The Science Behind Effervescence

Recently, I tried making effervescent caramels, which were not super successful. However, the chemistry behind the particular type of effervescence I created is pretty simple and easy to understand.

The two additives I combined for the effervescent coating were citric acid and sodium bicarbonate. Citric acid is commonly used for both flavoring citrusy foods and acidifying solutions because it is found in many citrus fruits. It is also very important physiologically; it is involved in the oxidation of fats and proteins and it also facilitates the conversion of carbohydrates into carbon dioxide and water during cellular respiration. (Cellular respiration is the process by which we convert food into energy.) At room temperature, it exists as a white powder, which makes it easy to use in molecular gastronomy.

Don’t get any funny ideas.

Sodium bicarbonate is the main ingredient in baking soda and is a very unique chemical compound. It is amphoteric, which means that it can react with both acids and bases to neutralize the pH of both. Because of this, though, it is neither a very strong base nor a very strong acid, meaning that it can really only neutralize weak acids and bases.

Luckily for this reaction, citric acid is a weak acid! The chemical reaction that takes place between sodium bicarbonate and citric acid is as follows:

As can be seen from the fabulous diagram, the rightmost hydroxide functional group (OH) on the sodium bicarbonate combines with the leftmost carboxylic acid (COOH) on the citric acid to form carbonic acid. The leftover parts bond to make sodium citrate. Carbonic acid is a fairly weak compound and will fall apart, yielding water and carbon dioxide. This carbon dioxide is what we feel as the fizzy part of the reaction.

So why are we able to coat caramel with this mixture of powders without the fizz escaping right away? The compounds need an aqueous environment (more plainly stated, they need to be dissolved in water) to be able to interact enough to react. When the caramels are ingested, the saliva in your mouth provide enough moisture to allow this reaction to occur. As long as they are dry, the reagents won’t undergo the acid-base reaction to form fizzy carbon dioxide.

Well, that was probably the simplest chemical reaction that I’ve researched so far. Effervescence is extremely easy to create, even for novices in molecular gastronomy. I’m going to continue reviewing molecular gastronomy as I design my lesson plans, so stay tuned for more posts.

The Science Behind Dry Ice and Liquid Nitrogen Ice Cream

Two cooling agents used pretty uniquely in molecular gastronomy are dry ice and liquid nitrogen, which are molecules that are commonly found in nature but are in a different form.

Dry ice is solid carbon dioxide, the gas version of which is in the air (at about a .035% concentration), in our bodies as we produce energy from glucose, and in carbonated drinks. The reason we typically only see carbon dioxide as a gas or as a solid has to do with its phase diagram:

The state of a substance (i.e. whether it’s a solid, liquid or gas) is dependent on two factors: pressure and temperature. Pressure on Earth is typically around 1 atmosphere, while temperature can vary depending on the location. As can be seen in the diagram, at 1 atm, carbon dioxide can either be a solid or a gas, but not a liquid; only at a pressure of 5.11 atm can we begin to see carbon dioxide in a liquid form.

Unlike water, which freezes at zero degrees Celsius, carbon dioxide freezes at -78.5 degrees Celsius. This is because of the great difference in their intermolecular forces. Water molecules experience hydrogen bonds with other molecules because they have such a strong dipole moment, meaning that the oxygen atom has such a strong pull on the hydrogen atoms’ electrons that the molecule develops a positive and negative charge. These charges are attracted to the opposite charges of other molecules, forming a tight-knit group of molecules, so water is a substance that is easy to freeze.While oxygen atoms have a great pull on carbon’s electrons in the carbon dioxide molecule, the symmetrical shape of the molecule gives a neutral charge on both sides. Because of this, carbon dioxide has few intermolecular forces and it is more difficult to force them to stick together as a solid as opposed to a free-flowing gas.

Liquid nitrogen, on the other hand, is usually seen in either liquid or gas form. Nitrogen, or N_2, has a completely symmetrical structure with an extremely strong triple bond between the two nitrogen atoms. It has virtually no intermolecular forces, especially because the two nitrogen atoms attract electrons equally well, so it is extremely hard to force nitrogen molecules into liquid or solid states. Unlike carbon dioxide, nitrogen can exist in all three states at 1 atm:

Its boiling point, at -195.8 degrees Celsius, is also lower than carbon dioxide’s freezing point, making liquid nitrogen colder than solid carbon dioxide.

Now, the question left is: why does pouring/adding dry ice or liquid nitrogen to a substance freeze it? The answer may seem simple (well, it’s cold!), but it’s a little more complicated than that. Temperature, be it in Fahrenheit, Celsius, or Kelvin, is a measure of energy (of the kinetic variety). When substances are surrounded by temperatures above their freezing/boiling points, the energy from that higher temperature will flow into the substance with the lower temperature in order to begin melting or subliming that substance. Until all of the substance is melted or sublimed, the unchanged portion of that substance will maintain the same temperature. It’s kind of confusing, so here’s an example: liquid nitrogen’s boiling point is at -195.8 degrees Celsius. If we pour one liter of it into a sorbet base at room temperature (about 25 degrees Celsius), the energy from the sorbet will go into evaporating the liquid nitrogen, thereby cooling the sorbet base and reducing the amount of liquid nitrogen. As long as there is liquid nitrogen in the bowl, though, its temperature will stay at -195.8 degrees. Eventually, when all of the liquid nitrogen has evaporated, the sorbet is probably at a temperature below the freezing point for water because so much energy has been removed from it. As such, at that point it would be a solid sorbet. It can be demonstrated by this graph:

Substances do not change temperature when they are in the process of changing phases. That’s the basic principle behind dry ice and liquid nitrogen ice creams.

The Science Behind Gelification

Before doing research on gelification, I assumed that it was a simple chemical process because it was so easy to do in the kitchen. I was completely wrong.

Additives that are involved in gelification belong to a category called hydrocolloids. A colloid is a substance that evenly disperses within another, so hydrocolloids dissolve and disperse evenly throughout water. Hydrocolloids used in molecular gastronomy are polysaccharides made up of glucose molecules, which are polar. Therefore, they attract to water molecules, as shown by this fabulous diagram:

It’s a little more complicated than a ribbon and balls.

Anyways, water is attracted to the glucose molecules and, through intermolecular forces, become semi-immobilized. This is what creates a gel. Based on the specific structure of the hydrocolloid molecules, the characteristics of the gel differ. Various properties include:

• Thermoreversibility: thermoreversible gels melt when heated and set when cooled again (except gels made from methylcellulose, which do the opposite). Thermoirreversible gels do not melt when heated.
• Tendency for Syneresis: Syneresis is the extraction of a liquid (often water) from a gel. Firmer gels tend to “weep” more, especially when being thawed after having been frozen or when pressure is applied to the gel. Agar agar gels are a notable example of gels with a tendency for syneresis.
• Freeze-thaw stability: This property kind of relates to syneresis. Some gels can be frozen and thawed repeatedly, but most degrade as their structural components are compromised. When multiple different kind of hydrocolloids are used in a gel, it is much more stable because of synergistic relationships between many of them.
• Clarity: Some gels are more transparent than others.

• Flavor release: This characteristic has to do with the textural properties of the hydrocolloid and how well it expresses the flavor of the liquid it has gelled. Gelatin, for example, melts at mouth temperature and therefore has great flavor release. Alginate is a very stable gel at body temperature, though, so it has poor flavor release.
• Shear reversibility: Shear is the force that occurs when objects take a parallel path in opposite directions, like the action done by scissors. Shear reversible gels reform after being broken by this type of force, but most gels can’t.
• Gel flow: hydrocolloids that thicken liquids are judged by the following flow properties:
  • Shear thinning: when mixed, most liquids thickened by hydrocolloids get less viscous. In this way, they are non-Newtonian liquids.
  • Yield point: These are liquids that act like a gel when at rest, but liquefy when sheared. These are called thixotropic fluids, a notable example of which is ketchup. It’s extremely hard to get flowing in those glass bottles, but when it does, it’s like a landslide.
  • Fluid gels: some gels look solid when on the plate, but act like fluids when in the mouth. Some agar gels look very solid but have a creamy, sauce-like texture in the mouth.

There are many different types of hydrocolloids (too many to list here!), most of which have different sets of properties. I’ll go into two of the most commonly used ones now: agar agar and carrageenan.

Carrageenan is an extract from red algae. There are three types of carrageenan: iota, kappa, and lambda, all of which have slightly different characteristics.

Iota carrageenan makes flexible, elastic gels in the presence of calcium ions. It is is fairly clear, but does not dissolve in cold water. My suspicion is that this is because iota carrageenan is not very polar, so the increased energy (or temperature) in the water molecules is necessary to develop intermolecular forces with the iota carrageenan molecules. As such, the solution must be heated to over 60 degrees Celsius to dissolve the carrageenan fully, and it will gel as it cools down.

Kappa carrageenan forms firm but elastic gels in the presence of potassium ions and brittle gels in the presence of calcium ions. The solubility and gelling processes are the same as iota carrageenan. It can be thinned with sugar and is thermoreversible.

Lambda carrageenan is not used for forming gels; rather, it is used solely as a thickening agent.

Agar agar is a hydrocolloid that forms heat-resistant gels while cooling between 32 and 43 degrees Celsius. It is able to retain its firmness up to about 85 degrees Celsius, or 185 degrees Fahrenheit, making it the optimum additive to make gels designed to be consumed while hot. It has much stronger gelling properties than gelatin, so a concentration of less than .01 molal is necessary. The firmness of these gels is directly proportional to the concentration of agar agar. High concentrations yield firm, brittle gels, while low concentration gels are “supple and fragile” (KitchenTheory.com). Agar agar is a very diverse hydrocolloid, also used to thicken pie fillings because of its heat resistance, stabilize ice creams in conjunction with other vegetable gums, create foams when put into a siphon. It is soluble only in water, not in alcohol or nonpolar liquids.

Well, that was a long winded post. Gels differ greatly based on the hydrocolloid(s) used to thicken them, so as you can imagine, there are near infinite combinations. I want to thank KitchenTheory.com, which really helped me understand the science behind gelification. I am working on trying liquid nitrogen ice cream next. Stay tuned!

The Science Behind Spherification

Earlier this week, I did spherification for the first time; you can find the link here. I searched into why this process occurs, and I found some interesting results.

The two main components involved in spherification are alginate strands, usually found in the additive sodium alginate, and calcium ions, which can come from calcium chloride, calcium lactate, or calcium gluconate. One of these ingredients is dissolved into a distilled water bath, while the other is dissolved in the liquid you want to spherify. It depends on whether you want to do basic spherification or reverse spherification based on the liquid’s properties.

The structure of an alginate strand is as follows:

When sodium ions (Na⁺) are bonded to the oxygens, the strand is fairly flexible and soluble in liquid. If a solution containing sodium alginate is dropped into a calcium ion bath, however, the calcium ions bond to the alginate strands, replacing the sodium ions. Calcium ions have a positive two charge (Ca²⁺), and therefore must make two bonds to complete their electron shell and become stable molecules. Because they are taking the place of sodium ions, they must make an additional bond to satisfy this requirement. The result looks something like this:

The molecules are all stuck together in a huge network! This eliminates their flexibility, making them more rigidly bonded together. To us, this looks and feels like a thin gel, which makes up the spheres’ thin membrane.

There are a few caveats that we have to keep in mind when spherifying liquids. First, if a liquid contains a non-negligible amount of calcium, we must do reverse spherification as opposed to basic spherification (reverse spherification is the process of mixing calcium ions into the liquid and making a sodium alginate bath instead of the other way around). Doing basic spherification with this type of liquid would cause the alginate molecules to prematurely react with the calcium in the liquid, forming a huge network of alginate strands within the whole body of the liquid, instead of just making a small membrane around it. Reverse spherification solves this problem by increasing the calcium concentration in the liquid so that when dropped in the alginate bath, only the outer calcium ions react with the strands of alginate, forming a sphere.

The second problem is when the liquid’s pH is too low. This may not seem like a significant problem at first, but can pose a real challenge to spherifying citrusy liquids. A low pH indicates a high concentration of hydrogen ions (H⁺), as shown by this chart:

When pH levels are below 5, the concentration of hydrogen ions is very large, and this increases their reactivity. When alginate molecules are in contact with a liquid with both calcium ions and a low pH, many of the molecules will react with the hydrogen ions instead of the calcium ions. Hydrogen ions have the same charge as sodium ions, so they will not need to bond with other alginate strands to complete their electron shells. As a result, the alginate strands will remain as flexible as they were with the sodium ions and no gel will form. We must add sodium citrate to liquids with low pHs to react with the excess hydrogen ions, allowing the calcium ions to react with the alginate strands more readily.

Finally, basic spherification does not last a long time because of one simple reason: there are too many alginate strands within the sphere to go unreacted for an extended period of time. The calcium ions will find a way to react with the remaining alginate molecules, and as they do so, the huge network of atoms that makes up the membrane will extend inwards. The liquid inside the membrane will eventually all be part of the network and it will feel more like a squishy gel than caviar. This does not happen with reverse spherification because the calcium ions within the membrane have nothing to react with; the alginate strands have either already all reacted or been rinsed of after spherification. This leaves the liquid on the inside with its original lack of a rigid structure.

Well, that was a long explanation of spherification. Even though it may sound complicated, it really is one of the most simple reactions in molecular gastronomy because it has so few reactants and products. Next time, I’ll be trying gelification. It’ll be fun!