Hello, everybody. Welcome again. My name is Erich Windhab from ETH in Zurich. I’m heading a laboratory on food process engineering, and I’ll give you a second course on extrusion, denoted as extrusion 2, which will go for the product’s development side. Food product development shall be the major topic, which I will introduce, then integrate extrusion processing on multiple structure scales, and finally, go into tailoring product properties by thermo-mechanical extrusion processing of the structure, with some subchapters where I will demonstrate different, let’s say, thermal processing aspects, combined with mechanical processing in different food product applications. Finally, I’ll summarise and conclude. Starting again with the S-PRO square scheme– process make structure, structure defines properties.
And starting with the consumer on the properties side, now we are specifically addressing aroma, flavour, texture aspects of food on the preference level side of the consumer. Safety is certainly an issue which has to be guaranteed. And nutritious aspects, health-supporting aspects, performance, and development support is what I would like to address in the context of functionality of the structures to be generated in extrusion processing. Structure is hierarchic from molecular via meso to macro scale. And certainly, the process to be considered is extrusion processing, as mentioned before. So we can define extrusion processing as a fluid mechanical operation of a viscous mass forcing it through a restriction or dye in order to generate or transform specific product structure and shape.
Starting with the hierarchic structuring, I would like to give you an idea of what can be done by extrusion processing on the different scales. And this can already start on a molecular, or let’s say, macro-molecular assembly scale, so forming some phospholipid structures, as shown in this first picture here. So already in this structure, we could embed some functional components, either between the fatty acid chains, which we have here for the phospholipid, or connected to the choline head, to the hydrophilic choline heads. From these types of self-assembling phospholipid structures, we may generate vesicle structures– vesicles in the size range between 10 and let’s say, several hundred nanometers.
Then go for the next scale, which is a multiple emulsion scale, so where 1/2000 microns can really go into large droplets with internal phase and surrounding phase. So this would be like a water– in oil in water type of an emulsion system. By the way, if you see a bit of a grey level and the little grey dots inside here, so the vesicles are filled in to this watery phase– water phase of this double emulsion, you can have several of these core aspects included, or these core components included in separate entities. You could even go for polarisation to dry the system and then embed into, let’s say, a food matrix, like a rice matrix shown here.
The question is how can all this be done? And I’m certainly showing it to you because the equivalent structure processing can be done by extrusion on all these length scales. We can really start on a macro-molecule scale and go through these type of structuring given in these figures along the series of length scales. In order to demonstrate this, I just choose a diagram which is showing the zero-shear viscosity of the matrices which we are treating and versus characteristic length scale, and really starting with a very small scale. So it’s carbon nanotube extrusion is possible done by a group of L.X. Zheng and co-workers, already in 2004.
So this is something which has been further cultivated and be followed in literature on nano extrusion. So from our work, we have done giant vesicle and vesicle extrusion in order to tailor make the size. So this is roughly 500 nanometers– 200 to 500 nanometers– pressing through pores and forming well-defined sizes of vesicles. If you want to go a bit more macroscopic, so this can be done in microfluidics channels, where we have a flow-focusing device in order to generate some droplets in a matrix fluid. And this can be repeated. This is a bit of a lab on a chip, constructed for doing these two or three steps of such type of flow-focusing emulsification behind each other.
And that means, with this, you can get these, let’s say, capsule in the capsule of the capsule type of structures. When we go a bit larger scale, so we can also and want to have deformed kind of entities, like gelled beads, or ellipsoidal gel beads or capsules, so we can do this eccentrically in a flow channel, which is a bit more microscopically shown here on a millimetre scale in order to look for upscaling characteristics and rules. When we go very low viscous, we can also say extrusion is also happening in spraying nozzles.
However, we have a turbulent flow generated at the outlet of a spraying nozzle, more or less pronounced depending on whether we mix air or gas already in two-phase nozzles into the flow, or we don’t, or at which position we do this so we can have an extrusion through these types of nozzles. And going to the highly viscous part– so we end up with rice kernels, like shown before, where we can have capsules embedded. Now this is typical cooking extrusion, which is done here and then cutting off these rice kernels at the outlet of the extruder.
And last but not least, very highly viscous and large-scale is ultra-low temperature ice cream extrusion, where you can generate ice cream with very small dispersed ice crystals, not interconnected, very creamy type of characteristics. I will go through this in some detail. But this shows you already the capacity of extrusion processing on different length scales. Now, with this, I’m just entering into giving you an overall look to a scheme, which starts with the input of any powder type of material. So kind of a processing chain, at the end of which the extrusion is doing a great job, or can do different jobs of major relevance. So solids are being produced in a well-defined way from raw material.
Extracts, maybe plant extracts, by special extraction and then coupling this for the generation of any type of capsules. So this is showing a membrane device in which such capsules are generated going into a spray, making a powder of these types of capsules. And finally, powder can also be sterilised, if necessary, if you go for infant formula, for example. And then mix the powder fractions. Go for granulation centering, maybe 3D-powder bed printing, or into extrusion. And in extrusion, so at the outlet of such a circuit of functional structure formation, we may have this type of multi-capsule powders in which we may have functional components embedded. And this may be, in the case of an instant drink, be re-dispersed.
Or let’s say, you should open up in the mouth, if there is aroma of flavours encapsulated. Or should open up, let’s say, in the duodenum for reaching all the small intestine, for reaching some specific receptors. And also may have to be tuned to survive the stomach passage in order to not release function components in the stomach. So this could be done with the rice kernels, let’s say, which are for fortified food production. There is another example demonstrated here with bakery products, so embedding this into dough masses which you can extrude and implement functional components or functional structures into such. Last but not least, there is also texturized plant protein, so so-called meat analogue.
So I would prefer to call it beyond meat analogues, because we can go much further than just simulating meat structure. So this is the chance to go in to such embedding functional capsules, capsule-like structures, into different food matrices, which then will have to be disintegrated in the mouth or in the stomach, and which can be observed by MRI, fMRI, and maybe there is also interest in the gut-brain interaction to look for a brain response– whether we are satisfied, or our body is satisfied on what we have eaten. So a little chocolate brain, which I show you here, causes quite a lot of a dopamine effect in the brain.
So just as an example for something that can be monitored throughout the digestive process. Because there, we have also the processing steps that disintegration steps of what we have created before and structured before. The function should certainly be released in a tailor-made manner, so at the receptors, in order to have a most efficient effect. Now let’s go a bit into a product development by extrusion. And so, I wanted to play through with you, just briefly, let’s say some different extrusion types where the mechanical treatments and structuring is superimposed by some thermal treatment, ultra-low temperature extrusion, cold extrusion, warm extrusion, hot extrusion, and wet extrusion, with a parameter set which you can see here.
Typically, for ultra-low temperature extrusion, we will see is ice cream– frozen desserts. And then we will see some pasta extrusion for the cold centering type of extrusion, some snack extrusion, starch-based snack extrusion for warm extrusion, and hot extrusion, and finally, the wet extrusion, which is a hot melt extrusion of proteins for the meat analogue type of products. Starting with the extrusion of pasta– so this shows a polymatik device. You see an extruder already in a combination of two extruder parts.
So a feeding extruder where you do a conditioning of flour and the components for pasta, mainly semolina, and then having what the pasta makers also call the press, the pasta press, which is another extruder to build up the pressure and press it and shape it through the nozzle. What you can see here, like spaghetti and other type of pasta coming out of multi-nozzle type of a multi-dye, extrusion dye set up under high pressure acting. What happens on the different length scales? We have started to think about, let’s say, looking specifically to macro, meso, and molecular scale. So the macro and the molecular scale are shown here depending on the water content at the beginning.
There is a dough of high-water content, and later on you may go for drying this type of pasta. But depending on the water content, you can see here or take out of this table what happens to the different structural building blocks on the different length scales here as a function of the water content. So it relates really to the granular system, then globular protein system on a meso scale, and then the molecules, the starch molecules and the protein molecules, and their interplay on the molecular scale.
When we go into the extruder with such a dough system, we can see that there is quite an impact of mechanical treatment because the mechanics is certainly the strength of the extruder to make the functional structure as a function of the mechanical treatment. And we can see here, just doubling the RPM of the screws, of the extruder screws, and if you compare to the pictures here you see a change in structures who have a more bubble-like or cavity-like structure, which is a bit more closed, in the case of the higher mechanical treatment. Just from an optical perspective, you can already see a lot. For the properties, there is quite an impact.
I just show you for the pasta, where would compare cooking loss, water uptake during cooking, the firmness of the cooking, and the stickiness of the final product as a function of the RPM of the extruder– means proportional to the mechanical power input into the system, or energy input. And that means, the more you treat, the more cooking loss you have, the less water uptake you have, the more stickiness is developed, and the less firmness is developed. So quality characteristics can be related to the structure and the structure can be related to the mechanical treatment in the extruder. Going for a snack extrusion, which is a hot extrusion, so we are looking on a molecular scale, as an example.
The molecular scale of starch dominated type of a system. And what we can see here is that the starch, the molecular weight with increasing mechanical energy input is decreasing. So we have a degradation of molecules due to these mechanical stresses acting. And the dependency on temperature can also be seen from this diagram. This is a model based from my work, from Miss Brümmer in 2002 already, but it is a nice example to look on the molecular scale and the related impact of extrusion. Again, molecular weights versus temperature and mechanical power input for a special type of starch, which shows a different behaviour than the one shown before.
But these type of diagrams give you a good orientation on what happens on the material scale. Looking at starch under optical conditions, one can see that under the polarisation microscope how it disintegrates as a consequence of gelatinization and modification happening under thermal impact. Important for these hot extruded snacks is a quality characteristics which is called expansion index VEI. And the expansion index, as you can see from this diagram, as a property relevant for the consumer, is very much depending on the water content. WG stands for water content. Lower water content means a more dry, highly viscous paste type of system.
As a consequence, the mechanical power inputs will be much higher under these conditions, keeping the extrusion conditions the same, but having a higher viscous or less water containing, highly viscous paste. And that means you have a stronger treatment and the molecular weight will certainly decrease, but with a positive effect on the volume expansion means the expansion, the steam, which is evaporating and generating steam bubble or air cell bubble type of structure, can better be stabilised by the starch being degraded by the mechanical stresses to lower molecular , weight as can be seen here.
Our next example it comes from high moisture. A meat analogue processing, which is a generation of a protein melts under high temperature and pressure conditions. And you see a long cooling die of an extruder, which is then trying to orient the fibular, or generate a fibular structure of the protein, and orient them in order to get these fibrous structure like meat on looks should have.
As you can see here, there is different isolates, protein isolates, from pea or from sunflower or from canola or mixtures also with fibre-rich systems in order to generate, let’s say plant-based health supporting material, which has a texture, a meat analogue-type texture, but certainly can also be further enhanced in aroma flavour aspects by embedding encapsulated material into these extruded matrices in the extruder. Last but not least, our ultra-low temperature extrusion– what we can do there. Extruding at very low temperatures, like in an ice cream or frozen dessert, allows us to keep ice crystals separated from each other because we can cool down to temperatures where only a maximum freeze concentrated solution is left as a serum.
And that means no free water, which will increase the ice crystals further, or lead to a high connectivity of three-dimensional ice crystal network. Conventionally, if you have an ice crystal slurry, from typical ice cream machinery, at minus 5 degrees c, and then filling into cups, it’s hardening, in so-called, hardening tunnels at minus 40 will give a more icy type of structure. So ice crystals grow together, form three-dimensional network, and this is certainly counterproductive from a sensory expectation perspective. Special characteristics of such extruders– so you do not want to have a steeped pressure profile. So what you can do is the typical pressure build-up in front of the dye can be compensated by your feet pump.
So to get a flat profile, gentle laminar mixing by a cording arrangement of the screws– just distributing the flows and gently mix at low shear rates, optimised heat transfer by narrow annular channels, or screw channels, with short characteristic length for the heat transfer and some further aspects or key aspects. The low shear rates should be below 30 reciprocal seconds roughly. A narrow shear rate distribution to have not locally large differences. And a narrow size distribution to have equal times for the treatment of the different structural building blocks in the system. What has to be adjusted, because when you cool down and get high viscosities you dissipate more energy proportional to the increase of viscosity.
So the management of the heat transfer to get rid of the dissipated heat, and on top, be able to freeze, cool further down to minus 15 to minus 18 degrees C is necessary, watts can be calculated as that heat transfer coefficients under optimised conditions of the extruder design of 400 to 500 watts per square metre and Kelvin are relevant. If you manage to produce this at minus 15 to minus 18, you may skip the hardening tunnel, go directly into packaging, and reduce the energy costs for such a system, for such a factory, up to 50% and more.
So this shows you two of these extruders with different screws, simple conveying screws, a twin screw, a single screw extruder for this ultra-low temperature extrusion. You have some evaporating cooling fluid around the barrel in order to generate the best heat transfer possible and reach the 400 to 500 watts per square metre, and counting. What is of importance to freeze the water is to see– that’s typically when we go to minus 5 in conventional processes. We may have frozen only 30% of the water. If you go to the minus 15 to minus 18, we reach roughly 70% to 75% of the water being frozen out.
And at 78 to 80, a typical ice cream recipes, it’s the maximum freeze concentrated solution reached. That means, not many crystals being available for further, or not much water being available to increase the crystals, furthering growth and let them grow together. So this is the concept of these high-pressure– of these ultra-low temperature extrusion for these frozen desserts. What it needs from a materials science perspective is the real logical information.
So the sheer viscosity is a function of shear rate, which can be described with so-called Herschel-Bulkley fluid model constitutive of real logical equation and consistency factor of flow exponent and the yield value, which are all depending on temperature, which can be measured by lab scale rheometry using, let’s say, a similar cooling and shear as in the extruder. What is of interest further, is power characteristics in order to see what the type of flow is at what Reynolds numbers, what power is necessary to generate the flows, or as a fingerprint characteristic of a flow apparatus so the power characteristics Newton as a function of Reynolds should be made available. This can be also measured and determined experimentally.
And the result of dispersing ice crystals are also air cells, or fat agglomerates. So this is of interest as the functionality of an extruder, of such a low-temperature extruder, in terms of refining the ice cream structure without melting it. So you have the dispersing characteristic’s diameter of the dispersed components, like ice crystals. In this case, it’s the air cells, so that the foam bubbles, or fat agglomerates, which are emulsified in the system. And you can see where it is the volumetric energy input. And you can see you for the extruder low temperature, freezer extruder, you can see the curve here as a characteristics what can be reached for the dispersed phases under such processing conditions.
With this, I would like to summarise length and timescales are important to be considered on different lengths and time scales. Extrusion can be done really from nano molecule of scale to macro scale. Extrusion principle can be treated in the same way. Encapsulation can be done in structures which are generated in extruders or embedded in such structures. Thermal treatment perspectives of y temperature range from ultra-low temperature to high melt– high-temperature melt extrusion for the meat analogues.
I played through the different types of these thermal treatments in pasta processing, in starch-based snacks processing, as well as, for meat analogues to demonstrate how these fibrous structures can be generated from the protein melts, and finally, ending up with the ice cream structure, finely dispersed structure under highly sustainable processing conditions, not making a hardening tunnel any more necessary. With this, I would like to finalise this presentation, which gave you a wide insight into the capabilities of extrusion processing, from length scale perspective, but also from the translation into real food products and their optimization with respect to desirable functionalities from a sensory, or from a health-supporting perspective.
With this, I would also like to thank my co-workers, which are listed in these last slides. And certainly, also thanks for the funding, which made it possible to move along this research. Pat, thanks a lot.