Sources of Food

For molds, the food of choice is organic matter (things that are or once were living).  Fungi will consume anything organic, yet many types have selective appetites.

Building materials including wood, paper, natural fabrics, leather, and even the starch in wallpaper paste are common examples of dead organic matter preferred by filamentous molds.  

Grasses, plants, trees, etc. provide examples of living organics for parasitic fungi and animal or human tissue provides a source of living nutrients for many pathogenic fungi.

Consumption
Fungi don't actually "eat" their food, but rather release enzymes to break-down complex organic compounds and absorb nutrients through their cell surfaces. 

By absorbing nutrients and obtaining carbon and energy necessary for growth from organic sources, fungi are classified as absorptive heterotrophs.  

During the process of enzyme release and nutrient absorption, molds also produce volatile organic compounds (VOC's).  Because they are produced during processes essential to growth, VOC's are termed Primary Metabolites.  (Since they are generated from a microbial source, some references list them as microbial VOC's or mVOC's.)

Representing a wide range of organic chemicals, VOC's are responsible for the 'moldy/musty' odor associated with fungal growth.

Volatile?

Volatile compounds are those that readily evaporate at common room temperatures and pressures. 

To illustrate the concept of volatility, place a drop of rubbing alcohol, water, and a heavy oil on a countertop.  The alcohol will evaporate quickly (volatile), the water more slowly (semi-volatile), and the oil is does not evaporate (non-volatile). 

Substrate and Nutrients
In most cases, the substrate mold is growing on provides the nutrients, however, in some instances the substrate is simply a foundation.

Mold growing on glass, ceramic tile, metals, or other inorganic materials is not obtaining nutrients from these substrates.  In these cases, mold is feeding on microscopic organic matter that is on the surface or trapped in tiny pores of the material.  

Bath-tile mold is an example:  Mold is typically consuming organic dust, dirt, debris, skin flakes, body oils, soap scum, etc., and the ceramic tile is simply a foundation for the colony.

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