Food goes bad and rottens with time. After zombie apocalypse we will no longer be able to count on the electric grid to keep the refrigirators running. And with the difficults that rise in providing food, there will be a great need to be able to let the food you have last long enogh. Over our history, a great deal of ways to let the food available last longer have been made.
Using the sun or other heat sources to remove water from food. This slow downs bacteria growth which prevents decay. This method is popular on many fruits, fish and meat. The meat is also often salted (see below). This method has the adavance of being easy and requiering little reasources. An obvious disadavantage is that the food has less water to satisfy thirst. An way to counter that can be to bury a hole in hot sand, keep the food there with a bucket in the middle, and seal the hole with waterproof cloth, wich slopes to the bucket. It will cause all the water which dries from the fruits to gather in the bucket. This will give you some water but this way is not the most effective drying method and you will proabably need to finish drying with other methods.
An way to extend the shelf life of dried meat and fish even further. The food is often smoked alongside this, meaning it is put under a fire, burning normally either wood or dried animal dung, and the smoke surronds the food, wich improves both ending time and taste.
One of the most popular food preservation methods of the modern day, canning involves cooking food and sealing it in sterile cans and boiling them after that. Canned food should be one of the primary targets when scavenging. However, trying to can your food can be dangerous as if it is not done properly, dangerous bacterias, such as the very deadly botulinium-bacteria, can grow there.
The most used method of the modern world. However, refrigirators and freezers requier a lot of electricity, which is hard to get in an undead world.