Thermal degradation kinetics of the phycocyanin from Spirulina platensis

Abstract

The cyanobacterium Spirulina platensis is a source of pigments, such as phycocyanin, which is used in the food, cosmetic and pharmaceutical industries. The thermal degradation kinetics of the liquid extract at pH values of 5, 6 and 7 was studied, evaluating its stability between 50 and 65 °C. The kinetic model was assumed and validated as being of the first order. Between 50 and 55 °C the extract was more stable at pH 6 and between 57 and 65 °C at pH 5, but was shown to be increasingly unstable at pH 7 as the temperature of the treatment increased. The addition of sorbitol between 10 and 50% (w/w) in the treatment at 62 °C for 30 min increased the half-life values of the phycocyanin extract, proving that its de-colorization was related to degradation of the protein chain.

Introduction

The cyanobacterium Spirulina platensis has been considered to be an important subject for biotechnological research due to its economical, ecological and nutritional importance [1]. S. platensis is a blue-green microalga which can produce large quantities of high value products such as phycobiliproteins [2]. Phycobiliproteins are a brilliantly colored family of water-soluble proteins bearing covalently attached, open-chain tetrapyrroles known as phycobilins [3]. On the basis of their visible absorption properties, the phycobiliproteins have been assigned to four spectroscopic classes: phycoerythrocyanin, phycoerythrins, phycocyanins and allophycocyanin [4].

Phycocyanin is the main pigment produced by the cyanobacterium S. platensis and may reach 20% in dry weight of the cell protein [5]. This phycobiliprotein is not only used as a natural, nutritious and coloring ingredient for foods and cosmetics, but also as a potential therapeutic agent in the treatment of oxidative diseases and as fluorescent marker in biomedical research [6], [7], [8].

The current tendency to use natural pigments has turned phycocyanin into an attractive bioproduct. In Europe, the search for natural dyes is growing since the artificial dyes are generally considered to be toxic or somehow dangerous [9]. Color is an important attribute related to the visual appeal and to the quality of nutritious products [10]. Thus excessive de-colorization during thermal processing compromises commercialization, and the kinetic parameters such as the reaction order, reaction constant and activation energy can supply profitable information on the change in quality of a food occurring during thermal processing [11].

The kinetics of color degradation have been considered to follow a first order reaction as shown by Ahmed et al. [12], Ahmed et al. [13], Gunawan and Barringer [14], Weemaes et al. [15] and Steet and Tong [16]. Due to the possible denaturation of the protein fraction of the phycocyanin and consequent loss of color, it is of interest to use stabilizing agents to maximize the shelf life of the protein solutions of greater biotechnical and pharmaceutical interest [17], [18]. Sugars and polyhydric alcohols have been used to stabilize proteins, and are now being used widely as stabilizing agents in the food industry as well as in pharmaceutical formulations, in concentrations inoffensive to the ingestion [18].

The objective of the present work was to study the thermal degradation kinetics of the aqueous extract of phycocynin from S. platensis, in the temperature range from 50 to 65 °C at pH values of 5, 6 and 7, and with the addition of a stabilizing agent under the conditions showing greater instability of the phycobiliprotein.