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 Barriers
August 14, 2020
Barriers

Professor Abbott’s Recommendation for Protecting Green Coffee.

Cells aren’t just bags of water; they are highly complex, concentrated solutions containing special molecules designed to keep proteins and nucleotides in controlled shape and size. If you dehydrate them carelessly, then this can create havoc. But the cells in seeds, just as those in (at one extreme) tardigrades or (a lesser extreme) hibernating animals, are designed to shut themselves down in a controlled manner so they are either at very low activity (hibernation) or switched off totally (tardigrades). A way we can do this in the lab is to add a sugar called trehalose. It never crystallises (unlike sucrose), so it never damages the cells. It just forms a tough glass that freezes everything in its stable state, ready to go back into action when water is gently added.

So seeds are somewhere between zero and super-low activity. To transport them, you need to keep O2 out because that can just generally react with biological stuff. In an active cell, oxidative damage is cleaned up automatically, but that’s not an option when things are switched off. Similarly, you don’t want water to arrive when the rest of the system is unprepared for action (e.g., there’s not enough O2 or CO2 for survival) because the proteins, etc., can start moving, but without the flow of the chemicals required to function properly.

A bag with low O2 and where water cannot escape (we don’t want to dehydrate beyond what the cell has done for the seed) nor enter is going to keep things fine, essentially forever.

 

Keeping Oxygen and Water Out of a Coffee Bag 

Here’s an apparently simple task: You have coffee in a bag and need to ensure that during storage and shipping no oxygen or water leaks into the contents with the possibility of damage to the flavour and texture. Because you want to be green, you need to do this with the minimum amount of material, preferably fully recyclable — which commonly means ‘single material’.

It turns out that only one single, practical material can do this, and that’s aluminium foil. As long as you can seal the edges of the foil and can ensure that the foil doesn’t get ripped, the barrier to both water and O2 is essentially perfect. And aluminium is fully recyclable. But for many reasons, this is not a solution that most people would find acceptable.

A chlorinated polymer such as PVDC is an impressively good barrier material on its own, but the polymer is not convenient for general packaging and, in any case, chlorinated polymers are rightly banned from casual consumer use because of their dire impact on recycling.

PET, as in fizzy drink bottles, is almost good enough on its own, but it’s not quite good enough as a thin bag material, and it’s relatively expensive.

Some polymers are a good barrier for water but not for oxygen, and some are the other way ‘round. Many are intermediate. In general, we want good barriers for both, and no single polymer can achieve this. (If it could, we’d all be using it).

What about making a better barrier via a thicker film? Suppose we needed a barrier that was 2x better. Okay, we can imagine using twice the thickness. But for packaging, we generally need 100x better, and we cannot use 100x the thickness!

So we have to go to a multi-layer barrier, and our decision then is a balance of priorities.

 

Multi-Polymer Bags

Here’s why we need multiple layers. For many reasons, polyethylene, PE, is an excellent, practical bag material. It is tough, lightweight, and made efficiently from common raw materials on a vast scale.1 It is totally resistant to water and therefore a great practical solution for when bags might get exposed to, say, some rain, and it is an excellent barrier to water vapour. Here’s the problem: It is a super-poor barrier for oxygen. Not even the most humble packaging film used in supermarkets can be realistically made from PE because the rate at which O2 gets through is unacceptably high.

As it happens, another common polymer called EVOH (polyethylene vinyl alcohol) is a sort of mix of PE and the commonly used PVOH, poly vinyl alcohol. It has exactly the opposite properties to PE. It is a wonderful O2 barrier, but it is easily damaged by water and is a poor water vapour barrier.

So, most common food packaging films are a sandwich of PE:EVOH:PE. It happens that EVOH is quite expensive, and at the same time, even a super-thin (2-µm) layer is an excellent barrier, so if the outer and inner PE are each 25 µm thick, then the EVOH is approximately 4% of the total. (We will come back to this figure shortly.)

A 1-µm EVOH layer would still, in theory, be an excellent layer, and using it would halve the cost of the EVOH portion and reduce it to 2% of the total. But in practice, putting down such a thin layer reliably is very hard, and you end up with pinholes. As we shall see, pinholes are a huge problem when you want a good barrier.

My understanding is that bags such as those used by GrainPro have to be of this multi-layer type to provide the practical barrier properties. The exact number, type, and thickness of layers is less important to us than the general principle of multiple layers that each fix a problem that cannot be solved by the other layers. If any single-layer material could do the job and be low cost, recyclable, etc., you can be sure that everyone would use it, as adding extra layers requires expensive and complex equipment.

 

Al, AlOx, and SiOx Layers

As mentioned above, aluminium (Al) is a perfect barrier material. If aluminium foil is not acceptable, then a thin layer of aluminium is the alternative. How thin? Just, say, 20 nm, i.e., 100x less material than in a multi-layer polymer bag. In terms of material use efficiency, it is hard to beat, and in terms of mass production, the Al industry is astonishingly efficient.

Because people don’t like the shiny aluminium look and because we often have good reasons to prefer transparent packaging, an alternative is to put down aluminium oxide (AlOx) or silicon oxide (SiOx) layers. These are not simple materials. If put down properly, these are wonderful barriers. But there’s a problem: Make them too thin and they aren’t a good barrier. Make them too thick and then they are too brittle and crack as the film is handled. It turns out that getting this balance right and doing it at an attractive cost is hard, so at the time of writing, EVOH is still the dominant technology, even though AlOx and SiOx are attractive because so little material is required.

The problem that everyone faces is the same as that of the EVOH barriers — pinholes. So we need to explain why pinholes are such a big issue.

 

The Pinhole Problem

The science is more complex than this simple example illustrates, but we can still get the principle. Let’s take a film with a defect-free EVOH, Al, AlOx, or SiOx barrier that lets through 1 molecule of O2 per cm² over a given time, and that rate is just acceptable for the package. Now, put a pinhole in the barrier so that O2 can get through at 1 million molecules per cm² of pinhole. If that pinhole is just 20 µm in diameter, then the area is ~3 millionths of a cm², so it will let 3 molecules through. So just one tiny pinhole has increased our oxygen diffusion rate from 1 to 4, making it unacceptable.

For EVOH, just coating thicker tends to decrease the pinholes, hence the ‘wasteful’ extra thickness used in most packaging. For aluminium, the situation is more complicated, but in general, a layer thicker than theoretically required is necessary. If you could make a pinhole-free aluminium for packaging, the thinness of the layer would make it look dark rather than shiny. And for AlOx and SiOx, going thicker hits the brittleness problem.

 

The Sealing Problem

A similar calculation can be done with bag seals. An apparently insignificant glitch in the sealing can allow more O2 through than the whole of the bag itself. This is not mere theory. I have been involved in food spoilage issues where everyone blamed the packaging material for being insufficiently good, when it turned out that poor maintenance of a humble heat-sealing unit was the root cause of the oxygen getting in.

Removing the air from a bag and then sealing it effectively is a far harder task than most people would imagine. Perhaps it’s not so hard to do on a single bag. But to do it day after day, year after year, requires a smart, reliable system — which is by no means an easy thing to set up.

 

Recycling

Let’s suppose we have a way to get all our plastic bags back to a recycling centre. In most countries this is simply not feasible, so debates about the bags’ intrinsic recyclability are irrelevant. The only debate is whether they go into a landfill or to thermal recycling2 — with strong passions expressed for and against each side. This debate is very different from that of recycling of the bulk of plastic packaging, which is PE and PET bottles, for which the choice to recycle  is a no-brainer.

Industrial applications have no problem sending large quantities of the same material from which GrainPro sacks are constructed back for recycling. Therefore, a coffee roaster receiving thousands of bags will be most unlucky if there is not a recycling route for this simple, relatively clean material.

Regulatory agencies have to make difficult decisions. If they say, ‘Only 100% pure polymers can be recycled’, then basically none would be recycled because the material will always contain some contaminants such as necessary printing on the package or minor residues inside. If they say, ‘We will allow 5% of junk to be included in recycling’, then they will be attacked by purists and welcomed by realists. In practice, the realists have been catered for, and the low levels of EVOH in multi-layer packages are seen as acceptable for recycled polymers. No one would ever notice low levels of AlOx or SiOx, and quite what to do with the 20 nm of aluminium is uncertain — it could be made to become invisible by its rapid oxidation to AlOx.

 

Waste Is Usually Worse Than Not Recycling

Arguably, a bag of coffee beans that has to be discarded because oxygen seeped in through the packaging via a pinhole or a poor seal is a greater loss to the planet than a bit of excess multi-layered plastic. And, given that in many parts of the world recycling plastic bags is in, any case, not practical or green, be a little kinder to those who make and use the bags.

We always have to be careful in our accusations about environmental issues. When it comes to making a cup of coffee, the part of the supply chain doing the most CO2 damage to the planet is us — from the energy required to heat the water to brew the coffee. This goes against all our green instincts — urely, it’s the fault of the fertilizers or transport or the roasters or … No — overwhelmingly, it’s us.

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