Latex yield and quality during storage of guayule (Parthenium argentatum Gray) homogenates

Abstract Extraction and purification of latex from guayule ( Parthenium argentatum Gray) require that harvested shrub first be homogenized in an alkaline aqueous buffer. The stability of the latex was investigated in a variety of homogenates and under different storage conditions. Neither the length of post-harvest storage (up to 5 weeks) nor branch diameter affected the concentration of the latex in homogenates. Latex concentration was not affected by the length of grinding time used to make the homogenate. However, latex concentration declined in acidic pH, and the initial latex concentration in homogenate prepared from defoliated shrub was below 4 mg/ml. This decline was less apparent in homogenates made from leafy shrub, suggesting a protective effect derived from the leaves. Storage at 4 °C prevented latex loss under all treatments. The quality of the rubber polymers in the latex fraction was investigated using size exclusion chromatography/multiangle laser light scattering detection. Polymer molecular weight and molecular radius declined in parallel, but both declined faster in the dilute homogenate generated from the second grind of the guayule bagasse than in the more concentrated homogenate from once-ground shrub. Degradation was greatly slowed in all treatments when the homogenates were stored refrigerated. Polydispersity values were low in all treatments, only slightly increasing over time, with the exception of homogenate generated by the second grind of shrub in the presence of leaves and stored at 24 °C. The relatively rapid polymer degradation of the latex fraction in this homogenate led to an increase in polydispersity followed by a decrease as the latex fraction was degraded to below detection levels. We conclude that guayule homogenate provides a stable environment for latex yield and quality, even at room temperature, for at least 13–16 weeks provided that the pH is basic and the concentration of rubber particles is at least 4 mg/ml. This is in contrast to the extractable latex content of harvested branches, which is prone to rapid coagulation and degradation in situ unless the branches are stored hydrated and refrigerated.