One idea is to use invertase, an enzyme derived from yeast, as an ingredient to bring down water activity in your truffle centers. Additionally, syrups such as honey will work similarly. Or you may also use potassium sorbate. Talk to your ingredients suppliers to determine the right amount to use for your specific truffle recipes.
Do note that if you claim that your products are all natural, you should pay close attention to how you choose to extend shelf life of your products. See this story on a bagel company who received a warning from the FDA for labeling their bagel as all natural because their blueberries contained potassium sorbate.