Tea can become slimy when it's been steeped too long or from contaminates in your water source, or the compounds in the tea.
The oily slimy tea is still safe to drink although it does not look good to some people.
The oily film on black tea is mostly made of some of the compounds in the tea – particularly, molecules called polyphenols and calcium carbonate.
Calcium carbonate is present in tap water, but its content varies from place to place.
A higher concentration of calcium carbonate will create a thicker film.
Once brewed the brewed tea will last for up to 5 days in the fridge before it spoils and should be thrown out after 5 days.
Tea should be refrigerated if not drinking or serving it within a few hours because it can start to grow bacteria.
If you leave tea out overnight then it will likely be spoiled and taste bad and then should be thrown out.
It is OK to leave the tea bag in the tea as you drink it.
However if you brew a large pot of tea then it's best to remove the teabag after a few hours or the tea may get stronger.
Other than that there's no harm in leaving the tea bag in the tea other than the tea getting stronger.
Dry tea does not go bad as long as it is kept dry and does not get moist.
However the longer you keep the dry tea or tea bags eventually the tea will lose it's flavor and won't be as good but it still is okay to brew the tea and drink it.
Tea sometimes tastes bad due to brewing it too long and making the tea bitter or the tea is being boiled at too high a temperature.
The best thing you can do with the tea you absolutely do not enjoy is to cold brew it.
Pour about 1-1.5 liters of lukewarm water over 1-2 spoons of tea leaves and steep overnight in the fridge.
It might surprise you with a completely new flavor profile and no bitterness at all.
Tea gets sweeter as it sits due to the tannins being released in the tea.
And tea contains many saccharides, such as tea polysaccharides, monosaccharides, polysaccharides, fructose, etc., this free monosaccharides and disaccharides can be dissolved in water, so we taste the obvious sweetness very easily.
Tea should be brewed for between 3 to 5 minutes.
If brewing tea in a pot then you should get the water boiling and then allow the tea bag to sit in the pot for 5 minute or so.
Or if you're using a regular tea pot then you can pour the hot water over the tea bag and allow it to sit and brew for around 5 minutes.
Steeping tea too long can be bad as it can increase the Caffeine in the tea which leads to the tea becoming bitter or sour tasting.
So you don't want to over steep the tea.
Steeping tea usually takes between 3 to 5 minutes which is enough time to make good tea.
Tea gets sour and spoils due to heat, humidity and air which then leads to bacteria growing in the brewed tea.
To keep your tea from souring you should keep it refrigerated if not drinking or serving the tea within a few hours.
Tea can also get sour taste when brewing it as a result of steeping or brewing it too long or because of too many tannins in the tea.
You can get sick from drinking old tea if the old tea has gone sour.
However it's safe to use old tea bags as long as they have no mold growth on them.
But sour tea if you drank it would likely just cause a bad taste in your mouth and possibly stomach cramping and stomach ache.
Tea does go sour.
As tea sets out at room temperature bacteria begins growing in the tea which turns the tea sour although it usually takes 12 hours of tea sitting out at room temperature for it to really get sour.
However it's best to refrigerate tea if not drinking it within a few hours to keep it from growing bacteria.
If you drink rotten tea the most that usually happens if you get a stomach ache, maybe vomit, and a bad taste in your mouth.
If the tea is really bad then you likely won't want to drink it anyway but if it was just a little sour then it may just hurt your stomach a little.
Bacteria can grow in tea.
When tea sets out at room temperature for too long it goes sour.
The sour tea is growing bacteria which is making it sour and if you have soured tea then it should be dumped out.
If you do happen to drink a bit of sour tea then you will likely be fine although you may get an upset stomach.
Tea leaves may be contaminated with coliform bacteria.
If iced tea is brewed at inadequate temperatures or in an improperly cleaned urn, or if it is stored for too long, it may grow coliform bacteria, most frequently Klebsiella and Enterobacter, and less commonly E. coli.
Tea that has been sitting around for way too long is also referred to as overnight tea as well.
When tea sits around for too long, a substance called TP (tea polyphenol) that it contains will start to oxidise, this darkens the tea.
In most cases, old tea will simply be less flavorful than it should be, and taste dull or stale.
While old tea isn't usually dangerous to consume, it's definitely a less than ideal tea drinking experience.
If your tea tastes like it's gone bad or if you spot visible mold or other signs of decomposition, steer clear.
Brewed tea will slowly lose its complexity of flavor if left at room temperature for more than a few hours.
The tea will eventually sour if left for more that 12 hours. It varies a lot among tea types, some will be fine for at 24 hours.