How to Keep Avocados Fresh

on Thursday, January 27, 2011

Have you ever wondered how to keep an avocado fresh and green and stop it from turning brown within a few hours?  I've read many pieces of advice over the years, including a recommendation to use citrus juice or to leave the pits in the avocado, or to mix guacamole with pits and lime juice to keep it green... but none of these techniques work for more than a few hours.  Why?

Limes preserve the green color because they contain vitamin C (ascorbic acid), a mild antioxidant.  Most fruits and vegetables contain one or more major antioxidants.  They use them as natural pesticides and preservatives for their little veggie bodies.

An antioxidant is exactly what it sounds like -- it is "anti-oxygen."  Oxygen is dangerous stuff.  It degrades anything it touches by "oxidizing" it.  Rust is metal that has been touched by oxygen.  In the same way, oxygen is harmful to the surface of the avocado and it turns brown when oxygen degrades it (also when enzymes work on it).  So to stop the degradation and browning, we have two choices:

1. Keep the avocado away from oxygen.  Put it under water or dip it in oil.  Or very tight contact with plastic wrap.
2. Cover the avocado with an effective antioxidant.   This has been my method of choice, and there are many cheap and effective antioxidants available.  By the way, "antioxidant" is another word for "preservative."  Food preservatives are simply antioxidants.  They can be purified from plants (we already know they contain natural antioxidants, that's why they are so healthy), or they can be manufactured from other raw materials.  The most effective antioxidants we know of today to protect fruit (avocados) from oxygen is sulfur, which is both an antioxidant and an antibacterial agent.

Many sulfur compounds are familiar to us if we have ever read a junk food container.  This is unfortunate, because sulfur is a natural compound.  When we make wine or beer, natural sulfur is created via fermentation.  Volcanoes produce sulfur, and many hot springs naturally contain sulfur compounds.  Combining sulfur with other elements yields myriad scary sounding chemicals: sodium metabisulfite, potassium metabisulfite, sulfur dioxide, sodium sulfite, sodium bisulfite, potassium bisulfite, potassium hydrogen sulfite.  These are all different forms of sulfur and perform the same basic function -- to put a little sulfur in, on, or near the food to keep bacteria and oxygen away from it.  Sulfur is used in beer and wine making to keep bad bacteria out and to keep the wine or beer from oxidizing (which would turn them to vinegar).  Sulfur in these applications is referred to as "sulfites."

Other antioxidants used as preservatives include BHA, and BHT, which are also antioxidants, and are ironically sold as both "health supplements" and also feared as "artificial preservatives."

The best known preservative is sulfur.  It is both antibacterial and an antioxidant.  To preserve our avocado and keep the oxygen away from its precious green meats, we need sulfur!  I have been using potassium metabisulfite for some time now and it works wonderfully well.  It is potassium bound to sulfur, and when it touches water it releases sulfur into the water.  Just put a little bit of the sulfur powder in water and wash the avocado with it.  When you want to eat the avocado you can remove the sulfur by washing or you can simply eat it, as I do.  If you are not allergic the sulfur is harmless.  After the wash the avocado will stay green and fresh and protected from rotting and bacteria for over a week.