Lactation and especially milk, which is the product of that unique mammalian process, are routinely encountered within our daily lives. Nevertheless, they often are poorly understood by many, even including many who are engaged in the business of producing milk. The overall course goal is to introduce fundamental concepts that form the basis for understanding the biology of lactation, the biology of the mammary gland, and the products of that important physiological process. As a learner in this course, you will be provided with a series of easily understood presentations that collectively will help you build a foundation for greater understanding of lactation. You will be able to engage with other learners so that you can extend your learning beyond the video presentations. Ultimately, you will be able to construct your own mental model for understanding the wide range of topics that relate to the biology of lactation. Upon completion of the course, you will be prepared to expand your knowledge and understanding of lactation from other sources and experiences as you pursue your individual interests. Before you start the course, I suggest that you identify a question or several questions about lactation that you already have on your mind. This could be from your own experiences, something you read about or saw, or something you have wondered about. Write down your question(s) and use that to help you decide how to engage with the content of this course. You might engage with the modules in the order they are presented, or start with a module that is of particular interest to you, or pick and choose modules in any order. I encourage you to engage in all of the types of learning activities that this course has to offer, including but not limited to, the discussion forums, quizzes, peer-review assignments, and concept maps and other learning aids.
Casein
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En provenance du cours de University of Illinois at Urbana-Champaign
Lactation Biology
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Module 3: Milk Composition
Milk is the product of the lactation process. Such a simple statement does not come close to doing justice to the complex nature of this biological fluid. In this module, we explore the major components of milk, as well as provide a basis for understanding milk as a product that we purchase in a store. Mechanisms of synthesis of milk components are introduced. And, several videos include discussions of the nature of differences in milk composition among mammalian species, including comparisons of cow milk with human milk.
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Dr. Walter Hurley, PhDProfessor Emeritus
Department of Animal Sciences
[SOUND]
Welcome.
We're going to continue thinking about milk composition and
looking at milk protein.
We talked earlier video about crude protein.
Now, we want to start breaking down and looking at specifically at milk proteins.
And we're going to focus, in this video, on caseins.
In another video we're going to talk about whey proteins.
Before we get going though, let me help you think about milk in another way.
This is an electron micrograph of milk and we have these great,
big milk fat globules, they're membrane bound.
You can't really see the membrane here but this electron dense material or
some other proteins that associate with those fat globules inside the cells and,
therefore, when they're secreted, they're still associated with that.
So milk is actually emulsion of fat globules.
Remember, of course, fat is of lower buoyant density than water.
This is why cream rises, so
this is a partially stable emulsion of milk fat globules.
If you take raw milk, set it out on the table, or in the refrigerator or whatever,
that cream's going to rise to the top.
So it's partially stable milk fat globule emulsion.
The black dots here represent the casein micelles.
So again, it's a suspension of casein micelles.
So these very, very densely packed granules called micelles of casein,
calcium, and phosphate, and we'll get to that here in just a moment.
And then, of course,
the rest of this aqueous component are the soluble components,
the whey proteins, lactose for example, and then many other kinds of protein.
So let's start looking specifically at the caseins.
Caseins are a group of proteins, they're more than one.
They're calcium phosphate-binding phosphoproteins.
So they're proteins, again there's several.
Many of them, most of them, are phosphorylated.
So they actually have a phosphate group attached to a serine residue.
And some cases they have multiple serine residues.
And those negatively charged phosphate groups then interact with calcium and
phosphate.
So it pulls together all this calcium phosphate and
again, we end up with this micelle structure.
Let's go to the next slide.
So let's break down and look at the caseins individually.
Very briefly in cow milk, recall that cow milk protein's about
3.5% protein, of which about 80% are the caseins.
And human milk, for example, which has a lower protein content to begin with, but
in addition, it's only about 40% casein.
So again, every species is a little bit different in terms of
the amount of protein and the breakdown of these different kinds of milk proteins.
And cow milk, we think of this as alpha S1, alpha S2.
These are multiphosphorylated, so that means they have a lot of different serines
that have phosphate groups on them.
Beta casein is typically called the major casein and therefore,
the major protein in milk because so much of cow milk is a casein.
It's also phosphorylated but nowhere near to the extent that the alpha S1 and
2 are phosphorylated.
Kappa-casein is glycosylated but more critically for our purposes.
It's a key protein in terms of stabilizing this micelle structure we're going to
talk about.
How do you keep that thing together and stabilized?
If you look at the ratio of these, about 4:1:4:1.
So the alpha s1, and the beta are the major ones.
Alpha s2, and the kappa are more minor in that
population of casein proteins in the micelle.
[COUGH] There's also one called gamma-casein.
This is really just a breakdown product of beta-casein.
So it's not a separate gene.
So, even as these things are being produced in the epithelial cell,
sometimes those peptide chains are being clipped.
And this particular one is a fairly large peptide, stays in the casein cell.
And so, sometimes people have identified that as gamma-casein.
Now, an important point here is if you go to another species, you might find
a little bit different terminology of what alpha s is, beta, and so on and so forth.
So other species use a slightly different designation for these.
So you kind of have to look at every individual species and say, okay,
which gene are we talking about here?
Call something else in another species.
This is what we see in cattle and goats and so on.
Go to the next slide.
Casein micelles, this is, again, what they look like in milk.
So here's casein micelle here, bunch of them here, one over here.
So they're very electron dense because they're so
tightly packed with calcium, phosphate, and the casein proteins.
So what I've done is I've taken one out here, and just basically blown it up but
also inverted it in terms of color.
So you can kind of see it's not homogeneous.
Rather, if you look carefully, you see some areas are lighter than other areas.
So it's not a homogeneous thing.
It's all these things packed in together.
If we look at the number of casein molecules that we might find in an average
micelle, it might be 10,000 casein molecules, casein protein molecules.
Calcium, about roughly 70% of the calcium in milk is going to be tied up,
in cow milk, is going to be tied up in that micelle.
That means the other 30% is going to be out here in the aqueous phase.
And about 50% of the phosphate's going to be tied up in that micelle.
So this has a lot to do with why milk has so much calcium in it because calcium
phosphate's not normally that soluble because you can package a lot
of that calcium into the suspension of these casein micelles.
Size and average diameters range from
about 50 to 250 nanometers or .05 to .25 micrometers,
averaging about 150 nanometers for example, in that ballpark.
So I have a little prop here, just to share with you.
This is just a plastic mesh sponge.
And so, if you can kind of take something like that and
think of that as the casein proteins are the plastic, kind of all intertwined here.
And then, in that would be the calcium phosphate.
So this is basically what I'm projecting to you here.
So it's, again,
just another kind of representation of what these things might look like.
Lets go to the next slide.
To take a quick look at what they look like in the cell.
So we have an epithelial cell here, mammary epithelial cell.
The lumen is up here, the lumen of alveolus.
The nucleus is here, the basal membrane would be down here.
We have a little bit of Golgi apparatus right here.
And then our secretory vesicles with lots of casein micelles in them,
they actually start to be formed in the Golgi apparatus.
They're phosphorylated, they start to form in there.
But then continue being formed in the secretory vesicles until they're finally
secreted out into the lumen of the alveolus as part of the milk.
Next slide.
Again, going back to what does milk look like.
Again, this idea of an emulsion of fat droplets in a suspension of casein
micelles.
And if we look, if we take a milliliter of milk,
we're going to get about 6x10 to the 14th casein micelles,
not molecules, but casein micelles per milliliter of that milk.
So lots and lots and lots of casein in that milk.
Let's go to the next slide.
So just to kind of wrap this up very briefly.
We talked about casein micelles.
And one of the key features of the casein micelle, again, this idea of
being stabilized, the kappa-casein stabilizing that as an example.
If for some reason that comes apart, all those different molecules of casein,
all those casein micelles kind of come apart.
They form this gelatinous material we call a curd.
And that's the basis of cheese, cottage cheese, yogurt and many,
many different kinds of milk products,
dairy products is the controlled manner of these casein micelles coming apart,
partially digesting the caseins, and so on and so forth.
And that's, again,
the basis of a lot of the milk products that we have available today.
In another video, we're going to go explore that a little bit more and give
you a couple of demonstrations on how we can make this casein micelle come apart.
[SOUND]
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