by Jeff Potter
Instructions for use. Dissolve 0.5% to 2% agar by weight in cold liquid and whisk to combine. Bring liquid to a boil. As with carrageenan, you can create a thicker concentrate and add that to a target liquid if the target liquid can’t be boiled. Compared to carrageenan, agar has a broader range of substances in which it will work, but it requires a higher temperature to set.
Use. Agar is a gelling agent, used in industry in lieu of gelatin in products such as jellies, candies, cheeses, and glazes. Since agar is vegetarian, it’s a good substitute in dishes that traditionally call for gelatin, which is derived from animal skins and bones. Agar has a slight taste, though, so it works best with strongly flavored dishes.
Origin and chemistry. Derived from seaweed. Like carrageenan, agar is a seaweed-derived polysaccharide used to thicken foods and create gels. When heated above 185°F / 85°C, the galactose in agar melts, and upon cooling below 90–104°F / 32–40°C it forms a double-helix structure. (The exact gelling temperature depends on the concentration of agar.)
During gelling, the endpoints of the double helices are able to bond to each other. Agar has a large hysteresis; that is, the temperature at which it converts back to a gel is much lower than the temperature at which that gel melts back to a liquid, which means that you can warm the set gel up to a moderately warm temperature and have it remain solid. For more information on the chemistry of agar, see http://www.cybercolloids.net/library/agar/properties.php.
Agar at the molecular level. When heated, the molecule relaxes into a relatively straight molecule (upper left) that upon cooling forms a double helix with another agar molecule (center). The ends of these double helices can bond with other agar double helices (upper right), forming a 3D mesh (left).
Technical notes
Gelling temperature
90–104°F / 32–40°C
Melting temperature
185°F / 85°C
Hysteresis
140°F / 60°C
Gel Type
Brittle
Syneresis
Yes
Concentrations
0.5%–2%
Synergisms
Works well with sucrose
Notes
Tannic acid inhibits gel formation (tannic acid is what causes overbrewed tea to taste bad; berries also contain tannins)
Thermoreversible
Yes
Chocolate Panna Cotta
Agar can be used to provide firmness, as this example shows. In a saucepan, whisk together and gently simmer (below boiling—just until small bubbles form on surface) for one minute:
3½ oz (100g) milk
3½ oz (100g) heavy cream
½ pod vanilla bean, sliced lengthwise and scraped
8 teaspoons (20g) powdered sugar
1 teaspoon (2g) agar powder
Turn off heat, remove vanilla bean pod, and add, briefly stir, and let rest:
3.5 oz (100g) bittersweet chocolate, chopped into fine pieces to assist in rapid melting
After a minute, add and whisk to thoroughly combine:
2 eggs yolks (reserve whites for some other recipe)
Pour mixture into glasses, bowl, or molds and store in fridge. The gel will set in as little as 15 minutes, depending upon the size of the mold and how long it takes the mousse to drop below agar’s setting point (around 90°F / 32°C).
Notes
The agar provides a firmness that creates a stronger mousse than that created when using gelatin, so you should plan to use this mousse in applications where firmness is a desired trait.
This chocolate mousse, while good by itself, really works better as a component in a dish. Example uses: roll a ball of the mousse in toasted nuts to create a truffle-like confection, spread a layer of the mousse into a prebaked pie crust and top with raspberries and whipped cream, or smear a thin layer of the mousse in the bottom of a bowl and place a small scoop of vanilla ice cream and some fresh fruit on top.
When working with a vanilla bean, use a spoon or the edge of a knife to scrape the seeds from the pod, and add both pod and seeds to your mixture. Scraping the bean helps get the vanilla into the mixture more quickly.
Rum Screwdriver Gel
In a small mixing bowl, measure out:
8 teaspoons (40g) rum
In a saucepan, whisk to combine, and bring to a boil and hold for an additional minute:
10 teaspoons (50g) orange juice
¼ cup (40g) sugar
1 teaspoon (2g) agar powder
Pour the hot liquid into the small mixing bowl, and stir thoroughly to combine. Transfer mixture to a glass, ice cube tray, or other food mold and store in fridge for 30 minutes or until set.
Notes
Yes, these are basically rapid-setting Jell-O shots. Using agar allows for a higher percentage of alcohol—you can gel rum by itself if careful—but make sure to leave enough juice in for flavor.
Play with substitutions. You can replace the rum and orange juice with fluids such as Malibu and coconut milk.
Making gels: Sodium alginate
The gels covered so far are all homogenous, in the sense that they are incorporated into the entire liquid and then set with heat. Alginate, however, sets via a chemical reaction with calcium, not heat, which allows for an interesting application: setting just part of the liquid by localized exposure to calcium.
This is done by adding sodium alginate to one liquid and calcium to a second liquid and then exposing the two liquids to each other. The sodium alginate dissolves in water, freeing up the alginate, which sets in the presence of calcium ions, which will only occur where the two liquids touch. Imagine a large drop of sodium alginate–filled liquid: the outside of the drop sets once it has a chance to gel with the assistance of the calcium ions, while the center of the drop remains liquid. It’s from this application that the technique called spherification is derived.
Instructions for use. Add 1.0% to 1.5% sodium alginate into your liquid (use water for your first attempt). Let the liquid rest for two hours or so to hydrate fully. It will be lumpy at first; don’t stir or agitate the liquid, as doing so will trap air bubbles in the mixture.
It’s probably easiest to add the sodium alginate a day in advance and let it hydrate in the fridge overnight.
In a separate water bath, dissolve calcium chloride to create a 0.67% solution (about 1g calcium chloride to 150g water).
Carefully drip or spoon some of your sodium alginate liquid into the calcium bath and let it rest for 30 seconds or so. (You can use a large "syringe" dripper or turkey baster to create uniformly sized drops.) If your shape floats, use a fork or spoon to flip it over, so that all sides of it are exposed to the calcium bath. Remove from bath, dip into another bowl of just water to rinse off any remaining calcium, and play.
Sodium alginate gels firm up over the span of a few hours, so you’ll need to make these near when you intend to serve them.
If your sodium alginate sets without exposure to the calcium bath, use filtered or distilled water. Hard water is high in calcium, which can trigger the gelling reaction.
Uses. The food industry uses alginate as a thickener and emulsifier. Since it readily absorbs water, it easily thickens fillings and drinks and is used to stabilize ice creams. It’s also used in manufacturing assembled foods; for example, some pimento-stuffed olives are actually stuffed with a pimento paste that contains sodium alginate. The olives are pitted, injected with the paste, and then set in a bath with calcium ions to gel the paste.
Origin and chemistry. Derived from the cell walls of brown algae, which are made of cellulose and algin. Alginates are block copolymers composed of repeating units of mannopyranosyluronic and gulopyranosyluronic acids. Based on the sequence of the two acids, different regions of an alginate molecule can take on one of three shapes: ribbon-line, buckled shape, and irregular coils. Of the three shapes, the buckled shape regions can bind together via any divalent cation. (A cation is just an ion that’s positively charged, i.e., missing electrons
. Divalent simply means having a valence of two, so a divalent cation is any ion or molecule that is missing two electrons.)
Alginate does not normally bind together (left), but with the assistance of calcium ions is able to form a 3D mesh (right).
Gel "Noodles" and Dots
This is really just a quick experiment to illustrate how to work with sodium alginate.
Create a 1% solution of sodium alginate and water. Add food coloring so that you can see the mixture as you work with it. Using a squeeze bottle, pipe out a strand into a bowl containing a 0.67% solution of calcium chloride in water.
Try making drops and other shapes as well. One food trend that’s still making the rounds is mini "caviar." The small drops of set sodium alginate liquids have a similar texture and feel as caviar but with the flavor of whatever liquid you use.
Once you’ve played with this using water, try using other liquids. Jolt Cola? Cherry juice? Keep in mind that liquids that are high in calcium or very acidic will cause the alginate solution to gel up on its own.
Spherification in shapes
Since sodium alginate sets via a chemical reaction, not a thermal one, you can freeze a liquid into a mold and then thaw it in a calcium bath to cause it to partially maintain its shape. The final shape won’t retain the crisp edges of the original frozen shape—it’ll swell and bloat out slightly—but you’ll still get a distinctive shape.
Note
Note that straight-up lime juice won’t work, because the alginate will precipitate out in the presence of strong acids. If you’re willing to experiment further, try using sodium citrate to adjust the pH.
Try freezing the liquid in a mold before setting the sodium alginate to get more complicated shapes.
Mozzarella spheres
What happens if you want to use sodium alginate in a food that already contains calcium? Depending upon the amount of calcium in the food, adding the sodium alginate straight to it would cause the liquid to set, giving you something similar to a brittle gel.
Swapping the chemicals—adding the calcium chloride to the food and setting it in a sodium alginate bath—doesn’t work; calcium chloride is nasty-tasting. Luckily, it’s the calcium that’s needed for the gelling reaction, not the offensive-tasting chloride, so any compound that’s food-safe and able to donate calcium ions will work; calcium lactate happens to fit the bill. This technique is called reverse spherification.
To create mozzarella spheres, mix 2 parts mozzarella cheese with 1 part heavy cream under low heat. Add around 1.0% of calcium lactate to this liquid and then set it in a water/sodium alginate solution of 0.5% to 0.67% concentration.
Ann Barrett on Texture
PHOTO USED BY PERMISSION OF ANN BARRETT
Ann Barrett is a food engineer specializing in food textures. She works for the Combat Feeding Directorate of the U.S. Army Natick Soldier Research, Development and Engineering Center (NSRDEC).
What does a food engineer do?
It’s like applied chemical engineering but for food. The training focuses on how to process food and how to preserve food, looking at food as material. I happen to have a specialty in food texture or food rheology; rheology means how something flows or deforms. My PhD topic was on the fracturability of crunchy food. How do you measure crunchiness or fracturability, and how do you quantitatively describe the way a food fails? When you chew a food and it breaks apart, can you describe that quantitatively and then relate that to the physical structure of the food?
Tell me a little bit about the NSRDEC.
There are several RDECs (research, development, and engineering centers) throughout the country. NSRDEC is focused on everything the soldier needs for survival or sustenance, aside from weaponry: food, clothing, shelters, and parachutes. The food part is largely driven by the fact that the military is potentially deployed in every kind of physical environment, so we need a wide range of foods to support soldiers operating in a wide range of situations. They have large depots of rations, and that drives a very long shelf-life requirement. Most of the food that we make needs to be shelf-stable for three years at 80°F / 26.7°C. That is not to say that the soldier will always eat something that’s three years old, but they definitely might. That drives a lot of the research here; foods that are shelf-stable but also good, that the soldiers will want to eat.
It must be really interesting to work with the constraints that you’ve got while trying to preserve flavor and texture. How do you go about doing that?
Well, it’s often one part experience or knowledge combined with two parts trial and error. There’s a lot of bench-top development here. Most of my experience has been in processing and engineering analysis of food, but I do have a project now where we’re trying to develop flavors for sandwich fillings. All flavors are chemicals, so you can replicate a natural flavor by knowing what the chemistry is.
For example, we’re working on a peanut butter filling for sandwiches. We’re trying to make a chocolate peanut butter flavor, a bit like Nutella. We have the peanut butter formula, and we’ve been looking at adding cocoa and at different chocolate flavors. We put three into storage to see how they would work, and two of them came out just okay, and one of them was delicious. When you’re developing something, you have to look at a number of different ingredients to see what works. There will be changes in both the flavor and texture of a food during long-term storage. Flavors tend to become less intense, or off-flavors might develop. Texture can degrade by moisture equilibrating, say in a sandwich, or by staling. There are a multitude of flavors that are commercially available, and also a multitude of ingredients that will adjust texture—for example, starches and gums for liquid or semi-solid foods, enzymes and dough conditioners for breads. So during development you need to optimize a formula to make sure the food is good after you make it and also good after storage.
Even with all the hard science, you still have some degree of, well, "Let’s just try it and see what happens?"
Oh, absolutely. You make a product up for a project, sample it, store it, and then sample it again. Everything is actually tasted here, and as a matter of fact, part of our duties is to go over and participate in the sensory panels because the food scientists here, the nutritionists, the dieticians, are all considered expert tasters. The first thing we do is make our product on the bench and then put it into a box that’s 120°F / 49°C for four weeks. Those conditions approximate a longer period of time at a lower temperature; it’s just a quick test to see if quality holds up. If the product holds up, next is 100°F / 38°C for six months; that’s supposed to approximate the quality you would get at three years at 80°F / 26.7°C. Then you have to check that it’s microbiologically stable, so it goes to the microbiologist for clearance, and then you can ask people to come and evaluate it. We rate the appearance, aroma, flavor, texture, and the overall quality.
How does the science of food texture work into enjoyment of food?
There are expected textural properties of whatever food category you’re dealing with. Sauces are supposed to be creamy; meats are supposed to be at least somewhat fibrous; bread and cake are supposed to be soft and spongy; cereals and crackers are supposed to be crunchy. When texture deviates from what’s expected, the food quality is poor. If you are going to measure and to optimize the texture of a product, you need to pinpoint the exact sensory properties you want.
For example, for liquids, flowability or viscosity is the defining physical and measurable characteristic. There are "thin" liquids and "thick" liquids, and you can often change thin to thick by adding hydrocolloids or thermal treatment. Solid foods come in many different textural types. There are elastic solids that spring back after deformation (Jell-O); there are plastic solids that don’t (peanut butter). Then besides "solid" solids there are also porous solids—think bread, cake, puffed cereal, extruded snacks such as cheese puffs. Porous foods have the structure of sponges, and like a wet versus a dry sponge, they can be elastic or brittle.
Somebody co
oking in the kitchen is actually manipulating these things both physically and chemically?
Yes, that’s exactly what cooking is. Take cooking an egg. The protein albumin will denature with heat, causing molecular crosslinking and solidification. Another example is kneading bread dough, which is a mechanical rather than a thermal process that makes the gluten molecules link up; that gluten network is what allows the bread to rise because a structure is developed that will hold gas liberated by yeast. And of course, every time you use cornstarch or flour to thicken a gravy or sauce, you’re employing a physico-chemical process. Heat and moisture will make the starch granules absorb water and swell and then bleed out individual starch polymers, which are like threads attached to the granules. The starch polymers then entangle, creating an interconnected structure that builds viscosity. That’s why your gravy gets thick.
Gravy
Flour (roux method)
Create a simple roux by melting 2 tablespoons (25g) of butter in a saucepan and then adding 2 tablespoons (17g) of flour. Stir while cooking over low heat until the roux sets and begins to turn light brown, about two to three minutes.
