Photostability of l-tryptophan in aqueous solution: Effect of atmosphere and antioxidants addition.

l-Tryptophan (l-Trp) is an amino acid important in nutrition, and mainly provided by food supplements. However, it is known to be unstable under light irradiation, which is an issue for the nutrition and feed industry. In the present study, the photostability of l-Trp was studied in acidic aqueous solutions under air and under an inert atmosphere, N2. The photodegradation was followed using UV-visible and fluorescence spectroscopy after photolysis. Moreover, molecular orbitals and bond dissociation energies calculations, and electron spin resonance spectroscopy were performed. From all these results, a photodegradation occurring through a free radical pathway was suggested. Interestingly, several antioxidants were tested to improve the photostability of l-Trp, especially during irradiation under air, since the l-Trp was evidenced to be much less stable under air than under N2. The results showed that sodium benzoate or EDTA were not efficient, but antioxidants such as chlorogenic acid, ascorbic acid or potassium sorbate improved significantly the photostability of l-Trp in acidic solutions.

Ascorbic Acid Derivatives as Potential Substitutes for Ascorbic Acid To Reduce Color Degradation of Drinks Containing Ascorbic Acid and Anthocyanins from Natural Extracts.

Ascorbic acid is widely used in the food industry as a source of vitamin C or as antioxidant. However, it degrades quickly in beverages at acidic pH and can accelerate the degradation of anthocyanins, natural dyes used in beverages, leading to a loss of color. In this work, we investigated the possibility to replace ascorbic acid by ascorbic acid derivatives to prevent its degradation effect on anthocyanins from natural extracts (black carrot, grape juice, and purple sweet potato). For this, the thermal and photolytic stabilities under air and under N2 of ascorbic acid (as reference) and of some ascorbic acid derivatives (3-O-ethyl-l-ascorbic acid, 2-O-α-d-glucopyranosyl-l-ascorbic acid, l-ascorbic acid 2-phosphate sesquimagnesium salt hydrate, l-ascorbyl 2,6-dibutyrate, glyceryl ascorbate, (+)-5,6-O-isopropylidene-l-ascorbic acid), soluble in aqueous model beverages, were studied alone and in the presence of anthocyanins from the natural extracts in citrate buffer at pH 3. The stability was followed by UV-visible spectrometry. To extend the investigation, some properties of the ascorbic acid derivatives (pKa, oxidation potential, bond dissociation energy, ionization potential) were also determined. Moreover, the addition of chlorogenic acid was examined to further stabilize the mixture of anthocyanins with 2-O-α-d-glucopyranosyl-l-ascorbic acid, a promising ascorbic acid derivative.

Thermal and Photochemical Stability of Anthocyanins from Black Carrot, Grape Juice, and Purple Sweet Potato in Model Beverages in the Presence of Ascorbic Acid.

Anthocyanins are natural dyes widely used in the food industry, but their chemical stability in beverages can be affected by the presence of additives. In the present paper, the interaction between anthocyanins and ascorbic acid (AA) is more particularly investigated. Ascorbic acid is an ubiquitous component in food products. In this study, the thermal stability at 43 °C and the photolysis stability in air and in an inert atmosphere (N2) of anthocyanins extracted from black carrot (BC), grape juice (GJ), and purple sweet potato (SP) were studied in the presence and absence of ascorbic acid (in citrate buffer at pH 3). Discriminating the main environmental factors (i.e., heat and light) affecting anthocyanin stability is a key point for better understanding the degradation pathways. The stability of the anthocyanins was followed by UV-vis spectrometry. Moreover, to understand the degradation mechanisms in both the presence and absence of ascorbic acid, various techniques such as fluorescence quenching, cyclic voltammetry, and electron-spin-resonance (ESR) spectroscopy were also used to furnish a full coherent picture of the chemical mechanisms associated with the anthocyanin degradation. In addition, molecular orbitals and bond-dissociation energies (BDE) were calculated to extend the investigation. Moreover, the effects of some supplementary stabilizers (chlorogenic acid, sinapic acid, tannic acid, fumaric acid, ß-carotene, isoquercitrin, myricitrin, green coffee bean extract, and rosemary extract) and sugars (sucrose, fructose, and glucose) on anthocyanins stability in the presence of ascorbic acid were examined.