A common occasion for ingredient substitution is the unavailability of a specific item required by a given recipe. Other reasons include the desire to introduce healthier alternatives to one's cooking, such as replacing or reducing fats, or because a dietary or physical condition actually demands the substitution.
In some cases, a cook simply lacks a critical ingredient and must improvise a replacement on the spot from items located in her pantry or refrigerator. Buttermilk, for example, can be approximated by adding a small amount of acid, such as in lemon juice or vinegar, to regular milk, whereas cocoa and vegetable oil replace unsweetened chocolate. To enhance the health of a recipe demanding bacon, the cook might consider Canadian bacon, turkey bacon or lean prosciutto. Rolled oats and bran cereal replace dried bread crumbs, and evaporated skim milk substitutes for cream.
For people with dietary disorders, ingredient substitution is a far greater concern. For instance, people with Celiac disease must be on constant vigil, replacing any ingredients derived from grains such as wheat, rye and barley; their digestive systems can't process them. Thus, when baking, Celiac sufferers often replace regular or all-purpose flours with amaranth, bean, corn and millet varieties. They frequently use arrowroot powder and xanthan gum as binding agents, and less-processed sweet ingredients such as dried fruit. Ultimately, however, any dietary adjustments prompted by health concerns should be outlined in collaboration with a physician.