Effects of Pre-treatments on Drying Characteristics of Bitter gourd

Abstract

Bitter gourd (Momordica charantia L.) is an internationally consumed vegetable containing high amounts of vitamins (C, A and E) and minerals (phosphorus, potassium, and calcium). It has many medicinal properties like anti-diabetic, anti-infective, antipyretic, anthelmintic, antifungal, androgenic, antiviral, antimalarial actions and is widely recommended in daily diet. Despite of many advantages, proper processing and value addition have not been explored yet. Drying with appropriate pretreatments can extend the shelf life and availability of the product for consumption throughout the year. The present work investigates various physico-chemical and textural properties of Bitter gourd and explores the effects of different pre-treatments on drying characteristics of Bitter gourd. The textural properties were dependent on the size and were significantly (P<0.05) different for small, medium and large size of Bitter gourds. However, there was no such difference for physical properties such as geometric mean diameter, sphericity, surface area, aspect ratio, bulk density, true density and coefficient of friction. Various drying characteristics like moisture ratio, drying rate, moisture diffusivity, shrinkage ratio and rehydration efficiency were determined. The drying of the pre-treated Bitter gourd was mathematically modelled for three temperatures (50, 60 and 70ºC). The dried samples were analysed for biochemical quality parameters like colour, vitamin C, total phenolic compounds, 2-2 diphenyl picryl hydrazyl, and ferric antioxidant reducing power. To improve the drying quality pre-treatments like hot water blanching, microwave blanching, ethyl oleate dipping and combination of these pre-treatments were given to Bitter gourds. At 70ºC, not pre-treated samples had maximum (190min) drying time whereas the combined ethyl oleate and microwave blanched samples had minimum (100min) drying time. Thus, ethyl oleate treated microwave blanched samples significantly (P<0.05) reduced the drying time at all three temperatures. Drying temperature had significant (P<0.05) effect on moisture ratio. Increased temperature reduced the drying time of the samples. Pre-treatment and drying temperature had significant (P<0.05) effect on the moisture diffusivity of Bitter gourd. The moisture diffusivity was found lowest in not pre-treated samples (2.65 to 4.85 x10-8m2/s) and highest in ethyl oleate treated microwave blanched samples (3.86 to 7.48 x10-8 m2/s). The moisture diffusivity was also dependent on temperature and the value increased from 3.86 to 7.48x10-8m2/s with increase in drying temperature from 50 to 70ºC. The activation energy was minimum (22.44 KJ/mol) for water blanched samples and was maximum (30.55 KJ/mol) for ethyl oleate treated microwave blanched samples. At 70ºC not pre-treated samples showed the highest (80.2 %) shrinkage ratio whereas ethyl oleate treated microwave blanched samples showed the lowest (52.06%) shrinkage ratio. As the drying temperature increased from 50 to 70ºC the shrinkage ratio increased significantly (P<0.05) from 70.3 to 80.2%. The lowest and highest rehydration ratio was found in not pre-treated and ethyl oleate treated microwave samples respectively. The rehydration ratio increased from 3.49 to 4.9% for not pre-treated samples as the drying temperature increased from 50 to 70ºC. Retention of various biochemical and nutritive parameters such as vitamin C, total phenolic compounds, 2,2, diphenyl picryl hydrazyl was highest for combined pre-treatment of ethyl oleate and microwave blanched samples. The overall drying kinetics of the pre-treated samples could be best fitted to Page Model.