This is a function of the freshness valve on the side of the bag. The valve is designed to let air out but not in. After being filled, our bags are stacked on pallets prior to being sent to your retailer. Bags on the bottom of the pallet tend to let more air out through their one-way valve due to the weight of product on top of them. Changes in temperature and atmospheric pressure may also cause a change to the air pressure in the bag which, upon reaching a set level, will cause the valve to open and release the excessive pressure. When this happens, the bag will take on a vacuum-packed appearance. There is nothing wrong with the food contained in such a bag, and it is completely safe to use as normal.