ice cream making with liquid nitrogen

An Introduction to Gas, Liquids and Solids in Food – States of Matter

When you prepare food you continuously change the states of our ingredients. You melt butter in a hot pan, boil down a runny stock into a thick syrupy consistency or transform a liquid custard into a solid tub of ice cream.

Without even realizing, you’re continuously switching between three* states of matter: solids, liquids and gases to make delicious food.

But how do those transitions even work? Knowing that will allow you to use them even more to your advantage!

Moving molecules – Kinetics

The whole world around is is filled with molecules. Molecules form the building blocks of our food as well. Carbohydrates, proteins, fats, they’re all molecules. Even though it may not seem like it since it happens at such a small scale, these molecules are moving all the time. They are never completely still.

Just how much all the matter around us moves depends on the temperature of that matter. The lower the temperature, the less movement. Heat, so a higher temperature, gives the energy to move.

Moving molecules contain energy. This energy is called kinetic energy. The lower the temperature the lower the kinetic energy of these molecules.

Absolute zero movement

There is one situation in which molecules would not move at all: at the lowest temperature possible, the absolute zero. For food this is quite irrelevant though since this absolute zero is at -273°C (-459°F).

Transferring kinetic energy

Molecules transfer kinetic energy to one another.If you place a few very hot and a few very cold molecules in the same room, they will become the same temperature over time (assuming an ideal system). This is because molecules continuously bump into one another. If one molecule has a lot more energy (aka, is hotter) than another one (which is colder) it will transfer some of that additional energy to the low energy molecule. After enough bumps, all molecules will move at about the same speed and thus have the same temperature!

steeping tea
A hot cup of water with a tea bag, there’s quite some movement of molecules within the water that is beautifully visualized thanks to the darker coloured tea.

States of matter

Even though molecules move all the time when above that lowest possible temperature, that does not mean they move freely. How freely they’re able to move depends on the state of matter of those molecules. There are three* main states of matter: solid, liquid and gas.

Solid

In a solid, molecules move, however, they move on their location. They might vibrate back and forth, shiver, etc., but they stay put. These molecules don’t have enough energy to move away from their spot. Sugar crystals are a solid. If you make a mound of sugar, it will stay in place. The crystals will pretty much stay put.

Liquid

Next up is a liquid, like the water in a glass. In a liquid the molecules have a lot more energy. In fact, they have enough energy to leave their spot and travel around within the liquid. However, they cannot completely ‘fly’ away from the liquid, they will stay within.

Another example of a liquid is olive oil at room temperature. When you pour the olive oil it will pour out of the bottle, but all of it will flow down, pulled down by gravity and end up in a puddle. None of it will float in the air.

Gases

In gases molecules get even more freedom. The molecules aren’t bound anymore by their fellow molecules. Instead, gas molecules can travel as far as they want until they hit a border. These borders are formed by liquids or solids, confining the gases. In order for molecules to be a gas they need even more energy to travel so freely.

maple syrup evaporator
Cooking maple syrup, you’re converting liquid into gas!

Changing phase

States of matter can easily transition into one another. This process is governed by temperature. A solid can transform into a liquid by heating up that solid. It’s what happens when melting butter. A liquid again can transform into a gas by heating it up. It’s what happens when bringing water to the boil and continuing to heat it, over time all water will be evaporated.

The reverse is also true. Cooling down a gas can transform it into a liquid and cooling it even further makes it into a solid.

Phase transition temperature

At which temperature these phase transitions occur is different for every molecule or molecule mixture. Water ice melts at 0°C (32°F), but that butter only melts when it’s in a hot pan. These temperatures are different because of how molecules interact, how well they hold onto one another and how well they attract (or repulse) one another.

Small, spherical molecules that don’t attract one another become a gas more easily than large bulky molecules. They need less energy to float around freely. An example of such a small molecule is hydrogen (H2). Hydrogen is a gas at room temperature. A sugar molecule on the other hand is a lot larger and forms structures with other sugar molecules. As such, it is solid at room temperature.

Knowing at which temperatures phase transitions occurs is something you use unconsciously when preparing food. We bring water to the boil (which essentially means we’re starting to evaporate it), freeze ice, etc., melt butter. You intuitively know what you should do to cause this phase transition.

peanut butter from the fridge - keeps its shape
Peanut butter that was stored at the fridge, see how well it keeps its shape after some has been taken out? It contains a lot of solid fat!

Phase transition energy

Converting phases takes a lot of energy, more energy than just heating up the molecules. Heating up water by just 1 degree Celsius from 99 to 100°C takes about 2kJ per kg of water. However, converting that boiling hot water into steam takes over 2000kJ per kg of water!

Going from solid to liquid or from liquid to gas always requires a lot of energy. The opposite is also true. Transitioning from gas to liquid or from liquid to solid results in the release of energy.

Phase transitions in food

Food is full of phase transitions! To name just a few:

  • Boiling of maple sap into maple syrup: liquid –> gas
  • Making ice cream: liquid –> solid
  • Roasting walnuts: liquid –> gas (you’re evaporating moisture!)
  • Melting lard in a pie crust in the oven: solid –> liquid
  • Storing peanut butter in the fridge: liquid –> solid

*The fourth state of matter is plasma which is relevant for physicists, but pretty rare to happen in food, unless you’re microwaving a grape

References

The engineering toolbox, Water vapor – specific heat, link

The engineering toolbox, Water – heat of evaporation, link

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