Corn gluten meal is a by-product of the manufacture of maize starch (and sometimes ethanol) by the wet-milling process (RFA, 2008). Corn gluten meal is a protein-rich feed, containing about 65% crude protein (DM), used as a source of protein, energy and pigments for livestock species including fish. It is also valued in pet food for its high protein digestibility (RFA, 2008). In the USA and Canada, corn gluten meal is also used as a fertilizer and pre-emergent weed killer (Pasupuleti et al., 2010; Christians, 1994).
The wet-milling process of maize is described in the figure above. The process yields 5 main products: maize starch, maize germ oil meal, corn gluten meal, corn gluten feed and maize steep liquor. After cleaning and removal of foreign material, the maize grain is usually steeped in water with sulfur dioxide (SO2) for 24-40 hours at a temperature of 48-52°C. The role of sulfur dioxide is to weaken the glutelin matrix by breaking inter- and intramolecular disulfide bonds. Steeping at 45-55° C favours the development of lactic acid bacteria that produce lactic acid, lowering the pH of the medium and thereby restricting growth of most other organisms. At the end of the steeping phase, the maize kernels contain about 45% water, having released about 6.0-6.5% of their dry matter as solubles into the steepwater, and have become sufficiently soft to be pulled apart easily with the fingers (BeMiller et al., 2009). After steeping, the maize kernels are coarsely ground so that the germs are separated from the endosperm and used for oil extraction. The extraction of oil from the germs yields maize germ oil meal. The remaining steeping water is condensed into a steep liquor. The endosperm undergoes further screenings that separate the fibre from gluten (protein fraction) and starch slurry. Fibre (bran) is mixed with steep liquor and maize germ oil meal to create corn gluten feed(ISI, 2008; RFA, 2011). The fibre-free endosperm is centrifugated in order to separate the starch fraction and the gluten, which have different densities, resulting in almost pure starch (99% starch), and corn gluten meal (CRA, 2006).
Note: it is important to note that corn gluten meal should not be mistaken for corn gluten feed, which contains about 22% crude protein rather than 65% and is nutritionally completely different. The name similarity of these products is an occasional source of confusion, particularly in papers written by non-native English speakers.