Conformational changes in serum pectins during industrial tomato paste production.

It is well-known that an irreversible decrease in serum viscosity occurs when tomato juice is concentrated by evaporation into paste. Several studies have suggested that the loss in serum viscosity is due to pectin depolymerization, caused by the high temperatures used during industrial tomato paste production. This study demonstrates that conformational changes in pectin may play a more important role than pectin depolymerization in the irreversible loss of serum viscosity during industrial tomato paste production. Samples of tomato juice, processing intermediates, and paste were obtained from a commercial producer in California. After dilution to 5 degrees Brix, tomato serum was obtained by centrifugation at 15000 g for 10 min. Weight average molecular weight (M(w)) and root-mean-square (rms) radius of the polymers in the tomato serum were determined using high-performance size-exclusion chromatography with multi-angle laser light scattering and refractive index detectors (HPSEC-MALLS-RI). Serum viscosity decreased throughout the juice concentration process, especially at later stages, where the processing temperature reached a maximum of 90-95 degrees C. In parallel with this decrease in serum viscosity, there was an increase in the soluble pectin concentration. Analysis of the M(w) distribution of the tomato serum showed that solubilization of pectin occurred across the entire polymer distribution range. The M(w) changed from 2.62 x 10(5) g/mol in the juice to 2.61 x 10(5) g/mol in the paste, indicating that minimal depolymerization occurred. However, the rms radius distribution indicated that the pectin conformation became more compact as the juice became more concentrated. Conformational plots revealed that serum pectins in the hot-break tomato juice and at the early stages of concentration behaved as extended coils, having shape factors of about 0.40. In processing intermediates taken from later stages in the process and in the paste, the shape factor changed to about 0.25, indicating a more compact conformation. This conformational change correlated with the observed decrease in serum viscosity in the paste production process. This result is consistent with a Flory-Fox-type relationship between viscosity, rms radius, and M(w). The conformational change may be due to increased polymer-polymer interaction brought about by the concentration process.