- 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
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
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
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.