Corn syrup is a staple in candy-making and some baking projects, but isn't generally used in home kitchens on a day-to-day basis. Because of this, the same container of corn syrup can sit in the pantry for months or even years. Because corn syrup can go bad, it's important to know the telltale signs that the bottle belongs in the trash.
According to the frequently asked questions section on the Karo Syrup website, corn syrup can be consumed safely as long as it appears normal. However, the website mentions that there is a best-by date stamped on each bottle, and that it's best if the product is consumed by that date. This date should be present on all containers of corn syrup. If the corn syrup is past that date or doesn't have a date stamped on it, the likelihood that it has spoiled is greater.
The Karo Syrup website states that it's normal for clear corn syrup to become yellowish over time, especially if it is stored in direct sunlight. This is not an indicator of spoiled corn syrup. As long as the corn syrup hasn't passed its best-by date, the site says it is still safe to consume.
If the corn syrup appears cloudy or there is a haze on the bottle, the product probably has begun to mold. This is a common thing with corn syrup, especially if the bottle isn't capped tightly. If either of these conditions appear in the corn syrup, throw it out.
Fermentation is also a common problem in open bottles of corn syrup, particularly if foreign food particles have made their way into the bottle. If the corn syrup appears to be bubbly or has a foamy film, it should be discarded.
If you aren't sure whether corn syrup has spoiled, open the bottle and sniff the syrup. If it smells like alcohol, bitter or oversweet, don't ingest it. This can also be an indicator that the corn syrup has either molded or fermented.