The term “pop,” also known as soda or soft drink, refers to a carbonated beverage typically made with water, sweeteners, flavorings, and carbon dioxide. Unlike perishable foods, pop does not “expire” in a way that makes it unsafe to consume if the container remains sealed. The high acidity and sugar content in the drink create an environment that is hostile to most harmful microorganisms, meaning the safety risk is extremely low. The primary change that occurs over time is a significant degradation in the quality, taste, and carbonation of the beverage.
Understanding the “Best By” Date
The date printed on a can or bottle of pop is not a safety expiration date but a quality indicator set by the manufacturer. This date, often labeled as “Best By,” “Best Before,” or “Best Taste Date,” signifies the point at which the company guarantees the product will be at its peak flavor and carbonation level. After this date, the manufacturer no longer promises the optimal sensory experience, but the product remains safe to drink.
These quality dates are voluntary and are not mandated by federal safety regulations. For regular, sugar-sweetened sodas, the quality may remain acceptable for up to nine months past the printed date when stored correctly. Diet sodas, however, have a shorter quality window, often only maintaining peak taste for about three months past the date due to the instability of artificial sweeteners.
The Science of Degradation: Fizz, Flavor, and Safety
The most immediate and noticeable form of degradation is the loss of fizz, which is the result of carbon dioxide ($\text{CO}_2$) escaping the liquid. Carbonation is achieved by dissolving $\text{CO}_2$ gas into the liquid under high pressure, and this gas slowly diffuses out of the container material over time. This loss of $\text{CO}_2$ also reduces the amount of carbonic acid, which contributes a sharp, tangy bite to the flavor profile.
Flavor changes also occur as the chemical components of the soda break down. In regular sodas, the flavorings and preservatives can degrade, sometimes leading to a stale or slightly metallic taste. For diet sodas, the artificial sweeteners, such as aspartame, are particularly unstable and can break down into compounds that result in a less sweet or even bitter flavor. This chemical breakdown, accelerated by heat, is the primary reason the taste of old pop is often described as “off.”
Storage and Packaging: Extending Shelf Life
The way pop is stored has a significant impact on how long it retains its quality. Storing the beverage in a cool, dark place is the most effective way to maximize its shelf life. Heat is the biggest enemy of pop quality, as elevated temperatures accelerate the rate at which $\text{CO}_2$ escapes and hasten the chemical breakdown of flavor compounds and sweeteners.
The type of container also plays a major role in preserving the drink’s quality. Aluminum cans and glass bottles offer a superior barrier against gas diffusion compared to plastic polyethylene terephthalate (PET) bottles. Aluminum cans, for example, have an annual $\text{CO}_2$ loss rate of only about 1% to 5%, meaning they retain carbonation much longer than plastic.
Plastic bottles are slightly permeable, allowing small amounts of $\text{CO}_2$ to escape through the material itself, which causes the pop to go flat more quickly. For this reason, pop packaged in cans or glass typically has a longer manufacturer-recommended quality period than the same product in a plastic bottle. Keeping the containers away from direct sunlight is also important, as light can break down artificial colors and certain flavor compounds.