Dyes in food chemistry: empirical use versus descriptor-based design

In the last decades a plethora of old molecules able to change their colour with the pH, like sulfonphtalein dyes do, has arisen a new success. Some of them are reported in Figure 1. While synthetized mostly around the middle of the past century, and mainly employed as acid-base indicators, sulfonphtaleins have been intensively used almost in each branch of chemistry, directly in solutions or immobilized on sensors. For example, in marine chemistry, the use of molecules such as thymol blue (Fig. 1a), but also o-cresol red and m-cresol violet, had started since the early 1980s, for accurate measurements of the pH of marine waters, where the potentiometric ones manifested all their limitations [1]. The not-trivial question of the effect of dyes’ impurities on the colour change has also been discussed [2]. In this specific context, the proposed solutions always refer to the peculiar conditions such as high salinity and low temperatures of ocean waters. In biochemistry and medicine, surprisingly, indicators still play a crucial role in the detection of cellular abnormalities. Bromocresol purple (Fig. 1b) was originally proposed as reagent for colorimetric of albumin detection [3], and it has been recently included into a commercial assay [4] demonstrating the endless interest towards these old reagents. In food science and technology, sulfonphtaleins have intensively exploited for their capability to sense the different acidity of molecules developed in the headspace over a packed proteinaceous food allowing to establish the spoilage level. Despite of this wide interest, the choice of these dyes, especially in implemented devices, is often totally empirical. It means that the behaviour of some, sometimes many, of them is experimentally tested, and the effect on the specific phenomenon registered. The choice of the selected dye is valid only under the selected conditions, and there are scarce attempts to understand the reason of the choice. Food science gives exemplary cases: authors claim to employ a pH indicator to detect biogenic amines when the choice of the dyes reflects the rather limited presence of basic molecules in volatilome [5]. Conversely, the selection is made in lab condition and the possible change of logK caused by modified conditions (embedding of dyes into a sensor that causes a different hydrophobicity [6] or change as function of T) is almost never taken into account. A systematic study of the effect of temperature on logK obtained by potentiometric measurements of the most common sulfonphtaleins is presented. The reactions are all exothermic, i.e. the protonation constants decrease increasing T. On this basis it is possible to rationalize some previous findings, and once again the equilibrium constants prove to be above all a practical and universal tool for designing and predicting the behaviour of the receptor according to its application. The authors would like to thank the National Recovery and Resilience Plan (NRRP), Mission 04 Component 2 Investment 1.5 – NextGenerationEU, Call for tender n. 3277 dated 30/12/2021; Award Number: 0001052 dated 23/06/2022.