Enzymatic assessment of pectin functionality in birch hemicellulose-based emulsifiers

Glucuronoxylans (GX) from birch and galactoglucomannans (GGM) from spruce are capable of forming and stabilising oil-in-water emulsions. Phenolic compounds co-extracted with wood hemicelluloses enhance the stability of an emulsion. The stabilising mechanisms of emulsions prepared from GGM have been studied in detail, while emulsions stabilised by GX require further investigation due to the complexity of their chemical compositions. On the contrary to GGM-based emulsions, pectin was found at the interface of GX-based emulsions. We hypothesise that pectin plays a role in emulsification and affects the bimodal droplet size distribution of an emulsion. Therefore, this study aimed to assess the functionality of pectin in GX-based emulsifiers with pectinase. First, pectinase treatment conditions were optimised, namely reaction medium, time, temperature and pectinase dosage. The enzymatic hydrolysis of pectin was confirmed quantitatively by gas chromatography (GC). To investigate the role of pectin in emulsion stabilisation, rapeseed oil-in-water emulsions were prepared from pressurised-hot-water-extracted (PHWE) spray-dried GX (sGX), sGX treated with pectinase (p-sGX), sGX treated with pectinase and centrifugation (c-sGX), ethanol-precipitated GX (eGX), eGX treated with pectinase (p-eGX), and eGX treated with pectinase and centrifugation (c-eGX). The stability of the 6 emulsions was measured in a 2-week accelerated storage stability test. Phenolic compounds originating from lignin were characterised by pyrolysis gas chromatography-mass spectrometry (pyrolysis GC-MS) to monitor the changes of lignin contents before and after pectinase treatment. It is concluded that the optimum conditions of pectinase treatment were: reaction medium: 0.1 M sodium acetate buffer (pH 4.5), reaction temperature: 50 °C, reaction time: 2 hours, pectinase dosage: 304 U per gram of birch hemicellulose. Results from GC analysis of free monosaccharides confirmed that only pectin was hydrolysed under the abovementioned pectinase treatment conditions and that the hemicellulose backbone structure remained intact. Emulsions prepared from sGX remained stable for over 2 weeks, while creaming was observed in emulsions stabilised by p-sGX and c-sGX during the second week of storage. Emulsions prepared from eGX, p-eGX, and c-eGX remained stable for over 2 weeks. Results from pyrolysis GC-MS indicated that pectinase treatment removed some of the phenolic compounds in GX. Therefore, it was concluded that associations existed among GX, lignin, and pectin. Pectin took part in the formation and stabilisation of emulsions prepared from sGX and eGX by connecting lignin to GX