Technical Abstract: Mannitol, formed mainly by Leuconostoc mesenteroides bacteria, is a sensitive marker of sugarcane and sugarbeet deterioration that can predict multiple processing problems. The delivery of consignments of deteriorated sugarcane or sugar beets to factories can detrimentally affect multiple process units, and even lead to a factory shut-down. An enzymatic factory method that is rapid, simple, and inexpensive is now available to measure mannitol in consignment juices at both sugarcane and sugar beet factories. This will allow factory staff to know which consignments of sugarcane or sugar beet will affect processing negatively or reject consignments that will cause unacceptable processing problems. Mannitol is directly measured on a spectrophotometer using mannitol dehydrogenase as the enzyme. The stability of the reagents, limited juice preparation, and linearity are described. The method is accurate compared to ion chromatography and precise, although precision decreases for both sugarcane and sugar beet juices with low mannitol concentrations. A strong polynomial relationship (R2=0.912) existed between mannitol and haze dextran (Alpha-(1'6)-Alpha-D-glucan) in juices obtained across a 3 month processing season at a sugarcane factory. Mannitol concentrations are usually higher than concentrations of monoclonal antibody dextran and other specific measures of dextran, which indicates (i) the usefulness and sensitivity of mannitol to better predict sugarcane deterioration from Leuconostoc and other bacteria than dextran, and (ii) the underestimation by sugar industry personnel of the relatively large amounts of mannitol present in deteriorated sugarcane. Greater than ~250-500 ppm/Brix mannitol in sugarcane juice predicts downstream processing problems. The increasing awareness of how mannitol detrimentally effects processing, e.g. its contribution to the Louisiana hard-to-boil massecuite phenomenon, is also fully discussed.