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.
Whey Proteins
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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.
Ministrado por
Dr. Walter Hurley, PhD
Professor Emeritus
[SOUND] So we just spent
some time looking at and
thinking about caseins.
A major class of the proteins that make up the milk proteins.
And now we want to look at the whey proteins, which are the other major set.
A lot of difference between species, many, many different whey proteins.
Although there's really only a small set of the major ones.
We'll cover some of those here in just a moment.
Different species have different sets of these things.
Some of them are common across species, some of them are not.
Again, a lot of variation among species, and
especially in terms of the whey protein.
So what we're going to do now is we're going to look again at cow milk.
So we're looking specifically at cow milk and
what are the major whey proteins in cow milk.
A major one is called beta-lactoglobulin.
So by its name you can tell it's a globular type of protein.
And cow milk, it makes up about 50% of the whey protein.
And we'll see a little bit later that whey proteins only make up about 20%
of the total protein.
So it's only about 10% of the total milk protein.
But it is the major whey protein in cow milk.
The next one is alpha-lactalbumin.
And this is about 25% of the whey protein.
So again about, what's that make it, about 5% of the total milk protein?
This one's particularly important in terms of synthesis of lactose.
It's part of that lactose synthase complex enzyme that's in the Golgi
apparatus in the secretory vesicles that's making lactose.
So this has a very important function in terms of production of milk.
What's the function of beta-lactoglobulin?
Well, frankly, we don't really know what that is.
It doesn't seem to have a major function in terms of the neonate.
Doesn't seem to have a major function in terms of synthesis of milk.
So it's still a little bit unclear as to what beta-lactoglobulin,
what it actually does.
What else is there, what else can we include in this pot of whey proteins?
Another one is lactoferrin, An iron-binding protein.
And again, in cow milk, there's not a lot of lactoferrin, but
there is some lactoferrin.
In human milk, there's a lot.
In fact, in human milk this is the major whey protein.
Human milk, for example, does not have beta-lactoglobulin.
In those few cases where beta-lactoglobulin has been identified in
human milk, it's probably from the woman ingesting cow milk.
And little tiny, tiny, tiny minute amounts of beta-lactoglobulin actually
making it into the breast milk of the woman.
So humans don't have this, the major milk whey protein in human milk is lactoferrin.
Other things, immunoglobulins, These are the antibodies.
And again, they're different IgGs, IgAs, IgMs, and so on.
Again, there's a whole range of these kinds of things in milk,
human milk as well.
And then certainly we can include a whole bunch of other ones down here.
And again, every species is a little bit different.
One example, for example, comparing cow milk, which I've indicated here.
Another, an enzyme activity we find in cow milk is called lactoperoxidase.
So these three put together are very antimicrobial.
Binds iron, again, different antimicrobial components.
Cow milk has a fair amount of lactoperoxidase in it.
Human milk, on the other hand, has relatively little lactoperoxidase but
has a lot of an enzyme called lysozyme.
And a lot of lactoferrin, a lot of lysozyme, a lot of immunoglobulin.
Very, very antimicrobial in human milk, even more so than in cow milk.
So again, you can see there's a lot of difference among species.
Let's take a look at the slide here to kind of give an indication of that.
And this is a slide where what we've done here is we've taken milks from different
species here, in different stages of lactation.
This is a lactating cow versus a cow that's not lactating at that
particular point.
We've run those through what's called an acrylamide gel.
And to make a long story short,
basically the higher molecular weight proteins are here.
The lower molecular weight proteins migrate down to here.
The caseins are kind of right along this band in the middle.
And you can see cow caseins, goat caseins, rabbit, and
mouse here in this particular case, mouse milk.
Beta-lactoglobulin runs right here,
you can see there's a lot of that in the goat and the cow.
And then as well, is this other physiological state,
the mammary squeezings from the dry cow,
the involuting cow mammary gland, alpha-lactalbumin.
And again, some of those are different sizes,
so some of these are actually not beta-lactoglobulin.
They're probably more alpha-lactalbumin.
Lots of diversity in terms of milk whey proteins.
What they are, how much there are, and so on and so forth.
So you really have to kind of focus on each individual species to really
understand the story about whey proteins, but there are many of them.
So as we think about these proteins in milk.
Again, caseins being a major class of the milk proteins.
Whey proteins being another class.
The proportions of those can be very, very different.
So let's make a little table over here.
So casein, and we'll just for the moment abbreviate whey proteins WP.
So we're looking at the proportions of these amongst two key species that
we're interested in, cow and human.
Human and cow milk.
Cow milk, about 80% of the total protein is casein, leaving
about 20% as whey protein, a lot of casein.
So when this stuff, the cow milk,
hits the intestine, it's going to be true for many species.
When that hits that stomach,
excuse me, when it hits the stomach, it forms that curd.
It forms a very tight curd, and so it's very slowly released.
Human milk, on the other hand, it's kind of the other way around.
It's only 40% casein and 60% whey protein.
And remember, human milk,
you have even lower concentration of total protein than cow milk.
So again, the curd in the infant,
the newborn infant, nursing the mom, consuming human milk.
That curd's going to be much more flocculent,
it's going to go through faster.
And it's just not going to set up the really hard curd that cow milk would be.
So again, people who are designing infant formulas based upon
proteins from cow milk need to be obviously taking that into consideration.
Very, very different composition of protein, as well as amount of protein.
And that will impact what happens actually in that stomach.
So we've taken a look a little bit at some of the milk proteins.
And we just want to review very, very quickly.
So the different major groupings of milk proteins are the caseins and
the whey proteins.
We've seen that there are multiple caseins, there are lots and
lots of different whey proteins.
This is usually secreted in a form called a micelle.
Again, many, many, many, many, what did I say,
10,000 molecules of caseins in each micelle.
Lots of calcium phosphate in the micelle, again, packaged.
And when that colloidal particle, sometimes people call it that,
comes apart, that's when we get the gelatinous material we call a curd.
Whether it's a cheese curd or some other kind of curd.
So this is really maintaining the stability of this in fluid milk.
And then managing or carefully controlling the disassociation of this
micelle is really the basis of most dairy foods and food products.
Whey proteins, again, we've seen that there is a lot of diversity among species.
In the cow milk, we have beta-lactoglobulin.
Pretty much all species are going to have alpha-lactalbumin.
because again, remember, that's part of this lactose synthase complex,
so it's involved in lactose synthesis.
Again, a very, very important protein.
Immunoglobulins, lactoferrin, lots of other proteins.
And again, each species, you kind of have to look at and see, well, what do they
have and how much in terms of proportions they have of those specific whey proteins.
So that's been a quick overview of milk proteins and
how they fit into milk composition.
And now what we want to do is we want to go on to another milk component and
take a closer look at that.
[SOUND]
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