FTIR Spectroscopy Measures Sugars in Apples

Richard Gaughan

One quantitative measure of the quality of fruit that can be employed to determine the optimum harvest time, to validate storage methods and to monitor processing is the relative amount of glucose, fructose and sucrose. High performance liquid chromatography can quantify the concentrations of these sugars, but it is destructive and time-consuming.

Fourier transform infrared spectroscopy quickly and nondestructively determines the relative concentrations of glucose, fructose and sucrose in apples, offering a quantitative assessment of their quality.

Now researchers at Zhejiang University in Hangzhou and at Jiangxi Agriculture University in Nanchang, both in China, have developed a Fourier transform infrared (FTIR) spectroscopy method that avoids these problems for the analysis of the sugar content of apples.

In an experimental evaluation of the technique, Yibin Ying of Zhejiang University and his colleagues used a Thermo Electron Corp. FTIR spectrometer to collect spectra from 130 Fuji apples. A source/detector fiber bundle delivered the illumination and captured the diffuse reflectance spectra. Each apple was measured at three locations equally spaced about its equator. Samples for calibration of the approach by comparison with high-performance liquid chromatography were created from a filtered solution of macerated apple gathered from each of the sample locations.

Using 100 of the apples, the researchers evaluated preprocessing techniques, spectral regions and mathematical models to calibrate the FTIR measurement method. Although the spectrometer gathered data from 4000 to 12,000 cm^–1, they determined that the best fit for fructose measurement was the 6000- to 12,000-cm^–1 region and that those for glucose and sucrose were the 4000- to 6000-cm^–1 and 8000- to 12000-cm^–1 regions.

The investigators then evaluated the remaining apples. The errors in sugar concentration, measured in percent by weight, were 0.3 percent for fructose and sucrose and 0.2 percent for glucose. The natural variation from apple to apple ranged from 10 percent to 5 percent for fructose, 5 percent to 2 percent for glucose, and 5 percent to less than 1 percent for sucrose.

The technique, already in use at Zhejiang University, was found to yield results comparable to those of high-performance liquid chromatography.

Journal of Agricultural and Food Chemistry, April 19, 2006, pp. 2810-2815.