Colour kinetics and rheology of coriander leaf puree and storage characteristics of the paste

Coriander leaves have been used as a food flavourant in various cuisines since ancient times. The colour degradation kinetics of coriander leaf puree was studied using a fraction conversion technique during thermal treatment at 50, 65, 80, 95 and 110 degreesC for up to 60 min. Blanched, comminuted coriander leaf puree was subjected to heat treatment at selected temperatures in an oil bath with agitation. Treated samples were removed from the bath at selected time intervals (0-60 min after come-up period), cooled immediately and analyzed for colour using a Hunterlab colourimeter. The rheological characteristics of the puree were evaluated using a computer-controlled Haake rotational viscometer at 50, 60, 70 and 80 degreesC and it was found that the Herschel-Bulkley model adequately represented shear stress-shear rate data. Temperature dependency of the consistency index and apparent viscosity at a shear rate of 100 s(-1) followed the Arrhenius relationship and the flow activation energy ranged between 17.2 and 17.9 kJ/mol. Coriander puree was converted to paste by adding common salt (2%) and the required volume of citric acid to adjust the pH to 4.2. The paste was then filled into glass bottles and stored at selected temperatures (5, 25 and 37 degreesC) for 6 months and was periodically evaluated for colour. total aerobic plate count and physicochemical properties. Coriander puree/paste colour was expressed in terms of tristimulus colour value a and combination (Lxaxb). First order reaction kinetics adequately described the changes in colour values during both thermal treatment of puree and storage of paste. The process activation energies were 29.3 and 22.1 kJ/mol, respectively, for a and (Lxaxb) during thermal treatment. The corresponding values during storage were 12.7 and 12.2 kJ/mol. The paste was microbiologically stable with no significant changes (P>0.05) in physicochernical characteristics during storage. (C) 2003 Elsevier Ltd. All rights reserved.