Thursday, January 18, 2018

Guest Post on Kitchen Torch Chemistry by Len

Kitchen Torch Chemistry

We have been having great fun with our new kitchen torch. I have found it amusing to look into what it does to food.  I have found that knowing a little about the chemistry of browning food has helped in finding ways to use the gadget.

When we brown food with the torch we are enabling two different chemical processes depending on what we are cooking: caramelization of sugar or browning of protein (the Maillard reactions). In both cases the goal is the same: the heat of the torch turns simple flavors into complex ones; that’s what cooking is mostly about. The background material to this is completely standard. A good source is Harold McGee’s classic book, On Food and Cooking, Chapter 14.

Caramelization:  Sugar has a very simple flavor, detected by receptors in the taste buds on the tongue. Table sugar is a colorless crystal which has no odor. If you heat it rapidly to 320F (160C) it melts to form a colorless liquid. However, around that temperature it also starts to break apart due to the heat. This is a slow process until about 338F (170C) when the rate of decomposition increases rapidly. The syrup turns brown – it becomes caramel and smells delightful. What’s happening is that the fragments of sugar recombine to make more than 100 different products, many of which can be sensed by our olfactory bulb. That’s how we sense complex flavors.

There’s are problem though: sugar burns at 350F (177C). We need to work in a narrow temperature range, which sounds tricky. In fact, it can be done, and  American children (or at least the patient ones) are trained to do it when they learn to toast marshmallows. Remember, brown them, don’t burn them.
Fig 1:  A torch is a great way to toast marshmallows in the winter.
Well, I guess we did burn them a bit.
The second problem is more serious. Water is usually present In food, and water boils at 212F (100C). If the torch is used on a wet surface, all the heat will go into evaporation, and the temperature will stick at 212F (100C), well below the caramelization temperature. A torch is a really bad tool for boiling water.

In crème brûlée this difficulty is avoided in an elegant way. The dish is made of a custard (flan or crème pâtissière) which is allowed to gel. Then sugar is put on top and caramelized. The custard is quite moist, but the surface need not be because the of the “skin” on the surface. A dense surface layer is quite common in gelation, and it seems to seal in the water. Making crème brûlée with a torch is dead easy, and the torch is an ideal tool. The contrast between the solidified caramel surface and the smooth custard underneath is wonderful. This has been one of my favorite desserts for a long time. See Mae’s previous post (link) for our success in the first try.

Encouraged, we decided to broaden our scope. We looked on the web and found two recipes that somehow made caramel in wet surfaces. The first is grapefruit brûlée, allegedly grapefruit with a nice crisp top. However, when you cut a grapefruit you release juice, and the surface is simply wet. We failed on our first attempt to make this dish. We followed a recipe that said to just put sugar on and torch it.  We did this, but the crisp texture of a caramel was simply not there. We made things worse by following our habit of pre-cutting the grapefruit sections to make them easier to eat.

Of course, there is a nice thing about this kind of experiment: you can eat the failures. A warmed-up, sugared grapefruit is fine for breakfast.

All undaunted we decided to use our insight into the mechanisms involved. We melted some butter and brushed it on the surface so that this would seal in the water long enough to make caramel. Then we sprinkled sugar on top and torched. It worked, and the dish was delicious.

Fig 2: Grapefruit brûlée made with melted butter.
Note the flake of caramel.
Even stranger was the claim that you can make oatmeal brûlée. We make oatmeal by boiling rolled oats in water in the microwave. It’s a wet dish. We did the straight-forward thing that recipes suggested (using drier oatmeal than we usually do). We failed. The result was sugared oatmeal – no bad thing, and we ate it, but not what we were after.

Following a suggestion by Evelyn we decided to seal this surface in a different way, by adding honey. Honey is a supersaturated solution of sugars (glucose and fructose). A thick honey doesn’t have much water, and is known to caramelize. We melted thick honey in the microwave, poured it on top of our oatmeal, and torched vigorously. We could easily smell caramel as we proceeded, and the result was extremely nice.

Fig 3: Oatmeal brûlée made with honey.
Caramelized honey has a nice, complex flavor.
Maillard Reactions: The Maillard reactions occur when a small sugar is heated in the presence of an amine (nitrogen-containing) group. Proteins are made of amino acids, and are usually the target of the reaction. The sugars occur naturally in the food. The temperature required for rapid reaction is 280-330F (140 – 165C). When the reaction proceeds many hundreds of reaction products are made. They are very complex: the wonderful odor and taste of grilled meat is produced this way, as is the brown crust of bread or cookies (they smell good too.)

The problem of moist surfaces is not usually present in this case. Broiled meat may be moist inside, but the surface is dry. We have shown in a previous post how we torched a steak after broiling. We put more black pepper on the steak than we usually do, and the result was quite interesting because we liberated the aromatic oils in the pepper. Our next experiment will be to use green peppercorns. There is a classic French recipe for steak with green pepper sautéed in a hot pan. We want to try broiling and then torching. Stay tuned.

The reductio ad absurdum of this kind of thing is something we have read about, sous-vide cooking of steaks. Professional chefs put steaks in plastic pouches and heat them in water baths at 135F (55C) to for an hour or two. There is no Maillard browning at all, but uniform cooking. Then the steak is torched on the surface. The photos we have seem make a steak look like an industrial product, some kind of plastic molding. We have no idea how these productions taste.

A simple example of Maillard is the browning of toast, and a torch can do it, though it’s a bit perverse. However, we can nicely combine the two browning reactions by making cinnamon toast. We buttered bread, sprinked on sugar, and torched. Excellent result.

Fig 4: Cinnamon toast came out very well.
Notes on technique: We use a butane torch made by Iwatani that we bought on Amazon. It’s fine for our purposes. Professional chefs use more powerful propane torches, the kind that plumbers use to solder pipes. I have been strictly forbidden to even think about bringing such a thing into our small, not very fireproof kitchen.

Fig 5. Our torch.
That said, I have found that I do want a pretty hot flame. I turn the gas all the way up and adjust the air so that the flame roars. The exception is for marshmallows and crème brûlée. For these dishes almost anything will do.

Do you need a torch? Of course not. Unless you want to solder pipes, anything a torch can do can be done with an oven broiler or a toaster oven. (Okay, not the marshmallow). But playing with fire is really cool, and I highly recommend it.

NOTE FROM MAE: The guest author of this post is my husband Len, a Professor of Physics at the University of Michigan and (now) dedicated kitchen torch user.


Angela said...

I always love having a new kitchen gadget! I end up trying to cook everything with it!

Jens Zorn said...

Very informative post, Len. Particularly encouraging in that even your less successful tries yield quite tasty results. I look forward to your article on cooking with CW lasers!

Judee Algazi said...

Great information to help understand the workings of the torch with food. The oatmeal brulee looks especially interesting. I bought one a year ago and have yet to try it! I thought it was only for creme brulee- You've inspired me.. thanks Mae

Tina said...

We have a torch! Our son gave us one quite a while back but I have only used it once. This looks like fun and wow, so informative. Thanks to your husband for guesting here!

Beth F said...

I have a smaller kitchen torch, which I don't use very often, but it's fun for meringue and creme brulee.

Debra Eliotseats said...

You are going torch-happy, Mae! :) Love the looks of those grapefruits!

Claudia said...

Very interesting, my next gadget might just be a torch:) The current one is a sous-vide device, which I'm enjoying.

Vicki said...

I have so many kitchen gadgets it's ridiculous. I may be adding a torch to my collection.

Geraldine Saucier said...

You have some great ideas for using a kitchen torch. I need to go and get one.

Carole said...

this is very interesting. I have a torch but haven't worked up the courage to give it a go yet Cheers