2 and left immersion for 30 minutes After 30 minutes, the solution was filtered with a filter paper 102 with a filter in size of 0.11cm to retain the extract, for the residue it would be sealed and kept in the freezer The extract of the third extraction will be examined for antioxidant activity first If the
21 absorbance of the sample is less than 0.2, it means that the concentration of the sample is still high, continue as in the 3rd time with the residue that has been collected after three times of extraction If the absorbance is greater than or equal to 1, the concentration of the sample is acceptable and the extraction completed; after that, transfer this extract into the previous total extract together
Figure 3.12 Diagram of extraction of samples
Soaking in Methanol (1 st time)
Soaking in Methanol (2 nd time)
Soaking in Methanol (3 rd time)
Analytical methods
The experiment was depended on Vietnamese Standardization (TCVN 9741:2013) and some documentaries [60], [61] to evaluate the moisture content of Curculigo orchioides samples
Dryer, which maintained at temperature 105 o C (at the laboratory area B, Ho Chi Minh City University of Technology and Education)
- The lid of the petri dish was opened and both were dried for about 1 hour in an oven at
105 o C Then are cooled in a desiccator After cooling to room temperature, the lid was closed and was weighed accurately to 0.001 g
- Sliced samples were weighed approximately 10 g immediately in a prepared petri dish with a lid on the side
- The petri dish with the measured sample and the lid opened was dried directly in the oven at 105 o C to constant weight for 4 hours The oven door should not be opened during the drying period
- Once the oven was opened and the petri dish was covered right after and transfered to the desiccator, the lid was opened (still leave the lid in the desiccator) then cooled it in a sealed desiccator for at least 30 min
- The petri were removed from the desiccator, covered and weighed to calculate the first moisture value This process was continued until the difference between the two weighings was not exceed 0.005 g.The experiment was performed in triplicates (n = 3)
- Determine the percentage of moisture according to the formula below:
G o : the weight of the sample before drying (gram)
G 1 : the weight of the sample after drying (gram)
W: the moisture percentage of dried samples (%)
The total phenolic content was determined by UV-VIS measurement with Folin- Ciocalteu’s phenol reagent and mentioned in TCVN 9745-1-2013 [62] This method is based on the ability to react with phenolic compounds of Folin-Ciocalteu’s phenol reagent The molybdostungtate complex salt mixture is very sensitive to reducing agents, so the presence of polyphenolic compounds in a slightly alkaline medium, it will be reduced to a blue compound with the strongest absorbance at 756 nm of wavelength
Figure 3.13 Reaction of evaluation of total phenolic content
Figure 3.14 The experiment of determining total phenolic content Table 3.1 Preparation of Gallic acid concentrations in the standard curve construction
The total Polyphenol content was performed according to theFolin-Ciocalteu method described by many authors [56] with some modifications
The reference standard was gallic acid against a blank sample was distilled water (a zero sample) and a control sample was solution without tested extract The concentration of gallic acid varied from 10, 20, 30, 40, 50 μg/mL were used to plot a calibration curve
On the tested sample,0.5 mL of the extract was added to react with 2.5 mL of Folin- Ciocalteu’s phenol, 10% for 3 - 8 minutes before pippeting 2 mL of Na2CO3 solution,
7.5%, then vigourously shaken The mixture was incubated at a dark place for 1 hour at ambient temperature and recorded the absorption at 765 nm The experiment was performed in triplicateand the results were expressed as mg Gallic acid equivalent (GAE) per one gram of dry weight (mg GAE/g DW)
The content of total polyphenol following Gallic acid equivalent was calculated by the formula below:
C: the concentration of the diluted working solution (mg GAE/mL) calculated based on the standard curve of Gallic acid
V: the volume of the extracted solution (mL) d: the dillution number m: the dry weight of the sample (g)
3.4.3 Determination of DPPH free radical scavenging activity
DPPH is a stable and purple free radical, it can accept electrons or hydrogen radicals to become stable and diamagnetic molecules Since the DPPH molecule has an odd electron, it has a deep purple color in ethanol or methanol and is strongly absorbed at 515 nm [63] The measurement is based on the ability of antioxidants to remove free radicals The odd electron of the nitrogen atom in DPPH is reduced by accepting a hydrogen atom from the antioxidant, resulting in reducing absorbance and reducing color [64]
Figure 3.15 Reaction of DPPH free radicals scavenging
Trolox (Axit (S)-(-)-6-hydroxy-2,5,7,8-tetrametyl- chroman-2-carboxylic)
Figure 3.16 The experiment of DPPH free radical scavenging
The DPPH free radical scavenging assay was carried outaccording to group of authors
Trolox is a strong antioxidant were used to prepare in the construction of the calibration curve Trolox concentrations were diluted in the range of 50, 100, 200, 400, 800 μM
The sample extract DPPH solution
Precisely, 24 mg DPPH was dilluted in 100mL of methanol and stored at a freezer Then 10 mL of stock solution were mixed with 45mL of methanol as the control and the result was read at 515 nm of wavelength was equal to 1.1±0.02
To the mixture, 150 μL of the tested extract and 2850 μL of DPPH solution against the blank was Methanol, then vortexed quickly and incubated at a dark room for 2.5 hours; finally, the absorbance was recorded The analysis was repeated 3 times and calculated as μM Trolox equivalent per one gram of the test sample in dry weight (μmol TE/g DW) The result of antioxidant capacity following Trolox equivalent is calculated by the formula below:
C: the antioxidant capacity of the diluted working solution (μmol TE/mL) calculated based on the standard curve of Trolox
V: the volume of the extracted solution (mL) d: the dillution number m: the dry weight of the sample (g)
3.4.4 Determination of ABTS free radical scavenging activity
ABTS is a free radical with strong green color absorption at 734 nm While adding a compound with antioxidant properties results in a loss of color accompaning with a decrease of absorbance [66]
Figure 3.17 Reaction of ABTS free radical scavenging
ABTS (2,2'-azino-bis (3-ethylbenzothiazoline-6-sulfonic acid)
K2S2O8 (Potassium persulfate Trolox (Axit (S)-(-)-6-hydroxy-2,5,7,8-tetrametyl- chroman-2-carboxylic)
Figure 3.18 The experiment of ABTS free radical scavenging
The ABTS free radical scavenging method was doneunder detailed description of the previous research [65]
The sample extract ABTS solution
Working solution of ABTS •+ was produced by mixing 10mL of ABTS and 10 mL of
K2S2O8 and kept 15 hours at room temperature in the dark The absorbance was adjusted to 1.1±0.02 at 734 nm by ABTS and methanol in the ratio of 1:30 The mixture included
150 μL of the extract and 2850 μL of ABTS reagent, shaken and left at room temperature in the dark condition and measured at 734 nm Methanol acted as a blank sample
Trolox was used as a positive standard in establishing the calibration curve with a row of 25, 50, 100, 200, 400 μM The results were expressed as μM Trolox equivalent per one gram of the test sample in dry weight (μmol TE/g DW)from 3 replicate measurements
3.4.5 Determination of Ferric reducing antioxidant power (FRAP)
Antioxidants are responsible for reducing the Fe 3+ complex to Fe 2+ in acidic medium Since then, the solution has an intense color corresponding to the amount of antioxidants in that material [67]
Figure 3.19 Reaction of Ferric reducing antioxidant power
CH3COONa.3H2O (Sodium acetate trihydrate)
CH3COOH (Glacial acetic acid)
Trolox (Axit (S)-(-)-6-hydroxy-2,5,7,8-tetrametyl- chroman-2-carboxylic)
Figure 3.20 The experiment of Ferric reducing anitoxidant power
Ferric reducing antioxidant power (FRAP) was measured by the previous description of authors [65]
In brief way, the FRAP reagent was prepared by combining TPTZ solution (10 mM) in HCl (36%, 40 mM), FeCl3 solution (20 mM), and acetate buffer (300 mM, pH 3.6) in the ratio of 10:1:1, kept warm at 37 o C before using A tested extract of 150 àL with 2850 àL of FRAP reagent Thereafter, the mixture was vortexed and left the mixture in the dark at ambient temperature for 30 minutes, the optical density was read at 593 nm The positive standard was Trolox against a blank tube was methanol Trolox solutions (25, 50, 100, 200,
400, 800 μM) were used for calibration Lastly, the FRAP values expressed as àM Trolox/one gram of the dry sample (μmol TE/g DW) with three replications.
Statistical analysis
The values were expressed as “mean ± SD” Statistical analysis was performed byOne Way - Analysis of variance (ANOVA) on SPSS 20 software, “p values < 0.05” were considered as significant
The sample extract FRAP solution
RESULTS AND DISCUSSION
Moisture content, TPC and antioxidant activities of the fresh Curculigo
As shown in the Table 4.1, the fresh Curculigo orchioides rhizomes (COR) contain a relatively high moisture content (81.83%) which is comparable to the moisture content reported in previous studies [2, 5, 9] The total phenolic content (TPC) of COR (8.58 mg GAE/g DW) is not too high compared to other phenolic rich plant sources but it exhibited very high antioxidant activities in all DPPH, ABTS and FRAP assays This result is not surprising as COR has been reported to contain phenolic glycosides which process strong antioxidant capacity such as curculigoside A, B, C and D and curculigine A and D [2] This suggests that COR is a medicinal ingredient which may provide potential benefits to human health Therefore, the identification of suitable drying methods for preserving those valuable characteristics of COR is necessary
Table 4.1.Characterization of the fresh Curculigo orchioides rhizome
Total phenolic content (TPC) mg GAE/g DW 8.583 ± 0.31
DPPH radical scavenging activity μmol TE/g DW 302.58 ± 0.69
ABTS radical scavenging activity μmol TE/g DW 164.96 ± 9.02
Ferric reducing antioxidant power μmol TE/g DW 62.014 ± 3.42
Effect of drying methods on the drying time and moisture content of the COR
As shown in Table 4.1, the time required for drying COR to the constant moisture content greatly varied among the drying methods The sun drying method required the longest drying time (up to 15 hours) while the convective drying at 100 o C and infrared drying could dry COR slices with the shortest periods The long drying time caused by sun drying is predictable as this is a traditional drying method depending on the heat from
32 sunlight with relatively low temperature Similarly, the vacuum drying method which was carried out at low temperature (40-60 o C) also extended the drying time up to 10 hours
Table 4.2 The require drying time for the 2mm and 5mm COR slices
2mm COR slices 5mm COR slices
Regarding the effect of the material size on the drying time, the thicker slides of COR needed more time to be dried in almost all drying methods compared to the time required for drying the 2mm COR slices However, the vacuum drying at 40 and 50 o C and the convective drying at 100 o C did not show any differences in the drying time between the two material sizes The result suggests that the utilization of very high temperature at 100 o C can supply a sufficient energy for evaporating water inside the COR slides regardless the variation in the material size Similar results were also observed in other published studies related to the drying of other plants [68-72]
Figure 4.1 Effect of different drying methods on the moisture content of the dried COR slices with the thickness of 2 mm (a) and 5mm (b)
CD60: Convective drying at 60 o C; CD80: Convective drying at 80 o C; CD100: Convective drying at 100 o C; VC40: Vacuum drying at 40 o C; VC50: Vacuum drying at 50 o C; VC60: Vacuum drying at 60 o C; IRD: Infrared drying at 80 o C; HPD: Heat pump drying at 40 o C; SD: Sun drying
Different letters above the column show significantly differences among the mean values (p < 0.05)
The equilibrium moisture content of the obtained dried COR are shown in Figure 4.1a and Figure 4.1b for the materials with the thicknesses of 2mm and 5mm, respectively It can be clearly observed that the infrared drying at 80 o C (IRD) and the heat pump drying at
40 o C (HPD) resulted in the dried products with exceptional low moisture content compared to the other drying methods The moisture contents of IRD and HPD dried COR slices were comparable and ranges from 5.1% to 7.2% In contrast, the minimum moisture content obtained by vacuum drying was relatively high (15.1% - 17.5%), especially for the vacuum drying at 40 o C (VC40) and 50 o C (VC50) of the 5mm COR slices (Figure 4.1b) Although the convective drying required relatively short time for the COR slides to reach the equilibrium moisture content, it could not reduce the moisture in the dried product to the low levels as which dried by IRD and HPD This phenomenon could be explain by the ability of the hot air flow to quickly evaporate and remove free water in the material in the convective drying oven However, to further remove moisture content of the material, it is necessary to evaporate the bonding water inside the drying medium, which could be done by the infrared irradiation in the IRD and the air very low relative humidity using in the HPD but not the hot air with normal humidity used in the convective drying The similar results were also reported for the drying of other medicinal plants and herbs such as spearmint [57], goldenberry [58], sour cherries [69] and lemon pomace [70]
For the comparison of the final moisture content between two sizes of the materials in each drying method, most of the drying methods dehydrated the 2mm COR slides to the lower moisture content compared to that of the 2mm COR slides, except for the case of sun drying (SD) The insignificant difference in the equilibrium moisture content of the 2mm-dried COR slides and 5mm-dried COR slides may be attributed to the low thermal energy obtained from the sunlight which was not enough for the heat demand to remove the bonding water even the drying time was extended to 15 hours for the drying of 5mm COR slides Interestingly, the increase in the drying temperature makes shorter drying times Besides, the moisture value in thin slices seem to decrease less than that of thick slices, so the storage capacity of thick samples be longer than that of thin samples It is noted that the humidity results of infrared drying and heat pump drying in each slice (2mm
35 and 5mm) have no significant difference, whereas IRD time is faster than that of other drying experiments [68].
Effect of drying methods on total phenolic content of the dried COR slices
The total phenolic contents (TPC) of the COR slices dried under different conditions are presented in Figure 4.2 It can be observed that considerably higher polyphenol amounts in the products were obtained from IRD and HPD The total phenolic contents of the IRD and HPD dried samples were 37.0 and 43.0 mg GAE/g DW and 24.2 and 26.2 mg GAE/g
DW for 2mm and 5mm dried slices, respectively Those were equivalent to the retention of 76.82% and 64.6% for 2mm slices, 80.04% and 67.72% for 5mm slices, respectively, compared to the initial phenolic content of the fresh COR material In contrast, CD80 and VD40 were the drying processes resulting in the lowest polyphenol retention of the dried samples compared with other drying methods In addition, SD was better in retention of TPC than others for the 2mm slices, whilst SD had no significant difference in TPC retention compared with CD60 and VD50 for the 5mm slices
In case of vacuum drying, the results of the present study of total phenolic content slowly decreased when the temperature was lowered TPC of samples dried at 60°C, 50°C, 40°C were 8.02 mg GAE/g DW, 10.8 mg GAE/g DW and 7.314 mg GAE/g DW, respectively, for thin slices (2mm) Correspondingly, the TPC of the dried slices dropped to 10.52 mg GAE/g DW in VD60 and to 8.345 mg GAE/g DW at VD40 in 5mm slices (Figure 4.2b) The results are in agreement with the report by Papoutsis et al (2017) for the vacuum drying of lemon (Citrus limon) pomace [70].On other study, the survey of the highest polyphenol content at vacuum drying of 50°C was in the pomegranate arils drying process [71]
Figure 4.2 Effect of different drying methods on total phenolic content of the dried COR slices with the thickness of 2 mm (a) and 5mm (b)
Different letters above the column show significantly differences among the mean values (p < 0.05)
Regarding convective drying method, the TPC was found in 2mm dried COR slices to be degraded to 10.197 mg GAE/g DW, 7.21 mg GAE/g DW and 8.44 mg GAE/g DW
37 when dried at drying temperature of 60 °C, 80 °C, 100 °C, respectively The convective drying of 5mm samples showed similar trend when the temperature was increased (14.753 mg GAE/g DW at 60°C, 8.385 mg GAE/g DW at 80 °C, 8.607 mg GAE/g DW at 100 °C) (Figure 4.2) The result of Katia et al (2013) [72] was contrary to our practice owing to dry products at a lower temperature which receives a greater drop TPC than products dried at higher temperature with a shorter time This was explained that the air circulation increase in a hot air dryer causes oxidation of some phenolic compounds due to oxygen exposure and high temperature thereby reduces TPC [69]
In general, the significantly higher retention of TPC was found for COR dried using IRD and HPD methods than using other drying methods It should also be noted that the concentration of nutrients in the remaining mass is increased during removing moisture
[73] Moreover, infrared drying and heat pump drying play a crucial in helping to keep good quality for the food products [68], [74] Furthermore, another study proved that in higher infrared drying temperature and in short processing time could preserve the highest phenolic compounds in peels and leaves of orange [75] These results were also in accordance with the other report of significant decrease in total phenolic compounds after drying in comparison with the initial value, but total phenolic compounds possessed higher values presented in dehydration at higher temperatures (such as 80°C) [76]
Among the analyzed samples, except for CD80, CD100, VD40 and VD60 for 2mm slices; CD80 and VD40 for 5mm slice, the dried products had higher TPC than the fresh ones For instance, when authors evaluated the content of phenolic compounds in tomatoes and pineapples after drying and also obtained higher values in the dried samples than in the fresh samples as a result of the bound phenolic compounds disengaging from the cellular constituents [77], [78] and generating free phenolic compounds, or conversion of mono-phenols to di- or tri-phenols [79] Similarly, the reports showed that the polyphenolic concentration of dried grapes was higher than that of fresh grapes [80],
[81] Likewise, another finding indicated that an increase in the total phenolic contents of grapes after drying, which was attributed to the increase in the dry matter concentration due to the water evaporation [82]
In summary, the IRD resulted in the dried COR products with the ighest TPC followed by HPD and SD while CD80 and VD40 caused the highest losses of TPC compared to the others.
Effect of drying methods on antioxidant activities
4.4.1 Effect of drying methods on DPPH free radical scavenging capacity of the dried COR slices
The DPPH radical scavenging capacity of the dried COR slices is illustrated in Figure 4.3 The results of the scavenging activity of DPPH radicals express a great drop in all the drying treatments of both sizes of dried slices compared to fresh material IRD dried sample was found to have better capability of DPPH scavenging than those from other conditions The IRD caused the loss of 49.1% in dried samples of 5mm and 51.48% of slice 2mm for DPPH radical scavenging capacity compared with the fresh rhizomes There was a downward trend of DPPH along with the rise of temperature within CD methods, in which dried samples at 100 o C of CD had the weakest DPPH scavenging ability which in both material thicknesses In comparison with vacuum drying conditions, the DPPH values for
VD at 60 o C were significantly higher than that for CD at the same temperature For CD dried samples, when the temperature ranged from 60 to 100 o C, there was a sharp reduction in DPPH antioxidant properties of dried slice 2mm (20.244, 18.244 and 1.244 μmol TE/g
DW, respectively) and similarly to dried slice 5mm (41.133, 27.244 and 3.244 μmol TE/g
DW, respectively) Besides, between CD60 and CD80 did not show considerable differences in drying of the 2mm slices
Figure 4.3 Effect of different drying methods on DPPH free radical scavenging capacity of the dried COR slices with the thickness of 2 mm (a) and 5mm (b)
Different letters above the column show significantly differences among the mean values (p < 0.05)
These results are in constrast to the results of the research on the influence of drying on murta (Ugnimolinae T.) berries [72], on banana [83] when they detected that hot air-dried
40 samples at a higher temperature will lead to shorter drying times and thus retain higher phytochemical amount and antioxidant properties than other samples dried at lower temperatures with longer time In other way, derivatives from certain reactions (i.e Maillard reaction) may be generated and accumulated at high temperatures, which might have an influence on the antioxidant properties of the final dried product [84]
For the vacuum dried samples, DPPH antioxidant activity rose slowly when temperature increasing from 40 to 60 o C, were 21.022 μmol TE/g DW at 40 o C, 55.133 μmol TE/g DW at 50 o C, 30.467 μmol TE/g DW at 60 o C, for samples size 2mm; and those for the size 5mm, were 32.133 ± 2.963 μmol TE/g DW at 40 o C, 81.467 ± 4.807 μmol TE/g
DW at 50 o C, 57.356 ± 11.423 μmol TE/g DW at 60 o C Theoretically, due to the degradation of biologically active constituents at high temperatures, the decrease of antioxidant activity at high temperature [85] However, in the case of VD, the samples dried by VD40 showed the lowest DPPH scavenging activity compared to those dried at 50 and 60 o C This phenomenon may due to the much longer exposure of the samples dried by VD40 to the oxygen and light at high moisture content [86] Interestingly, the sun drying (SD) in this study showed positive effects of the retention of DPPH radical scavenging activity of COR compared to the CD and VD The antioxidant capacities of samples dried under sunlight of two sample sizes of 2mm and 5mm were 23.07% and 24.28% compared to the fresh material, respectively
Generally, dried thin slices had DPPH radical reduction lower than dried thick layers, predicted that these thin layers were easily affected by various elements of drying conditions than thick layers Furthermore, the depletion of antioxidant properties as a result of peeling, cutting and slicing were mentioned [87]
4.4.2 Effect of drying methods on ABTS free radical scavenging activity
From the results provided in the Figure 4.4, it can be seen that ABTS antioxidant power varied significantly among the investigated drying methods and temperatures for COR slides of both sample sizes Among the different drying methods, IRD and HPD exhibited noticeably high ability to scavenge ABTS free radicals, with retention rates of 67.31%, 55.44% (2mm) and of 69.94%, 62.23% (5mm), respectively, compared to that of the fresh rhizomes As expected the sun drying (SD) also produced the dried products with very high
ABTS antioxidant ability, which was the third highest for the 2mm slides after IRD and HPD but for the SD 5mm slides, their ABTS antioxidant was comparable to the HPD which was just lower than the IRD dried samples
Surprisingly, revert trends of the change of antioxidant power along with the increase in drying temperature were observed for CD and VD The increase of drying temperature from 60 to 100 o C of the CD caused great reduction in the antioxidant activity of the dried samples while a significant evolution of that was achieved with the rise in VD temperature from 40 to 60 o C The CD100 and VD50 caused lowest ability of the dried samples to scavenge ABTS radical cation Three CD60 dried samples had ABTS antioxidant capacity of 218.96 μmol TE/g DW of 2mm slices or 280.76 μmol TE/g DW of 5mm slices while that of the the other two conditions at 80 and 100 o C were 164.36 and 120.16 μmol TE/g
DW for 2mm slices and 202.36 and 180.36 μmol TE/g DW for 5mm slices, respectively These results correspond to the study that investigated convective drying to produce berries powder with enhanced polyphenol contents and preserved antioxidant potential [88] Regarding vacuum drying conditions, it is possessed a rapid increase in relevance to free radical scavenging when raising the temperature, which is similar to the finding demonstrated the effect of lemon pomace dried under higher vacuum drying temperatures [70].
Figure 4.4 Effect of different drying methods on ABTS radical scavenging capacity of the dried
COR slices with the thickness of 2 mm (a) and 5mm (b)
Different letters above the column show significantly differences among the mean values (p < 0.05).
4.4.3 Effect of drying methods on Ferric reducing antioxidant power
The data presented in Figure 4.5 indicates the influence of drying methods on the ferric reducing antioxidant potential (FRAP) of Curculigo orchioides rhziomes (COR) All of these drying techniques caused the significant losses of Ferric-reducing antioxidant capacity in the dried products with various degrees In this study, dried samples obtained by VD40 were found to have the weakest ferric reducing power (10.013 μmol TE/g DW and 18.300 μmol TE/g DW, in order of 2mm and 5mm slices) The IRD dried samples exhibited highest value of FRAP (117.062 μmol TE/g DW, size of 2mm and 125.157 μmol TE/g DW, size of 5mm)
Similar to DPPH and ABTS scavenging properties, the convective 5mm samples of CD60 had FRAP value significantly higher than the 2mm ones which were 89.538 μmol TE/g DW and 30.919 μmol TE/g DW, respectively However, we did not notice any differences between convective drying (60 o C, 80 o C, 100 o C) and vacuum drying (50 o C and 60 o C) of slice 2mm The higher retention capacity of the ferric reducing potential was
60 o C of CD in size 5mm by 30.74% compared with the initial material A downward trend
43 in ion ferric reducing power was observed when temperature was raised from 80 to 100 o C for the CD for 2mm slices This trend is in accordance of the previous report for drying of dried garlic slices under thermal condition [89] For samples dried by vacuum dryer, there is a decline in FRAP analysis after lowering temperate, at 40 o C were the lowest (10.013 μmol TE/g DW in models 2mm and in models 5mm, with 18.3 μmol TE/g DW), exception for VD at 60 o C of slice in size 5mm was greater than 23% higher than the original sample These could be interfered by faster deactivation of oxidase enzyme (polyphenol oxidase) at higher temperature [87] In agreement with DPPH and ABTS assays, dried slices of 2mm were also lower values than that of 5mm, apart from values in CD at 60 o C greater than in
Figure 4.5 Effect of different drying methods on FRAP antioxidant capacity of the dried COR slices with the thickness of 2 mm (a) and 5mm (b)
Different letters above the column show significantly differences among the mean values (p < 0.05)
Consequently, the IRD resulted in the dried COR slides with the highest antioxidant capacity followed by HPD and SD while CD100 and VC40 caused the lowest antioxidant capacity of the dried COR In addition, the sample size of 5mm could help retaining better antioxidant capacity compared to the thinner COR slices (2mm).
CONCLUSION AND RECOMMENDATIONS
Conclusion
The research results were shown that drying methods had considerable effects on drying time, equilibrium moisture content, total phenolic content and antioxidant capacities of
Curculigo orchioides rhizomes (COR) The change in the drying temperature of convective drying (CD) and vacuum drying (VD) caused severe impacts on the losses of TPC as well as antioxidant activities of the sample sizes of COR The CD100 and VC40 drying methods resulted in the lowest TPC and antioxidant capacity of the dried samples Infrared drying at 80 o C (IRD) was found as the most suitable method drying of COR because of its short drying time and the highest retention of TPC and antioxidant activities compared to the other investigated methods In addition, heat pump drying (HPD) and sun drying (SD) should also be considered as the potential options for the drying of COR because their economic benefits while they can still produce relatively high quality dried COR Regarding the impacts of sample size, although the slide thickness of 5mm required longer time to be dried, it could retain TPC and antioxidant capacity better the 2mm slices of COR in most of the drying processes In conclusion, the infrared drying at 80 o C for 5mm slices is recommended for dehydration of Curculigo orchioides rhizomes in order to retain bioactive compounds and biological activities for preservation and further utilization as a medicinal material.
Recommendations
As this study limited at investigating drying methods with the available equipment in the laboratories of the university, other drying techniques are suggested for further studies on the dehydration of Curculigo orchioides rhizomes
In addition, to maximize the retention of valuable phytochemicals as well as biological activities of Curculigo orchioides rhizomes, the optimization of drying conditions is recommended.
[1] Lợi Đ.T Prof., Những cây thuốc và vị thuốc Việt Nam, Hà Nội: Nhà xuất bản Y học,
[2] K S N Jitendra Mehta, "A review on ethanomedicines of Curculigo orchioides Gaertn (Kali Musli): Black gold," International Journal of Pharmacy & Biomedical
[3] M Asif, "A Review on Phytochemical and Ethnopharmacological Activities,"
Mahidol University Journal of Pharmaceutical Sciences, vol 39, no 3-4, pp 1-10,
[4] Ngọc N.B et al., "Ba hợp chất phenolic glycoside phân lập từ thân rễ của cây Sâm Cau (Curculigo orchioides Gaertn.) thu hai tai Tay Nguyen," Tạp chí Dược học, vol
[5] Agrahari, A.K et al., "Studies on the Anti- inflammatory properties of Curculigo orchioides Gaertn Root tubers," International Journal of Pharmaceutical Science and Research, vol 1, no 8, pp 139-143, 2010
[6] Prior R.L and Cao G., "In vivo total antioxidant capacity: comparison of different analytical methods," Free Radical Biology and Medicine, vol 27, no 11-12, pp 1173-1181, 1999
[7] May N.V., Giáo trình Sấy nông sản, 2nd ed., Hanoi: Nhà xuất bản Khoa học và Kỹ thuật, 2004
[8] Viện Y Dược học châu Á, "Sâm cau với tác dụng của Sâm cau và cách dùng trị bệnh tốt nhất," Nấm Lim xanh Việt Nam, 26 12 2012 [Online] Available: https://namlimxanh.vn/sam-cau-la-gi-tac-dung-cach-dung-nhan-biet-sam-cau-that- gia.html#Sam_cau_la_gi [Accessed 12 February 2013]
[9] Anupama, "Kali Musli (Curculigo orchioides) Information, Benefits, Uses and Warning," Bimbima, 30 6 2016 [Online] Available: https://www.bimbima.com/herbs/kali-musli-curculigo-orchioides-information- benefits-uses-and-warning/53/ [Accessed 30 June 2016]
[10] "Diversity of Ethnomedicinal Plants in Boridand Forest of District Korea, Chhattisgarh, India," Scientific Figure on ResearchGate, [Online] Available: https://www.researchgate.net/figure/Curculigo-orchioides-Gaertn_fig3_273492774 [Accessed 28th Dec 2021]
[11] "Sâm Cau đen (Tiên Mao)," Nông sản dược liệu vùng cao, [Online] Available: https://nongsanduoclieu.vn/do-ngam/sam-cau-den-tien-mao.html [Accessed 2017]
[12] Tú L.N et al., Hóa học thực phẩm, Hà Nội: Nhà xuất bản Khoa học và Kỹ thuật,
[13] Mrad N.D et al., "Influence of air drying temperature on kinetics, physicochemical properties, total phenolic content and ascorbic acid of pears," Food and Bioproducts
[14] S I R Kanti B P., "Plant polyphenols as dietary antioxidants in human health and disease," Oxidative Medicine and Cellular Longevity, vol 2, no 5, p 270–278, 2009
[15] Reddan J.R et al., "Propyl gallate is a superoxide dismutase mimic and protects cultured lens epithelial cells from H2O2 insult," Experimental Eye Research, vol 76, no 1, pp 49-59, 2003
[16] Robins R J., "Phenolic acids in foods: an overview of analytical methodology,"
Journal Agricultural and Food Chemistry, vol 21, no 10, p 2866–2887, 2003
[17] Maga J A., "Simple Phenol and Phenolic compounds in Food," Critical Reviews in
Food Science and Nutrition, vol 10, no 4, pp 323-372, 1978
[18] N M Shahidi F., Food Phenolics: Sources, Chemistry, London: Lancaster, Pa :Technomic Publishing Company, 1995, pp 247-260
[19] Toma´s-Barbera´n, F A.; Espı´n, J C, "Phenolic compounds and related enzymes as determinants of quality in fruits and vegetables," J Sci Food Agric, vol 81, pp 853-
[20] Ferna´ndez-Zurbano P et al., "Role of Hydroxycinamic acids and flavanols in the oxidation and browning of white wines," J Agric Food Chem., vol 46, pp 4937-
[21] Jacob R A.; Burri B J., "Oxidative damage and defense," Am J Clin Nutr., vol 63,
[22] Stacewicz-Sapuntzakis, M et al., "Chemical composition and potential health effects of prunes: a functional food?," Crit ReV Food Sci Nutr., Vols 251-286, p 41, 2001
[23] Benjamin C et al., Encyclopedia of Food and Health, Oxford, UK: Academic Press,
[24] Macheix J J et al., Fruit phenolics, USA: CRC Press, 2018
[25] Manda S M et al., "Phenolic acids act as signaling molecules in plant-microbe symbioses," Plant Signaling & Behavior , vol 5, no 4, pp 359-368, 2010
[26] BYJU'S, "2015," 2020 [Online] Available: https://byjus.com/chemistry/phenolic- acid/
[27] Kumar S and Pandey A K., "Chemistry and biological activities of flavonoids: An overview," The Scientific World Journal, vol 2013, pp 1-16, 2013
[28] Griesbach R J., "Biochemistry and genetics of flower color," in Plant Breeding Reviews, vol 25, Jules Janick, Ed., John Wiley & Sons Inc., 2005, pp 89-100
[29] Panche A N et al., "Flavonoids: An overview," Journal of Nutritional Science, vol
[30] Rupasinghe V P H et al., "Chapter 8 - Chemopreventive Properties of Fruit Phenolic Compounds and Their Possible Mode of Actions," in Studies in Natural Products Chemistry, vol 42, RELX Group, Elsevier B.V, 2014, pp 229-266
[31] Ronald R W et al., Ed., "Polyphenols in the Prevention and Treatment of Vascular and Cardiac Disease, and Cancer," in Polyphenols in Human Health and Disease, vol 2, Elsevier BV, Academic Press, 2014, pp 809-1383
[32] Parage C et al., "Structural, functional, and evolutionary analysis of the unusually large stilbene synthase gene family in grapevine," Plant Physiology, vol 160, no 3, p 1407–1419, 2012
[33] Riviere C et al., "Natural stilbenoids: distribution in the plant kingdom and chemotaxonomic interest in Vitaceae," Nature Product Reports, Vols 1317-1333, no 11, p 29, 2012
[34] Cooper R and Nicola G., Natural products chemistry: sources, separations and structures, Taylor & Francis: CRC Press, 2014
[35] Nahar L and Saker S D., Chemistry for pharmacy students: general, organic and natural product chemistry, John Wiley & Sons, 2019
[36] Robert L Bradley, "Chapter 6: Moisture and Total Solids," in Food Analysis, 4th ed., New York, Springer Science+Business Media, 2010, pp 95-104
[37] T PoóS , E Varju, "Drying characteristics of medicinal plants," International Review of Applied Sciences and Engineering, vol 8, no 1, pp 83-91, 2017
[38] J Müller and A Heindl, "Chapter 17: Drying of Medicinal Plants," Medicinal and Aromatic Plants: Agricultural, Commercial, Ecological, Legal, Pharmacological and Social Aspects , vol 17, pp 237-252, 2006
[39] Brun C.A., "Gingsen," in Cooperative Extension Program, Washington State
[40] Van Dalfsen, B., "Ginseng Dryer Operation," Farm Mechanization Fact Sheet, British Columbia, Canada, 1998
[41] "Phân loại các phương pháp sấy trong Công nghệ thực phẩm," EnvitechCorp, 29 Oct
2020 [Online] Available: http://envitechcorp.vn/2020/10/29/phan-loai-cac-phuong- phap-say-trong-cong-nghe-thuc-pham/ [Accessed 29 Oct 2020]
[42] "Sấy nông sản_Bài 3: Phương pháp và thiết bị sấy," Kỹ thuật Chế tạo - Phát triển Công nghiệp, 16 10 2017 [Online] Available: https://kythuatchetao.com/say-nong- san-bai-3-phuong-phap-va-thiet-bi-say/ [Accessed 16 10 2017]
[43] James G Brennan, Ed., "Chapter 3: Evaporation and Dehydration," in Food Processing Handbook, Weinheim, Wiley - VCH, 2006, p 104
[44] McMinn W.A.M and Magee T.R.A, "Principles, Methods and Applications of the Convective Drying of Foodstuffs," Food and Bioproducts Processing, vol 77, no 3, pp 175-193, 1999
[45] Calín-Sánchez A et al., "Comparison of Traditional and Novel Drying Techniques and Its Effect on Quality of Fruits, Vegetables and Aromatic Herbs," Foods, vol 9, no 9, p 1261, 2020
[46] Gardner A.W., Industrial Drying, Sc Sons Ltd., London, UK: Billing, 1971
[47] Dilip M Parikh, "Vacuum Drying: Basics and Application," Chemical Engineering, vol 122, no 4, pp 48-54, 2015
[48] Moses J.A et al., "Novel drying techniques for the food industry," Food Eng Reviews, vol 6, no 3, pp 43-55, 2014
[49] Goh L.J et al., "Review of heat pump systems for drying application," Renewable and Sustainable Energy Reviews, vol 15, no 9, pp 4788-4796, 2011
[50] Hawlader M.N.A and Jahangeer K.A., "Heat pump design for food processing," in
Handbook of Food Process Design, New York, Wiley Online Library, 2012, pp 578-
[51] Kudra T and Mujumda A.S., Advanced drying technologies, CRC Press, 2009
[52] Chou Siaw Kiang and Chua Kian Jon, "Heat Pump Drying Systems," in Handbook of Industry Drying, 3rd ed., Arun S Mujumdar, Ed., Oxfordshire, UK, Taylor &
[53] Chua K.J et al., "Heat pump drying: recent developments and future trends," Drying
[54] Praveen Kumar and SK Shukla, "Effect of Curculigo orchioides in experimental,"
Journal of Pharmacognosy and Phytochemistry, vol 8, no 1, pp 1012-1016, 2019
[55] Quang H T et al., "Study on genetic diversity of Curculigo orchioides Gaertn populations from Vietnam, an endangered medicinal herb," Plant Cell Biotechnology and Molecular Biology, vol 19, no 7-8, pp 293-301, 2018
[56] Hieu L T et al., "In vitro antioxidant activity and content of compounds from Curculigo orchioides rhizome," Hue University Journal of Science: Natural Science, vol 129, no 1B, pp 71-77, 2020
[57] Orphanides A et.al, "Effect of drying method on the phenolic content and antioxidant capacity of spearmint," Czech J Food Sci., vol 31, no 5, p 509–513, 2013
[58] Jessica L et al., "Effect of dehydration temperature on physico-chemical properties and antioxidant capacity of goldenberry (Physalis peruviana L.)," Chilean journal of agricultural research, vol 73, no 3, pp 293-300, 2013
[59] Vinh N.Q et al., "Ảnh hưởng của nhiệt độ sấy đến hàm lượng polyphenol tổng số và khả năng kháng oxi hóa của đài hoa bụp giấm [An effect of drying temperature on total polyphenolic content and antioxidant activity of roselle calyxes (Hibiscus Sab L.]," Tạp chí Khoa học Đại học An Giang, vol 4, no 3, pp 74-78, 2014
[60] Dzung N.T Assoc Prof., Writer, THỰC TẬP QUÁ TRÌNH VÀ THIẾT BỊ
[Performance] Ho Chi Minh City University of Technology and Education, 2014
[61] TCVN 9741-2013, ISO 7513:1990, "Chè Hòa Tan Dạng Rắn - Xác Định Độ Ẩm (Hao Hụt Khối Lượng Ở 103 Độ C)[Instant tea in solid form – Determination of moisture content (loss in mass at 103oC)]," Jan 2012 [Online] Available: https://vanbanphapluat.co/tcvn-9741-2013-che-hoa-tan-dang-ran-xac-dinh-do-am- hao-hut-khoi-luong-o-103-c#toan-van [Accessed Jan 2012]
[62] TCVN 9745-1:2013, ISO 14502-1:2005, "Determination of substances characteristic of green and black tea - Part 1: content of total polyphenols in tea - Colorimetric method using Folin-Ciocalteu reagent," Jan 2013 [Online] Available: https://vanbanphapluat.co/tcvn-9745-1-2013-xac-dinh-chat-dac-trung-che-xanh- che-den-ham-luong-polyphenol [Accessed Jan 2013]
[63] Châu T.T.H., "A research on the effect of the concentration of ethanol co-solvent on the total content and the antioxidant activity of polyphenol in custard apple (annona squamosa l.) peels by using the supercritical CO2 extraction method," Tạp chí Công Thương - Khoa học và Công nghệ, Hoàn Kiếm, Hà Nội, 2020
[64] Mishra K et al., "Estimation of antiradical properties of antioxidants using DPPHradical dot assay: A critical review and results," Food Chemistry, vol 130, no
[65] Thaipong K et al., "Comparison of ABTS, DPPH, FRAP, and ORAC assays for estimating," Journal of Food Composition and Analysis, vol 19, no 6-7, pp 669-
[66] Phong L.Q et al., "Đánh giá hoạt tính kháng viêm, kháng khuẩn và ức chế enzyme α-glucosidase in vitro của nấm Ophiocordyceps sinensis giàu selen," Bản B của Tạp chí Khoa học và Công nghệ Việt Nam, vol 62, no 8, 2020
[67] Benzie F.F.I and Strain J.J., "The Ferric Reducing Ability of Plasma (FRAP) as a Measure of ‘‘Antioxidant Power’’: The FRAP Assay," ANALYTICAL BIOCHEMISTRY , vol 239, no 1, pp 70-76, 1996
[68] Riadh M.H et al., "Infrared Heating in Food Drying: An Overview," Drying Technology, vol 33, no 3, pp 322-335, 2014
[69] Wojdylo A et al., " Effect of convective and vacuum–microwave drying on the bioactive compounds, color, and antioxidant capacity of sour cherries," Food Bioprocess Technology, vol 73, no 3, p 829–841, 2014
[70] Papoutsis K et al., "Effect of vacuum-drying, hot air-drying and freeze-drying on polyphenols and antioxidant capacity of lemon (Citrus limon) pomace aqueous extracts," International Journal of Food Science and Technology, vol 52, no 4, p 880–887, 2017
[71] Karaaslan M et al., "Drying kinetics and thermal degradation of phenolic compounds and anthocyanins in pomegranate arils dried under vacuum conditions," International
Journal of Food Science and Technology, vol 49, no 2, p 595–605, 2014
[72] Katia R et al., "Changes in bioactive compounds and antioxidant activity during convective drying of murta (Ugnimolinae T.)," International Journal of Food Science and Technology, vol 49, no 4, p 990–1000, 2013
[73] Mujumda A.S , "HANDBOOK OF INDUSTRIAL DRYING," Drying Technology, vol 6, no 3, pp 571-573, 2007
[74] Kivevele T and Huan Z., "A review on opportunities for the development of heat pump drying systems in South Africa," Heat pump drying systems in South Africa, vol 110, no 5-6, pp 1-11, 2014
[75] Bejar K et al., "Effect of infrared drying on drying kinetics, color, total phenols and water and oil holding capacities of orange (Citrus Sinensis) peel and leaves,"
International Journal of Food, vol 7, no 5, 2011
[76] López J et al., "Effect of Air Temperature on Drying Kinetics, Vitamin C,," Food Bioprocess Technology, vol 3, no 5, pp 772-777, 2010
[77] Chang C.H et al., "Comparisons on the antioxidant properties of fresh, freeze-dried and hot-air-dried tomatoes," Journal of Food Engineering, vol 77, no 3, p 478–485,
[78] Da Silva D.I.S et al., "Changes of antioxidant constituents in pineapple (Ananas comosus) residue during drying process," Industrial Crops and Products, vol 50, pp 557-562, 2013
[79] Bennett L.E et al., "Total polyphenolics and anti-oxidant properties of selected dried fruits and relationships to drying conditions," Journal of Functional Foods, vol 3, no 2, pp 115-124, 2011
[80] Bellincontro A et al., "Different postharvest dehydration rates affect quality characteristics and volatile compounds of Malvasia, Trebbiano and Sangiovese grapes for wine production," Journal of the Science of Food and Agriculture, vol 84, no 13, p 1791–1800, 2004
[81] Karakaya S et al., "Antioxidant activity of some foods containing phenolic compounds," International Journal of Food Sciences and Nutrition , vol 52, no 6, pp 501-508, 2001
[82] Serratosa M.P et al., "Changes in hydrophilic and lipophilic antioxidant activity in relation to their phenolic composition during the chamber drying of red grapes at a controlled temperature," J Agric Food Chem, vol 59, no 5, p 1882–1892, 2011
[83] Vu H.T et al., "Effects of drying conditions on physicochemical and antioxidant properties of banana (Musa cavendish) peels," Drying Technology, vol 35, no 9, pp 1141-1151, 2017
[84] Miranda M et al., "Influence of temperature on the drying kinetics, physicochemical properties, and antioxidant capacity of Aloe Vera (Aloe Barbadensis Miller) gel," J
[85] Nicoli M et al., "Influence of processing on the antioxidant properties of fruit and vegetables," Trends in Food Science & Technology, vol 10, no 3, pp 94-100, 1999
[86] Rabeta M.S and Vithyia M., "Effect of different drying methods on the antioxidant properties of Vitex negundo Linn Tea," International Food Research Journal, vol
[87] Lim Y.Y and Murtijaya J., "Antioxidant properties of Phyllanthus amarus extracts as affected by different drying methods," Food Science and Technology, vol 40, no
[88] Bustos M C et al., "The Influence of Different Air-Drying Conditions on Bioactive Compounds and Antioxidant Activity of Berries," J Agric Food Chem., vol 66, no
[89] Zhou L et al., "Drying of garlic slices (Allium sativum L.) and its effect on thiosulfinates, total phenolic compounds and antioxidant activity during infrared drying," Journal of Food Processing and Preservation , vol 41, no 1, p 2016, e12734
The calibration curve of Gallic acid in evaluation of TPC
Absorbance of Gallic acid to measure total Polyphenol content
The calibration curve of Gallic acid in evaluation of TPC
The calibration curve of Trolox in DPPH free radical scavenging capacity
Absorbance of Trolox to determine DPPH free radical scavenging capacity
The calibration curve of Trolox in DPPH free radical scavenging capacity
The calibration curve of Trolox in ABTS radical scavenging capacity
Absorbance of Trolox to measure ABTS antioxidant activity
The calibration curve of Trolox in ABTS radical cation scavenging capacity y = -0.0009x + 0.9634 R² = 0.9948
The calibration curve of Trolox in FRAP assay
Absorbance of Trolox in Ferric reducing antioxidant power
The calibration curve of Trolox in FRAP method y = 0.0021x + 0.1699 R² = 0.9991
Effect of Drying methods on moisture content of each thickness :
Drying_method N Subset for alpha = 0.05
Means for groups in homogeneous subsets are displayed a Uses Harmonic Mean Sample Size = 3.000
Drying_method N Subset for alpha = 0.05
Means for groups in homogeneous subsets are displayed a Uses Harmonic Mean Sample Size = 3.000
Effect of Drying methods on Drying time of each thickness:
Drying_time_2mm N Subset for alpha = 0.05
Means for groups in homogeneous subsets are displayed a Uses Harmonic Mean Sample Size = 3.724 b The group sizes are unequal The harmonic mean of the group sizes is used Type
I error levels are not guaranteed
Drying_time_5mm N Subset for alpha = 0.05