- Preservation of food by freezing is a common method
of processing. Of relatively recent origin is the freeze-drying
method. This method has several advantages over
other methods of preservation, but is not free of problems,
one of which is the maintenance of appearance and
In the present study, the effects of freezing, thawing,
freeze-drying, and reconstitution on the physico-chemical
properties of Northwest variety strawberries
with particular reference to textural characteristics
were investigated. Very little prior work is reported
with regards to the effect of these factors on fruits.
Strawberries were frozen at two different rates of
freezing - slow frozen at 0°F and quick frozen in a
blast freezer at -20°F. Frozen berries were examined
by physical and chemical methods to evaluate the changes.
In a separate study, berries were thawed under standard
conditions of relative humidity and temperature and the drip collected for analysis. Thawed berries were examined
by both physical and chemical methods. To study the reconstitution
behaviour of the berries, they were freeze-dried
and reconstituted under standard conditions of berry
to water ratio, temperature of water and time of reconstitution.
Reconstituted berries and the remaining
solutions were then examined. Measurements of the texture,
color, total solids, alcohol insoluble solids, pectins,
cellulose, ash, and constituents of ash were made
on raw, frozen, thawed, and reconstituted berries. The
results indicated the following conclusions:
1. An increase in percent soluble solids, pH, and
titratable acids and a decrease in total solids and AIS
of the berries were observed upon freezing. Slow frozen
berries showed more pronounced effects. Smaller berries
were observed to change the least. No significant differences
were found in the other constituents.
2. Strawberries lose weight upon thawing depending
on the chemical composition of the berries. An inverse
relationship was observed between weight loss and total
solids, AIS, pectic substances, cellulose, ash and mineral
content of the berries. With gradual increase in the
weight loss upon thawing, there was a progressive softening
of the berries. A direct relationship was found between
weight loss and the amount of pectic substances,
sugars, titratable acids, ash, and minerals found in the drip. Slow freezing resulted in higher weight loss and
smaller berries lost the least.
3. A linear relationship exists between weight loss
of the berries upon thawing and the area of the drip.
This method offers a convenient and quick procedure for
evaluation of the quality of frozen strawberries, with the
added advantage of collecting the drip and storing it for
further analysis at a convenient time.
4. Ability to reconstitute was found to be related
to rate of freezing and size of berry. Quick freezing
and smaller berries gave the best results.
5. Direct relationship was found between the degree
of reconstitution and total solids, AIS, pectic substances,
cellulose, ash, and minerals. The amount of
pectic substances, sugar, titratable acids, ash, and minerals
leaching out of the berries into the reconstituting
solution were smaller and were independent of the degree
6. Individual berries showed a large variation in
their chemical constituents, which was responsible for
the differences in the thawing and reconstitution behavior
of Northwest strawberries.