Determination of quality parameters of tomato paste using guided microwave spectroscopy

Guided microwave spectrometry was evaluated for in-line determination of quality attributes, soluble solids content, titratable acidity and Bostwick consistency of tomato pastes in continuous processing. The spectrometer was tested for two processing operations, dilution and evaporation. A total of 34 tomato paste samples were prepared by blending 9 different tomato pastes together in various proportions with final Bostwick consistency ranging from 2 to 11 cm. During the processing operation, spectra of 5 samples were obtained when different soluble solids contents were achieved. At the same time, standard reference methods were used to measure the parameters of interest on the same samples taken from the processing flow loop. The spectrum was recorded from 136 to 2690 MHz in 2 MHz steps. Partial Least Square (PLS) was employed to produce prediction models for each quality attribute from the collected microwave spectra. Cross-validation was performed to validate the calibration models. These models demonstrated good predictability of the soluble solids content, titratable acidity and Bostwick consistency in both dilution and evaporation conditions, as shown by their high correlation coefficient (r) and low root mean square error of cross-validation (RMSECV) values. Interval PLS (iPLS) was applied to optimize the models by selecting the most relevant and non-noisy regions in the spectra. The iPLS models showed improved accuracy for prediction of all three of the parameters, especially for Bostwick consistency (r > 0.92 and RMSECV < 0.7 cm). These observations suggest that guided microwave spectroscopy gave accurate estimation of soluble solids content, titratible acidity and Bostwick consistency of tomato paste. Different processing conditions required varying prediction models for the changes in the dielectric properties of tomato pastes.