Sweet and sour candy can be either hard or chewy, and there is often a sour powder coating the outside of this type of candy. Underneath the powder, the center of the candy is usually sweet, or at least sweeter than the coating. Some types of candy are made in the opposite way, with a sweet coating on the outside and a sour candy underneath. Candy is generally one of the more popular types of sweet and sour foods, along with dishes made with some Chinese spices designed to give a sweet and sour flavor.
Acid levels in the candy typically determine the sourness of sweet and sour candy. Citric acid, which is the acid found in many fruits, including lemons and oranges, contributes to the sour taste in many types of sweet and sour candy, for instance. It is a weak acid, though it can break down the connective tissues in foods such as steaks to make them more tender. This is the reason some form of citric acid is almost always an ingredient in meat marinades. Citric acid is also commonly used in soft drinks to provide a refreshing bite that gives some types of soda a unique taste and quality.
The powdery coating on the outside of most sweet and sour candy is usually citric acid in powder form. How much citric acid is used will generally determine whether the candy is mildly sour or almost unbearably bitter. It is also often added as ingredient in the main part of the candy, particularly if most of the candy is also meant to be sour. Citric acid is usually the ingredient that makes candy sour, regardless of whether it is a chewy type of candy or a hard variety that is meant to be eaten slowly.
Sweet and sour candy can cause a variety of responses from those who eat it, including extreme dry mouth upon tasting the candy. Many non-candy foods, including the sweet and sour sauce used on a variety of Chinese dishes, also rely on the mix of sour and sweet, but with a result that is usually far less sour. Citric acid may still be used, though it is often used in the form of the actual fruit or fruit juice. Vinegar is a common ingredient in these types of sweet and sour foods, however, and often the one that provides the most tangy taste in these dishes.
Citric acid may be the most common ingredient for producing sweet and sour candy, but a variety of other acids can also be used instead of it or along with it. Ascorbic acid, phosphoric acid and lactic acid are examples of other sour ingredients. The acidic ingredients in sweet and sour candy raise the pH level of the candies, making some of the most sour varieties potentially damaging for the enamel of the teeth. A little baking soda in water used as a rinse or a snack of a low pH food such as milk or cheese can neutralize the acid, though someone who only eats sweet and sour candy occasionally is probably not going to suffer tooth damage from it. Experts generally agree that it is best to wait at least 30 minutes after eating very sour candy to brush one's teeth to avoid scratching the enamel, which may be temporarily softened by the acid.