0
Glycerol as feed ingredient
Glycerol is a byproduct of the industry of bio-diesel fuels, representing roughly 10% of the crude oil used for that purpose. Therefore, large quantities are expected to be available in the near future for the different markets that can have a potential interest for it. Between others, animal nutrition, and namely swine nutrition, seems to have a window for using it (in some cases for reasons that differ from the simple evaluation of its pure nutritional properties).
Glycerol is present in the Community Register of feed additives ex Reg. CE 1831/2003 , n° E 422, group emulsifiers and stabilizers, without any restriction regarding doses or animal species.
Crude glycerol (that is the market – available material) is a gelatinous liquid, variable in opacity, slightly sweet-tasting, and promptly and completely water soluble.
Its quality parameters have not been extensively studied up to now, and generally are highly dependent on the type of industrial processes used in the different sites of production.
As per general, the content in pure glycerol would have to be between 85 and 88% (even if some contracts have a baseline at 80%), methanol would not have to exceed 0,5% (even if commercial sources generally are between 0,1 and 0,3%).
Crude glycerol has an ash content of 5-7%, for about 50% being NaCl: this salt content would have to be kept in account, when large quantities are used.
The Technical Group of Assalzoo (Association of italian feed compounders) in 2007 has attributed to crude glycerol an energy value of 3650 Kcal/DE per kg for fatteners, and of 3500 Kcal/DE per kg in weaners. Lammers et al. (2008) in the US has given a value of 3350 Kcal/DE. It is therefore evident that a clear, shared information about the energy value of glycerol today does not exist.
If we consider that glycerol does not have any further nutritional value than energy (no protein, no fiber, only the cited amount of NaCl) it can be concluded that the identification of the exact energy value for glycerol is crucial for its optimal use, when simply based on the nutritional profile.
On the other side, bibliography shows clearly that the performance parameters are not negatively influenced when glycerol is added to the feeds in percentages not exceeding the 10% ( … assumed to be able to include this quantity in commercial feed operations …): diets need to be carefully re-balanced not only on the energy side, but also from the protein/aminoacid point of view. In gross, it can be said that glycerol can easily substitute the starch contained in the cereals, up to 15% of cereals, corresponding to a 10% of glycerol in the diet. Obviously, the non-starch content of the cereals substituted would have to be replaced from other ingredients containing some protein, some fibers, etcetera ….
The use of glycerol as feed ingredient is therefore strictly linked from one side to its market price, and from the other side to the price of the protein feeds/additives that have to be used to balance the lack of protein consequent to the use of glycerol in the diets.
Glycerol as functional feed
Glycerol is a pure source of energy originated neither from polysaccharides, neither from fats: in this point relies the possibility to use it as functional feed, oriented to the improvement of the carcass quality, and namely to the reduction of the unsaturation degree of the deposit fats.
A target of less than 15% of linoleic acid in the subcutaneous fat of the pigs is common to almost all the swine production types in the world: unfortunately this result can be hardly achievable if the diets of the pigs contain more than 1,7/1,8% of C 18:2. And this is a common fact, for example, when “new” ingredients like DDGS are used in non cosmetic quantities.
Also on this point several are the bibliographic references: between others a paper has been published from the italian researchers Della Casa et. al. in 2008 :
Table 1: Acidic composition of the subcutaneous fat of heavy italian pigs fed with different levels of glycerol (5 or 10% of the feed) for different times (all growing period, or only finishing phase)
| % |
Control |
Glyc 5 % all |
Glyc 10 % all |
Glyc 5 % finish |
Glyc 10 % finish |
| Oleic acid |
44.47 |
45.66 |
46.29 |
45.26 |
45.02 |
| Linoleic acid |
13.65 |
13.24 |
12.91 |
12.97 |
12.6 |
| Mono unsatur. tot |
46.88 |
48.18 |
48.85 |
47.82 |
47.63 |
| Polyunsatur. tot |
14.52 |
14.1 |
13.78 |
13.81 |
13.4 |
(Della Casa et al., 2008)
Glycerol is also used in sport nutrition to hyperhydratate the athlets before the performance (Coutts et al., 2002). Is similarly possible to “prepare” with proper doses of glycerol the animals for transportation (the hydratating dose is a single administration 1,2 g. of pure glycerol per kg of body weight, provided 2 hours before the event, followed from the ingestion of water,: not impossible to do in pig units). An excellent source of information on glycerol is the “Biodiesel feeds sector” in the website of Minnesota University, where echos of this second, and quite new, nutraceutical application of glycerol can be found.
