Freezing is a great way of preserving our food without the use of any added ingredients and with minimal loss of essential nutrients. Clarence Birdseye was the first to develop the commercial freezing of foods in 1923. However, not all foods freeze very well. This practical looks at some of the differences between them.
Get some examples of different fruits, say strawberries, apple, banana, black currants. Take a few of each and freeze them in an ordinary freezer. Leave them for about one week. Take some other samples of these fruits and cut them open with a very sharp knife (scalpel would be ideal but a craft knife is good too). Using a microscope on low power or a good magnifying glass, study the structure inside.
When the fruits have been in the freezer for several days, bring them out and let them thaw. Some might be quite messy so use plates to collect any juices. Compare the differences in internal structure of the fruits with what happens when they are thawed out.
Strawberries have a large, cellular structure with lots of water. When they're frozen slowly, as in a domestic freezer, the water damages that structure so on thawing, they are soft and mushy. Great if you want to make freezer jam (just freeze them with some sugar) but not so good if you want to decorate a flan. Apple pieces freeze well because the cells are much stronger and contain a little less water than strawberry cells. Even so, when thawed out they may be a bit softer than when they started. Quick freezing in liquid nitrogen (-196 oC) preserves the structure much better. The ice crystals that form are very small and stay inside the cells so that they don't destroy the walls.
You could make a list of fruits that freeze well and those that don't. Compare their structure and see if this relates to their freezability. It is also possible to extend this work to a much wider range of foods and examine what works and what doesn't.