Impacts of different drying strategies on drying characteristics, the retention of bio-active ingredient and colour changes of dried Roselle

    Impacts of different drying strategies on drying characteristics, the retention of bio-active ingredient and colour changes of dried Roselle Thing Chai Tham, Mei Xiang Ng, Gan Shu Hui, Lee Suan Chua, Ramlan Aziz, Luqman Chuah Abdullah, Sze Pheng Ong, Nyuk Ling Chin, Chung Lim Law PII: DOI: Reference:

S1004-9541(17)30162-3 doi:10.1016/j.cjche.2017.05.011 CJCHE 836

To appear in: Received date: Revised date: Accepted date:

7 February 2017 26 May 2017 27 May 2017

Please cite this article as: Thing Chai Tham, Mei Xiang Ng, Gan Shu Hui, Lee Suan Chua, Ramlan Aziz, Luqman Chuah Abdullah, Sze Pheng Ong, Nyuk Ling Chin, Chung Lim Law, Impacts of different drying strategies on drying characteristics, the retention of bio-active ingredient and colour changes of dried Roselle, (2017), doi:10.1016/j.cjche.2017.05.011

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ACCEPTED MANUSCRIPT Separation Science and Engineering IMPACTS OF DIFFERENT DRYING STRATEGIES ON DRYING CHARACTERISTICS, THE ☆

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RETENTION OF BIO-ACTIVE INGREDIENT AND COLOUR CHANGES OF DRIED ROSELLE

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Thing Chai Tham1, Mei Xiang Ng2, Gan Shu Hui3, Lee Suan Chua4, Ramlan Aziz5, Luqman Chuah Abdullah6, Sze Pheng Ong7, Nyuk Ling Chin8 and Chung Lim Law9

Department of Chemical Engineering, The University of Nottingham, Malaysia Campus

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1,2,3,7,9

Jalan Broga, 43500 Semenyih, Selangor D.E, Malaysia

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Tel.: +6(03)-89248169 Email: [email protected]

Institute of Bioproduct Development, Universiti Teknologi Malaysia, 81310, UTM, Johor Bahru,Skudai,

Department of Chemical and Environmental Engineering, Universiti Putra Malaysia, 43400 UPM Serdang, Selangor, Malaysia

Department of Process and Food Engineering, Universiti Putra Malaysia, 43400 UPM Serdang, Selangor,

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Malaysia

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Abstract: The drying kinetics of Roselle (Hibiscus sabdariffa. L) of variety Terengganu (UMKL-1) and the quality attribution of Roselle were studied. The experiments were conducted using four different drying methods, including solar greenhouse drying (SD), solar greenhouse with intermittent heat pump drying(SIHP), hot air drying (HA) and heat pump drying (HP). Among the four drying methods, HP achieved the highest drying rate at a range from 0.054 g H2O·(g DM)-1·min

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to 0.212 g H2O·(g DM)-1·min

while SD had the

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lowest drying rate, measured at 0.042 g H2O·(g DM)-1·min . The analysis on colour kinetics revealed that there is no significant colour loss (p >0.05) observed from HP’s dried Roselle. Greater amount of flavonoids compounds i.e. protocatechuic acid was found in SD and SIHP dried finished product whereas HP’s dried Roselle contains

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Supported by the Ministry of Agriculture (MOA), Malaysia (NER 30001).

higher percentage of catechin as compared to other drying methods.

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Keywords: Hibiscus sabdariffa L., drying, heat pump, total colour change, protocatechuic acid, catechin

1.0 Introduction 1.1 Roselle and its medicinal uses

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Roselle (Hibiscus sabdariffa L.) is recognized as a tropical shrub which belongs to the family Malvaceae. Roselle can be found in tropic and sub-tropic regions such as India, Indonesia and Malaysia. This herbaceous

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subshrub can grow up to 2.4 m tall with cylindrical red stems. The flowers of Roselle are typically red in calyx consisting of five large sepals with a collar (epicalyx) of 8 to 12 pointed bracts around the base[1]. The fleshy

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base of the flower (the calyx) can be processed into food products[2, 3] to make syrup, refreshing drinks, jellies,

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wine, jams and natural food colourants [4-6]. Roselle has been widely used in local medicines and food. The leaves or calyces are traditionally prepared in fusion as they are rich in anthocyanins, which has antioxidant

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property and is useful in diuretic and sedative treatment [7-10]. In fact, the flavonoid compounds of Roselle varies between studies, probably due to genetic, environmental, ecology and harvest conditions of the plant[11]. Studies have reported that the concentration of ascorbic acid of Roselle is 2.5, 3 and 9 times higher compared to

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that of blackcurrant, grapes and citrus, respectively[12]. It is also recommended to use Roselle as a folk remedy for abscesses, bilious conditions, cancer, cough, debility, dyspepsia, dysuria, scurvy and strangury [13, 14]. Roselle contains polyphenols of the flavonol and flavanol type in simple or polymerised form. Protocatechuic acid (PCA), a phenolic compound containing a 3,4-dihydroxy substructure, is a compound that naturally occurs in the dried flower of Roselle. It has demonstrated strong healing functions owing to its strong antioxidant and antitumor promotions effects[15, 16] .Besides, the antibacterial effects of PCA against food spoilage bacteria was investigated by Chao et al[17]. Other than PCA, this research paper also considers another type of flavonoid, named as catechin. Catechin is a flavan-3-ol, a type of natural phenol, antioxidant as well as plant secondary metabolite. Cocoas and teas, especially those species derived from the tea plant Camellia sinensis are rich in catechin[18, 19]. Catechin, the naturally occurring flavonoid has been proven in preventing human plasma oxidation [20]and

ACCEPTED MANUSCRIPT inhibiting the oxidation of low density lipoprotein[21].Moreover, other researchers highlighted the potent antioxidant of catechin in cardiovascular and metabolic health[22]. In this paper, the effects of drying techniques on the retention of two bio-active compounds i.e. PCA and catechin in dried Roselle were examined and

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presented.

an economical and clean method for the preservation of

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Over the years, solar energy is well-known as

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1.2 Drying of Roselle

agricultural product [23, 24]. Roselle is dried traditionally using solar greenhouse drying (SD) method in which the calyces are naturally dried by spreading over mats or plastic sheets placed directly on the open floor area[25].

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With SD method, the calyces are gathered up and kept securely in store during sundown or rainy day and the similar drying process is repeated on the next day until the weather gets better. A typical SD will take

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approximately three to four days to dehydrate the crops as it is dependent on ambient condition such as temperature and relative humidity, RH [2, 26]. Nevertheless, the quality of the dried crop from this drying

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technique is questionable due to the exposure to contaminations, dirt, pest infestation and other external calamities such as rain, and loss by birds. According to Plotto [27] and EcoCrop [28] , rain or high humidity

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during the drying process can lower the quality and yield of the calyces. Thus, a change in weather condition can

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result in poor productivity and inconsistency in drying quality of dried products. Thus far, Roselle’s drying technique still dominant by SD [2] and following by hot air drying (HA) [26, 29]. However, there is a lack of research in advanced drying technique to alleviate the aforementioned issues. In this paper, heat pump system

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(HP) is considered as an alternate drying solution for Roselle. Unlike SD, HP able to operate independently and maintain the optimum drying condition for a drying chamber. This will help to mitigate those adversities found in SD. Positive results have been reported from various drying investigations on different products including biomaterials with HP [30-33].Similarly, HP has been utilized for herbs drying such as Jew’s mallow, spearmint and parsley [34]. Although dried Roselle have been used and consumed extensively in pharmaceutical or food industry for a long period, literature review shows a lack of information about raw material processing, especially the drying process. Hence, the present work aims at investigating the drying characteristic of Roselle with SD, SIHP, HA and HP. In this case study, a locally built low temperature heat pump assisted dryer was developed to support the drying of Roselle with and without solar drying system. Besides, this research outlines the influence of both ambient condition and drying location on the drying rate of Roselle which are discussed in the following section. With the applications of four different types of drying techniques, this paper: (a) examine the drying

ACCEPTED MANUSCRIPT kinetics of Roselle calyxes and petals; (b) investigate the colour change of dried Roselle; (c) the retention of bioactive ingredient compounds in Roselle.

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1.3 Weather condition

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A tropical country like Malaysia experiences warm-humid equatorial climate, characterised by both high

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temperature and high humidity throughout the year. Temperature at sea level range from 21°C to 32°C, while the annual rainfall varies from 2,000mm to 2,500mm [35]. Furthermore, Malaysia has RH ranges between 70% and 90% [36]. It is rare to have a full day with complete clear sky although this nation has abundant sunshine. On

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average, Malaysia receives about 6 hours of sunshine per day[37].Therefore, the effect of change in weather, especially changes in ambient condition must be taken into account to relate the drying rate of the sample.

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Effects of weather change on both temperature and RH were measured and recorded.

2.0 Materials and Methods

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2.1 Preparation of Material

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Fresh Roselle of the variety Terengganu (UMKL-1) with a harvest maturity (approximately 85-100 days after sowing) were purchased from the local farmer located at Senggarang, Johor, Malaysia. The seed’s capsule is

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removed from the fresh calyx with a seed’s capsules removing tool. Then, fresh Roselle’s calyces are preserved in a chest freezer (CFR 400B, Ardo, minimal freezing temperature: -24°C, UK) with a room temperature

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controlled below 10°C. For comparison, the Roselle’s calyx is further processed into two forms: one group as whole (remain uncut) and another group is cut into petals with a pair of scissors. The dimension of the Roselle was measured with Vernier callipers (Kern Germany, with an accuracy of ±0.01 mm). The calyx consisting of five large sepals has a dimension of( 5± 1.5) cm long and (2.5 ± 0.5) cm of base width. The petal has a thickness of (3.4 ± 0.3) cm of sepal and (6.0 ± 0.2) cm long. During the experiment, the Roselle calyx and petals were prepared with initial sample mass of (20 ±1.5) g (each sample has three replications) and spread evenly on the stainless steel fine wire mesh tray for drying. 2.2 Drying strategies The experiments are carried out in four different types of operating conditions with various dryers including SD, SIHP, HP and HA. Temperature for each experiment was maintained below 45°C to define the low temperature drying for Roselle calyx and petals. On the other hand, the RH of dryers was recorded below 40% in average during the experiment.

ACCEPTED MANUSCRIPT 2.2.1 First condition: Solar drying (SD) and solar with intermittent heat pump drying (SIHP) SD for Roselle was conducted in a solar greenhouse dryer (SGD) located at Sendayan Commodity Development Centre (SCDC), Seremban, Malaysia. The SGD is designed as natural convection mode with no active air

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provided to the system. A hemisphere SGD was built with steel structure and covered with transparent thin sheet

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made from polyethylene. The transparent sheet captures the incoming solar radiation and acts as a heat collector.

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According to Abdullah and Mursalim [38] ‘Solar radiation is transmitted into the entire drying chamber through the transparent structure and since the resulting long wave radiation created within the chamber is opaque to the transparent wall and roof, the accumulated energy will heat the incoming air from the ambient thereby increasing

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the chamber temperature. The accumulated heat that transferred and conducted to the interior of room is used for diffusion activities of water and vapour from the centre to the surface of drying specimen. Meanwhile, the

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remaining amount of energy is applied for the water evaporation at the surface or lost to the ambient by radiation or convection. Under ambient condition, this process continues until the dried product reaches equilibrium

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moisture content. On the other hand, heat pump was introduced in SGD to improve the drying time and efficiency. As shown in Fig.1, the heat pump included an evaporator (refrigerant: R-134a), a condenser unit

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(water and air cooled), an expansion valve, compressor and an axial blower. The system takes in moisture laden

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air inside the SGD where the humid air is directed towards the cool surface of the evaporator. Then, the moist air is cooled down to its dew point and liquid water is removed. After the water removal, the process air becomes dry. The temperature of the dry air is increased by hot water flowing through a heat exchanger. An axial blower

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is supplying constant dry air to the system. The blower tilt approximately 45° from horizontal position to ensure the dry air is distributed across the drying shelves inside the SGD. In intermittent mode, the HP ceases to operate if room RH falls below 50%. The layout of the SGD and drying shelves was illustrated in Fig.1. Basically, the even span SGD facial facing 266° in west direction and 13 units of drying shelves are arranged in series and parallel inside the SGD. Meanwhile, the sample coding in both SD and SIHP are labelled in RRXYZ, where RR: Roselle sample; X: The shelf’s number (2,7 &11); Y: calyx (1) or petal (2) and Z: sample’s number. 2.2.2 Second conditions: Low temperature heat pump assisted dryer (HP) A locally built vertical two chamber HP was applied to examine the drying process of Roselle. As shown in Fig. 2, the HP has an evaporator (BEM-013, refrigerant: R-134a), a condenser unit (water cooled and air cooled), an expansion valve (TEN 2, Danfoss, Denmark), bi-cylinder compressor for cooling (2hp, TFH 4525Y, coolant: R134a, manufacturer: Tecumseh, France) and a 2 hp axial blower. Similarly, the main objective of HP is to maintain drying chamber’s RH and temperature at optimal conditions by extracting excess humid air from wet

ACCEPTED MANUSCRIPT product inside the drying chamber. The system takes in moisture laden air inside the drying chamber which the moist air was directed to the cool surface of the evaporator. Then, the moist air was cooled down to its dew point and condensate which later turn into dehumidified dry air. Waste heat is recovered from the condenser to

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increase the dry air temperature. If necessary, temperature of the dry air can be elevated through an auxiliary

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heater. This extra heat from dry air creates more vapour pressure that enhance the water diffusion from the

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product. Meanwhile, an embedded blower is blowing constant flow of dry air at high volume to remove the free water from the product. Additionally, the vertical drying chambers are air tight to prevent any potential heat losses to the ambient. The HP is equipped with a user-friendly logic panel and user can pre-set drying parameters

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for respective experiment. In this HP experiment, the desired RH was set at minimum of 20%, and room temperature controlled by auxiliary heater.

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The arrangement of Roselle calyx, C and petal, P inside the HP was illustrated in Fig.3; T, M and B is referring to top, middle and bottom of the HP, respectively. The air flow circulates inside the dryer is from bottom to top

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across the drying shelves.

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2.2.3 Third conditions: Hot air drying (HA)

HA were performed in a laboratory scale hot air circulation universal oven (UNB 500, Memmert, Germany,

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range: 30-220°C with accuracy of 0.5°C). The drying temperature was controlled at 35°C, 40°C and 45°C respectively and (31±0.5)% for oven humidity. The temperature and RH inside the oven were assumed constant

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throughout the drying process.

2.3 Measurement of drying room temperature and humidity The drying parameters including temperature and RH were measured with sets of thermos-hygrometer. Two sets of thermohygrometer (EE220, E+E Elektronik GES.M.B.H,Austria, , measuring range (0…100% RH, accuracy ±1.5%RH)/(-40…80°C, accuracy ±0.1°C) ; HygroFlex3-HF3, Rotronic AG, Switzerland, measuring range (0...100% RH, accuracy ±2.0%RH) (-10…60°C, accuracy ± 0.3°K) were installed at both front and rear of SGD to measure temperature and RH over drying time. For HP, a thermos-hygrometer (Omniport 30, multifunctional hand held meter with logprobe, E+E Elektronik GES.M.B.H,Austria, logprobe 31, for RH(0…100% RH, accuracy ±2%RH); temperature (-40…180°C, accuracy± 0.2°C) , was used to measure the temperature and RH inside the chamber. 2.4 Drying characteristic and kinetic profiling

ACCEPTED MANUSCRIPT Each drying sample has three replications and the moisture content loss is determined in accordance to AOAC standard[39]. An analytical balance (ME204; sensitivity 0.1 mg, Mettler Toledo, USA) was used for consecutive weight measurement of the Roselle’s samples. Later, the water activity of dried sample was measured with water

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activity meter (Pawkit, AquaLab, USA; accuracy:± 0.2). Meanwhile, dried Roselle sample weighed two grams

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each from respective drying test was carry out in a universal laboratory oven (UNB 500, Memmert, Germany) at

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105°C for 24 hours to obtain bone dry weight and determine the moisture content in dry basis (db.) The moisture content, Xd of Roselle was determined with reference to the bone-dry weight, Wd expressed in equation (1) as shown below:

𝑊𝑡 − 𝑊d × 100% 𝑊d

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𝑋d (% 𝑑𝑟𝑦 𝑏𝑎𝑠𝑖𝑠) =

(1)

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For the purpose of graphical presentation, the moisture ratio was defined based on the moisture content (Eqn. 2)[40]. Consequently, the comparison between various set of drying conditions can be done. The moisture ratio,

∅=

𝑋𝑖 − 𝑋eq 𝑋0 − 𝑋eq (2)

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∅ of Roselle during drying experiments was calculated by following equation:

X0, represents the original moisture content, Xi is the moisture content at time t, and Xeq is the equilibrium moisture content. Xeq is determined when three consecutive reading were obtained from the experiment. The

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initial and final moisture content were tabulated respectively in Table 1. -1

The drying rate (g H2O·(g DM)-1·min ) was determined by approximation of the derivatives to finite differences as shown in Eqs. (3)-(4)[41, 42]. *Drying rate at 𝑡 = 𝑡0 (first point), d𝑋 d𝑡

=

𝑋1 −𝑋0 𝑡1 −𝑡0

(3)

*Drying rate at 𝑡 = 𝑡𝑛 ; 𝑛 = 1, … , 𝑁 − 1 (intermediate point), d𝑋 𝑋𝑛+1 − 𝑋𝑛−1 = d𝑡 𝑡𝑛+1 − 𝑡𝑛−1

(4)

*Drying rate at 𝑡 = 𝑡f (last point),

(5)

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d𝑋 𝑋𝑓 − 𝑋𝑓−1 = d𝑡 𝑡𝑓 − 𝑡𝑓−1

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Where 𝑡 is time (min) and 𝑋𝑜 and 𝑋𝑓 are the moisture content (g H2O·(g DM)-1·min ) at initial and equilibrium

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conditions, respectively.

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2.5 Colour determination

Various types of drying techniques will have different effects on the physicochemical properties of a dried

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product[43, 44]. In this study, the physical property of Roselle i.e. colour is measured with a handheld colour meter (LC100, The Tintometer Ltd, England). Colour was measured at three different sides of Roselle calyx with three replications each on the sample and averaged to obtain the mean and standard deviation reading.

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Colour data of Roselle calyces and petals is expressed in 𝐿∗ , 𝑎∗ and 𝑏 ∗ values where 𝐿∗ presented as the lightness

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coefficient which range from (0 for black to 100 for white), 𝑎∗ ranged positive value of 60 indicates redness, and negative value of -60 indicates greenness). Meanwhile, 𝑏 ∗ (positive value of 60 indicates yellowness while

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negative value of -60 indicates blueness). Then, 𝑎∗ and 𝑏 ∗ were further derived into hue angle via Eq. (6). Generally, fresh Roselle has positive 𝑎∗ and 𝑏 ∗ indicates redness and yellowness of the calyces. Total colour

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change of the sample was calculated by using Eqn. 7 [45, 46]. 𝑏∗

(6)

Hue angle = tan−1 (𝑎∗) 1

∆𝐸 ∗ = [(𝐿∗ − 𝐿∗𝑜 )2 + (𝑎∗ − 𝑎𝑜∗ )2 + (𝑏 ∗ − 𝑏𝑜∗ )2 ]2

(7)

in which high degree of colour degradation of dried product was expressed by a high value of ∆𝐸 ∗ . 2.6 Statistical analysis The data of colour and retention of bio-active ingredients were reported in mean ± standard deviation (SD) of triplicate determination. Statistical calculation by SPSS version 22.0 software (IBM Corp, USA) was carried out. Significant differences (𝑝 < 0.05) between means were evaluated by one way ANOVA and Tukey’s range test. 2.7 Quality analysis for bio-active ingredient of Roselle 2.7.1 Extraction process

ACCEPTED MANUSCRIPT Bio-active ingredients from dried Roselle were extracted by reflux extraction method. Reflux extraction method is the most commonly used method to obtain bio-active ingredient from herbs [47, 48]. Dried Roselle was grinded into powder formed prior to the extraction. Two grams of powder were added into 100 ml methanol

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in a 250 ml round bottom flask equipped with a cooling condenser followed by boiling for 60 minutes. The

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a tight glass bottle and then stored in fridge for further analysis[49].

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supernatant was kept dry and filtered by using a membrane filter to get a clear extract. The extracts were kept in

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2.7.2 HPLC-PDA

Dried extract was dissolved in methanol (2.5 ml) and 10μl was taken for injection. All samples extracts

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were filtered with 0.45 µm nylon filters prior to injection. Two bio-active ingredients i.e. PCA and catechin were analyzed by a high-performance liquid chromatography (Waters Alliance e2695, Milford, MA) system

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combined with a photo diode array (Waters 2998, Milford, MA). A C18 reserved phase Xbridge column (5 µm

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pore size, 4.6 x 250 mm) was used for the separation process. The elution solvents were classified as mobile phase-A (0.1% acetic acid in distilled water) and mobile phase-B (acetonitrile), respectively. The sample were

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eluted according to the following binary gradient: 0-6.5 min - 20% B, 6.5-10 min –80% B, 10-15 min, 80% B, 15-16 min, 20%B, 16-20 min, 20%B. Each sample extract was analysed at the mobile phase with the flow rate of 0.8 mL/min and detector wavelength at 30 o C. Data were integrated by the software of Empower 3 (Waters

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Corporation, USA). All bio-active ingredients were quantified using the external standard method.Quantification was based on the peak area[50]. A serial of standard solution with different concentration ranging from 1 – 500 ppm was prepared for calibration curve. 3.0 Results and discussion 3.1 Drying characteristic of Roselle In this study, the drying profile of Roselle comprises the reduction of moisture content and the change of drying rate with time were reported. From Table 1, the initial moisture content for both calyx and petal for each respective experiment was indifferent. However, petal of HP and HA have higher initial moisture content in comparison with both SIHP and SD sample. Whereas, calyx of SD had the lowest initial moisture content among others. The variation of initial moisture content of Roselle samples between studies, probably due to harvest conditions, ecology, conditions of crop, soil type, and environmental effect such as rainfall [11, 51, 52].

ACCEPTED MANUSCRIPT In terms of final moisture content, both HP and HA samples show consistent results for both calyx and petal. This is because the sample was dried in a controlled and confined environment i.e. minimum interruption from ambient change. Conversely, both SD and SIHP samples have bigger variance in final moisture content when

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environmental factor such as weather change influences the stability of final moisture content at the end of

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drying period. As shown in Table 1, corresponding water activities for SIHP and SD were both 0.51; whereas

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HA and HP were measured at 0.52 and 0.54, respectively. Despite slight difference of final moisture content between various drying strategies, however, all the dried samples have approximate water activities well below the maximum threshold (𝑎w < 0.60).These results showed that the low temperature drying able to reduce the

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moisture contents to a safe level that could prevent microbial spoilage and minimize deteriorative reactions[53]. 3.2 SD

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In Fig. 4, the dried Roselle demonstrated five different stages of moisture reduction trend. The first 400 minutes shows a drastic reduction of moisture followed by modest reduction at between 500 to 1500 minutes. Again, the

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moisture loss was reported in between 1500 to 2000 minutes before steady plateau was formed near the end of

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the drying session. A significant change in moisture content for the first 400 minutes was recorded for petals whereas calyx has slightly lower moisture reduction rate. This drastic change was due to petals (RR721 and

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RR11_21) received higher coverage of solar irradiation especially in the middle and rear of SGD. Additionally, the available surface of petals exposed to the sunlight is bigger than the calyx. According to Fatouh et al.[34], whole plants required longer drying time than that of leaves and drying time increased with increasing surface

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area and small size herbs exhibited short drying hour. By comparison, more water loss can be observed from petals than calyx sample. On the contrary, drying temperature was the key parameter that significantly influenced the drying rate[54-56]. Hence, the SGD’s temperature also contributes to the rapid drying of sample. In Fig.9, the temperature of SGD was above 40 °C between 12 pm and 5pm while highest temperature was recorded during 2pm to 3pm.Similar outcome was reported by Rabha et al.[57] where the ambient temperature was found highest during 11 am to 2 pm. In this hour, the solar irradiation position is directly vertical to the sample at the noon while slightly slanted during late afternoon. In terms of drying rate, both calyx and petal at front of SGD (RR211 and RR221) recorded highest drying rate at -1

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0.036 g H2O·(g DM)-1·min and 0.042 g H2O·(g DM)-1·min , respectively at the first 20 minutes of drying. In Fig. 5, the sharp increase in drying rate within a short duration was in fact due to the sample position at the SGD in which samples received maximum amount of solar irradiation and causing temperature raise and rapid water

ACCEPTED MANUSCRIPT losses. In the meantime, calyx samples only have proximate moisture reduction trend and slower moisture loss rate when compared to that of petals. It was observed that an unsteady drying rate trend happened from 0 to 335 minutes. The major cause for this

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unusual trend was due to weather interference such as cloudy day and rainfall that will increase the room RH.

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Besides, this phenomenon can be explained by the shifting of solar irradiation pointing from west of SGD (refer

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to Fig.1) during late afternoon. In this case study, increasing trend of drying rate was observed in subsequent days falls between 1500-2000 minutes and at 3000 minutes, respectively as an effect of hot weather during the mid-day. However, these drying rates were insignificant in comparison with the drying rate obtained from the

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first day of drying as most of the free moisture was removed. Low drying rate was observed at 695-1500 minutes and 2100-2800 minutes, respectively suggested the sample’s moisture migration was dominated by internal

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diffusion. On the other hand, the total drying time for petals was 67.5% shorter than the calyx in the same drying condition. In fact, agricultural product dried with SD usually consumed longer drying hours e.g. four days for tomato[58] ,35 h for Roselle[59], 193 h for ghost chilli pepper [57].

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Fig.6 illustrated the variation of temperature and relative humidity recorded in SGD when SD was employed.

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Average temperature recorded from the SGD from SD was( 40.16±7.24)°C, whereas RH was (37.56 ±15.06) %. As aforementioned, the interference of cloudy weather has adverse effect toward the drying temperature and

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RH[60]. A reduction of SGD temperature is causing the increase of RH. Indirectly, this will slow down the whole drying process. It was observed that the room RH can reach more than 70% over the night as shown in Fig. 9. This is not an ideal drying environment for hygroscopic material such as herb or spices due to its

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susceptibility of reabsorb moisture from the surrounding air when humidity is high[61]. The risk of microbial contamination is high if drying by rain or slow drying rate[62]. 3.3 SIHP The performance of a SIHP was influenced by four ambient parameters such as solar irradiation, ambient relative humidity, ambient wind velocity and ambient temperature[63]. Moisture loss profile of both petals and calyx in Fig. 7 exhibited four major falling rate period with a sharp moisture reduction in the first 300 minutes and followed by a constant plateau. The constant plateau indicated a gradual decrease of moisture removal from samples especially at night. Obvious moisture reduction with a steep slope was recorded on the next day of drying and similar pattern occurred in the third and fourth drying period due to overnight drying. Similar result to SD in terms of quicker moisture removal rate of petal than calyx was found in SIHP too.

ACCEPTED MANUSCRIPT Fig.8 depicted highest drying rate was achieved by petal located at middle (RR721) with 0.059 g H2O·(g DM)1

·min-1; however, calyx exhibited lower drying rate despite sharing the same drying location. In comparison to

SD, SIHP’s sample had consistent drying rate in the first 300 minutes. In addition, the fluctuation of drying rate

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has been minimized and average drying rate for both petal and calyx was slightly higher than SD’s samples. On

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the next day, the drying rate for both calyx and petal once again increased and reached as high as 0.017 g H2O·(g

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DM)-1·min-1 for petal and calyx located at the middle section. Similar trend of drying rate was formed in others samples and drying rate of calyx was marginally improved thereafter. Unlike SD, SIHP drying shows constant drying rate in 1700-2400 minutes’ region followed by a small increase of drying rate before it reaching

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equilibrium moisture content. This slow drying process can be explained by the domination of internal moisture diffusion mechanism over the external free surface evaporation[63]. With SIHP, the room RH’s was maintained

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at desired lower level and hence the water removal rate was consistent even at night drying[34, 64]. Evidently, the drying rate of SD with heat pump assisted had higher drying rate than SD alone.

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In this case study, the heat pump was set up to operate intermittently when drying room RH rises above 55%.

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This has assisted in the continuity of the drying operation at night. When using the SIHP strategy, average temperature and RH inside the SGD were measured at (40.0±6.97)°C and (37.96± 14.17) % respectively. The

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SGD’s drying condition for SIHP was fairly similar to SD deduced a fair comparison between these two experiments. In Fig.9, the room’s RH with HP operated intermittently measured at 57.8%-67% (during night) which was slightly lower than SD (58.4%-75.5%). On the other hand, a huge spike of RH measured at 75.5%

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was due to heavy rain that causing higher ambient RH in that particular of drying period. In a nutshell, solar dryer integrated with heat pump warrants sustainable drying performance and shorter drying time with controlled drying condition at low RH. Additionally, the drying rate was extended beyond sunset. Similar observation with different products was reported by Yahya et al.[65],Gan et al.[66] ,Best et al.[67] and Seyfi et al.[68]. 3.4 HP In this experiment, a vertical type two chamber HP was applied to dry Roselle sample without expose them to solar irradiation. As shown in Fig.10, petals demonstrated faster moisture reduction rate in comparison to the calyx flower which have similar outcomes obtained from both SD and SIHP strategy. Besides, it is noteworthy to highlight that the moisture reduction rate was higher for those sample located near the bottom shelf of HP, especially at the first 1000 minutes. At this stage, moisture particles were picked from surface by convective heat transfer. In the air flow design of HP (Fig.3), samples located at bottom shelf received fresh dry air and greater air flow than the top sample. The air turbulence created by recirculating fan inside the dryer helps in moisture

ACCEPTED MANUSCRIPT removal[69, 70]. Eventually, this sample experiences rapid water migration and loss of moisture content. After 1200 minutes of drying hours, the moisture reduction shown insignificant variation as the dried sample either top or bottom has gradually approaching equilibrium moisture content. With HP, the drying time of calyx was

T

shorter by 45.04% and 37.60% in comparison with SD and SIHP respectively. This was supported by the fact

IP

that lowering air relative humidity in the drying air would create a greater drying force for moisture removal on

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the solid surface hence, the increase the drying rate and shorter drying hours[71, 72].

Both petals and calyx were subjected to substantial drying rate as shown in Fig. 11. Petals exhibited significant -1

-1

drying rate ranging from 0.103 g H2O·(g DM)-1·min to 0.212 g H2O·(g DM)-1·min followed by calyx which

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was slightly lower than petals. Similarly, the average drying rate for top petal and calyx was lower than bottom sample due to air flow design as aforementioned[70]. However, this phenomenon was only apparent in the

MA

beginning of the drying test. The drying rate of petals and calyx approximate to each other as soon as the drying time reached 400 minutes and above. In other word, the Roselle samples was reaching the falling rate drying

D

period in which the change of moisture was insignificant. In a nutshell, the average drying rate for HP was found

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two to three folds faster than SD and SIHP strategy. Other successful testimonials with HP drying were demonstrated by Minea et al. [73] ,Chua et al. [74] and Fatouh et al.[34], respectively.

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For herb application, HP is recommended to operate at temperature lower than 50 °C[75, 76]. It was clearly shown from Fig. 12 that the HP chamber humidity well controlled between (21.1± 3.52) %RH and drying room temperature at (32.1±2.0)°C. The temperature and RH trend demonstrated an increasing and decreasing trend,

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respectively during the drying test. However, there was a slight drop in temperature measured in the morning (8 am) of the second day of drying due to a lower ambient temperature at the test venue. Also, the RH recorded below 20% before the sample reach equilibrium moisture content. A sustainable low RH and controlled temperature environment has enhanced the dehydration process of Roselle, thus accelerated the drying rate and drying time is reduced. A reduction in drying time using HP was also reported by Phoungchandang et al. [77] in the drying of lime leaf too. Moreover, the HP helps in minimize the rehydration issue, especially overnight drying through continuous drying at controlled environment.

3.5 HA A universal drying oven (HA) has 30 % shorter drying time in comparison with HP. The oven’s humidity level and temperature were assumed constant over the drying period. The variation of moisture ratio demonstrated that the higher the drying temperature, the faster the moisture reduction in the sample. As illustrated in Fig. 13, petal

ACCEPTED MANUSCRIPT has faster moisture removal rate at 45 °C, however there was no difference in terms of moisture reduction rate in between 35 °C and 40 °C. Saeed et al.[29] reported that at different temperatures (35,45,55 and 65°C ), the drying time was reduced with increased temperature. This suggested that temperature beyond 40°C can improve

T

the moisture removal rate for Roselle.

IP

On the other hand, the drying rate of petals was substantial in the first 30 minutes as illustrated at Fig.14. This

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was mainly due to the continuous hot air circulation inside the oven removes the surface water from the petals which have bigger area exposed to hot air in comparison to calyx. Similarly, the drying rate for the petals dried -1

at 45°C reached highest at 0.11 g H2O·(g DM)-1·min . This was followed by the petal and calyx drying at 40 -1

NU

°C and 45 °C respectively with a drying rate of range of 0.10 g H2O·(g DM)-1·min

or a 10% dropped in the

average drying rate. Likewise, the drying impact of HA was more prominent in petal than the calyx which also

D

3.6 Colour analysis of dried Roselle

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applied to SD, SIHP and HP strategies.

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Table 2 demonstrated the colour parameters CIE L*, a* and b* values and other derived parameters such as total colour change (ΔE), chroma and hue from different drying methods. Total colour changes, ΔE from solar drying

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(both SD and SIHP) exhibited marginal difference between calyx and petal. Both experiments suggested higher colour change was found in Roselle petal. Conversely, total colour change from HA’s dried calyx was slightly higher (both 35°C and 40°C) than SD and SIHP’s dried calyx. Whereas, HA’s petals exhibited lower colour

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changed when 40°C and 45°C was applied, respectively in drying. Instead, the HP dried Roselle are showing consistent colour results particularly in L*, b* and ΔE regardless calyx or petal (p>0.05). The degree of redness (a*) from HP dried sample was higher than sample from others drying strategy, especially with SD’s sample. According to Wong et al.[78], the calyxes comprises of brilliant red pigments of four anthocyanins including dephinidin 3-sambubioside and cyanidin 3-sambubioside as the major pigments and delphinidin 3-glucoside and cyanidin 3-glucoside as the minor ones. The anthocyanins are derivative of the basic flavylium cation structure which are highly reactive and involve in decolourization of the anthocyanin pigments. The rate of anthocyanin decomposition depends on factors such as temperature, oxygen, temperature and etc.[79, 80]. Similarly, drying temperature significantly induced the increase of a* and b* colourimetric parameters due to non-enzymatic browning reaction, which turned the samples more reddish and yellowish when the temperature rise.[44]. This phenomenon was further demonstrated by HA Roselle calyx whereby the a* and b* increased proportionally

ACCEPTED MANUSCRIPT with temperature. On the other hand, the increases of chroma value from all the samples was due to the present of high level of oxygen which stimulated the enzymatic browning reaction between the oxygen[45, 81]. The hue value for dried Roselle measured at between 12° and 19° was indicating a dark red colour of final product. In

T

conclusion, low temperature drying for Roselle warrant a total colour change within 10% (mean value) whereas

IP

HP assures consistency of colour for both petals and calyx.

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3.7 Quality analysis

In Fig.15 (a), HA yield lowest percentage solution of PCA in comparison with other drying methods. The average PCA% was merely (0.0176±0.0006)wt%. The reason for this is unknown but probably caused by the

NU

reaction change such as oxidative degradation during the drying period. Literature reviews suggested that the stability of polyphenols compound can be altered by different conditions such as light, high temperature ,oxygen,

MA

solvents, the presence of enzymes, proteins, metallic ions, or association with other food constituents[82]. Cheng’s investigation in phenolic compound in high temperature water suggested the decomposition of PCA increased with rising temperature and the acids became less stable with longer heating time[83]. In this case

D

study, both SIHP and SD demonstrated high retention of PCA with a highest percentage solution from calyx

TE

measured at SIHP:0.1058 wt% and SD:0.1211 wt%, respectively. From Fig.15 (b), it was noticed that the catechin retention from HPwas two to three times higher than other drying strategies. SD and SIHP have average

CE P

catechin content in both calyx and petals whereas HA recorded low catechin value .According to Li et al, catechin degradation kinetics was affected by RH and temperature, but temperature was the dominant factor[84].

AC

Thus, with lower RH and consistent low drying temperature, the HP can retain more bio-active compounds. By comparing different drying strategies, HA is a less desirable drying method particularly when higher retention of bio-active ingredient is desired. On the other hand, while retaining good amount of PCA content, HP’s dried Roselle also contain high retention of catechin acid among the other drying techniques. Alternately, the solar drying (both SD and SIHP) produced balanced compounds retention between PCA and catechin acid respectively as exemplified in Fig.15.

4.0 Conclusions By comparing the drying kinetics of four different drying methods, the highest drying rate was achieved by HP, followed by HA, SIHP and SD. The results suggested confine dryer such as HP and HA can speed up the drying process and increase the output of the product, meanwhile SD has poor drying performance due to environmental factor e.g. weather condition and product rehydration during overnight drying. Likewise, HP had yield consistent

ACCEPTED MANUSCRIPT red colour of dried Roselle for both calyx or petal which is an added value property to finished product. Although HA had significantly reduced the total drying time of Roselle, the retention of bio-active ingredient was undesirable. HPLC analysis revealed that both SD and SIHP have significant retention of PCA in dried

T

Roselle (𝑝 < 0.05) whereas higher catechin acid was detected in HP(𝑝 < 0.05), followed by SD and SIHP,

IP

respectively. Overall, HP or heat pump assisted solar drying (SIHP) is a better drying option for Roselle as dried

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Roselle could have better colour quality and high retention of bio-active compounds.

5. 6. 7.

8.

9.

10. 11.

12.

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D

4.

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3.

CE P

2.

Ross, I.A., Medicinal plants of the world: Chemical constituents, traditional and modern medicinal uses. Vol. 1. 2003: Humana Press Inc. Tjukup Marnoto., Endang Sulistyowati., Budiyastuti, P., Sumarwoto, P., M.Syahri., Bambang Sugiarto., Yusuf Hanafi., Girman., and Kristianingrum., Drying of Rosella (Hibiscus sabdariffa) flower petals using solar dryer with double glass cover collector. International Journal of Science and Engineering, 2014. 7(2): p. 150-154. Ashaye, O.A., Studies on moisture sorption isotherm and nutritional properties of dried Roselle calyces. International Food Research Journal 2013. 20(1): p. 509-513. Duke, J.A., Handbook of Energy Crops. 1983, Center for new crops and plants products: Purdue University,Indiana. Esselen, W.B. and Sammy, G.M., Applications for roselle as red color food colorant. Food Product Development, 1975. 9: p. 37-40. Beristain, C.I., García, H.S., and Vazquez, A., Foam mat dehydration of Jamaica (Hibiscus sabdariffa L.) instant drink. Drying Technology, 1993. 11: p. 221-228. Carbajal, O., Waliszewski, S.M., Barradas, D.M., Orta, Z., Hayward, P.M., Nolasco, C., Angulo, O., Sanchez, R., Infanzon, R.M., and Trujillo, P.R.L., The consumption of Hibiscus sabdariffa dried calyx ethanolic extract reduced lipid profile in rats. Plant Food for Human Nutrition, 2005. 60: p. 153-159. Akindahunsi, A.A. and Olaleye, M.T., Toxicological investigation of arqueousmethanolic extract of the calyces of Hibiscus sabdariffa L. Journal of Ethnopharmacology, 2003. 89: p. 161-164. Arroyo, S.F., Inmaculada, C., Rodriguez, M., RaulBeltran-D., Federica, P., Jorge, J., Vicente, M., Antonio, S.C., and Alberto, F.G., Quantification of the polyphenolic fraction and in vitro antioxidant and in vivo anti-hyperlipemic activities of Hibiscus sabdariffa aqueous extract. Food Research International, 2011. 44: p. 1490-1495. Patel, S., Hibiscus sabdariffa: An ideal yet under exploited candidate for nutraceutical applications. Biomedicine & Preventive Nutrition, 2014. 4: p. 23-27. Da-Costa-Rocha, I., Bonnlaender, B., Sievers, H., Pischel, I., and Heinrich, M., Hibiscus sabdariffa L. – A phytochemical and pharmacological review. Food Chemistry, 2014. 165: p. 424-443. Musa, Y., Engku Ismail EA., Yahaya,H., Manual of Roselle's Cultivation Technology (Manual teknologi penanaman rosel). 2006, Kuala Lumpur: Institut Penyelidikan dan Kemajuan Pertanian Malaysia (MARDI).

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Table 1: Corresponding water activity, initial and final moisture content of calyx and petals in different drying techniques Average final Water activity Drying strategies Average initial moisture moisture content/% aw content/% (db.) (db.) SD Calyx

712.02 ±109.01

13.05±1.44

Petal

896.55±149.68

10.99±1.53

D

SIHP

973.28±139.42

19.41±4.77

TE

Calyx Petal HA

956.46±46.76

16.16±0.22

1028.55±43.73

15.55±1.93

1043.20±72.22

15.46±1.80

1078.84±72.16

16.22±1.02

1042.40±82.90

16.38±0.09

CE P

Calyx Petal HP Calyx

0.51

0.52

0.54

AC

Petal

0.51

Table 2: Colour kinetics under different drying conditions and parameters

Drying methods Fresh Roselle

Total colour change, ΔE

Colour Parameters L*

a*

Chroma

Hue

b*

18.433±2.57a

17.36±2.57 b

5.58±1.12c

15.20±3.54a

17.12±0.65a

Calyx

21.38 ±4.45b

11.67±3.77a

3.97±2.77f

7.28±2.67b

12.61±3.86 d

18.19±0.84b

Petal

22.54±4.47 c

13.49±4.98a

3.48±2.29f

8.95±5.11c

14.19±4.64 e

14.76±0.41c

Calyx

20.57±1.87a

15.95±3.80 b

4.82±2.18c

6.47±2.78 a

16.77±3.91 a

16.22±0.51d

Petal

24.40±0.95c

17.81±3.33 b

3.70±2.53f

8.59±3.04c

18.50±3.53 b

12.52±0.77e

19.53±2.11a

17.51±4.10 b

3.74±2.99f

8.53±3.40 c

18.23±4.12 b

12.55±0.55e

-

SD

SIHP

HA (calyx) at 35 °C

ACCEPTED MANUSCRIPT at 40 °C

22.03±0.99c

17.12±5.09 b

4.76±2.07 c

8.39±4.16 cd

17.86±5.19 b

15.15±0.35c

at 45 °C

21.74±1.31b

19.30±3.68c

5.03±1.72 c

6.57±2.02 a

20.03±3.60 bc

14.35±0.70c

at 35 °C

21.20±3.08 b

19.41±6.30 c

6.18±3.02 d

8.96±5.33 c

20.43±6.51 bc

17.56±0.52ab

at 40 °C

20.68±2.36 ab

16.59±2.86 b

5.29±1.84 c

7.56±3.92 b

17.49±2.83 b

17.82±0.16ab

at 45 °C

21.37±2.90 b

18.08±2.82c

5.71±1.31 c

7.61±2.31 b

19.71±3.17 b

17.14±0.34a

23.19±0.92 c

21.42±2.90 d

6.56±2.29 d

7.27±2.32 b

Calyx

IP

HP

T

HA (petal)

21.98±3.85 c

16.97±0.36ad

SC R

18.80±0.23b 24.53±1.28 c 18.44±4.28 b 6.30±2.49 d 7.79±4.01 bd 19.96±4.41 b Petal Mean values ± standard deviation (n=3 replications) within the same column with the same letter are not

NU

significantly different (p>0.05)

MA

Front Entrance

AC

CE P

TE

D

HP

S1 S2 S3

S4

S5

S6

S7

S8

S9

S10

S11

S12

S13

Rear

D

MA

NU

SC R

IP

T

ACCEPTED MANUSCRIPT

AC

CE P

TE

Fig. 1: The layout of SGD (from top view), location of drying shelves and heat pump

Fig. 2 : Low temperature heat pump assisted dryer (HP)

ACCEPTED MANUSCRIPT

Moist air to dehumidify

T

Heat pump assisted dryer

IP

Shelf 1 T_C

Shelf 3 M_C

NU

Shelf 4 M_P

SC R

Shelf 2 T_P

Shelf 5 B_C

Dry air to drying chamber

D

MA

Shelf 6 B_P

TE

Fig. 3: Position of Roselle samples and direction of air flow

CE P

1.2

0.8

AC

Moisture ratio (dimensionless)

1

0.6

RR211

RR711

RR11_11

RR221

RR721

RR11_21

0.4

0.2

0 0

500

1000

1500

2000

2500

Drying time (min) Fig. 4: Changes in moisture ratio with time using SD strategy

3000

3500

ACCEPTED MANUSCRIPT 0.045 0.04

RR221

RR721

0.02 0.015

T

RR711

IP

0.025

RR211

SC R

0.03

RR11_11

RR11_21

NU

Drying rate, (gH20/gDM.min)

0.035

0.01

0 0

500

1000

MA

0.005

1500

2000

Drying time (min)

2500

3000

3500

45 40 35 30

70 60 50 40

25

30

20 20

15

T

10

RH 10

Time

Fig. 6: Variation of temperature and RH in SGD (SD)

Relative humidity,%RH

Room Temperature ,(°C )

50

CE P

55

80

AC

60

TE

D

Fig. 5: Changes in drying rate with time using SD strategy

ACCEPTED MANUSCRIPT 1.2

RR211

RR711

RR11_11

RR221

RR721

RR11_21

T

Moisture ratio (dimensionless)

1

SC R

IP

0.8

0.6

NU

0.4

0 0

500

1000

MA

0.2

1500

2000

2500

3000

D

Drying time (min)

TE

Fig. 7: Changes in moisture ratio with time using SIHP strategy

CE P

0.07

0.05

AC

Drying rate, (gH20/gDM.min)

0.06

0.04

RR211

RR711

RR11_11

RR221

RR721

RR11_21

0.03

0.02

0.01

0 0

500

1000

1500

2000

2500

Drying time (min) Fig. 8: Changes in drying rate with time using SIHP strategy

3000

3500

ACCEPTED MANUSCRIPT 60

80 70

50

30

40

20

20 T

NU

10

30

10

RH 0

MA

0

Relative humidity,(%RH)

T

50

IP

40

SC R

Temperature,(°C)

60

D

Time

TE

Fig.9: Variation of temperature and RH in SGD (SIHP)

CE P

1.2

0.8

AC

Moisture ratio, (dimensionless)

1.0

0.6

T_P

M_P

B_P

T_C

M_C

B_C

0.4

0.2

0.0 0

200

400

600

800

1000

1200

1400

1600

Drying Time (min)

Fig. 10: Changes in moisture ratio with time using HP strategy

1800

2000

ACCEPTED MANUSCRIPT 0.25

T IP

0.15

T_P T_C

M_P

B_P

M_C

B_C

SC R

Drying rate, (gH2O/gDM.min)

0.20

0.10

0.00 200

400

600

800

1000

MA

0

NU

0.05

1200

1400

1600

1800

2000

Drying Time (min)

D

Fig. 11: Changes in drying rate with time using HP strategy

27.0 25.0 23.0 21.0

20.0 19.0 15.0 17.0 10.0

15.0

Time Fig. 12: Temperature and moisture evolution trend in the HP chamber

Relative humidity, %RH

25.0

29.0 RH(%)

CE P

30.0

Temperature (°C )

AC

Temperature, (°C)

35.0

31.0

TE

40.0

ACCEPTED MANUSCRIPT 1.2

1

0.6

T

IP

HA35_P

HA40_P

HA45_P

HA40_C

HA45_C

SC R

Moisture ratio

0.8

HA35_C 0.4

0 200

400

600

800

MA

0

NU

0.2

1000

1200

1400

Time (Minutes)

TE

D

Fig. 13: Changes in moisture ratio with time using HA strategy

Drying rate, (gH20/gDM.min)

CE P

0.14

0.12

AC

0.1

0.08

0.06

HA35_P

HA40_P

HA45_P

HA35_C

HA40_C

HA45_C

0.04

0.02

0 0

200

400

Time (Minutes) 600 800

1000

Fig. 14: Changes in drying rate with time using HA strategy

1200

1400

ACCEPTED MANUSCRIPT 0.16

a

0.14 Protocatechuic Acid

T

b

c

IP

bc

0.10

SC R

Protocatechuic acid, %w,w

0.12

0.08 0.06

d

d

d

MA

0.02 0.00

HA 35

HA 40

HA 45

e

NU

0.04

e

SIHP_C

SIHP_P

SD_C

SD_P

HP_C

HP_P

Drying strategies

TE

D

(a)

8.00

Catechin

Catechin,%w/w

5.00 4.00 3.00

c

AC

7.00 6.00

d

CE P

9.00

a

a

b

b

SIHP_C

SIHP_P

b

b

SD_C

SD_P

a

2.00 1.00 0.00 HA 35

HA 40

HA 45

Drying strategies

(b)

HP_C

HP_P

ACCEPTED MANUSCRIPT Fig. 15: Retention of PCA (a) and catechin (b) in dried Roselle calyx and petals under different drying strategies. Oven drying with different temperature (HA35, HA40, HA45); SIHP (solar with intermittent heat pump drying for Roselle calyx, C and petal, P); SD (solar drying for Roselle calyx, C and petal, P) and HP (heat pump drying

T

for Roselle calyx, C and petal, P)

AC

CE P

TE

D

MA

NU

SC R

IP

Vertical bar and line graph indicated with the same letter are not significantly different (p>0.05)

ACCEPTED MANUSCRIPT Highlights

CE P

TE

D

MA

NU

SC R

IP

T

A unique drying technique with heat pump for Roselle processing is proposed. The color kinetics of dried Roselle from different drying techniques were compared. The study has demonstrated the potential of innovative drying for Roselle due to its noteworthy drying efficiency as well as retention of bio-active ingredient.

AC

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