Food Safety – Is Guacamole Safe When Sitting Out?

Fat doesn't spoil like other foods. No bacteria can live in fat. Going rancid is a chemical reaction in which the fat molecules break down. So "leaving out" is not a problem.

The major factors in going rancid are light and air. The more light hits your fat, the sooner it goes rancid. Also, rancidity occurs when the fat is oxidized, meaning that if you prevent contact with air, your fat will last longer.

Both factors are minimized by transferring the fat to a tightly closed opaque container. With liquid oils, you use a dark colored bottle. As bottles are impractical for solid fat, just strain your grease into a jar, close it, and put it in a cupboard. The shelf life should be many months, probably more than a year.

Also, don't worry that you can get something nasty when the grease goes bad unnoticed. Rancid fat isn't a big safety risk, and ingesting it in small amounts is not problematic. If it stinks, throw it out. Before that, there is no problem eating it.

The above assumes pure, well-strained fat. Pieces of fried meat left over in the fat are a safety risk, even tiny ones.

First, you have to distinguish between food safety issues and other ways of food "going bad". The likely thing is that your avocadoes were safe to eat (= not full of pathogenous bacteria) on the day you threw them out.

However, people cannot tell when food is full of bacteria, and tend to throw it out when it is too changed from its original state. Or they dislike the taste and texture of the changed state. This is common with fruit and vegetables. They are still alive after being picked, their metabolism goes on, but many of the necessary nutrients and water aren't delivered by the roots and stems any longer. So they change their color, taste and texture, and become mushy, wilted, or black,or change in other ways which people dislike, so they proclaim the produce "spoiled" and throw it away.

If this is what happened to the avocadoes, there is a good chance that making it into guacamole would have let you enjoy the avocadoes a day later. A pureed avocado doesn't metabolize the way a whole avocado does, and the changed pH and other ion concentration (salt) changes the chemical reactions going on in the cells. It is now no longer a living fruit, but prepared food. The pH and cold storage also inhibit mold from growing, which can happen on ripe fruit held at room temperature.

This is something specific to avocadoes and guacamole and not a generic statement about mixing random ingredients, you cannot make such generalizations in principle.