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: [email protected]
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
3
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
7
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
12
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
16
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
17
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