How to make the most flavorful vegetable stock

What makes a good vegetable stock? My quick answer is always: the one brimming with flavor that works for you.

Stocks form the basis of soups, savory sauces, flavorful pilafs (pulaos) and other grain dishes, and they can transform a braised vegetable or meat dish from good to excellent. In addition, you can tailor a stock to suit your whims, even once you've begun cooking. This is where herbs and spices reveal their playfulness. Depending on the situation, I'll throw in a pinch of saffron threads, which impart their floral aroma and sunset hues. Scrunched bits of fresh Makrut lime leaves or lightly cracked spices such as star anise can also add a new layer of flavor.

Yet stocks aren't mythical creatures, but rather ones composed by an intuitive understanding of the chemistry of flavor. A cosmic principle is at work: Things must be destroyed in order for new things to be created. Ingredients transform into new forms of texture, color, aroma and taste that only a rapid boil can draw out. Broadly speaking, flavor can be built into a vegetable stock at three major points: when you select ingredients, during cooking and in a final garnish of seasoning just before serving.

The start: Your choice of vegetables will affect flavor, but even their age will sway flavor and texture. The classic components of the mirepoix - carrot, onion and celery - are rich in flavor molecules waiting to be unleashed. After storage, a carrot will taste much sweeter than it did the day it was harvested. Carrots, like many root vegetables, contain large reserves of starch, and over time, an enzyme called amylase cuts the starch to release sucrose, or table sugar. With less starch, the carrot won't thicken the stock as much, but the stock will taste sweeter.

Onions are the second most important ingredient. They're famous for their sulfurous molecules that make us tear up when we take a knife to them, and heat changes these molecules to new substances that enhance flavor and give aroma. Glutamate, for instance, adds much-desired savory character to stock. Similarly, hidden inside onion petals are long chains of fructose, the chief sugar in honey, which heat helps release. Celery likewise releases a potent panoply of aromatic substances, such as sedanolide, that build flavor while enhancing the perception of umami and sweetness contributed by other ingredients.

Now this is where I stray. I like to add one other ingredient: an extract of dried shiitake mushrooms. Dried shiitake adds a very important umami or savoriness by contributing two molecules: glutamate and a nucleotide called GMP that's present in the cells' RNA. In fresh shiitake, these two substances are present in very low amounts, but during drying, the glutamate is concentrated and GMP is released through an enzyme reaction. The two molecules amplify each other's umami. To extract the most flavor, I first prepare a "mushroom tea," steeping chopped dried shiitakes in water held at a particular temperature for a couple of minutes. You could throw the mushrooms in directly with the water, but I've found that letting them presoak buys the enzyme a bit of time to bump up the umami by a few extra notches.

The second stage: Heat creates and releases flavor molecules from the vegetables. I "dry" brown my vegetables - cooking them without oil or added water over low heat and use the water released from the vegetables as they cook. This gives you better control over two important reactions: caramelization and the Maillard reaction. During caramelization, sugars produce bittersweet substances and caramel-colored pigments. The Maillard reaction occurs between certain sugars, such as the sucrose and fructose in the carrots and onions, and the amino acids of the proteins in the vegetables. This reaction produces new aroma and taste molecules, as well as brown pigments that add to the stock's color.

Heat does one more thing: It breaks the vegetables down. Once my vegetables reach the desired degree of browning, I add water to bring the flavor molecules together, and then I stir in the mushroom tea. The degree to which the vegetables are browned can vary; my preference is for a darker toffee color, but I avoid getting them too dark, as the stock might taste bitter.

A stock provides a lot of opportunities to build flavors to suit your taste and cooking needs. If you're using spices or dried herbs, add them just before the mushroom tea. I use fresh herbs at the end, just before serving. Dried onion, garlic and tomato powders teem with umami molecules and can be used to build a richer, more savory note, while slices of fresh ginger and chilies can add a layer of heat and aroma. The final stage: Once the water draws out the flavor molecules, it's time to strain the spent vegetables. If you don't plan to use the stock the same day, you can reduce the liquid further and freeze it for up to a month. Whether I'm freezing or not, I add salt to amplify the savory flavors.

A few finishing touches in the form of a tablespoon or two of freshly minced scallions or chives, or such herbs as cilantro or parsley, will add color, flavor and texture; even the zest of a lemon will liven things up with its bright aroma. Such ingredients provide creative freedom, while the core of the stock is where flavor molecules come together in perfect harmony.

UMAMI-PACKED BROWN VEGETABLE STOCK

SERVINGS: 4; Healthy

Serve with couscous.

Ingredients

• 1 medium yellow onion (about 10 ounces), diced
• 1 head garlic, excess "paper" removed, halved across
• 1 pound carrots, diced
• 8 ounces celery (about 3 ribs), diced
• 3 ounces dried shiitake mushrooms, chopped
• 3 cups tap water, plus 2 cups ice water
• Fine sea salt

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Steps

Add the onion and garlic halves to a large stockpot or Dutch oven with a heavy lid. Cover the pot and set over medium-low heat. Cook, stirring occasionally, until the onions start to brown, 3 to 4 minutes. Add the carrots and celery, cover and cook, stirring occasionally, until the vegetables are tender and turn a deep toffee color, 30 to 45 minutes. Do not let the vegetables blacken or the liquid will taste bitter.

While the vegetables cook, prepare the mushroom "tea." In a large, heavy-bottomed stockpot or Dutch oven over low heat, bring 3 cups water to 158 degrees, using an instant-read or candy thermometer. Add the shiitakes and maintain the temperature between 140 and 158 degrees for 30 minutes. Use the ice water to lower the temperature of the mushroom infusion if it gets too hot.

After 30 minutes, add the mushroom tea, along with the mushrooms and any remaining ice water, to the browned vegetables in the stockpot.

Increase the heat to high and bring the contents of the pot to a rolling boil. Remove from the heat and let cool for 10 minutes.

Strain the stock through a fine-mesh sieve into a large bowl, gently pressing on the vegetables to extract any extra liquid (discard the vegetables after). Season to taste with salt and use or store.

NOTE: If you want to concentrate the stock for storage, rinse out and dry the stockpot. Return the strained stock to the stockpot, bring to a rolling boil over high heat, then reduce the heat to low and simmer, scraping down the sides of the pot with a spatula (to return the flavorful substances that accumulate there), until you get 1 cup, about 45 minutes. Use right away, or transfer the concentrate to an airtight freezer-safe storage container or zip-top bag.