Hot Air Drying and Quality of Red and White Varieties of Onion (Allium cepa)

Ayoola Olalusi

doi:10.4236/jacen.2014.34b003

Abstract

This study was undertaken to investigate the drying characteristics of red and white varieties of onion (Allium cepa) in a convective dryer. The drying characteristics of onion slices were examined at air temperatures of 50℃, 60℃ and 70℃ and sample thicknesses of 2 mm. During the drying experiments, air velocity was held stable at 0.9 m/s. The effects of air temperature on the drying characteristics and quality parameters such as vitamins C and D, macro minerals (Mg, Na, P, Ca and K), non-enzymatic browning and thiolsulphinate of the dried onion slices were determined. For each of the plots of the drying curves from the mechanical drying tests, there was an absence of or just a very brief appearance of a constant rate period because onions are hygroscopic and hygroscopic foods tending to quickly enter the falling rate period; drying time decreased considerably with increased temperature. The time required to reduce the moisture content to any given value was dependent on the drying conditions, being the longest at an air temperature of 50℃, 0.9 m/s and the shortest at an air temperature of 70℃ and air velocity of 0.9 m/s. The loss of vitamin C, which is a thermo-sensitive compound, was the least at an air temperature of 50℃.

Highlights

Onion (Allium cepa) has been widely used even in ancient times as seasonings, for medical uses and as foods [1]
The graphs of moisture content versus drying time and drying rate versus drying time at air velocity of 0.9 m/s and air temperature of 50 ̊C, 60 ̊C, 70 ̊C are shown in Figure 1 and Figure 2 for the red and white onion varieties respectively
The dehydration rate of onion slices was found to be greatly dependent on the air temperature

Summary

Introduction

Onion (Allium cepa) has been widely used even in ancient times as seasonings, for medical uses and as foods [1]. The importance of onions lies in the flavour that it imparts to several other dishes. Onion is characterized by its distinctive flavour and pungency which are due to sulphur containing compounds available in the scales of bulbs. Onion is a basic flavouring in the kitchen. It is used as a vegetable, or as a spice to bring out the flavour of other dishes without overpowering them. Onions are the indispensable vegetable, the strong (yet sweet) cornerstone of modern cooking, not just in our culture but around the world. Onions have been thought to have certain medicinal and disease prevention powers; modern science has begun to show that there may be some fact in the ancient lore. In the United States, products that contain onion extract are used in the treatment of topical scars [21]; some studies

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