No other food products focus consumer attention as those that are prepared for consumption by children. Of current interest are the natural and added sugar contents of processed baby foods and juices.
Identification and quantification of natural sugars was recently investigated in baby food products by mid-IR Fourier transform infrared (FTIR) spectroscopy. Using horizontal attenuated total reflectance (HATR), neat baby food samples were analyzed without need for extensive sample preparation. By use of the HATR technique it was demonstrated that high sensitivity could be easily achieved without significant effect from water content. Factor space chemometric analysis was used to establish a robust method that allowed the confident measurement of sugar concentrations in these food products.
The method was developed using a training matrix of three naturally occurring sugars, fructose, glucose, and sucrose. It was confirmed using a verification matrix and was found to be readily applicable to the evaluation of sugar quantities occurring in commercial baby food products. Several commercial products were analyzed with this method and quantities of fructose, glucose, and sucrose were determined.
Baby food samples pose a unique challenge for FTIR analysis because of the strong IR absorption by water. Figure 1 shows the FTIR absorption spectra of water (red) and a 5 percent aqueous glucose solution (black) acquired using a HATR accessory with a trough liquid plate. The strong absorption bands due to water can be seen between 3,800 to 2,800, 1,700 to 1,550, and below 900 centimeter (cm)-1. However, absorption from the glucose can be seen in the fingerprint region between 1,486 and 963 cm-1. This water-free absorption region suggests that quantitative sugar analysis in aqueous solutions may be feasible.
FTIR spectra of aqueous solutions of 5 percent fructose (blue), 5 percent glucose (green), and 5 percent sucrose (black) are shown in Figure 2. The sugars have characteristic absorption bands that appear in the 1,486 to 963 cm-1 range. The absorption bands overlap making quantitation by least squares or traditional multivariate analysis routines difficult
Chemometric factor-space analysis was utilized to establish simultaneous calibration curves for the three-sugar aqueous mixtures. Partial least squares (PLS) was selected as the factor-space routine of choice. The use of a factor-space analysis routine increased the number of dimensions in the analysis. This allowed for each sugar component to be assigned to a specific dimension or factor-space. In addition, noise in the spectra (e.g. water absorption) was also treated by the additional dimensions. By using the factor-based routines, the components that attributed to analytical noise (e.g. water absorptions) could readily be identified and separated out in the quantitation.
A training set of samples was prepared to cover the full three-dimensional quantitative space required for analysis of the baby foods. Since there were three sugar components of interest, fructose, glucose, and sucrose, a three-dimensional training matrix was created (Figure 3). Aqueous sugar samples were prepared to cover the eight corners of the matrix, the face centered positions of the matrix, and the matrix center.
In addition to the training matrix, a verification matrix (Figure 4) of aqueous sugar samples was prepared using random concentrations of each of the three sugars, fructose, glucose, and sucrose. The verification matrix was used to evaluate the validity of the calibration method.
FTIR Absorbance spectra were acquired of the training matrix standard samples using parameters of 4 cm-1 resolution, Happ-Genzel apodization, and the averaging of 32 scans. Figure 5 shows the spectra and demonstrates the complexity of the overlapping absorption bands.