Turning Milk Green: Thermal Pasteurisation Alternatives

The traditional heat-based method of milk pasteurisation, by which milk is heated to 71.7°C for around 15 seconds to destroy harmful bacteria and reduce the numbers of spoilage organisms to extend shelf-life, is a mature and highly developed technique. However, scientists and industry groups are constantly looking for new methods of making the energy-intensive process more environmentally friendly, further extending shelf-life and producing a product with a taste as close to untreated milk as possible.

An example of this was a recent workshop held by British dairy industry group Dairy UK in conjunction with the Carbon Trust and green developer Camco to explore innovative new ways of cutting emissions and making the dairy industry more efficient. Among other issues, the pasteurisation process was an important point of discussion. We talk to Ed Komorowski, Dairy UK’s technical director, to find out about the new technologies being put forward and whether they might improve on a well-established formula.

Chris Lo: Why are alternatives being sought to thermal pasteurisation? What are the disadvantages of the standard process?

Ed Komorowski: There are two possible approaches to alternatives. One is a replacement for pasteurisation, and the second approach is an addition to pasteurisation. And as an addition to pasteurisation – one example of a technique that is currently in use is the filtered milk technique. That process is undertaken and then followed by pasteurisation, and the benefit it gives in terms of filtered milk is a longer shelf-life. So these techniques could, if they are carried out in addition to pasteurisation, give a longer shelf-life.

If there were a standalone technology, then depending on the technology, there may be less energy used, and it may also result in a product with characteristics such as a preferred taste. What I think it’s fair to say is that pasteurisation is a highly developed, very efficient technique and therefore the hurdle to an alternative technology replacing it is extremely high.

CL: Ultraviolet pasteurisation might hold the key to pasteurising milk in a more environmentally friendly way, as well as increasing shelf-life. How does this technology work?

EK: Well, the theory is that you choose UV light in frequencies that affect the DNA of the bacteria in the milk, and because you’re specifically affecting the DNA by the proper choice of UV, then you don’t need the same energy as you would to heat the milk.

But some of the issues to be overcome are that you wouldn’t want to use excess UV because that might cause off-flavours in the product or it might cause other chemical changes. So you’d want to choose the amount of UV to be sufficient but not excessive.

What isn’t known is which pathogens are the most UV-resistant. This is one of the beauties of pasteurisation – we know that the TB bacterium is the most heat-resistant pathogen, so you can set your conditions to kill the TB bacterium, and that way you’re confident you’re killing all the pathogens and also killing spoilage organisms. When you move to UV, you’d have to say to yourself, "what is the condition that will destroy all pathogens, and what about spoilage bacteria and other changes?" It’s certainly appealing in principle, but it has its complications. And obviously with milk being opaque, you’ve also got to ensure that your UV gets through to all the milk and there is no shadowing.

So there are a lot of issues before you can move to that. And perhaps the other thing is that a new technology might well be considered a "novel" technology, as defined in the Novel Food Regulation (Regulation EC 258/97 of the European Parliament and the Council). If that was the case, you would have to obtain regulatory approval – another hurdle.

CL: Is the lack of research the main obstacle blocking or delaying the adoption of UV for pasteurisation in the UK?

EK: I think yes, more research, but the development of all the controls that would be necessary as well. There are a number of things. Heat destroys bacteria – pathogens and spoilage. Heat destroys enzymes and we use this for what is called the phosphatase test, so by carrying out the phosphatase test on milk you can demonstrate you have carried out the correct heat treatment.

The pasteurisation plants have inbuilt controls – temperature monitoring equipment and divert valves, so if the temperature is not met the milk doesn’t go forward. You’ve got all of that associated with pasteurisation and a well-proven technique. So you would have to replicate all of that, but in ways that would be appropriate to UV. It’s certainly not a quick and easy solution, but it may ultimately have some merit under some circumstances or perhaps eventually as a real alternative.

CL: Implementing that sort of technology sounds expensive – do you think that plant owners and the dairy industry in general would be willing to make that kind of investment for the sake of a greener pasteurisation process?

EK: I think so, yes, but with a big qualification, because what a plant owner would not be prepared to do is in any way compromise on the safety of the product, and no compromise at all in terms of consumer acceptability of the product. It’s all very well us talking about these new technologies and seeing the possible benefits in terms of energy – a consumer might well react against new technologies. Already you’ve got consumers who protest about pasteurisation, so you can certainly imagine that with a new technology, people are not just going to rush out and say, "this is exactly what we want, it cuts energy." I’m sure they will come up with all sorts of questions about it.

CL: Microwave and pulsed electric field pasteurisation are also being discussed as potential alternatives for the industry – could you explain these methods and what makes them attractive?

EK: Microwave technology would, in principle, remove the need for a large boiler system and the system to get heat to your pasteuriser, but on the other hand it’s a system that uses electricity and therefore it’s not clear if it would save energy. One of the beauties of pasteurisation is the fact that is carried out in a high-temperature short-time heat exchanger. What that does is it allows the heated product to heat the incoming milk and the energy is transferred from the hot to the cold and as a result the cold is heated, so it’s an exceedingly efficient process.

This process of the hot product heating the cold and itself being cooled is called regeneration. Regeneration effects for pasteurisation are over 90%. You only need a fraction of the energy required.

Now if you take microwave, you will not have this regeneration so you’ll have to build in your own technology to overcome that, so microwave isn’t necessarily a solution. I think that it may have applications for other products, but I’m not sure microwave is the way forward in terms of milk.

Pulsed electric field is what’s known as a cold sterilisation process. Bacteria and spores are polarised and stretched to destruction through an electrical field. It’s not known what would be required to ensure that all bacteria in milk could be destroyed. So I think there are quite a lot of unknowns. It’s one thing, perhaps, to plan the technique to water but when it comes to milk, it may mean you need much more energy. Again, it comes back to these fundamental questions: how much energy? What bacteria would be most resistant to this process? How do we ensure that we have set it such a way that we can be confident we will destroy all pathogens? What about spoilage organisms? What about enzymes? There are a lot of unknowns about this as well.

CL: It sounds like most of the alternative techniques are at the theoretical stage right now, at least on a commercial scale?

EK: I think that’s right. As an industry, we’re forward-looking; we’re trying to stretch ourselves to all environmental challenges. So we certainly want to keep our eyes open to these things, but I don’t think they are solutions if you take a time scale of two to three years – it’s not going to be that sort of time scale.

CL: Some groups, especially in the US, think that pasteurisation is an unnecessary process as raw milk contains unique beneficial bacteria and can be produced hygienically under the right conditions. What is Dairy UK’s opinion on this issue?

EK: There’s no doubt that you can have healthy animals and obviously we want all animals to be healthy. But it is possible that with some pathogens – and E coli O157 is an example – an animal carrying it shows no physical effects whatsoever. So you can’t have the same confidence that what you will end up with is milk with absolutely no pathogens. The other thing, of course, that pasteurisation also does is destroy spoilage organisms and extends shelf-life. So if you have raw milk, and let’s assume it’s pathogen-free, it will not keep for very long because of spoilage organisms.

Obviously, what heat also does is have a certain effect on the taste – a very mild effect, but I can understand people saying that they prefer the taste of raw milk. But frankly pasteurisation is such a useful technique – it produces a very safe product.

CL: These issues and more were discussed in a workshop as part of Dairy UK’s industrial energy efficiency accelerator programme, and it has been announced that a report will be compiled based on the group’s findings. Can you give an update on the report’s findings on pasteurisation alternatives?

EK: We’re still awaiting the report, but obviously we attended the actual meeting, and what we are aware of is technologies in various stages of development. We’re following them, but we don’t want to see them today as an alternative to pasteurisation. I think what we would much more likely see are possible applications in addition to pasteurisation, where they might well have the benefit of extending shelf-life.

I think a limitation of pasteurisation is the fact that it’s a gentle heat treatment and it doesn’t kill thermodurics (heat-resistant bacteria). These heat-resistant organisms are not harmful but they can limit shelf-life. So potentially these techniques could be useful in that they may well mean you can limit the number of thermodurics and bacterial spores, getting a longer shelf-life.

I think what we would see initially is that sort of application. It may be that a company that is considering that application would be treating it with some sort of commercial secrecy while it is being evaluated. So we’re not necessarily going to see something before it hits the marketplace.