Visualization of pectin in carrot pomace using FTIR microspectroscopy: feasibility of internal standardization

Abstract

Pectins are a group of polysaccharides commonly used in industry as gelling, emulsifying, and stabilizing agents. Traditionally obtained from apple or citrus peel, the increasing global demand for pectin calls for the study of alternative sources [1], [2] . Although currently employed in animal feed, carrot pomace, a by-product obtained after juice production, is considered a good target for pectin extraction [3], [4] . Nevertheless, modifications on the chemical structure during biomass storage have been reported [5] , and there is a limited knowledge on changes occurring during pectin extraction. Industrial exploitation of carrot pomace requires the development of robust analytical methods to collect information about the variation in composition through the sample. FTIR microspectroscopy allows the acquisition of spectral and spatial information simultaneously but is not widely implemented in biomass samples due to the inherent limitations of the technique. Uncontrolled physical and chemical phenomena such as changes in sample size, humidity, or instrumental drift are responsible for spectral variance, hindering the potential of this technique for quantitative analysis [6] . Data pre-processing methods can remove the inherent variation in FTIR- spectra but can generate artifacts [7] . Normalization of the spectra based on the signal of an internal standard could overcome this limitation [8] . Nevertheless, internal standardization is not usually employed on biological samples due to the difficulty of finding compounds exhibiting no spectral interference. This work aims to study the feasibility of internal standardization to develop a spectroscopic method for pectin determination on carrot pomace. In a first step, a calibration model for pectin determination was prepared, enriching carrot pomace residues obtained after pectin extraction with different percentages of pectin and a constant amount of internal standard. Subsequently, the calibration model was tested on samples subjected to internal standardization and analyzed by FTIR microspectroscopy. Our results show the potential of this approach in FTIR imaging, where internal standardization could provide a method to quantitatively assess the spatial distribution of pectin in complex biomass matrices. The findings open up an attractive prospect of using FTIR microspectroscopy for mapping changes of target compounds at different stages of extraction.