This document summarizes research on drying tropical fruit pulps using spouted beds. It finds that the composition of the fruit pulp significantly impacts the fluid dynamics and performance of the drying process. High starch and lipid contents promote stable fluidization and high powder production efficiency. In contrast, high reducing sugar concentrations result in unstable operation and very low powder yields. The article also explores optimizing powder production based on pulp composition and developing new products by drying mixtures of pulps with adjusted formulations.

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Drying of Tropical Fruit Pulps: Spouted Bed
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Drying of Tropical Fruit Pulps: Spouted Bed Process
Optimization as a Function of Pulp Composition
S. C. S. Rocha
a
, J. S. Souza
b
, O. L. S. Alsina
c
& M. F. D. Medeiros
b
a
School of Chemical Engineering – UNICAMP , Campinas, SP, Brazil
b
Department of Chemical Engineering – UFRN, Lagoa Nova , Natal, RN, Brazil
c
Department of Chemical Engineering – UFCG , Campina Grande, PB, Brazil
Published online: 11 Aug 2011.
To cite this article: S. C. S. Rocha , J. S. Souza , O. L. S. Alsina & M. F. D. Medeiros (2011) Drying of Tropical Fruit Pulps:
Spouted Bed Process Optimization as a Function of Pulp Composition, Drying Technology: An International Journal, 29:13,
1587-1599, DOI: 10.1080/07373937.2011.585442
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3. Drying of Tropical Fruit Pulps: Spouted Bed Process
Optimization as a Function of Pulp Composition
S. C. S. Rocha,1
J. S. Souza,2
O. L. S. Alsina,3
and M. F. D. Medeiros2
1
School of Chemical Engineering – UNICAMP, Campinas, SP, Brazil
2
Department of Chemical Engineering – UFRN, Lagoa Nova, Natal, RN, Brazil
3
Department of Chemical Engineering – UFCG, Campina Grande, PB, Brazil
Results of drying of tropical fruit pulps in spouted beds (SBs) are
presented, focusing on the effects of fruit pulp composition on the
SB fluid dynamics and process performance and the development
of new products formulated from mixtures of pulps with varied com-
position. It was verified that high starch and lipid contents favored
stable fluid dynamics and high powder production efficiency, while
high reducing sugar concentrations resulted in bad dynamic regime
and very low powder production. Powder production efficiency was
statistically correlated with the pulp composition. Also, drying of
mixtures of fruit pulps with addition of starch and lipids was inves-
tigated. Results of fluid dynamics, drying performance, and sen-
sory tests of yogurts enriched with the powders revealed promising
potential of the SB for obtaining high-quality fruit powders.
Keywords Drying of fruit mixtures; Drying performance;
Spouted bed drying of pastes; Spouted bed with inert
particles
INTRODUCTION
Fruit cultivation is one of the most diversified and
important agroindustrial activities in Brazil. Due to the
country’s size (nearly half of South America) and different
types of climate, a wide variety of fruits, ranging from trop-
ical fruits to those considered cold climate fruits, such as
apples, pears and peaches, are produced. Out of the total
annual production, it is estimated that 14% (about 5
million tons) constitutes little-exploited tropical fruits, such
as umbu (Spondias tuberosa), hog plum (Spondia lutea), red
mombin (Spondia purpurea), hog plum mango (Spondias
dulcis), sour sop (Annona muricata), sapodilla (Manilkara
achras), and mangaba (Hancornia speciosa), among others.
Tropical fruits are tasty and aromatic and, in addition to
being hydrating, provide energy and are rich in vitamins
and mineral salts, mainly calcium, iron, and phosphorous.
Despite a significant consumption of fruit in Brazil, both in
their natural form and as juices (the best form to maximize
their nutrients) or prepared as sweets, jams, compotes, ice
creams, etc., and the surge in agribusiness and exports,
there is still a high level of fruit wastage, mainly in
cyclically produced seasonal fruits.
On the other hand, dry fruit consumption has increased
significantly in recent years, mainly in health foods, such as
granola, enriched cereals, and whole wheat breads. How-
ever, most drying techniques used require long exposure
to heat, with losses in heat-sensitive nutrients and irrevers-
ible changes in physical and chemical characteristics. As a
consequence, the rehydration process does not regenerate
the natural characteristics of dried fruits.[1,2]
Aiming at increasing the useful life of fruit without alter-
ing its nutritive and sensory characteristics, new technolo-
gies for fruit processing have been developed and
introduced in the agribusiness sector. With these technolo-
gies, wastage can be minimized and fruit consumption can
be increased during the out-of-season period and fruits
exploited as raw material in the manufacture of industria-
lized products, such as sweets, ice creams, baby foods, etc.
Some fruits cultivated mainly in the northeast of Brazil,
such as hog plum, umbu, red mombin, and Surinam cherry
(Eugenia uniflora), among others, are very acidic and juicy
with a low pulp=pit ratio, which renders them unsuitable
for dry fruit production. These fruits, whose consumption
is mainly in the form of juice and ice cream, are depulped
and commercialized frozen, requiring large storage and
transportation space. Conservation by freezing results in
high energy costs and adds no value to the product.
The development of fruit powder through post-harvest
processing ensures a product with low water content,
greater stability, and prolonged storage under ambient
conditions. Among the techniques used in fruit powder
production are lyophilization, encapsulation of juices by
co-crystallization with sucrose, and spray drying as well
as fluidized bed and spouted bed (SB) drying with inert
particles.
Lyophilization is complicated and costly. Though
already studied as an alternative for obtaining dried
fruit,[3,4]
this technique is more commonly used in the
Correspondence: S. C. S. Rocha, School of Chemical
Engineering-UNICAMP, P.O. Box 6066, Campinas, SP, Brazil
13083-970; E-mail: rocha@feq.unicamp.br
Drying Technology, 29: 1587–1599, 2011
Copyright # 2011 Taylor & Francis Group, LLC
ISSN: 0737-3937 print=1532-2300 online
DOI: 10.1080/07373937.2011.585442
1587
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4. drying of heat-sensitive products of high commercial value,
such as medicines, dry extracts, etc.
Spray drying is considered a viable alternative in fruit
powder production. It is known to produce large amounts
of solids, and the nutritional characteristics of the product
are maintained owing to the short contact time of the raw
material with the heating gases inside the dryer. However,
as in co-crystallization,[5]
additives and=or adjuvant are
added to the fruit juice formulation as emulsifiers, thick-
eners, and wall materials. Generally, wall material is used
in the formulation to retain volatile ingredients by micro-
encapsulation and also to avoid degradation of the vita-
mins and browning. Despite the importance of adjuvant
in the above-mentioned processes, their presence causes
changes in the characteristic fruit flavor, significantly
modifying the original fruit composition.
Fruit powder production with the minimum addition of
adjuvant, using simpler low-cost drying techniques, has
been the subject of study in Brazil and in other coun-
tries.[6–9]
Drying of tropical fruit pulps in SBs with inert
particles has been extensively studied in Brazil on account
of the excellent quality of the powdered products obtained,
simplicity and ease of operation of the equipment as well as
low building, set-up, and maintenance costs.[9–12]
Also,
advances in the SB technology to dry pastes and suspen-
sions with inert particles as well as new equipment design
have been recently published showing importance and
advantages of the technique.[13–18]
In this work, a summary of results of SB drying of trop-
ical fruit pulps is presented, with emphasis on three aspects:
(i) the effects of fruit pulp composition on SB fluid dynamic
behavior and process performance; (ii) optimization of pow-
der production as a function of composition of the pro-
cessed pulp; (iii) development of new products formulated
from mixtures of fruit pulps with adjusted composition.
EFFECTS OF PULP COMPOSITION ON SPOUTED BED
DRYING OPERATION
The conventional SB paste dryer, shown in Figure 1, has
a drying chamber consisting of a cylindrical column with a
conical base. A gas, usually air, is injected into the column
at the inlet orifice. When gas velocities over the minimum
spout velocity are used, the inert particles start a cyclic
movement, ascending in the central region and falling
along the annulus of the bed. Particles in the spout move
in an upward direction, and after reaching the top of the
bed in decelerated motion, form a fountain just above the
annulus. The particle path in the annulus is directed
towards the base, returning to the central channel or spout.
This cyclic movement of particles creates three distinct
regions: the spout – a dilute phase with high porosity; the
annulus – a low porosity moving bed; and the fountain –
a region above the bed where particles change the direction
of their vertical motion from upwards to downwards. This
gas-particle contact configuration induces high rates of
heat and mass transfer between gas and particles, allowing
efficient drying of pastes and suspensions.[19]
The suspension feed is sprayed, dropped, or injected into
the bed of inert particles, usually at the top or bottom of
the column. Feeding can be continuous, batch-wise, or
intermittent. The liquid spreads over the surface of the par-
ticles and, after drying, forms a thin film covering them.
The inter-particle attrition and slippage cause the breakage
of the film, resulting in a fine powder that is entrained by
air and collected in the cyclone.
The effects of the paste-like material feeding on SB fluid
dynamic parameters and drying performance have been
studied since the 1980s.[6,20–29]
Traditional work on this subject[20]
verified that, upon
adding water and glycerol to the bed of inert particles,
there was a reduction of about 10% in the minimum spout-
ing flow rate, Qms, which was attributed to the decrease in
the number of particles entering the spout region in beds of
wet particles. Other work[6]
analyzed the influence of umbu
pulp feeding on Qms, maximum and stable spouting pres-
sure drop, DPmax and DPssp. The findings showed that an
increase in the mass of umbu pulp fed resulted in a signifi-
cant increase in DPmax, no significant variation in DPssp,
and a significant decrease in Qms. However, other
authors[25]
observed an increase in Qms when pure water
or an aqueous alumina suspension was added to a bed of
inert particles. This effect was more pronounced for the
alumina suspension. By analyzing the fluid dynamic beha-
vior of an SB with the addition of acerola pulp, similar
tendencies were reported.[20,26]
FIG. 1. Scheme of a spouted bed dryer with inert particles (with per-
mission of Taylor and Francis).
1588 ROCHA ET AL.
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5. The complex fluid dynamic behavior of the SB with
paste-like materials, which seemed to result in discrepant
observations by different authors, is a function of paste
feeding mode (continuous or intermittent), its physico-
chemical properties, bed geometry, and paste-inert wett-
ability characteristics.
A critical analysis made in[28]
led to the following conclu-
sions: (i) a critical feed flow rate exists, above which the
inter-particle cohesion forces caused by the formation of
liquid bridges are significant; (ii) the cohesion forces depend
on the paste and inert properties, and are higher for particles
of smaller sizes and lower sphericity; (iii) when these forces
are significant, Qms increases with the paste feeding flow rate
until the bed collapses; (iv) Qms decreases when cohesion
forces are negligible due to the formation of a thin layer
of paste on the inert surface, causing particle slippage.
Studies on drying of different pastes in SBs, such as foods
of vegetable and animal origin, organic and inorganic
chemical products, bioproducts, and medicines, emphasized
material characteristics as one of the factors that signifi-
cantly influences the process.[7,21]
Analyses of the drying
behavior of different vegetable products have related par-
ticle adherence and spout regime collapse to the sticky char-
acteristics of fruit pulps and vegetable juices due to their
high sugar contents (glucose, fructose, and sucrose). How-
ever, none of the works available in the literature could
quantify and properly explain these interactions.
Medeiros and co-workers[29]
focused on the influence
of the chemical composition of fruit pulps on the fluid
dynamics and drying operation in SBs. Since the chemical
composition of the pulp directly affects the properties of
the paste, it is expected to influence both drying perform-
ance and product quality. To quantify these effects, an
experimental methodology was developed.
Experimental Methodology
Different tropical fruit pulps (umbu, hog plum, hog
plum mango, sweetsop, red mombin, acerola, and mango),
without any chemicals or water added, were analyzed to
identify and quantify their main constituents. They were
characterized by the following contents: reducing and
non-reducing sugars, fibers, starch, pectin, total solids, sol-
uble solids, lipids, and water. pH and citric acid percentage
were also determined. All these analyses followed standard,
well-established methods from the literature.[30]
Starch and
pectin were identified as significant components, apart
from reducing sugars, lipids, and fibers.
According to preliminary drying experiments five signifi-
cant constituents (reducing sugars, lipids, fibers, starch,
and pectin) were defined as independent variables for ana-
lyzing the drying process. Natural mango pulp was
adopted as a standard pulp and its composition could be
modified, when needed, by adding known amounts of
reducing sugars, starch, pectin, lipids, and fibers. A 25-1
fractional factorial experimental design with three repli-
cates at the central point was adopted to study the
effect of the variables, as shown in the six first columns
of Table 1.
TABLE 1
Results of drying modified pulps according to the experimental design
Pulp Cw
sugar (%) Cw
lipids (%) Cw
fibers (%) Cw
starch (%) Cw
pectin (%) gpd (%) vpd (%) Loss (%) hp-dry (degree)
1 7.67 0.77 0.50 0.52 1.82 14.06 68.95 16.99 24.5
2 19.95 0.77 0.50 0.52 0.67 0.00 97.07 2.93 31.0
3 7.67 6.84 0.50 0.52 0.67 22.10 22.64 55.26 23.0
4 19.95 6.84 0.50 0.52 1.82 0.00 23.05 76.95 20.5
5 7.67 0.77 2.01 0.52 0.67 12.19 75.02 12.79 31.5
6 19.95 0.77 2.01 0.52 1.82 0.00 94.89 5.11 26.5
7 7.67 6.84 2.01 0.52 1.82 27.77 9.09 63.14 21.0
8 19.95 6.84 2.01 0.52 0.67 0.00 58.37 41.63 20.0
9 7.67 0.77 0.50 4.66 0.67 20.83 49.08 30.09 21.0
10 19.95 0.77 0.50 4.66 1.82 9.11 58.14 32.75 26.0
11 7.67 6.84 0.50 4.66 1.82 49.65 19.87 30.48 25.0
12 19.95 6.84 0.50 4.66 0.67 6.69 33.07 60.23 23.0
13 7.67 0.77 2.01 4.66 1.82 26.70 37.29 36.01 27.5
14 19.95 0.77 2.01 4.66 0.67 5.89 66.12 27.99 27.5
15 7.67 6.84 2.01 4.66 0.67 27.57 12.56 59.87 22.0
16 19.95 6.84 2.01 4.66 1.82 23.88 6.72 69.40 19.5
17 13.81 3.81 1.26 2.59 1.24 16.09 16.69 67.22 23.0
18 13.81 3.81 1.26 2.59 1.24 15.86 18.58 55.56 22.0
19 13.81 3.81 1.26 2.59 1.24 19.49 11.05 69.46 20.0
DRYING OF TROPICAL FRUIT PULPS 1589
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6. Because efficiency was defined in relation to the total
solids in the pulp fed, the water content was no longer an
independent variable. For that reason, the pulp composi-
tions were parameterized in relation to the water content
(Ci
w
¼ Ci=Cw) in order to normalize, to get easier compar-
isons between the factor influences, and to obtain more
generalized results. The maximum levels of concentration
for each component were those found in tropical fruit
pulps, whereas the minimum levels corresponded to those
of the mango pulp. Materials used to modify and control
the pulp composition were glucose and fructose (as reduc-
ing sugars), soluble starch, citric pectin, olive oil (as lipids),
fibers (extracted from the natural mango pulp), and
distilled water.
Six different responses were chosen to evaluate process
performance. The first three, namely, Qms, DPssp, and hp-fl,
the drained angle of repose for the pulp-wetted inert parti-
cles, are related to bed fluid dynamic behavior and insta-
bility. The other three variables, mainly used to optimize
the pulp composition, were the drained angle of repose of
the inert particles after drying, hp-dry, the powder pro-
duction efficiency, gpd, and the retention of powder on
the inert particle surface (adhered or adsorbed onto the
particle surface), vpd.
The values of gpd and vpd are defined as follows:
gpd ¼
Mpd 1 À mpd
À Á
Mpp 1 À mpp
À Á Â 100 ð1Þ
vpd ¼
MpdÀret
Mppð1 À mppÞ
 100 ð2Þ
Besides the responses to the factorial experimental design
(Qms, DPssp, hp-fl, hp-dry, gpd, and vpd), the bed dynamics
were observed every minute during the first 10 minutes
from the pulp feeding to the bed.
To perform the tests, a cone-cylindrical SB dryer made
of stainless steel and having acrylic windows was used, fol-
lowing the scheme presented in Figure 1. The conical base
had included angle of 60
, height of 13 cm and inlet orifice
of 3 cm, and the cylindrical part of the bed had 18 cm
of diameter and height of 72 cm. More details on the
equipment and methodology can be found in.[31]
High-density polyethylene particles (dp ¼ 3.9 mm,
qap ¼ 950 kg=m3
, and sphericity ¼ 0.76) were chosen as
inert. The static bed porosity of these particles is 0.29
and their drained angle of repose is 19.5
. From the DP
vs. Q curve obtained for characterizing the SB of inert par-
ticles (without pulp), the minimum spouting conditions
were determined as Qms0 ¼ 17.04 Â 10À3
m3
=s (0.67 m=s)
and DPms0 ¼ 670 Pa at Tgjin ¼ 70
C and Minert ¼ 2.5 kg.
Influence of the Chemical Composition of the
Pulp on Spouted Bed Fluid Dynamics
Based on preliminary tests, the following operating con-
ditions were selected for carrying out the experiments:
Minert ¼ (2.500 Æ 0.005) kg, Mpp ¼ (50 Æ 1) g, top ¼ 40 min,
Tgjin ¼ (70 Æ 1)
C, and Q=Qms ¼ 1.25 Æ 0.05. The DP vs. Q
curves were obtained for the SB of inert with pulp accord-
ing to the following procedure: (i) spouting the bed of
inert particles by air at Q=Qms0 ¼ 1.25 and Tgjin ¼ 70
C
until reaching the steady stable regime (i.e.,
DPms0 ¼ DPssp ¼ constant and Tgjout ¼ constant); (ii) feed-
ing the pulp over the fountain region using a syringe during
about 1 min; (iii) monitoring DP, Q, Tgjout and annulus and
fountain heights during and after feeding the pulp until
steady state was re-established; (iv) decreasing Q slowly
and recording its value and the corresponding pressure
drop as well as any other parameters that were changing;
and (v) registering as Qms the lowest Q value for which
the fountain was still observed. For comparison, distilled
water was also used as a standard liquid fed into the SB
of inert particles.
Changes in the fluid dynamics became evident within a
few minutes after injection of pulp into the bed of inert.
For almost all experiments, the SB flow regime stabilized
about 10 minutes after injection of the pulp, by which time
most of the pulp moisture had evaporated. Such behavior
was not observed for pulp 2, in which case the SB was char-
acterized by a diluted and lower height fountain and ser-
ious trends to collapse the spout. The very poor quality
of the fluid dynamic exhibited when injecting pulp 2 is
due to agglomerates formation caused by the high concen-
tration of sugar in this pulp, which resulted in channelling
and reduced pressure drop.
Pulps 1, 2, 3, 5, and 9 (see Table 1) were chosen to ana-
lyze the effect of the chemical composition on the SB stab-
ility and hydrodynamics, since these pulps have the
maximum concentration of each individual main compo-
nent: pectin, reducing sugars, lipids, fibers, and starch.
The presence of water resulted in a sharp decrease in DP.
An expansion of the annulus and an increase in the foun-
tain height were also observed in this case. However, as
drying proceeded, initial values of DP were gradually
restored. Similar behavior was also obtained with the
addition of mango pulp. Immediately after feeding the
mango pulp, DP sharply decreased. As drying continued,
DP increased. However, the SB steady condition was
attained at a DP smaller than DP0. These observations con-
firm that, apart from the first instantaneous effects of water
and mango pulp feeding, the influence of the mango pulp
on the bed fluid dynamics was still present when practically
all of the water had already evaporated, which may be
attributed to powder retention on the surface of the inert
particles.
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7. All other pulps also caused a sharp decrease in DP
immediately after being added to the bed of particles.
According to Spitzner Neto and Freire,[27]
this decrease is
explained by the agglomeration, which together with the
pulp viscosity, jeopardizes particle circulation in the bed,
increasing the air flow rate in the spout region. With a
greater resistance in the annulus region, the air flow rate
and the voidage in the spout region increase, leading to a
decrease in the pressure drop across the bed. The higher
the fluid dynamic instability brought about by the pulp,
the larger is this reduction in DP.
It was verified that water resulted in a decrease in pres-
sure drop of about 12% in relation to the dry bed, while for
the fruit pulps, the reduction ranges from 24% (for pulp 3
with the highest content of lipids) to 80% (for pulp 2 with
the highest content of reducing sugars). This corroborates
that the reducing sugar content in pulps contributes more
to instabilities in SB dynamics than the lipid content.[31]
Similar behavior of increasing fountain height just after
the addition of liquid was observed for all pulps, except
for pulp 2. After feeding pulp 2, the fountain height
decreased and oscillated (around 85% of the initial value)
as drying proceeded. With regard to the annulus, its expan-
sion, measured by the increase in height, was observed after
feeding each one of the 19 pulps. The greatest expansion was
recorded for pulp 2. This annular bed expansion is explained
by a decrease in the circulation rate, which caused a redistri-
bution of particles inside the bed, with higher solids concen-
tration in the annulus, resulting in its expansion.
Values of minimum spouting flow rate, Qms, ranged
from 13.0 Â 10À3
m3
=s to 19.6 Â 10À3
m3
=s (corresponding
to minimum spouting velocities in the range of 0.51 m=s
to 0.77 m=s) and hp-fl varied between 1.21 and 2.01
for
all the pulps. It was verified that, although these para-
meters varied within a narrow range for all the pulps, the
highest Qms corresponded to pulp 2, whereas the lowest
value of hp-fl was obtained for pulp 1. The results suggest
that high starch and lipid content favor bed flowability,
as expected due to their lubricant characteristic. Neverthe-
less, a more detailed statistical analysis was made to con-
firm the observed trends.
Drying Performance
The last four columns in Table 1 show the results
obtained for the drying parameters that are related to pro-
cess performance. The loss refers to the percentage of pow-
der mass retained in the equipment, i.e., adhering to the
walls, dispersed at the column outlet, and eventually lost
in the cyclone.
In the work by Medeiros et al.[9]
and Medeiros,[29]
the
results of the statistical analysis of the fractional factorial
design for confidence level of 95% were presented, which
showed that all the pulp components, except fibers, exert
significant effect on gpd. Reducing sugars cause a decrease
in gpd and this effect is the most significant one. Starch, pec-
tin, and lipids favor gpd, with starch concentration being
the most influential. This result is in accordance with
the dynamic results for reducing sugar, starch, and lipid
concentrations.
As regards vpd, the same components show significant
effects, and as expected, such effects are opposite to those
on gpd.
Pulps having high concentrations of reducing sugars form
highly adherent films on the surface of inert particles, and
interparticle attrition and impacts, are not sufficient to break
these films as drying proceeds. In addition, due to the sticky
characteristics of reducing sugars, particle agglomeration
occurs, compromising the regime stability and even leading
to SB collapse. Lipid concentration, followed by starch, exerts
the most important effects on vpd. The important negative
effectoflipids onvpd isrelatedtotheir lubricant characteristic,
which interferes with the bed dynamics, enhancing particle
circulation and thus facilitating breakage of the adherent film.
For hp-dry, only the lipid concentration was signifi-
cant.[9,29]
According to the results of the fractional factorial
design, fiber concentration does not significantly affect dry-
ing performance. Therefore, this variable was excluded
from the statistical analysis, and the experimental design
could then be rearranged into a complete 24
factorial
design with three replicates at the central point.
Based on this 24
complete factorial design, a predictive
model was obtained for gpd at a confidence level in 95%.
As shown by Medeiros and co-workers.[9]
the percentage
of explained variation of gpd predicted by the model is
93.73%. The three last lines in Table 1 show results of the
replicates at the central condition of the experimental
design. Regression and residue analyses showed that repro-
ducibility of the data was satisfactory for all responses ana-
lyzed.[29]
More details about the regression and residue
analyses can be found in.[29]
In the following equations, Ci
w
¼ Ci=Cw is expressed in
wt% and gpd in mass percentage of powder produced in
relation to the solids content in the pulp feed. The model
is represented by the following equation:
gpd ¼ 15:68 À 9:71 Á
Cw
sugar À 13:81
6:14
þ 4:31 Á
Cw
lipids À 3:81
3:03
þ 5:89 Á
Cw
starch À 2:59
À Á
2:07
À 2:36 Á
Cw
sugar À 13:81
6:14
Á
Cw
lipids À 3:81
3:03
þ 2:55 Á
Cw
starch À 2:59
À Á
2:07
Á
Cw
pectin À 1; 24
0:57
ð3Þ
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8. resulting in
gpd ¼ 17:40 À 11:10Cw
sugar þ 3:17Cw
lipids þ 0:17Cw
starch
þ 0:52Cw
pectin À 0:13Cw
sugarCw
lipids þ Cw
starchCw
pectin ð4Þ
The correlation represented by Equation (4) was assessed
using experimental data related to the drying of different
natural tropical fruit pulps, as shown in Figure 2. Drying
of red mombin pulp resulted in the only significant devi-
ation from the gpd prediction given by Equation (4). This
deviation can be explained by the different drying con-
ditions used in processing this pulp (lower Q and Tgjin with
Q=Qms ¼ 1.05 and Tgjin ¼ 50
C) and by agglomeration pro-
blems during this test.
In Equation (4) it can be seen that interactions between
the components of the pulp resulted in changes to the pulp
properties in a way that could enhance or jeopardize drying
performance. Analyses of these properties’ changes on a
fundamental point of view were not possible yet in this
work. Preliminary tests were made trying to relate drying
behavior to the pulp viscosity and surface properties of
the pulp-inert in the spouted bed. However, changes of
these properties during drying and difficulties to correlate
data of the properties for different fruits led to the appli-
cation of the methods adopted in this work. Use of statisti-
cal analysis helped in determining the influences of
interactions of the components of the pulps on the process
performance, which was later verified for several kinds of
natural fruit pulps, as shown in Figure 2. From a practical
point of view, the development of this work was important
to subsidize decisions on adding starch and lipids to act as
adjuvant and enhance process performance, making it feas-
ible to treat tropical fruit pulps in SBs.
The promising results obtained from the statistical mod-
elling of gpd led to the proposition of an optimized pulp
composition, which maximizes the efficiency in an ampli-
fied range of concentrations of reducing sugars, lipids,
starch, and pectin, now covering the range encountered
in many tropical fruit pulps. As the mango pulp was the
basis for the modified pulp compositions, concentrations
of the components below the ones encountered in the stan-
dard mango pulp could not be tested with the experimental
design. According to Equation (4), the highest efficiency
should occur for lowest sugar content and highest lipid,
starch, and pectin contents.
After application of an optimization routine, the opti-
mum pulp composition was determined by means of con-
ventional optimization methods available in literature
and commercial software.[9]
The objective function was
the optimized pulp, which would then have the following
composition: Cw
sugar ¼ 5.52%, Cw
lipids ¼ 14.69%, Cw
starch
¼ 4.93%, and Cw
pectin ¼ 2.78%, resulting in a maximum gpd
of 81%.
A pulp having the optimized composition was prepared
and a drying experiment was carried out at Q=Qms ¼ 1.22
and Tgjin ¼ 70
C, with five intermittent pulp feedings into
the bed of inert particles. The experimental results obtained
for gpd are shown in Figure 3. Excellent drying perfor-
mance with uniform powder production was achieved in
this experiment. Although the efficiency was lower than
that predicted by the optimization procedure (81%), gpd
of about 70% was considered an excellent result, reproduc-
ing the drying behavior obtained with natural tropical fruit
pulps, but showing a significantly higher powder pro-
duction rate and efficiency.
This result for the optimized pulp composition moti-
vated the development of the next step of the research uti-
lizing mixtures of fruits, taking advantage of the different
natural pulp compositions to generate the optimized com-
position with the help of some oil and starch additives. It is
FIG. 2. Efficiency of powder production – Tests with natural fruit pulps
and modified pulps – Observed values as a function of the predicted values
(with permission of Taylor and Francis) (color figure available online).
FIG. 3. Efficiency of powder production – Tests with modified pulp of
optimized composition (with permission of Taylor and Francis).
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9. worth mentioning that powder mixtures of fruits have
shown very good market acceptance.
DRYING OF MIXTURES OF TROPICAL FRUIT PULPS
IN A SPOUTED BED
The choice of a mixture of tropical fruit pulps is
explained by the functionality of the mixture produced by
the synergy of the individual compositions. This powder
mixture, with natural flavor, aroma, and functional com-
ponents, may result in products of sensory and nutritional
quality that will make their way onto the market.
The optimum composition obtained in the previous
work[29]
was the basis for preparing mixtures of pulps.
The mixture formulations included pulps of mango, which
has high fiber and carotenoid contents with aroma preser-
vation; umbu, which has high lipid, vitamin C, and com-
plex B contents; and red mombin, which has a high
starch content. Additives such as commercial cornstarch,
pectin (citric pectin from Merck1
), and lipids were added
to the mixture of pulps aiming at reaching the optimum
composition that makes the spouted bed drying feasible.
It is worth highlighting that, although the concentration
of lipids identified as optimal for fruit pulp drying is high,
the mixtures were formulated to contain around 2% of
lipids due to their adverse health effects.
EXPERIMENTAL METHODS
Mango, red mombin, and umbu pulps without any addi-
tives, even water, were acquired at the local market. The
pulps were packed in 100 ml plastic bags and stored in cold
chambers at À18
C. The physicochemical characteristics of
the pulps (reducing and total reducing sugars, proteins,
total solids, moisture content, lipids, pH, and acidity
expressed as the percentage of citric acid) were determined
by standard methods. Data on fruit composition were con-
sidered in the mass balances.
Different products were tested as a lipids source: olive
and Brazil nut oils, coconut milk, heavy milk, palm fat
powder, and palm olein.
Heavy milk is an emulsion of fat in milk, having a large
amount of milk fat.
Coconut milk is extracted from mature coconut pulp. It
is rich in protein, lipids, calories, carbohydrates, vitamins
A, B1, B2, B5 and C, and mineral salts, mainly potassium
and magnesium.
Olive oil is well recognized for its benefits to health and
for having large amounts of monounsaturated fat, which
reduces the risk of coronary diseases.
Brazil nuts are rich in X-6 and vitamin E, and ideal for
consumption in salads and on fish. Their main advantages
are their high protein content, good fiber content, high con-
tent of fatty acids of vegetable origin (X), and ideal concen-
tration of essential minerals such as selenium.
Palm fat is a vegetable fat that substitutes hydrogenated
and animal fat in preparing diverse products in the food
industry. It is totally free of trans fat, making the final pro-
ducts more healthy. Also, it is commercialized encapsu-
lated in carbohydrate, which enhances dissolution and
product flavor. Finally, the palm olein is one of the richest
sources of vitamin E, helping in the reduction of the circu-
lating cholesterol, among other benefits to health.
All formulas contained around 30% mango, umbu, and
red mombin pulps; 1.3 to 1.5% pectin; 1.3 to 1.5% corn
starch; 6% water; and the contents of different types of
lipids as shown in Table 2.
Drying experiments were conducted to define adequate
formulations of the pulp mixture in terms of lipid source.
The mixture formulations were submitted to drying
under fixed operating conditions. These fixed conditions
were defined based on the results obtained in the previous
work[29]
on the drying of modified tropical fruit pulps: an
inert load of 2.50 Æ 0.01 kg, Tgjin of 70 Æ 2
C. Feeding of
the mixtures into the bed was intermittent, with a volume
of 100 ml added during 20 minutes, followed by an interval
of 15 minutes before the next feeding. The same air flow
rate (20% above the minimum spouting for the bed of inert
at 70
C) was maintained in the drying experiments of the
different mixtures. The results were analyzed in terms of
drying performance and sensory tests.
Drying performance was evaluated by process efficiency
and the fraction of solids retained in the bed (Equations (1)
and (2), respectively).
The same methodology as that used to formulate indus-
trialized yogurts with the addition of fruit pulps was
applied to prepare samples of yogurts incorporating the
powders obtained from the drying of the mixtures shown
in Table 2. Proportions of 93.4% yogurt (natural, skimmed,
without sugar, and with a thick consistency), 1.8% powder,
and 4.8% sugar were maintained in the samples. Samples
were tasted by 20 tasters at the Food Engineering and
Nutrition Schools of UFRN (Federal University of Rio
Grande do Norte in Brazil).
After establishing the appropriate types of lipids, new
drying runs were done with the mixtures containing the
TABLE 2
Formulation of the mixtures of fruit pulps
Formula name Lipid composition
FOLIVE Olive oil (1.3%)
FHMILK Heavy milk (5.5%)
FCOCOM Coconut milk (5.5%)
FBRNUT Brazil nut oil (1.3%)
FPALMF Palm fat (powder) (2.0%)
FPALMO Palm olein (2.0%)
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10. chosen lipid sources, aiming at having a sufficient amount
for their characterization.
Physical and physicochemical characteristics were
determined for the dry powders obtained from these mix-
ture formulations. Solubility and reconstitution time as
well as the properties of the product after reconstitution
were also evaluated.
Physicochemical Characterization of the Mixtures of
Pulps and Products (Dry Powders)
All characterization analyses as described below were
done in triplicate.
The moisture content was determined by the oven
method until reaching constant weight using an oven with
air circulation at 70
C.[30]
Determinations of pH, lipid content, titratable acidity,
total soluble solids, residue by incineration, protein con-
tent, and vitamin C content (mg AA=100 ml of sample)
were made according to standard methods in the litera-
ture.[30]
Reducing sugars were determined by the method
described in[32]
and water activity was obtained directly in
an analyzer that applies the dew point principle.
Percentage of vitamin C loss was obtained relating the
AA (ascorbic acid) contents found for the powder and
the pulp mixtures in natura, expressed on a dry basis.
VCloss ¼ 100 À 100x
VCpd
VCpp
ð5Þ
Physical Characterization of the Mixtures
Measurements of surface tension of the pulp mixtures
were taken with a Kru¨ss tensiometer by the well-known
ring method, and density was obtained by pycnometry.
A digital rheometer (RheoStress – from Haake, model
RS-150, sensor geometry of co-axial cylinders, model
DG-41, with a thermostatic bath, also from Haake,
model K20) was utilized to analyze the rheological beha-
vior of the mixtures and the reconstituted powder. An
interval of 15 to 150 sÀ1
of rate of shear strain was covered
in the tests, due to equipment limitations (instabilities
occurred for lower values of shear stress). Data on shear
stress as a function of rate of shear strain were adjusted
to the Power Law model, which represents the rheological
behavior of fruit pulps.[33]
Powder Characterization
Dry powder was characterized by its solubility, reconsti-
tution of the pulp mixture, angle of repose, and Hausner
factor.
Powder solubility was determined by the method
described in[34]
and the procedure specified in[22]
was
applied to evaluate the time of reconstitution of the powder
to the pulp mixture.
Powder mixture flowability was analyzed by the static
angle of repose and Hausner factor. The latter was determ-
ined as the ratio of the experimentally observed tapped and
free apparent densities.
RESULTS AND DISCUSSION
Physicochemical Characterization of the
Pulp Mixture Formulations
The results of characterization of the pulp mixtures with
the addition of corn starch, pectin, and the different types
of lipids are shown in Table 3.
Total soluble solids expressed as
Brix are lower than
the values found in other work[35]
for pasteurized and ster-
ilized pure´e of various fruits (27.0
Brix). Also, the formula-
tions in this work had lower solids contents, and higher
moisture contents, than the pulps modified and analyzed
by Medeiros.[29]
The difference can be explained by the
low solids content and very high moisture content of the
umbu pulp used in this work. This result points to the good
performance of the drying process using the mixture for-
mulations proposed here, as reducing sugars (which corre-
spond to almost the total solids content in fruit pulps)
interfere in a negative way in the spouted bed drying of
fruit pulps.
The pH of the different mixtures shown in Table 3 is
compatible with the pH range of the modified pulps (from
2.9 to 4.2) studied in.[29]
It is known that the pulps’ proper-
ties are influenced by their compositions and pH. For
example, some properties of fruit pulps having a high
reducing sugar content (mainly glucose) are altered when
pH is reduced to values lower than 3. This effect may not
show up in the mixtures analyzed in this work due to the
low sugar content and pH 3. Also, the pH of a food is
an important factor as it can indicate the growth, survival,
or destruction of microorganisms present. According to the
pH range (3.2 to 3.9), the mixtures are classified in the
group of very acidic foods, in which microbial development
is restricted to yeast and mold, with possibly some lactic
and acetic bacteria.
With respect to surface tension, one can observe that
there was no significant variation between the different
TABLE 3
Physical and physicochemical characterization of the pulp
mixture formulations
TSS (
Brix) TTA (%) pH r (dyn=cm)
FOLIVE 11.3 1.1 3.3 57.4
FHMILK 10.0 0.8 3.2 59.9
FCOCOM 10.7 0.8 3.3 62.6
FBRNUT 11.4 0.9 3.2 57.7
FPALMF 13.1 0.9 3.3 62.6
FPALMO 13.1 0.9 3.9 51.6
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11. formulations; the measurements are close to the umbu pulp
surface tension (62.1 dyn=cm).
Water activity was similar for all mixtures analyzed and
very high, as expected (mean value: 0.986 Æ 0.003).
The results also showed that the different types of
lipids in the mixtures did not affect their densities
(1.061 Æ 0.003 g=ml). The densities obtained in this work
are slightly lower than those of the natural fruit pulps that
compose the mixtures; they are compatible with the ones
determined for the modified pulps used by Medeiros.[29]
Drying
Figure 4 contains plots of the mass of powder (accumu-
lated) collected during the pulp mixture drying. The gra-
phic shows that for most of the experiments the mass of
powder increased linearly with time, indicating that the
production rate was practically constant.
The effect of different types of lipids on powder pro-
duction is more evident for the palm fat powder and the
heavy milk. The largest and smallest collections of powder
were obtained with the formulas including FPALMF and
FHMILK, respectively. The production rate estimated from
the linear model corresponds to 0.355 g=min (21.3 g=h)
and 0.257 g=min (14.4 g=h), respectively. The straight lines
almost overlap for the other lipid sources with an average
powder production of 0.3 g=min (18 g=h).
The results obtained for the moisture contents of the
pulp mixtures in natura (mpp) and of the powder (mpd) as
well as the process efficiency (gpd) and the material reten-
tion in the bed (vpd) are shown in Table 4. It was verified
by experimental observations that the drying of some mix-
tures resulted in high levels of material retention inside the
bed (mainly for the formulation using heavy milk) or
material adhering to the dryer walls (as for the mixture
having coconut milk).
No significant variation is observed for the pulp mois-
ture content, which corresponds almost completely to the
average of the moisture contents of the fruit pulps which
the formulations are composed.
Powder moisture contents were in the range of 4.11 to
8.05%. These values are compatible with the ones found
by Medeiros and co-workers.[9]
As shown in Table 4, drying efficiencies were in the
range of 35.6 to 52.3%, which is higher than the drying effi-
ciencies obtained for drying of umbu, mango, and red
mombin pulps using the same equipment and inert.[9]
In the drying of FHMILK there was higher retention of
material in the bed than the drying yield, which agrees with
experimental findings. Heavy milk is the only animal fat
source used in the mixture compositions proposed in this
work. Interactions between this type of fat and the other
components of the pulps may be the reason for the
adhesion of the film of this mixture to the inert particles.
Drying of FCOCOM resulted in low material retention;
however, the drying efficiency was also low due to the loss
of powder attached to the bed walls in accordance with the
experimental reported data.
For the drying of FPALMF and FPALMO, the efficiencies
were higher and, specifically for the FPALMO, the material
retention was very low (13.8%). Stable fluid dynamics were
observed during the drying experiments. Bed pressure drop
and heights of the fountain and of the annulus remained
stable during the drying runs, showing only alterations
inherent to those in the intermittent feeding of the pulp
mixtures.
Apart from process performance, results of sensory test-
ing of the yogurts prepared with the addition of the dried
mixtures were also used as a criterion to select the appropri-
ate mixture formulations. Yogurts containing dried FHMILK,
FCOCOM, FPALMF, and FPALMO were approved in the sen-
sory test. For the other yogurts, characteristic odor of the
lipid source utilized negatively influenced the tasters.
Despite their sensory acceptance, the mixtures with
addition of heavy milk and coconut milk were also
FIG. 4. Powder production during drying of the pulp mixture formula-
tions (color figure available online).
TABLE 4
Drying results for the mixtures of fruit pulps
mpp (%) mpd (%) gpd (%) vpd (%)
FOLIVE 82.9 4.11 44.3 30.5
FHMILK 83.1 4.69 35.6 58.9
FCOCOM 83.4 8.05 43.3 14.3
FBRNUT 83.4 7.58 39.4 31.4
FPALMF 82.3 6.27 52.3 26.2
FPALMO 82.2 7.30 49.1 13.8
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12. discarded, the former due to the low drying efficiency and
the latter because of the high level of material retention
during the drying tests. Thus, FPALMF and FPALMO were
selected to continue this research, due to their good SB dry-
ing performance and good score in the sensory analysis.
Analyses of composition of these two formulations were
performed and the results are shown in Table 5.
Powder Characterization
To validate the mixtures selected, an analysis of the
dried powders was carried out. Product quality was evalu-
ated through physical and physicochemical characteriza-
tion and ascorbic acid contents. Results are shown in
Table 6.
The average ascorbic acid content of the powders from
FPALMF and FPALMO was 10 mg AA=100 g of sample. This
value is significantly lower than those found for powders
obtained from umbu, red mombin, and mango pulp dried
in a SB at 70
C without any additives (68.4, 52.52, and
32.28 mg AA=100 g of sample, respectively).[11]
For the pulp mixtures FPALMF and FPALMO in natura,
the average ascorbic acid content was of 2.98 mg AA=
100 g of sample, which is much lower than the value found
for the fruit pulps without additives. This low ascorbic acid
concentration is due to the modification of the composition
of the mixtures by addition of lipids, starch, and pectin.
For evaluation on a dry basis, the loss of vitamin C was
37% (average), which is of the same order of magnitude
and slightly lower than the vitamin C loss recorded in other
work.[11]
Powder moisture contents were compatible with the
results for dried Surinam cherry pulp without additives
(8.12%) and with the addition of 15% maltodextrin
(7.64%).[36]
The acidities (expressed as citric acid content) found for
FPALMF (5.42%) and FPALMO (4.65%) were lower than the
ones obtained for green acerola powder dried in an oven
and lyophilized (7.68 and 8.50%, respectively),[37]
and
higher than the value reported for powder from pineapple
bagasse (2.58%).[38]
Powders obtained from the two mixture formulations
showed good solubility, of the same order of magnitude.
Averages of the static angle of repose were 49
for the
two mixture formulations. Angles of repose lower than
45
are characteristic of free-flowing powders, while angles
of repose above 50
suggest cohesiveness. The value
obtained in this work is at the limit between free flowability
and cohesiveness.
Mean free apparent densities obtained for FPALMF and
FPALMO powders were 0.29 g=ml and 0.21 g=ml, respec-
tively. These values are of the same order of magnitude
as those for other powders from fruit pulps with and with-
out additives obtained in a spray dryer and in a SB.[36,39]
Tapped densities were evaluated for the powders from
the two mixtures, showing values of 0.40 g=ml and
0.33 g=ml for FPALMF and FPALMO, respectively. The
Hausner factor was calculated as the ratio of the tapped
to the free apparent densities, resulting in 1.46 and 1.58
for FPALMF and FPALMO. The Hausner factor is related
to the cohesive forces of a particulate material; if the factor
is lower than 1.25, the material can be classified as
free-flowing. Hausner factors higher than 1.4 are typical
of cohesive materials.[40]
Analysis of the powder characterization reveals simi-
larity between the products of the two mixture formula-
tions, FPALMF and FPALMO. The powders had low
moisture contents; ascorbic acid contents were not high
in the mixtures and important losses of vitamin C were
reported. Acidity analysis classifies the powders as acids,
consequently making microbiological contamination diffi-
cult. The Hausner factor indicates that the powders of both
formulations are cohesive.
Experiments on pulp reconstitution resulted in an aver-
age time of 315 s for complete reconstitution. A similar
result was found for the reconstitution of tomato powder
obtained in SB.[22]
It is important to note that drying tem-
perature influences reconstitution time, as verified in other
work.[22,41]
Reconstitution times were higher for higher
drying temperatures. The bridges formed between the
particles during drying could be more rigid at high
TABLE 5
Compositions of FPALMF and FPALMO
Parameters FPALMF FPALMO
TRS (%) 13.0 Æ 0.2 13.9 Æ 0.2
RS (%) 11.83 Æ 0.45 11.02 Æ 0.06
Proteins (%) 0.69 Æ 0.01 0.85 Æ 0.03
Lipids (%) 2.29 2.27
Incineration residue (%) 0.53 Æ 0.01 0.51 Æ 0.01
mpp (%) 82.3 82.2
Ascorbic acid
(mg AA=100 ml of sample)
2.76 2.97
TABLE 6
Powder characterization
Parameters FPALMF FPALMO
Ascorbic acid
(mg AA=100 ml of sample)
10.0 Æ 0.2 9.7 Æ 0.2
mpd (%) 8.51 Æ 0.55 7.30 Æ 0.04
Acidity (% citric acid) 5.42 Æ 0.25 4.65 Æ 0.04
Solubility (%) 60.15 Æ 0.001 67.82 Æ 0.006
Angle of repose (
) 49 Æ 1 49 Æ 1
HF 1.46 Æ 0.06 1.58 Æ 0.02
Reconstitution time (s) 300 Æ 1 330 Æ 1
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13. temperatures, altering the reconstitution and solubility of
the final product.
A summary of the properties of the reconstituted and in
natura pulp mixtures (FPALMF and FPALMO) is presented in
Table 7.
No significant alterations can be seen between the recon-
stituted and in natura pulp mixtures. The small variations
observed can be explained by the higher moisture content
of the reconstituted pulp and probably by the drying
process with volatile losses.
Results of the rheological characterization (fit to the
power law model) of the mixture formulations, in natura
and reconstituted, are shown in Table 8.
Similar results for each mixture, in natura and reconsti-
tuted, confirm the potential of SB drying for tropical fruit
pulp mixtures.
CONCLUSIONS
For most of the pulps, a sharp decrease of the bed pres-
sure drop was obtained just after the addition of pulp into
the bed. As drying continued, the pressure drop increased
until a steady condition, which was attained at a smaller
pressure drop than the initial one. Observations confirmed
that, apart from the first instantaneous effects of pulp fed,
an influence of the pulp on the bed dynamics was still
present when almost all water was evaporated, which was
attributed to powder retention on the surface of the inert
particles.
Pulps with high lipid and=or high starch contents
resulted in stable spouting regime.
The higher the fluid dynamic instability brought about
by the pulp, the larger was the reduction in the bed pressure
drop, as verified for the pulps having high reducing sugar
contents.
Analysis of the fluid dynamics suggested that high starch
and lipid contents favored bed flowability and reducing
sugars resulted in bad dynamic regime.
Repetitions at the central condition of the experimental
design resulted in appropriate reproducibility of powder
production efficiency.
For some conditions of the experimental design there
was no powder produced, as in the runs with pulps con-
taining high reducing sugar and low starch concentrations.
The highest efficiency was obtained for the pulp with low
reducing sugar and fiber contents, and high pectin, lipid,
and starch contents.
Statistical analysis of the fractional factorial design for
95% of confidence level revealed that all the components,
except fibers, exerted significant effect on efficiency of pow-
der production. Reducing sugars caused a decrease in the
efficiency and this effect was the most significant. Starch,
pectin, and lipids favored powder production, the starch
concentration being the most influential.
A predictive statistical correlation was obtained for
efficiency of powder production as a function of the
pulp composition. Regression and residue analyses as
well as comparisons of predicted and experimental
values of efficiency of powder production for all the
modified pulps, for natural mango pulp, and for differ-
ent tropical fruit pulps dried in the same SB dryer
attested a good fit of the proposed correlation to the
experimental data.
An optimized pulp composition was determined, which
would result in a maximum efficiency of powder pro-
duction of 81%.
In the drying of mixtures of mango, red mombin, and
umbu pulps with addition of starch and different lipid
sources, formulas with palm fat powder and palm oil
resulted in higher efficiencies and, specifically for the palm
oil, material retention in the bed was very low. Stable fluid
dynamics were observed during the drying experiments.
Bed pressure drop and heights of the fountain and annulus
stayed stable during the drying runs.
Powders from the mixtures of pulps with adjusted com-
position (using palm fat powder and palm oil as lipid
sources) had good solubility in water, intermediate cohes-
iveness, and moisture contents, citric acid percentages
and reconstitution times compatible with the results
found in the literature for other pulps. Also, yogurts
TABLE 7
Characterization of the mixtures: in natura and
reconstituted
FPALMF FPALMO
Parameters In natura Rec. In natura Rec.
mpp (%) 82.3 86.1 82.2 86.1
TSS (
Brix) 13.1 12.8 13.1 12.2
TTA (%) 0.9 0.9 0.9 0.9
pH 3.3 3.4 3.9 3.8
qpp (kg=m3
) 1062 1057 1060 1058
r (dyn=cm) 62.6 45.8 51.6 48.2
Rec. – reconstituted.
TABLE 8
Rheological parameters (power law model) for the pulp
mixtures in natura and reconstituted
Parameters
K n R2
FPALMF – in natura 2.56 0.30 0.998
FPALMF – rec. 2.95 0.30 0.995
FPALMO – in natura 4.00 0.38 0.996
FPALMO – rec. 3.17 0.38 0.996
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14. containing these powders obtained good scores in sensory
analyses.
SB drying of the pulp mixtures (with palm fat powder
and palm oil) resulted in a high-quality product for use
by industry in the preparation of enriched foods. Consider-
ing that palm oil is a Brazilian product of moderate price
that has been replacing the powder of palm fat in the
Brazilian industries, the mixture including palm oil was
chosen to continue this research. The next step will be to
analyze process performance for this fixed formulation of
the pulp mixture, aiming to optimize process efficiency
through modifications of the operating conditions and
the dimensions of the SB drier.
NOMENCLATURE
Ci
w
¼ Ci=Cw Conc. component i=conc. comp. w (%)
d diameter (m)
K parameter (power law model) (NÁmÀ2
Á s)
M mass (kg)
m moisture content, wb (%)
n parameter (power law model) (À)
P pressure (NÁmÀ2
)
Q flow rate (m3
Á sÀ1
)
RS Reducing sugars (%)
R2
Correlation coeficient (À)
T temperature (
C)
t time, (s)
TRS Total reducing sugars (%)
TSS Total soluble solids (
Brix)
TTA Total titratable acidity (%)
VC vitamin C content (mg AA=100 ml of sample)
Greek Letters
D variation (À)
h angle of repose (
)
g efficiency (%)
v retention (%)
q density (kg Á mÀ3
)
Subscripts
0 initial
ap apparent
g gas
in inlet
loss loss
max maximum
ms minimum spouting
out outlet
p particle
pd-ret powder retained
p-dry dry particles
p-fl wetted particles
pp pulp
ssp stable spouting
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DRYING OF TROPICAL FRUIT PULPS 1599
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