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Look at the outer skin of the avocado. Check the color. Hold the avocado in your hand and apply gentle pressure. Using the stem of the avocado to check for ripeness. Cut the avocado open. Taste the avocado.
If there is visible mould, or a rancid odour, the avocado is not safe to eat and should be discarded. If the avocado is severely dented and marked, or has squashed areas it has likely gone bad. Different varieties of avocado have different skin colours. The most common variety of avocado, the Hass, will change colour to very dark green or purple colour when ripe.   If a Hass avocado has reached a deep black colour, it may be past its prime. Most other commercially available avocados, like the Bacon, Fuerte, Gwen, Pinkerton, Reed and Zutano retain a green colour even when ripe. Take care not to bruise the fruit with your fingertips. A ripe avocado will yield slightly with light force. If slight pressure leaves an indent in the avocado, it is a sign it has gone bad. Some people advocate checking for softness by pushing in or removing the stem. If the stem moves easily it means the avocado is ripe. Once the stem is removed the colour of the flesh is also revealed.  This method may be effective when judging softness but not when judging colour. To get a good indication of the quality of the flesh more surface area needs to be seen. If you are examining avocados to purchase, you should avoid tampering with the fruit in this way. Removing the stem can compromise the quality of the fruit for other prospective purchasers. If you already own the avocado, this is the quickest way to determine if it has gone bad. The flesh should be a light green colour. If the flesh is black or brown, the avocado should not be eaten. If there is some minor bruising in the form of small, isolated areas of brown discolouration, the avocado is okay to eat. If you have thoroughly inspected the flesh but are not sure whether the avocado has gone bad, it is safe to perform a taste test. Avoiding any brown spots, try a small amount of the green flesh. Avocado should be creamy, mild and subtly sweet. If it smells or tastes musky or off, it has gone bad.