As an expert sommelier and brewer, I have had plenty of experience with syrups, including fruit-flavored ones. When it comes to the shelf stability of fruit simple syrup, it is important to note that it is generally not as long-lasting as unflavored syrup. The natural sugars and flavors from the fruit can cause the syrup to spoil more quickly.
Unflavored syrup, such as a simple syrup made with just sugar and water, can be made in advance and stored in an airtight container in the fridge for up to a month. This is because there are no additional ingredients that can potentially spoil or alter the syrup's quality. I have personally made unflavored syrup in large batches to have on hand for cocktails and other recipes, and it has remained perfectly fine for the entire month.
However, when it comes to flavored syrups, especially those made with fruits, it is best to use them within two weeks of when they're made. This is because the natural sugars and flavors from the fruit can provide a breeding ground for bacteria and yeast, leading to fermentation or spoilage. I have learned this the hard way by keeping a batch of fruit syrup for too long, only to find it had a funky smell and taste.
To ensure the best shelf stability for fruit simple syrup, there are a few things you can do. First, make sure to use fresh, ripe fruit when making the syrup. Overripe or spoiled fruit can introduce unwanted bacteria into the syrup, increasing the chances of spoilage. Secondly, it is important to strain the syrup thoroughly after infusing it with fruit. Any leftover fruit particles can also contribute to spoilage.
In my experience, it is best to store fruit syrup in airtight containers in the fridge rather than at room temperature. The cooler temperature helps to slow down any bacterial growth and prolong the shelf life. I have also found that using smaller containers, such as glass jars or bottles, can be beneficial. This way, you can use up one container at a time without exposing the entire batch to air and potential contamination.
While unflavored syrup can be stored in the fridge for up to a month, fruit-flavored syrups should be used within two weeks. It is important to use fresh fruit, strain the syrup thoroughly, and store it in airtight containers in the fridge to maximize shelf stability. By following these guidelines, you can enjoy delicious fruit syrups without the risk of spoilage.