Reductio ad sapidum: Reduction fundamentals

One of the techniques that cropped up a lot in the menu for my recent dinner party was reduction: the carrot flan called for reduced blood orange and carrot juices; the plum glaze and foie gras sauce on the duck dish both involved reduction; and the sauce bordelaise on the beef dish was based on reduced veal jus, which was itself based on a heavily reduced dark veal stock.

Those few spoonfuls of sauce, enough for no more than 8 servings, were the ultimate reduction of about 5 pounds of veal bones, plus some wine and assorted vegetables. Why go to all the trouble of reducing liquids so far, at such expense? The answers, as they usually are in cooking, are flavour and texture. Flavour, because reduction concentrates the sapid and aromatic molecules in the liquid, resulting in a bigger burst of flavour in a smaller volume, and texture, because it concentrates non-volatile components like sugars and gelatin, thickening the final liquid. A spoonful of stock will spread out to cover the whole plate; a spoonful of reduced demiglace will still pool, but will tend to confine itself to a smaller area. (And when it’s chilled, it sets up into a pretty firm gel, like in the photo up top.)

Reduction presents two major challenges, though: first, when you boil the water out of a liquid, you can also boil off a lot of volatile aromatic molecules with it. This is usually explained by pointing out that all the molecules that are making your kitchen smell so good are doing so by leaving the sauce! Second, when you increase the temperature, you also drive up reaction rates, which means that you’re not only concentrating the sugars in your blood orange juice, you may be affecting them chemically as well. Coagulation can be a problem here; when I boiled the carrot juice for the carrot flan, I got a layer of orange scum on top of a clear liquid.

So what’s the best approach to reducing liquids: quickly over high heat, or at a gentle simmer over a period of many hours? For me, the jury is still out. On the one hand, low and slow may limit some of the reactions, but it takes a lot longer. High and fast may cause more reactions and release more volatile molecules, but it’s finished sooner. (Another great way to reduce liquids quickly is to increase the surface area by putting it in as wide a pan as is practical. Be careful you don’t reduce it too far and burn it, though.) For this meal, I tended to make my reductions over high heat, largely because time was of the essence when prepping this much food.

Of course, there are other, more expensive pieces of equipment for reduction, notably the rotary evaporator, and I imagine that reverse osmosis will one day enter the culinary market. But for now, open-pot boiling is the easiest approach.

How do you prefer to reduce liquids: high and fast, or low and slow?

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2 Responses to “Reductio ad sapidum: Reduction fundamentals”

  1. It’s low and slow for me. I strain the pressure cooked stock into a wide vessel such as a le Creuset 8 liter or a Dutch oven, and let it barely simmer on the lowest setting of an induction burner for several hours, while I do other things. This usually produces a deep, well flavored, jelled stock, and will go into a demi glace if I leave it long enough.
    The strained bones and meat usually are good for a second soup stock, with a few added aromatics.

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