Fages, Jacques

Galet, Laurence

Oulahna, Driss

Ré, Maria Inês

Vu, Tuyet-Oanh

English

Ré, Maria-Inês

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Agglomeration of a fine and hydrophobic powder by spray drying

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Agglomeration of a fine and hydrophobic powder by
spray drying
Tuyet-Oanh Vu, Laurence Galet, Driss Oulahna, Jacques Fages, Maria-Inês Ré
To cite this version:
Tuyet-Oanh Vu, Laurence Galet, Driss Oulahna, Jacques Fages, Maria-Inês Ré. Agglomeration of a
fine and hydrophobic powder by spray drying. WCPT4 -World congress on particle technology 4, Jul
2002, Sydney, Australia. 6 p. ￿hal-01782654￿
1 
AGGLOMERATION OF A FINE AND HYDROPHOBIC 
POWDER BY SPRAY DRYING 
 
Vu T.O.1, Galet L.1, Oulahna D.1, Fages J.1, Ré M.I.2 
 
1 Ecole des Mines d’Albi-Carmaux, Laboratoire de Génie des Procédés des Solides Divisés, 
UMR CNRS 2392, Campus Jarlard, F81013 Albi-France 
2 Instituto de Pesquisas Tecnológicas do Estado de São Paulo, Agrupamento de Processos 
Químicos, Divisão de Química, CEP 05508-901, São Paulo-SP-Brazil 
 
 
INTRODUCTION 
 
Agglomeration has wide applications in the food industry since the formation of larger and 
permanent aggregates of finely divided solids allows better flow characteristics and an 
improved control of solubility [1, 2]. In the case of cocoa, the main reason for agglomeration 
is to enhance its dispersion rate for instant beverages. ‘Instant’ means the agglomerated cocoa 
powder can be dispersed in water or in milk by stirring the product in the liquid for a short 
time [3, 4, 5]. 
The low dispersibility of cocoa powder in liquid is due to its physical properties (small 
particle size diameter, high cohesivity) and its composition (10-12% cocoa butter content). A 
previous study [6] has shown that cocoa dispersion kinetics can be improved by increasing 
the slowest step of the dispersion, the powder wetting by the liquid. Among several 
possibilities for achieving this increasing the grain size and porosity by agglomeration 
appears to have many advantages. 
Several techniques have been used to produce instant cocoa powders which require additives 
such as binders and surfactants [7, 8] or more recently, a continuous thermal agglomerator 
[9]. And alternative is spray drying, a technique widely used in the food and pharmaceutical 
fields to dry heat-sensitive materials. In this work, spray drying has been investigated as a 
potential technique to obtain agglomerated cocoa powder from finely dispersed cocoa 
powder. 
 
 
MATERIALS AND METHODS 
 
 
CHARACTERIZATION OF AGGLOMERATED AND NON- 
AGGLOMERATED COCOA POWDERS  
 
The cocoa powder used here was provided by Cargill Ltda., Brazil. It is a low fat cocoa 
powder (10-12% of cocoa butter) and it is insoluble in water.The non-agglomerated raw 
powder and the resultant spray dried granules are both characterized in the same way. The 
mean diameter was measured using a Malvern laser diffraction instrument with edible oil as a 
dispersant phase to separate the granules without breaking them. The true density was 
measured with a Micromeritics Accupyc helium pycnometer. The aerated (ρa) and packed 
(ρp) densities were measured with a Hosokawa powder tester. These two densities were used 
to calculate the following indexes : the Carr index (IC) that is representative of the powder 
flowability and the Hausner index (HR) that corresponds to a measure of the cohesivity 
between the grains or the granules : 
2 
IC =
ρp − ρa
ρ p  
H R =
ρp
ρa  
The intra-particular porosity of the grains and the intra-granular porosity of agglomerated 
granules were measured with a Micromeritics mercury porosimeter using a maximum 
pressure of 413 MPa. The inter-particular or inter-granular porosities were calculated from 
these porosities and the measured values of true and aerated densities. The morphology of the 
grains and the granules were observed with a scanning electronic microscope (S.E.M.). 
 
 
AGGLOMERATION BY SPRAY DRYING 
 
0
50
100
150
200
250
300
350
400
0 5 10 15 20 25
vi
sc
os
ity
 (c
p)
mass fraction (%)
 
Figure 1. Dynamic viscosity vs. mass volume of cocoa dispersions in demineralized water at 
20°C 
suspension feed
hot air
1. atomizing
2. liquid/air contact
3. drying
hurricane
fine granules
big granules  
Figure 2. Experimental spray dryer 
 
Suspensions of cocoa powder in demineralized water were prepared with different solid 
concentrations and their dynamic viscosities were measured with a Brookfield DVII 
viscometer in a cell controlled at 20°C. The aim was to identify the suitable solid 
3 
concentration in aqueous suspension for the agglomeration tests since, the initial solid 
concentration in the feed and the resultant suspension viscosity can strongly influence the 
physical characteristics of the final dried particles from a spray drying processes [10]. From 
the data obtained (Figure 1), a mass fraction of 20% was chosen for the spray drying 
agglomeration tests because the viscosity of a 20% mass fraction suspension is still low 
enough not to hinder droplet formation. 
A rotary atomizer APV Nordic Anhydro PSD52 with a drying capacity of 7 kg water per hour 
was used in the experiments (Figure 2). The speed of rotation of the atomizer was 30 000 
rpm. The outlet temperature was 90°C, and the inlet temperature and the suspension flow 
were chosen to keep the outlet temperature constant.  The large granules of the agglomerated 
powder were collected at the bottom of the drying chamber and the fine granules were carried 
by the air flow and collected in a cyclone.  The two size population of granules were obtained 
and called “large spray dried granules” and “fine spray dried granules” and they were 
characterized in the same way as for the non-agglomerated cocoa powder. 
 
 
RESULTS AND DISCUSSION 
 
 
DIAMETERS AND SIZE DISTRIBUTIONS 
 
The characteristic diameters and the span of the size distribution for the powder and the 
granules are given in Table 1. It can be seen that the spray drying is a good method for 
agglomerating cocoa powder as the mean diameter changes from 15.7 µm to 40.8 µm for the 
large granules and to 24.8 µm for the fine ones. We can also note that there is a wide size 
distribution for the non-agglomerated cocoa powder (span ~ 3), but narower for the 
agglomerated granules. Agglomeration by spray drying reduces the amount of fine particles..  
 
 
Material 
d10 
(µm) 
d50 
(µm) 
d90 
(µm) 
Span 
(d90-d10)/d50 
     
     
Non agglomerated cocoa powder 5.9 15.7 51.4 2.9 
     
Large spray dried granules 23.2 40.8 65.0 1.0 
     
Fine spray dried granules 7.3 24.8 46.2 1.6 
     
Table 1. Mean diameters and span measured by a laser diffraction in edible oil 
 
 
PARTICLE AND GRANULE MORPHOLOGY 
 
Figures 3 and 4 show the differences between non-agglomerated cocoa grain powder and the 
spray dried granules. It can be seen that the non-agglomerated cocoa grains are irregular, and 
the spray dried granules are predominantly spherical and porous (Table 3). Their rounded 
shape can be attributed to drying,by atomisation. The suspension is broken into small liquid 
droplets by the rotary atomiser, and these spherical droplets are instantly dried by the co-
current flow of hot air . The solids contained in these droplets remains aggregated, the 
original particles being still identifiable (Figure 3c and 3d), and the liquid volume initially 
4 
between the powder grains is substituted by air volume, that induces a porosity in the formed 
agglomerates. 
 
(a) (b) 
Figure 3. S.E.M. for raw cocoa powder, magnitude 150X and 750X 
 
(a) (b) 
Figure 4. S.E.M. for spray dried granule powder, magnitude 150X and 750X 
 
 
FLOWABILITY AND COHESIVITY 
 
The non-agglomerated cocoa powder is very cohesive probably because of its irregular slab 
shaped particles. These irregularities produce an entanglement of the grains, and then fragile 
agglomerates. The agglomerates produced by spray drying are spherical and solid interactions 
between granules are less important than for the grains leading to a lower cohesivity for 
granules than for grains and consequently improved flowability (Table 2). Due to its physical 
characteristics, the agglomerated powder is easier to handle. 
 
 
Material 
ρtrue 
(g.cm-3) 
ρaerated 
(g.cm-3)
ρpacked 
(g.cm-3)
IC 
 (%)
Flowabilit
y 
HR 
(-) 
Cohesivity 
 
        
Non-agglomerated 
powder 
1.45 0.28 0.54 47.9 Very bad 2.1 Very 
strong 
        
Large spray dried 1.45 0.31 0.54 41.5 Medium 1.4 Strong 
5 
granules 
        
Fine spray dried granules 1.45 0.30 0.54 44.8 Bad 1.8 Very 
strong 
        
Table 2. Densities and mechanical properties of cocoa powders 
 
 
POROSITIES 
 
The non-agglomerated cocoa powder is characterized by an inter-particular porosity, that can 
be calculated from its true and aerated densities. However, the spray dried cocoa granules 
present an intra-granular porosity, equivalent to the inter-particular porosity of non-
agglomerated powder. The results are shown in Table 3. 
 
 
 
Material 
Intra-
particular 
porosity 
(%) 
Intra-granular
porosity 
(%) 
Inter-
granular 
porosity 
(%) 
Sum of 
porosities 
(%) 
     
Non-agglomerated powder 0 no 83.0 83.0 
     
Large spray dried granules 0 17.5 60.8 78.3 
     
Fine spray dried granules 0 17.2 62.2 79.4 
     
Table 3. Porosites for cocoa grains and granules 
 
 
CONCLUSIONS 
 
Spray drying appears to be an attractive method for the preparation of cocoa powder 
agglomerates which seems to come close the desired properties for rapid dispersing 
beverages. Briefly, the technique brings about modifications to the raw cocoa powder, by 
increasing the mean diameter of the original grains, inducing an intra-granular porosity and 
decreasing the cohesivity between the resulting cocoa granules. The spray drying process thus 
modifies some physical properties of a cocoa powder that are the important characteristics in 
the dispersion kinetics of a powder in a liquid. Quantification of the dispersion kinetics is 
presently under evaluation. 
 
 
ACKNOWLEDGEMENT 
 
We would like to thank Cargill Ltda. Brazil for providing the cocoa powder. 
 
 
REFERENCES 
 
6 
[1] Dumoulin E. and Bimbenet J.J. (1993) Qualité des poudres obtenues par pluvérisation. 
Cahier de l’AFSIA, Séchage par pulvérisation, 8. 
 
[2] Cordier F. (2000) Etude de la granulation humide des poudres cosmétiques par assiette 
granulatrice et mélangeur à haut-cisaillement. PhD. ENSM Paris-France 
 
[3] Due Jensen J. (1973) Methods of instantizing powders for the preparation of food 
drinks. The Manufacturer Confectioner. 
 
[4] Schubert H. (1993) Instantization of powdered food products. International Chemical 
Engineering, 33(1), pp 28-45. 
 
[5] Kyaw Hla P. (1999) Wetting behaviour of instantized cocoa beverage powders. 
International Journal of Food Sciences and Technology, 34, pp 335-342 
 
[6] Vu T.O, Galet L., Oulahna D., Fages J. (2001), Improving the dispersion kinetics of a 
cocoa powder by size enlargement. Proceedings of the 7th International Symposium on 
Agglomeration, Albi-France, pp 739-748 
 
[7] Wieland H. (1972) Cocoa and chocolateprocessing. Park Ridge, NJ: Noyes Data 
Corporation. In: Omobuwajo et al., Journal of Food Engineering, 46, pp 73-81. 
 
[8] Due Jensen J.(1975) Agglomeration, instantizing, and spray drying. Food Technology, 
29, pp 60-71 
 
[9] Omobuwajo T.O., Busari O.T., Osemwegie A.A. (2000) Thermal agglomeration of 
chocolate drink powder. Journal of Food Engineering, 46, pp 73-81 
 
[10] Oneda F. and Ré M.I. (2001) The effect of formulation variables on the dissolution 
and physical properties of spray-dried microspheres containing organic calcium salts. 
Proceedings of the 7th International Symposium on Agglomeration, Albi-France,  2,pp 707-
717 


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Agglomeration of a fine and hydrophobic powder by spray drying

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