The Effect of Deprotonation on the Reaction Kinetics of an Oxygen Scavenger Based on Gallic Acid

Astrid F Pant,Matthias Reinelt,Didem Özkasikci,Siegfried Fürtauer

doi:10.3389/fchem.2019.00680

Abstract

Oxygen scavengers based on gallic acid (GA) usually contain a base that establishes the alkaline conditions necessary for the humidity-induced scavenger reaction. Here we measured the effect of sodium carbonate (Na2CO3) and sodium hydroxide (NaOH) on the reaction kinetics of such scavengers. The time-dependent oxygen absorption of aqueous GA solutions (pH 2.9–13.8) was determined and the results were described using a second-order kinetic model. We calculated the degree of deprotonation (DoD) of GA in the solutions from the species distribution curves, and assessed the effect of the DoD on the reaction kinetics. This revealed that both the reaction rate coefficient k and the scavenger capacity n were significantly affected by the DoD. If the DoD fell below 0.25, there was no significant reaction. Although k increased with the DoD, n reached a maximum at DoD = 0.6–0.7. In principle, target DoD values can be achieved using any base, but a strong base is more efficient because lower quantities are required. In our experiments, the amount of Na2CO3 required to reach the maximum DoD was more than twice that of NaOH. Our results provide the basis for the functional design of active packaging systems incorporating optimized GA-based oxygen scavengers.

Highlights

Gallic acid (GA), known as 3,4,5-trihydroxybenzoic acid, is a natural polyphenol found in many plants either as a free acid or as a component of tannins (Shahidi and Naczk, 2003; Belitz and Grosch, 2013)
O2 scavengers based on GA have a high O2 absorption capacity and can be integrated into packaging films (Ahn et al, 2016; Pant et al, 2017)
We found that the amount and type of base added to an aqueous GA solution changes the O2 absorption properties of the system in terms of the reaction rate constant and the maximum absorption capacity

Summary

Introduction

Gallic acid (GA), known as 3,4,5-trihydroxybenzoic acid, is a natural polyphenol found in many plants either as a free acid or as a component of tannins (Shahidi and Naczk, 2003; Belitz and Grosch, 2013). When combined with a base, GA can be used as an oxygen (O2) scavenger for packaging applications, preventing the oxidation of sensitive food products (Langowski and Wanner, 2005). O2 scavengers based on GA (e.g., a mixture of GA and sodium carbonate) have a high O2 absorption capacity and can be integrated into packaging films (Ahn et al, 2016; Pant et al, 2017). Depending on the film structure, the scavenger can either improve the barrier function of the packaging material or absorb residual O2 from the headspace. The scavenger function is activated by humidity derived from the product or in the environment (Wanner, 2010). The O2 scavenging function of GA-based O2 scavengers relies on the autoxidation of GA in alkaline solution. GA is a weak polyprotic acid (Figure 1) with four acidic protons that can be transferred to an acceptor base

Objectives

Methods

Results

Conclusion