Posted by Eric Troy on 08 Mar 2014 20:04
How many times do you just use half an avocado, and you want to save the rest in the fridge for a day or so? Often, right? But, if you leave half of a sliced avocado in the refrigerator, it will turn brown pretty quickly, through oxidation. The most frequently repeated tip for keeping your sliced avocado from turning brown is to leave in the pit. The pit, we are told, will protect the avocado from oxidation, somehow. It it true? Nope. Not at all. Yet, this idea is so prevalent that some people even drop a pit into a bowl of prepared guacamole to keep it from turning brown!
What Causes Avocado Browning?
Like apples and some other fruits and vegetables (bananas, pears, peaches potatoes)1, the flesh of avocados, once exposed or cut into, turns brown through a process known as enzymatic browning.
Since exposure to oxygen, which reacts with certain chemicals in the fruit, is involved, it is also sometimes simply referred to as oxidative browning or just oxidation. Avocados contain enzymes. One particular enzyme (and some others) called polyphenol oxidase, in the presence of oxygen, reacts with other compounds in the fruit called phenols. Usually, these chemicals are kept compartmentalized. The plant enzymes are in the cell cytoplasm and the phenols are in the vacuole, but when the fruit is damaged, as when cut, they intermingle. The enzyme oxidizes the phenolic compounds, which become quinones.
These quinones react with proteins and this forms brown or gray polymers. It is not only cutting into fruit that causes this browning. Any type of damage to the cells of the fruit, such as bruising, can result in enzymatic browning. There are a lot of other complex interactions that can occur, but this is the gist of the browning problem. The brown color, by the way, although gross, does not make the fruit dangerous to eat and does not change its nutritional value. By the way, a similar thing happens to cut or torn lettuce.
The avocado pit has no magical properties that protect the avocado flesh from enzymatic browning. The only part of the avocado that will not turn brown when you leave the pit in is the part of the avocado that the pit is touching, which is the place it is sitting when you slice the avocado in the first place. It's packed into it's little space nice and tight, so, if you leave it there, it will basically help keep oxygen away from the round divet it occupies. There is nothing mysterious or scientific about it. In fact, if you remove the pit and then put it back where it was before storing the avocado half, you'll let in enough air underneat the pit to make that space turn brown as well.
How to Keep An Avocado Half From Turning Brown
There are two ways to do this. First, you can rub a bit of lemon juice on the exposed flesh of the avocado half. Then wrap it tightly in plastic wrap, before putting it in the fridge. The lemon juice will help keep it from turning brown for perhaps a day or two. Lemon juice works, as well, on apples, pears, or any fruits subject to "fruit rust," which is what chefs call the browning. Another rubbing method that is sometimes mentioned is olive oil instead of lemon juice. The idea is that the oil will basically create seal to protect the avocado from air. When you are using sliced avocado in a salad or other cold preparation, olive oil works pretty well to keep them looking nice and green in the dish. This will only work, of course, in dishes where you don't mind including olive oil in the mix. Nothing will stop the process of oxidation indefinitely though.
The second method, and this one might work a bit better, is to store the avocado half with the sliced part down, in a container of acidulated water. What is acidulated water? It's water to which you have added lemon juice, or some other kind of acid ingredient, like vinegar or white wine. But, in this case you want lemon juice, because it won't ruin the flavor of the avocado. The basic formula for acidulated water is one tablespoon of lemon juice for each cup of water. This will work for two or more days (but there are no guarantees). Invariably, there will be some brown spots that you will have to cut away, even in the best of circumstances.
Working With Large Amounts of Avocado: How to Keep them From Turning Brown
The problem with avocados is they turn brown very quickly. So, let's say you want to make a huge bowl of guacamole for a party, but you want it to be as fresh as possible when you serve it. You have a conundrum. You need to get your ingredients ready to go so you can mix and mash them all up at the last minute, rather than spending a lot of time away from your guests. You want to go ahead and get all your avocados removed from the peel (you remove avocado flesh from the peel rather than the other way around) and get it all cut up in cubes so that it's ready to combine with all the other guacamole ingredients.
Most of the other ingredients will be fine. You can chop your onion, cilantro, tomato, garlic, chile pepper, or whatever ingredients you want, and let them sit in the bowl waiting for the avocado. You can have your lemon, lime, or both ready to go. But you can't leave out your cut up avocado or it will be a brown mess by the time you serve it. No problem. Just put your chopped, sliced, or just scooped out avocado into a bowl of ice water until you are ready to use it. This will work for a few hours, keeping the avocado nice and green. The water will keep oxygen molecules from coming into contact with enzymes, and the cool temperature will slow down any reactions that take place. Obviously, you can't use this for mashed avocado.
If this method works to store avocado for short periods prior to use, why wouldn't it work for long-term storage? If chefs swear by it for larges-scale prep work, why use any of the methods above? We started with a half an avocado we didn't want to use right away? Can't we just put it into an ice water bath and keep it in the fridge? Well, according to an article at 52 Kitchen Adventures, this works quite well!
Speaking of guacamole, it will, of course, turn brown as well. And, it doesn't matter that there is lemon or lime juice in it, or oil. There are many reported methods of keeping guacamole from turning brown overnight. I think the number one method of keeping it from turning brown is to eat it all right away. But, that is not reality. Who doesn't want some leftover guacamole for the next day. Faith from "The Kitchn" claims to have found the the best way to keep guacamole from turning brown. Hint: It involves water.
You may be thinking that, if oxygen is the culprit here, why not just vacuum pack the avocado slices, or other fruit slices. Well, assuming you have one of those fancy new-fangled vacuum packing gadgets, it may not work out the way you hope. Think about it. If vacuum packing, which has been around for years, were the solution to non-brown sliced fruit, then sliced apples wouldn't be such a new food item in the produce department.
You'll notice that pre-sliced packaged apples do not come in vacuum packed bags. So what is the problem? The problem is you can't have your cake and eat it too. For one thing, there is going to be some oxygen in the fruit, so you can't shut down the browning process entirely. The other problem is that even when it is cut into slices, the fruit continues to respire. You see, vegetable and fruits are a bit different than other foods. They continue to respire even if you change the atmosphere around them. This is true if you remove the atmosphere, or if you modify it with some type of inert gas. First, they will use up all the oxygen they still have available. While this happens, CO2 will build up and if it build up enough, this will change and harm the fruit. Then, when there is no oxygen available, the fruit can swith from aerobic respiration (requiring oxygen) to anaerobic respiration, which is called anaerobosis. The result of all this is that weird odors and off-flavors will develop in the fruit.
Also, there is one other danger, however minimal: Storing a fruit in a non-oxygen package can promote the growth of anaerobic bacteria which can cause severe food poisoning. One of these is Clostridium botulinum which causes the dangerous condition botulism.
Not All Apples Turn Brown Quickly When Cut, but What About Avacodo Varieties
You probably already realize that not all fruits turn brown when cut or damaged. For instance, citrus fruits and tomatoes do not, because they contain a lot less of the enzymes responsible. It turns out that not all varieties of apples have the same levels of these enzymes either, so not all apples brown at the same rate. Apples subject to quick browning will turn brown within an hour. Other varieties can take a longer, sometimes a whole lot longer, to brown. Apple varieties that are better at holding their color once cut and do not turn brown as quickly are:
- Golden Russet
- Maiden's Blush
- Sweet Sixteen
- Tydeman's Red
Of course, you will probably not be able to find most of these exotic varieties of apples in your supermarket, except perhaps for Cortlands. However, Fuji and Gala apples, two tender-sweet apples that are common in supermarkets, also do not brown too quickly and can be good for fresh apple dishes such as salads. I have read that there are varieties of avocados that do not turn brown when cut, or that turn brown very slowly. However, in my research, I have not yet been able to ascertain what varieties these are. It may not matter to most of us, since we don't have access to them, but if you know anything about this, let me know in the comments!