How storage of specialty feeds affects protein quality

Most specialty feeds, and particularly those designed for young animals, contain relatively high concentrations of sensitive ingredients.

Vasyl Helevachuk | Dreamstime

Most specialty feeds, and particularly those designed for young animals, contain relatively high concentrations of sensitive ingredients like amino acids, lactose and other simple sugars, vitamins, and several sources of oils and fats. All these highly digestible ingredients are required to support fast-growing animals in the phase of gastrointestinal development.

All such ingredients are, by necessity, very expensive, especially when compared to simple ingredients like corn, wheat and soybeans. Examples of sensitive and expensive protein-rich ingredients include animal plasma, egg antibodies, fish meal, whey protein concentrate, skim milk and soybean protein concentrate. These specialty ingredients contain a great amount of protein (amino acids) that is not only expensive but also highly prone to deterioration when stored under unfavorable conditions. Such conditions exist inside facilities where young animals are reared. One example is pig nurseries where the temperature is 28 to 32 C (82.4 to 89.6 F) and humidity from 50 to 90 percent. Young broilers, too, are kept in rooms where high temperature zones tend to coincide with water and feed sources. Of course, feeds exposed to open air (open bags), sunlight and external humidity will be prone to deterioration as well.

Understanding the Maillard reaction

The Maillard reaction is what makes our bread crust brown, but this is just the surface of the issue. The end result of the Maillard reaction in stored feed is that lysine is rendered unavailable for digestion by the animal. When protein and amino acids are exposed to conditions of increased temperature and humidity, the process of the Maillard reaction takes place. More commonly referred to as the “browning” reaction, this non-enzymatic process occurs even at room temperature, but at a very slow pace.

The Maillard reaction is involved in many food and feed applications such as cooking, baking, pelleting and extrusion. In a controlled situation, it is a desirable process because it creates aroma and flavor. But when browning becomes excessive (such as in burnt pellets) or when it is not controlled (such as during storage), then protein quality is reduced without benefit.

The end result of the Maillard reaction in stored feed is that lysine is rendered unavailable for digestion by the animal.

The Maillard reaction involves binding of free amino groups to the carbonyl group of reducing sugars. Free amino groups exist in all crystalline amino acids and at the end of protein molecules. Amino acids that are fully bound to proteins are not reactive because they contribute their amino group to the peptide bond, but hydrolysis frees protein-bound amino acids. Thus, hydrolysates containing many short-chain peptides are very susceptible to browning. Lysine is a unique amino acid in that it has two amino groups and thus, even protein-bound lysine is reactive. Although all amino acids are affected, lysine is used as an indicator for the evaluation of protein quality deterioration in the Maillard reaction. On the other hand, reducing sugars include glucose (dextrose), fructose in molasses and lactose in milk products. Starch and sucrose are not reactive unless hydrolyzed. Although sugar losses are greater and more intense than lysine losses, the deterioration of protein quality is most problematic because of cost concerns.

Learn more about the Maillard reaction: The Maillard reaction’s impact on animal nutrition

The intensity of the Maillard reaction increases as time, temperature, humidity and alkalinity increase. Also, the initial Maillard reaction products have autocatalytic properties that intensify the effects of heat treatment. Storage of heat sensitive material for prolonged periods of time initiates a second cycle of Maillard reactions. Residual feed moisture also affects the extent of the Maillard reaction during storage. A 10 percent moisture level in milk powder stored for 10 weeks at 30 C resulted in a 20 percent loss in lysine bioavailability. There is an inverse relationship between temperature and time such that as temperature declines, more time is needed for the same loss of protein quality and vice versa.

Lipid oxidation

That lipid oxidation results in methionine and tryptophan destruction is often overlooked because lipid oxidation as a cause of amino acid damage has received little attention from the feed industry. With most diets for young animals being enhanced with fats and oils to increase energy intake and facilitate the pelleting process, lipid oxidation can become problematic when it comes to protein quality.

Oxidation of lipids or oxidative rancidity is a natural process that occurs when unsaturated fatty acids take up oxygen. Such fatty acids exist in all fats and oils, but certain vegetable oils such as soybean and corn oil are especially rich in them. Lipid oxidation is enhanced by exposure to air (open bags), light, high temperatures and certain inorganic minerals such as iron and copper. The reaction is autocatalytic and, once started, oxidized fatty acids continue to form and peroxides (end products) accumulate. Antioxidants added to the feed can only delay the process by stabilizing reactive fatty acids.

Keeping the feed fresh is old but frequently ignored advice.

Oxidized fatty acids, also referred to as free radicals, react not only with other fatty acids but also with amino acids rendering them unavailable to the animal. Methionine and tryptophan are particularly susceptible to oxidation by free radicals. Methionine and tryptophan (along with lysine) are limiting amino acids in most animal diets, and they are often added in the form of feed-grade amino acids. In a recent study, flaking alone reduced the availability of methionine and tryptophan in wheat, rye, barley and oats by as much as 26 percent. This decrease in amino acid destruction was highly correlated with lipid oxidation products formed during the flaking process.

Practical implications

Keeping the feed fresh is old but frequently ignored advice. Freshness in diets designed for young animals is even more important because these diets are loaded with free amino acids, sugars and lipids — ingredients highly susceptible to quick deterioration once exposed to an adverse environment. Keeping feed fresh and feeding often and little prevents stale feed and a waste of money, while preserving animal health and performance.

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