Tomato juices and tomato juice concentrates : a study of factors contributing to their gross viscosity

The gross viscosity of tomato juice and tomato juice concentrates was found to be determined primarily by the water insoluble solids (WIS) content. The serum viscosity did not contribute to gross viscosity. The WIS consisted of whole tomato cells, vascular bundles and skin fragments. In general the WIS could be fractionated into 40-45% pectin, 25-30% hemicellulose and 30-35% cellulose. Highly branched as well as more linear pectin fragments were found to be present in tomato WIS. Xylans, arabinans and (arabino)galactans were involved in the attachment of pectic substances to the cell wall matrix. The presence of xylans, xyloglucans, (gluco)mannans and a limited amount of galactan in the hemicellulose was indicated. Pectin fragments, characterized by a low content of arabinose and galactose containing side chains, and esterified in such a way that they were degraded by both PG and PL, made an important contribution to gross viscosity. Cellulose was found to make a relatively smaller contribution to gross viscosity than these pectin fragments. The rigidity of the cell walls, caused by the cellulosic structure, seemed to influence gross viscosity at higher WIS concentrations. The differences which exist naturally in WIS composition were found to be too small to cause large differences In gross viscosity. Higher hot break temperatures were found to result in a more effective removal of WIS from skin material. The influences of hot break temperature, finisher temperature and size of finisher screen opening on the finisher operation were closely related to the efficiency of the finisher. The concentration of a tomato juice by evaporation resulted in a loss in gross viscosity, after dilution of the paste to the original strength. The causes for this "dilution-loss" were studied and could be related to the simultaneous concentration of tomato cells and, presumably, ions. The centrifugation-serum-concentration method resulted in a significantly decreased "dilution-loss".