Bacteria can grow on butter although it's not as likely as other dairy products.
Butter's high fat and low water content make the butter less susceptible to bacteria growth but it's still possible.
Butter's main source of microorganisms is cream that can be sweet, raw, sour or pasteurized.
Yeast and molds can cause discoloration and off flavors on the butter and psychrotrophic Gram negative bacteria can cause proteolytic and lipolytic changes.
Salmonella can also grow in butter at 25 C and is not eliminated by freezing or refrigeration.
When butter has gone bad the butter will often have mold, have an unusually shiny gloss to it, a strange smell, sour or off putting taste and visible discoloration.
Butter can get fungus and mold although fungus and mold is not as common in butter as a result of the butters high fat content.
Although if the butter is exposed to moisture and is left for a long time, the mold can grown on the butters surface.
If you see any type of mold on the butter the butter is not safe to eat and should be thrown out.
Butter does go bad if left in the heat.
The high temperatures can cause the butter to go rancid and even melt and cause the butter to develop an off flavor and bad smell.
It's best to keep your butter stored in the refrigerator unless you're gonna use it within a day or two.
Butter can be left at room temperature for one to two days.
When kept in the refrigerator butter will last around 2 months and 6 to 9 months when frozen.
Butter quickly goes rancid in the heat due to oxidation which is accelerated by warm or hot temperatures.
Melted butter does go rancid and even non melted butter does eventually go rancid especially when left out at room temperature or a long time.
Exposure to heat, oxygen and light can accelerate the oxidation process of the melted butter and non melted butter and cause it to develop an off flavor and turn rancid.
Oxidation of the butter is when the fats in the butter break down when they're exposed to air and cause it to go rancid.
Melted butter and other butter often goes rancid within a few days if not refrigerated.
The white stuff in melted butter is simply milk solids that are made of fat and milk proteins.
Butter is made of water, fat and protein that form an emulsion and when the butter melts the emulsion then separates into layers.
And the white foam which floats to the top is made of milk proteins and the white stuff or solids that settle to the bottom of the melted butter is just milk solids.
Butter melting including on bread is not a chemical change and is instead a physical change.
The butter melting including on bread is a physical change and not a chemical change because the butter is simply changing from the state of being a solid to the state of being a liquid.
And the chemical composition of the melted butter still remains the same.
This means that the butter that is melted can be reversed by cooling the butter back down to a solid state.