XPS analysis of food products: toward chemical functions and molecular compounds

This contribution extends to typical food products the approach followed before to analyze microbial surfaces by XPS. Wheat-flour-based cakes with different formulations (absence of oil or egg products) and related nonbaked mixtures were analyzed by XPS. The C 1s, N 1s and 0 1s peaks can be decomposed into 4,2 and 3 components, respectively, with contributions that are due to well-identified chemical functions, typical of classes of biochemical compounds: proteins, carbohydrates, phospholipids (PL) and neutral lipids (NL). These attributions are supported by the analysis of ingredients, and by the 1 : 1 relationships between concentrations of elements and/or functional groups which demonstrate the consistency of the information regarding carbohydrates, proteins and PL: C 1s (286.2 eV) and O 1s (532.4 eV) contributions due to alcohol and ether; uncharged nitrogen (399.6 eV), and C 1s (287.6 eV) or O 1s (532 eV) contributions due to amide; phosphorous and positively charged nitrogen (402.3 eV) which are markers for PL. The concentrations of elements or functions ratioed to total carbon can be modeled on the basis of the known composition of model biochemical compounds, which leads to an evaluation of the surface composition expressed in wt% of these classes of compounds. Thereby, it was shown that surface accumulation, as compared to the bulk, increases in the order proteins < NL < PL. Moreover, the surface enrichment of PL compared to triglycerides was found to be increased after baking. Copyright (C) 2008 John Wiley & Sons, Ltd.