Impact of differing population levels of Rhyzopertha dominica (F.) on milling and physicochemical properties of sorghum kernel and flour

Authors: Park, Seok Ho; Arthur, Franklin; Bean, Scott; Schober, Tilman

Submitted to: Journal of Agricultural and Food Chemistry
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/19/2008
Publication Date: 12/1/2008
Citation: Park, S., Arthur, F.H., Bean, S., Schober, T.J. 2008. Impact of differing population levels of Rhyzopertha dominica (F.) on milling and physicochemical properties of sorghum kernel and flour. Journal of Stored Products Research 44: 322-327.

Interpretive Summary: Sorghum is receiving increased attention as a food grain in the United States, especially for persons suffering from celiac disease, and storage of food grade sorghum could increase as this market develops. Rhyzopertha dominica (F.), the lesser grain borer, is a major economic insect pest of many stored grains, including sorghum, but there is little published data regarding the attractiveness of sorghum as a food source for R. dominica. The objectives of this study were to determine the attractiveness of sorghum as a food source for R. dominica, and to quantify the damage to sorghum quality parameters caused by different levels of R. dominica infestation. The results showed that R. dominica will readily feed on sorghum, and the infestation of R. dominica not only affects the physical properties of the sorghum kernels but also affects the chemical composition during storage. We found that initial population of bugs, culture temperature, and their relation affected the number of progeny and feeding damage. As the initial population increased, the number of progeny, percentage of insect-damaged kernels, and feeding damage increased. R. dominica infestation caused decrease in sorghum protein content and kernel hardness, and increase in starch pasting viscosity.

Technical Abstract: The importance of the storage of sorghum increases as the demand grows for sorghum as a substitute for gluten-based products. Rhyzopertha dominica F., the lesser grain borer, is a major economic insect pest of many stored grains, and can also infest sorghum. However, few studies have investigated susceptibility of sorghum to R. dominica and effects of insect population and resulting damage on the physicochemical properties of sorghum kernels and flour. Mixed-sex 1-2 week old R. dominica adults were placed on 150 g of sorghum in 0.24 liter glass jars at population levels of 0, 10, 20, 40, and 80, and held these jars at 27 °C and 32 °C, 57% relative humidity. We found that main effects, initial population level and temperature, and the interaction were significant for the number of F1 progeny and frass weight (P<0.01). A strong positive correlation was also found between initial populations, F1 progeny, percentage of insect-damaged kernels (IDK), and frass weight. The feeding damage as calculated by frass weight significantly decreased abrasive hardness and milling yield. Kafirin content decreased probably due to mandibular grinding and mastication and/or the proteolytic enzymes produced by R. dominica as the level of the initial population and infestation increased. Initial population and culture temperature significantly affected most pasting properties, and overall pasting viscosity increased with initial population, F1 progeny, and percentage of IDK at 32 °C. The direct reason for an increase in pasting viscosity appears to be the disintegration of protein structure that tightly covers starch granules and restricts swelling. Results indicate that infestations of R. dominica can have a severe negative impact on the milling quality of sorghum and also alters the physicochemical properties of sorghum flour.