The plastic is quite thick. I've even had goats get into where it's stored and jump onto the bags with sharp hooves and still not puncture it. Not something I want them doing, but it's happened. We've never opened a bag that was spoiled, but my dealer will replace any bags that are bad when opened. You do find yeast colonies, which are fantastic natural probiotics, but it is very obvious that it is not mold. It smells a little like yogurt. There is more info about it on their website. The way Chaffhaye smells it would be virtually impossible not to know if it has spoiled, IMO. It is a very distinctive, sweet, and pleasant smell. As far as using it in a reasonable amount of time- my goats go through it too fast to worry about it not being fresh. I've had bags open for as long as 10 days even in the heat and humidity of Summer without spoilage though.<br /><br />__________ Sun Dec 16, 2012 10:40 am __________<br /><br />So far so good! They certainly enjoy it.