Guacamole contains a mash of avocados, peppers, tomatoes, cilantro and seasonings that require refrigeration after preparation. If it's left unrefrigerated, bacteria is a primary concern for the raw mix. These bacteria can include salmonella and E. coli, both of which cause serious illness. At room temperature, these bacteria multiply at rapid rates, rendering the guacamole unsafe to eat within a matter of hours. If you have no idea how long the guacamole has been sitting out, don't eat it.
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Air Temperature Matters
Guacamole can only sit at room temperature a short while before it begins to spoil. Outside of refrigeration, guacamole keeps for about two hours before the microbes that lead to foodborne illness begin to multiply to dangerous levels. If the air temperature is 90 degrees Fahrenheit and above, the guacamole requires refrigeration after one hour. If you eat unrefrigerated guacamole after these recommended safe times, you risk getting sick.
If It’s Brown
If you notice that the guacamole that you are serving has turned brown, don't fret. Just because unrefrigerated guacamole turns brown, that doesn't mean it's bad. The brown that you see is caused by oxidation, a reaction between the guacamole and the surrounding air. Use a spoon to skim away the browned top layer -- as long as it hasn't been sitting longer than the recommended times.
Play it Safe
After preparing guacamole, put it in the refrigerator and leave it there until serving time. Refrigerate once again within two hours of serving -- one hour if its a hot day. Store the mixture in a bowl with a piece of plastic wrap spread over top. Press the plastic wrap lightly against the surface of the guacamole to keep the air out. Homemade guacamole keeps up to three days in the refrigerator. Commercially prepared varieties keep up to five days.
Refrigeration doesn't help if the guacamole is contaminated with bacteria from the start. While small amounts of bacteria are naturally present on foods, in high amounts they become dangerous. Wash your hands with warm water and soap, scrubbing for a full 20 seconds before handling guacamole ingredients. Rinse the ingredients prior to preparation and make sure all cutting boards, utensils and serving containers are clean and sanitized before you begin.