In the past several months, a recurring question from manufacturers of treats and semi-moist petfoods has come up: “Can we use ‘natural’ glycerin in our foods?” The quick answer is yes, but why are they asking this? There seems to be confusion creeping into the market; maybe some sort of controversy is brewing about glycerin. But first, some background is in order.
We have known about glycerin for centuries. It was first identified in 1770 by Sheele and got its name from Chevreul in 1811 after glyceros, the Greek phrase for sweet. Other common names for glycerin include glycerol, 1,2,3 - propanetriol or glycyl alcohol.
Its molecular structure was characterized in 1883 by Bethlot and Luce, and very early on it was learned that glycerin can be nitrated to produce nitroglycerine. This is an extremely unstable liquid that explodes with the slightest jiggle. In an effort to stabilize nitroglycerine but retain its explosive properties, Alfred Nobel, of the Nobel Prize fame, combined it with diatomaceous earth to create dynamite (1866). In the armed conflicts that followed, this drove demand for glycerin—most of which was produced from tallow.
Today, explosives are an insignificant fraction (less than 2%) of glycerin’s use. Instead, it is now used in some 1,500 different applications from adhesives, ceramics and cleaning solutions to industrial lubricants, preservatives and antifreeze solutions. Food and beverage applications make up a significant volume, accounting for about 10% of the glycerin market.
What makes glycerin so ubiquitous? It is a natural part of metabolism, where it is converted to glycerol-3-phosphate (G3P), then dihydroxyacetone phosphate (DHAP). DHAP is at the intersection to pyruvate, malate and (or) oxaloacetate and entry into the citric acid cycle for generation of ATP, to the pentose phosphate pathway for ATP generation and nucleotide synthesis and to glycogen synthesis via glycogenolysis. Glycerin (glycerol) is well-recognized by animals’ physiology; whether derived endogenously from body fat stores, dietary fats or dietary glycerol, there is no difference in how the body utilizes it.
Chemically, it is a low molecular weight (92.09) three-carbon sugar alcohol (polyol) that makes up the backbone of triglycerides—also known as fat or oil. This simple polyol has one secondary and two primary alcohol groups per molecule, which allows it to serve as both solvent and reactant. It is soluble in water and alcohol (ethanol). Glycerin will react with a wide array of compounds such as acids (organic and inorganic), aldehyde and esters, and it also facilitates polymer formation—for example, polyesters.
As an ingredient, glycerin is an odorless, colorless, hygroscopic, sweet, viscous liquid. In food applications, it serves as a sweetener (about 60% the sweetness of table sugar), plasticizer, emulsifier and humectant. Glycerin has a density of 1.262 g/mL, a boiling point of 290ºC, a melting point of 18.2ºC and freeze point (66.7% solution) of -45.5ºC.
In petfoods, the primary use for glycerin is to bind free water (a humectant) in soft-moist/semi-moist dog and cat foods. In this application, humectants are generally included at 10 to 18% to help control water activity below 0.65 (Rokey, 2003). This is an important threshold to retard mold growth. The slightly sweet taste of glycerin also provides a small measure of palatability to dog foods.
Glycerin can be derived from both natural and synthetic means. Natural glycerin is derived from fats and oils by one of three different methods:
Until recently, most natural glycerin was produced exclusively as a co-product from soap production, and the largest share of that was from saponification of vegetable oils—thus the use of the common marketing term “natural vegetable glycerin.”
Synthetic glycerin is produced when propylene is chlorinated to allyl chloride, then converted to dichlorohydrin and finally to glycerin with dilute sodium hydroxide and sodium carbonate.
Given the current volume of natural glycerin in the market, the economics of synthetic glycerin are unfavorable.
Among all sources, two primary product categories or grades exist: crude and refined. Crude glycerin is generally sold in concentrations from 75-90% and may contain significant process contaminants. Refined glycerin ranges from 99.5% "technical grade" to 96% United States Pharmacopeia, 99.5% USP and 99.7% USP. USP Kosher grades are also available at 99.5% and 99.7%.
Use of the natural refined USP glycerin should be no issue for petfoods. However, there has been a tremendous amount of crude glycerin coming into the market from biofuels production, since 1 gallon of biodiesel yields 1 lb. of glycerin. This places significant supply side pressure on the glycerin market to find new uses for this ingredient, such as in animal feeds.
In 2007 (in this publication), Dzanis provided an overview of discussions being held within the Association of American Feed Control Officials relative to biofuel co-products, including glycerin. The challenge is the production of biofuels by transesterification leads to a significant amount of residual methanol (wood alcohol) and sodium that remains in the glycerin co-product stream. The US Food and Drug Administration and EFSA (Europe) have taken a hard look at whether this should disqualify glycerin from use in animal feeds. In the US, the USP grades of glycerin are considered generally recognized as safe (GRAS), but this does not extend to the crude glycerin from biodiesel production.
In a petfood application, there is no research published on the use of crude glycerin from biofuels. However, in production livestock there have been a number of studies conducted over the past five to 10 years, generally recognizing that dietary utilization of crude glycerin (about 87%) by animals (mink, swine, chickens) is acceptable at levels of 5-15% of the diet and that the energy value of glycerin is comparable to starch (Slausen and Sandbol, 2005; Cerrate, 2006; Lammers et al., 2008; Kerr et al., 2009; Mendoza et al., 2010).
So, it would seem the confusion regarding use of natural glycerin is not really about the safety of glycerin itself but the entry of a new supply stream into the market from biofuels production. The original refined grades of glycerin remain safe and effective for use in petfoods. The biofuels source has yet to be completely vetted for use in animal feeds and, given the nature of its potential contaminants, will need significant verification for safety before we consider using it in pet diets.
Read more columns by Dr. Aldrich at
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Dear GregGreat to see, that you have found our mink research usefull.However there seems to be something wrong with the reference.At least it should be Clausen & Sandbol and I also think that the year should be 2009.That reference can be found at page 120 at the following link:http://www.njf.nu/filebank/files/20100304$200514$fil$8iQhPszOr16UG7V64Wom.pdfA later report in Danish with an English abstract can be found at page 74 (actual page in the printed version is page 73):http://issuu.com/kopenhagenfur/docs/faglig_aarsberetning_2010?mode=window&pageNumber=1The digestibility trial mentioned in the first report was published later in Danish, but with an English abstract and the abstract can be found on page 4 at this link:http://issuu.com/kopenhagenfur/docs/sammendrag_2009for odd reasons the uploaded version of this years Annual Report only contains the abstracts.I'll be happy to try to answer any possible questions.best regardsPeter Sandbol
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