Perhaps you may have noticed; most bread sold in plastic bags retain their freshness for more than a week, while the bagel you just bought in the morning simply becomes hard to bite at the end of the day. Since their basic ingredient is the flour, there must be a reason for this difference. 

Of course, if we also keep the bagel in the bag, it will remain fresh for some time more. However, nobody with a decent taste would ever like to eat that bagel a week later. Yet the bread, although stale, is still edible even after many days. 

Before delving into the reasons and roots of this difference, there is one more thing to consider. It is hard to stop once one starts eating a bagel; but two slices of bread are enough to keep the stomach full. This distinction comes not only from the product but also the efforts to keep the product fresh. At this stage, the first idea that comes to our mind is the addition of preservatives in the case of bread. In fact, the explanation is not as simple as it seems. At least, those chemical materials, which will maintain body integrity for many years following death, are irrelevant when it comes to bread.