Stability of curcuminoids in turmeric oleoresin : effects of light exposure antioxidants and metal chelation

Abstract

Despite the increasing demand for natural colourants, their sensitivity to light is a major disadvantage and could pose restrictions to their utilization as food colourants in industry. Turmeric (Curcuma longa L.) is a tropical plant native to southern and south eastern tropical Asia. The most active colour components in turmeric roots are curcuminoids, which are used in many food and pharmaceutical preparations. However, curcuminoids are unstable and have been replaced by stable synthetic dyes such as sunset yellow. Hence, the purpose of this study was to evaluate the effect of storage of turmeric oleoresin with and without protection from external environmental conditions, including sunlight during daytime, on the stability of curcuminoid pigments, measured by high-performance liquid chromatography and correlated with L*a*b* colour space values and colour evaluation by a trained sensory panel. Turmeric oleoresin powders were placed in labelled, clear plastic jars and exposed to external environmental conditions during the day for a period of 10 weeks. Control samples were stored in a refrigerator at 4 °C. To determine the effect of protection of turmeric oleoresin powders from external environmental conditions on curcuminoid stability, jars were covered with aluminium foil and stored outside and exposed to external environmental conditions for a period of 10 weeks. Control samples were refrigerated at 4 °C. Over time of exposure to external environmental conditions, curcuminoid pigment degradation was evidenced, with 100% degradation observed after 9 weeks of storage, relative to the control. Curcuminoids were less sensitive to degradation when shielded from external environmental conditions, with a 22% reduction in degradation observed relative to turmeric oleoresin powders without aluminium foil protection after 10 weeks of exposure to external environmental conditions. Significant sensory differences were observed between turmeric oleoresin powders with and without protection from external environmental conditions compared to the control samples. In an attempt to enhance curcuminoid stability in the presence of external environmental conditions, the effect of antioxidants, tertiary butylhydroquinone (TBHQ) and ascorbic acid on the stability of curcuminoid pigments in turmeric oleoresin, was studied. The effect of divalent ion-curcuminoid complexation on the stability of curcuminoid pigments in turmeric oleoresin when exposed to external environmental conditions, was also investigated. Antioxidants were homogenously distributed onto the powdered turmeric oleoresin base at 0.02% (m/m). Samples were exposed to external environmental conditions for a period of 10 weeks. According to the total colour difference results (L*, a* and b* values), the addition of both TBHQ and ascorbic acid did not improve the colour stability of the curcuminoid powders after 10 weeks of exposure to external environmental conditions. The presence of ascorbic acid had no significant influence on curcuminoid stability, with the degradation trend following that of the control, which did not contain antioxidants. The effect of complexation of curcumin with divalent ion (Mg2+) by mechanical mixing was also investigated. After 10 weeks of exposure to external environmental conditions, no significant improvement in curcuminoid stability was observed after the addition of magnesium ions to turmeric oleoresin powders when compared to the control which did not contain magnesium ions. Correlation analysis was conducted to determine the relationship between percentage curcuminoid degradation, sensory scores and colour ratings of turmeric oleoresin powders that were exposed to external environmental conditions. A strong negative linear relationship was observed between percentage curcuminoid degradation and rating of colour. A perfect positive relationship was observed between chroma and b* values. Chroma also had a strong positive relationship with L*. Percentage curcuminoid degradation showed a weak positive relationship with both a* and hue angle values. Exposure to external environmental conditions negatively affected the colour stability of curcuminoid pigments in turmeric oleoresin powders and the level of pigment degradation was dependent on time of storage. Stabilization strategies investigated in this study, such as Mg2+- curcuminoid complexation as well as the addition of antioxidants (TBHQ and ascorbic acid) did not significantly improve curcuminoid stability in turmeric oleoresin powders. Constant storage of ingredients and products containing turmeric oleoresin at low temperature (<5 °C), together with light protection is vital to retard colour degradation.