- 1 Introduction
- 2 Physical Environment
- 3 Biological Interactions
- 4 Microbial Processes
- 5 Key Microorganisms
- 6 Current Research
- 7 References
Kitchen waste is defined as left-over organic matter from restaurants, hotels and households . Tons of kitchen wastes are produced daily in highly populated areas. Kitchen wastes entering the mixed-municipal waste system are difficult to process by standard means, such as incineration, due to the high moisture content . Furthermore, organic matter can be transformed into useful fertilizer and biofuel . New disposal methods that are both environmentally and economically efficient are being developed which rely on various forms of microbial decomposition.
Kitchen waste is a nutrient rich, or eutrophic, environment containing high levels of carbohydrates, lipids, proteins, and other organic molecules which can support abundant populations of microorganisms . The anaerobic nature of kitchen wastes is typical for a eutrophic environment, because aerobic bacteria deplete oxygen through respiration at a faster rate than oxygen can be replenished by diffusion. Although the presence of water is essential for bacteria growth, the high moisture content in kitchen waste exacerbates the anaerobic condition as oxygen is insoluble in water and it is hard for oxygen to diffuse through water . Kitchen waste is usually acidic due to the action of acid fermentation bacteria such as lactic acid bacteria . As lactic acid can act as an uncoupler in acidic environment, it is toxic to other bacteria, thus a buffer is usually added into kitchen wastes to make the environment less acidic. Overall, the high moisture and nutrient level make kitchen waste an ideal environment for anaerobic biodegradation.
A syntrophic and nutritionally mutualistic relationship exists among the organisms in kitchen waste environment forming an anaerobic food web. Hydrolytic enzymes break down complex molecules into monomers which can be used by fermentative bacteria. Products of fermentation can be further reduced to methane by methanogens . In addition, Pseudomonas_aeruginosa produces a biosurfacant called rhamnolipid which can increase bioavailability of nutrients in kitchen waste for other bacteria . Researchers also found that lactic acid bacteria can suppress the growth of food poisoning bacteria such as Staphylococcus aureus, and Bacillus cereus . Therefore, bacteria within kitchen waste environment work together as well as compete with each other for resources in the system.
In general, four steps occur in anaerobic digestion including hydrolysis, acid fermentation, acetogenesis, and methanogenesis . In the hydrolytic step, complex molecules such as polysaccharides and proteins are broken down into monomers such as monosaccharides and amino acids by extracellular enzymes. The hydrolytic step can be facilitated by adding artificial enzymes. In acid fermentation step, lactic acid bacteria ferment monosaccharide and produce lactic acid, which can be used industrially to make various commercial products such as plastic . Additionally, acid-tolerant bacteria can be used to ferment sugar to produce ethanol, which is thought to be a promising new energy source . Acetogenic bacteria can take simple sugars and make acetate or it can act as hydrogen consumer and facilitate fermenting processes of other bacteria. Methanogens use products from fermentation step such as hydrogen and carbon dioxide to produce methane which can be used as energy . Furthermore, organic nitrogen is converted to ammonia which can be used more readily by plants as fertilizers .
Lactic acid bacteria (LAB)
Lactic acid bacteria are one of the main groups of acid fermenting bacteria in kitchen waste environment. One example of lactic acid bacteria is Lactobacillus_plantarum, which is gram positive, and rod-shaped. Lactobacillus_plantarum ferments on glucose present in kitchen waste and produces lactic acid .
Acetogenic bacteria can act as primary fermenters that use sugars to generate acetate as well as acting as hydrogen consumers allowing possible secondary fermenters to grow. One example of acetogenic bacteria is Clostridium spp., which are gram positive and form endospores. Clostridium spp. is also responsible for producing the odour of kitchen waste due to sulphur compounds released .
Other key microorganisms
Staphylococcus_aureus, and Bacillus_cereus are two gram positive, food poisoning bacteria present in kitchen waste. Pseudomonas_aeruginosa produces surfactant, rhamnolipid that increases bioavailability .
Current anaerobic biodegradation method involving gathering organic wastes such as kitchen wastes into chambers with controlled environment, allowing anaerobic bacteria to work on the organic wastes, and collecting the biogas such as methane produced to use as energy. It is a cleaner and more efficient alternative to aerobic composting, as anaerobic digestion is generally an energy producing process, whereas composting is mostly energy consuming. In addition, methane produced from composting is released into environment where methane produced in anaerobic digester is used as fuels. 
Codigestion of kitchen waste and cattle manure to increase biogas production
Codigestion of kitchen wastes with cattle manure offers a better carbon and nitrogen nutrient balance which creates a positive synergy within the system. Kitchen wastes tend to be acidified by the action of acid fermenting bacteria, which can inhibit further anaerobic digestion, whereas cattle manure offers basic buffers such as bile that neutralizes the environment. It is found that codigestion of kitchen waste with cattle manure generates 44% more methane than anaerobic digestion of kitchen waste alone. 
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