[Photos: Rachel Leah Blumenthal / Eater]
Harvard University's annual Science & Cooking public lecture series brings chefs from around the world to lecture on the intersection of science and cooking. And Eater Boston editor Rachel Leah Blumenthal is on the scene. This week: Joanne Chang of Boston's Flour Bakery + Cafe and Myers + Chang.
Last night was only the second lecture of the season, but Joanne Chang's talk undoubtedly smelled better than any of the others possibly could, thanks to a simple demonstration of sugar water heating up. (Plus, attendees got to eat sugar cookies, buttercream, and caramel sauce.)
When Chang gave a lecture last year, she illuminated some of the science underlying four types of baked goods, from cream puffs to pie crusts. This time around, she delved into one of the key building blocks of nearly all desserts: sugar. It's a subject she's been thinking about a lot lately; next spring, she'll release a cookbook called Baking with Less Sugar, featuring recipes that call for low amounts of sugar or use alternatives like honey and maple syrup.
To begin the lecture, Chang examined sugar's numerous roles in desserts (no, it's not just for adding sweetness.) Then, she explained (and demonstrated) the different stages of heating sugar, from the thread stage to caramelizing.
What Does Sugar Do?
Sugar's most obvious role is to sweeten, but are there any desserts where that's its only purpose? Chang could only come up with chocolate and was able to contradict any other suggestions from the audience.
Another reason it's used, she continued, is to be creamed into butter, resulting in light and fluffy cakes and cookies. When the sugar and butter are mixed together, the sugar crystals dig air pockets into the fat. Think about a farmer hoeing the land, aerating it, she said. To reinforce the point, Chang showed photos of two cakes, one made with half as much sugar. The latter was a couple inches shorter — much denser than the light, fluffy one made with a full portion of sugar.
Sugar is also useful in desserts because it's hydrophilic. Like salt, it attracts water. This is particularly useful in terms of shelf life — packaged cookies take a long time to get stale because the sugar helps keep moisture in. For a quick and tasty demonstration of sugar's affinity for water, sprinkle some sugar on strawberries. Wait a bit: some juice will be pulled out of the strawberries.
Sugar is also vital to the formation of popsicles, ice creams, and other frozen treats because it lowers the freezing point. This means that ice cream, stored at zero degrees Celsius in a standard freezer, is "frozen" enough to keep its form but soft enough to eat. But too much sugar in a popsicle, for example, creates a soupy mess.
And meringue? Sugar's working hard there too, stabilizing beaten egg foam. Chang demonstrated by beating egg whites alone and comparing them to a beaten mix of egg whites and sugar. The latter was a lot more stable. Egg whites are mostly made of water and protein, and when beaten, the proteins trap air bubbles, making a temporarily fluffy substance that eventually deflates. But when sugar is added, it acts like packing peanuts, she explained. Because of sugar's affinity for water, the two combine into a syrup that protects the foam bubbles, keeping the mixture fluffier for a longer time. This is essential for desserts like lemon meringue pie and angel food cake, as well as buttercream frosting.
Sugar also tenderizes by inhibiting gluten development. As discussed last year, two proteins in wheat flour, gliadin and glutenin, don't really interact until liquid is added. Then, they bond to form gluten, a web-like protein composite that gives bread its great chewy texture. Great for bread...but not so much for cake. Similar to its role in protecting the foam bubbles in a beaten egg foam, sugar helps shield gliadin and glutenin from bonding with each other, thus inhibiting the formation of gluten and yielding tender cakes.
The Maillard reaction, where food browns and gets a deeper flavor, depends on sugar as well. With enough heat (somewhere around 310 degrees Fahrenheit, generally), proteins and sugars reconfigure themselves into ring-like structures that reflect light differently, giving off a brown color. This posed a problem in photographing recipes for Chang's upcoming cookbook. No sugar, no browning — does a pale sour cream cake look appetizing?
And finally, sugar plays a role in making pastries crispy. When a cookie, for example, is baking, the sugar liquefies at first and then recrystallizes as water evaporates, adding crunch.
From Candy to Croquembouche
When heated with water, sugar has a number of distinct stages that can be described in terms of candy-making. Each stage corresponds to both a temperature range and a resulting ratio of sugar to water, so it's possible to carefully manipulate the speed at which the reaction occurs by starting with more or less water. Chang's bakers, for example, start with the minimum necessary water to save time. But if time is no concern, start with more water so it's easier to avoid crystallization (which will almost definitely ruin the batch). Sugar can crystallize at any stage of the reaction, but a couple ways to avoid it are to watch out for any sugar crystals stuck on the side of the pan (they can act as a seed, causing a chain reaction of crystallization, so make sure everything gets dissolved) and don't move the syrup while boiling. Once it caramelizes, you're safe.
The names of the stages refer to how a bit of the hot syrup will perform if dropped into cold water. First, sugar reaches thread stage, which occurs between 230 and 235 degrees Fahrenheit. At this point, the mixture is a syrup that is 80% sugar. It's good for sweetening tea or glazing fruits. When dropped in cold water, it forms a thread.
Soft-ball stage happens between 235 and 240 degrees Fahrenheit and a sugar concentration of 85%. At this point, it forms a soft ball when dropped in cold water, but that ball will flatten in your hand. Fudge and fondants are made at this stage.
At 245 to 250 degrees and 87% sugar, it's the firm-ball stage, at which point the syrup will form a ball that doesn't immediately flatten but will if you squeeze it. This (and not the later caramelizing stage) is where soft, chewy caramels are made. Firm-ball syrup can also be mixed with egg whites to make Italian meringue, which is more stable than the French meringue demonstrated earlier (the egg whites with straight-up sugar). Italian meringue and butter form buttercream.
And hard-ball stage comes next, with the temperature ranges beginning to widen out. This one lasts from 250 to 265 degrees, and the sugar concentration is up to 92%. The ball will be hard enough to hold its shape unless you really squash it. This is the stage for gummies, marshmallows, and rock candy.
Then the sugar moves out of the ball stages and into soft-crack stage between 270 and 290 degrees. Most of the moisture is gone and the sugar concentration is 95%. The syrup will now form flexible threads in cold water, and you can bend them a little before they break, perfect for salt water taffy.
Finally, the hard-crack stage occurs between 300 and 310 degrees with a sugar concentration of 99%. The syrup forms brittle threads in cold water, and it's used for things like lollipops and nut brittles.
Beyond hard-crack, the sugar progresses past candy and into caramelization. At this stage, you might make praline, for example, by mixing nuts into the syrup, pouring it all out on a tray, and letting it cool. You could also make caramel sauce by adding cream and, optionally, butter, salt, and vanilla. Chang provided a hint for making caramel sauce: don't trust the color you see just by looking in the pan. Tilt the pan, and then look at the sheen on the bottom. That's how you'll know when it reaches the proper brown shade. Chang also repeated a popular demonstration from last year, making spun sugar for a croquembouche (basically flicking caramelizing sugar over a tower of pastry balls).
While the precise numbers at each stage show why a candy thermometer is virtually essential to the candy-making process, if you have enough practice, you might learn to go thermometer-free. Chang is able to hear differences among stages; for example, there's an audible thickening of the mixture between soft-ball and firm-ball. You can also carefully test by dropping bits of the mixture into cold water and feeling for effects noted above. (Beware of burns.)
— Rachel Leah Blumenthal
The Harvard Science & Cooking lecture series continues next Monday, September 22, with Mark Ladner of Del Posto presenting "Al Dente: When Plastic Meets Elastic." (His new gluten-free pasta truck, Pasta Flyer, will be parked outside from 11:30 a.m. to 9 p.m. for those looking to buy an on-theme lunch or dinner.) Seating for the free event is first come, first served, and more details can be found on the Harvard website.