I left some coffee unfinished... Should I keep it well and drink it tomorrow?

There are times when I buy and drink Americano in large quantities. The amount is more than I expected, so I can't finish it all at once and leave some behind. I hate to throw away the leftover coffee, but is it okay to store it and drink it later?

It is acceptable to drink the Americano you had in the morning in the afternoon. Americano is composed only of shots extracted from coffee beans and water, making it less prone to spoilage compared to other foods. The process of brewing the beans with hot water sterilizes it, resulting in a low initial contamination level. Additionally, Americano is difficult to undergo decay, rancidity, or spoilage. Bacterial growth and protein deterioration cause spoilage, fat deterioration causes rancidity, and other components including carbohydrates cause spoilage as well. Since Americano contains little to no protein, fat, or carbohydrates, it is less likely to spoil. It also exists in an acidic environment (pH 4.8 to 6), which bacteria dislike.

Even in environments where bacteria find it difficult to proliferate, mold can still grow. Mold can thrive even in environments lacking nutrients and that are acidic, as long as oxygen is present. Mold present on coffee beans can survive hot water brewing and transfer into the coffee. Americano has fewer nutrients, so mold grows more slowly compared to other foods, but if left at room temperature for about five days, mold can be seen on the surface. Lattes or cappuccinos containing milk have richer nutrients than Americanos, so they spoil much faster. It is best not to leave these coffees at room temperature for more than two hours.

The risk of coffee going bad depends on whether you open the lid and sip directly from it. Bacteria in saliva can transfer to the coffee, increasing the likelihood of spoilage and speeding up the process. When the lid is opened, mold spores floating in the air may settle on the surface of the coffee. It usually takes about five days for mold spores to germinate and grow to a visible size. Therefore, even if mold is not visible immediately, you cannot be completely assured of safety. Coffee that has been touched by the mouth should be finished within 24 hours, no matter how slowly you drink it. If a day has passed, it is best to discard it.

If the coffee has not been touched or opened, it can be kept at room temperature for 5 days. If stored in the refrigerator, it can be kept for up to a week. The Korea Food & Drug Administration considers liquid coffee safe when the total bacterial count is 100 CFU/mL or less. According to research by Professor Seong-hee Hwang from the Department of Bio-Food Industry at Semyung University, bacteria were detected at an average of 78.4±29.7 CFU/mL when coffee purchased from a cafe was stored at room temperature for 5 days, and 51.2±32.1 CFU/mL when stored in the refrigerator for 5 days. Reheating the stored coffee in the microwave once will kill the bacteria, making it safer to drink.

<Source: Health Chosun>