As with the first article in this comprehensive series about the use of organic acids in feed I will use a cup of coffee to explain acid binding capacity of feed and how it may be costing you more and earning you less than you realise.
Last time we looked at how pH and pKa are related, where we used different sweeteners in coffee as an analogy for acids and temperature as an analogy for pH. If you recall we found that an acid is present in either its dissociated or undissociated form in any solution. The level of dissociation depends on the pH of that solution.
Now again, go get a cup of coffee for this one. Don’t worry I’ll wait.
Got coffee? Good, as you know by now, I like my coffee with milk and if you recall we added a teaspoon of sugar (acid) to our coffee. Lets assume that tempature of my coffee 85°C, or in the case of acids, if the pH is around 4-5 and we have added formic acid (our sugar). Formic acid has a pKa of 3,75 which in temperature terms was 75°C. Our pH is slightly higher than the pH where half of the acid is dissociated, the pKa of our formic acid. From the previous article we know that at pH 4-5 there is more formic acid in dissociated form than in undissociated form, because the pH of the solution is higher than pKa of formic acid.
This is important to know because when we get into how acids exert their effects in the next articles you will learn that that it is primarily the undissociated molecule that can penetrate bacteria and disturb their growth or kill them. In a stomach with feed, our physiologically relevant pH is 4-5. That means that there is a tendency to be less undissociated formic acid available for murdering bacteria. Now the pH scale is logarithmic so, for every increase or drop in one pH level there is 10 times more or less H+ concentration. In the case of our formic acid this means that at a pH of 4,75 there is a tenfold decrease in undissociated formic acid, because the pKa of formic acid is 3,75.
So far so good, we have formic acid in our solution that is slightly more dissociated than undissociated or ‘whole’.
As I said I like to add milk to my coffee and some parts of the milk are capable of interfering with the sweetness of my coffee. If I take a café latte, because I’m from Copenhagen, I won’t taste the sweetness at the same level because it is overridden by the other ingredients. I am not tasting the sweetness of H+ or of the the other part of the dissociated formic acid molecule, the anion HCOO–. That’s because the sodium, calcium or zinc in my milk will bind my anion HCOO– to form sodium-, calcium- or zinc formate. Likewise, other ingredients like protein or amino acids are looking to bind my dissociated proton (H+). The key here is that if these dissociated molecules are bound to other molecules the possibility of my formic acid anion HCOO– to become formic acid again (HCOOH) by binding H+, is no longer available. Also when my H+ dissappears my pH will increase and my possibilities to sweeten my coffee with sugar become more limited.
This is what is referred to as acid binding capacity (ABC) or buffering capacity. Normally in the mature animal this isn’t a problem because in the stomach we have another acid that acts like our sweetener pill. Hydrochloric acid increases the level of H+ available in excess levels and gives a lot of excess H+. The formic acid anion HCOO– will bind with any excess H+ and voila you have formic acid HCOOH and a ‘whole’ molecule that gives the antibacterial effect. The problem comes in if we have feed with high acid binding capacity that has a lot of ingredients. They will steal the excess H+ and reduce their availability for our formic acid anions.
Illustration of acid binding capacity
In the solution ingredients like protein, chloride, oxygen or others bind the free protons (H+) from our dissociated acid. The anions (A-) are bound to zinc, sodium from our added organic ingredients.
In young animals that are “still figuring out” how to produce acid and don’t produce enough hydrochloric acid this problem is made worse, because a lot of the ingredients we use in piglets, like fishmeal, skim milk, whey, etc have a high Acid Binding Capacity. Some of you may also have noticed I slipped in zinc in my list of ingredients that bind HCOO– earlier. In fact, zinc has the highest ABC of all the minerals we give our piglets. If we look at inclusion levels of medicinal levels of zinc and normal levels of formic acid, lactic acid, propionic acid or blend of acids at 2,5 kg/ton and 5 kg/ton of feed respectively, you can see that zinc all but cancels the effect of the acids. No wonder the acids don’t always work if we are aren’t careful enough to make sure they aren’t bound.
|Ingredient||ABC @ pH 4||g/kg feed||ABC 0,4 kg feed /day|
|* Blend of 50% formic, 30% lactic and 15% propionic|
So how do we fix this?
Well, since pH is on a logarithmic scale, it means if we shift pH by 1 we have 10 times the amount of bacteria killing molecules available. For a start, we need to find alternatives to medicinal levels of zinc oxide by 2022 due to EU legislation. Unfortunately, there is no single solution to replacing zinc oxide. However of solutions that have been tested that show the most promise for zinc replacement organic acids seem to be an important element.
Also thinking about what form of acids are in can have an influence. In feeds where we use calcium formate we can replace this with formic acid or other acids that have a 2-3 times less acid binding capacity. Finally, being aware of reducing ingredients such as sodium bicarbonate, limestone and other acid binding ingredients like fishmeal to a bare minimum can go a long way to helping your organic acids work.
In the next article in our series we will take a closer look at which acids work best against what type of microorganism and what the difference is between and acid and acid salt.
In the meantime, leave your comments or questions below and I will answer them as best I can.