You can tell when simple syrup has gone bad because it starts to grow cloudy. When properly made, simple syrup is clear in appearance. Any cloudiness is a sign that bacteria is starting to grow, and the syrup needs to be thrown away. If it’s been left for a while, the syrup may start to smell bad. It should then be disposed of immediately. Bacteria might be growing because there was an issue in making the syrup, it was stored improperly, or it’s been left too long.
Another way simple syrup can go bad is crystallization. This is when crystals begin to form in the syrup. This isn’t a health risk like bacteria, but it’s still a sign that there’s a problem with the syrup. If you’re making your own simple syrup, be aware that crystallization can occur at any stage.