Production of high-value chemicals from starch food waste by chemical conversions

With the acceleration of industrialization and urbanization, living standards in China have risen significantly, leading to substantial changes in consumption patterns. Consequently, the scale of the catering market has expanded annually, resulting in a corresponding increase in urban kitchen waste. Kitchen waste, the primary component of municipal waste, possesses both harmful and resource-rich properties. Traditional food waste treatment methods, such as incineration, landfill, and anaerobic fermentation, are extensive in scale, however, they may also suffer from several issues, e.g. secondary environmental pollution, low resource utilization rates, limited value-added outcomes. Advanced methods and technologies for high-value utilization are under development, severely restricting the utilization of food waste. The harmless and resourceful utilization of food waste will become a new trend in the future. Starch is the main component of kitchen waste, such as flour food, rice and potatoes. Thus, the development of chemical conversion methods to convert starch-rich waste is urgent. In this paper, the characteristics, existing treatment technologies and pretreatment methods of starch food waste are systematically introduced. The composition of kitchen waste is complex, including common components such as starch, oil, protein, and fiber, as well as potential impurities like plastics and metals. To achieve high-value conversion of starch-rich food waste, it is crucial to arrange appropriate pre-sorting, pre-processing, and other pretreatment processes based on the specific characteristics of different food waste sources and subsequent conversion requirements. The resource utilization technology for starch-based food waste is primarily categorized into three types: non-biological treatment, biological treatment, and chemical treatment. These technologies convert starch food waste into various products, enabling the diversified utilization of food waste. Then, the catalytic system for the conversion different starch food wastes to chemicals such as glucose, 5-hydroxymethylfurfural (HMF), levulinic acid (ester), γ-valerolactone (GVL) and lactic acid (ester) were introduced. Researchers have made great progress by using acid catalysts to catalyze the hydrolysis of starch raw materials to produce glucose, HMF and levulinic acid. The reaction mechanism can be regarded as: H⁺ combines with the oxygen atom of the glycoside bond to protonate and generate conjugated acid. Then, the conjugated acid breaks the C−O bond under the attack of water molecules, and generates hydroxyl, and finally the protonated glucose removes H⁺ to produce glucose. Glucose can be converted into HMF through two pathways: (1) Glucose is isomerized at acid sites to produce fructose, which is then dehydrated to form HMF. (2) Glucose is directly dehydrated to produce HMF under high-temperature conditions. Levulinic acid is produced from the further hydrolysis of glucose or HMF, while levulinate and GVL could be produced through alcoholysis reaction. For lactic acid (ester) production, both acid and basic catalysts showed excellent reactivity in the conversion of starch. Starch was firstly transformed into glucose, and then further isomerized to produce fructose. Fructose underwent inverse aldol condensation reaction to produce glyceraldehyde intermediates, and further underwent the addition and isomerization reactions to produce lactic acid (ester). Finally, the existing challenges and prospects in the chemical conversion of starch food waste into high-value chemicals were outlined. Presently, chemical treatment of kitchen waste remains primarily at the laboratory research stage, featured with high costs and suboptimal economic returns. Nonetheless, chemical treatment of kitchen waste typically boasts high rates of resource recycling and entails a lengthy project industry chain.