I prepare natural flavoured sugar syrups, I add glucose (10% of sugar quantity) to covert this sugar into invert sugar so that the syrup doesn’t crystallise. However the shelf life is too short, 2-4 weeks and I see bacteria developing. How to increase the shelf life without using chemical compounds? If I need to add anything as preservative I have decided to add only natural things. Can anyone suggest how to increase the shelf life without changing the taste?