Composting is a natural biological process that converts organic waste materials into a rich, stable soil amendment. This transformation relies on the coordinated efforts of billions of microorganisms, primarily bacteria and fungi. The general answer to whether moldy food can be composted is yes, but this depends on the type of composting system used and the specific food item involved. Mold is simply a visible form of fungus, and these organisms play a part in the decomposition cycle.
The Role of Fungi in Composting
Fungi, including the visible molds, are the specialized recyclers within the composting ecosystem. They possess the unique ability to break down the most resilient structures in plant matter that bacteria cannot easily digest. These structures include complex carbohydrates like cellulose and the highly durable polymer lignin, which gives plants their woody stiffness.
Fungi excrete powerful enzymes directly into the compost matrix to dissolve these compounds externally before absorbing the simpler nutrients. This action makes them primary decomposers, particularly important in breaking down “brown” materials such as wood chips, dried leaves, and paper. By initiating the degradation of this tough, carbon-rich material, fungi make the resulting simpler compounds available for bacteria to consume. This sequential breakdown ensures that the pile continues to decompose efficiently, transforming waste into usable compost.
How Composting Temperature Affects Mold Spores
The temperature achieved within a compost pile is the single most important factor determining the fate of mold spores and mycotoxins. Composting is categorized into two main styles based on temperature: cold (passive) and hot (active) composting. Cold composting operates at ambient temperatures and cannot reliably eliminate harmful spores. In these passive systems, mold spores can survive and potentially spread when the finished compost is applied to the garden.
Hot composting employs a thermophilic phase where microbial activity generates significant heat. This phase requires the internal core temperature of the pile to reach a minimum of 131°F (55°C). Sustaining this temperature for at least three days with regular turning is necessary to destroy the majority of human and plant pathogens, fly larvae, and weed seeds. The high heat denatures the proteins in most mold organisms, effectively killing them.
While the heat kills most molds, some types, like Aspergillus fumigatus, are known to be thermotolerant. Though active growth ceases around 133°F (56°C), its hardy spores can survive higher temperatures. However, the intense heat greatly reduces their overall numbers, and hot composting significantly reduces the risk associated with adding moldy food by actively sanitizing the material.
Specific Moldy Foods to Avoid
Certain moldy food items should be excluded from any composting system, regardless of the temperature achieved, due to risks inherent in the source material. Moldy meat, bones, and dairy products attract nuisance pests such as rodents, raccoons, and flies. Dairy, in particular, breaks down slowly and can create dense, anaerobic pockets within the pile, which inhibits the beneficial, oxygen-requiring decomposition process.
Moldy oils and grease should not be added, as they coat other organic matter, creating a water-resistant barrier. This barrier suffocates the microbes and prevents them from accessing the food source, slowing down the aerobic composting process. Pet waste, even when moldy, carries a high risk of harboring harmful bacteria, viruses, and parasites that are often not completely neutralized in a standard backyard compost pile.
The concern in these cases is not primarily the mold itself, but the nature of the material combined with the mold, which attracts unwanted visitors and disrupts the pile. For vegetable scraps, fruits, and bread, the mold is generally acceptable. However, items like meat, dairy, and oils introduce secondary problems that undermine the health and safety of the composting operation and should be discarded through other means to maintain a functional and sanitary compost system.