meat discolouration

How packaging changes the colour of (red) meat

When unwrapping a piece of meat the other day I noticed that it wasn’t just one red colour. Instead, I saw brown, purplish and red/pink colour all in just one piece of meat! Was anything going wrong here? Was my meat going bad?

But no, nothing was wrong with the meat. We had actually bought it not that long before using and it still smelled perfectly fine and fresh. These colour changes are perfectly normal for meat. Which colour you see and how many different colours, all depend on the way the meat is packed.

The colour of meat

Before delving deeper into this we have to take one step back: what gives meat its colour? As we found for fruits & vegetables, the colour of meat is caused by a specific molecule, in the case of meat it’s myoglobin. Myoglobin is a protein and quite a large complex molecule. The main function of myoglobin isn’t to give muscle meat colour of course. Instead, myoglobin plays an essential role in getting oxygen into the muscles for them to do their work.

Myoglobin can perform this role thanks to a specific section of the molecule, called the heme group. This heme group can bind an iron ion and it’s this iron ion that can then bind oxygen. If you remember what chlorophyll, the molecule that makes plants green, looks like, you will notice they’re quite similar!

Myoglobin gives colour

The exact conformation of the heme group depends on whether or not oxygen is present and bound to the iron ion. That again determines the colour of meat! With these types of molecules, their exact configuration is highly important for giving a specific colour.

There are roughly three colours the molecule + ion can be, all depend on the presence or absence of oxygen:

  1. No oxygen present: the myoglobin does not bind any oxygen and turns a purple colour (the form of the molecule is then called: deoxymyoglobin).
  2. Little oxygen present: the meat will turn brown (the state of the myoglobin is now called: metmyglobin).
  3. Plenty oxygen (e.g. in the air): the meat turns red (oxymyoglobin is present, the red variant).
  4. There is a fourth option which makes the meat pink: the presence of carbon monoxide (CO). It will compete with the oxygen for that spot on the iron ion.
Red meat from supermarket MAP
A fresh piece of meat from a recently openend ‘modified atmosphere pack’, notice the bright red meat colour. These types of packs tend to contain quite a lot of oxygen.

Packaging meat

Now that we know what determines the colour of meat, it is time to look at the type of packaging. The packaging your meat sits in will determine the gas composition that surrounds the meat and as a result the colour! There are three most common ways of packaging meat:

  1. Vacuum sealed, in other words, all air has been pulled out and the plastic sits tightly around the meat.
  2. Modified atmosphere packaging, you might not recognize this immediately, but just about all the meat in supermarkets packed in a plastic tray with a sealed plastic layer have been packed this way. It means that the air in which the meat lies does not have the same composition as the air around us does. More on this later.
  3. Hand wrapped in a bag or tray by a butcher.

What’s important in this story is the composition of the air. As we will see, the air composition highly influences the colour of the meat. More or less oxygen influences how the meat looks like. So how does that look like for our packages?

  1. Vacuum sealed: there is no air, thus no oxygen
  2. Modified atmosphere packaging: it all depends on the product that’s inside, the producers can put whichever gas is best for the product. In the case of red meat this generally means there’s quite a bit of oxygen, more than in regular air.
  3. Hand wrapped: this pack contains the same air as the one around us, thus roughly 20% oxygen, a little carbon dioxide and roughly 80% nitrogen.

What’s the best packaging technique for red meat?

So based on these phenomenon there are clear advantages and disadvantages of the different types of packaging of red meat. Vacuum packed meat will make meat look a very dark red, sometimes even purplish which not all consumers might appreciate.

In a hand wrapped package of meat the amount of oxygen will get deplted over time. As a result, this type of packaging has the result that it turns brown over time. You will especially see this for minced mat. The outside of the minced meat in this type of package may still be red whereas the inside will have turned brown, since oxygen was not able to come in.

Modified atmosphere packaging however can be used in such a way though that the meat remains a beautiful red over time. It can do so by either eliminating oxygen or by strongly increasing the oxygen content.

Unfortunately, the packaging world isn’t as easy as that. The air composition of a pack of meat doesn’t only influence the colour, it also influences shelf life. Generally speaking, a vacuum packed piece of meat keeps a lot longer than the other two packs and hand wrapped keeps for the shortest amount of time. Also, adding too much oxygen to a MAP piece of meat can increase the growth rate of bacteria, thus causing the meat to spoil faster. What’s more, different cuts of meat tend to react slightly different to different packaging methods.

Multiple colours in meat

So how about that piece of meat I menioned at the start, with all those different colours? What had happened there?

This steak type piece of meat had been bought at a butcher who hand wrapped the meat in a little tray. When taking the meat out of the pack a day later there were three different layers clearly visible.

Meat different colours

Now that we know the relationship of oxygen with colour of red meat, we can explain what we saw. The piece of meat that sat at the top of the steak was a nice pink/red. There is oxygen at the top so it keeps its colour. In the middle though we saw a very dark red colour, possibly because there is no oxygen in there at all. The bottom part however, the part that touches the tray, was completely brown. There must have been some oxygen in contact with that part before it got packaged and then in the pack that was reduced quite a bit!

What was interesting to see that just keeping the meat out on the colour made it become an even red/pink again. apparently, the oxygen was quick to re-enter the muscle and give it back its colour!

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