Although not a perfect test, your senses are usually the most reliable instruments to tell if your eggnog has gone bad. According to Eatbydate.com some common traits of bad eggnog are discoloration (which you will have to have noted your original eggnog color to begin with because it can range from very white to beige to very yellow in color), a lumpy texture and a very distinctive sour smell (which you will notice upon removing the lid or opening the carton).
These changes occur from lactic acid producing bacteria.
All types of eggnog last for a shorter period of time if they are not stored properly. So pay attention to the expiration date.
Whether you make it yourself or opt for a convenient carton from the grocery store, one thing is certain - the holidays aren't complete without eggnog! Check out some delicious ways you should be eating and drinking eggnog to get your fill during the holidays at Thekitchn.com.