Analysis of the effect of crystal size and color distribution on fluorescence measurements of solid sugar using chemometrics

Fluorescence from sugar crystal samples has previously been used to obtain information about factory imprint and sugar quality. Solid-phase fluorescence has potential as a fast screening method, but the spectra are highly influenced by the measurement geometry and sugar crystal sample. The aim of the present study was to examine how the fluorescence measurements are related to the sugar crystals for a better understanding of both. Initially, five sugar samples of varied composition were sieved into five crystal size fractions. Fluorescence excitation-emission landscapes of the fractions were measured with solid transmission and reflection techniques and in solution. The transmission fluorescence was quenched at ultraviolet wavelengths, and light scatter highly influenced the reflection fluorescence. Principal component analysis (PCA) showed that large crystals favored the transmission fluorescence, whereas smaller crystals improved the reflection fluorescence measurements. The multi-way method PARAFAC (parallel factor analysis) was used to resolve spectra of individual components from the fluorescence landscapes. Transmission and solution components had similar spectral profiles at higher wavelengths, characterizing a colorant and a colorant intermediate. The resolved components of the reflection data were very influenced by scatter. Color predictions based on a few significant wavelength variables equaled the model results of full-spectrum models using partial least-squares regression (PLS). The variables corresponded to wavelength maxima of the resolved colorants and ultraviolet wavelengths characterizing colorant precursors.