In vitro oral processing of raw tomato: Novel insights into the role of endogenous fruit enzymes

During consumption of fruits, the breakdown of the fruit tissue due to oral processing (chewing, mixing with saliva) may activate or increase the rate of endogenous enzyme activities via the disruption of the cell wall, cellular de‐compartmentalization and particle size reduction allowing the enzymes to reach their substrates. The aim of this study was to investigate the activity of one such endogenous fruit enzyme (pectin methylesterase (E.C. 3.1.1.11) during in vitro oral processing of raw tomatoes and associated changes in viscosity and microstructure. Oral processing of tomatoes purees was examined in the presence of artificial saliva at 37 ○C. In vitro oral processing was followed using immunofluorescence microscopy, apparent viscosity measurements, spectrophotometric and titrimetric techniques. Results demonstrated that pectin methylesterase had slight but significant activity in the tomato fruit during in vitro oral processing generating methanol as a function of oral processing time, which was further evidenced using immunolabelling techniques to detect methylated pectin epitopes. A significant shear‐thinning behaviour of the tomato puree was observed due to dilution and/or endogenous fruit enzyme activity. These results suggest that activity of other fruit enzymes, such as polygalacturonase, which catalysed the depolymerisation of unmethylated pectin chains might have resulted in a decrease in viscosity, which compensated for the increased potential for gel formation (if any) caused by PME. These interesting insights on role of endogenous fruit enzymes might pave the way to the understanding of fruit viscosity modification occurring in the mouth and help in rational design of new fruit based products.