I do not concur the defrosting and refrozen in all cases. I go along with the following:
Why Ice Crystals Form
The most common reason large ice crystals form in your frozen food is slow, uneven freezing. Ice crystals do not form during more rapid freezing because the food molecules cannot form a six-sided snowflake before the freeze is complete. Slower freezing results in larger, less-manageable ice crystals that cause damage to the food. In the case of fresh fruits and vegetables, ice crystals can form because you have not dried the produce completely. This extra water on the surfaces of the fruits and vegetables eventually leads to a mushier product.
Preventing Ice Crystal Formation
You don't want to go to all of the trouble of freezing food just to end up with a mushy mess. Prevent large ice crystals from forming by taking the following steps: Encourage food to freeze rapidly. If your freezer has a quick freeze shelf, use it for items you wish to freeze. Never stack items you wish to freeze, instead distribute them throughout your freezer to allow for more uniform freezing. When the items have frozen solid, you can move them around and stack them as you wish. Dry fresh foods thoroughly before freezing. If you are freezing strawberries or green beans from your garden, it is important to be sure they are completely dry to avoid ice crystal formation. Keep your freezer at or below a temperature of 0 degrees Fahrenheit if possible, as lower temperatures keep frozen foods more stable.
Read more :
http://www.ehow.com/info_8119164_causes-ice-crystals-frozen-food.html