In a lot of posts on the this website we regularly mention specific food physics terms. However, not all of you might be familiar with the concept of water activity or dispersions. To solve the problem of having to explain all concepts in every post (which wouldn’t be very enjoyable reading), we’ve written a few posts, including this one focussing on food physics, to help you get started with those basics!
Food physics is one of the main food science disclipines that we discuss here on the blog. Food physics comes in handy when you’re looking at the flow of material, aerating a foam, determining the water content of a food or when you’re looking into melting or evaporating something. A general understanding of the basic terms will help understand these concepts a lot better.
For ways of applying food physics in the real world have a look at the page dedicated to food physics. There we’ve collected all our food physics posts for you to browse through covering a wide range of topics. In this post we won’t necessarily apply the knowledge, instead, we’ll focus on teaching you the basics to help you understand those other posts!
If you’re interested in taking your food science knowledge to the next level, consider signing up for the food science basics course. There we discuss food physics, along with the other fields within food science!
What is food physics?
Before we get started, let’s discuss the definition of food physics. Very simply said, food physics studies the physics of food. This might sound obvious, but, try to remember your (high school) physics courses and the topics you’ve discussed. Most likely, you’ve learned something about phases (such as water, ice and vapour), you might have also discussed thermodynamics, or electromagnetic waves (entrance: microwaves!) and astronomy. Whereas we won’t be applying astronomy in foods all those other concepts do come back in food physics.
An important distinction between food physics and food chemistry is that within food chemistry reactions tend to take place. Molecules can bump into each other and come out different than they came in. Within food physics, no reactions take place. Instead, we are looking at the same molecules, but look at how they behave under different conditions. Also, we’re not necessarily looking at living things such as micro-organisms, that belongs to the field of food microbiology.
1. Phase transitions & Kinetics
One of the big differences between food physics and food chemistry is the transformation and reaction of molecules. In chemistry molecules react and change, whereas within food physics they don’t. Instead, we’re looking at how these molecules behave in different environments. There are a lot of different situations physics looks into, but one of the main parameters is: temperature.
Even humans start behaving different at higher or lower temperatures. We might slow down or speed up. Molecules do the same, but it’s a bit more straight forward. We have to start with the concept that molecules are always moving (unless they are at the lowest temperature possible, -273°C). The lower the temperature, the less the molecules will move. The higher the temperature the faster they move around.
This speed of molecules is again linked to the amount of kinetic energy of the molecules, described in the law of kinetics. We won’t dive into this here, but it is useful to understand. The faster a molecule moves, the more energy it has.
If two molecules bump into another (and don’t react, imagine ping pong balls), the faster molecules will transfer part of its energy to the slower moving molecule. This is exactly what happens if you mix boiling hot water with cold water or when you bake a steak. The fast and slow moving molecules will move next to and into one another. In the end all will move at a new average speed which is somewhere in between that of the hot and cold molecules. As a result, the water will reach an intermediate temperature and the steak will be hot through since the molecules have all passed on their energy into the meat.
Phases of materials
When zooming out a little from those molecules we will see a group of molecules moving alongside each other. They might either fill up a complete space, sit together at the bottom of a glass or they might form a solid block. This behaviour is described by phases. You might have recognized the gas, liquid and solid in those descriptions. Each molecule type or combination of molecules will be in a certain phase at a certain temperature.
As an example, water becomes ice below 0°C, butter is solid in the fridge, but melts in a warm pan and cinnamon powder is a solid. There are more complex phases than the basic three (solid, liquid, gas), but for simplicity we’ll stick with those for now.
A material will be solid at a temperature lower than the liquid, and it only forms a gas at an even higher temperature. When we think back of that kinetic energy concept, molecules in a solid have least kinetic energy, whereas those in a gas have most. Take care that this only goes up when comparing the same molecules, as soon as you start changing those, things become more complicated.
This kinetic energy is important to explain why phase changes occur. There are a lot of forces at play here. The molecules don’t only move around, they tend to be attracted in some way to the other molecules. How large this attraction is differs strongly per molecule. Also, sometimes molecules might be so large, it’s hard for them to move around each other, they block each other’s way. These, among with various other factors, influence the phase of these molecules at a certain temperature.
If the molecules barely attract one another and are small and spherical (thus not blocking one another), they won’t stay as close as let’s say a huge set of molecules that strongly attract one another. The first molecules will probably become a gas far more easily than the second set.
For a material to go from one phase to the other, it will need enough energy for the molecules to break free a little more. In a solid, the molecules vibrate/move at a fixed place, whereas in a solid, they can roam around, but will stick together. In a gas on the other hand, the molecules have gotten so much energy that they aren’t limited anymore by their relationships. Instead, they are limited in their movement by the air they’re in. They will fill up a space evenly.
Phases & phase transitions in food
Bread dough: bread dough contains quite a bit of water. When a bread is put into the oven. This water will heat up. Once it’s become warm enough, this has enough kinetic energy, it will evaporate and turn into a gas!
Olive oil in the fridge: At room temperature olive oil is a liquid. However, if you place it in the fridge, you can see white flocks in the olive oil. These are fats that have solidified. Nothing’s permanently changed though, once you take it out of the fridge the olive oil will return back to ‘normal’, that is, liquid.
2. Water activity
Water activity is one of those scientist terms we use a lot on this blog. The concept water activity is crucial for shelf life determination as well as for quality control of crispy and crunchy properties of a food.
A formal definition of water activity is: The water activity is the partial vapor pressure of water divided by the vapor pressure of pure water. This describes a formula used for its determination (we’ve done a deep dive on water activity as well). In simpler terms the water activity is a measure for the availability of water. It defines how much ‘unbound’ water is available. ‘Unbound’ water is water that is available, for instance for micro organisms to use or it can move around easily (we discussed this relationship between water activity & micro organisms before). Water can be ‘bound’ in various ways, for instance, it can be part of a very dry crispy structure through which it cannot move or escape. Another way is because of the presence of salt or sugar, these two components dissolve in water and ‘force’ the surrounding water molecules around them, that way making them unavailable.
The scale of water activity goes from 0 to 1,0. If something has a water activity of zero (I wouldn’t know of an example in food) it doesn’t have any available water. Pure water, without any additions, has a water activity of 0,99-1,0.
A quick side note, the water activity is not the same as the concentration of water! Products with the same water content might have a completely different water activity.
Influencing water activity
Within cooking and food we are continuously playing with this water activity. In some cases we want it to go up, in other cases we want it to go down. A low water activity tends to be good for crispiness, it also makes it harder for micro organisms to grow in. At a water activity below 0,91 a lot of bacteria (though not all) cannot grow anymore for instance. A higher water activity will generally make something softer or more palatable. The target strongly depends on the product and its use.
In thermodynamics the ‘activity’ of a food determines how molecules will behave, whether they have enough ‘chemical potential’ for a certain movement or reaction. If a food has a high water activity it will have a higher so called chemical potential. Within thermodynamics though, things like to go to an equilibrium where possible. In other words, if you place a high water activity next to a low one, thermodynamics laws say that these will even out over time. This is not good news if you’re trying to make a dry crispy crust with a moist filling (=low + high water activity). The crust will become soggy and the filling becomes dry.
We’ve discussed that issue in another post. We should now understand the relevance of water activity in food so it’s time to look at how to influence it.
Remove water through heat: drying a product, either by heating it, or leaving it in a dry environment will result in water evaporating and thus a lower water activity. Baking a moist pie crust in the oven is a great example here.
Bind more water: in food salt and sugar are very common ways of lowering water activity of foods. There are a lot of other molecules out these though that can be used as well. As an example, a sugar solution with 50% sugar and 50% water, the water activity will be a little below 0,92. You need quite some sugar to lower the water activity drastically. A salt solution with 10% salt, the rest water, has a water activity of 0,94. But just imagine, this will taste really very salty!
3. Food physics & dispersions – Foams, emulsions and gels
Physics is not about transforming molecules, but it is all about mixing molecules in a smart way. This last section, focussing on dispersions is a great example of that. Dispersions describe mixtures of at least two components. However, instead of these two reacting, they ‘float’ around each other. They don’t react, but they do interact. The amount and type of molecules present stays the same.
Food physics spends a lot of time analyzing and studying these dispersions. Molecules can interact in very interesting ways, forming unique structures. There are a lot of different dispersions, but for now we’ll focus on those most relevant for food and this blog.
Foams: A foam essentially is a liquid phase (e.g. cream) mixed with a gas (e.g. air). This results in a complex structure of a lot of air bubbles surrounded by a liquid. Some foams are very stable and will hold up for several days (e.g. baked meringue or marshmallows), other foams collapse within a matter of minutes (e.g. whipped cream or eggnog foam). Food physics studies how to stabilize or influence these foams.
Emulsions: Emulsions are made up of two liquids floating around each other. The most common example in food is that of oil and water. They can be emulsified. That is, the droplets of one will be evenly spread through the other, but they won’t react. In the case of a water & oil emulsion, stabilizers actually have to be added to keep the emulsified, thus mixed well.
Gel: In a gel liquid particles sit trapped in a solid. The solid particles prevent the liquid (e.g. water) from moving. In food these solid particles are often large branched molecules which hold onto a liquid. Using gelatin or making a mousse is a great example of this.
Suspension: The last common emulsion in food that we’ll discuss here is that of solid particles floating in a liquid (the reverse of a gel). In a suspension the solid particles can be stably dispersed, they won’t clump together. However, this tends to be a challenge, in a lot of cases the solids either want to move up or down the liquid, as is the case in chocolate milk.
This should have you covered for a lot of posts on this blog whenever we talk food physics. If you’re interested in learning more browse through the posts in which we apply food physics, sign up for the food science basics course or sign up for our newsletter (bottom of page) to stay in touch.
Phases & phase transitions: the ideal gas law, learn more about how gases behave.
The values for water activity of sugar and salt solutions come from UC Davis.
Have a look at my post on popping popcorn, a great example of applying physics!