Modelling kinetics of watercress (Nasturtium officinale) colour changes due onication treatments to heat and thermos

Watercress (Nasturtium officinale) colour changes due to blanching by heat and a combined treatment of heat/ultrasound (thermosonication) were studied in the temperature range of 82.5 to 92.5 °C. The application of thermosonication was intended to enable less severe blanching treatments and, therefore, improve the quality of the blanched product. The thermosonication blanching processes promoted changes of the green colour (a parameter) at a higher rate (Pb0.05), when compared with the heat blanching processes. No significant differences (PN0.05) were detected between heat and thermosonication blanching processes in terms of the colour parameters L, b and TCD changes. In both treatments, a fractional first order model fitted well the experimental data for L, a and b (RH 2 =0.99; RTs 2 =0.99) and TCD (RH 2 =0.92; RTs 2 =0.96) colour parameters. The chlorophylls content showed no significant differences (PN0.05) between thermally treated and thermosonicated watercress samples. The present findings will help to evaluate the effectiveness of thermosonication as a novel process to replace the classical heat treatment. Industrial relevance: Conventional blanching commonly results in severe losses or destruction of nutrients due to process intensity and extended process times. Consequently, the attempts to use the synergistic effects of heat and ultrasound (at least) for enzyme inactivation are of high relevance. The results, although not conclusive, indicate, that they may aid optimization of blanching processes. Watercress (Nasturtium officinale) is a vegetable, consumed raw or cooked in various kinds of recipes. Under refrigerated storage it has a short shelf-life of approximately 7 days, but by freezing a longer period for distribution and storage can be achieved. Prior to the freezing process, blanching has an important role in the inactivation of enzymes, microorganisms and in the stabilisation of colour. However, the degree of thermal treatment during blanching can also have adverse effects on the quality of frozen vegetables. It causes irreversible modifications to the cellular structure, many vitamins and minerals can leach into the blanching medium, and green chlorophylls may convert to yellow brown pheophytins. For this reason many alternative ⁎ Corresponding author. Tel.: +351 22 5580058; fax: +351 22 5090351. E-mail addresses: [email protected] (R.M.S. Cruz), [email protected] (M.C. Vieira), [email protected] (C.L.M. Silva). methods have been developed, but the conventional hot water blanching is commonly used (Katsaboxakis, 1984). The colour quality attribute strongly influences the consumer in the acquisition of the final product. In order for colour to be quantified objectively, the visual observing components needed to be taken into account are the object, light and the observer (Hunter Lab, 2000). Colour can be measured by a tristimulus colorimeter, represented by three co-ordinates in the colour scale. The L parameter represents lightness (L=0 black; L=100 white), a (red to green) andb (blue to yellow) (Ávila&Silva, 1999; Francis&Clydesdale, 1975; Lau, Tang, & Swanson, 2000; Martins & Silva, 2002; Nisha, Singhal, & Pandit, 2004; Tijskens, Schijvens, & Biekman, 2001). In the thermal processing of green vegetables, changes in colour, from bright green to olive green, are the result of the conversion of chlorophylls into pheophytins and pyropheophytins (Ihl, Monsalves, & Bifani, 1998; Lau et al., 2000; Martins Table 1 Initial characteristics of raw watercress