The relationship of processing method to the physicochemical changes which occur in processed soft fruits Public Deposited

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  • Large amounts of plant tissue are used in the production of many canned, frozen and dried foods. Some of this material is unsatisfactory in appearance and texture, despite careful processing from high quality raw materials. In the present study, the changes induced in soft fruits by controlled heating and freezing treatments were of prime concern. Strawberries and tomatoes were canned and frozen in 60° B sucrose syrup, stored for 3-6 months and examined by physical and chemical methods. Measurements of the texture, pectins, hemicelluloses, cellulose, nitrogen, ash and ash constituents, were made on raw, frozen and canned samples. The tissues were sliced, macerated with 0.4 percent sodium hexametaphosphate, and screened. The cell suspensions were dialysed and stored at 35°F, with toluene as a preservative. The cell suspensions were analyzed similarly to the tissue samples, and the reactivity of the cell suspensions with sugars and electrolytes was investigated. The results indicated the following conclusions: 1) No significant difference was found between the effects of canning and freezing on the texture of the fruits, as determined by the Shear Press, although major differences were found between the fresh and processed samples. 2) Drained weights were lower when the fruits were canned than when frozen, especially in the strawberries. 3) Histological changes brought about by the treatments were more marked in the frozen than in the canned fruits. Freezing caused severe breakage of cell walls, plus some cell separation, whereas canning tended to cause more shrinkage of cells without much breakage or separation. Adhesion between cells was increased in the heated packs. Damage to cells increased as cell size increased. The study showed that more emphasis should be placed on selecting varieties of fruits for processing which have a smaller mean cell size, and thicker cell walls. 4) Aqueous suspensions of parenchyma cells were found to be a useful means of studying the several mechanisms involved in textural and drained weight changes. 5) No sorption affinity was found between dialysed, depectinized parenchyma cells and sucrose or glucose. 6) Dialysed cells were negatively charged, highly hydrated, and formed viscous suspensions at low concentrations. 7) The addition of small amounts of cations, particularly H⁺ ion and Al⁺⁺⁺ ion, precipitated cell suspensions, by suppressing the ionization of the carboxyl group or by salt formation. 8) A new hypothesis was proposed which assigned an important role to the acids and other electrolytes of fruits, in governing the textural and drained weight characteristics of such products. It was proposed that the quantity of acids and other electrolytes which become distributed throughout the cell during processing, causes a reduction of cell hydration including a shrinkage of cell walls, which could be partially reversed during subsequent storage.
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