As a result of chemical or physical reactions when heating/cooking foods:
1.will it make certain fruits(example plums,durian,banana,apple) to change from sour to sweet or vice –versa?
2.will it help to make foods that has gone slightly bad to be safer for eating?
Please also state the reasons for your answers.
Physical changes are changes that affect the form of a chemical/compound/element but not it's chemical composition. For instance, water (H2O) can be in the form of water vapor, ice, or liquid water. In all three cases, the chemical composition is the same - that is, it is still H2O. However, the three are different forms of water. When it is hydrolyzed, water forms hydrogen gas (H2) and oxygen gas (O2). In this case, it no longer is water. This is a chemical reaction.
Chemical changes (like the water example given above) are changes that involve a chance in the composition of the substance.
Cutting a paper into smaller pieces is a physical change since it still is paper - no change in composition. Burning paper, on the other hand, would entail changing the composition of paper - it turning into burnt paper or ashes.
In terms of food, chemical reactions occur during ripening of fruits, or when fruits go bad. A change in flavor during cooking or heating of food indicates that a chemical reaction has taken place. Most chemical changes are not reversible just as ashes can't be changed back to paper (*note however, that chemical reactions can be reversible via another chemical reaction). That said, one cannot change back the flavor of fruits once they are cooked or heated (of course, this is barring the use of additives).
At the same time, food that go bad undergo chemical changes. Some compounds in them might have disintegrated or have changed. Hence, it is not possible to make them good again. This is analogous to cooking egg. Once cooked, you cannot make it uncooked.