Hello, my name is John Mathai. I am a Ph. D. student in the Hans H. Stein Monogastric Nutrition Laboratory. Today I'm going to take this opportunity to share some data with you from an experiment we conducted last year comparing the digestibility of various plant and animal proteins. 

We’ll begin with a little background. There’s a lot of question and debate about the quality of proteins from different sources. And this extends beyond swine and poultry diets, but even, if not especially, in the human diet. Often in human nutrition we see that argument boil down to: How do plant sources of protein compare to animal sources of protein? How do diets based on animal protein compare with diets based on plant proteins?

But we know that we do not have a requirement for protein, per se, but we have a requirement for indispensable amino acids. And so when we look at these ingredients we can look at the concentration of amino acids. However, we know that the concentrations of amino acids can vary dramatically among different protein sources. And even within sources they can show significant variation. For example, if we are discussing animal proteins as a whole, we know that there are major differences among the concentrations of amino acids of egg proteins, milk proteins, and/or meat proteins. Or, if we are discussing plant proteins as a whole, we know that the concentration of amino acids vary between sources, as well. An oilseed, like soybean, has a very different amino acid profile than a cereal grain, like corn. And even still, we know that the value of these proteins to an organism is beyond their concentrations, but extends into what we would consider the bioavailability of those amino acids.

There’s a quote here from the FAO that I like to use that describes protein quality as such: The nutritive value of a protein depends on its capacity to provide nitrogen and amino acids in adequate amounts to meet the requirements of an organism. Now, as swine nutritionists, we take this even a step further, by viewing ingredients on their abilities to furnish specific amino acids in terms of digestibility. And in practice, we do this by using standardized ileal digestibility values for amino acids determined using cannulated pigs.

And so knowing this, we wondered if by using this measure, we could differentiate between the quality of various dairy and plant proteins. And maybe elucidate the debate of protein quality among different sources.

So, to do this, we procured four dairy proteins: Skim milk powder (SMP), milk protein concentrate (MPC), whey protein concentrate (WPC), whey protein isolate (WPI) and four plant proteins: soy flour (SF), soy protein isolate (SPI), pea protein concentrate (PPC), and wheat. Now, I know these ingredient are a little esoteric, especially for their use in any swine diets. So, I thought it would be helpful to go through, just very briefly, the process of how these are produced. 

Now these diagrams you’ll see are oversimplifications, but they will help us to understand these proteins. So for the dairy proteins we start with whole milk. If we remove that fat from the whole milk, we are left with skim milk. And then if we take that skim milk and then we spray dry it we are left with skim milk powder (one of our other protein sources). But if we take that liquid skim milk and we ultra-filter to remove lactose and the soluble minerals we are left with milk protein concentrate (another one of our protein sources).

Now the other whey proteins are also derived from whole milk, but the process is a little bit different. If we split that whole milk into its curd fraction—this is the fraction with casein that would be used for cheese production—one of the by-products (at least it was traditionally seen as a by-product) is whey. So if we take this whey and we remove most of the fat via filtration, you are left with whey protein concentrate. If we remove the remaining fat, lactose, and soluble minerals via ultrafiltration, you are left with whey protein isolate.

We see a similar pattern for our plant protein sources. For our soy products, we begin with dehulled soybeans. If we remove the oil from these soybeans we are left with white flakes. Then if we grind these white flakes down so that 97% of the content passes through a 100-mesh screen, we are left with soy flour. And then if we take that soy flour and solubilize the proteins in water, and then take that solution and precipitate that protein and spray dry it, we are left with SPI. The pea protein concentrate in our experiment uses dehulled field peas and the same method. And the wheat used in this experiment was just whole ground wheat. I just want to make a quick note that this is the way the soy flour in our experiment was created; however, there are other techniques to create full-fat soy flours and reduced fat soy-flours as well, at least for human consumption. 

So, now that we have covered the ingredients in some depth, we will go into the study design itself. For this experiment we used nine ileal cannulated barrows with initial body weights of about 26kg. These pigs were allotted to a 9 x 9 Latin square of nine diets and nine periods. The pigs had a seven-day adaptation to the diets and for two days we collected ileal effluent samples. And throughout the experiment the pigs were fed at three times their maintenance requirement.

Each of the ingredients I described previously were the sole source of amino acids in their respective diets, with the exception of wheat. Wheat has a low crude protein and in order to make a wheat diet more palatable and feasible, the wheat diet included soy flour. Therefore all the results reported for wheat are via the difference procedure, as opposed to the direct. All of our diets were formulated to have a CP percentage of 17%, and what may seem a little unusual here, we formulated the diets to have the same calcium and digestible phosphorus levels. And that’s because we were using dairy proteins that have a very high concentration of calcium and phosphorus, and we wanted to eliminate the effects that may have on amino acid digestibility. And finally we had one N-free diet and this was to allow us to determine endogenous losses and calculate our standardized ileal digestibility values.

So, it’s important to understand these ingredients as a whole, and to understand their composition before we look at the digestibility results. And in this presentation our dairy proteins are on the left side of the graphs in dark orange, and the plant proteins are on the right side of the table in lighter orange. And we can see here, the CP concentration levels are highly variable among these sources, as we’d expect. If we look at our wheat here, remember a cereal grain, we see that it has a very low concentration of crude protein. And if we look at our dairy proteins on the left we see that, in general, as the processing level increases of our ingredients the CP levels increase. The same is true for soy.

So bearing in mind that CP values are very different I will present the lysine, methionine, threonine, and tryptophan concentrations of these ingredients on a gram per 100g crude protein basis, so that we can look at these amino acid concentrations a little more objectively. So, looking at lysine here we see that our dairy proteins, especially the whey proteins, have higher concentrations than our plant proteins.

With methionine, also, we see that the concentration is greater in dairy proteins than in our plant proteins.

When comparing threonine, we see that the concentration is greater in the whey proteins than in all other proteins. However, only slightly higher when considering the skim milk powder and milk protein concentrate compared with the plant proteins.

We see a similar trend as threonine with tryptophan, in that it is highest in the whey proteins, but similar in the other ingredients.

So, now that we have an idea of the concentration of amino acids in the ingredients, we will move on to the digestibility results.

So, if we’re looking at the digestibility of lysine, we don’t see many significant differences except that the digestibility of lysine is greater in both whey protein isolate and soy protein isolate than in soy flour. And we also see the concentration of lysine is greater in all ingredients tested than in wheat.

However, if we look at methionine, we see a little more separation between our ingredients. WPI has a greater digestibility of methionine than all other ingredients. And all dairy proteins other than skim milk powder have greater digestibility of methionine. In the plant proteins we see that the digestibility of methionine is greater in soy protein isolate than in soy flour, and greater in both soy products than in other plant proteins.

Looking at threonine we see that the digestibility is greater in milk protein concentrate, whey protein isolate and soy protein isolate than in soy flour, skim milk powder and wheat. It is also interesting to note that both skim milk powder and wheat have statistically equivalent digestibilities of threonine.

And finally if we look at tryptophan, we see that the digestibility is greater in whey protein isolate than in soy protein isolate. And that the tryptophan digestibility is greater in whey protein isolate, whey protein concentrate, milk protein concentrate, and soy protein isolate than all other proteins.

By looking at these data we see that the digestibility of indispensable amino acids are generally greater in dairy proteins than in these plant proteins. And as a result, we see that dairy proteins can potentially be higher quality protein sources than plant proteins when fed to pigs. And what this experiment really highlights, something that I think we all know, is that the concentration and digestibility of amino acids can vary significantly, even among high quality protein sources.

So if we look here again at tryptophan digestibility data and our two isolates, WPI and SPI, we see that even among two very high crude protein ingredients, there is a significant difference in digestibility. So, we can see that there can still be some segregation even among high quality sources.

So, with that I would like to thank my lab for their support throughout this experiment. And I would also like to thank you for your attention and remind you that if you liked this presentation and would like to hear more like it, please visit our website at nutrition.ansci.illinois.edu.