Determination of amorphous/rubbery states in freeze-dried prebiotic sugars using a combined approach of near-infrared spectroscopy and multivariate analysis

Abstract

Galacto-oligosaccharides (GOS) and lactulose are well-recognized prebiotics widely used in functional food and pharmaceutical products, but there is still a lack of knowledge regarding their physical–chemical properties. In this study, a physical–chemical approach on two GOS of different composition (GOS Cup Oligo H-70® and GOS Biotempo) and lactulose was assessed. Mid infrared and Raman spectra of the freeze-dried sugars allowed their structural characterization in the amorphous state, lactulose, showing the main spectral differences. Freeze-dried sugars were then equilibrated at 4 °C at relative humidity (RH) ranging from 11% to 80%. Near-infrared reflectance spectra were registered in each condition in the 900- to 1700-nm region. A principal component analysis (PCA) was performed on the three sugars equilibrated at different RH. In all the three sugars, the groups observed explained more than 95% of the variance and were related with the RH of the samples. According to the loading plots of PC1, the main differences related with RH were observed in the 1380- to 1500-nm region. As the amorphous states are very sensitive to changes in temperature and moisture content, and the moisture content is related with the parameter T-Tg (T: storage temperature; Tg: vitreous transition temperature), an effort was made to determine this parameter directly from the NIR spectra. To this aim, a partial least square model (PLS) was defined. Tg values obtained by differential scanning calorimetry (DSC) were used to calculate the T-Tg values of reference. The model was validated with an independent set of data. The mean of predicted values fitted nicely T-Tg obtained from DSC (correlation = 0.966; R2 = 0.934), thus supporting the use of the PLS model to investigate unknown samples. The stability of amorphous sugars in foods and pharmaceuticals is of practical and economical importance because it affects different quality attributes of foods, including texture, aroma retention and shelf life. Therefore, predicting T-Tg, a parameter that is independent on the sugar investigated, directly from their NIR spectra is of utmost importance to determine the shelf life of food and food-related products and up to our knowledge has never been determined hereto.