Curious Cook

Caramelization: new science, new possibilities

For me, the epitome of stovetop alchemy is  making caramel from table sugar. You start with refined sucrose, pure crystalline sweetness, put it in a pan by itself, and turn on the heat. When the sugar rises above 320°F/160°C, the solid crystals begin to melt together into a colorless syrup. Then another 10 or 20 degrees above that, the syrup begins to turn brown, emits a rich, mouth-watering aroma, and adds tart and savory and bitter to its original sweetness.

That's the magic of cooking front and center: from one odorless, colorless, simply sweet molecule, heat creates hundreds of different molecules, some aromatic and some tasty and some colored.

How does heat turn sugar into caramel? Heat is a kind of energy that makes atoms and molecules move faster. In room-temperature table sugar, the sucrose molecules  are jittery but standing in place, held still by the forces of attraction to their neighbors. As the sugar heats up in the pan, its molecules get more and more jittery, to the point that their jitters overcome the attractive forces and they can jump from one set of neighbors to another. The solid crystals thus become a free-flowing liquid. Then, as the temperature of the sugar molecules continues to rise, the force of their jittering and  jumping becomes stronger than the forces holding their own atoms together. The molecules break apart into fragments, and the fragments slam into each other hard enough to form new molecules.

That's what I've thought for many years, along with most cooks and confectioners and carbohydrate chemists: heat melts sugar, and then begins to break it apart and create the delicious mixture we call caramel.

And we've all been wrong.

It turns out that, strictly speaking, sugar doesn't actually melt.  And it can caramelize while it's still solid. So proved chemist Shelly Schmidt and her colleagues at the University of Illinois in studies published last year.

It's dismaying to think that so many could be so wrong for so long about such a basic ingredient and process! But it's also a rare opportunity to rethink the possibilities of the basic. Here's a plateful of possibilities; scroll down for more.

Professor Schmidt's group made their discovery when they tried to nail down the precise melting point of sucrose. The figures reported in the technical literature vary widely, and it wasn't clear why.

The melting point of a substance is the temperature at which it turns from a solid into a liquid while maintaining its chemical identity. When solid ice turns into liquid water, for example, the molecules of H₂O move fast enough to escape the attractive forces of their neighbors, but they're still H₂O. And it doesn't matter how fast the substance heats up: the melting point is the same. Ice melts at 32°F/0°C. Always.

After careful analysis, Professor Schmidt found that whenever sugar gets hot enough to turn from a solid into a liquid, some of its molecules are also breaking apart. So sucrose doesn't have a true melting point. Instead it has a range of temperatures in which its molecules are energetic enough to shake loose from their neighbors, and a range in which the molecules jitter themselves apart and form new ones. And these two ranges overlap. Whenever sugar gets hot enough to liquefy, it's also breaking down and turning into caramel. But it starts to break down even before it starts to liquefy. And the more that sugar breaks down while it's still solid, the lower the temperature at which it will liquefy.

When we make caramel standing at the stove, we use high heat to liquefy and then brown the sugar in a few minutes, and the liquefying temperature can be upwards of 380°F/190°C. But Professor Schmidt's group found that when they ramped up the heat slowly, over the course of an hour, so that significant chemical breakdown takes place before the solid structure gives way, the sugar liquefied at 290°F/145°C. 

I made the caramelized sugars in these photos by putting crystals and cubes in my gas oven at around 250°F/125°C, shielding them with foil above and below to avoid temperature extremes from the cycling heating element, and leaving them there overnight and longer. In the large sugar crystals, which I got in a Chinese market, it's clear that breakdown and caramelization is fastest in the center. That may be because the center is where impurities get concentrated as the crystals are made, and the impurities then kickstart the breakdown process. 

Caramel makers have long known that, as is true in most kinds of cooking, the key to caramelization is the combination of cooking temperature and cooking time. But the the temperatures have typically been very high, the times measured in minutes. Now we know that you can caramelize low and very slow and get something different. Sugar breakdown even occurs at ambient storage temperatures, though it takes months for the discoloration and flavor change to become noticeable. For a manufacturer this is undesirable deterioration. But for a cook in search of interesting ingredients, it could be desirable aging.

In a follow-up to her initial scientific reports, Professor Schmidt wrote in Manufacturing Confectioner that

from a practical point of view, caramelization can be thought of as browning of sucrose by applying heat for a length of time. Thus it may be possible to better control the caramelization reaction by identifying the time-temperature conditions that optimize the production of desirable caramel flavors compounds, while minimizing undesirable ones. Confectionery manufacturers and sugar artisans, armed with this new scientific knowledge, may be able to push their craft in unforeseeable directions.

For example: aged sugar, roasted sugar, caramel-center crystals. Let the pushing begin!

 

Schmidt, S.J. Exploring the sucrose-water state diagram. Manufacturing Confectioner, January 2012, 79-89.

Lee, J. W. et al. Investigation of the heating rate dependency associated with the loss of crystalline structure in sucrose, glucose, and fructose using a thermal analysis approach (Part I). J Agric. Food Chemistry 2011, 59: 684-701.

Lee, J. W. et al. Investigation of thermal decomposition as the kinetic process that causes the loss of crystalline structure in sucrose using a chemical analysis approach (Part II). J. Agric. Food Chemistry 2011, 59: 702-12.

 

 

 

 

 

 

 

 

 

 

 

Sugar from the garden

While browsing among the vegetable starts at a nursery in Santa Cruz last year, I came across a flat of sugar beets. I'd never tasted sugar beets before. They're a special variety of Beta vulgaris bred for sugar production, with none of the colorful pigments of vegetable beets that would further complicate the manufacture of pristine white crystals. So I bought some seedlings and planted them. They grew well through nearly a year of benign neglect. Last month I dug them up and tried them out.

The beets I grew were irregular in shape and size, the largest weighing in at about a kilogram, or over 2 pounds trimmed of leaves and small roots. Both raw and roasted they had a mild beet flavor and were very sweet indeed. I used my refractometer, a handy instrument that measures the concentration of dissolved materials in liquids, to get a rough idea of the sugar levels in their juice. Most of the sugar beets ran between 15% and 18% dissolved solids, while store-bought red and chioggia beets were closer to 5%.

I was especially curious to see what a home version of beet sugar would be like. Refined white table sugar is manufactured from beets and from sugarcane by extracting the juices from the raw materials, evaporating off their water, and separating the sucrose sugars from everything else, including a host of other plant chemicals and byproducts of the evaporation process. That separation process involves the use of mineral lime, carbon dioxide, charcoal made from various materials (sometimes animal bone), and centrifuges.

Not for the casual sugar-maker! Instead I figured on making an unrefined sugar, a beet version of the delicious palm and cane jaggeries that come from Asia, or the cane panela (piloncillo, papelón) of Latin America, or North American maple sugar.

But I had my doubts about whether unrefined beet sugar would be anything close to delicious. I'd read that unlike the molasses left over from cane sugar manufacture, beet molasses is fit only for livestock feed. I wondered whether that's because the beet residues are intrinsically unpleasant, or are somehow made so by the particular way beets are handled. Where sugar cane grows above ground and is processed shortly after harvest, beets are dug from the soil, have that distinct earthy odor, and may be stored for months in piles 20 feet high, where they remain alive and can deteriorate. Even fully refined beet sugar sometimes ends up with off flavors.

I ended up making small batches of beet sugar in three different ways. Each time I started by washing the beets, and then trimming and peeling them--two steps not in the industrial flowchart that I hoped would minimize off flavors.

The first time around I ran the beets through a juicer, and got about half the starting weight in juice. Rather than boiling it down over high heat, I gently evaporated it in a gas oven set at 250^oF. (Gas ovens are well vented, so evaporation-slowing humidity doesn't build up.) The juice quickly turned an unappealing brown-gray and developed a strong beet odor, probably due to browning enzymes and perhaps also enzymes that generate earthy-smelling volatiles. After a few hours, I had a beety sweet syrup that cooled into a dull-colored paste. It was edible, but not especially nice.

Next I shredded the beets in a food processor, simmered the shreds in three times their weight in water to extract their juices, strained out the shreds, and evaporated the liquid down. This syrup and paste had a pleasantly mild beet aroma, but they were still an unfortunate grayish brown.

Finally I tried precooking the beets to kill the enzymes before I shredded them and extracted the juices. I sliced the beets, rinsed them to remove enzymes from the surfaces, then microwaved the slices in batches so that they reached the boiling point quickly, in a couple of minutes. The syrup developed only a faint beet aroma and a light gray color that soon disappeared into a cool-toned brown.

All of the beet syrups and pastes were mainly sweet, but with an edge of acidity and saltiness from the other plant materials that were extracted and concentrated along with the sugars. When I cooked them down to the point that they turned dark brown, all had the pronounced acidity and bitterness  of cane molasses along with its characteristic aroma, which mostly masked any beetiness. 

So it is indeed possible to make a good unrefined sugar from a vegetable that can be grown almost anywhere. True, it's not very efficient to do so: evaporating off water burns a lot of energy for the amount of sugar you end up with. (At least you can use the beets efficiently: the greens are essentially the same as chard, and you can squeeze or pan-dry the spent shreds to remove excess moisture, add salt and a little of their own sugar to restore some taste, and then toss with starch or beaten egg to make beet hash browns or latkes). 

But it's an interesting process and product. Along the way I learned that in the beet-growing areas of Germany, Zuckerrübensirup is sold as a spread and to flavor pumpernickel bread dough and sauerbraten.  Earthy sweetness has its uses. 

Curious Cook in the New York Times: Making Candies in the Microwave

In my last Curious Cook column for 2010, I write about the advantages of cooking down candy mixes in the microwave oven, which is more forgiving in several ways than the stovetop. I also give recipes for nut brittle, New-Orleans-style pralines, and saffron-flavored Turkish delight.

 For any home cook who enjoys candy-making or would like to give it a try, I strongly recommend Peter Greweling's recent book Chocolates and Confections, in the Culinary Institute of America's "At Home" series (John Wiley & Sons, 2010).  His Chocolates and Confections: Formula, Theory, and Technique for the Artisan Confectioner (Wiley, 2007) delves deeper into the technical details.

And for an eye-opening review of just how complex the chemistry of candy syrups is, take a look at the review cited below.