| Commercial products usually contain preservatives. There are some that are natural so even organic foods could have preservatives. Handling that is safe for store-bought foods may not be safe for homemade. Nor is cooking alone a preservative. Bacteria are all around us, so even if a food could be completely sterilized by home cooking -- itself not likely -- unless it was also hermetically sealed, it could easily be reinfected just from being out in the air. What matters is the amount of available water. Bacteria need not just warmth but water to grow. That's why dry foods, for example cold cereal, can last a very long time, while the same food in a moist form, even a well-cooked one like hot cereal, will go bad much more quickly. Jerky lasts much longer than a juicy steak, even a well-done one. Cookies last longer than cakes. Sugar holds water, making it unavailable to bacteria, so foods with a lot of sugar last longer than less-sugary versions. Sugar is the reason "preserves" are preserved -- see what happens if you use only just enough sugar for flavor, or substitute artificial sweetener. Put a lot of sugar in your cream cheese frosting and it will last a good deal longer than plain cream cheese. Make it with part butter (which has far less water in in than cream cheese) and it should last even longer. But don't count on it lasting as long as a store-bought version, especially one that didn't need refrigeration in the first place.
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