PSI stands for Protein Solubility Index and is a measure of the solubility of protein in maize (corn). PSI can be used to predict maize digestibility, with a low PSI value leading to a reduced digestibility and thus poor feeding value. The main cause of low PSI is heat damage during drying of maize harvested at higher moisture (as shown in the SEM images below). Drying results in massive structural change, loss of granulation and formation of poorly soluble starch / protein / fibre complex.
PSI is determined as the percentage solubility of maize protein in a specific salt solution. This method was originally designed for the starch industry - low PSI means low starch yield - and has recently been adopted by the animal feed industry. For protein feedstuffs such as soybean and canola meals, heat damage can be estimated by measuring reactive lysine (click here to read a post about the importance of reactive lysine).
Picture A - Undried corn starch
Picture B - Dried corn starch
Photographs courtesy of University of Poznan, Poland
PSI is one of the parameters in the AB Vista Feed Quality Service for maize (corn) which was developed in collaboration with Aunir.