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Drying 2004 Proceedings of the 14th International Drying Symposium (IDS 2004)So Paulo, Brazil, 22-25 August 2004, vol. B, pp. 876-883

876

DRYING OF BANANA PASTE IN ROTARY DRYER WITH INERT BED

Rebeca Y. C. Padilla, Jos R. Limaverde and Jos R. D. Finzer ,2

1Federal University of Uberlndia Chemical Engineer Program

Av. Joo Naves de vila, 2160 Bloco K, CEP 38405-100, Uberlndia MG [email protected], [email protected], [email protected]

2Associated Faculties of Uberaba Food Engineer Course

Av. do Tutuna 720, Bairro Tutunas, CEP 38061.500, Uberaba-MG - Brazil

Keywords: banana paste drying, rotary dryer with inert bed, additive, banana flour.

ABSTRACT

The aim of this is work is to study the drying of banana "nanica" paste (variety,belonging to the subgroup Cavendish) in Rotary Dryer with Inert Bed (RDIB). Asbanana is a perishable fruit, one of the techniques used to enlarge its lifetime forconsumption, reducing packing and transportation costs, improving sensorial attributesand preserving the nutritional value is the banana drying process. Preliminary tests were

performed to adjust the operational process parameters. For the final experiments ofdrying, an experimental planning (central composed planning) was carried out, havingas variables: the drying time and the mass of fed paste into the dryer and, as responses,the value of the moisture content and the production of banana flour. As a result of thetests performed following the experimental planning, it was obtained a beige-coloredproduct, composed of powder and banana flakes, and characteristic aroma of banana.The influence of variables in the banana drying process was investigated by means ofamultiple regression for the experimental results. The best operating condition madepossible a product with 8.0% moisture content (wet base) and a flour productionefficiency of 72.9% and finally the final product was characterized.

INTRODUCTION

The banana, besides constituting an expressive carbohydrate source (highly energetic), it is still rich inpotassium, sodium, phosphorus, chlorine, magnesium, sulfur, silicon, calcium; vitamins: A, B1, B2, Cand B3 (niacin) that are essential for the operation of the human body (Padovani, 1989).Brazil the second largest world producer of banana produces eight million tons a year, and only exports2% and losses are about 40% (Knapp, 2001).


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Banana flour is used in the pap making, soups, as enrichment of milk, in ice creams and other foods. Itcan also be used in the bread making, where it is mixed in the proportion of 1/3 of banana flour to 2/3 ofwheat flour (Padovani, 1989).Among the conventional dryers used in banana drying the following are distinguished: the traysdryer, where the time of drying is significant; the drum dryer, where the product does notpresent good quality, above all for not presenting uniformity in the drying; the spray dryer, thatit makes possible a high quality product; however, the capital investment and operational costs

are significant, what makes unfeasible its use for medium and small capacity industries(Hufenuessler & Kachan, 1986; Maskan, 2000; Nury et al., 1973).The rotary dryer with inert bed (RDIB) is an alternative technically viable, because in the processing ofother foodstuffs good quality powdered products were obtained (Finzer et al., 1993).In this work the description of the methodology used for banana paste drying (with and without additive)in RDIB, as well as the results of the drying tests are presented.

MATERIALS AND METHODS

Raw material

The raw material used in this work is the banana "nanica" variety of the subgroup Cavendish with 75%of ripening (yellow color with green tips) that is more appropriate for the flour obtaining (Manica,1997).

Banana paste preparation

In the paste preparation several tests were carried out by adding different water ratios: 5%, 10%, 15%,and 20% in relation to banana mass. The objective was to obtain a paste with possibility of pumpingeasily. The most appropriate banana paste was that prepared with 10% water. The preparation way of thepaste was the following:- selection of bananas with 75% of ripening;- wash in chlorinated water (4 ppm), at temperature of 45C, for preliminary cleaning of bananas(Manica, 1997);

- five minute blanching in boiling water to reduce the number of microorganisms and enzymesinactivation, followed by cooling for three minutes (Dandamrongrok et al., 2000);- skin elimination and then cut in disk-like slices of approximately 2 cm in thickness;- immersion in 0.2% sodium bisulfite (NaHSO3) solution during five minutes, to avoid oxidation of theproduct;- drainage of the solution for five minutes;- triturating in a knife rotary crusher. Addition of: i) water to obtain the content of 10% of the total pulpmass; ii) 1% of additive Dimodan HO-1, a distilled monoglyceride (to improve the texture; to reduce theviscosity and the sugar crystallization), and iii) 0.4% of ascorbic acid to avoid the darkening during thedisintegration step.

Drying tests

To select the inert materials to be used in the RDIB, qualitative tests of coating and drying were carriedout in a laboratory oven. Different materials were tested: stainless steel spheres, porcelain spheres andtechnyl cylinders. The porcelain spheres had been the chosen ones because they had not presentedoxidation of product and presented a better heat transfer than the "technyl" cylinders. Besides, theyallowed the obtaining of a more homogeneous paste film on surface, during the coating before the drying.The product presented oxidation when stainless steel spheres were used.


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Rotary dryer with inert bed

The drying tests of banana paste were carried out in R&D Laboratory of Chemical EngineeringFaculty of Federal University of Uberlndia. Figure 1 is a schematic representation of the unit of dryingand its accessories.

A Steel ribbon F Blower K BalanceB Gear G Air discharge L Peristalt ic pumpC Rolling H Orifice plate and Manometer M AgitatorD Engine I Electric resistance N CycloneE Teflon ring J Voltage transform O Temperature display

Figure 1 - Schematic unit of the Rotary Dryer with Inert Bed and accessories.

In this dryer, banana paste is fed into the drying chamber that contains inert bed of porcelain spheres (2.8to 3.0 cm indiameter). In the operation the inert materials are covered for a layer of banana paste.

The inerts facilitate the drying when they enlarge the contact surface between the material and drying air.The chamber rotation propitiates a great number of collisions and shear stress (inert-inert, and inert-wall),facilitating the mixture and the paste drying. The mechanical shocks and the shear stress provide thegrinding of the product during the drying. The dry material is conveyed by the drying air and separates in acyclone.The angular speed of the drying tunnel was maintained at 18 rpm. Previous studies showed that in thisoperational condition the dryer presents good operational performance (Yoshida et al.,2001).In the drying tests the operation conditions were: air temperature of 70C (for not exceeding thedegradation temperature of the banana's vitamins); the air mass flow of 5.3 kg/min (for making possible agood heat transfer convection coefficient). These operational conditions are to make easy the powderdischarge (Limaverde Jr., 2000). After preliminary experiments the drying tests ware carried out

according to the factorial planning (central planning).

Characterization of the final product

The banana flour was characterized by following contents: moisture, ash, total carbohydrate; lipids,and proteins according to the rules of the Adolfo Lutz Institute (1985); and caloric values were calculatedbyusing sum of carbohydrates, lipids and proteins specific calories (Ascar, 1985).

ABE

O

N

C

D

FG

H

I

J

KL

M


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RESULTS

Drying test of banana paste without supporting.

The rotary dryer with inert bed was assayed initially with the following operational conditions: feeding0.25 kg of banana paste (without additive) by cycle, with initial moisture of 79.5% (wb). The feeding time

by cycle was of approximately 120 s. Each operation drying cycle had duration of 35 min, and during 15min it was operated with the drying air temperature of 70C and the 20 remaining min with drying airtemperature of 53C. Five consecutive drying cycles were effected. The airflow was of 8.8x10-2 kg/s. Theresults are shown in Table 1. It can be observed that, in these operational conditions, an average flourproduction of 2.92x10-3 kg was obtained, with average moisture content of 9.26% (wb); and the averageproduction efficiency of 5.2%, (ratio of separated powder in the cyclone by the amount of solids in thefeeding). The banana flour mass in each cycle was not significant and its color was dark.At the end of the last cycle the dryer was disassembled and observed that most amount of the fed materialwas contained in the drying tunnel. All the inerts were covered with moist paste and adhered to thecentral axis and paddles.

Table 1 - Performance of rotary dryer without additive.

Drying air

Cycle

Operation

time

(min)Temperature

(C)

Flow rate

(kg/s)

Total feed

pastelike

(kg)

Flour

production

(kg) x103

Flour

moisture

% (wb)

Production

efficiency

%

11520

70,353,3

8,8x10-2 0,25 4,5 8,9 8,0

21520

71,153,3

8,8x10-2 0,25 3,0 10,2 5,3

31520

70,554,0

8,8x10-2 0,25 3,2 8,9 5,7

41520

70,454,9

8,8x10-2 0,25 2,1 9,4 3,7

5 1520 70,452,9 8,8x10-2 0,25 1,8 8,9 3,2

Drying test of banana paste with additive

Technical information regarding the additive (Dimodan HO-1) used in the drying of pastelike materialsshowed the easiness to detach the dry material from surfaces. This is an indication that the productioncapacity should increase when the additive is used. Based on this information and on the preliminary testsother drying experiments in the same operational conditions were conducted. The results are shown inTable 2, where it can be observed an average flour production of 28.3x10-3 kg with average contentmoisture of 7.7% (wb) and average production efficiency of 49.0%. The banana flour production waspractically stabilized after the first cycle. The obtained product, in powder and flakes form, presented a

beige coloration and a characteristic banana aroma.After last cycle operation the dryer was disassembled and it was observed in the spheres and walls of thedrying tunnel that a little amount of paste left adhered. This was an indication of stability of theproduction, and the dryer capacity was 0.25 kg banana paste by drying cycle. Losses of banana powderoccurred on cyclone overflow.


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Table 2 - Performance of rotary dryer with additive.

Drying air

Cycle

Operation

time

(min)Temperature

(C)

Flow rate

(kg/s)

Total feed

pastelike

(kg)

Flour

production

(kg) x103

Flour

moisture

(% ) wb

Production

efficiency(%)

11520

70.553.7

8.8x10-2 0.25 21.7 7.7 37.7

21520

72.152.0 8.8x10

-2

0.25 31.6 8.0 54.7

31520

72.153.2

8.8x10-2 0.25 27.2 7.6 47.3

41520

72.752.6

8.8x10-2 0.25 30.4 7.7 52.7

51520

72.251.4

8.8x10-2 0.25 30.4 7.7 52.7

Figure 2 shows the comparison of the bananaflour production with and without additive at thesame operational conditions. It can be observedthe difference of the flour accumulated

production and the additive influence in thepaste-like banana drying. This is fundamental forthe process technical viability. For the processingtime operation of 175 min using additive theproduction was approximately ten times largerthan in the operation without it. 0 50 100 150 200

0

20

40

60

80

100

120

140

160With additiveWithout additive

Accumulatedproduction(kg).103

Drying time (min) Figure 2 - Comparison of the production of accumulatedflour with and without additive.

Drying tests of factorial planning.

The test results of the factorial planning - PCC (central planning) are showed in Table 3. Each testcorresponds to two drying stages. In the first stage the temperature of the drying air was maintained at 70C with duration of operation time as showed in the table. In the second stage, in all tests the drying airtemperature was reduced to 53C and maintained for 20 minutes.

Table 3 - Results of the drying tests, with the conditions of each experiment

Codified

variablesTest

X1 X2

Total paste-

like load

(kg)

Drying time

1st. stage

(min)

Product moisturecontent

(%)

Banana flour

production

(kg)

1 -1 -1 0.25 15 7.7 0.0282 -1 1 0.25 45 7.3 0.036

3 1 -1 0.75 15 11.0 0.0544 1 1 0.75 45 8.0 0.1225 -1.0781 0 0.23 30 7.5 0.0346 1.0781 0 0.77 30 10.0 0.0887 0 -1.0781 0.50 14 10.6 0.0108 0 1.0781 0.50 46 8.3 0.0839 0 0 0.50 30 9.5 0.08610 0 0 0.50 30 9.3 0.085


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Figure 3 presents the comparison of theaccumulated banana flour production as afunction of accumulated drying time (where itcan be seen in Test 4), the largest accumulatedproduction of 0.61kg. Test 7 presents thesmallest accumulated production of banana flour

(0.050 kg), in which the drying time wasinsufficient.

Multiple regression analyses response

Using data presented in Table 3 a multipleregression was carried out for the responses ofmoisture content and flour production.

0 50 100 150 200 250 300 3500

50100150200

250300350400450500550600650

Test 1Test 2Test 3Test 4Test 5Test 6Test 7

Test 8Test 9Test10

Accumulatedprodution(kg).103

Drying time (min)

Figure 3 - Comparison of the accumulated production ofthe ten factorial planning (PCC) tests.

Moisture content

The obtained correlation, Equation (1), relates the banana flour moisture as a function of the codifiedvariables.

U = 9.40 + 1.08X1 0.94 X2 - 0.75X12 - 0.64X1X2 (1)

To illustrate the effects of the process variableson banana flour quality as moisture content (U)the response surface is presented (Figure 4).When the feeding of the paste (X1) increases andthe drying time decreases (X2), the moisturecontent increases. The maximum region at thesurface response for the moisture contentcorresponds to the largest feeding levels of paste-like banana. The condition where the moisturecontent level is within the expected value, lowerthan 8%, corresponds to the region where thefeedings should be lower than 0.33 kg ofpastelike banana.

7.487

7.881

8.276

8.67

9.065

9.459

9.854

10.248

10.643

11.038

above Figure 4 - Response surface for the moisture content asfunction of X1 and X2.

Flour production

The Equation (2) presents the correlation found to represent the production of banana flour as afunction of the codified variables.

P=0.074+0.027X1+ 0.024X2 0.018X22+0.015X1X2 (2)

The value of the correlation coefficient (0.90) indicates the quality of the obtained adjustment of theexperimental data in the response production of banana flour.To illustrate the effects of the process variables in the banana flour quality regarding the flour production(P) the response surface is presented in Figure 5. It was shown that when the pastelike feeding increases


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(X1), the production increases; decreasing the drying time (X2), the production decreases. The maximumregion for the flour production corresponds to the largest levels of feeding of the paste and to drying time.

0.024

0.0330.042

0.052

0.061

0.071

0.08

0.089

0.099

0.108

aboveFigure 5 - Response surface for the banana flourproduction as a function of X1 and X2.

Banana flour characterization

The obtained product of the Test 4 was used

to banana flour characterization because it

presents the better characteristics as colour,aroma, production and mainly the moisturecontent (6-8%).The results of the physicochemistry analyses are

shown in Table 4. As reference of banana flourcomposition percentages published by De Martinet al. (1990) are appropriate. Comparing theresults of banana flour obtained in the currentwork with suitable data, it is found that theypresent similarity, especially in the moisturecontent that is a decisive parameter of the qualityof this product type. Analyzing the data in Table4, it is found that the banana flour caloric valueobtained in this work is within the range ofvalues published in the literature.

Table 4 - Banana flour quality

Data published by De Martin et al. (1990)Analyzed items Measured value

Gross Michel species Unidentified species

Moisture content (wb) (%) 8.03 8.50 5.00Ash content (%) 2.63 2.00 3.06Total carbohydrate content (%) 65.66 82.20 79.90Lipids content (% 2.81 1.05 2.30Proteins content (%) 3.79 2.00 4.98Caloric value (kcal/100 g) 303.10 346.50 283.00

CONCLUSIONS

- The largest efficiency of production of dried banana, in the rotary dryer, happened with the addition ofDimodan HO-1 to the paste. The powder production without the presence of additive in the paste wasnot technically viable.- The adjustment of the empiric equations of moisture content and flour production indicated that about98% of the variability of data was explained by the equation proposed for the answer humidity contentand 90% for the answer production of banana flour.- Analyzing the results obtained, the best operational conditions for accomplishment of the drying ofbanana's paste in rotary dryer with inert bed can be selected: drying air flow rate 8.8x10-2 kg/s; drying airtemperature, operating for 45 minutes at average temperature of 70C and 20 minutes at average

temperature of 53C; banana paste feed at the rate of 4.2x10-3 kg/s; total banana paste feed of 0.75 kg bycycle. Under these operating conditions it was obtained an average production of 0.122 kg of banana flourcorresponding to powder production efficiency of 72.9%.- The product obtained was in powder form and flakes beige in color and characteristic aroma of banana.- The results obtained: moisture content of 8.0% (wb) it is within the established range for this producttype (6 to 8%).


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ACKNOWLEDGMENT

This work received support from: 1) CNPq (National Council of Research); 2) Danisco Brasil Ltda forDimodan HO 1 supply

NOTATION

X1 Codified variables (paste feed) -X2 Codified variables (drying time) -U Moisture content %P Banana flour production kgwu Wet basedb Dry base

LITERATURE

Ascar J.M. (1985), Anlise Percentual, Unisinos, Rio Grande do Sul, Brasil, Vol 1.

Dandamrongrak, R., Young, G., Mason, R. (2002), Evaluation of various pre-treatments for the dehydration of bananaand selection of suitable drying models, Journal of Food Engineering, Vol. 55, pp.139-146

De Martin,Z.J., Travaglini, D. A.,Okada, M., Quast, D. G., Hashizume, T. (1990), Processamento: produtos,caractersticas e utilizao. In: Instituto de Tecnologia de Alimentos. Banana-cultura, matria prima,processamento e aspectos econmicos: serie frutas tropicais no. 3, 2da ed., ITAL, Campinas, Brasil

Finzer, J. R. D.; Burjaili, M. M.; Limaverde, J. R. (1993), Alternativa tecnolgica de secagem de materiais pastosos -secador rotatrio com recheio de inertes, Cincia & Engenharia, Vol. 2, pp. 97-119

Hufenuessler, M.; Kachan, G. C. (1986), Estudo da viabilidade de desidratao de pur de banana em secador tipojorro, Cincia e Tecnologia de Alimentos, Vol. 1, pp. 31 41

Instituto Adolfo Lutz. (1985), Normas analticas, Mtodos fsicos e qumicos para anlise de alimentos, 3rd ed., SoPaulo, Brasil, Vol. 1, pp. 21-53

Knapp, L. (2001), Projeto pode substituir farinha por banana, O Estado de So Paulo, 26 de novembro,www.estadao.com.br.

Limaverde Jr, J. R. (2000), Secador rotatrio com recheio de inertes aplicado a secagem de matrias pastosas,Uberlndia: Dissertao, Mestrado Universidade Federal de Uberlndia, Programa de Ps-Graduao emEngenharia Qumica, 78 p.

Manica, I. (1997), Fruticultura tropical 4: Banana, Cinco Continentes Editora Limitada, Porto Alegre, Brasil.

Maskan, M. (2000), Microwave/air and microwave finish drying of banana, Journal of Food Engineering, Vol. 44, pp. 71 18

Nury, F. S.; Brekkle, J. E.; Bolin, H. R. (1973), Fruit. In: Von Arsdel, W. B.; Copley, M. J.; Morgan, A. I. Fooddehydration: practices and applications,The AVI Publishing Company, Westport, Vol. 2Padovani, M. I. 81989), Banana um mercado crescente para este alimento milenar. 2 a ed., ICONE, So Paulo, Brasil

Yoshida, L. M.; Limaverde J. R.; Finzer, J. R. D.(2001), Secagem de resduo da formao de cido ctrico em secadorrotativo com recheio de inertes, XXIX Congresso Brasileiro de Sistemas Particulados, So Joo Del Rei MG.Anais de ENEMP cd- rom.

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