Drying of hot chilli using solar tunnel drier

Solar Energy 81 (2007) 85–92 www.elsevier.com/locate/solener

Drying of hot chilli using solar tunnel drier M.A. Hossain a

a,*

, B.K. Bala

b

Farm Machinery and Postharvest Process Engineering Division, Bangladesh Agricultural Research Institute, Gazipur-1701, Bangladesh b Department of Farm Power and Machinery, Bangladesh Agricultural University, Mymensingh-2202, Bangladesh Received 28 July 2004; received in revised form 9 June 2006; accepted 9 June 2006 Available online 24 August 2006 Communicated by: Associate Editor Istvan Farkas

Abstract A mixed mode type forced convection solar tunnel drier was used to dry hot red and green chillies under the tropical weather conditions of Bangladesh. The drier consisted of transparent plastic covered flat-plate collector and a drying tunnel connected in series to supply hot air directly into the drying tunnel using two fans operated by a photovoltaic module. The drier had a loading capacity of 80 kg of fresh chillies. Moisture content of red chilli was reduced from 2.85 to 0.05 kg kg1 (db) in 20 h in solar tunnel drier and it took 32 h to reduce the moisture content to 0.09 and 0.40 kg kg1 (db) in improved and conventional sun drying methods, respectively. In case of green chilli, about 0.06 kg kg1 (db) moisture content was obtained from an initial moisture content of 7.6 kg kg1 (db) in 22 h in solar tunnel drier and 35 h to reach the moisture content to 0.10 and 0.70 kg kg1 (db) in improved and conventional sun drying methods, respectively. The use of a solar tunnel drier and blanching of sample led to a considerable reduction in drying time and dried products of better quality in terms of colour and pungency in comparison to products dried under the sun. The solar tunnel drier and blanching of chilli are recommended for drying of both red and green chillies. 2006 Elsevier Ltd. All rights reserved. Keywords: Blanching; Chilli; Collector; Colour; Pungency; Solar tunnel drier

1. Introduction Hot chilli is an important spice and condiment in the tropics and subtropics. It is an indispensable item in the kitchen for every day cooking in Bangladesh. Hot chilli is dried to make chilli powder and to store it for both short and long term storage. A large quantity of chilli is lost during the production season when the supply is abundant. Farmers do not get a proper return from their harvest during the peak period of harvest due to the low market price of the abundant supply of chilli. Sometimes they just have to sell the fresh chilli at a price far below the production cost. Whereas, the price of dried chilli always remains high even at the harvesting season. There is an increasing inter*

Corresponding author. Tel.: +88 2 9252407; fax: +88 2 9262713. E-mail addresses: [email protected], [email protected] (M.A. Hossain). 0038-092X/$ - see front matter 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.solener.2006.06.008

est in dried hot chilli for both the local market and foreign market. Hossain (2003) reported that chilli is a potential cash crop in Bangladesh and has a good export market. In Bangladesh, chillies are traditionally sun dried. The farmers expose their chillies to the open sun on a mat, earthen floor, cemented floor or on a tin shed. In this method, drying cannot be controlled and a relatively low quality dried product is obtained. Drying rate is very slow and takes 7–15 days, depending on the weather conditions (Hossain, 2003). In this traditional sun drying method, chillies become contaminated with dust, dirt, rainfall, animals, birds, rodents, insects and microorganisms. Under these conditions, losses can be as high as 40–60% of total quantity (Mangaraj et al., 2001). Usually no pre-treatment are used before the drying of chilli. As a result, the colour of most of the dried chilli becomes dull red. As an alternative to conventional sun drying, solar drying is a promising alternative for chilli drying in

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M.A. Hossain, B.K. Bala / Solar Energy 81 (2007) 85–92

Nomenclature db DR G

dry basis drying rate, kg kg1 h1 solar flux density, W m2

Bangladesh, because mechanical drying is mainly used in industrial countries and is not applicable to small farms in developing countries due to high investment and operating costs (Mu¨hlbauer et al., 1993). Solar energy for crop drying is environmentally friendly and economically viable in developing countries. The natural convection solar drier appears to have potential for adoption and application in the tropics and subtropics. It is suitable at a household level for drying of 10–15 kg of fruits and vegetables. But the natural convection solar drier suffers from limitations due to extremely low buoyancy induced airflow inside the driers (Bala and Woods, 1994). In addition, comparatively high investment, limited capacity and the risk of crop spoilage during adverse weather conditions have up to now prevented the wide acceptance of natural convection solar driers (Schirmer et al., 1996). Trim (1982) designed and constructed a tent type forced convection solar drier for drying of red chilli at Food Research Institute, Republic of Korea. The size of the drier was 4.25 m long and 2 m radius with a drying area of 22 m2. The capacity of the drier was to dry 300 kg of fresh chilli in 3–4 days. A forced circulated indirect solar drier for commercial drying of red chillies was designed and installed at Sarder Patel Renewable Energy Research Institute, Gujrat, India. The system consisted of 12 m2 flat plate air heater, a 0.75 kW electric blower and a drying chamber to hold 60–65 kg of chillies per batch. The system dries red chillies with 12% initial moisture content to 4% (db) final moisture content at an operating temperature of 60 C (Philip et al., 1993). Tiris et al. (1996) developed a multi-rack type mixed mode solar dryer at the Ege University, Turkey. The drying curves of the solar dried products were compared with traditional sun-drying results. Use of the solar dryer reduced the drying time by factors of 1.7, 2.2, 1.8 and 2.22, respectively, for sultana grapes, green beans, sweet peppers and chilli peppers. A commercial inclined box type solar drier was designed and developed at Central Arid Zone Research Institute, Jodhpur, India. About 80–100 kg of green chilli could be dried from moisture content from 85% to 6% (wb) in 3.5–4 days (Thanvi, 1999). Hollick (1999) developed a commercial solar drier for drying fruits, vegetables and spices. The roof type solar collector of 1120 m2 area was connected with a rack type drying chamber. The drier would able to dry 4 kg of chilli per day per m2 of the collector area from the moisture content 80–5% (wb). Mangaraj et al. (2001) dried red chilli by different methods such as open sun drying, greenhouse type solar drying, solar cabinet dry-

M T wb

moisture content kg kg1 (dry basis) temperature, C wet basis

ing and mechanical drying at Indian Agricultural Research Institute, New Dehli, India. From the experimental results it was inferred that open sun drying took 150 and 102 h, green house type solar drier took 90 and 66 h, solar cabinet drier took 54 and 36 h while mechanical drying took 26 and 16 h to reduce moisture content from 300% to 9% (db) for unpunched and punched chillies, respectively. Recently, researchers are opting for forced convection solar tunnel drying for drying of various crops (Pangavhane and Sawhney, 2002). Mu¨hlbauer and his associates developed a forced convection solar tunnel drier at Hohenheim University, Germany. One photovoltaic module is required to operate the fan independent of electric grid. It is used for commercial drying of fruits, vegetables, cereals, grain, legumes, oil seeds and spices. Even fish and meat can be dried properly in the forced convection solar tunnel drier (El-Shiatry et al., 1991; Esper and Mu¨hlbauer, 1996; Schirmer et al., 1996 and Bala, 2000). The forced convection solar tunnel drier, therefore, may be considered for adoption and application in Bangladesh for drying of hot chillies. 2. Description of the solar tunnel drier A Hohenheim type solar tunnel drier was redesigned, fabricated and installed at the Department of Farm Power and Machinery, Bangladesh Agricultural University, Mymensingh, Bangladesh. The drier basically consisted of a plastic sheet covered flat plate solar air heating collector, a drying tunnel unit, two dc fans and a 40 W photovoltaic module. The drier was 20 m long and 1.80 m wide. The solar collector unit was connected in series with the drying tunnel as shown in Fig. 1. The collector and drying chamber were made of plain metal sheets and wooden frames in a number of small sections and were joined together in series. These sections can be opened easily for transportation from one place to another. Glass wool was used between the two metal sheets at the bottom of the drier as an insulation material to reduce the heat loss from the bottom of drier. The collector was painted black to facilitate absorption of solar radiation. The drying area of the drier unit was same as that of the collector. Both the collector and the drying units were covered by 0.2 mm thick transparent UV stabilised plastic sheet. The plastic sheet was fixed on the collector side of the drier to the metal frame using U-type aluminium channel and rubber rope. At the drying unit one end of plastic sheet was fixed to a metal tube, which allows rolling


Fig. 1. Solar tunnel drier: 1. air inlet; 2. fan; 3. solar module; 4. solar collector; 5. side metal frame; 6. outlet of the collector; 7. wooden support; 8. plastic net; 9. roof structure for supporting the plastic cover; 10. base structure for supporting the drier; 11. rolling bar; 12. outlet of the drying tunnel.

of the plastic sheet up and down for loading and unloading of the drier. To prevent the entry of water into the drier during rain, both lateral ends of the plastic cover were fixed at 15 slope. A 40 W solar module was installed at the inlet of the solar collector as a power source to operate the fans, which supplied air over the product. The whole system was placed horizontally on tables made of iron angle frame 0.8 m above the ground floor for ease of loading and unloading of the products. Plastic net of 2 mm · 2 mm size was used as a tray and the tray was placed 75 mm above the floor of the drier. The drier was installed at a place free of shade, particularly for the period of 8:00 a.m. to 4:00 p.m. 3. Experimental procedure Three experimental runs in full load conditions were carried out for red chilli and another three for green chilli during the period of February–April in the years 1999 and 2000. The whole pods of red and green chillies were water blanched in hot water at about 85 C for 3–4 min. The blanched chilli was then spread on a bamboo mat to drain out excess water. After the draining out of excess water and cooling to ambient temperature, the chilli was weighed and spread out over the tray in a single layer in the solar tunnel drier. To compare the performance of the tunnel drier with that of conventional sun drying, three control samples of blanched chilli and another three samples of unblanched chilli were also placed on trays in a single layer beside the drier in the open sun. Drying was started after completion of the loading, usually at 9:00 a.m. and discontinued at 4:00 p.m. Weight loss of both the samples in the solar tunnel drier and the control samples in the open sun were measured during the drying period at 1 h interval with an electronic balance (Model CT1200-S, OHAUS Corporation, Florham, USA: accuracy ±.01 g). In the afternoon,

87

after 4:00 p.m., the samples in the solar tunnel drier were kept in the drier and the control samples were kept in a room at ambient conditions. These control samples were again put out in the sun next morning usually at 9:00 a.m. Then both the solar and sun drying samples were subjected to dry under the same weather conditions. A K-type thermocouple (Chromel–Alumel) was used to measure the drying air temperature along the flow direction of the air inside the collector and drier at seven fixed points. A solar meter (Model 776, Dodge Products, Houston, Texas, USA: accuracy ±1.5%) was used to measure the solar radiation at the position of the PV module. Relative humidity and temperature of the ambient air were measured with a digital humidity/temperature meter (Model Fluke 51, John Fluke MFG, Co. Inc., USA: accuracy ±2.5%). Velocity of drying air was measured with a vane type anemometer (Model Taylor 3132, Taylor Instruments, Toronto, Canada: accuracy ±.01 m/s) at the outlet of the drier. The ambient temperature, ambient relative humidity, temperature at seven points in the drier, relative humidity at the inlet and outlet of the drier, air flow rate at the out let of the drier, solar radiation, generated voltage and flow of current of the PV module were recorded at 1 h intervals during the solar drying of chilli. The moisture content of the chilli sample was measured at the starting and end of each run of the experiment by drying the samples in an air ventilated oven at 105 C for 24 h. After completion of drying, the dried chilli was collected, cooled in a shade to the ambient temperature and then sealed it in the plastic bags. About 80 kg of fresh red and green chillies were dried to about 20 kg. 4. Colour measurement The hot chilli pods were destalked, sliced longitudinally into two halves and the seeds were removed from the placenta. The sliced pods were oven dried at 58–60 C for 2 days and ground using a grinding mill (Model 1/T-204, ¨ Hler, Goldon) equipped with a 1-mm screen. The chilli BU powder was sealed in plastic bags and stored at 20 C until processed. The colouring strength of chilli was determined by the internationally accepted EOA (Essential Oil Association of USA) method. The EOA method based on the absorbance of a 0.01% w/v solution of the extract in acetone at 458 nm and multiplied by 61,000, gives the EOA colour value (Verghese et al., 1992). The absorbance was measured using a UV spectrophotometer (Model UV1201, SHIMADZU, Japan). 5. Pungency test Four grams of hot chilli powder was extracted with acetone till a colourless acetone solution was obtained. The volume was then made up to 100 ml with acetone. The extract was kept 3 h at room temperature. After 3 h, 5 ml of acetone was taken in a beaker and heated on a water bath till fully dry. To this, 5 ml of 0.1 N NaOH solution


was added followed by 3 ml of 3% phosphomolybdic acid solution and was kept at room temperature for 1 h. Finally optical density values were measured at 650 nm with the help of a UV spectrophotometer. The value of optical density is considered to be the pungency index of hot chillies (Mangaraj et al., 2001). Different values of optical density were obtained for different chilli samples. The sample for which the optical density was higher is considered to contain more capsaicin or more pungent.

40 35

Temperature rise, °C

88

2

R = 0.88

30 25 20 15 10 5 0

6. Statistical analysis

0

100

200

300

400

500

600

Solar radiation, Wm-2

Fig. 3. Air temperature rise at the outlet of the collector as a function of solar radiation.

70

Drying tunnel

Collector

60

Temperature, °C

The colour values and pungency indices of solar, improved sun dried and conventional sun dried chillies were statistically analysed using randomised block design (RBD). The data of colour values and pungency indices of red and green chillies obtained experimentally in the year 1999 and 2000 were analysed by analysis of variance using the software SPSS 9.0. The mean differences of colour values and pungency indices were graded by Duncan’s Multiple Range Test (DMRT).

50

Initial mc = 75.14% Intermediate mc = 39.16% Final mc = 5.99%

40 30 20

7. Performance of collector and drier

10

550

70

500 450

60

400

50

350 300

40 30

250 200

Solar radiation

150 100

Ambient temperature

50 09:00

20 10

Collector outlet temperature

10:00

11:00

12:00

13:00

14:00

Temperature, °C

Solar radiation, Wm-2

The variations of solar radiation, ambient air temperature and air temperature at the outlet of the collector for a typical day during solar drying of red chilli are shown in Fig. 2. The variation of ambient air temperature during the drying period (February–April, 1999 and 2000) varied from 20 to 35 C while the air temperature at the outlet of the collector varied from 40 to 66 C. During the drying period, air temperature at the outlet of the collector was observed to be much higher than the ambient air temperature. The rise in air temperature at the outlet of the collector above ambient air temperature against solar radiation, during solar drying of red chilli, is shown in Fig. 3. The average air temperature rise at the outlet of the collector over ambient air temperature was found to be 21.62 C during solar drying of chilli. It is observed from Fig. 3 that air

15:00

0 16:00

Time of a day, h Fig. 2. Variations of solar radiation, ambient temperature and drying air temperature at the outlet of the collector with time of a typical day during solar tunnel drying of red chilli (1 April 2000).

0

2

4

6

8

10

12

14

16

18

20

Length, m

Fig. 4. Temperature profile along the length of the collector and drier measured at different moisture contents for a typical experimental run during solar tunnel drying of red chilli (1 April 2000).

temperature rise in the drier increased linearly with the increase in solar radiation. The following regression equation was developed for air temperature rise at the outlet of the collector with solar radiation. DT ¼ 19:418 þ 0:077G

ðR2 ¼ 0:88Þ

ð1Þ

It is observed from Fig. 4 that the temperature profile along the length of the drying tunnel not only depends on solar radiation but also on the moisture content of chilli to be dried. At the beginning of drying, when the moisture content of the hot chilli was high, the air temperature decreased along the length of the drying tunnel. Due to the evaporative cooling on the surface of the chilli, the air temperature in the drier dropped. When the amount of energy required for evaporation of moisture from the chilli became less than the received energy then the air temperature in the drier increased. After completion of the initial stage of drying, drying air temperature in the drier is found almost constant along the length of the drier. This indicated uniform thermal stress throughout the drier and the constant air temperature inside the drier provided uniform drying throughout the length of the drier. 8. Drying of red and green chillies Blanched chilli sample was dried in the solar tunnel drier and at the same time blanched and unblanched chilli sam-


Moisture content, kgkg-1 (db)

3 2.5

Solar tunnel drying Improved sun drying Conventional sun drying

2 1.5 1 0.5 0 0

5

10

15 20 Drying time, h

25

30

35

70

80

60

70 60

50

50

40

40 30 Ambient temperature Drier temperatue Ambient relative humidity Drier relative humidity

20 10 0 09:00 10:00

11:00

12:00 13:00

14:00 15:00

30 20

Relative humidity, %

Temperature, °C

Fig. 5. Solar tunnel, improved sun and conventional sun drying of red chilli for a typical experimental run (6–10 April 2000).

10 0 16:00

Time, h Fig. 6. Ambient and average drier temperature and relative humidity for a typical day during solar tunnel drying of red chilli.

vection. Moreover it lost heat energy to the surrounding environments. So, temperature (average) in the drier was higher than the ambient temperature and corresponding relative humidity in the drier was lower than the ambient relative humidity (Fig. 6). As a result, the drying rate of red chilli in the drier was found to be higher than that of in open sun. The drying rate of improved sun drying sample was found higher than that of conventional sun drying sample although they were both dried in the open sun. This is due to the fact that the sample of improved sun drying method was blanched and the tissues of this blanched chilli was cooked partially and the cell wall of chilli pods became soft, more permeable to moisture diffusion. As a result, transfer of moisture in the blanched sample was higher than that of unblanched sample. A comparison of drying of red chill of present study with Mangaraj et al. (2001) (solar green house type drier) and Kaleemullah and Kailappan (2005) (rotary drier) is given in Fig. 7. It is observed from the figure that the drying rate of present study was higher than those of Mangaraj et al. (2001) and Kaleemullah and Kailappan (2005) but they follow the similar pattern. This may be due to that the initial moisture content of present study was (2.8 kg kg1 (db)) lower than Mangaraj et al. (2001) (3.0 kg kg1 (db)) and Kaleemullah and Kailappan (2005) (3.2 kg kg1 (db)) and also the chilli varieties were different. Also, the drying air temperatures of present study was 50–55 C and those of Mangaraj et al. (2001) and Kaleemullah and Kailappan (2005) were 55 and 50 C, respectively. The comparison of moisture content of green chilli in the solar tunnel drier with those obtained by improved and conventional sun drying method of drying for a typical experimental run conducted in the year 1999 is shown in Fig. 8. Green chilli was dried to a moisture content of 0.06 kg kg1 (db) from 7.6 kg kg1 (db) in 22 h of drying in solar tunnel drier as compared to 35 h of drying in open sun by improved and conventional sun drying methods for comparable samples to a final moisture content of 0.105 and 0.70 kg kg1 (db), respectively. There is a relationship between drying rate and moisture content of the product during drying. Drying rate versus moisture content during solar tunnel drying of red chilli and conventional sun drying of green chilli are shown in


ples were dried in the open sun. For comparison of drying rate of red and green chillies in the solar tunnel drier with that of in the open sun, the blanched chilli samples were dried in solar tunnel drier and in the open sun. Farmers of Bangladesh dry chilli without applying any pre-treatments and hence unblanched sample was dried in the open sun as a control sample. In this study, blanched sample dried in the solar tunnel drier is termed as ‘solar tunnel drying’ and those dried in the open sun by blanching and unblanching are termed as ‘improved sun drying’ and ‘conventional sun drying’, respectively. The drier was loaded with 80 kg of blanched chilli. The changes of moisture content with drying time for a typical experimental run for solar tunnel drying, improved sun drying and conventional sun drying of red chilli is shown in Fig. 5. Moisture content of red chilli reached to 0.05 kg kg1(db) from 2.85 kg kg1 (db) initial moisture content in 20 h of drying in solar tunnel drier while it took 32 h to bring down the moisture content of similar samples to 0.09 kg kg1(db) by improved sun drying and 0.40 kg kg1 (db) by conventional sun drying method. This is due to the fact that the chilli in the drier received energy from the radiation from the sun and more energy transported from the collector by forced convection, while the samples dried in the open sun received energy from only the incident solar radiation and less energy transported from surrounding environment by natural con-

89

3.5

Present study Mangaraj et al. (2001) Kaleemullah and Kailappan (2005)

3 2.5 2 1.5 1 0.5 0 0

5

10

15

Drying time, h

20

25

30

Fig. 7. Comparison of drying of red chillies of present study with other investigators.

90


Solar tunnel drying

7

Improved sun drying

6

Conventional sun drying

-1

0.35

8

Drying rate, kgkg-1 h

Moisture content kgkg-1 (db)

9

5 4 3 2

2

R = 0.95

0.3 0.25 0.2 0.15 0.1 0.05

1 0 0

10

20

30

0

40

0

Drying time, h

2

4

6

8

10


Fig. 8. Solar tunnel, improved sun and conventional sun drying of green chilli for a typical experimental run (15–19 February 1999).

Fig. 10. Drying rate of green chilli as a function of moisture content during conventional sun drying.

For improved sun drying of green chilli: 0.4

DR ¼ 0:1229 þ 0:2396LnðMÞ

2

0.35

ð6Þ

For conventional sun drying of green chilli:

-1

0.3

Drying rate, kgkg-1h

ðR2 ¼ 0:93Þ

R = 0.95

DR ¼ 0:1137 þ 0:0732LnðMÞ

0.25

ðR2 ¼ 0:95Þ

ð7Þ

It is also evidence from the equations that for both red and green chillies, drying rates were observed higher in solar tunnel drier followed by improved sun drying and conventional sun drying methods.

0.2 0.15 0.1 0.05

9. Quality of dried red and green chillies

0 0

0.5

1

1.5

2

2.5

3


Fig. 9. Drying rate of red chilli as a function of moisture content during solar tunnel drying.

Figs. 9 and 10, respectively. It is observed from the figures that drying rate of chillies was higher at higher moisture content and it decreased logarithmic as moisture content reduced. At the initial stage of drying, moisture content of chilli was high and more moisture was evaporated from outer surface and outer layers of chilli. As the drying process proceeded, the moisture on the surface decreased and the evaporation zone moved from the surface into inside of chilli and less evaporation took place and hence drying rate reduced with the drying time as well as moisture content. The following regression equations were developed for drying rate with the moisture content of red and green chillies. For solar tunnel drying of red chilli: DR ¼ 0:2598 þ 0:0796LnðMÞ ðR2 ¼ 0:95Þ

ð2Þ

For improved sun drying of red chilli: DR ¼ 0:1371 þ 0:0593LnðMÞ ðR2 ¼ 0:94Þ

ð3Þ

For conventional sun drying of red chilli: DR ¼ 0:0708 þ 0:0638LnðMÞ ðR2 ¼ 0:92Þ

ð4Þ

For solar tunnel drying of green chilli: DR ¼ 0:1399 þ 0:3793LnðMÞ ðR2 ¼ 0:97Þ

ð5Þ

The colour and pungency are considered to define the quality of dried hot chillies as these properties reflect the consumers’ acceptance and therefore the market price. The mean colour value and pungency of dried red hot chilli for different experimental runs conducted in the years 1999 and 2000 are shown in Table 1. There was no significant difference between the colour value of the solar tunnel dried red chilli and improved sun dried red chilli. The reason might be that both solar and improved sun dried red chilli was water blanched before drying. The mean colour values obtained from conventional sun dried red chilli was significantly lower than those obtained from solar tunnel and improved sun dried red chilli both in the years 1999 and 2000. As colour and pungency has a strong positive correlation i.e. more is the colour, more pungent is the chilli. Since the conventional sun dried red chilli was not blanched and the colour as well as the pungency may be reduced due to enzymatic browning. Production of enzymes in fruits and vegetables leads to discolouration, loss of vitamins and breakdown of tissues. Most of the enzymes are inactivated at 70 C temperature (Scanlin, 1997). Also blanching reduces the enzymatic browning and prevents the loss of colour of chilli (Chung et al., 1992). Chilli samples in solar tunnel and improved sun drying method were water blanched at 80 C for 3 min. So, colour as well as pungency losses of solar tunnel and improved sun drying samples may be prevented. Higher pungency index was found for the solar tunnel dried red chilli than that of conventional sun dried red chilli but they


91

Table 1 Colour and pungency of solar tunnel, improved sun and conventional sun dried red chillies (mean ± standard error, n = 3) Drying method

Solar tunnel drying Improved sun drying Conventional sun drying Significance

Colour (EOA colour unit)

Pungency index

Year 1999

Year 2000

Year 1999

Year 2000

4306.0 ± 93.8a 4176.8 ± 87.1a 3682.4 ± 95.2b p 6 0.01

3751.8 ± 79.3a 3646.7 ± 106.9a 3278.46 ±82.4b p 6 0.01

2.317 ± 0.007 2.285 ± 0.005 2.302 ± 0.006 Not significant

2.355 ± 0.008 2.343 ± 0.005 2.331 ± 0.007 Not significant

Means in the same column with same letters are not significantly different from each other by DMRT.

Table 2 Colour and pungency of solar tunnel, improved sun and conventional sun dried green chilli (mean ± standard error, n = 3) Drying method

Solar tunnel drying Improved sun drying Conventional sun drying Significance

Colour (EOA colour unit)

Pungency index

Year 1999

Year 2000

Year 1999

Year 2000

1380.9 ± 86.3a 1324.3 ±79.8a 1064.7 ± 98.2b p 6 0.05

1636.4 ± 98.1a 1491.9± 86.5a 1109.7 ±102.4b p 6 0.05

0.866 ± 0.005 0.849 ± 0.007 0.894 ± 0.004 Not significant

0.926 ± 0.003 0.889 ± 0.006 0.890 ± 0.004 Not significant

Means in the same column with same letters are not significantly different from each other by DMRT.

are not statistically different (Table 1). Mangaraj et al. (2001) found higher pungency from solar dried red chilli than that of sun dried red chilli. It is also found in Table 1 that the colour values of red chillies in the years 1999 and 2002 were different. This may due to the variations of chilli samples grown in years 1999 and 2000. The mean colour value and pungency of dried green chilli of different experimental runs conducted in the years 1999 and 2000 are given in Table 2. There was no significant difference between the colour of the solar tunnel dried green chilli and improved sun dried green chilli. The mean colour values obtained from conventional sun dried green chilli was significantly lower than those obtained from solar tunnel and improved sun dried green chilli both in the years 1999 and 2000. The values of pungency index for solar tunnel drying, improved sun drying and conventional sun drying samples of green chilli were almost similar and statistically alike. 10. Conclusions Average air temperature rise in drier was about 21.62 C above the ambient temperature and it was almost constant in the drier. The solar tunnel drier can be used to dry up to 80 kg of fresh chillies. In all the cases, the use of this drier led to considerable reduction in drying time in comparison to that of conventional sun drying, and the products dried using this drier were of better quality as compared to their conventional sun dried counterparts. This drier can be easily constructed using locally available materials. This can be operated by a photovoltaic module independent of electrical grid. The photovoltaic system has an advantage because the temperature of the drying air in the drier is automatically controlled by the solar radiation and photovoltaic combination. Blanching retained more colour than that of unblanched samples for red and green chillies with-

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