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Thermal decomposition and kinetic studies of solid riboflavin using model-free methods

Posted on 21. February, 2015.

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Riboflavin (vitamin B2) has conjugated double bonds and nitrogen in its ring structure and commercially can be produced by chemical synthesis or microbial fermentation. It is present in most living systems, and in milk, eggs, meat products and vegetables which are essential for normal growth and development of the body, production and regulation of essential hormones, and formation of red blood cells.

Moreover, riboflavin has an essential role in metabolising carbohydrates, proteins, lipids, and is crucial for the development of biological energy in the electron-transport system. Riboflavin is an active part of the coenzymes of flavin mononucleotide (FMN) and flavin adenine dinucleotide (FAD), which catalyse various redox reactions and play central roles in several dehydrogenases and oxidases.
The process of thermal decomposition, primarily involving temperature and heating rate, influences riboflavin in food processing. It is relatively stable during thermal food processing and various food storage conditions but in the presence of light it can be very easily reduced and oxidised by accepting or donating an electron or hydrogen atom. The investigation of the kinetics and reaction mechanisms of solid compounds is a demanding task with complex results arising from a great variety of features such as the formation and growth of new nuclei, diffusion of reaction products, and the physical state of the reagents.

In this paper, the thermal stability and decomposition kinetics of riboflavin were investigated by nonisothermal thermogravimetric analysis (TGA) experiments in an inert atmosphere. For kinetic analysis, riboflavin was heated from room temperature to 800 oC with five different heating rates (5,10,15,20 and 30oC min-1). From the thermal decomposition process, it was found that there are two main stages of pyrolysis. In the DTG thermograms, the temperature peaks at maximum weight loss rate changed with varying heating rate. The kinetic parameters of decomposition including apparent activation energy (Ea) and lnA (pre-exponential factor) under an inert atmosphere have been evaluated from the model-free isoconversion methods of Kissinger–Akahira–Sunose (KAS), Flynn–Wall–Ozawa (FWO) and Friedman over the whole temperature domain. It was found that values of Ea and lnA decrease with increasing conversion (α). The reaction order does not have a significant influence on the process because of the high value of the pre-exponential factor.
The apparent activation energy distributions with conversion calculated by these methods ranged between 15.18 and 236.55 kJ mol-1, andvaried over a broad range in a complex manner depending on the heating rate.

Read the full article in  Progress in Reaction Kinetics and Mechanism, Volume 40, Number 1, 2015, pp. 86-94.

Authors: Naushad Ahmad, Manawwer Alam and Meshary Ayad N. Al-Otaibi

Keywords: riboflavin, nonisothermal degradation, solid-state kinetics, isoconversional methods


Image: Riboflavin