* To whom correspondence should be addressed. E-mail: jasahmed@ redi!mail.com Lebensm.-Wiss. u.-Technol., 34, 380 } 383 (2001) Thermal Kinetics of Colour Degradation and Storage Characteristics of Onion Paste J. Ahmed and U. S. Shivhare* Department of Food Science & Technology, Guru Nanak Dev University, Amritsar-143005 (India) (Received February 29, 2000, accepted February 13, 2001) The kinetics of colour degradation at selected temperatures and ewects of packaging materials and storage temperatures on colour of onion paste were determined. Colour change during thermal processing of onion paste followed xrst-order reaction kinetics. The tristimulus colour value combination La/b adequately described the colour degradation. Dependence of the rate constant during thermal processing of onion paste obeyed the Arrhenius relationship. The activation energy for onion paste was estimated to be 16.2 kJ/mol. The total colour of onion paste was signixcantly awected (P40.05) by packaging materials, temperature and duration of storage. The paste was more stable at low temperature (5 3C) than at higher temperatures (25 3C) with respect to colour, and colour degradation was minimum when the paste was packed in a high-density polyethylene pouch.  2001 Academic Press Keywords: onion paste; kinetics; colour; packaging materials; storage Introduction Onion (Allium cepa L.) is valued for its therapeutic prop- erties and possesses a strong characteristic aroma and #avour which makes it an important ingredient during food processing (Augusti et al., 1996; Dron et al., 1997). The characteristic #avour of onions comes primarily from volatile organic sulphur compounds released en- zymatically by the action of allinase on several naturally occurring amino-precursors during comminution. The primary reaction products are thiosul"nates, which are heat labile and dissociate on heating to form di- and tri-sul"des and other sulphur compounds that have been associated with cooked onion #avour (Block &O'Conner, 1974). Onion paste is one such product that is convenient to use and could retain the original colour and #avour in a semi-solid form. Colour is an important attribute because it is usually the "rst property the consumer observes (Saenz et al., 1993). Various factors are responsible for the loss of colour during processing of food products. These include non-enzymatic and enzymatic browning and process conditions such as pH, acidity, packaging material and duration and temperature of storage. To optimize the process, it is important to determine the kinetic para- meters (reaction order, reaction rate constant, activation energy) for colour change (Weemaes et al., 1999). The colour kinetics of food products is a complex phenom- enon and dependable models to predict experimental colour change, which can be used in engineering calcu- lations, are limited. Therefore experimental studies and application of various simpli"ed models to represent the behaviour are required. Anthocyanin is the major pigment responsible for colour in red onions and the major compounds identi"ed were cynidin-3-glucoside, cyanidin-3-diglucoside and peonidin glucoside (Kalra, 1987). Onions contain reduc- ing sugars and amino acids that enhance non-enzymatic browning during thermal processing (Berk, 1980; Shin & Bhowmick, 1994). Several authors have studied the col- our kinetics of food materials during thermal processing in terms of changes in Hunter tristimulus colour values ¸, a and b (Shin & Bhowmik, 1994; Berry, 1998; Kajuna et al., 1998; Nanke et al., 1999; Weemaes et al., 1999; Ahmed et al., 2000). Hence, if the kinetics of colour degradation is determined and the order of colour change is established, the total colour can be used to evaluate quality of food material during thermal processing. No information is available on thermal kinetics of colour degradation and storage behaviour of onion paste. The present study was undertaken to investigate the kinetics of colour degradation of onion paste at selected processing temperatures using the Hunter colour scale values (¸, a, b) and the e!ects of temperature and packag- ing material on colour during storage of onion paste. 0023-6438/01/060380 #04 $35.00/0 doi:10.1006/fstl.2001.0771  2001 Academic Press All articles available online at http://www.idealibrary.com on 380