Composting leftovers is conditional, depending heavily on the ingredients and the system used. Leftovers refer to cooked or prepared foods, which behave differently than raw scraps in a compost environment. Success in a traditional backyard pile relies on whether the ingredients are purely plant-based and how they are handled. Understanding the composition of your meal and the capabilities of your compost system is the first step in diverting this material from the landfill.
Composting Leftovers: The Plant-Based Acceptables
Plain, cooked plant materials are welcomed in a well-managed backyard compost pile because they are high in nitrogen, which drives microbial activity. Acceptable items include steamed or boiled vegetables, plain cooked rice, pasta, oatmeal, and bread products that have not been coated in fats or dairy. Since cooking begins to break down the cellular structure, these materials can decompose faster than their raw counterparts.
Preparation is necessary before adding these items to the bin. All cooked scraps should be cut or broken down into small pieces to maximize the surface area available for microorganisms. This step accelerates decomposition significantly, preventing materials from clumping together.
It is important to balance the high moisture and nitrogen content of cooked food by immediately mixing it with “brown” materials, such as dried leaves, shredded paper, or wood chips. This carbon-rich material absorbs excess moisture and provides structure, preventing the wet scraps from compacting and becoming slimy. Burying the scraps deep within the center of the pile and covering them completely helps insulate the materials and deter common pests.
The High-Risk Exclusions: Meat, Dairy, and Oils
Certain leftover ingredients should never be introduced into a standard home composting system due to the risks they pose. This category includes all cooked and raw meat, fish, bones, dairy products, and concentrated fats or cooking oils. These substances are chemically complex, containing high concentrations of protein and lipids that break down too slowly in a typical backyard setting.
The primary concern is the attraction of nuisance pests, such as rodents, raccoons, and flies, which are drawn to the strong odors of decomposing animal products. These pests can scatter the compost materials, introduce pathogens, and create sanitation issues. Burial or covering the scraps does not reliably deter scavengers once they detect the scent of meat or dairy.
The decomposition of animal proteins and fats often leads to anaerobic conditions within the pile, which harms the composting process. Fats and oils can coat the compost particles, creating an impenetrable barrier that smothers aerobic microbes and slows down the overall breakdown. The resulting foul odors are the putrid scent of rotting food caused by a lack of oxygen, not the earthy smell of healthy compost.
Another issue is the potential for pathogens, as meat and dairy can harbor bacteria like Salmonella and E. coli. A typical home compost pile rarely reaches the sustained high temperatures (130 to 160 degrees Fahrenheit) required for the thermal sanitization necessary to kill these human pathogens. Introducing these materials risks contaminating the finished compost, which could then spread bacteria to garden soil and food crops.
Specialized Methods for Processing Difficult Food Waste
For those who wish to divert all food waste, including high-risk materials, specialized processing methods offer practical alternatives.
Bokashi
One effective option is Bokashi, an anaerobic fermentation process that utilizes a sealed bin and bran inoculated with beneficial microbes. The system “pickles” food waste, including meat, dairy, and oils, which eliminates the odors and pest attraction associated with their decomposition. Once fermentation is complete, the acidic material must be buried in the garden or added to a regular compost pile for final breakdown. This two-stage approach allows high-risk leftovers to be safely pre-treated.
Trench Composting
Another method is Trench Composting, which involves burying food scraps, including small amounts of meat or dairy, directly into a trench dug 8 to 12 inches deep in the soil. The soil’s natural microbial community breaks down the material below the surface, which contains odors and prevents pests from accessing the waste.
Vermicomposting
Alternatively, Vermicomposting, or worm composting, uses red wiggler worms to process food scraps in a contained system. While worms will eat many leftovers, this system is best kept to plant-based scraps, as large amounts of meat and dairy can overwhelm the worms and turn the bin anaerobic.