About a week ago, I decided to dig in to MIT’s free, online Kitchen Chemistry class. The first lesson involves preparing salsa and guacamole – two tasty and nutritious treats I love to make. I, however, decided to focus on the guacamole portion of the lesson right now because I rarely make salsa when tomatoes are not in season – I’ll save that one for the summer. (Call me a tomato snob, but I don’t like them if they are not locally grown and vine-ripened. They just don’t have that deep, rich tomato taste to me.) Without any further ado, here is what I learned about avocados:
Avocados are full of folate (good for heart health), vitamin K, dietary fiber, vitamin B6, vitamin C, and copper and have more potassium than a medium banana. They also contain nutritious monounsaturated fat (oleic acid) which is thought to increase good cholesterol (HDL) and decrease the bad (LDL).
Ripe avocados are firm with a little give but not squishy and have darkened, unblemished skin. Notably, avocados do not ripen on the tree - it is not until after they are picked that they begin the ripening process. This makes them very shippable and durable but a pain to get just right. Everyone who has cooked with avocados knows they are usually rock hard when found in the store and using avocados in a recipe takes some pre-planning. Everyone also probably knows that stashing them in a brown bag at room temperature will ripen them...but why? Gas is the answer - ethylene gas to be more specific. Fruits and other parts of plants naturally produce this during key stages of growth. Some fruits, such as bananas, produce copious amounts of ethylene. In fact, placing an overripe banana in the brown bag with an unripe avocado will supercharge the process. (This works to ripen tomatoes too.) To cease, or at least slow, ripening, place the avocados in the fridge. The coolness will put them in a sort of suspended animation - for a few days.
Once the avocado is cut open, it will begin to rapidly brown. Some fruits turn brown when exposed to the air due to a process called oxidation which is, in essence, a reaction by fruit enzymes (found in bananas, apples, potatoes, avocados and more) to the oxygen molecules in the air. This chemical reaction (akin to rust in metal) can be slowed or stopped by destroying the enzyme (through cooking), lowering the pH of the fruit (with ascorbic acid like orange juice or lemon juice) or physically blocking oxygen exposure (covering with water or plastic wrap). In guacamole, the addition of lemon or lime juice to the recipe accomplishes this as does pressing plastic wrap against the surface of the final product. I also found some recipes that suggest adding mayonnaise to the guacamole - which makes sense - as mayo is a stable emulsion of oil, egg yolk and either vinegar with a pH of 2.4 or lemon juice with a pH of 2.2. (<7 is acidic, >7 is base, 7 is neutral) (I’ve heard mashing the interior of the pit and adding it to the dip will do the same, but I’ve never tried it.)
Well, all this fancy science talk has made me hungry. I think I'll go mash some avocados! Here is my favorite Cook's Illustrated Chunky Guacamole recipe:
3 medium avocados, ripe (preferably Hass - the kind most commonly seen in the store)
2 tablespoons minced onion
1 medium clove garlic, minced
1 small jalapeño chile, minced (1 to 1 1/2 teaspoons)
1/4 cup minced fresh cilantro leaves
1/4 teaspoon table salt
1/2 teaspoon ground cumin (optional)
2 tablespoons lime juice
1. Halve one avocado, remove pit, and scoop flesh into medium bowl. Mash flesh lightly with onion, garlic, jalapeño, cilantro, salt, and cumin (if using) with tines of a fork until just combined.
2. Halve and pit remaining two avocados, and prepare. Gently scoop out avocado into bowl with mashed avocado mixture.
3. Sprinkle lime juice over diced avocado and mix entire contents of bowl lightly with fork until combined but still chunky. Adjust seasoning with salt, if necessary, and serve.