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Experimental production of annatto powders in spouted bed dryer
Journal of Food Engineering 59 (2003) 93–97 www.elsevier.com/locate/jfoodeng
Experimental production of annatto powders in spouted bed dryer I.K. Shuhama, M.L. Aguiar 1, W.P. Oliveira, L.A.P. Freitas
*
Industrial Physics Laboratory, Faculdade de Ci^ encias Farmac^ euticas de Ribeir~ ao Preto––USP, Via do Caf e s/n, Ribeir~ ao Preto, SP, 14040-903, Brazil Received 9 July 1998; received in revised form 29 August 2001; accepted 22 October 2002
Abstract A new method for the production of annatto powders by drying the alkaline aqueous extracts in a spouted bed was studied. The effects of drying conditions such as air inlet temperature and extract feed rate on the properties of the powder obtained were investigated. The powder properties taken as quality parameters were the bixin and moisture content and the particle size distribution. Results on total through-put, bixin content and recovery obtained showed that spouted bed drying of aqueous extracts is advantageous when compared to extraction by mechanical attrition. Also the data showed that above the drying temperature of 80 °C the bixin content decreases sharply. The particle sizes of the powder were always below 60 lm. Ó 2003 Elsevier Science Ltd. All rights reserved. Keywords: Convective drying; Bixa orellana sp.; Extracts; Bixin
1. Introduction The use of annatto (Bixa orellana, sp.) seeds as a source of natural coloring (Ingram & Francis, 1969) is an ancient art, since its pigments were manipulated by Brazilian native tribes for skin and artifacts painting. The seeds are coated by a layer of coloring substances, which can be obtained by extraction with solvents, like alkali or oil, or by mechanical attrition. The main pigments are bixin and norbixin, dicarboxilic acid monomethyl esters, which represents 90% of the total pigments (Preston & Rickard, 1980). Among the current uses of this foodstuff is the coloring of butter, mayonnaise, seasonings, cheese and also for baking purposes (Anand, 1983; Madsen, 1981; Ramamurthy & Krishna, 1982). Since the recent prohibition on use of the synthetic dye Tartrazine (FD & C-Yellow 5) in several countries (Andres, 1980) there is an increasing international interest for the annatto pigment due to its lack of toxicity. Among the methods applied for obtaining annatto pigments there are the extraction from seeds with aqueous solutions of sodium hidroxide, potassium hidroxide or other suitable solvents such as propylene glycol, oil or pure water (Tong, 1984). An alternative
*
Corresponding author. E-mail address: [email protected] (L.A.P. Freitas). 1 Address: Universidade Federal de S~ao Carlos, S~ao Carlos, SP, 13565-905, Brazil.
method is the mechanical attrition of the seeds for wearing of the pigment layer, which is usually performed in ball mills (Barreto, Jaeger, & Massarani, 1989), or in spouted beds (Massarani, Passos, & Barreto, 1992). The present report proposes a new method for the preparation of annatto powders based on the alkaline extraction from seeds and further drying in a Spouted Bed with inert particle bodies. Spouted Bed Dryer was chosen because it is a low cost and high productivity dryer that could be easily disseminated for this purpose. Experiments were carried out to verify the applicability of Spouted Bed Dryers for the drying of aqueous alkaline extracts and alkaline-acidified extracts from annatto seeds. The effects of drying conditions such as the air temperatures and extract feed rates on the powder characteristics were determined. The results were compared with the data reported by Barreto et al. (1989) and Massarani et al. (1992).
2. Materials and methods The seeds were purchased from a single plantation located in Sao Paulo State, Brazil. The bixin content in the seeds was 1.0%, as determined by WHO/FAO (1975) procedures. For the extract preparation, 200 g of seeds were washed five times using portions of 200 ml aqueous NaOH (0.05, 0.1, 0.25 and 0.5 N). Each wash was performed at
0260-8774/03/$ - see front matter Ó 2003 Elsevier Science Ltd. All rights reserved. doi:10.1016/S0260-8774(02)00433-8
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I.K. Shuhama et al. / Journal of Food Engineering 59 (2003) 93–97
Nomenclature A Cb Cpw Cpg dpv e Ge L Mp Rb Tb Tei Tgi
absorbance (–) bixin concentration (M L3 ) specific heat of water (L2 T3 °C) specific heat of air (L2 T3 °C) volumetric mean diameter (L) molar absorptivity (Mol L1 ) volumetric flow rate of annatto extract (L3 T1 ) length of sample cell (L) seed charge in the bed (M) bixin recovery (%) bulk bed temperature (°C) inlet extract temperature (°C) inlet air temperature (°C)
35 °C for 15 min in a mechanically stirred beaker. The solid content of the extract obtained was always under 4%, which is low for direct drying in the spouted bed. Thus, two different methods were used to increase the solids content prior to the drying experiments: (a) the water content was reduced in a low pressure distillation system at 40 °C, and the extraction solutions were concentrated fivefold, obtaining the extracts ABE (aqueous alkaline extracts); (b) the extracts were treated with an aqueous solution of hydrogen chloride 0.2 N and the suspension was centrifuged and concentrated fivefold, obtaining the extracts AAE (aqueous acidified extracts). The spouted bed unit consisted of a cylindrical body with internal diameter of 9.3 cm and height of 22.0 cm. The conical base had an internal angle of 63° and the diameter of the air inlet orifice was 2.5 cm. A peristaltic pump was used to feed the extracts into the bed through a 3.1 mm diameter tube mounted centrally at the top of the cylindrical column. A scheme of the drying apparatus is shown in Fig. 1, with some details of the spouting bed used. For drying the extracts, 0.3 kg of glass spheres with average diameter of 2.6 mm were used as inert bodies. The total pigment content, bixin plus norbixin, was determined by spectrophotometry using a Spectropho-
Fig. 1. Scheme of the spouted bed dryer assembly.
Tgo Veq We Wg Wp ddp u k g X
outlet air temperature (°C) equipment volume (L3 ) mass feed rate of annatto extract (M T1 ) mass feed rate of air (M T1 ) powder production rate per volume of equipment (M L3 ) standard deviation of bixin powder diameter (L) water content in the extract (M M1 ) latent heat of water evaporation (M L2 T1 M1 ) thermal efficiency (–) powder moisture content (M M1 )
tometer Micronal (Model B-380). The method used was that proposed by FAO (WHO/FAO, 1975). The solvent used for the analysis was an aqueous solution of NaOH 0.1 N. The maximum absorbance was at 453 nm. In order to determine the experimental deviation in the bixin content analysis, two samples of annato powders were analyzed with four repetitions and the maximum deviation observed was less than 5%. The moisture content in the powder was determined by weight loss after 24 h in a vacuum oven at 60 °C and absolute pressure of 360 mm Hg. The samples (2 g) were weighed in a precision balance (0.1 mg). The powder size distribution was determined with a particle sizer Galai (Model CIS-1). This analyzer uses the ‘‘time of flight’’ technique with scanning laser (Otten & Fayed, 1997).
3. Results and discussion Fig. 2 shows the bixin and total solids content in the extracts obtained for different NaOH concentrations. As
Fig. 2. Concentration of bixin in the extract versus NaOH concentration in the aqueous solution.
I.K. Shuhama et al. / Journal of Food Engineering 59 (2003) 93–97
95
Fig. 3. Scheme of the preliminary studies for extraction and drying in SB.
can be seen the highest extraction efficiency was obtained with the 0.1 N NaOH solution. For this NaOH concentration the carotenoids represented almost 15% of the total solids extracted. This observation is in agreement with the literature (Ruiz & Wood, 1971). Based on this, the further drying studies were carried out using the extracts obtained with 0.1 N NaOH solutions as solvents. The concentrated ABE and AAE were then dryed in the Spouted Bed at 60 °C. Fig. 3 shows a diagram of the preliminary drying results, which shows that the AAE presented sticky behavior during drying, causing the agglomeration of the inert particles in the bed. The AAE were then dried in vacuum oven. The ABE did not present stickiness, resulting in a powder with good aspect. Further experiments were carried out with the ABE to determine the effect of spouted bed dryer conditions on powder properties. The extracts ABE were dryed at 60, 80, 100 and 120 °C and at extracts feed rates of 1, 2, 3 and 4 ml/min. Air inlet flow rate was kept constant at 0.5 m3 /min. This air inlet flow rate corresponds to 60% above the minimum spouting air velocity (Epstein & Grace, 1997). Bixin contents in the powder versus the volumetric extract feed rate, Ge , are shown in Fig. 4a. The extract feed rate did not affect the final bixin content, Cb , in dried powder, which is on the contrary of what should be expected for heat sensitive biological products
Fig. 4. Bixin content of the powder versus: (a) extract feed rate; (b) air inlet temperature.
(Strumillo & Adamiec, 1991). Probably in the range of extract feed rates studied here, the residence time of the pigment and the outlet air temperature were not altered significantly. In order to determine the influence of drying conditions on Cb , the bulk bed temperature is more indicated than the air inlet temperature. Since heat and mass transfer in the dryer is very fast, bed temperature is quite uniform and is close to the outlet air temperature. Bed temperature can be calculated by the heat and mass balance in the dryer: Tb ¼ Tgi
We ½ðCpw ðTb Tei ÞÞ þ ðkðu X ÞÞ Wg Cpg
Fig. 4b shows the bixin content in the powder versus bed temperature, Tb . The curves obtained for extract feed rates of 1 and 3 ml/min showed that the bixin contents for Tb from 60 to 80 °C were close to 13%, while the contents for Tb between 100 and 120 °C were about 6.5%, indicating that the highest decrease in bixin content occurred between 80 and 100 °C. This shows that above 80 °C the thermal degradation rate of bixin and norbixin increases significantly. The increasing degradation rate of bixin in NaOH solutions with temperature was previously known (Ruiz & Wood, 1971), although the sudden increase in degradation rate between 80 and 100 °C was never reported. Massarani et al. (1992) reported a significant increase in degradation rates above 60 °C during the mechanical extraction of annatto powder in spouted beds. The little difference between the present data and those reported by Massarani et al. (1992) may be explained by the longer residence times needed for the mechanical extraction. It is well known that the extend of degradation of thermosensitive materials depends not only on the temperature itself, but also on the time of exposure to this temperature. For illustrative purposes the data of Massarani et al. (1992) were also plotted in Fig. 4b. Fig. 5a and b present the moisture content in the powder versus the extract feed rate and bed temperature, respectively. As shown in Fig. 5a, there is a tendency to an increase in moisture content with increasing
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Fig. 5. Moisture content of the powder versus: (a) extract feed rate; (b) air inlet temperature.
Fig. 6. Thermal efficiency versus: (a) extract feed rate; (b) air inlet temperature.
Table 1 Average diameter and mean deviation obtained from the particle size distribution
The thermal efficiency of the drying process was defined as the ratio of the heat used for drying to the total heat loss of the gas, given by:
We (ml/min)
Tgi (°C)
dpv (lm)
ddp (lm)
1.0 1.0 1.0 1.0 2.0 2.0 3.0 3.0 3.0 3.0 4.0 4.0
60 80 100 120 80 100 60 80 100 120 80 100
20.8 21.6 65.5 27.8 29.7 43.9 43.2 37.6 53.4 21.1 34.1 47.0
16.3 16.0 19.8 23.1 18.3 22.9 15.3 26.7 28.9 9.7 17.7 23.5
extract feed rate. This may be explained by the decrease in air outlet temperature with increasing liquid feed rate. It can be seen in Fig. 5b that the powder moisture content decreases as the bed temperature increases. This can also be attributed to the air outlet temperature. The average particle volumetric diameter and standard deviation obtained for the powder samples are shown in Table 1. The powder size analysis indicated a log-normal distribution. Table 1 shows that the diameters ranged from 20.8 to 65.5 lm. The average diameters obtained show the complex influence of the process parameters on this variable, reflecting the non-trivial balance in the kinetics of layer depositions on inert bodies surface and the mechanisms of drying, attrition and agglomeration in the bed.
g¼
We ½ðCpw ðTb Tei ÞÞ þ ðkðu X ÞÞ Wg ½Cpg ðTgi Tgo Þ
The thermal efficiencies are shown in Fig. 6, as a function of extract feed rate (a), and bed temperature (b). As can be noted the thermal efficiency varied in the range between 20% and 75%, and increased with the extract feed rate and decreased with Tb . This can be explained by the increase in Tb with increasing air inlet temperature and decreasing extract feed rate, allowing higher heat losses to the surroundings, thus decreasing the thermal efficiency of the process. Table 2 shows the results obtained in this work as compared to the commercial powder available on the market (Meer Co., 1980) and those obtained by mechanical extraction in ball mills (Barreto et al., 1989) and by attrition in a spouted bed (Massarani et al., 1992). The powder obtained in this work has a similar bixin content as the others, but with a narrower size distribution and finer particles. The bixin recovery obtained was superior to that of the ball mills and similar to that obtained by spouted bed attrition (Massarani et al., 1992). The powder production rate per unit of equipment volume, Wp , may be used as a parameter for the comparison of the performance of the processes under consideration. Information provided by Barreto et al. (1989) and Massarani et al. (1992) were used to estimate their equipment internal volumes. This showed
Table 2 Powder and process characteristics for the drying in spouted bed and others Characteristic
Massarani et al. (1992)
Barreto et al. (1989)
Veq (L) Mp (kg) Cb (%) dpv (lm) Rb (%) Wp (kg/m3 )
135 6 <16.1 <147 89 1173
1 0.335 <11.4 ND 57 320 900
120 12.5 <23.9 ND 77 1010
Meer Co. (1980)
This work
ND ND 8–12 <248 ND ND
1.65 0.3 <13.3 <230 90 2060
I.K. Shuhama et al. / Journal of Food Engineering 59 (2003) 93–97
that the lab scale ball mill has by far the highest powder production rate per volume, Wp , but at higher scale the productivity of this equipment decreases sharply. The Wp obtained for the process proposed in the present study is higher than that obtained during mechanical attrition in the spouted bed and ball mill (120 l scale). The data for the annatto mechanical extraction in spouted beds (Massarani et al., 1992) also showed a tendency of Wp to decrease with the increasing scale, i.e., higher seed charge in the bed. This shows that the process proposed here is competitive with the processes of mechanical extraction when powder quality and productivity are considered as comparison parameters. Another advantage in this process is that the powder obtained is soluble in water.
4. Conclusions The results of the experimental data showed that it is possible to prepare annatto (Bixa orellana, L.) powders from aqueous alkaline extracts by drying in spouted beds. The powders have a high content of the pigments, comparable to that found in commercial products (Meer Co., 1980) and that obtained by attrition in spouted beds (Massarani et al., 1992). The advantages of the proposed method are the higher productivity and the smaller particle sizes obtained (ultra-fine powders), as shown in Table 2. It was also shown that the process parameters studied did not influence most of the powder properties investigated. However, the data showed a sharp decrease in pigment content above 80 °C, indicating that the extracts cannot be dried above this temperature. The results suggest that the process proposed herein could be used in industrial scale to prepare annatto pigments. The economical importance of this method is very significant, since the SBD technology can be easily disseminated in annatto producing countries. The methodology could be applied even in a small scale by urucum (Bixa orellana, sp.) farmers, allowing them to sell a powder with higher dye content instead of the raw seeds.
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Acknowledgements Financial support from CNPq and FAPESP for this project is gratefully acknowledged.
References Anand, N. (1983). The market for annatto and other natural colouring materials, with special reference to the United Kingdom (29 p.). London: Tropical Development and Research Institute. Andres, C. (1980). What are alternatives to use of FD & C yellow 5 in food? Food Processing, 41(5), 48–49. Barreto, D. W., Jaeger, L. M., & Massarani, G. (1989). Production of Bixin Concentrates. In D.J. Sartori, A.M. Silveira (Eds.), Proceedings of XVII meeting on flow through porous medium (pp. 175–184), Sao Carlos, Brazil. Epstein, N., & Grace, J. R. (1997). Spouting of particulate solids. In L. Otten & M. E. Fayed (Eds.), Handbook of particulate science and technology (pp. 532–567). New York: Van Nostrand Reinhold, Chapter 10. Otten, L., & Fayed, M. E. (1997). Handbook of particulate science and technology. New York: VanNostrand Reinhold. Ingram, J. B., & Francis, B. J. (1969). The annatto tree (Bixa orellana, L.). A guide to its occurrence, cultivation, preparations and uses. Tropical Science, 11(2), 97–102. Madsen, P. T. (1981). Colouring margarine with annatto, a natural colour of vegetable origin (13 p.). CHR HansenÕs Laboratorium A/S. Massarani, G., Passos, M. L., & Barreto, D. W. (1992). Production of annatto concentrates in spouted beds. Canadian Journal of Chemical Engineering, 70, 954–959. Meer Co. (1980). Annatto Technical Information Catalog, North Bergen, NJ. Preston, H. D., & Rickard, M. D. (1980). Extraction and chemistry of annatto. Food Chemistry, 5, 47–56. Ramamurthy, M. K., & Krishna, R. S. (1982). Manufacture of cheese colour from annatto seeds. Indian Dairyman, 34(12), 263–264. Ruiz, H., & Wood, G. P. (1971). The separation of annatto pigments by sephadex. Tropical Science, London, 13(3), 211–214. Strumillo, C., & Adamiec, J. (1991). Advances in drying of biosynthesis products. In Proceedings of the XIX meeting on flow through porous medium (vol. I, pp. 17–61), UNICAMP, Campinas, Brazil. Tong, L. B. (1984). Preparation and the chemistry of natural food colours from annatto plant. MARDI Research Bulletin, 12(3), 298– 304. WHO/FAO (1975). Toxicological evaluation of some food colours, enzymes, flavour enhancers, thickening agents and certain other. WHO Food Additives Series, Geneve 6, 43–46.
Experimental production of annatto powders in spouted bed dryer I.K. Shuhama, M.L. Aguiar 1, W.P. Oliveira, L.A.P. Freitas
*
Industrial Physics Laboratory, Faculdade de Ci^ encias Farmac^ euticas de Ribeir~ ao Preto––USP, Via do Caf e s/n, Ribeir~ ao Preto, SP, 14040-903, Brazil Received 9 July 1998; received in revised form 29 August 2001; accepted 22 October 2002
Abstract A new method for the production of annatto powders by drying the alkaline aqueous extracts in a spouted bed was studied. The effects of drying conditions such as air inlet temperature and extract feed rate on the properties of the powder obtained were investigated. The powder properties taken as quality parameters were the bixin and moisture content and the particle size distribution. Results on total through-put, bixin content and recovery obtained showed that spouted bed drying of aqueous extracts is advantageous when compared to extraction by mechanical attrition. Also the data showed that above the drying temperature of 80 °C the bixin content decreases sharply. The particle sizes of the powder were always below 60 lm. Ó 2003 Elsevier Science Ltd. All rights reserved. Keywords: Convective drying; Bixa orellana sp.; Extracts; Bixin
1. Introduction The use of annatto (Bixa orellana, sp.) seeds as a source of natural coloring (Ingram & Francis, 1969) is an ancient art, since its pigments were manipulated by Brazilian native tribes for skin and artifacts painting. The seeds are coated by a layer of coloring substances, which can be obtained by extraction with solvents, like alkali or oil, or by mechanical attrition. The main pigments are bixin and norbixin, dicarboxilic acid monomethyl esters, which represents 90% of the total pigments (Preston & Rickard, 1980). Among the current uses of this foodstuff is the coloring of butter, mayonnaise, seasonings, cheese and also for baking purposes (Anand, 1983; Madsen, 1981; Ramamurthy & Krishna, 1982). Since the recent prohibition on use of the synthetic dye Tartrazine (FD & C-Yellow 5) in several countries (Andres, 1980) there is an increasing international interest for the annatto pigment due to its lack of toxicity. Among the methods applied for obtaining annatto pigments there are the extraction from seeds with aqueous solutions of sodium hidroxide, potassium hidroxide or other suitable solvents such as propylene glycol, oil or pure water (Tong, 1984). An alternative
*
Corresponding author. E-mail address: [email protected] (L.A.P. Freitas). 1 Address: Universidade Federal de S~ao Carlos, S~ao Carlos, SP, 13565-905, Brazil.
method is the mechanical attrition of the seeds for wearing of the pigment layer, which is usually performed in ball mills (Barreto, Jaeger, & Massarani, 1989), or in spouted beds (Massarani, Passos, & Barreto, 1992). The present report proposes a new method for the preparation of annatto powders based on the alkaline extraction from seeds and further drying in a Spouted Bed with inert particle bodies. Spouted Bed Dryer was chosen because it is a low cost and high productivity dryer that could be easily disseminated for this purpose. Experiments were carried out to verify the applicability of Spouted Bed Dryers for the drying of aqueous alkaline extracts and alkaline-acidified extracts from annatto seeds. The effects of drying conditions such as the air temperatures and extract feed rates on the powder characteristics were determined. The results were compared with the data reported by Barreto et al. (1989) and Massarani et al. (1992).
2. Materials and methods The seeds were purchased from a single plantation located in Sao Paulo State, Brazil. The bixin content in the seeds was 1.0%, as determined by WHO/FAO (1975) procedures. For the extract preparation, 200 g of seeds were washed five times using portions of 200 ml aqueous NaOH (0.05, 0.1, 0.25 and 0.5 N). Each wash was performed at
0260-8774/03/$ - see front matter Ó 2003 Elsevier Science Ltd. All rights reserved. doi:10.1016/S0260-8774(02)00433-8
94
I.K. Shuhama et al. / Journal of Food Engineering 59 (2003) 93–97
Nomenclature A Cb Cpw Cpg dpv e Ge L Mp Rb Tb Tei Tgi
absorbance (–) bixin concentration (M L3 ) specific heat of water (L2 T3 °C) specific heat of air (L2 T3 °C) volumetric mean diameter (L) molar absorptivity (Mol L1 ) volumetric flow rate of annatto extract (L3 T1 ) length of sample cell (L) seed charge in the bed (M) bixin recovery (%) bulk bed temperature (°C) inlet extract temperature (°C) inlet air temperature (°C)
35 °C for 15 min in a mechanically stirred beaker. The solid content of the extract obtained was always under 4%, which is low for direct drying in the spouted bed. Thus, two different methods were used to increase the solids content prior to the drying experiments: (a) the water content was reduced in a low pressure distillation system at 40 °C, and the extraction solutions were concentrated fivefold, obtaining the extracts ABE (aqueous alkaline extracts); (b) the extracts were treated with an aqueous solution of hydrogen chloride 0.2 N and the suspension was centrifuged and concentrated fivefold, obtaining the extracts AAE (aqueous acidified extracts). The spouted bed unit consisted of a cylindrical body with internal diameter of 9.3 cm and height of 22.0 cm. The conical base had an internal angle of 63° and the diameter of the air inlet orifice was 2.5 cm. A peristaltic pump was used to feed the extracts into the bed through a 3.1 mm diameter tube mounted centrally at the top of the cylindrical column. A scheme of the drying apparatus is shown in Fig. 1, with some details of the spouting bed used. For drying the extracts, 0.3 kg of glass spheres with average diameter of 2.6 mm were used as inert bodies. The total pigment content, bixin plus norbixin, was determined by spectrophotometry using a Spectropho-
Fig. 1. Scheme of the spouted bed dryer assembly.
Tgo Veq We Wg Wp ddp u k g X
outlet air temperature (°C) equipment volume (L3 ) mass feed rate of annatto extract (M T1 ) mass feed rate of air (M T1 ) powder production rate per volume of equipment (M L3 ) standard deviation of bixin powder diameter (L) water content in the extract (M M1 ) latent heat of water evaporation (M L2 T1 M1 ) thermal efficiency (–) powder moisture content (M M1 )
tometer Micronal (Model B-380). The method used was that proposed by FAO (WHO/FAO, 1975). The solvent used for the analysis was an aqueous solution of NaOH 0.1 N. The maximum absorbance was at 453 nm. In order to determine the experimental deviation in the bixin content analysis, two samples of annato powders were analyzed with four repetitions and the maximum deviation observed was less than 5%. The moisture content in the powder was determined by weight loss after 24 h in a vacuum oven at 60 °C and absolute pressure of 360 mm Hg. The samples (2 g) were weighed in a precision balance (0.1 mg). The powder size distribution was determined with a particle sizer Galai (Model CIS-1). This analyzer uses the ‘‘time of flight’’ technique with scanning laser (Otten & Fayed, 1997).
3. Results and discussion Fig. 2 shows the bixin and total solids content in the extracts obtained for different NaOH concentrations. As
Fig. 2. Concentration of bixin in the extract versus NaOH concentration in the aqueous solution.
I.K. Shuhama et al. / Journal of Food Engineering 59 (2003) 93–97
95
Fig. 3. Scheme of the preliminary studies for extraction and drying in SB.
can be seen the highest extraction efficiency was obtained with the 0.1 N NaOH solution. For this NaOH concentration the carotenoids represented almost 15% of the total solids extracted. This observation is in agreement with the literature (Ruiz & Wood, 1971). Based on this, the further drying studies were carried out using the extracts obtained with 0.1 N NaOH solutions as solvents. The concentrated ABE and AAE were then dryed in the Spouted Bed at 60 °C. Fig. 3 shows a diagram of the preliminary drying results, which shows that the AAE presented sticky behavior during drying, causing the agglomeration of the inert particles in the bed. The AAE were then dried in vacuum oven. The ABE did not present stickiness, resulting in a powder with good aspect. Further experiments were carried out with the ABE to determine the effect of spouted bed dryer conditions on powder properties. The extracts ABE were dryed at 60, 80, 100 and 120 °C and at extracts feed rates of 1, 2, 3 and 4 ml/min. Air inlet flow rate was kept constant at 0.5 m3 /min. This air inlet flow rate corresponds to 60% above the minimum spouting air velocity (Epstein & Grace, 1997). Bixin contents in the powder versus the volumetric extract feed rate, Ge , are shown in Fig. 4a. The extract feed rate did not affect the final bixin content, Cb , in dried powder, which is on the contrary of what should be expected for heat sensitive biological products
Fig. 4. Bixin content of the powder versus: (a) extract feed rate; (b) air inlet temperature.
(Strumillo & Adamiec, 1991). Probably in the range of extract feed rates studied here, the residence time of the pigment and the outlet air temperature were not altered significantly. In order to determine the influence of drying conditions on Cb , the bulk bed temperature is more indicated than the air inlet temperature. Since heat and mass transfer in the dryer is very fast, bed temperature is quite uniform and is close to the outlet air temperature. Bed temperature can be calculated by the heat and mass balance in the dryer: Tb ¼ Tgi
We ½ðCpw ðTb Tei ÞÞ þ ðkðu X ÞÞ Wg Cpg
Fig. 4b shows the bixin content in the powder versus bed temperature, Tb . The curves obtained for extract feed rates of 1 and 3 ml/min showed that the bixin contents for Tb from 60 to 80 °C were close to 13%, while the contents for Tb between 100 and 120 °C were about 6.5%, indicating that the highest decrease in bixin content occurred between 80 and 100 °C. This shows that above 80 °C the thermal degradation rate of bixin and norbixin increases significantly. The increasing degradation rate of bixin in NaOH solutions with temperature was previously known (Ruiz & Wood, 1971), although the sudden increase in degradation rate between 80 and 100 °C was never reported. Massarani et al. (1992) reported a significant increase in degradation rates above 60 °C during the mechanical extraction of annatto powder in spouted beds. The little difference between the present data and those reported by Massarani et al. (1992) may be explained by the longer residence times needed for the mechanical extraction. It is well known that the extend of degradation of thermosensitive materials depends not only on the temperature itself, but also on the time of exposure to this temperature. For illustrative purposes the data of Massarani et al. (1992) were also plotted in Fig. 4b. Fig. 5a and b present the moisture content in the powder versus the extract feed rate and bed temperature, respectively. As shown in Fig. 5a, there is a tendency to an increase in moisture content with increasing
96
I.K. Shuhama et al. / Journal of Food Engineering 59 (2003) 93–97
Fig. 5. Moisture content of the powder versus: (a) extract feed rate; (b) air inlet temperature.
Fig. 6. Thermal efficiency versus: (a) extract feed rate; (b) air inlet temperature.
Table 1 Average diameter and mean deviation obtained from the particle size distribution
The thermal efficiency of the drying process was defined as the ratio of the heat used for drying to the total heat loss of the gas, given by:
We (ml/min)
Tgi (°C)
dpv (lm)
ddp (lm)
1.0 1.0 1.0 1.0 2.0 2.0 3.0 3.0 3.0 3.0 4.0 4.0
60 80 100 120 80 100 60 80 100 120 80 100
20.8 21.6 65.5 27.8 29.7 43.9 43.2 37.6 53.4 21.1 34.1 47.0
16.3 16.0 19.8 23.1 18.3 22.9 15.3 26.7 28.9 9.7 17.7 23.5
extract feed rate. This may be explained by the decrease in air outlet temperature with increasing liquid feed rate. It can be seen in Fig. 5b that the powder moisture content decreases as the bed temperature increases. This can also be attributed to the air outlet temperature. The average particle volumetric diameter and standard deviation obtained for the powder samples are shown in Table 1. The powder size analysis indicated a log-normal distribution. Table 1 shows that the diameters ranged from 20.8 to 65.5 lm. The average diameters obtained show the complex influence of the process parameters on this variable, reflecting the non-trivial balance in the kinetics of layer depositions on inert bodies surface and the mechanisms of drying, attrition and agglomeration in the bed.
g¼
We ½ðCpw ðTb Tei ÞÞ þ ðkðu X ÞÞ Wg ½Cpg ðTgi Tgo Þ
The thermal efficiencies are shown in Fig. 6, as a function of extract feed rate (a), and bed temperature (b). As can be noted the thermal efficiency varied in the range between 20% and 75%, and increased with the extract feed rate and decreased with Tb . This can be explained by the increase in Tb with increasing air inlet temperature and decreasing extract feed rate, allowing higher heat losses to the surroundings, thus decreasing the thermal efficiency of the process. Table 2 shows the results obtained in this work as compared to the commercial powder available on the market (Meer Co., 1980) and those obtained by mechanical extraction in ball mills (Barreto et al., 1989) and by attrition in a spouted bed (Massarani et al., 1992). The powder obtained in this work has a similar bixin content as the others, but with a narrower size distribution and finer particles. The bixin recovery obtained was superior to that of the ball mills and similar to that obtained by spouted bed attrition (Massarani et al., 1992). The powder production rate per unit of equipment volume, Wp , may be used as a parameter for the comparison of the performance of the processes under consideration. Information provided by Barreto et al. (1989) and Massarani et al. (1992) were used to estimate their equipment internal volumes. This showed
Table 2 Powder and process characteristics for the drying in spouted bed and others Characteristic
Massarani et al. (1992)
Barreto et al. (1989)
Veq (L) Mp (kg) Cb (%) dpv (lm) Rb (%) Wp (kg/m3 )
135 6 <16.1 <147 89 1173
1 0.335 <11.4 ND 57 320 900
120 12.5 <23.9 ND 77 1010
Meer Co. (1980)
This work
ND ND 8–12 <248 ND ND
1.65 0.3 <13.3 <230 90 2060
I.K. Shuhama et al. / Journal of Food Engineering 59 (2003) 93–97
that the lab scale ball mill has by far the highest powder production rate per volume, Wp , but at higher scale the productivity of this equipment decreases sharply. The Wp obtained for the process proposed in the present study is higher than that obtained during mechanical attrition in the spouted bed and ball mill (120 l scale). The data for the annatto mechanical extraction in spouted beds (Massarani et al., 1992) also showed a tendency of Wp to decrease with the increasing scale, i.e., higher seed charge in the bed. This shows that the process proposed here is competitive with the processes of mechanical extraction when powder quality and productivity are considered as comparison parameters. Another advantage in this process is that the powder obtained is soluble in water.
4. Conclusions The results of the experimental data showed that it is possible to prepare annatto (Bixa orellana, L.) powders from aqueous alkaline extracts by drying in spouted beds. The powders have a high content of the pigments, comparable to that found in commercial products (Meer Co., 1980) and that obtained by attrition in spouted beds (Massarani et al., 1992). The advantages of the proposed method are the higher productivity and the smaller particle sizes obtained (ultra-fine powders), as shown in Table 2. It was also shown that the process parameters studied did not influence most of the powder properties investigated. However, the data showed a sharp decrease in pigment content above 80 °C, indicating that the extracts cannot be dried above this temperature. The results suggest that the process proposed herein could be used in industrial scale to prepare annatto pigments. The economical importance of this method is very significant, since the SBD technology can be easily disseminated in annatto producing countries. The methodology could be applied even in a small scale by urucum (Bixa orellana, sp.) farmers, allowing them to sell a powder with higher dye content instead of the raw seeds.
97
Acknowledgements Financial support from CNPq and FAPESP for this project is gratefully acknowledged.
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