Can lemon sodas go bad?

Question

So, I just opened up a bottle of lemon-flavored soda that had been sitting in my family's garage for a few years, and I noticed that its flavor seemed different than it normally is. I wasn't sure if it was safe to consume or not, so I tipped my glass out into the sink after a couple of sips, and then looked at the bottle and noticed that the color looked a little yellowish, rather than completely transparent like normal. One of my family members tried a small glass of it, noted that it lacked carbonation, and said it probably just tasted strange because it was flat.

The best before date on the bottle was in 2018, and we live in an area where temperatures in the summer can reach 30-40 degrees C, though I'm uncertain exactly how hot the garage gets. It wouldn't have been exposed to direct sunlight, though.

Can bottles of soda go off, either due to chemical changes in the soda, or due to the plastic in the bottle breaking down? It was sealed the entire time, so I don't think that any bacteria or wild yeast would have gotten into it, right?

It’s a regular soda, not diet. Ingredients on its label are “carbonated water, sugar, acidity regulator (330), natural flavour, preservatives (211, 202).”

Answer 1

Acidity regulator E330 is citric acid. I found no indication that it's chemically instable at temperatures < 100°C and a study found it stable over a period of 5 months.

Additive E211 is sodium benzoate that's used to stop bacterial and fungal growth in acidic foods and drinks. A study found that over a period of 90 days 4 - 8% of sodium benzoate degrades. I'm no chemist and don't know what chemicals the result of this degradation are, but they could influence the taste of the soda and loose the ability to stop bacteria growth.

Another study found that in peach puree treated with sodium benzoate, the amount of sucrose (common sugar) decreased and the amount of glucose and fructose increased. Although all 3 are different types of sugars, there is a notable difference in taste.

Additive E202 is potassium sorbate, another food preservative and probably the culprit for the yellow color. Food Reviews International writes:

However, in solutions and in foods, [potassium sorbate] undergoes autoxidation during storage, forming carbonyls and other compounds. Many factors (e.g., pH, temperature, packaging, water activity, and composition of food) influence its stability. Degradation of sorbic acid is associated with development of browning in foods. Acetaldehyde and /3‐carboxylacrolein have been reported to be the major degradation products of sorbic acid in aqueous solutions. β‐Carboxylacrolein is found to be responsible for sorbate‐induced browning in foods as it reacts with amino acids and proteins to form brown pigments.

Plastic bottles are known to let the carbonation of carbonated water escape over a long time. Carbonation means that carbonic acid is solved in water, which then forms little bubbles of carbon dioxite (CO₂). As long as the carbonic acid is still dissolved in the soda, it adds a slightly tart flavor to it, which we are used to. Once most of it evaporated, the overall flavor of the soda changes to that of a stale soda (which it basicly is).

As to the "natural flavor", since it's not named, we cannot know for sure what it was. But there's a good chance that it also degenerated at least partially over time.

So, to answer your question: Yes, lemon sodas can go bad. That's why there is a "best before" date printed on them. As far as I understand the chemistry involved, your soda wasn't poisenous, but it was chemically altered enough to be considered "gone bad".