The Science of the Most Important Ingredient: Water
“Why is Aunt Minnie in the hospital?” is a rhetorical question the physicist Richard Feynman asked. He was making the point that to answer a question, you have to assume the person asking the question possesses certain background knowledge. There could be a pretty straightforward answer to the question: Aunt Minnie slipped on ice. But if the questioner doesn’t know what ice is, they might ask: “Why is ice slippery?” The answer to that would lead to a whole new world of knowledge.
Ice is slippery because water is strange: it melts when it is compressed. A person’s weight melts the ice under them so the surface becomes slippery. Water is one of the very few substances in this world that expands when frozen and shrinks when melted. Why does water behave this way? Because of the Hydrogen bond.
The Hydrogen Bond
Water’s chemical formula is H₂O. In the water molecule, the oxygen atom forms one bond with each of the two hydrogen Atoms. Chemists call these bonds covalent bonds, which consist of a pair of shared electrons. The oxygen nucleus is bigger and pulls on those electrons harder than the hydrogen nucleus, so those electrons shift closer to the oxygen nucleus. This makes the oxygen side of the bond slightly negative and the hydrogen side of the bond slightly positive, like a little magnet. When two water molecules come together, the negative side of one molecule’s H-O bond is attracted to the positive side of another molecule's H-O bond. This intermolecular attractive force is the hydrogen bond. It is responsible for the unique chemical and physical properties of water.
In liquid water, the angle between the two H-O bonds is 104.5 degrees. As temperature drops, the water molecules become less restless and form more Hydrogen bonds with each other. When water freezes, the molecules settle into a crystalline structure and snap into a grid, where the angle between the two H-O bonds widens to 109 degrees. Each water molecule occupies more space in ice than in liquid water. That’s why water expands when it freezes.
A polar covalent bond doesn’t form every time a Hydrogen atom shows up. For instance, the C-H bond prevalent in organic compounds is not considered polar, therefore fat molecules don’t form hydrogen bonds with each other, or with water. That’s why oil does not dissolve in water.
But water is actually a very powerful solvent. For a solute (the substance to be dissolved) to dissolve in a solvent, the solvent molecules have to attach to the solute molecules, wrap them up and pull them away from other molecules of the solute. In the case of salt, because the bond between the sodium and chloride atoms is an ionic bond (They don’t share a pair of electrons. The sodium atom has completely given up an electron to the chloride atom), the little magnets in water molecules tear the sodium ion away from the chloride ion. They exist as separate ions in the solution. In the case of sugar, the sucrose molecules don’t break apart, but they do form intermolecular bonds with the water molecules. Individual sucrose molecules are pulled away from each other by the water molecules that swarm them and dissolve into water.
It takes energy to break the existing bonds among sucrose molecules and the hydrogen bonds among water molecules. As temperature rises, more energy means more sugar dissolves in water. Sugar solution of higher concentrations, when cooled, result in different candy texture. The candy thermometer is a convenient tool to indirectly measure the sugar concentration by measuring the temperature of the syrup.
Boiling Water
Technically, what the candy thermometer measures is the syrup’s boiling point, which is another unusual characteristic of water. Normally the boiling points of lightweight molecules are low. Due to low intermolecular forces, it doesn't take much kinetic energy for a molecule to escape into the air. For instance, the boiling point of H₂S (a heavier molecule than H₂O) is -60⁰C. Water’s boiling point is unusually high because of the Hydrogen bond. It takes 4.18 Joules of energy to raise the temperature of one gram of water by one degree. This number is called the specific heat capacity of a substance. In comparison, the specific heat of Copper is 0.385 J/g K, air:1J/g K, meat: 1.7 J/g K, olive oil: 2 J/g K. Therein lies the answer to one of life’s mysteries: why water takes so long to boil? The highest power consumption of a countertop appliance in the United States is 1500W. To boil 1Kilograms of water beginning at 20⁰C ambient temperature, and asuuming a generous 80% heating efficiency, it will take 4.18 x 1000 x (100–20) / (1500 x 0.8) = 278 seconds or almost 5 minutes with a countertop water kettle.
When there are foreign substances in water, they elevates the water's boiling point. The extra molecules get in the way of the escaping water molecules, so they need to have higher energy to break free. The macro manifestation of higher water molecule energy is higher temperature. However, during normal cooking, the salt concentration is not high enough to meaningfully change the boiling point of the liquid.
So how does water boil? On the stovetop, a pot of water is heated from the bottom. As the water in the bottom heats up, it rises up and is replaced by the colder water that sinks to the bottom. These movements are called convection currents. The French have a word frémir, meaning to quiver or to tremble, that refers to this pre-bubble stage when the surface of the liquid is visible moving.
The first bubbles show up around the bottom and the side of the pot. These are not steam. They are air dissolved in water. Unlike sugar, air’s solubility decreases as temperature increases (that’s why you should keep your opened champaign in the fridge). As the temperature rises further, some hot water near the bottom turns into steam and starts to float up. But most of them can’t reach the surface because the cold water they encounter along the way condenses them back to liquid water. When you see a lot of bubbles rise to the top and pop, the water is boiling at 100⁰C. Measuring boiling water is a great way to calibrate your thermometer.
Freezing Water
Another common way to calibrate thermometers at home is to use an ice bath. The temperature of the ice bath should be 0⁰C under atmospheric pressure. This leads to an observation: water can remain liquid at 0⁰C.
When water forms ice, it starts at nucleation sites where a few water molecules cluster together to form a small crystal. Over time, more and more water molecules join and the ice crystals grow in size. If the temperature is below 0⁰C but there are no nucleation sites, ice crystals will not form and water remains liquid. The nucleation sites can be impurities or physical disturbances. For undisturbed pure water, ice does not form until -39⁰C, when a phenomenon called homogenous nucleation happens. Modernist chefs have taken advantage of this property of water and created interesting dishes. Here is an example: https://www.youtube.com/watch?v=N1cZfOyqC78. Controlling the growth of ice crystals during the freezing process has a profound impact on the texture of frozen, and previously frozen, food.
Impurities get in the way of water molecules joining the ice crystals, which makes it harder for water to freeze. The result is freezing point depression. Ice cream is not one big hard block of ice because there is so much sugar dissolved in it the ice crystals can not grow very big. On the other hand, when the water molecules do join together and form crystals, they squeeze out the impurities. Freezing can purify water, or extract dissolved substances, depending on your goal.
Most of our food is water. About 70% of beef and chicken is water. 93% of spinach is water. When we freeze food, we are freezing water. water in food exists in two different places: inside cells and outside cells. The water outside cells is relatively pure and freezes at around -1⁰C. The water inside cells has more solute like proteins, sugars, and salt, so it doesn’t freeze until under -20⁰C. Before the temperature drops below -20⁰C, more and more water molecules migrate out of cell walls and join the ice crystals outside cells. This makes the remaining liquid even more concentrated, and its freezing point further depressed. The growing crystals and the dehydrated cells lead to damaged texture and ruptures at the cell level.
The worst way to freeze food is to freeze them slowly. To freeze food, you need to transfer heat from inside the food to outside the food. This heat transfer path consists of two parts: inside the food the heat is conducted. Outside the food, the heat is transferred by convection. The conductive heat transfer coefficient is a physical characteristic of the food, that is to say, you can’t change it without changing the food. So the only way to speed up the freezing is to accelerate the convective heat flow out of the surface of the food. According to Newton’s law of cooling, there are two ways to do this: increase the convection heat transfer coefficient or increase the temperature difference. You could change the former by increasing the air flow rate. Restaurants use something called the blast chiller. It’s like an anti air fryer. Instead of hot air, it forcefully circulates very cold air around the food.
A large difference in temperature between the food and its environment can be accomplished by cryogenic freezing, which involves immersion or spraying the food with liquid nitrogen. Liquid nitrogen’s boiling point is about -196⁰C under atmospheric pressure.
For home cooks without special equipment, a practicle method is to cut the food into small pieces to both shorten the conduction path and increase the surface area for convection.
The key to great mouth feel of ice cream is keeping the ice crystals small. There is a lot of dissolved sugar in ice cream, which interferes with the growth of ice crystals. Also, the churning of ice cream makers helps keep the crystals small by both physical disturbance and incorporating air. (Do you know FDA requires that ice cream can not have more than 50% air?) Commercial ice cream makers add ingredients that bond with free water in the mixture so the water molecules are not free to join ice crystals. Some animals have antifreeze proteins in their body. These proteins bind to small ice crystals to inhibit their growth. That’s why hibernating bears don’t freeze solid.
But there is a more interesting way to freeze water quickly: manipulating the pressure.
Phase changes of water
Intuitively it’s quite obvious why applying pressure to a substance changes its freezing point: The enthalpy of the system increases, the entropy of the system decreases, which changes the Gibbs free energy…OK, maybe it’s not so obvious. Fortunately, you don’t have to take a class in thermodynamics to understand the phase diagram of water, which contains all the useful information for cooking.
This diagram shows what state water is in at any temperature/pressure combination. The unit of the pressure is atm: the standards atmospheric pressure. Below the AE line water is gas. Between AE and AD water is liquid. Left to the AD line water is solid. Water is special in that the AD line has a negative slope. At 1 atm and 0⁰C, if pressure is increased, water contracts and turns from solid into a liquid. In the phase diagram of most other substances, the boundary between solid and liquid has a positive slope, as in, the phase diagram of Nitrogen (figure 2).
A few points of interest on the phase diagram of water are: C is the boiling point: 100⁰C at 1 atmospheric pressure. A is called the triple point where the three phases (gas, liquid, solid) of water exist in equilibrium.
With the knowledge of the phase diagram, we can play different tricks with water. The tool home cooks most famliar with is the pressure cooker, aka instant pot). In a typically pressure cooker, the pressure is raised to 2 atm/30 psi, under which the water boiling temperature becomes 121⁰C. At this temperature, you can soften potatoes for mashed potato in about 1/3 of the time.
Normally when we dry food, water evaporates from the surface of the food. Water inside the food moves to the surface in liquid form, damaging cells and membranes along the way. If we could find a path in the phase diagram where ice turns into gas, without becoming liquid first, we can avoid the damage. Not only is the texture preserved, but also flavor loss is minimized because evaporating water vapor takes few flavor molecules along with it. The process of freeze drying is so tender that it’s widely used to preserve flowers.
The blue path in figure 3 illustrates how it can be done. Freeze drying is a tricky process. The food has to be first frozen quickly to avoid damage during the freezing process. Then the pressure is dropped to about 6mPa, and the food is heated slowly to let the ice sublimate. The heating has to be slow because if vapor pressure builds up in the freeze dryer, all of a sudden you are at a different point in the phase diagram and water becomes liquid.
By the way, sublimation doesn’t just happen at low pressure. It also happens at normal pressure. Freezer burn is due to sublimation. Evaporation happens when a water molecule gains enough energy to break free. It doesn’t know it’s in an ice lattice, or it’s in the freezer, or what pressure it’s under. As long as the temperature is not absolute zero, there is always a non-zero chance some molecules will gain enough energy.
Expensive sushi grade fish needs to be frozen and frozen well on fishing boats, not only for preservation but also for killing parasites. The Japanese sushi industry seems to incubate a lot of interesting innovations in freezing technology. One flash freezer uses an electromagnetic field to control the growth of ice crystals, another creates special airflow so that the cold front hits all surfaces of the food simultaneously.
Looking at the phase diagram, we can see two ways for water to move from the liquid phase into the solid phase: horizontally lower the temperature and vertically lower the pressure. The speed of temperature change depends on the heat transfer process, which happens slowly. But the speed of pressure change can happen at the speed of sound. Theoretically, lowering the temperature of water to less than 0⁰C under high pressure, water will stay liquid. If we drop the pressure suddenly, liquid water turns solid and we can create uniformly small ice crystals. This is the blue path in figure 4. Experimentally it’s been shown to preserve the texture of potatoes and tofu better than a blast chiller. Commercial equipment for pressure shift freezing doesn’t seem to be available yet, but active research is reported to be underway.
Understanding the science behind common phenomena can lead to magical solutions like pressure shift freezing. There are tremendous opportunities in applying well-known scientific principles to solve real problems for cooks.
Ideas for a better fridge
No. We are not talking about making the fridge door a big iPad. And if there is a screen on the fridge, what I would like to see is not the humidity and temperature of my neighborhood, but the humidity and temperature inside my fridge. And not the measurement at just a single point, but at multiple locations.
As we saw earlier, it’s important to freeze food quickly. The current home freezer is fine for storing frozen food but is probably the worst way to freeze food. There should be a separate blast chiller chamber. It may be too energy intensive for the whole freezer to be kept at ultra-low temperatures, but after a “pre-cool” period, the chiller temperature can be dropped for the duration of blast chilling.
But the point of the blast chiller is not low temperature, but rapid temperature drop. Blast chilling is not just for making ice cream and freezing fish for sushi. It’s also critical for food safety. We have mentioned that 5⁰C to 55⁰C is the danger zone for bacteria reproduction. If you have hot food out, they may stay in the danger zone too long. It’s even worse if you put them in the fridge because it will warm up the whole fridge and bring other food items’ temperature up to the danger zone. In a well-managed professional kitchen, if the food stays in the danger zone for up to 4 hours, it will have to be thrown away. Department of Health Guidelines state that to safely Blast Chill food its temperature must be reduced from +70⁰C to +3⁰C or below within 90 minutes. CDC report that improper cooling (including improper cooling in the fridge) is, by far, the number one cause of bacterial growth leading to food borne illness. There is also a culinary benefit: a quick chill thickens and gels juices before they can leak out.
It’s not just the temperature drop process we should worry about. Improper thawing also could leave the food in the danger zone for too long. A fundamental problem with thawing is that ice conducts heat faster than water. When food is frozen, the outside turns ice first and speeds up conduction. During thawing, the outside turns liquid first and slows down conduction. The blast chiller can work in reverse. With careful control heating and cooling steps, and accelerated airflow and evaporation, it can speed up thawing.
It may even be possible for the chiller to use the pressure shift freezing technology. The fridge is a unique appliance in the home kitchen because 1. It’s huge. 2. It already has a powerful compressor that could be used to generate both high pressure and vacuum. Add an edge sealer, and it becomes a vacuum sealer. Add a scale, a camera, an RFID tag in the reusable bag and some image recognition algorithm, and even a cook who is too lazy to bother putting labels on food can have quantifiable and traceable information for every package of food in the freezer.
Speaking of lazy cooks, when are we going to get a foot-operated fridge door?
Enjoyed this article? 🌟 Help spread the knowledge by forwarding it to a friend and don’t forget to follow me on Medium for upcoming posts. Your support is much appreciated — thank you!
