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Some protostellar systems show little or no millimetre line emission ofcomplex organics. This can be interpreted as a low abundance of thesemolecules, alternatively they could be present in the system but are not seenin the gas. The goal is to investigate the latter hypothesis for methanol. Wewill attempt to answer the question: Does the presence of a disk and opticallythick dust reduce methanol emission even if methanol is abundant in the icesand gas? Using the radiative transfer code RADMC-3D, methanol emission linesfrom an envelope-only model and an envelope-plus-disk model are calculated andcompared with each other and the observations. Methanol emission from theenvelope-only model is always stronger than from the envelope-plus-disk modelby at least a factor 2 as long as the disk radius is larger than 30 au (for L=8L$_{\odot}$). In most cases, this is due to lower temperatures and, hence, thesmaller amount of warm methanol inside the snow surface of theenvelope-plus-disk model. The intensities drop by more than an order ofmagnitude for models including high mm opacity dust grains and disk radii of atleast 50 au (for L=8 L$_{\odot}$) due to continuum over-subtraction. The lineintensities from the envelope-only models overproduce the observations ofprotostars with lower methanol emission even with large dust optical deptheffects. The envelope-plus-disk models can explain the bulk of theobservations. However, they can only reproduce the observations of sources withhigh luminosities and low methanol emission when dust optical depth effectsespecially continuum over-subtraction in the disk becomes effective. Therefore,both the effects of disk and dust optical depth should be considered to explainthe observations. In conclusion: Absence of methanol emission does not implyabsence of methanol molecules in either gas or ice.

Importance of source structure on complex organics emission II. Can disks explain lack of methanol emission from some low-mass protostars?