Effect of Two Stage Drying Mode on the Quality Attributes of Dried Onion Slices
Year: 2015

Effects of processing variables such temperature (during first stage) cut off time and tempering period on quality attributes such as ascorbic acid, reducing sugar, total sugar, rehydration ratio, dehydration ratio and colour of dried onion slices was carried out to develop dried onion slices so as to reduce spoilage and post harvest losses of onion. Response Surface Methodology (RSM) was employed for the experimental design. Results showed that Ascorbic acid decreases with increase in temperature. There was increase in rehydration ratio by decreasing temperature and cut off time. The full second order model was found inadequate in describing the reducing sugar, total sugar, dehydration ratio, L* a* and b* due to lower values of R2 and F. Key wordsresponse surface methodology, onion dehydration, ascorbic acid, rehydration ratio I.INTRODUCTION Onion (Allium cepa L. is an important vegetable crop in the world. Onion is being extensively cultivated all over the world, especially in china, India, Netherlands, Pakistan, Bangladesh and Australia. India is the second largest producer of onion with an area of 554.15 'ooo ha) and production of 9.5 MT next to China[1] About 35% to 40% produces lost due to lack of proper post harvest practices and management for onions in India. Thus, there is a need for processing it into stable products in order to minimize the losses during the postharvest handling and storage [5,12] Onions are used for their flavor, aroma, and taste, being prepared domestically or forming raw materials for a variety of food manufacturing processes (dehydration, freezing, canning, and pickling) Onions were among the earliest vegetables to be produced, canned, dried, and frozen. Dehydration is the most important method for preservation of onions with an ultimate aim of improving storability by reducing its moisture content. Removal of water from onions prevents microbial growth and thus makes storage without refrigeration possible. The dried onion has become a standard ingredient used as flavour additive in a wide variety of processed foods such as ketchup, sauces, soups, salad dressings, sausage and meat products, potato chips, crackers and other snack items. The fundamental objective to create a shelf stable product, in food drying is to dry a product but without affecting the desired quality attributes. The ascorbic acid, sugars, rehydration ratio and colour of the dried onion are considered the most important quality attributes affecting the degree of acceptability of the product by the consumer. The rehydration characteristics of a dried product are widely used as a quality index. Rehydration is a complex process and indicates the physical and chemical changes caused by drying and treatments preceding dehydration [4,7].Ascorbic acid is probably the most liable of all the vitamins contained in the foods. The loss was occurred because it is water soluble and is rapidly destroyed by heat, light and oxidation. Losses vary from 10 to 50 during preparation of the food for drying due to washing, leaching and blanching. If vegetables were sulphited before drying, retention is increased  Hence, the International Journal of Latest Trends in Engineering and Technology (IJLTET) Vol. 5 Issue 3 May 2015 149 ISSN: 2278 621X present research work was carried out to investigate the effect of various process conditions such temperature (during first stage) cut off time and tempering period on nutritional quality of onion slices using two stage drying. II.MATERIALS AND METHODS Fresh well graded, dark pink colored, good quality onions were procured from local market, peeled and cut into slices of thickness 4 mm and pre treated with 0.25 KMS for 5 min [9] 100 g of onion slices were taken as test sample. The Tray dryer (cabinet type) was used for drying studies of onion. Dryer consists of an air delivery system, air heating system, drying chamber with trays, temperature controller etc. The treated samples were spread uniformly in a single layer on the tray of drier. Drying of onion slices during first stage was carried out at 70, 80, 90 0C for 30, 50, 70 min, then these samples were tempered for a period of 0, 20 40 min and dried at 600C in second stage till attainment of final moisture content of 4 to 6 (db) The dried samples were then packed in polythene bag to avoid moisture migration from atmosphere as these dried slices are hygroscopic in nature.  Response Surface Methodology (RSM) was employed for the experimental design. The RSM allows design of experiments with minimum number of experiments, without affecting the accuracy of results and determine the interactive effects of the variables on the responses  It provides statistically acceptable results and can be used for optimization of the process [2,3] Incomplete Composite Block (Box Behnken) design was used to design the experiments with three variables, each to be examined at three levels. The three independent variables were high temperature during first stage (Td) cut off time (tc) and tempering period (tp) The levels of temperature were 70,80 and 90oC for first stage, 30,50 and70 min for cut off time and 0,20 and 40 min for tempering period. The experimental design resulted in 17 combinations on which experiments were performed (Table 1) Quality determination For estimation of quality attributes such as ascorbic acid, reducing sugar and non reducing sugar of the dried onion slices standard methods were adopted. Ascorbic acid Ascorbic acid in the sample was estimated by 2, 6dichlorophenol indophenol visual titration method  Total Sugar and Reducing Sugar Total sugar and reducing sugar content in the sample was estimated by Lane and Eynon method  Colour Colour of sample was determined using the digital camera and Adobe Photoshop 7.0 .The sample was kept under the light source such that the light intensity over the food sample should be uniform. Digital camera (Sony 7.2 mega pixels) was used to capture the image. Adobe Photoshop 7.0 was used to obtain color parameter L* a* b* the preferred color model for food research. L* is the luminance or lightness component, which ranges from 0 to 100, and a* (from green to red) and b* (from blue to yellow) are the two chromatic components, which range from 120 to +120. To convert Lightness, a and b values obtained from the Histogram window to L* a* b* following formulas were used.  L* (Lightness/250) x100 (1) a* (240a/255) 120 (2) b* (240b/255) 120 (3) Rehydration Ratio For the rehydration ratio test, dehydrated samples (2 g each) were dipped in boiling water for 25 minutes and contents were then filtered through Wattman No.4 filter paper. The rehydrated onion sample was then weighed and the weight recorded as WR. The rehydration ratio (RR) was computed using the following equation.