Influence of sugars and polyols on the thermal stability of purified tomato and cucumber pectinmethylesterases: a basis for TTI development

Abstract

Tomato and cucumber pectinmethylesterases (PME) were extracted and purified by affinity chromatography using kiwi PME inhibitor (PMEI) as a ligand. The optimal pH ionic strength combination, which led to a maximal thermostability of purified cucumber PME was identified as 6.7 and 1.25 M NaCl, respectively. The effect of sugars such as sucrose and trehalose and the effect of polyols including mannitol, sorbitol, lactitol and glycerol on the thermostability of purified PME was evaluated based on the residual enzymic activity after isothermal treatments performed in temperatures between 55 and 77 °C. Under all conditions, purified tomato PME showed a first order inactivation curve, while purified cucumber PME showed a biphasic inactivation curve. Purified tomato PME showed a maximal thermostability in citrate buffer, pH 4.5 complemented with glycerol 50% (v/v) ( D 77 °C -value of 6.62 min and z-value of 5.34 °C). The heat stable fraction of purified cucumber PME exhibited a maximal thermostability in Bis–Tris buffer, pH 6.7, 1.25 M NaCl complemented with glycerol 60% (v/v) ( D 75 °C -value of 2.79 min and z-value of 5.23 °C). The potential of both PME to be used as time–temperature integrators (TTIs) to monitor pasteurization process is discussed. Finally, when calculating the equivalent number of hydroxyl groups ( nOH) provided by each stabilizer per volume unit of enzyme solution, it could be observed that the D-value for purified tomato PME was exponentially related to the nOH. This observation may be used as a predictive tool for enzyme stabilization. The influence of the OH source on the thermal stability was also investigated in terms of nature of the OH source (nature of the molecule and level of polymerization of a polyvinyl alcohol, PVA).