A Rapid tool for the stability assessment of natural food colours.

Natural food colours lack stability under a number of conditions such as pH variation, oxidation, hydration, heat treatment and, most importantly, exposure to daylight. Stability tests to assess shelf life of natural colours under light irradiation can be time consuming. Thus, an accelerated test carried out under high light intensity irradiation that can be related to normal daylight irradiation conditions is highly desirable. Samples of various natural colouring solutions were prepared in aqueous model matrices at a range of pH values to mimic the majority of food matrices, pasteurised and irradiated under normal D65 light (0.2 W/m(2)) at 25°C, and in parallel under high light intensity irradiation (30 W/m(2)) at 3 different temperatures (25, 35 and 45°C). Similarly to the already known Q10 parameters for temperature, acceleration factors QL for irradiation, were determined and used for the first time to obtain a link between colour degradation under normal and accelerated conditions. It was possible, using these acceleration factors, to greatly reduce the time required to predict and compare the shelf life stability for a series of natural colours in aqueous model systems.

Flavonoids: hemisynthesis, reactivity, characterization and free radical scavenging activity.

Phenolic compounds form one of the main classes of secondary metabolites. They display a large range of structures and they are responsible for the major organoleptic characteristics of plant-derived-foods and beverages, particularly color and taste properties and they also contribute to the nutritional qualities of fruits and vegetables. Phenolic compounds are also highly unstable compounds which undergo numerous enzymatic and chemical reactions during postharvest food storage and processing thus adding to the complexity of plant polyphenol composition. Among these compounds flavonoids constitute one of the most ubiquitous groups of all plant phenolics. Owing to their importance in food organoleptic properties and in human health, a better understanding of their structures, their reactivity and chemical properties in addition to the mechanisms generating them appears essential to predict and control food quality. The purpose of this work is an overview of our findings concerning the hemisynthesis, the reactivity and the enzymatic oxidation of some flavonoids and shed light on the mechanisms involved in some of these processes and the structures of the resulting products. The free radical scavenging activity of some of the synthesized compounds is also presented and a structure-activity relationship is discussed. The first part of this review concerns the synthesis and structural characterization of modified monomeric flavanols. The use of these compounds as precursor for the preparation of natural and modified dimeric procyanidin derivatives was then explored through different coupling reactions. The full characterization of the synthesized compounds was achieved by concerted use of NMR and ESI-MS techniques. The free radical scavenging activity of some of the synthesized compounds was investigated. The second part of this review concerns the enzymatic oxidation of several flavonols by Trametes versicolor laccase. Most of the major oxidation products have been isolated as pure compounds and their structures unambiguously established through spectroscopic methods. Correlation between the structure of the oxidation product and the substitution pattern of the starting materials allows mechanistic features of this transformation to be elucidated.