Effect of pretreatment and temperature on air-drying of Dioscorea alata and Dioscorea rotundata slices

Journal of Food Engineering 80 (2007) 1002–1010 www.elsevier.com/locate/jfoodeng

Effect of pretreatment and temperature on air-drying of Dioscorea alata and Dioscorea rotundata slices Kolawole O. Falade *, Taiwo O. Olurin, Ebenezer A. Ike, Ogugua C. Aworh Department of Food Technology, University of Ibadan, Ibadan, Nigeria Received 10 November 2004; received in revised form 31 May 2006; accepted 23 June 2006 Available online 16 October 2006

Abstract Effects of pretreatment and drying conditions on yam varieties, namely Dioscorea alata and Dioscorea rotundata, in a fabricated laboratory scale hot air drier at temperature range of 50–80 C and constant air velocity of 1.5 m2/s were investigated. Mass transfer during air-drying of yam slices was described using Fick’s diffusion model. Drying took place entirely in the falling rate period. Temperature dependency of moisture on diffusivity was illustrated by the Arrhenius relationship. Over the range of temperature, moisture diffusivities varied from 9.92 · 108 to 1.02 · 107 and 0.829 · 106 to 1.298 · 105 m2/s for D. alata and D. rotundata, respectively. Activation energy for drying of D. alata and D. rotundata varied from 25.25 to 46.46 and 41.75 to 72.47 kJ/mol, respectively.  2006 Elsevier Ltd. All rights reserved. Keywords: Dioscorea alata; Dioscorea rotundata; Effective moisture diffusivity; Blanching; Sulphiting; Air-drying

1. Introduction Yams (Dioscorea spp.) are an important source of carbohydrate for many people of the sub-Sahara region, especially in the yam zone of West Africa (Akissoe, Hounhonigan, Mestres, & Nago, 2003). Yams belong to the genus Dioscorea of the family Dioscoreaceae; which has about 600 species of which D. alata L.; D. cayenensis Lam, and D. rotundata Poir, has the greatest economic importance. Yams are wide spread and are one of the major stable foods in many tropical countries (Akanbi, Gureje, & Adeyemi, 1996; Omonigbo & Ikenebomeh, 2000). Apart from a moisture content of between 50% and 80%, which makes it susceptible to deterioration, yam tubers consist mainly of starch, sugars, protein and fibre (Opara, 1999; Rasper & Coursey, 1967). To overcome the high perishability of fresh tubers due to high moisture contents, yams are processed into flour by peeling, slicing and blanching and sun-drying (Akissoe et al., 2001). Dried and *

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milled yam is less perishable and can be consumed throughout the year, mainly as a thick paste called amala (Akingbala, Oguntimehin, & Sobande, 1995; Akissoe et al., 2001). The main quality attributes of amala are colour, texture and taste (Akissoe et al., 2003; Hounhouigan, Kayode, Bricas, & Nago, 2003), which may be affected by yam cultivar and processing conditions. The flesh of the yam species used is usually white, but the colour of the processed flour ranges from creamy white to dark brown (Akissoe et al., 2003). The discoloration phenomenon has been attributed to enzymatic browning, due to the action of polyphenol oxidase (Almentros & Del Rosatio, 1985) and the production of polyphenols and derived products (Osagie & Opoku, 1984). Some pretreatments or practices employed to abate these problems include sulphiting and blanching. These pretreatments will in no doubt affect the drying characteristics and energy requirements during drying of yam slices. Although, considerable amount of food materials are dried artificially in heated mechanical air-drying systems (Das, Das, Rao, & Jain, 2001; Doymaz, 2004a, 2004b; Maskan, 2001; Senadeera, Bhandari, Young, & Wijesinghe, 2003), there

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Nomenclature d D0 Deff Ea k M0

half (d/2) the characteristic dimension of food measured along the line of air flow pre-exponential factor or moisture diffusivity constant (m2/s) effective moisture diffusivity (m2/s) activation energy (kJ/mol) drying constants initial moisture content (kg H2O/kg dry solids)

is little or no information on the influence of the pretreatments on the energy requirements during the drying of yams. Thus the objectives of this work is to investigate the effects of variety and processing conditions on the drying characteristics, and activation energy for drying of white yam (D. rotundata) and water yam (D. alata).

Me Mi N n t Tabs Rg

equilibrium moisture content (kg H2O/kg dry solids) instantaneous moisture content at time t (kg H2O/kg dry solids) drying constants constant drying time (h) absolute temperature of drying air (K) in Eq. (4) universal gas constant (kJ/mol K)

Blanched slices were immediately cooled in cold water (25 C) for 5 min to remove excess heat. Moreover, fresh batches of 50 · 20 · 10, 50 · 20 · 20 and 50 · 20 · 30 mm slices were dipped in 0.5% sodium metabisulphite solution for 2, 4 and 5 min, respectively. 2.2. Drying procedure

2. Materials and methods 2.1. Drying pretreatments, procedure and conditions White yam (Dioscorea rotundata Linn) and water yam (Dioscorea alata) were purchased from a local Market in Ibadan, Nigeria. Average moisture content of D. rotundata and D. alata were 71.32 ± 1.4% and 69.44 ± 0.2% (wet basis), respectively. Yams were washed, manually peeled and cut into rectangular slices having the dimensions of 50 · 20 · 10, 50 · 20 · 20, 50 · 20 · 30 mm using very sharp stainless steel knives. The 50 · 20 · 10 mm slices were blanched at 80 C for 5 min; 50 · 20 · 20 and 50 · 20 · 30 mm slices were blanched at 100 C for 2 min.

Yam samples were transferred into the drier (Fig. 1), designed and fabricated at the Department of Mechanical Engineering, University of Ibadan, Ibadan, Nigeria. Drier was allowed to run for about an hour prior to loading of samples to allow the heated air to stabilize at the desired temperatures (50, 60, 70 and 80 C) and at constant air velocity (1.5 m s1). Pre-weighed samples were loaded into the drier and removed at regular intervals until three consecutive weights were constant, indicating equilibrium condition. Moisture contents at equilibrium were determined according to the method of AOAC (1984). Drying experiments were conducted in triplicate and average values reported.

Fig. 1. Isometric view of the fabricated air drier used for the drying of yam slices.

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2.3. Determination of drying characteristics Drying curves were fitted into the simple exponential model (Eq. (1)). Simplifying the general series solution of Fick’s second law generally leads to the model. The simple exponential model is the first term of a general series solution of Fick’s second law (Doymaz, 2005). It is generally assumed that the mechanism of moisture migration during thin layer drying of food materials is characterized by diffusion as described by Fick’s second law of diffusion (Babalis & Belessiotis, 2004; Doymaz & Pala, 2002; Luikov, 1966; Maskan, 2001; Saravacos, 1995; Senadeera et al., 2003). The simple exponential model was used to describe the drying okra (Doymaz, 2005) MR ¼

m  me ¼ aðktÞ m0  me

ð1Þ

where MR = moisture ratio, m = moisture content at time t (kg water/kg dry matter), me = equilibrium moisture content (kg water/kg dry matter), m0 = initial moisture content (kg water/kg dry matter). The diffusion based model for an infinite slab is based on the assumptions that the system is isotropic (the diffusion properties are constant in each direction) and, the initial moisture, the final moisture content is equal to its equilibrium moisture content at the given conditions of drying.

Moreover, the drying surface reaches its equilibrium value instantaneously, and that there was no shrinkage of product during the course of drying was applied:   1 2 m  me 8 X 1 2 p Dt exp ð2n  1Þ ¼ ð2Þ m0  me p2 n¼1 2n  1 4L2 Eq. (2) can be written as   1 8 X 1 p2 Dt exp ð2n  1Þ2 MR ¼ 2 p n¼1 2n  1 4L2

where L is the thickness of the yam slices; D is the moisture diffusivity. Karatas (1997) and Senadeera et al. (2003) reported the correlation between the drying conditions and the values of the effective diffusivity using Arrhenius type equation:   Ea D ¼ D0 exp  ð4Þ RT abs where D0 = diffusion coefficient; Ea = activation energy (kJ/mol); R = universal gas constant (8.314 J/mol K); Tabs = absolute air temperature (K). 2.4. Statistical analysis Results were evaluated for analysis of variance (ANOVA) using Excel Windows XP.

1.2

Fresh D. rotundata Blanched D. rotundata Sulphited D. rotundata Fresh D. alata Blanched D. alata Sulphited D. alata

1

Moisture ratio

0.8

0.6

0.4

0.2

0 0

5

10

15

ð3Þ

20

25

30

35

40

Drying time (Hrs) Fig. 2. Effect of pretreatment on moisture ratio of 10 mm D. alata and D. rotundata during air-drying at 50 C.

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1005

2

1.8

Fresh D. rotundata Blanched D. rotundata Sulphited D. rotundata Fresh D. alata Blanched D. alata Sulphited D. alata

1.6

Drying rate (kg water/hr)

1.4

1.2

1

0.8

0.6

0.4

0.2

0 0

0 .5

1

1.5

2 .5

2

3

Moisture content (kg water/ kg d. s.)

Fig. 3. Effect of pretreatment on drying rate of D. alata and D. rotundata during air-drying at 50 C.

1.2

10mm D. rotundata 20mm D. rotundata 30mm D. rotundata 10mm D. alata 20mm D. alata 30mm D. alata

1

Moisture ratio

0.8

0.6

0.4

0.2

0 0

5

10

15

20

25

30

35

Drying time (Hrs)

Fig. 4. Effect of slice thickness on moisture ratio of sulphited D. alata and D. rotundata during air-drying at 60 C.

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10mm D. rotundata 20mm D. rotundata 30mm D. rotundata 10mm D. alata 20mm D. alata 30mm D. alata

0.7

0.6

Drying rate (kg water/ hr)

0.5

0.4

0.3

0.2

0.1

0 0

0.5

1

1.5

2

2.5

Drying time (Hrs) -0.1

Fig. 5. Effect of slice thickness on the drying rate of sulphited D. alata and D. rotundata during air-drying at 60 C.

3. Results and discussion 3.1. Effects of pretreatment and variety on moisture ratio and drying rate of yams Moisture contents and moisture ratio of yam slices decreased with increasing drying time. Moisture ratio of blanched D. alata and D. rotundata were higher than fresh (untreated) and sulphited samples. Fig. 2 showed the effect of pretreatment on moisture ratio of 10 mm D. alata and D. rotundata air-dried at 50 C. Blanching may have caused the gelatinization of yam starches, resulting in decreased rate of moisture movement from within the material to the surface during air-drying. Similar result was reported during air-drying of blanched banana by Dandamrongrak, Mason, and Young (2003). Typical drying rate curve for D. alata and D. rotundata showing the effect of pretreatment are illustrated in Fig. 3. Generally, drying rates decreased with decreased moisture contents, and drying occurred in the falling rate period. Initially, drying rates were highest when moisture contents were largest, after which the drying rate decreased steadily with decreased moisture contents. This trend could be due to the removal of free moisture near the surface of the yam slices at the early stages of drying. Blanched yam slices showed lower drying rates compared to sulphited and untreated yams slices. Results are in agreement with obser-

vations in the drying of prickly pear fruit (Lahsasni, Kouhila, Mahrouz, & Jaouhari, 2004), potato (Srikiatden & Roberts, 2003) and banana (Dandamrongrak et al., 2003). Blanching of potato did not increase the rate of drying because of starch gelatinization (Alzamora & Chirife, 1980) that resulted in reduced porosity (Mate, Quartert, Meerdink, & van’t Riet, 1998). Also, blanching did not result in reduced drying time in banana due to the effect of starch gelatinization (Dandamrongrak et al., 2003). It is obvious from Fig. 4 that increasing the thickness of yam slices resulted in decreased moisture ratio, thus the drying time is decreased. The time required to dry yam slices to specific moisture is influenced by the thickness, being fastest with the 10 mm and longest with the 30 mm. Moreover, drying rate decreased with increased thickness of yam slices (Fig. 5). Decrease in moisture ratio and drying rate as the thickness of yam slices increased is due to the effect of the exposed surface area resulting in increased diffusion path of moisture out of the yam slices during airdrying. 3.2. Effect of temperature on moisture ratio and drying rates of yam slices Moisture ratio decreased with increasing drying temperature. Fig. 6 shows the effect of temperature on moisture ratio of 20 mm blanched D. alata during air-drying. Similar

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1.2

1

50 C 60 C 70 C 80 C

Moisture ratio

0.8

0.6

0.4

0.2

0 0

5

10

15

20

25

30

35

Drying time (Hrs) Fig. 6. Effect of temperature on moisture ratio of 20 mm blanched D. alata during air-drying.

trend was exhibited by D. rotundata (result not shown), and by fresh and sulphited samples during drying. Generally, higher drying temperatures resulted in steeper curves and shorter drying times. The time required to reduce the moisture ratio to any given level was dependent on the drying temperature. According to Doymaz (2005), the effect of drying air temperature was most dramatic with moisture ratio, moisture ratio decreased rapidly with increased drying air temperature. There was no constant rate period during the drying of yam slices (Fig. 7). Drying occurred predominantly in the falling rate period. This showed that diffusion is the dominant physical mechanism governing moisture movements in the yam samples. Similar results were reported for greenbean (Rosello, Simal, SanJuan, & Mulet, 1997), okra (Doymaz, 2004b; Gogus & Maskan, 1999), red chilli (Gupta, Ahmed, Shivhare, & Raghavan, 2002), Carrot (Doymaz, 2004a; Prabhanjan, Ramaswamy, & Raghavan, 1995) and eggplant (Ertekin & Yaldiz, 2004). 3.3. Effect of pretreatment on effective moisture diffusivity of yam slices A graph of ln MR (moisture ratio) against time was plotted, as extrapolated from Eq. (3). From its slope, effective moisture diffusivity (Deff) was estimated. Calculated mois-

ture diffusivities for D. alata and D. rotundata varied with pretreatment (Tables 1 and 2). Generally, effective moisture diffusivity increased with increased drying air temperature, but Deff increased with increased thickness of yam slices. Blanched D. alata yams showed lower Deff values compared to the sulphited and untreated samples. Deff values of D. alata of similar thickness (e.g., 10 mm) was in the order: control > sulphited > blanched. However, untreated D. rotundata slices exhibited lowest, while sulphited slices recorded the higher moisture diffusivity (m2/s). Comparably low moisture diffusivity values of blanched D. alata and D. rotundata could be due to resistance to moisture migration as a result of gelatinization of starch granules. Dandamrongrak et al. (2003) and Njitang and Mbbofung (2003) reported similar results for drying of banana and taro respectively. Moisture diffusivity decreased with increased thickness of D. rotundata slices. Thinner D. rotundata slices (10 and 20 mm) showed higher moisture diffusivities. This result is in agreement with previous studies by Senadeera et al. (2003) on potato. Results indicated that diffusion is the most likely physical mechanism governing moisture movement in the D. alata and D. rotundata. Drying during falling rate period is governed by water diffusion in the solid. This complex mechanism involving water in both liquid and vapour states is often

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0.8

0.7

50 C 60 C 70 C 80 C

Drying rate (kg water/ hr)

0.6

0.5

0.4

0.3

0.2

0.1

0 0

0.5

1

1.5

2

2.5

3

Moisture content (kg water/ kg d.s.) Fig. 7. Effect of drying temperature on drying rate of 20 mm blanched D. alata during air-drying.

Table 1 Moisture diffusivity (m2/s) of D. rotundata slices at various thicknesses Moisture diffusivity (m2/s)

Temperature (C)

Pretreatments

Dry bulb

Wet bulb

10 mm

20 mm

50 60 70 80

27 31 34 36

1.120E06 2.867E06 4.902E06 8.029E06

0.9662E06 1.652E06 3.313E06 5.797E06

50 60 70 80

27 31 34 36

1.386E06 2.548E06 5.121E06 1.041E05

1.008E06 2.150E06 5.122E06 1.0281E05

50 60 70 80

27 31 34 36

3.392E06 5.449E06 9.008E06 1.298E05

2.771E06 4.625E06 8.862E06 1.121E05

characterized effective moisture diffusivity (Al Hodali, 1997). Effective moisture diffusivity varied from 9.92 · 108 to 1.02 · 107 m2/s and 0.829 · 106 to 1.121 · 105 m2/s for D. alata and D. rotundata, respectively. These values lie within the general range of 1011–

30 mm 1.004E06

0.829E06

2.452E06

Fresh Fresh Fresh Fresh Blanched Blanched Blanched Blanched Sulphited Sulphited Sulphited Sulphited

106 m2/s reported by Zogzas, Maroulis, and MarinosKouris (1996) and Marinos-Kouris and Maroulis (1995) for food materials. Srikiatden and Roberts (2003) also reported the moisture diffusivity range for potato, carrot core, carrot cortex and apple as 4.68 · 1010–1.02 · 10–9,

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Table 2 Moisture diffusivity (m2/s) of D. alata slices at various thicknesses Moisture diffusivity (m2/s)

Temperature (C)

Pretreatments

Dry bulb

Wet bulb

10 mm

20 mm

50 60 70 80

27 31 34 36

1.110E07 1.917E07 1.93E07 4.00E07

1.36E08 2.32E07 2.36E07 5.85E07

50 60 70 80

27 31 34 36

9.21E08 1.23E07 1.41E07 2.84E07

1.25E07 1.83E07 2.18E07 2.82E07

50 60 70 80

27 31 34 36

1.53E07 2.37E07 2.54E07 4.01E07

1.30E07 2.61E07 2.30E07 5.97E07

30 mm Fresh Fresh Fresh Fresh

1.634E08

Blanched Blanched Blanched Blanched

1.303E08

Sulphited Sulphited Sulphited Sulphited

2.128E08

Table 3 Effect of pretreatment and thicknesses of D. alata and D. rotundata slices on activation energy (Ea) for mass diffusion during air dying Fresh

D. rotundata Ea (kJ/mol) D. alata Ea (kJ/mol)

Blanched

Sulphited

10 mm

20 mm

10 mm

20 mm

10 mm

20 mm

59.94 43.03

69.45 46.25

63.11 25.26

72.47 35.14

41.71 35.13

43.48 46.46

6.42 · 109–1.48 · 109 m2/s, 6.8 · 1010–1.21 · 109, 9 9 2 1.01 · 10 –2.06 · 10 m /s, respectively. The lower values recorded in yams could be due to differences in the structure and composition of yams and, drying facility and conditions. 3.4. Activation energy for drying of yam slices The natural logarithm of calculated moisture diffusivities (ln D) were plotted against the reciprocal of the absolute temperature (1/Tabs). Activation energy (kJ/mol) for drying was calculated from the slope with high correlation coefficients (R2 = 0.8112–0.9866 for D. alata; R2 = 0.9539– 0.9980 for D. rotundata), indicating a good fit (Table 3). Activation energy for drying is the energy required to initiate mass diffusion from a wet food material during drying (Mittal, 1999). Activation energy for drying ranged from 25.26 to 46.46 and 41.75 to 72.47 kJ/mol for D. alata and D. rotundata respectively, depending on pretreatment and thickness of yam slices (Table 3). Effect of variety was significant (p < 0.05) on activation energy. However, pretreatment and thickness showed no significant effect (p < 0.05) on activation energy for mass diffusion. The energy of activation (Ea) obtained in our work (Table 3) are comparable with existing literatures by Sabarez and Price (1999) for prune (57.00 kJ/mol), Doymaz (2004a) for carrot (28.39 kJ/mol), Doymaz (2005) for okra (51.26 kJ/mol), Bon, Simal, Rosello, and Mullet (1997) for potato (20.0 kJ/mol) and Park, Vohnkova, and Brod (2002) for mint (57.0 kJ/mol). Values of the energy of activation lie

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