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Food Research 3 (2) : 102 - 107 (April 2019)
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Influence of process variable on integrated power-temperature drying process
for rambutan seed fat yield: a case study
1 Ahmad, S., 1* Anuar, M.S., 1Taip, F.S., 1Shamsudin, R. and 2Ab Mutalib, S.R.
1Department of Process and Food Engineering, Faculty of Engineering, Universiti Putra Malaysia, 43400,
UPM Serdang, Selangor, Malaysia.
2Food Technology Programme, School of Industrial Technology, Faculty of Applied Sciences, 40450,
Universiti Teknologi Mara Shah Alam, Selangor, Malaysia.
Article history:
Received: 28 May 2018
Received in revised form: 5
October 2018
Accepted: 5 October 2018
Available Online: 6
November 2018
Integrated power-
Rambutan seed,
Fat yield,
Microwave finished oven,
Oven finished microwave
The effect of process variable namely; integrated sequence, variable intensity, drying time,
and initial moisture content were studied on integrated power-temperature drying process
for the rambutan seed fat yield. This study examined the effect of these parameters on fat
yield and correlation between process variable on the integrated power-temperature drying
process. Rambutan seeds that were dried at two different integrated sequences (microwave
finished oven (MFO); oven finished microwave (OFM)) at three microwave power levels
(250, 600 and 1000W) with four different microwave time exposures (5,10, 20 and 60
mins) and two oven temperatures (45oC and 58oC) dried up to 1% of final moisture
content for both level of initial moisture content (high and low). It was noted that fat yield
was related with sequence, microwave power and microwave exposure significantly.
Results were discussed in terms of sequence influence and relation between process
variable on fat yield and how it is portrayed as an effective drying method for rambutan
seed. These results will aid the integrated drying process development for agricultural
products. The fat yield obtained through an efficient cost-effective drying process can help
to promote the use of the rambutan fat yield in the industry, particularly the food industry.
1. Introduction
A short drying time leads to an effective drying
process. Shorter drying time is often associated with
having a lower energy consumption and therefore lower
processing cost. Materials subjected to short drying time
could retain its initial structure and nutrient content.
Accordingly, the challenge to provide a shorter drying
time to preserve product quality has become the main
priority in drying process development. In order to
reduce the drying time, countless interventions have been
implemented including combining drying equipment
which has different drying mechanisms to enhance the
drying process in order to shorten the drying time.
Examples of normal combined drying process such as
involving both the convective and radiation drying on
pumpkin (Ceclu et al., 2016), microwave and vacuum
drying on edamame (Hu et al., 2006) and kiwifruit slice
(Tian et al., 2015), convective and microwave drying on
slice and pureed of strawberries (Venkatachalapathy and
Raghavan, 2000), peeled longan (Varith et al., 2007),
canola seed (Hemis et al., 2015), apple slice
(Aghilinategh et al., 2015) and American ginseng (Ren
and Chen, 1998), as well as hot air and vacuum drying
on lettuce (Yuan et al., 2015).
The combination of these equipment is done through
various ways, namely either concurrently or sequentially;
referring to the implementation of both mechanisms into
a single apparatus or for the case when each dryer is used
sequentially. However, concurrent and sequential drying
techniques were not properly differentiated in previous
studies as they used the term combined or assisted
alternately ambiguously. Previous research focused on
the effect of combined drying mechanism on drying
time, energy and cost consumption, as well as drying
characteristics and behavioural of material. Significant
improvements when using a combined drying
mechanism was clearly presented via microwave-
convective drying as it provided a shorter drying time,
better drying characteristics and behaviour as well as
sustaining the dried quality product as shown in previous
research on slice and pureed strawberries
(Venkatachalapathy and Raghavan, 2000), peeled longan
(Varith et al., 2007), canola seed (Hemis et al., 2015),
apple slice (Aghilinategh et al., 2015) and American
ginseng (Ren and Chen, 1998).
103 Ahmad et al. / Food Research 3 (2) (2019) 102 - 107
eISSN: 2550-2166 © 2018 The Authors. Published by Rynnye Lyan Resources
Due to the ability to provide a competitive level in
terms of drying time, cost and dried quality product, the
microwave- convective drying was applied in this study.
The drying combination applied in this study refers to
the second type of drying combination, namely,
sequential combination that involved sequence rotation
in the dryer equipment. To avoid confusion on the type
of combination drying mechanism used, the combined
sequence applied is termed as integratedin this study.
As combination sequence used in this study is
microwave and forced convection oven, thus, the term
drying combination utilized in this study is the integrated
power-temperature drying process. Advantages of the
combined drying mechanism has been observed on
previous research, however, a higher technology is
required in combining drying equipment to embed two
different drying mechanisms as well as the need of
practised labour and expensive production cost. As the
material used in this study is rambutan seed which is
agricultural waste and purpose of drying is for
sustainable resources to produce a by-product for food
(Lannes et al., 2003; Issara et al., 2014) and cosmetic
(Lourith et al., 2016) industries as well as for medicinal
purposes (Soeng et al., 2015). Thus, the low cost of
drying process is required to ensure converted process of
agricultural waste into by-product is effective and
competitive in term of production cost and functional.
Accordingly, integrated power-temperature drying
process is applied in this study. Application of integrated
power-temperature in this study should be able to reduce
the drying time of rambutan seed that normally required
32 up to 48 hours under convective drying. A longer
drying time exposure would most probably change the
material structure, which in turn potentially degrade the
dried quality product. However, in improving drying
process through integrated power-temperature, there is
some potential process variable that is believed to affect
drying behaviour as well as the fat yield, which is the
response in this study.
Therefore, the aim of this study is to examine the
sequence influence on either microwave finished oven or
oven finished microwave method that will provide an
optimal drying condition as derived based on higher fat
yield. The potential process variable, namely, are the
integrated sequence (microwave finished oven (MFO)
and oven finished microwave (OFM)), variable intensity
(microwave power and the oven temperature), drying
time (microwave exposure and oven exposure) and
initial moisture content (high and low level). In addition,
the pattern of relation between fat yield and these four
process variables as well as the most significant process
variable influence were also determined. As expected
there is a significant effect of sequence applied on fat
yield and all the process variables show a strong
correlation and could affect the variability of fat yield
with a higher percentage in multiple regression analysis.
Thus, optimistically, these findings can give a new
dimension to the industry upon the need for integrated
drying variables in the drying process to increase the
efficiency in terms of product quality. The scope and
range studied are relatively lab-scale, thus; it is expected
that the readers can assume the finding as a baseline data
and subsequently need a proper scaling-up to industrial
production level for adaptation purposes. The fat yield
obtained through an efficient cost effect drying process
can help to promote the use of the rambutan fat yield in
the food industry.
2. Materials and methods
2.1 Materials
Rambutan was obtained from Taman Pertanian
Universiti (TPU), Universiti Putra Malaysia. Fruit
harvested were kept in zip-lock polyethylene bag at 8.5°
C in cool room prior deseeded. Fruit was deseeded
manually, and the seed was washed and left at air room
temperature (24-25°C) to dry up surplus water at seed
surface prior kept in double polyethylene zip-lock bag at
4°C in chiller model Protech SD-700 (Advanced
Scientific, Malaysia). The initial moisture content of 5 g
of seed was measured using a moisture analyzer
(OHAUS MB45, UK) at 105°C for each batch prior to
the drying process.
2.2 Drying procedure
Rambutan seed was dried by two cases of integrated
drying variable based on different sequences; case 1;
microwave finished oven (MFO) and case 2; oven
finished microwave (OFM) respectively. In MFO, 20 g
of rambutan seed with varied in initial moisture content
(high; 30-45% and low; 20-25% kg H2O/ kg dry basis)
were dried by three different microwave powers 250,
600 and 1000 W at 5, 10, 20 and 60 mins respectively
and followed by automatic electric oven drying at two
different temperatures of 45°C and 58°C until the final
moisture content reached 1%. The drying time required
in automatic electric oven drying was based on the time
needed to achieve 1% of final moisture content. After the
drying time required in the automatic electric oven was
known, then, case 2 (OFM) drying was performed. In
OFM, 20 g of rambutan seed with varied in initial
moisture content (high; 30-45% and low; 20-25% kg
H2O/ kg dry basis) were dried by automatic electric oven
at two different temperatures; of 45°C and 58°C based
on the previous drying time obtained in the second stage
of case 1 (MFO) drying and finished by microwave at
similar power levels and drying times. Weight loss was
recorded every 5-min and 20-min intervals during
Ahmad et al. / Food Research 3 (2) (2019) 102 - 107 104
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microwave and automatic electric oven respectively.
Weight loss was measured using an analytical balance
AY220 model (Shimadzu, Japan) with precision up to
0.0001 g. All samples were dried in duplicates in order to
obtain an accurate, reliable, and robust experimental
data. All experiment involved will be explained briefly
by as Figure 1.
2.3 Fat yield determination
The extraction process of rambutan seed was carried
out by soxhlet extraction method at 70-80°C for 8 hrs
upon constant particle size, 500 µm of grounded
rambutan seed dried with 1:10 ratio (dried powder:
solvent). Hexane was used as an extractor agent prior to
being evaporated by rotary evaporator at 40- 55°C. The
organic residue obtained was then dried for 1 hr at
103±2°C to calculate the fat yield. The fat yield from the
rambutan seed was then measured using equation (1).
Where a is the weight of round bottom flask after
extraction; b is the weight of round bottom flask before
extraction; and c is the weight of sample before
3. Results and discussion
3.1 Effect of integrated sequence drying variable on fat
Both integrated sequences; MFO and OFM did not
fall within a similar group even when it was tested within
a similar drying variable range and was given a different
set of fat yield means. This can be clearly explained by
the significant P value of 0.001 (t-test) at alpha 0.05. In
addition, there is significant effect of different drying
method (MFO: microwave finished drying; OFM: oven
finished microwave) on fat yielding of dried rambutan
seed by given a highly significant P value <0.0001 at
alpha 0.05. Therefore, there is a need to perform the test
for the mean separation to observe the different
population means between drying method at alpha 0.05.
According to tukeys mean separation, microwave
finished oven (MFO) and oven finished microwave
(OFM) sequences bringing their own capital letter B and
A, respectively. This indicates there is a significant
different between population means of sequences at
alpha 0.05 and these sequences could significantly
affected on fat yielding of dried rambutan seed. Besides,
as OFM given a higher mean value compared to MFO,
37.897 and 32.316, respectively. Hence, OFM provide a
better drying process of rambutan seed in terms of
sequence in obtaining a higher fat yield compared to
MFO. This may be due to effect of the combination of
drying mechanism applied where it started with
convective drying at higher level of moisture content and
finished up by microwave drying at low moisture content
which approaching to 1% of final moisture content that
has been set up as cut point for drying process in this
A higher mean fat yield of rambutan seed obtained in
OFM, 37.897, compared to previous studies that
procured 34.34±0.77 at 4.81% final moisture content by
Figure 1. Experimental involved in integrated power-temperature drying process
105 Ahmad et al. / Food Research 3 (2) (2019) 102 - 107
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hot air dryer with 488 mins drying time or equivalent to
more than 8 hrs (Chimplee and Klinkesorn, 2015) was
observed. A possible explanation for this due to
integrated drying mechanisms of microwave and
convective has overcome the drawback between each
other and leading to a shorter drying time that is
approximately less than two hours in total drying time
and as well as increasing the extraction yield of
rambutan seed fat. The higher fat yield obtained from the
rambutan seed through the OFM sequence drying
process could feasibly enhanced the functionality of
rambutan seed fat in various industries such as mixing it
with cocoa butter up to 30% in chocolate manufacturing
in order to reduce cocoa butter usage (Manaf et al., 2013;
Zzaman et al., 2014), providing a competitive defatted
rambutan seed flour which is more nutritious than
existing wheat flour (Eiamwat et al., 2015; Eiamwat et
al., 2016), an ability to offer a modest fat source that had
similar properties to vegetable oils which can potentially
be used in personal care industry and cosmetics
ingredient (Lourith et al., 2016), as well as an ability to
be a renewal source of petrodiesel by having 160
biodiesel yield kgha-1 (Winayanuwattikun et al., 2008).
Overall, this study highlights the need for enhancing the
rambutan seed fat extracted with a shorter drying time
that would provide a promising economical drying
process through the lower energy consumption exhibited
by the oven finished microwave sequence drying
process. A similar finding also found for Thompson
seedless grape as a better drying condition was obtained
under a combined drying mechanism between
microwave and convective dryers involved (Kassem et
al., 2011).
3.2 Relations of process variable involved on fat yield
Correlation of each process variable on the fat yield
of rambutan seed under integrated power- temperature
drying process is shown in Table 1. Two out of the five
process variables involved insignificantly effects on the
fat yield by giving insignificant P value at alpha 0.05.
Oven temperature (OT) is insignificantly affecting the fat
yield under integrated drying process is probably due to
the rambutan seed has reached the 1% of final moisture
content, prior to entering the oven drying phase.
Accordingly, when the subsequent drying process is
using forced convection drying oven, it does not make
any significant changes to the fat yield as observed in
microwave finished oven (MFO) sequence. A similar
trend can be observed for the oven finished microwave
(OFM) sequence drying process was performed. This is
because the drying time in oven is short due to the same
drying exposure has been applied in the OFM to seek
uniformity between MFO and OFM for comparison
purposes. A short period of oven drying in the MFO is
caused by the residual percentage of moisture content
after microwave drying is near the 1% final moisture
content as the cut point of the drying process in this
study. Therefore, the rapid drying process reaching the
1% of final moisture content in the MFO is due mainly
to the drying mechanism applied during the first phase
involving microwave drying. Microwave drying
mechanism involves collisions between moisture
particles within the material that in turns generate heat
internally and help facilitates the moisture removal from
the rambutan seed. The seed normally consists of bound
water and stored at deepest layer of material and need a
longer drying time to release moisture particle under
normal convective drying conditions. Thus, a longer
drying time required if dried by forced convection during
oven drying as water removal process occurs mainly due
to the temperature gradient between the air and the
surface material. The removal of water molecules
happens until equilibrium exist in terms of the
temperature between air and material. Thus, the
convective drying mechanism required a longer drying
time. Accordingly, a shorter drying exposure in the OFM
unable to change the fat yield from the rambutan seed.
This situation is clearly shown in the integrated drying
process at higher microwave power level with longer
microwave exposure up to 1 hour. Therefore, it is
proposed to re-evaluate the range studied of microwave
exposure and microwave power level in assessing the
effect of integrated power-temperature drying process
particularly for low moisture content material such as
rambutan seed in this study.
Whereas, initial moisture content insignificantly
affected the fat yield probably due to the level of
moisture content is affecting the drying time. So, if the
drying time has been set up, as in this study, thus, the
initial moisture content could not significantly affect the
fat yield.
Although the sequence, microwave power and
microwave exposure significantly affected the fat
yielding, it is represented in terms of a weak correlation
as can be seen from the correlation for each process
Indicator Sequence (SQ) Oven Temperature
Power (MP)
Exposure (ME)
Initial Moisture
Content (IMC)
r 0.24942 0.03334 0.28264 0.24240 -0.09021
P value 0.0005 0.6462 < 0.0001 0.0007 0.2134
Table 1. Correlation value of each process variable involved on fat yield in integrated power-temperature drying process
Ahmad et al. / Food Research 3 (2) (2019) 102 - 107 106
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variables involved 0.24942, 0.28264 and 0.24240,
respectively (Table 1). As these process variables
significantly affected the fat yield, the most influence
process variable upon fat yield could be predicted and in
turn, the equation model consisting of process variable
can be developed. This equation model is very important
as baseline data for the subsequent work that focuses
similar relationship but at a different range of process
variables as well as for different materials which have
similar structures.
Based on the assessment of multiple regression
stepwise selection method, the microwave power
becomes the most influential process variable on the fat
yield from the rambutan seed compared with other
process variables by giving a highly significant P value
and the highest of F value, 16.50 compared to 13.70 and
13.82 for sequence and microwave exposure
respectively. The similar for P and F values were
obtained for both probability levels and confidence
intervals that have been studied which were at alpha 0.15
and 0.05 and confident interval of 85% and 95%
respectively. These two levels of probabilities and
confident intervals have been studied to evaluate if the
range of the acceptance error has been increased; a
higher value of R2 and lowest value of Cp could be
obtained. However, a similar regression value has been
obtained, by given only 20.09% (Table 2) in fat yield
variability could be explained by the linear regression as
presented in equation (2);
4. Conclusion
Based on the study conducted, oven finished
microwave (OFM) exhibited a higher mean of fat yield,
37.897, compared to microwave finished oven (MFO),
32.316. Therefore, OFM can provide a promising
alternative drying mechanism in terms of obtaining a
higher fat extracted at a shorter drying time. In terms of
the fat yield, an equation model presenting the
relationship between fat yield and the process variable
was developed and it showed that the microwave power
as the most significant process variable. Therefore, it is
expected that the model may provide a baseline data to
researchers in improving the drying process via the
integrated drying variables for agricultural products
especially for the rambutan seed to obtain a higher fat
yield for use in the industry, especially the food industry.
The authors gratefully acknowledge Universiti Putra
Malaysia (UPM) for IPS research grant (vote no.
9464600) and Ministry of Higher Education as well
as Applied Science Faculty, UiTM, Shah Alam for
financial sponsored throughout this study.
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ResearchGate has not been able to resolve any citations for this publication.
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The study was conducted to investigate fatty acid composition, rheological properties and crystal formation of rambutan fat and cocoa butter. The results showed that lauric acid, palmitic acid, and stearic fatty acid in rambutan fat were less than cocoa butter, but oleic acid found almost the same. The crystal formation of cocoa butter was not complex at 25°C, while rambutan fat and their mixture shown complicated network of crystal form. The Newton, Bingham and Casson plastic rheological models was used to describe fat flow in this experiment and the result showed that rambutan fat had higher viscosity than cocoa fat. Based on the results the study recommended that mixture proportion up to 30% rambutan seed fat can be used as a cocoa butter substitute whereas higher proportion completely alters original cocoa butter properties. Therefore, there is feasibility of using the rambutan fat to substitute cocoa butter and the mixtures of the two fats in suitable proportion in chocolate manufacturing.
Rambutan is commercialized for fresh consumption and industrially processed leaving seed as a major residue. Rambutan seed from the industry is worthy of attention for certain industrial applications and feasibility. Extractive yields and fatty acid compositions of rambutan seed fat obtained under different extraction conditions were studied to assess possible applications on an industrial scale. Maceration in n-hexane for 1h was shown to be feasible for rambutan seed fat extraction (30.12±0.04%). Re-use of the solvent gave non-significantly different extractive yields (p>0.05). Oleic and arachidic acids were exhibited as the major fatty acids (31.08±0.75% and 28.65±0.72%) followed by gondoic, palmitic, stearic, isooleic, behenic, linoleic and palmitoleic acids. The physicochemical properties of the fat feasible for an industrial practice were determined, including acid (4.35±0.00mg KOH/g), iodine (44.17±0.30g I2/100g), peroxide (1.00±0.00g/g), saponification (246.73±0.10mg KOH/g) and unsaponified (0.10±0.00%) values. This bio-fat with a moisture content of 1.77±0.12% was melted at 46.05±0.05°C. Stable bar and liquid soaps containing rambutan seed fat were developed. Such application demonstrates the potential of rambutan seed fat as a raw material for the cosmetic and personal care industries. The extraction method was modified to meet requirements for industrial feasibility. This unconventional bio-fat with a specification in terms of fatty acid profiles and physicochemical properties is proposed. Furthermore, the fat is comparable with other vegetable oils and cosmetic ingredients, and is compatible with other cosmetic ingredients.
This study experimented on microwave-vacuum (MWV) drying of kiwifruit (Actinidia deliciosa) slices as a way to improve drying productivity and the quality of dried fruit. The effects of microwave power density, sample thickness and vacuum degree on the drying characteristics of kiwifruit slices were studied. The results showed that these three factors had significant effects on the vitamin C and chlorophyll contents and on the rehydration capacity of dried kiwifruit slices, as well as the total drying time. The processing parameters of MWV drying of kiwifruit slices were optimized by using Box–Behnken design with a quadratic regression model built by response surface methodology. The optimum processing parameters were experimentally determined as microwave power density at 7.7 W, sample thickness of 6.0 mm and MWV drying for 11.5 min at a vacuum degree of −90 kPa. High-quality dried kiwifruit slices can be created under these conditions using this drying process.Practical ApplicationsKiwifruit (Actinidia deliciosa) has become a popular fruit because of its bright green flesh, high level of vitamin C and because it contains a wide variety of phytonutrients, including carotenoids, lutein, phenolics, flavonoids and chlorophyll. It needs some form of preservation to extend its shelf life. Microwave-vacuum (MWV) drying is a novel alternative that combines the advantages of both microwave drying and vacuum drying. The objectives of this study were to investigate the effect of microwave power density, sample thickness and vacuum degree on the MWV drying characteristics and quality of MWV-dried kiwifruit slices, and to evaluate a suitable drying model for describing the drying process. In addition, the process parameters of MWV drying were optimized by response surface methodology. This present study provides the theoretical basis for industrial-scale production.
Rambutan (Nephelium lappaceum L.) kernels contain a high amount of fat and it has a potential to be used as a new alternative source of edible vegetable fat. Drying of fat seed can improve the fat yield and prolong the seed storage shelf life. Therefore the drying behaviour of rambutan kernels was investigated using a hot air dryer for 6h at each of three temperatures, 45, 55 and 65°C. Based on coefficient of determination (R2) and root means error (RMSE), modified Henderson and Pabis model were found to be the best thin-layer drying model for rambutan kernels (R2 > 0.99 and RMSE < 0.02). The effective moisture diffusivity of rambutan kernels, estimated from a modified equation of Fick’s second law of diffusion, was 2.56  10-10 to 3.68 × 10-10 m2 s-1. The result of fat extraction significantly indicates the effect of low moisture rambutan kernels on the fat yield enhancing (P < 0.05).