Impact of treatment with oxidative bleaching agents on walnut (Juglans Regia L.) shell chromatic parameters

Abstract

Walnuts were formerly marketed in an unbleached condition, but the demand of the trade has led to the general practice of bleaching. Some walnuts are still sold in an unbleached condition, but most of those destined for out-of-state markets must be bleached. The objectives of this study are to present the effect of different bleaching agents on walnuts (Juglans Regia L.) shell. Materials and methods. For color quantifying it was used the most popular color space system for measuring color of agricultural products – the Commission International de l’Esclairage (CIE) color system which uses the L*, a*, and b* indices. Also, the total color difference (E), a single value which takes into account the differences between the L*, a*, and b* of the sample and standard was calculated. Results and discussion. Analyzing the obtained values for cromatic parameters for each of used bleaching agents it was esteblished that those vary depending on the bleaching agent, concentration, pH of the medium, temperature and retention time of walnuts in bleach. The concentration has a major significance, with its growth reduces the time required to achieve an expected result. The pH of the medium plays an important role in the catalytic oxidation. Speed of walnut shell bleaching and brightness (L) increase with increasing pH from 3 to 10. The studied agents form the following series of bleaching activity: Na2B2O4(OH)4 >Okoron 12>H2O2 > Ca (ClO)2. It has been established that the optimal conditions for bleaching with Na2B2O4(OH)4 are: C = 5%, pH = 10; t =60 ºC. Conclusions. All bleaching agents proved to be suitable to lighten the walnut shell. In comparison with the widely used H2O2 the ageing process developed simultaneously, but there were differences in the total lightness change, the Sodium perborate treated samples stayed lighter.