Reactive lysine: A Test for Soybean Quality

Lysine is the first limiting amino acid in diets for swine and second limiting for poultry. It is used as a reference to adjust the content of other amino acids in diets for these animals and therefore, determines performance. Monogastric animals can’t synthetize lysine, which is why the essential amino acid needs to be included from protein sources in diets.

Including soybean meal in practical diets requires heat-treatment to inactivate the anti-nutritional factors.  After heat processing, a laboratory analysis is performed to determine anti-nutritional factors.  How well they are reduced depends on temperature and duration of heat-treatment. If the temperature is too high during processing, or heat processing is too long, the soy turns brownish.  This causes the Maillard reaction to occur resulting in lysine-sugar complexes, which are insoluble and damage lysine.

Specific methods have been developed to determine the reactive lysine which indicates the amount of lysine available for animals. According to this, it is considered that the accurate indicator of optimum soybean processing is conducting feeding trials with animals to determine digestible lysine and performance and should be used as the gold standard for comparing product quality. For example, studies have been conducted to predict the reactive lysine in the rumen-undegradable protein fraction (RUP) or escape protein of soybean meal for ruminants  which is available for milk production.

You may be asking how useful are laboratory tests in getting to know the reactive lysine value?  Data from the reactive lysine assay can be used to make meaningful quality decisions about the soymeal after heat processing. For example, it has been demonstrated that excessive heat processing using autoclaving reduces both the content and digestibility of total (from 27.5 to 19.2 g/kg) and reactive lysine (from 23.4 to 11.7 g/kg) of soybean meal which compromises animal performance when these damaged proteins are included in diets. Also, estimation of reactive lysine may be an optional tool to know how much of the amino acid that is added to the diet is highly digestible and useful for production and other body functions.

Further research is needed to understand how reactive lysine values of various feed ingredients are screened as part of quality control after processing. In the future, determining reactive lysine can be an additional indicator of quality before performing animal studies.