Evaluation of spray-dried and freeze-dried red pitaya powder as a

functional natural colorant in a model juice system

S.W. Chan1*, Y.J. Lee1, P. K. Lim1 and C.P. Tan2

1School of Biosciences, Taylor’s University, No. 1, Jalan Taylor's, 47500 Subang Jaya

Selangor, Malaysia

2Department of Food Technology, Faculty of Food Science and Technology, Universiti Putra

Malaysia, 43400 UPM Serdang, Selangor, Malaysia

*Corresponding author:

Dr. Chan Sook Wah

School of Biosciences,

Taylor’s University,

No. 1, Jalan Taylor's,

47500 Subang Jaya Selangor,

Malaysia.

Tel: +603- 5629 5553

Fax: +603- 5629 5311

E-mail address: sookwah.chan@taylors.edu.my

1

Abstract

Red pitaya (Hylocereus polyrhizus) is an exotic fruit which belongs to the cactus family of

Cactaceae. It is cultivated in many tropical countries such as Thailand, Cambodia, Vietnam,

China, Australia and Malaysia. Pitaya fruit has recently drawn considerable attention due to

its health beneficial claims such as antioxidant activity and the potential to reduce the risk of

hypercholesterolemia and hypertension. Particularly, betacyanin in red pitaya is a potential

source of natural colorant as an alternative to synthetic colorant. Consumption of synthetic

colorants has been associated with many health problems such as liver and kidney damage. In

this work, red pitaya powder was obtained via different processing methods (spray drying and

freezing drying). Their physicohemical and antioxidant properties were compared and the

stability of red pitaya betacyanin was analyzed by applying into a model juice system. As

compared to spray-dried powder, freeze drying produced powder of higher yield with

significantly (p < 0.05) lower moisture content, water activity; significantly (p < 0.05) higher

bulk density, tapped density and particle size; better flowability and lower cohesiveness.

However, betacyanin content and antioxidant activities of both spray-dried and freeze-dried

red pitaya powder were comparable. Significant (p < 0.05) changes in betacyanin were

observed during 4 weeks storage study. Betacyanin was best preserved in model juice at

storage temperature of 4 °C, but completely degraded at room temperature and 40 °C after

one week storage. With these findings, it can be concluded that betacyanin from red pitaya

powder could be a potential natural colorant for chilled food products.

Keywords: red pitaya, spray drying, freeze drying, betacyanin, natural colorant

2

Introduction

Hylocereus polyrhizus or better known as red pitaya fruit is originated from America and

belongs to cactus family and plant order of Caryophyllales (Phebe & Chew 2009). This

species have its peel and flesh in purple-red color. The flesh contains small black seeds which

distributed evenly and its texture is delicate and juicy (Jamilah et al. 2011). Red pitaya fruit is

cultivated worldwide in Malaysia, Bangladesh, China, Vietnam, Taiwan, Thailand, Australia,

Israel and others (Ee et al. 2014a; Jamilah et al. 2011). In Malaysia, red pitaya fruit is

consumed as fresh fruit as well as commercially manufactured to produce juices or jams

products. The deep purple color of red pitaya flesh is contributed by the betacyanin pigment.

It is reported that betacyanin has antioxidant properties which making it suitable to be used as

food colorant and serves as functional ingredient in food product (Lee, Wu & Siow 2013).

One of the factors that affect the customers’ food choice is color. Synthetic colorant has been

applied for ages in order to improve and enhance the food appearance. In Malaysia, 14

synthetic colorants have been permitted to be applied as coloring substance in food (Food Act

1983 (Act 281) & Regulations (as at 1st March 2014). However, due to the reported allergic

and intolerance cases after consumption of synthetic colorant and the potential health hazard,

natural colorant has recently gained market attention in order to replace synthetic colorant in

the food application. According to Food Act 1983 (Act 281) & Regulations (2014), beet red

is one of the permitted natural coloring substances to be used in food. Beet red, synonym to

beetroot red is a coloring substances extracted from red beets roots. It is composed of mainly

betalain pigments, purple-red betacyanins of which account for 75-95% of betanin (FAO

2012). Despite the rich betacyanin content in beet red, red pitaya betalain is more preferable

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due to the presence of geosmine and pyrazine in beet root which contribute to “earthy” taste

(Chik et al. 2011).

Common methods suitable used to dry heat labile pigment are spray drying and freeze drying.

Spray drying converts fluid materials into dry solid particles by atomizing the feed fluid into

a hot gas medium to obtain powder instantaneously (Phisut 2012). Spray drying process is

commercially used to produce fruit juice powders of good quality and facilitate easier

transportation and storage. Freeze-drying refers to lyophilization is a drying process where

the water is frozen at very low temperature and subsequently transformed from solid state

into the vapor state through sublimation under low pressure. It became one of the best

options for the production of heat-sensitive materials because this process does not involve

heat (Ciurzyńska & Lenart 2011). Spray drying is more commonly used in the production of

powder food colorant as it is more economical. However, it might increase loss of antioxidant

content as this process involves high temperature. Freeze dried powder often give better

properties but it involves high operating cost (Laokuldilok & Kanha 2015; Lee, Wu & Siow

2013). There are various studies reported on the physiochemical properties and antioxidant

properties of red pitaya fruit as well as production of spray dried red pitaya powder (Ee et al.

2014a, 2014b; Lee, Wu & Siow 2013; Liaotrakoon et al. 2012; Woo et al. 2011). However,

there is limited research on the production of freeze dried red pitaya powder and the

comparison of physiochemical properties between spray dried and freeze dried red pitaya

powder. Besides, study on the stability of red pitaya betacyanin in the food model system is

very scarce. Hence, this research is aimed to compare the physiochemical and antioxidant

properties of red pitaya powder produced by spray drying and freeze drying. The effects of

temperature and light on the stability of betacyanin of spray dried and freeze dried red pitaya

powder in a model juice system were analyzed.

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Materials and Methods

Raw materials

All fresh red pitaya fruits used were purchased from fruits shop ‘Eats More Fruits’ located in

Kota Kemuning, Shah Alam, Selangor.

Materials, chemicals, and reagents

Maltodextrin with dextrose equivalent (DE) 10% - 13% was obtained from V.I.S Foodtech

Ingredient Supplies Sdn. Bhd., Kepong, Kuala Lumpur. Citric acid anhydrous was purchased

from Bake with Yen Sdn. Bhd., Puchong, Selangor. Sugar (coarse sugar) was obtained from

Central Sugars Refinery Sdn. Bhd., Shah Alam, Selangor. Methanol, trifluorocetic acid

(TFA), acetonitrile, Folin-Ciocalteu’s (FC) reagent, sodium carbonate anhydrous, denatured

ethanol, 1M sodium hydroxide, di-sodium hydrogen phosphate and iron (III) chloride were

supplied by Merck, Germany. Sodium nitrite, aluminium chloride-6-hydrate, trichloroacetic

acid and potassium ferricyanide were obtained from Bendosen Laboratory Chemicals,

Norway. Gallic acid, catechin and betanin (red beet extract diluted with dextrin) were

procured from Sigma-Aldrich, USA. Potassium di-hydrogen phosphate was obtained from

Hamburg Chemical, Germany. All chemicals and reagents used were of analytical grade or

high performance liquid chromatography (HPLC) grade. Distilled water was used throughout

the analyses.

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Preparation of red pitaya juice

Peel of red pitaya fruits was removed and the flesh was cut into smaller pieces. The flesh was

then blended using blender to obtain the puree. The puree was filtered 3 times using muslin

cloth to remove seeds before subjected to spray drying and freeze drying.

Spray drying of red pitaya juice

The filtered juice was mixed with distilled water at ratio of 1:2 (150 g of red pitaya juice

mixed with 300 g of distilled water). 15% (w/w) of maltodextrin (67.5 g) was added to the

red pitaya juice mixture and homogenized using blender. The red pitaya juice mixture was

then spray dried using laboratory scale spray dryer (Lab-Plant SD-06, Labplant UK Ltd.,

UK). The spray drier was equipped with 0.5 mm spray nozzle (215 mm OD × 500 mm long).

The pump speed of spray dryer was maintained at 11.58 mL/min, inlet temperature at 140 °C,

fan setting at 4.3 m/s and pressure at 2 bars. The powder obtained was stored in Schott bottle

at room temperature until further analysis.

Freeze drying of red pitaya juice

The filtered juice was mixed with same amount of maltodextrin as added into spray dried red

pitaya juice (150 g of red pitaya juice mixed with 67.5 g of maltodextrin) and homogenized

using blender. The red pitaya juice mixture was then poured into Schott bottle and frozen in

ultra-low temperature freezer (Model: DW-86L388, Haier Group, China) at –80 °C for

overnight. The red pitaya juice mixture was then freeze dried using benchtop freeze dryer

(Model: FreeZone 4.5L, Labconco, USA) at –50 °C under pressure below 0.110 mBar for 55

6

hours. The dried red pitaya product was then ground using dry mill. The powder obtained

was stored in Schott bottle at room temperature until further analysis.

Yield

The yield of red pitaya powder was calculated using equation below:

Yield (%) = Weight of powder (g) Solid content of juice (g ) + Weight of maltodextrin added (g )

×100

Solid content of juice is the dry weight of juice.

Moisture content

The moisture content of red pitaya powder was determined using electronic moisture analyzer

(Model: MOC63u UniBloc, Shimadzu, Japan).

Water activity

The water activity (aw) of red pitaya powder was determined using water activity meter

(Model: Decagon AquaLab LITE, Decagon Devices, Inc., USA).

Bulk and tapped density

For bulk density determination, 5 g of powder was measured and gently poured into a 25 mL

graduated measuring cylinder. The volume occupied by the powder was recorded and used to

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calculate the bulk density according to equation below (Jinapong, Suphantharika & Jamnong

2008; Saifullah et al. 2014; Victória et al. 2013):

Bulk density = mass of powder (g ) volume occupied (mL )

For tapped density, 5 g of powder was poured into a 25 mL graduated measuring cylinder.

The samples were repeatedly dropped manually 100 times by lifting and dropping the

cylinder under its own weight at a vertical distance of 10 cm. The volume occupied by the

samples after tapped was recorded and calculated using equation below (Jinapong,

Suphantharika & Jamnong 2008; Saifullah et al. 2014; Victória et al. 2013):

Tapped density = mass of powder (g ) volume occupied after tapped (mL )

Flowability and cohesiveness

Flowability and cohesiveness of the powders was estimated using Carr index (CI) and

Hausner ratio (HR) (Jinapong, Suphantharika & Jamnong 2008). CI and HR were calculated

from the bulk and tapped densities of the powders using equation as follows:

CI (%) = ρ tapped - ρbulk ρ tapped

×100

HR = ρ tapped ρbulk

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Flowability and cohesiveness of red pitaya powder classification are presented in Tables 1

and 2, respectively.

Particle size

Particle size of spray dried and freeze dried red pitaya powder was identified using laser

diffraction method (Model: Mastersizer 2000, Malvern Instruments Ltd, UK). The condition

set was air pressure at 1 bar, vibration rate of feed at 30% and refractive index at 1.44. The

result was reported as surface-weighted mean diameter D [3, 2] (µm) and span for the

measurement of particles size distribution.

Powder morphology

Morphology of spray dried and freeze dried red pitaya powder was observed using scanning

electron microscope (SEM) (Model: JSM 6400, JEOL Ltd., Japan). SEM stubs were attached

with double-sided adhesive tape and a thin layer of powder was stuck on the tape before

subjected for drying. The samples were then coated with a thin layer of gold and the

morphology was observed at 2000× magnification.

Color

The color of the powder and juice (L*, a*and b* values) was measured using colorimeter

(Model: ColorFlex EZ, Hunter Associates Laboratory, Inc., USA).

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Betacyanin content

The betacyanin content of the powder was identified using high performance liquid

chromatography (HPLC) (Model: Prominence UFLC, Shimadzu, Japan). Betacyanin in

sample was separated using Thermo Scientific Hypersil Gold column, (5 µm, 150 × 4.6 mm).

The mobile phase used for elution is an isocratic solution contains a mixture of 0.5%

trifluroacetic acid (TFA) (90%) and acetonitrile (10%). Isocratic solution contains a mixture

of 0.5% trifluoracetic acid in water (90%) and acetonitrile (10%) was used as the mobile

phase. The column oven condition was set at 25 °C, absorbance of 536 nm, flow rate at 1.00

mL/min for 15 minutes per each injection with 20 µL injection volume (Jamilah et al. 2011;

Rizk, El-kady & El-bialy 2014). All samples were filtered using 0.45 µm nylon syringe filter

prior to injection. The result was reported in mg/mL and the calibrated betanin standard curve

equation was y = 146234x + 2639.8 (R² = 0.9999).

Total phenolic content (TPC)

TPC was determined using Folin-Ciocalteu (FC) method adopted from Lim, Lim & Tee

(2006) with slight modification. Sample (0.3 mL) was mixed with 1.0 mL of FC reagent and

stand for 3 mins. Then, 0.8 mL of 7.5% (w/v) sodium carbonate anhydrous solution was

added into the mixture and vortex. The mixture was incubated in dark for 2 hours and the

absorbance was recorded at 765 nm using UV-Vis spectrophotometer (Model: GENESYS 10

UV, Thermo Scientific, USA). Denatured ethanol (70%) was used as blank. The polyphenol

concentration was expressed as milligram of gallic acids equivalents (GAE) per 100 g of dry

weight. The calibrated standard curve equation for gallic acid was y = 15.338x + 0.0801 (R²

= 0.9945).

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Total flavonoid content (TFC)

Estimation of the TFC in crude extracts was performed according to the procedures described

by Thoo et al. (2010) with slight modifications. Sample (0.25 mL) was added into test tube

followed by 1.25 mL distilled water and 75 µL of 5% sodium nitrite. The mixture was

incubated for 6 mins and 150 µL of 10% aluminium chloride-6-hydrate solution was added

into the mixture. The mixture was incubated for 5 mins and 0.5 mL of 1M sodium hydroxide

and 250 µL of distilled water were added into the mixture. The absorbance was then recorded

using UV-Vis spectrophotometer at wavelength of 510 nm and distilled water was used as

blank. The flavonoid concentration was expressed as milligram of catechin equivalents (CE)

per 100 g of dry weight. The calibrated standard curve equation for catechin was y = 0.0024x

+ 0.0112 (R² = 0.9996).

Ferric reducing antioxidant power (FRAP)

The ferric reducing power of the fruit extracts was determined by using the potassium

ferricyanide–ferric chloride method (Lim, Lim & Tee 2006). Sample (1 mL) was mixed with

2.5 mL of 0.2M phosphate buffer (pH 6.6) and 2.5 mL 1% potassium ferricyanide. After that,

it was incubated at 50 °C for 20 minutes. After that, 2.5 mL of 10% trichloroacetic acid was

added to the incubated mixture. The incubated mixture (2.5 mL) was transferred into new test

tube. 2.5 mL of distilled water and 0.5 mL 1% iron (III) chloride were added into the mixture

and the sample was incubated for 30 minutes. The absorbance of the mixtures was then

recorded using UV-Vis spectrophotometer at wavelength of 700 nm and 70% denatured

ethanol was used as blank. The polyphenol concentration was expressed as milligram of

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gallic acids equivalents (GAE) per 100 g of dry weight. The calibrated standard curve

equation for gallic acid was y = 21.581x + 0.1902 (R² = 0.9935).

Stability studies: temperature and light

Stability studies were conducted by incorporating spray dried and freeze dried red pitaya

powder into model juice. 600 mL of model juice was prepared with powder concentration of

5 % (30 g) , brix value around 9 – 10% (addition of sugar around 32 g) and pH value between

3.1 – 3.2 by using citric acid (approximately 0.71 g). Model juice was pasteurized at 98 °C

for 5 sec before hot filled into universal bottle and held for 5 minutes before it was cooled

under running tap water. Model juices were stored in five different conditions: 4 °C with light

and without light, room temperature with light and without light and 40 °C. Betacyanin

content of juices incorporated with spray dried and freeze dried red pitaya powder was

determined and sampled on weekly basis until week 5. The results were expressed in

betacyanin retention (%) based on the following formula:

Betacyanin retention (%) =

Betacyanin content of fresh sample - betacyanin content of weekly sample Betacyanin content of fresh sample

× 100

Statistical analysis

Results were analyses using Statistical Package for Social Sciences (SPSS) Version 21

software (IBM, SPSS Inc.). All results are reported in mean ± SD of triplicate determinations.

Independent t-test was used to determine significant differences between two samples and

One-way analysis of variance (ANOVA), followed by Tukey’s post hoc test, was used to

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conclude significant differences between three or more samples at level of p < 0.05. Repeated

measures was used to determine significant changes over 5 weeks of storage stability study of

model juice at level of p < 0.05.

Results and Discussions

Preliminary study

It was the first attempt for this research team to produce red pitaya powder using freeze

drying. Therefore, a preliminary study was done in order to identify the suitable method to

obtain an ideal sample. Pure filtered red pitaya juice was freeze dried at the initial step.

However, the end product turned out to be unsatisfied due to high stickiness of the ground

powder. This might be due to the incomplete removal of moisture from the red pitaya juice as

it contained high moisture and sugar content. High sugar or soluble solid will decrease the

drying rate hence it might require longer time to obtain a powder of better quality (Heldman

& Lund 2007). Therefore, based on several other studies done by Estupiñan, Schwartz &

Garzón (2011), Mehrnoush, Mustafa & Yazid (2012), and Murali et al. (2014), maltodextrin

was added into the red pitaya juice before subjected to freeze drying.

Physiochemical properties of red pitaya powder

Yield

In food industry, yield is an important economic consideration. The higher the yield is, the

more efficient the process is. It was observed that the yield of freeze-dried red pitaya powder

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(96.05%) was around three times higher compared to spray-dried powder (33.25%) (Table 3).

As red pitaya juices contain high sugar naturally, it might create stickiness problem during

spray drying (Lee, Wu & Siow 2013). Although maltodextrin was added to increase the glass

transition temperature of the powder, however the end result showed that the yield of spray

drying is still not satisfied. Product loss during spray drying process was largely due to the

sticking of product on the wall of drying chamber and cyclone hence led to decrease in yield.

During freeze drying, the red pitaya juices were frozen in the bottle and the end product was

all collected within the same bottle. Therefore, there is no or insignificant product loss.

Moisture content and water activity (aw)

As shown in Table 3, there is a significant difference (p < 0.05) between the moisture content

of these two red pitaya powders in which freeze-dried red pitaya powder (12.36%) has lower

moisture content compared to spray-dried red pitaya powder (13.03%). Ibarz & Barbosa-

cánovas (2003) stated that freeze-dried product with moisture content lower than 2% can be

obtained suggesting that the freeze drying time in this research can be prolonged in order to

obtain freeze-dried red pitaya powder of even lower moisture content. Apart from moisture

content, water activity (aw) is another important parameter to determine and predict shelf

stability of the food product as it will affect the safety of food. The water activity of spray-

dried and freeze dried red pitaya powder was reported as 0.412 and 0.289, respectively (Table

3). Since both water activity were lower than 0.50, therefore the samples would have lower

risk of spoilage due to microbial reactions.

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Bulk and tapped density

Spray-dried red pitaya powder has significantly (p < 0.05) lower bulk density (0.31 g/mL)

and tapped density (0.49 g/mL) compared to freeze-dried red pitaya powder (bulk density:

0.65 g/mL; tapped density 0.84 g/mL) (Table 3). Measurement of bulk density and tapped

density is essential for food powder as it is related to the physicochemical properties,

geometry, size and surface characteristics of the individual powder particles. This will allow

the adjustment for storage, processing, packaging and distribution conditions of the food

powder (Barbosa-Canovas et al. 2005).

Bulk density is the mass of particles that occupies a unit volume while tapped density refers

to the bulk density of a powder that has been settled into closer packing state by means of

tapping (Barbosa-Canovas et al. 2005). The bulk density is dependent on moisture content,

particle size and the distribution of the particle (Barbosa-Canovas et al. 2005; Goula &

Adamopoulos 2005; Grabowski, Truong & Daubert 2006; Koç & Kaymal-Ertekin 2014).

Higher moisture content in the powder will increase the tendency of powder to stick together,

hence more interspaces between the powder and therefore resulting in a larger bulk volume

and decrease in powder bulk density (Goula & Adamopoulos 2005). Hence, this justifies that

spray-dried red pitaya powder has lower bulk density due to its higher moisture content as

compared to freeze-dried red pitaya powder. Smaller particle size will have larger surface

area. This will increase the cohesive force as there is increase in contact surface area between

the particles and lead to more interspace between particles hence decrease in bulk density

(Grabowski, Truong & Daubert 2006; Jinapong, Suphantharika & Jamnong 2008; Phisut

15

2012). Lower bulk density powder is undesirable because it requires larger volume of

package (Koç & Kaymal-Ertekin 2014).

Flowability and cohesiveness

According to the classification of flowability and cohesiveness based on Carr Index (CI)

(Table 1) and the Hausner ratio (HR) (Table 2), it was observed that spray-dried red pitaya

powder has bad flowability (36%) and high cohesiveness (1.6) while freeze-dried red pitaya

powder exhibited a fair flowability (22%) and intermediate cohesiveness (1.3). According to

Jinapong, Suphantharika & Jamnong (2008), powder with better flowability often have a

higher bulk density. This was in agreement with the current research that freeze-dried red

pitaya with better flowability has higher bulk density compared to spray-dried red pitaya

powder. This is because the forces between interparticles of freeze-dried powder decreased,

hence making the powder densified and have higher packing in between the particles (Saw et

al. 2013). Particle size and particle size distribution also displayed major impact on the

powder flowability and cohesiveness. It was expected that increase in particle size will have

better flowability and decrease in cohesiveness (Jinapong, Suphantharika & Jamnong 2008).

This observation was agreed in this research in which freeze-dried red pitaya powder with

significantly larger particle size has better flowability and cohesiveness. To further explain,

smaller particles size will have higher surface area and resulted in higher cohesion and

attrition force formation that lead to resistance in powder flow (Victória et al. 2013).

Particle size

Freeze-dried red pitaya powder has significantly higher (16.00 µm) surface-weighted mean

diameter, D [3, 2] compared to spray-dried red pitaya powder (10.16 µm) (p < 0.05) (Table

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3). Besides this, spray-dried red pitaya powder displayed significantly wider span (8.06)

compared to freeze-dried red pitaya powder (2.97) (p < 0.05). Span value measures the

particle size distribution. Large particle span results in large contact area and will have low

flowability (Xu et al. 2007). As mentioned above, particle size and size distribution are the

factors that affect powder flowability. Evidenced by the larger particle size and wider size

distribution of freeze-dried red pitaya powder, thereby it has improved flowability.

Powder morphology

Under the view of SEM, irregular fragments and porous structure were observed in freeze-

dried red pitaya powder (Figure 1B) while spray-dried red pitaya powder exhibited a more

regular and smooth spherical shape (Figure 1A). This occurrence is largely due to the

different processing methods. For freeze-dried red pitaya powder, it formed dried and porous

cake which needed to be ground mechanically in order to obtain the fine powder. As for

spray-dried red pitaya powder, the feed liquid passes through pressure nozzle and atomized

into droplets hence spherical powder was obtained.

Color

The color parameter (L*, a* and b*) of spray-dried and freeze-dried red pitaya powder was

compared. Low L* values indicated darker color in powder due to the purple-reddish color in

red pitaya powder (Phebe & Chew 2009). Freeze-dried has significantly lower L* value

indicating the powder is darker in color. Despite that, the a* value which is an indication of

redness intensity (Ee et al. 2014a), did not display significant difference between these two

powders although freeze-dried red pitaya powder has slightly higher redness intensity. The

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color difference between the powders reflected different processing methods will affect the

color characteristic of the powder. Since betalain contributing to the red-purple color in red

pitaya is a natural pigment, therefore it is prone to thermal degradation (Chapman 2011;

Socaciu 2008). Spray drying which utilizes hot air to dehydrate the moisture is expected to

degrade the betalain in red pitaya while freeze drying process which uses low air environment

and low temperature is expected to better preserve the color of the powder.

Betacyanin content

Two major peaks were eluted from the betanin standard represent betanin (tR = 3.7 min) and

isobetanin (tR = 4.4 min) as seen in Figure 2A. These retention times were used as reference

to identify the betacyanin content in the spray-dried and freeze-dried red pitaya powder.

Betacyanin content of spray-dried and freeze-dried red pitaya powder was reported as 3.99

mg/mL and 3.88 mg/mL, respectively with no significant difference (p > 0.05) (Table 3).

Besides this, three extra peaks observed in Figures 2B and 2C indicated the presence of

compounds other than betacyanin pigment in the red pitaya powder. Studies done by

Stintzing, Schieber & Carle (2002) and Wybraniec et al. (2001) have found out more than

five betacyanins present in red pitaya pulp such as phyllocactin, isophyllocactin, hylocerenin,

isohylocerenin, betanidin, and isobetanidin. Therefore, this explained the presence of other

prominent peaks after 7.0 min (Figures 2B and 2C). Characterization of the unknown peaks

was unable to be conducted in this experiment due to unavailability of the commercial

standards.

Total phenolic content (TPC) 

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Total phenolic content of both spray-dried (88.10 mg GAE/100 g) and freeze-dried red pitaya

powder (93.64 mg GAE/100 g) was not significantly different (p > 0.05) (Table 3). However,

it was observed that spray-dried red pitaya powder has slightly lower TPC. Phenolic

compounds were described as heat labile substances (Lee, Wu & Siow 2013). Therefore,

decrease in TPC of the spray-dried red pitaya powders can be due to the usage of high

temperature during spray drying. This was supported by Lee, Wu & Siow (2013) and Orak et

al. (2012) in which a significant loss of phenolic compounds was reported due to thermal

degradation in spray-dried white and red dragon fruit powder and strawberry tree dried using

hot air. Besides this, betacyanins pigments has phenol structure in the molecule will also

contributed to the TPC (Jamilah et al. 2011; Zainoldin & Baba 2009). Since natural pigment

is heat sensitive, therefore destruction of betacyanin will also lead to reduction in TPC.

Total flavonoid content (TFC)

Total flavonoid content for spray-dried and freeze-dried red pitaya powder was reported as

23.58 mg CE/ 100g and 29.21 mg CE/ 100 g, respectively. Similar to trend observed in TPC,

freeze-dried powder has slightly higher flavonoid content but is not statistically significant (p

> 0.05). Flavonoid belongs to one of the categories in phenolic compound family (Nunes

2012). Since phenolic compound is heat sensitive, therefore spray drying process is expected

to destruct some flavonoid compound in red pitaya powder.

Ferric reducing antioxidant power (FRAP)

Ferric reducing antioxidant power for spray-dried and freeze-dried red pitaya powder was

measured as 35.59 mg GAE/100 g and 34.43 mg GAE/100 g, respectively and their reducing

19

power are almost similar with no significant difference (p > 0.05). The reducing capability of

red pitaya powder is contributed by the presence of betacyanin, phenolic compound and

flavonoid (Jamilah et al. 2011; Saeed, Khan & Shabbir 2012).

Stability of red pitaya betacyanin in model juice system

As seen in Table 4, no significant difference in betacyanin retention among spray-dried and

freeze-dried sample. Significant changes were observed under storage at different

temperature for both set of model juices. Storage at 4 °C exhibited the highest betacyanin

retention while storage at room temperature and 40 °C displayed no retention in betacyanin

after one week of storage. This indicated that storage temperature significantly affected the

stability of betacyanin in the model juice. In general, storage at 4 °C displayed similar trend

in both powders, where betacyanin content decreased gradually during 4 weeks storage. As

for storage at room temperature and 40 °C, betacyanin in model juice was degraded

completely after one week storage. No significant effect of light on retention of betacyanin

was observed in this study.

Degradation of betacyanin in this research can be described by the theory that betacyanin is a

heat sensitive natural pigment. Liaotrakoon et al. (2013) who studied on the influence of the

thermal processing on the betacyanin content of dragon fruit puree concluded that

temperature has strong influence on the betacyanin content. Increase in temperature will

accelerate the degradation of betacyanin. In this research, although effect of thermal

processing on betacyanin was not investigated, however, the result showed that temperature

abuse during storage significantly decreased the betacyanin retention. Besides that, result in

this research was in accordance with research done by Woo et al. (2011), who stated storage

20

temperature significantly affected the betacyanin stability in spray-dried red pitaya. Besides,

it is also suggested by the researchers where storage under light exposure will affect the

betacyanin retention. However, no significant effects of light on betacyanin retention were

observed in this study, hence this statement was contradictory to this research.

Conclusion

5.0 CONCLUSION AND RECOMMENDATIONS

This study investigated the physicochemical and antioxidant activities of freeze-dried and

spray-dried red pitaya powder and the stability of red pitaya betacyanin in a model juice

system. Higher yield of red pitaya powder was obtained from freeze drying process. Freeze-

dried red pitaya powder has significantly lower moisture content and water activity.

However, since both powder reported water activity lower than the critical limit (aw < 0.50),

therefore both are expected to have lower risk of spoilage due to microbial reactions. Spray-

dried red pitaya powder has significantly lower bulk density and tapped density compared to

freeze-dried red pitaya powder. Lower bulk density of powder is not favorable as it requires

larger volume during packaging. It was also observed that freeze-dried red pitaya powder

with larger particle sizes displayed better flowability and lesser cohesiveness compared to

spray-dried powder. Overall, freeze drying process produced better physiochemical

properties red pitaya powder but no significant difference in term of betacyanin content and

antioxidant activities as compared to spray-dried powder. On the other hand, the storage

study showed that betacyanin content was well preserved at storage in refrigeration. Increase

in storage temperature will degrade the betacyanin hence causing discoloration in the model

juice. In general, red pitaya powder is suitable to be used as natural colourant in pasteurized

product and storage at refrigeration temperature is recommended. The versatility of red pitaya

21

powder as a natural food colorant can be exploited further in future by incorporating it into

more other food model systems such as cultured milk, jelly and pastille.

Acknowledgements

Financial support provided by Taylor’s Research Grant Scheme (TRGS) from Taylor’s

University (TRGS/ERFS/1/2015/SBS/005) for this study is gratefully acknowledged.

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List of Tables

Table 1 Flowability of powder based on Carr Index (CI)

CI (%) Flowability

< 15 Very good

15–20 Good

20–35 Fair

35–45 Bad

> 45 Very bad

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Table 2 Cohesiveness of powder based on Hausner ratio (HR)

HR Cohesiveness

< 1.2 Low

1.2–1.4 Intermediate

> 1.4 High

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Table 3 Physiochemical and antioxidant properties of spray-dried and freeze-dried red

pitaya powder

Physiochemical propertiesSpray-dried red pitaya

powder

Freeze-dried red pitaya

powder

Yield (%) 33.25 ± 7.04b 96.05 ± 2.78a

Moisture content (%) 13.03 ± 0.87a 12.36 ± 2.76b

Water activity (aw) 0.412 ± 0.01a 0.289 ± 0.13b

Bulk density (g/mL) 0.31 ± 0.00b 0.65 ± 0.05a

Tapped density (g/mL) 0.49 ± 0.03b 0.84 ± 0.03a

Carr Index 36 ± 2.71a 22 ± 2.68b

Haunser Ratio 1.6 ± 0.08a 1.3 ± 0.05b

Particle size (D [3,2], µm) 10.16 ± 3.95b 16.00 ± 5.36a

Colour , L* 67.00 ± 0.26a 59.06 ± 4.41b

a* 34.61 ± 1.03a 37.58 ± 6.52a

b* -18.94 ± 1.17a -20.16 ± 3.91a

Betacyanin content (mg/mL) 3.99 ± 0.32a 3.88 ± 0.57a

TPC (mg GAE/100g) 88.10 ± 3.16a 93.64 ± 4.30a

TFC (mg CE/100g) 23.58 ± 3.37a 29.21 ± 4.27a

FRAP (mg GAE/100g) 35.59 ± 1.80a 34.43 ± 3.44a

Results are reported in mean ± SD of triplicate determinations. Mean values in the same row with different superscripts are significantly different (p < 0.05).

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Table 4 Betacyanin retention (%) in model juice incorporated with spray-dried (SD) and freeze-dried (FD) red pitaya powder

Sample Storage conditionBetacyanin Retention (%)

Week 0 Week 1 Week 2 Week 3 Week 4

SD 4 °C with light 100.00 ± 0.00a 62.46 ± 5.47b 49.80 ± 2.00c 40.41 ± 3.29d 37.70 ± 5.70d

4 °C without light 100.00 ± 0.00ab 79.32 ± 17.95b 70.00 ± 5.00c 63.26 ± 19.08c 44.71 ± 3.65d

RT with light 100.00 ± 0.00 a 0.00 ± 0.00 b - - -

RT without light 100.00 ± 0.00 a 0.00 ± 0.00 b - - -

40 °C 100.00 ± 0.00 a 0.00 ± 0.00 b - - -

FD 4 °C with light 100.00 ± 0.00 a 57.88 ± 0.21b 53.09 ± 3.00c 49.54 ± 4.61c 54.59 ± 8.88d

4 °C without light 100.00 ± 0.00 a 58.78 ± 0.36b 54.80 ± 3.40c 52.28 ± 1.53bc 31.64 ± 15.25bc

RT with light 100.00 ± 0.00 a 0.00 ± 0.00 b - - -

RT without light 100.00 ± 0.00 a 0.00 ± 0.00 b - - -

40 °C 100.00 ± 0.00 a 0.00 ± 0.00 b - - -

Results are reported in mean ± SD of triplicate determinations. Mean values in the same row with different superscripts are significantly different (p < 0.05).

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List of Figures

Figure 1 Scanning electron micrographs (SEM) of red pitaya powder: (A) spray-dried, and (B) freeze-dried at 2000× magnification

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(B)(A)

Figure 2 Chromatogram of (A) betanin standard, (B) spray-dried and (C) freeze-dried red pitaya powder with peak retention time indicated

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Betanin

Isobetanin

Betanin

Isobetanin

Betanin

Isobetanin