Jenny L. Brichta

Post-harvest stability of latex in different sizes of guayule branches.

Abstract Commercial development of hypoallergenic latex from Parthenium argentatum (Gray) for the manufacture of latex medical and household goods, is hampered by the lack of information on latex stability in the harvested shrub prior to processing. In this paper, we investigate the effect of post-harvest storage on extractable latex content of guayule branches. We found that harvested guayule branches can be stored at 4°C for at least 2 weeks without compromising latex yield, provided that dehydration does not occur. When stored hydrated at 4°C, latex levels declined between 2 and 5 weeks in all branch sizes with the most latex lost from the smallest branches and the least from the medium ones. The latex levels declined due to a combination of conversion of latex into solid rubber in situ and rubber degradation, depending upon the branch size. Overall rubber degradation from the latex and solid rubber pools in the branches was substantial in the smallest branches, but was not seen in the largest. Latex levels declined more quickly at warmer temperatures, even when the branches were hydrated, and were adversely affected by even slight dehydration.

Some Rheological Properties of Latex from Parthenium argentatum Gray Compared with Latex from Hevea brasiliensis and Ficus elastica

Latex was purified from Parthenium argentatum Gray (guayule), Hevea brasiliensis Müll. Arg. (the Brazilian or para rubber tree), and Ficus elastica Roxb. (the Indian rubber tree) in ammonium alginate at pH 10. The rheological properties of the different latices (rubber particle suspensions) were determined and compared using flow temperature ramps. Latex from all three species became more viscous with increasing rubber particle concentration and decreasing temperature. At any particular temperature and concentration, latex from F. elastica was by far the most viscous, whereas the H. brasiliensis latex was the least viscous. In addition, the tendency for the latex to coagulate increased with increasing temperature and increasing particle concentration. F. elastica latex was highly sensitive to temperature, H. brasiliensis latex was the least sensitive, and P. argentatum latex demonstrated intermediate properties. The underlying causes of these differences in latex rheology are not clear but may partially relate to the particle size (largest in F. elastica and smallest in H. brasiliensis), the particle size distribution, and/or to the considerable differences in the biochemical components of the monolayer biomembrane that surrounds the various rubber particles. Differences in the molecular weight of the rubber contained within the rubber particles seem less likely to play a role because the particles remain intact in this study.