Physical and chemical attributes of a genetically modified fruit pectin.

摘要

Pectin is an important polymer used in the food industry as a thickening and gelling agent. Though pectin is ubiquitous in plants, chemical and structural differences among pectin molecules prevent most from being viable for industrial use. Enzymes found naturally in fruit cell walls during the ripening process impair many desirable attributes of fruit pectins, rendering them unsuitable for industrial applications. Pectin methylesterase (PME) is one such enzyme whose expression can be altered during ripening through the use of recombinant genetic engineering. Reduction in levels of PME results in increased degree of methylation and molecular size of pectin, greatly increasing the favorable quality attributes of processed tomato products, primarily viscosity. Reduction in PME activity also alleviates calcium deficiency-related disorders such as blossom end rot, generating savings from greater harvest yields. We hypothesize that an optimum level of PME activity exists such that the processing attributes and quality characteristics of tomato fruit pectins are optimized for industrially viable use. Steady-state shear ramps were performed to discern differences in rheology between prepared tomato samples of varying PME activity and ripeness stage, for whole tomato juice, separated juice serum, and tomato juice solids reconstituted with both water and tomato serum. Our results show that a reduction in PME activity greatly increases the final viscosity of prepared tomato samples, and the maturation process decreases overall viscosity through the degradation and demethylation pf plant cell walls and pectins. The difference in viscosity for juice was more distinct than that for serum, indicating the importance of PME activity on the fruit cell wall and water-insoluble pectins. Nuclear Magnetic Resonance (NMR) spectra and Gas Chromatography (GC) both indicated the presence of galacturonic acid residues in tomato serum, and NMR showed that these galacturonic acid residues are indeed from pectin. Furthermore, NMR spectra confirmed the effectiveness in down-regulating PME and its impact on the degree of methylation of serum pectins, confirming with rheological measurements that increased DM of pectin yields higher viscosity products. Practical application of this information will prove valuable in understanding the effect of genetically regulating enzyme activity in vivo and determining the optimal level of PME for creating tomato products with desirable quality attributes. Furthermore, the present work seeks to use tomato pectin as a model system for elucidating information regarding the relationship between genetic alterations of pectin and the effect on its chemical structure which affects the physical and functional attributes.