听力材料：
All foods will eventually spoil if they’re not preserved. Food preservation is the process of preventing or slowing the natural decay of edible plants and animals, spoilage that results from microorganisms—bacteria, molds, and yeasts—that are present in the food.
The most natural method of preserving food is drying. Nature preserves the seeds for next year’s crop by drying the outer layers of fruits and vegetables that are exposed to the sun and air. We copy nature’s effects by exposing food to warm air, a method that removes all the water that would otherwise lead to spoilage. Microorganisms need moisture to grow, so removing the water will stop the microorganisms from growing.
Drying is a very ancient form of food preservation. Prehistoric people dried grains, fruits, and nuts in the sun. When fire was discovered, they learned to smoke meat and fish by the fireside. Native Americans dried and chopped meat, then mixed it with fat. The lack of moisture in the meat kept it from spoiling. Modern industrial science has perfected the techniques of drying and applied them to the mass production of a variety of foods, such as powdered eggs and milk, peas, mushrooms, coffee, soups, and salad dressings.
Another method of food preservation is curing. Curing is done by adding ingredients, such as salt, sugar, spices, or lemon juice. Curing is one of the oldest methods of food preservation and is still used in the production of many meats and fish. The ingredients used in curing have unique reactions with the food they’re added to. For example, salt slows the growth of microorganisms and removes some of the water from the food. Sugar counteracts the hardening that’s sometimes caused by salt. Lemon or lime juice added to raw fish will preserve the fish for a short time without removing all the moisture.
Question：
Using points and examples from the talk, explain how the two methods of food preservation keep food from spoiling.
