I worked at a dog food store, and the vacuum bags will slowly get air over time, especially if stored upright rather than flat. I wouldn't worry. I have gotten bags of food that last my dog 6+ months and have never had it go bad. I try to keep it somewhat airtight, but I have just rolled down the top of the bag. You can smell if it is bad. Usually the fat goes rancid first. (This is living in northern West Virginia and in Connecticut.)
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First time Chihuahua owner with many years of dog experience!