Determination of sucrose in sugar-cane juice and molasses by flow-injection spectrophotometry

Abstract

An automated procedure is proposed for the spectrophotometric determination of sucrose in sugar-cane juice and nonlasses. The previously diluted and filtered sample is introduced into a flow-injection analyzer designed with two merging streams, producing two samples zones. One zone is transported directly towards the merging-stream confluence; the other zone reaches this site after flowing through a heated coil in which partial and reproducible sucrose inversion is attained under controlled conditions of acidity and temperature. At the confluence point, a buffered periodate stream is added to oxidize the sugar. The consumption of periodate, which mainly reflects the fructose content, is measured spectrophotometrically as a transient lowering of the iodine concentration, produced by reaction of periodate with iodide. The two processed zones proceed sequentially to the flow cell and two peaks are recorded. The sucrose content in the sample is proportional to the difference in peak heights. System dimensioning and the effects of temperature, pH, reagent concentrations, flow rates and the presence of other reducing sugars in the sample are discussed. With the proposed system, about 30 sugar-cane juice samples can be analyzed per hour, and sample clarification is not required. Precise results (r.s.d. <1%), in agreement with those obtained by h.p.l.c., are achieved for sucrose contents of 11–14% (w/v) in cane juice. Modifications of the system for analysis of molasses (16–52% w/w sucrose) are described.