Epsilon Archive for Student Projects

Abstract

The growing population ultimately puts pressure on the food industry to become more effi-cient and to reduce postharvest food waste. The leading method of reducing postharvest decay of fruits has been through fungicide application. However, because of the public per-ception towards fungicide usage there is now a demand for new methods, that are environ-mentally friendly and unharmful for human consumption. The use of antimicrobial com-pounds and activities of microorganisms isolated from natural crop that possess antagonistic activity towards postharvest pathogens, has therefore become an emerging field of research. Earlier research has demonstrated that a yeast strain within the genus Rhodotorula marked antagonistic activity towards several postharvest pathogens, when the living culture was ap-plied onto fruit surface. The oleaginous yeast R. babjevae has been found to coproduce in-tracellular lipids and extracellular polyol esters of fatty acids (PEFA) when exposed to ni-trogen limitation and an excess of carbon. The amphiphilic PEFA has been shown to possess several bioactivities, one of them being antifungal, thus making it a possible biocontrol agent.

Because of the aforementioned research on species in the Rhodotorula genus as potential biocontrol agents, one indicating the antagonistic activity of the living yeast cultures and one towards the potential antagonistic activity of the PEFA, the experiment was conducted to evaluate the difference. The aim of the experiment was therefore to evaluate the inhibitory effect of the living yeast culture of R. babjevae, and of its extracellular PEFA towards the postharvest pathogens Penicillium expansum, Botrytis cinerea and Aspergillus flavus. The yeast where cultured in a nitrogen limited medium with an access of glucose, to promote lipid production. During incubation of the yeast culture, the progress in biomass was meas-ured through optical density (OD) and samples were analyzed using HPLC to measure the glucose concentration. Intracellular lipid production was recorded by micrographs. A disk diffusion bioassay was prepared with two different cultures of living yeast culture (3 and 6 days old) and two concentrations of a PEFA pellet, on MEA plates inoculated with the molds. Continuous registration of the antagonistic activity followed during the incubation in the dark at 25 °C for 5 days. The results from the disk diffusion assay showed antagonistic activity from both the living yeast cultures towards B. cinerea, although neither of the two PEFA concentrations inhibited the mold. A. flavus and P. expansum had overgrown the plates after 2 days of incubation, probably because of too concentrated spore solution. The antagonistic activity towards B. cinerea could be explained by its slower growth rate, which provided the yeast time to acclimatize to the new environment. The antagonistic activity of R. babjevae marked promising characteristics towards becoming a potential biocontrol agent under commercial conditions.