07 Food Science

Monday November 22, 2004

With the start of the third week at the CCA, our class has settled in to a friendly camaraderie. Safety and Sanitation is now firmly behind us, and we’ve gained enough experience in the kitchen to feel comfortable with the fiercely hot commercial ranges and 10”-long German-steel knives that we each carry in our knife bag.

Everyone looks forward to Food Science, our second two-week lecture-based class. Unlike Safety and Sanitation, a dry subject that dealt mostly with bacteria and food-borne illnesses, Food Science promises to teach us things that will make us better cooks, such as how to serve the crispiest lettuce, or why red onions sometimes turn blue when they're cooked.

Indeed, during our Food Science lectures, Chef Steve provides examples of many real-world situations where it’s important to understand what’s happening to the food at a molecular level. He argues that this type of understanding is what makes the difference between an experienced chef and a novice cook. By studying Food Science, he believes that we can jump past the years of trial and error that other cooks must go through to learn the same thing.

Below, I've included some of Chef Steve's more interesting food science examples. If you're interested in reading more about these kinds of topics, check out Cookwise, by Shirley O. Corriher.

When Do Poached Apples Become Applesauce?

Chef Steve's first compelling example came from two recipes – one for poached apples, and one for applesauce. Each recipe uses the same ingredients, and differs only in the specification of when the sugar should be added – add it in the beginning of the process to make poached apples, and add it at the end of the process to make applesauce.

Why does such a subtle difference lead to dramatically different results? Think of the juice inside an apple’s cells as little more than concentrated sugar-water. When you cook an apple in plain water, the sugar inside the apple’s cells acts like a magnet, pulling water in through the cell membranes. This process is called osmosis, and it occurs because nature wants to dilute the sugar-water inside the cells so that it has the same concentration as the water outside the cells. All of the extra water flowing into the cells raises the pressure inside their walls until, eventually, they burst and the apple breaks down into a sauce.

If you add sugar to the water before cooking, the apple will keep its shape because the sugar water outside its cells is similar in concentration to the juice inside. Because there is no difference in sugar concentration, osmosis does not force water inside the apple’s cell walls, and they maintain their integrity. At the end of the cooking process, you end up with a poached apple that still has its original shape.

How Can You Re-Crisp Lettuce?

Osmosis also plays a role in the crispiness of lettuce leaves: if you leave lettuce out on the counter for much more than 30-40 minutes, the leaves droop and begin to look flaccid. This occurs because the dryer air is sucking the water out of the leaves’ cells.

If you soak the lettuce in cold water for 20-30 minutes, water will rush back into the plant cells and cause them to perk up again. Just make sure that you dry the lettuce thoroughly with a salad spinner before you serve it – otherwise the moisture on the leaves will dilute the flavor of whatever dressing you add.

Why Do Chopped Onions Make Us Cry?

Chefs chopping onions often look miserable – they have red, sniffly noses, and tears stream uncontrollably down their cheeks. Why? Like other plants, onions are made of individual cells. When you cut through an onion, you cut through the cell walls that keep sulfur-containing molecules inside the cells from enzymes in the spaces between. When these two substances mix together, they produce sulfuric acid which rises up from the onion and makes your eyes water.

Many home remedies exist for preventing onion-induced tears. One of the most successful includes cutting the onion under a stream of cold water – the sulfur compounds dissolve in the water and are washed down the drain before they can cause you to cry. Another way to prevent tears is to put the onion in the freezer for 10-15 minutes before you cut it – the cold temperatures slow down the enzymes so fewer sulfuric-acid reactions take place. Chef Steve’s favorite solution, however, is to simply make sure that you’re using a sharp knife when you’re chopping an onion – duller knives will pierce through many more cell walls, resulting in more sulfuric acid and more tears.

Why Does Red Cabbage Sometimes Turn Blue During Cooking?

Many rules of food science are based on the acidity of the foods that are being cooked. For example, the color compounds in red onions, red cabbage, beets, cherries, and other dark red fruits turn blue when exposed to alkaline substances. Although most foods are either neutral or acidic, acids can easily evaporate during cooking, leaving an alkaline pH and an unappetizing blue vegetable.

The solution? Add a teaspoon of lemon juice or cream of tartar to heighten the amount of acid in the dish. The extra acidity of those substances should be enough to tip the pH scale back under 7, and return the vegetables to their original color. Other high-acid ingredients that can help include vinegar, sour cream, buttermilk, apples and tomatoes.

How Can You Keep Vegetable Colors Bright?

Heat directly affects the color as well as the texture of many fruits and vegetables. When cooking, the trick is to retain as much of the natural color as possible without sacrificing texture.

Blanching vegetables by dropping them briefly into boiling water (less than a minute), and then plunging them into cold water to stop the cooking, causes a dramatic change in color. Caterers often blanch vegetables for raw vegetable platters to get brilliant colors and a more attractive presentation. The vegetables are still essentially raw, just much brighter in color.

Blanching works because fresh vegetables have air between the cells. This film of air clouds the true bright colors of vegetables. When you drop a vegetable into boiling water, the air expands, floats to the surface, and bubbles away. The vegetable suddenly becomes much brighter in color. Broccoli and green beans turn from a pale whitish green to a brilliant vivid green. Even the change in color of bright-colored vegetables like carrots is apparent.

If you want to cook the vegetables completely, understand that green vegetables can take only 7 minutes of cooking before they will turn from bright green to a drab olive-brown. This change occurs because of the reaction of the natural acids in the vegetable with its chlorophyll. Just as breaking the cell walls of the onion produces a chemical reaction that causes our eyes to tear up, cooking a vegetable breaks down the cell walls that protect the cholophyll from the naturally occurring acid in the plant. If your vegetable is too thick to cook completely in 7 minutes, try cutting it into smaller, thinner pieces.

Last: 06 Basic Skills Pt. 2
Next: 08 Nutrition

Cook green vegetables for less than 7 minutes in order to retain their bright color.

In this journal entry:

When Do Poached Apples Become Applesauce?

How Can You Re-Crisp Lettuce?

Why Do Chopped Onions Make Us Cry?

Why Does Red Cabbage Sometimes Turn Blue?

How Can You Keep Vegetable Colors Bright?