Natural Blue Food Colour: (Natural Food Colorant Production and Membrane Separation)

Research output: ThesisPh.D. thesis


In recent years, there has been a growing tendency to avoid the use of artificial colorants and additives in food products, especially after some studies linked their consumption with behavioural changes in children. However, the incorporation of colorants from natural origin remains a challenge for food technologists, as these are typically less vivid and less stable than their synthetic alternatives. Regarding blue colorants, phycocyanins from cyanobacteria are currently in the spotlight as promising new natural blue colorants. Phycocyanins are proteins which blue colour results from the presence of the chromophore phycocyanobilin (PCB), a covalently attached linear tetrapyrrole. The applications of phycocyanins as food colorants are however limited, as they show poor stability in certain conditions of pH, light and temperature. Cleavage of PCB from the protein followed by careful product design is a possible approach to help overcoming the present stability issues of phycocyanin-based colorants. This thesis presents a process for cleavage of PCB from phycocyanin by solvolysis in alcohols as a first step towards the formulation of a new natural blue colorant.
A pre-treatment dia-ultrafiltration step for purification of phycocyanins from small-size additives was developed. The filtration process was optimized in terms of molecular weight cut-off, trans-membrane pressure, process temperature and feed concentration. In order to interpret the permeate flux behaviour observed for different feed concentrations, a mathematical model was developed. The model is based on permeate flux variations caused by three resistances in series: the membrane resistance, the fouling resistance and the concentration-polarization boundary layer resistance. Provided the initial conditions, the calculated parameters, and a set of differential equations, the model was able to predict the behaviour of the flux with a confidence level above 95 %.
Three different methods of cleavage of PCB by solvolysis were evaluated in this study: conventional reflux, sealed vessel heated in an oil bath, and microwave assisted reaction. The optimal method was found to be the sealed vessel method, as it produced an equal yield as the conventional method, but with a faster reaction rate and higher purity. The microwave assisted method, however, resulted in increased product degradation. Based on the reaction products observed, two competitive mechanisms are suggested: a concerted E2 elimination, and a SN2 nucleophilic substitution.
The stability essays on PCB showed that it is most stable when stored in dry form or in ethanol. PCB showed poor stability in water, although at moderately low temperatures and higher purity the degradation rates decreased. PCB was also found to be more sensitive to pH than phycocyanin. Regarding the stability with time, PCB showed a similar stability at pH 3, and worse at pH 5 and pH 7. The change from blue to green colour in acid conditions was attributed to protonation of the chromophore. However, the effect of protonation could be both prevented and reversed, by entrapment of PCB in sodium dodecyl sulphate micelles.
The results showed that cleavage of PCB from phycocyanin can be achieved by the sealed vessel method in a faster and in a more environmentally friendly way than by the conventional method. Some insights into the stabilization of PCB are also provided.
Original languageEnglish
Publication statusPublished - 21. Jun 2017

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