Colostrum can be refrigerated for only about 1 week before quality (immunoglobulin or antibody concentration) declines.  If you store colostrum, unfrozen be sure that the refrigerator is cold (33-35°F, 1-2°C) to reduce the onset of bacterial growth.  If the colostrum begins to show signs of souring, the quality of the colostrum is reduced.  The immunglobulin (very large protein) molecules in colostrum that bring passive immunity to the calf will be broken down by the bacteria, reducing the amount of immunity that the colostrum can provide.  Thus, it is important that colostrum be stored in the refrigerator for only a week or less. 
 
How long can the frozen colostrum be stored?  We often answer this question flippantly by saying, "just as long as you would store frozen fish to eat!"  Colostrum may be frozen for up to a year without significant breakdown of the immunoglobulins.  However this is one example where improved technology is not in our favor.  Frost-free freezers are not the best for long-term colostrum storage.  They go through cycles of freezing and thawing that can allow the colostrum to partially thaw.  This can greatly shorten colostrum storage life.  Freezing colostrum in 1 or 2 quart bottles or 1 quart in 1 or 2 gallon zip-closure storage bags is an excellent method of storing colostrum.  Many producers have had great success using the zip-closure bags.  Use two bags to minimize the chance of leaking, and lay them flat in the freezer.  By laying the bags flat, the rate of thawing can be increased, thereby reducing the delay between time of calving and feeding.  The freezer should be cold (-20°C,  -5°F) - it's a good idea to check your freezer occasionally.  Much more information about colostrum use and transfer of passive immunity is available from the OSU Fact Sheet F-3358 Disease Protection of Baby Calves.