Physico - chemical stability of tomato products

The effect of some physical processes and enzymatic hydrolysis on the physicochemical properties of tomato suspensions was studied.Concentration degree has a large effect on the apparent viscosity and the storage modulus of suspensions after being diluted to a standardized water insoluble solids level. Besides decrease in average particle size, microscopic fracture of the cellulosic microfibrillar network during concentration are thought to be responsible for this phenomenon. By wet sieving it was shown that the bulk of the particles has a size between 45-180 μm. The tomato cell wall seems to be highly deformable. The 90- 180 μm wet sieve fraction had highest apparent viscosity and yield stress as well before as after homogenization.Homogenization of tomato suspensions as well as of strawberry sauce led to an increase in the apparent viscosity and storage modulus, whereas that of apple sauce led to a decrease in the apparent viscosity. The difference in behaviour upon homogenization is due to a difference in fracture behaviour of the plant cells, which is probably a result of the cell wall structure, especially the microfibrillar cellulose structure.Incubation of tomato suspensions with highly purified well specified polysaccharide degrading enzymes resulted in a decrease in rheological parameters. By homogenization the apparent viscosity increased to higher values compared to that of the non enzyme treated tomato suspension. The enzyme preparations gave rise to more serum separation. Hydrolysis of diluted hot break paste by pectin esterase from oranges or fungi resulted in a much higher yield stress, which is probably due to the formation of a calcium pectinate network. The apparent viscosity became only slightly higher. Serum viscosity increased initially, after which it decreased to about 50% of the original value.The main physical problem of tomato suspensions is the formation of a serum layer on top of it. Several mechanisms are responsible: uniaxial compression of the weak particle network due the gravitational force and drainage of serum as a result of unevenness in the surface. The physical behaviour of tomato suspensions can be better understood by considering the tomato cell wall as a concentrated, composite gel consisting of cellulose microfibrils embedded in a "jelly" matrix of pectic and hemicellulosic substances