J. Agric. Food Chem. 1997, 45, 166−171
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Direct Spectroscopic Sucrose Determination of Raw Sugar Cane Juices Fre´de´ric Cadet* and Bernard Offmann Laboratoire de Biochimie, Faculte´ des Sciences, Universite´ de la Re´union, 15 Avenue Rene´ Cassin, B.P. 7151, 97715 Saint-Denis Messag Cedex 9, Re´union, France
A more accurate, less time-consuming, and nonpolluting spectroscopic method than the currently used HPLC or polarimetric methods is proposed for the routine quantitative determination of sucrose in complex biological samples. Opaque raw sugar cane juices representative of a sugar cane harvest are analyzed by Fourier transformed mid-infrared attenuated total reflectance, and the spectral data are processed by principal component analysis (PCA) and principal component regression (PCR). The most suitable region for the measurement of sucrose was found to be the 1250-800 cm-1 region. The spectroscopic representation of the first axis as assessed by PCA in this spectral region featured characteristic absorption bands of sucrose. By PCR on the spectral data from a calibration set, a prediction equation was established to predict sucrose content in unknown samples. Good overall predictions were obtained. The values of the predicted sucrose concentration were more accurate (bias ) 0.041 g/100 mL) than those obtained by direct polarimetry (bias ) -0.163 g/100 mL). The method is validated on a panel of 1267 samples representative of a sugar cane harvest. Keywords: Raw sugar cane juices; mid-FTIR; PCR; sucrose INTRODUCTION
Sucrose is the principal product of the photosynthesis process and is the most widespread sugar in the plant kingdom. Sucrose is a renewable chemical resource and has considerable commercial value in the chemical, microbiological, pharmaceutical, and food industries. One of the most essential concerns in the sugar cane industry is to determine the price of the sugar canes that are delivered and sold to the miller. The price of sugar cane is estimated by its sucrose content. Not only is the efficiency of the method used to determine sucrose content in sugar cane critical, but the delay and costs of the analyses are also important. The officially adopted methods for the routine analysis of sucrose content in sugar cane juice vary from country to country. In some countries the classical and physical refractrometric and polarimetric methods are used, while other countries use chromatographic methods such as HPLC or GLC (Wong Sak Hoi, 1982; Honda, 1984; Meade and Chen, 1985). In all cases, routine analysis can only be done on processed juices. As for example, for polarimetric measures, the raw juices that are obtained from pressed sugar cane at high pressure contain fibers and impurities that make the solution opaque. These juices need to be processed with lead acetate, which precipitates impurities, and then filtered on a cellulose filter so as to obtain clarified juices ready for analysis. About 20 min is necessary to conduct these operations. The most used methods currently are polarimetry (Schneider, 1985) and HPLC (Clarke, 1985). It has been shown that polarimetric measurements underestimate sugar content (Meade and Chen, 1985; Brokensha et al., 1978; Cadet et al., 1991). Measuments by HPLC are not precise enough, and the results are not easily reproducible for routine analysis (Rouch et al., 1995). * Author to whom correspondence should be addressed [telephone Intl (262) 93 82 02; fax Intl (262) 93 81 66]. S0021-8561(96)00700-5 CCC: $14.00
Spectroscopic methods such as near-infrared or midinfrared can be easily adapted for routine use for industrial control of food products. Rapid analytical methods have developed considerably since the advent of near-infrared reflectance spectroscopy (NIR) between 1960 and 1970 (Norris, 1978). More and more constituents and products are now being analyzed according to this method (Osborne, 1981; Williams and Norris, 1987). The spectroscopic method is most widely used in the food industry for the quantitative measurement of major biochemical products. Few studies have focused on the use of the MIR range for analysis of food products since mid-infrared reflectance (MIR) spectra are complex and since water contained in biological products is a strong IR absorber. However, with the advent of Fourier transform infrared spectroscopy in parallel with the use of powerful microcomputers and with the advent of new spectroscopic techniques such as attenuated total reflectance (ATR), MIR spectroscopy has developed considerably. ATR is an analytical technique that has great potential with regard to analysis of food products (Depecker et al., 1985; Crocombe et al., 1987; Van de Voort and Ismail, 1991; Cadet et al., 1991). There are different mathematical methods that can be used for the processing of complex infrared spectral data. Only recently has principal component analysis (PCA) been used for the study of transmission MIR spectra (Antoon et al., 1979; Gillette and Koenig, 1982). PCA was used for the first time for the study of diffused reflection near-IR spectra by Bertrand et al. (1984) and Cowe and McNicol (1985). The aim of this work was to evaluate PCA and principal component regression (PCR) mathematical treatments for the description and processing of mid-infrared spectral data of raw sugar cane juices. This method has been tested on 1267 samples obtained throughout a sugar cane harvest time (4 months). © 1997 American Chemical Society
J. Agric. Food Chem., Vol. 45, No. 1, 1997 167
Determination of Sucrose in Raw Sugar Cane Juices MATERIALS AND METHODS Raw Sugar Cane Juices. Sampling of sugar was done by coring. The average core was about 7 kg. After pulverization, a subsample of approximately 1 kg was removed. A hydraulic press was used to extract juice from the samples obtained from the coring and from the disintegrator. The sample was pressed for 2.5 min at 250 bar. Mid-infrared ATR Spectra. Mid-Fourier transform infrared (mid-FTIR) spectra were collected on a Michelson 100 Fourier transform spectrophotometer. ATR spectra were obtained with a Specac Overhead ATR system. The crystal of the reflectance element is made from zinc selenide, a material that is quite inert to water; it is quite rapidly cleaned between samples by spraying with water and then dried with filter paper. The data were recorded from 700 to 5000 cm-1 in 4 cm-1 increments at log(1/R), in which R is the ratio of the reflected intensity for the background to that of the sample. Although the ATR experiment does involve the reflection of the radiation within a crystal, the interaction of the radiation with the sample is the transmittance of radiation through the sample; this depth of penetration is wavelength dependent, but it is passing through a finite layer of the sample. For this reason, plots can be read according to absorbance (or transmittance). The combination of four scans resulted in an average spectrum. The intensity the spectra was low; the highest peaks had log(1/R) values
