In 2002, much of the 1.35 million tons of corn gluten meal was consumed by the livestock market. However, a fair amount was used in petfood. Corn gluten meal appears to be "below the radar" as an ingredient targeted as a "bad guy" probably because consumers wonder what it is: corn, gluten or something else. They likely won't find it on grocery store shelves when shopping for dinner.
Corn gluten meal (CGM) is a co-product from corn wet milling, the process responsible for production of corn sweeteners and corn syrup. In the wet milling process, corn is cleaned and then steeped in a water/sulfur dioxide mixture at 50°C. This acidic stew activates the inherent lactobacillus organisms, which begin to break down the corn seed.
The water and germ are then separated from the flint and protein by a water cyclone and the resulting material is ground. Hulls are removed by screening and the protein is separated from the starch by centrifugation. The protein (gluten) mash is then dried and sized. Corn gluten consists of four major classes of protein: Albumins, globulins, glutelins and zein (prolimines). While corn gluten doesn't have the same level of functionality as wheat gluten for bread production, it does retain some functional proteins that are beneficial in extrusion.
Why is corn gluten meal used in petfoods? Most of the available CGM contains 60% protein. Thus, it serves a purpose as an economical high-protein ingredient. On a cost-per-unit protein basis, CGM costs about 10% less than petfood-grade poultry by-product meal, but about 20% more than soybean meal. It is a reasonable source of methionine, but low in lysine and arginine. Because of this, CGM is typically paired or complemented with another protein source. Additionally, when compared to other proteins, CGM has a low level of ash (< 2.0%) and a full complement of vitamins and xanthophylls such as zeaxanthin and lutein.
Whether CGM has some intrinsic urine acidifying properties is still a bit unclear.
Case, et al. (1990) reported that when fed to puppies, the nutrient availability of CGM was superior to poultry by-product meal. In general terms, protein digestibility of CGM in dogs is high (avg. 88%; Yamka, et al., 2004). Relative to other studies, this ranks CGM even with poultry meal and better than soybean meal. Recent work in the dog has tried to determine if an animal-based protein is better than or different from vegetable proteins. Corn gluten meal is often the reference vegetable protein used in these studies.
Research reported by Wakshlag, et al. (2003) demonstrated that dogs fed a 28% protein diet where the protein was supplied exclusively by CGM had greater lean body mass loss in 10 weeks than dogs fed a similar diet containing chicken protein. The researchers speculated that this was due to the higher cysteine:methionine ratio and greater arginine concentration in the chicken-based diet. The loss in lean body mass was not observed when a combination of CGM and chicken as fed. This may be due to the complementarity of lysine and arginine from the chicken with the methionine and cystine of CGM. In another study, muscle turnover was influenced by a CGM-based diet (Helman, et al., 2003), which further points to the value of pairing CGM with other protein sources rather than using it as the sole source.
The nutritional value of CGM in cat foods is a bit different story, given that CGM is often used to help boost the protein level without adding a lot of ash, as one often encounters with poultry, meat or fish meals. Corn gluten meal is also reputed to be urine acidifying, which for cat foods is a big consideration. Thus, selecting CGM as an ingredient for cat foods seems like a good idea. However, recent research has blurred this story. Fanuba, et al. (2001) reported that cats fed CGM-based diets had a comparable nutritional value and urine acidifying effect to that of a fish meal diet; however, fecal DM% was higher and the HCl-insoluble urine content (potential crystal forming) was lower when compared to fish meal.
In a second study, Fanuba, et al. (2002) reported that CGM was not as digestible, nor did it improve urinary predisposition to crystal formation relative to meat meal. The authors also indicated that CGM-based diets might reduce calcium and magnesium availability relative to meat meal. Whether CGM has some intrinsic urine acidifying properties is still a bit unclear. Nutrient availability of CGM appears to be comparable to fish meal but slightly less than meat meal.
Thus, corn gluten meal is used in petfood because it is an economical protein source that works well as a complement to other protein ingredients. It is also beneficial in cat and specialty foods where the high level of protein must be off-set with low or controlled levels of ash. Its digestibility is comparable to other protein meals (vegetable and animal), and it contains a high level of vitamins and xanthophylls (shown to be beneficial to canine immune system health).
Considering cost, nutrient availability and other features that CGM brings to companion animal diets, it is certainly worth more consumer education on its value, as well as continued use as a complementary protein source in dog and cat formulas.