Food Science Basics in 6 weeks – part 4: Food Physics

Welcome to week 4 of the ‘Food Science Basics in 6 weeks’ course! If you’ve just dropped in mid-way, no problem. Keep on reading if you’d like to learn more about food physics or hop on to the first week of this programme. I think it’s easiest if you follow the courses 1 to 6, but since the weeks are pretty independent (except for week 2 and 3), feel free to shuffle it up!

Want to dive deeper? Want to take quizes and do exercises with the theory you’re learning in this mini course? Sign up for the course version of this blog posts series. It’s free as well, but will give you more chances to practice. Also, the course is improved continuously, using your feedback (so give me feedback!).

This week we’ll be zooming in on another topic of food science: food physics. This week we’ll be zooming in on the following:

  • Phase transitions: ice, water and gas; the basics!
  • Water activity – so important for reasons you’ll learn below
  • Mixtures of components and phases

The basics: food physics

Before we get started though, we will have to understand what food physics actually is. In the first week you’ve read a little about that already, but I guess we’ll have to refresh your memory a little. So start by reading a little more on other pages and remember to return once you’re finished!:

  • What is food physics on foodcrumbles.com?
  • Browse through this page, it gives you loads of fascinating examples.
  • Boston University has a nice article about a food physics course.
  • A good teaser to start with, several fun food phenomenon explained using physics from the physics.org website.

1. Phase transitions & kinetics

One of the big differences between food physics and food chemistry is that chemistry is all about transformations and reactions of molecules. Different molecules go in than those that come out. However, with physics, we generally keep the same molecules, however, based on their environment, these molecules might behave avery differently!

1.1 Temperature & kinetic energy

The first we’ll discuss is the effect of temperature, which is one that plays a role in so many phenomena. Temperature itself is again related to kinetic energy, so let us start with that.

Molecules are always moving, unless they are at the lowest temperature possible (-273C). Any temperature above that, which is the case for our foods, and molecules will be moving. The speed at which they move depends on the temperature of the molecules. The higher the temperature, the more the molecules will move around!

The faster the molecules move, the more energy these molecules have as well. This is defined by 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, the more likely it is to bump into others with sufficient speed to cause for instance a chemical reaction!

If a molecule moves at a higher speed than another molecule and they bump into each other, the faster molecule 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. 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.

1.2 Phase of molecules

Now that we’ve learned that molecules move faster at a higher temperature we can go to the next step: phases. The most common example of the different phases is water: ice is the solid phase, water is the liquid phase and vapour is the gaseous phase.

Using this example, you will probably be able to tell that the solid phase is always the one at the lowest temperature, whereas the gas is always the warmest phase.

There are two main things that play a role here: on the one hand it’s the effect of temperature we discussed above. The molecules in a gas do indeed move faster than in a solid, since they’re warmer. The second thing is the attraction between molecules. Molecules, small as they are, exert quite a lot of different types of forces on one another. We won’t discuss which now, but fact is, molecules quite attract one another.

In a solid the energy of the molecules (the kinetic energy) is not enough to overcome these attractive forces to allow molecules to move around instead. The molecules pretty much sit at the same spot and vibrate there. When a solid is heated, the molecules will vibrate more, but they will pretty much stay where they are. Only when the temperature is high enough for the molecules to go through the phase transition will they start moving more freely. In a liquid molecules don’t stay put at their place anymore, but they aren’t able to float around completely free. They will still stay together. During a phase transition molecules get a boost of extra energy. This will happen again when going from liquid to gas. In a liquid the molecules still have to stay around one another. In a gas however, they are able to float around quite freely!

1.3 Example

Let’s discuss with an example. Take an ice cube, try pouring water out of the ice cube. Won’t work will it? All the molecules will stay put in this little cube, they stay at their spot.

Now take the liquid water. You can actually pour liquid water, but, the water will stay at the bottom of your glass. It will not fill up the complete glas, spreading around.

That’s what happens when the water evaporates from your pan! The water will go everywhere!

These three different phases and the effect of temperature on molecules can be seen everywhere in food. You’ll see next week how important temperature is for microbiology and last week you’ve seen that temperature influences chemical reactions. In food and cooking we use phase transitions all the time, melting things (e.g. melting sugar), evaporating moisture (e.g. baking bread) or solidifying molecules (e.g. cookie dough in the fridge).

Want to see an example of working with different phases? Read my post on the ideal gas law!

2. Water activity

The next important phyiscs topic we’ll discuss has all to do with water: the water activity. Water activity agains plays a role in a lot of different processes. A lot of micro-organisms require a minimum water activity to be able to grow and a lot of chemical processes don’t take place when the water activity is too low (or too high, though that’s less common)!

2.1 Definitions

So what is this water activity? Let’s get formal first: the water activity is the partial vapor pressure of water divided by the vapor pressure of pure water.

If you’re not that much into physics you might raise your eyebrows on this one hearing all the complicated terms. Let’s explain it in other words as well. The water activity defines how much of the water in a product is available, or, in other words, unbound.

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 note for those wondering. The water activity is not the same as the concentration of water! Products with the same water content might have a totally different water activity, based on how the water is ‘bound’.

2.2 Influencing water activity

So how does the water activity of a product go up or down? Water activity can be reduced by getting rid of water. So if you dry a piece of meat or dry a cookie, the water activity will go down.

Another way to lower the water activity is to ‘bind’ more water. The two most common ways to bind water are sugar & salt. Sugar and salt will dissolve in water. Water molecules will sit around these molecules. Even though the water will not literally bind to the sugar, it will float around it, by ‘floating’ around it is not available anymore.

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!

2.3 Water activity & movement of molecules

If two products with a different water activity are place on top of each other, water molecules will start moving from the product with the highest water activity to that with the lower water activity.

Let’s take a dry biscuit that you cover with strawberry jam. Unfortunately, you’ve forgotten about your biscuit with jam and when you return a few hours later your biscuit is soggy. The water has moved from the jam into the biscuit!

 

Water activity plays a huge role in product development and food safety. A lot of micro-organisms (especially the disease causing ones)  cannot grow anymore at water activity lower than 0,91. As mentioned with the biscuit + jam snack, if you want your product to stay crunchy and moist, you will have to come up with a way to overcome the movement of water.

3. Foams, emulsions and gels

Physics is not about transforming molecules, but it is all about mixing molecules in a smart way! Food physics is especially interested in mixing different phases, without the molcules actually dissolving or reacting. Instead, the different components will ‘float’ around each other.

There are a lot of different possible combinations, to name just a few (find the whole list on Wikipedia):

  • Gas in liquid: this is a foam!
  • Liquid in a gas: a mist
  • Liquid in liquid: an emulsion, think of water and oil, these don’t mix well and don’t dissolve in one another.
  • Liquid in a solid: a gel

Since we’ve already covered quite some ground today we will not zoom in the science of these so-called dispersions. Instead, read several of the posts advised below in which we’ll be focussing on foams.

Meringues, ready to go into the ovencutting marshmallowsFoam 1: Meringue – a foam which is baked in the oven to stay stable

 

Foam 2: Marshmallows – a foam which is stabilized with gelatin

chocolate mousse with creme patissierewhipping up Italian meringueFoam 3: Chocolate mousse – stabilizing a foam with solid chocolate fats

 

Foam 4: Italian meringue – stabilizing a foam using heat and sugar

 

Of course, there’s a lot more food physics to be discovered, this is just a very first basics course to give you a glimpse of it all. Interested in learning more? Have a look at my post on popping popcorn, a great example of applying physics!

 

That’s it for this week! Do not forget to do this week’s quiz. As usual, it doesn’t only test your knowledge, but will give you some interesting assignments to help you really understand all that you’ve learned!

 

The values for water activity of sugar and salt solutions come from UC Davis.

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