Yeasts have been used for centuries in biotechnological applications, for food processing industry (wine, beer, cider, bread, cheese, etc.). For the past few decades, they have also been studied with the aim of producing oil to replace fossil resources. Through a genetic modification of baking yeast (S. cerevisiae), i.e. expression of proteins involved in oil storage in seeds, we obtained strains able to produce more lipids. To better understand the mechanisms involved in oil accumulation in these yeasts, we analysed these strains and control strains (with low oil content) by infrared microscopy (Fourier Transformed Infrared, FT-IR). This technique provides an overview of the cellular metabolism thanks to a spectral fingerprint of biological macromolecules (lipids, nucleic acids, sugars, etc.). FT-IR can replace traditional biochemical tests, which use large quantities of biomass and solvents. Large populations can be tested (high throughput approaches). Combined with the high spectral and spatial resolution offered by synchrotron light, FT-IR can be used for an analysis at the single cell scale, for statistical and population heterogeneity analyses. Thanks to the presence of the biology lab in the SOLEIL facility, we were able to develop a protocol allowing the FT-IR analysis of freshly-sampled cells at different growth times. A drop of the cell suspension was dried on a ZnSe ATR hemisphere 4 mm in diameter. The spectra were recorded using the Continuum XL microscope available on the SMIS beamline, with a spatial resolution of 4x4 µm, which is the mean size of a cell. The spectra were then analysed (spectrum correction and statistical analyses) to evaluate spectrum variations between strains. We have shown that an increase in oil content leads to significant metabolic changes, in particular on carbon pools. An inverse correlation between oil content and carbohydrates reserves (glycogen) was revealed. This inverse correlation between storage lipids and storage carbohydrates has been confirmed by biochemical analysis.