To　improve　the　economic　gain　of　anaerobic　fermentation,　this　study　proposes　a　novel　chain　elongation-based　biotechnology　of　converting　food　waste　(FW)　into　medium　chain　fatty　acids　(MCFAs)　and　long　chain　alcohols　(LCA).　These　two　fermentative　products　are　more　profitable　due　to　their　higher　energy　density　and　easier　separation　compared　to　short-chain　fatty　acids　(SCFAs).　The　necessity　and　the　cost　for　externally　dosing　an　electron　donor　(ED)　for　chain　elongation　were　eliminated　by　inoculating　different　yeast　levels　(2.25　×　107,　3.37　×　107,　6.74　×　107,　and　8.99　×　107　cells　per　FW　ml　Saccharomyces　cerevisiae)　into　the　fermenters.　In　situ　ethanol　production　was　then　facilitated　and　was　subsequently　utilized　to　produce　MCFAs.　Overall,　MCFA　formation　was　enhanced　by　yeast　inoculation　compared　to　the　control.　533　mg　of　COD　per　L　MCFAs　were　generated　in　the　batch　test　with　2.25　×　107　cells　per　FW　ml　S.　cerevisiae.　Higher　but　similar　MCFA　yields　(∼1500　mg　COD　per　L)　were　obtained　in　batch　reactors　with　higher　yeast　inoculations.　The　greater　endogenous　ethanol　and　SCFA　supply,　enriched　chain　elongating　organisms,　along　with　the　promoted　CE　pathway　based　on　the　increasing　gene　abundance　jointly　contributed　to　the　higher　MCFA　production　from　FW　with　yeast　inoculation.　More　interestingly,　a　high　accumulative　butanol　level　(1228　mg　COD　per　L)　was　observed　under　8.99　×　107　cells　per　FW　ml　S.　cerevisiae　inoculation.　The　more　abundant　genes　encoding　the　acetone-butanol-ethanol　platform　and　the　lower　conversion　from　butanol　to　caproate　likely　induced　such　a　high　butanol　yield　according　to　metagenomics　analysis.　©　2023　The　Royal　Society　of　Chemistry.