Recently, we made a variety of cocktails from Ottolenghi and Scully's NOPI book. Now, we are left with two sugar syrups. According to the book, they can be stored a few weeks in the fridge. Skip ahead a month or two, and we're now left with two bottles of syrup that I'm not sure whether they are still safe to consume. As far as I'm concerned, the sugar acts as a preservative (like in jams) and it should be no problem; however, I'm a bit reluctant to ignore the advice of a distinguished chef - but perhaps he was just referring a change in flavor.
The syrups in question are 50/50 water and sugar (in weight), infused with cloves or saffron, each boiled for about 10 minutes. There is also a recipe with fresh ginger that is not cooked but blitzed, which may be more of a problem than the others.
How long can a syrup be preserved? What, if any, are the signs of a spoiled syrup?