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Raw materials for Encapsulation – Carbohydrates

Prof. Dr.-Ing. Erich J. Windhab (ETH Zürich)

Raw materials for Encapsulation I - with focus on carbohydrate capsule and gel systems
Hello, everybody. And welcome for this lecture course on raw materials for encapsulation, where I will focus in this first part of this lecture on carbohydrates. Subchapters, which I will give you after an introduction, is on carbohydrates in encapsulation processing, then followed by exemplary description of galactomannans film and gel forming. Then I’ll have a subchapter– comparative testing of carbohydrate, film, and gel properties for encapsulation, and finalise with conclusions and a short summary. With this, I would like to start again, with the S-PRO square scheme. So because process make structure, structure defines property. And from a consumer perspective, we go in reverse engineering approach– backwards. That means we start from consumer expectations for encapsulating materials or encapsulation technologies.
This is controlled release, and mass transfer of active components– aroma, flavour, nutrients– and also the protection of such functional components. In addition, we would like to have certainly some powder characteristics from a techno-functional perspective, which give satisfying handling. Structure-wise we start with capsules and carriers on the macro side, followed by mesoscale macromolecular networks and macromolectular assemblies. And on the micro side, for the structure we have the carbohydrate and encapsulated functional component molecules. Processing relates to encapsulation– mainly related to the technologies of spray drying, spray chilling, fluidized bed coating, and extrusion. Let’s have a look at the basic structure classes of encapsulated compounds and encapsulated characteristics in food. We have irregularly shaped microcapsules.
We have core shell microcapsule, polynuclear microcapsule, insoluable matrix microspheres with pores for the release of the functional components, and soluble matrix microspheres. And in this table we have, let’s say, listed the ability of some of these structures to give best protection against oxygen, water, pressure, heat, and light interaction. And you see for the structures mentioned on top, how excellent or less good or poor these may be, so we can sort accordingly. Going into the materials for encapsulation– so we have mainly carbohydrate polymers, proteins, and lipids. I will focus in this lecture on the carbohydrate polymers, which is quite a widespread family. I will go into some details of selected ones.
All is related to the spray drawing, spray chilling, extrusion, fluidized bed, and microsphere capsule generation as typical technologies we want to apply this for. Looking for the other– or in comparison to the other big class of macromolecules on the food grade side for encapsulation– the proteins– so the polysaccharides and carbohydrates, which is the focus here– is in particular the underreactivity. So also, the high molecule of weight, non-surface activity, thickening, water holding capacity, and temperature insensitivity as major differences to the proteins of relevance in the context of encapsulation.
When we look at the different types of carbohydrates– which is starch and derivative, cellulose, plant exodates, plant extracts, and so forth– we can relate this to the listing– or a bit, let’s say, differentiated listing of the technological processes mentioned before. What is important for everybody to apply is to go through a checklist first in order to get an orientation. Possible purposes for encapsulation can be increase of shelf life, masking of taste, simplification of handling, controlled targeted release, and improvement of appearance. And aspects which one should explore or think about for having a better orientation and tailor making the capsules is functionality of encapsulating in the product.
Restrictions may be for the coding material, concentration of the encapsulate, type of release intended, and the stability requirements– as well as, certainly, cost constraints, because economy has to fit as well. A list of representative examples is given in this table, where we have the different shell materials, and then some aspects of regulatory status, chemical class, encapsulation process, and application– just to give an overview what’s already available. When we go for the carbohydrates now into very specific details, we want to know about the source, which is typically marine, plant, or microbe, biological or animal related. And for the structure– structure is most important as we know, because it codes the properties.
And this is why linear, or branch structure– and on the linear side, neutral or charged– the branched ones are mostly charged. So it is of relevance for the best selection. Going for methodologies– how to characterise the ability to go for a good encapsulation characteristic. So as from a substance perspective, it’s solubility, rheology, transition temperatures– let’s say, melting, or glass transition temperature. Stability– under pH temperature conditions. Surface activity, film forming ability– of major importance, because we want to encapsulate with films and the gellation ability– because sometimes we just have gels, spheres, and maybe not necessarily coded layers. So this can be analysed by a variation of methods. I will go into some details of some of these exemplarily.
Now, going forward, in order to sort it a little bit towards, let’s say, main characteristics, I decided to go a bit for one family, of galactomannans as carbohydrates– because one can nicely show up these process structure property relationships. Let’s start with a structure. So structure is mannose chains with side chains of galactose. And what is interesting in these galactomannan family is that the average degree of substitution– the so-called degree galactose changes from locust bean gum, 25%– tara gum, guar gum, to feno greek, which is up to 97. So, we have an increasing substitution, with the galactose side change, which makes a difference in molecular weight, but also in the properties.
As we will see, molecular weight distribution, shown on the right. So we see from locust bean gum, to tara, to guar, to feno greek, we have an increase in the molecular weight– a clear shift. When we look at a phase diagram, which we can accordingly derive– so we have the molecule weight axis, and we have the degree of galactose substitution. We can see the islands here, from the locust bean gum, tara, guar, to the feno greek. And we also have, let’s say, a bit of a dotted line here, because the locust bean gum is on the borderline between an unstable domain and a thermodynamically stable domain.
This relates to these different phases builds, depending on the degree of substitution with galactose. So we can see a phase one, which is more densely packed– mostly chains with low galactose content. And the less densely packed ones, with high galactose content. And this also leads, then– so when we have like for the locust bean gum, we have a low content of galactose. So this leads towards these more densely packed structures, which gives more gel-like conditions under same temperature and concentration conditions, compared to tara and to guar gum. So in this direction, we also have a change in rheology.
When we want to analyse– and for the detail size exclusion, chromatography gives us a good, let’s say, plausibility, to start on a molecule of scale. And what you can see here is refractive index, wide-angle light scattering, to see something, or get some information about length scales of structure. Also, low-angle light scattering with viscometry and intrinsic viscosity, as part of the viscometry, as well as the molecular weights. So we can see, we can get all these informations, and getting the distributions, with these chromatographic technique– which is very suitable and very comfortable. Just to extract some of the most important aspects– intrinsic viscosity, which is kind of the viscosity of a molecule in its liquid surrounding.
So we can see that when we look at versus molecular weight of this intrinsic viscosity, we can see how this aligns. This depends certainly, also, on the galactose substitution. And we can derive an equation, which is shown here, which includes the galactose content– besides, let’s say, the molecular characteristics– intrinsic viscosity, molecular characteristics of the pure mannose chain part. The other one is the molecular radius. So it’s a viscometric radius, which is also increasing with the number of units, or the molecular weight, according to this master curve shown in this diagram. When we go for processing of galactomannans gel, or layer systems, film systems, we have to prepare this from the seed.
And I wanted to demonstrate here how much processing can impact on the quality of these molecular characteristics. So when we have different, let’s say, milling methods for the seeds– for the endosperm of the seeds– soaking first, to go for a so-called rubbery milling. That means we soak in water in order to have more rubbery characteristics, which allows for a more gentle disintegration of the seed– compared to a boil method and dry milling. And so we can see how the larger molecular weight parts are very shrinking by the dry milled situation. And so where we have fragments here on the lower molecular weight side, when we do the dry milling.
This has a big impact on the rheology, on viscosity functions versus shear rate as we can see here. Because what we can see derived from the soaking rubbery milling in yellow is close to a decayed higher zero shear viscosity, compared to the boiled and dry milling ones. So at the very beginning of the processing, we decide about whether we can keep what nature has synthesised for us, or not. When we look at this in a diagram, which is typical for this characterization of molecular, viscous, and molecular weight characteristics. So you can see here the zero shear viscosity versus concentration times intrinsic viscosity.
And along these lines, we can see that commercial products– the black dots– and the so-called homemade, most gently treated ones here high up– so is quite far away, and on the better side, in terms of viscous characteristics, or increasing viscosity– because finally the locust bean gum is a thickener, which has been introduced here in this diagram. So when we want to come back to the wider range of galactomannans and other carbohydrates, we can see that there is common rules– just some shifts of the curve for the intrinsic viscosity for the viscometric radius, as well as for the molecular weight distribution, depending on the type of plant families where the seeds and these galactomannan materials come from.
Finally, and last but not least, when we generate the encapsulated films or gels, we have to test them mechanically. Certainly determining or depending on the thickness, the tensile strength, steel strength, puncture strength, and also elongation break point are of interest and of importance. We do this with these mechanical testing machines. Another thing, if we have the films formed, we want to know about oxygen permeability– which has to be as low as possible, in order to avoid oxidative effects. And we can see here, the comparison of films with 2%, carboxymethylcellulose 3%. Then we have sodium alginate, which is the SA, different percentage, and potato starch as the third one.
What is also important is water resistance during storage, against the moisture during the storage. So water resistance, also, in an acid environment or a alkaline environment, which has to be taken into account. So for the carbohydrates I have mentioned before, this is compared here. And also of interest, oil permeability, or uptake of oil into these type of structures, which should be low as well in order to not act as a sponge for oily material. Last but not least, here is some optical appearance aspects, which is turbidity of the films. Some of the films should be, let’s say, in parks. In some cases, you want to have them transparent. And you can also sort them accordingly.
And then finally, also look at the structure of these different films. Sometimes one can nicely see, let’s say, channels, which also give a bit of an idea on permeability, or related to permeability, this type of structure for these films. If one does a complete comparison– so I have the big list, where there is also proteins included. But here in the red frames, you have only the polysaccharides. And you see, with respect to tensile strengths and so forth, all the criteria I mentioned before, you have the ranking for the best on the left-hand side, for the worst on the right-hand side.
And this is all related in the red boxes for these polysaccharides, the carboxymethylcellulose, sodium alginate and potato starch, in comparison, at the different percentages. With this, I would like to briefly summarise. So we have seen that these materials for encapsulation either are relevant for a film coating, or for microporous gel type of structuring. So we have major characteristics of the carbohydrates, which differentiate them from proteins, for example, their low reactivity, high molecular weight, low surface activity, thickening, and water holding capacity, and, in particular, also their temperature insensitivity. The galactomannan carbohydrate family was shown exemplarily, in order to give an idea of how these molecular structure, property, and impact of processing relationships looked like.
And these can certainly be played through for other families of carbohydrates. And finally, the functional differentiation with the testing of mechanical aspects and mechanical characteristics, but also permeability aspects and resistance against water, finalises this kind of characterization. With this, I would like to thank you for your attention.
Carbohydrates as a raw material for encapsulation. Carbohydrate capsule and gel systems as focus
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