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Effect of Pretreatments on Drying Kinectics and Quality of Solar Dehydrated Chillies

M. A. Hossain
Farm Machinery and Postharvest Process, Engineering Division, Bangladesh Agricultural Research Institute, Gazipur- 1701, Bangladesh

B. L. D. Choudhury
Department of Biochemistry, Bangladesh Agricultural University Mymensingh-2202, Bangladesh

B. K. Bala
Department of Farm Power and Machinery, Bangladesh Agricultural University Mymensingh-2202, Bangladesh

Abstract/ Executive Summary:

The effect of different pretreatments on drying rate and quality of solar dehydrated red and green chillies were investigated. Red chillies were blanched, sulphited and treated with sodium hydroxide and dipsol (water based emulsion) whereas, green chilli were blanched and treated with sulpher dioxide, sodium hydroxide, and sodium bicarbonate. Moisture diffusivities of red and green chillies for different pretreatments were determined to compare the drying rate of each of the pretreated samples. The Drying rate of red chilli subjected to blanching, sulphiting and sodium hydroxide treatments were found higher than those of the control and dipsol treated samples. For green chilli, the higher drying rates were found for all treated samples over the control sample. The highest colour values and glossyness were found for blanched and dipsol treated samples of red chilli. Chemical additives (sulpher dioxide, sodium hydroxide and dipsol) significantly reduced the pungency of red chilli. The highest colour value and glossyness were found for blanched and sulphited samples of green chilli. Pungency of green chilli was not reduced significantly by applying pretreatments. Therefore, it is recommended for blanching of both red and green chillies before solar drying to increase the drying rate and for the retention of better colour, glossyness and pungency without application of chemical additives.

Keywords: Colour, Capsaicin, Moisture diffusivity, Pretreatment, Pungency, Solar drying

Location: Bangladesh Agricultural Research Institute, Gazipur

Start Date:

End Date:

Program Area: Farm Mechanization

Commodity: Chilli

Objectives:

To determine the effect of different pre treatments on drying rate, colour, glossyness, and pungency of solar dried red and green chillies.

Methodology:

Freshly harvested red and green chillies of variety Balujuri (Mymensingh local) was procured from the local market. They were cleaned and washed with clean water and spread on a plastic filter tray to drain out excess water. The fresh chilli was then sealed in plastic bags and stored in a refrigerator at a temperature of 8°C. Before starting each experiment, the required chilli sample was brought out from the refrigerator and held until the sample attained room temperature. Whole chilli pods were selected for experiments, because in Bangladesh chilli is dried as whole pod without cutting or slicing (Elias and Hossain, 1984). About three liters of fresh water was taken in a pan and heated up to a temperature of 88±2°C (FAO, 1995). When the desired temperature was obtained then 500 g of chilli sample was taken in a cotton bag and was immersed into hot water and kept for three minutes. At this time heating was continued so that the temperature of the water did not decreased below the desired temperature. When the blanching was completed the bag was brought out from the hot water and dipped into normal cool water to prevent over-blanching. After cooling, the bag was brought out from water and the chilli sample was spread on a plastic tray to drain out excess water. Both red and green chillies were blanched following the same procedure. Nine grams of sodium metabisulphite was mixed in three liters of fresh water (9 g sodium metabisulphite in 3 liter water≈2000 ppm SO2) and heated to a temperature of 80°C. Then 500 g of chilli sample was taken in a cotton bag and immersed in the solution and held for three minutes. When the treatment time was completed the bag and contents were brought out from the hot solution and dipped again into cold water to prevent over-heating. After cooling, the bag was brought out from the water and the content was spread on a plastic tray to drain out excess water. This treatment was applied for both red and green chillies. The chilli sample (500 g) was dipped in 2% sodium hydroxide solution at 80°C for three minutes. When the treatment was completed then the bag and contents were brought out from the solution and dipped in normal water to cool the sample and wash out excess alkali. After cooling the chilli sample was spread on a plastic tray to drain out excess water. Sodium hydroxide treatment was used both red and green chillies. This treatment was applied only for green chilli. A green chilli sample (500 g) was taken in a cotton bag and dipped in three liters of 10% sodium bicarbonate solution at 80°C for 3 minutes. After the completion of treatment time the bag with the chill content was brought out from the hot solution and was dipped in water to cool the sample and wash out excess salt. Dipsol is a water-based emulsion containing potassium carbonate (2.5%), refined groundnut oil (1%), gum acacia (0.1%) and butylated hydroxy anisol (BHA) (0.001%) (Laul, 1977). About 250 ml of dipsol is required for the treatment of one kg of red chilli. For the preparation of 250 ml of dipsol, 6.25 g potassium carbonate was dissolved in 100 ml of distilled water and 0.25 g gum acacia was dissolved in 100 ml distilled water separately. BHA (2.5 mg) was slowly mixed in 2.5 ml refined groundnut oil. Then the water-based solutions were mixed with the BHA-mixed groundnut oil and made homogeneous by stirring. The volume of the emulsion was made up to 250 ml by adding an excess amount of distilled water. Then fresh red chilli sample was dipped in dipsol solution for five minutes. After completion of the immersion time, the chilli sample was spread on a plastic tray to drain out excess solution. Only the red chilli was treated with dipsol solution. A solar tunnel drier was designed, fabricated and installed at the Department of Farm Power and Machinery, Bangladesh Agricultural University, Mymensingh, Bangladesh. The drier basically consists of a plastic sheet covered flat plate solar air heating collector, a drying tunnel unit, two dc fans and a 40 watt photovoltaic module. The drier was 20 m long and 1.80 m wide. The solar collector unit is connected in series with the drying tunnel. The details of the solar drier have been described by Hossain (2003). The treated and the control chilli samples were spread on a plastic net in thin layer in a solar tunnel drier. Each of the treatment was replicated three times. Three experimental runs were conducted for red chilli and another three for green chilli. Drying was started usually at 9:00 am and discontinued at 4:00 pm for each day. Important parameters affecting the performance of the drier were measured. Weight loss of the product during the drying period was measured with an electronic balance. All these data were recorded at onehour interval. The moisture contents of the chilli were measured at the start and end of each run of the experiments by gravimetric method (AOAC, 1995). Sensory evaluation or panel test is an important aspect of quality assessment of a product. The colour and glossyness of dried red and green chillies were determined by sensory evaluation. Fifteen experienced panelists were asked individually to evaluate the colour and glossyness of coded samples of pretreated red and green chillies. The samples were presented in identical containers coded with 3 digit random numbers. A Hedonic scale (1-9) was used to evaluate the degree of liking or disliking of the samples. The pungency or capsaicin content of chillies was determined by solvent extraction and photometric method using the procedure given by Sadasivam and Manickam (1997). The chilli pods were destalked, sliced longitudinally into two halves and the seeds were removed from placenta because pungency and colour of chilli are concentrated in its pericarp and placenta. The sliced pods were oven dried at 58 to 60°C for two days and grounded using a grinding mill equipped with a 1- mm screen. Dry chilli powder of 500 mg was collected in a 100 ml volumetric flask. Ten milliliter acetone was added to the volumetric flask and shaken in mechanical shaker for 3 hours. The contents were centrifuged (10000 rpm) for 10 min. One milliliter of clear supernatant was pipetted into a test tube and evaporated to dryness in a hot water bath. 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 0.01% w/v solution of the extract in acetone at 458 nm with the reading multiplied by 61000 is the EOA colour value (Verghese et al., 1992). Five milligram chilli powder was extracted in 500 ml of acetone. Then the absorbance of the extracted solution was measured at 458 nm with the help of a UV spectrophotometer against a reagent blank.

Source of Information: Journal of South Pacific Agriculture, Volume 12:Nos. 1 & 2, 2005

Article Link (Online Journal):

Funding Source:

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Achievement/Findings:

The drying rate of red chilli subjected to blanching, sulphiting and sodium hydroxide treatments were found to be higher than those of the control and dipsol treated samples. For green chilli, the higher drying rates were found for the treated samples over the control sample. The highest colour values and glossyness were found for blanched and dipsol treated samples of red chilli. Chemical additives significantly reduced the pungency of red chilli. The highest colour values and glossyness were found for blanched and sulphited samples of green chilli. Pungency of green chilli was not reduced significantly by applying any pretreatments. Therefore, it is recommended that blanching should be used for both red and green chillies before solar drying to increase drying rate and for retention of better colour, glossyness and pungency without application of chemical additives. This will reduce the cost of chemicals and health hazards and be more acceptable to consumers.

Knowledge Management: Journal