Comparison of the adsorption behaviour of catechin onto cellulose and pectin

Highlights

• Thermodynamics & kinetics of catechin adsorption onto fibre was studied.

• Adsorption of catechin onto pectin was higher than onto cellulose.

• The adsorption was predominantly a physisorption spontaneous process.

• Catechin adsorption onto fibre was accompanied by increased fluorescence.

Abstract

The adsorption behaviour of catechin onto cellulose and pectin was compared. The adsorption of catechin onto the two fibres involved an initial fast adsorption phase followed by a slower adsorption as the sites became saturated and the systems moved towards equilibrium. The adsorption capacity of pectin for catechin (20.71 ± 2.24 mg/g) was significantly greater than that of cellulose (2.41 ± 0.05 mg/g) after equilibration for 24 h at 37 °C. The Langmuir and Freundlich models were applied to obtain the quantitative information about the adsorption of catechins to pectin and cellulose. Thermodynamic data derived from the isothermal adsorption carried out at the temperatures of 27 °C, 32 °C, 37 °C and 42 °C suggested that the adsorption was spontaneous and the binding was driven predominantly by physisorption. Fluorescence experiments confirmed the adsorption of catechins onto cellulose and pectin. The results showed that catechin adsorption capacity and adsorption mechanism were different for pectin and cellulose.

Introduction

Dietary fibre is an important food component which has benefits for gastrointestinal and cardiovascular health (Anderson et al., 2009). The beneficial effects of dietary fibre in human health has been attributed to its absorptive properties (Boeing et al., 2012, Slavin, 2013). Dietary fibre absorbs excessive oil and fat in the digestive tract, which is then excreted from the human body (Kaczmarczyk, Miller, & Freund, 2012), balances the pH in the gastrointestinal tract (Dziedzic, Górecka, Kucharska, & Przybylska, 2012) and absorbs undigested food residues and increases defecation, resulting in weight loss (Eswaran, Muir, & Chey, 2013).

Dietary fibre also absorbs other dietary components, such as protein (Borisenkov et al., 2013, Dhital et al., 2015), polyphenols (Bohn, 2014, Phan et al., 2015) and divalent minerals (Faure, Koppenol, & Nyström, 2015). Dietary fibre acts as a carrier and protects unstable active molecules in the gastrointestinal tract until it reaches the desired site for further absorption, but many beneficial ingredients are also excreted together with dietary fibre (Macagnan, Da Silva, & Hecktheuer, 2016).

The interactions between polyphenols and fibres has consequences for food processing and health (Renard, Watrelot, & Bourvellec, 2016). The binding of some polyphenols to insoluble fibres increases the antioxidant activity of the fibre (Çelik & Gökmen, 2014). The ability of fibre to carry bioactives into the gut is considered to be an important physiological function of dietary fibre (Sauracalixto, 2011). After ingestion, part of the polyphenols will be transferred across the intestinal wall and the rest with the fibre will be delivered into the colon, where the action of gut microflora digests the fibre, releases polyphenols and converts them into more bioactive components (Clifford, Van Der Hooft, & Crozier, 2013). Understanding the interaction and adsorptive properties of dietary fibre for bioactive molecules such as polyphenols will help provide insights into the carrier properties of dietary fibre. It is important to consider the interactions of polyphenols with dietary fibre as this will influence the bioaccessibility and bioavailability of bioactives (González-Aguilar, Blancas-Benítez, & Sáyago-Ayerdi, 2017). There is still insufficient data on the interactions of polyphenols with fibre. Systematic studies on the adsorption of various polyphenols and different fibres will provide some insights into how different polyphenols interact with different fibres. This data, combined with further investigations on how these interactions enhance the production of bioactive phenolic acids by colon bacteria will be important for the development of new products and dietary recommendations for improving health (Edwards et al., 2017).

Dietary fibres are classified into insoluble dietary fibre and soluble dietary fibre. Insoluble dietary fibre is usually obtained from cereals and legumes and includes cellulose, xylans and lignin (Rosin, Lajolo, & Menezes, 2002). Soluble dietary fibre includes pectin, alginates and some plant polysaccharides from fruits, seafood and vegetables (Grigelmo-Miguel, & Martin-Belloso, 1999). For this study we chose to compare the adsorption of catechin, onto an insoluble dietary fibre (cellulose) and a soluble dietary fibre (pectin). Catechin represents a common polyphenol found in many plants (Arts, van de Putte, & Hollman, 2000). Cellulose and pectin are amongst the most common fibres, and are present in the cell walls of plants.

Previous studies have shown that the adsorption of catechin onto cellulose is a dynamic process, which is affected by the type of cellulose and catechin concentration (Adt, Flanagan, D'Arcy, & Gidley, 2017), as well as the environmental conditions, such as pH and temperature (Costa, Rogez, & Pena, 2015). First and second-order kinetic models and Langmuir and Freundlich adsorption model have been applied to study the adsorption behaviour of catechin onto cellulose (Phan et al., 2015). The adsorption is considered as a physical process (Ye et al., 2009). The interactions between catechin and pectin and application as natural antioxidants and functional food ingredients have been examined. Pectin has been used as taste masking agents to reduce catechin astringency (Hayashi, Ujihara, & Kohata, 2005). Pectin was used as an adsorbent to recover and stabilize the polyphenols (Schieber et al., 2003). The formation of pectin-catechin complexes is influenced by degree of esterification of the pectin and pH value (Zhao, Diao, & Zong, 2013). However, while the previous studies shed light on the differences in the adsorption behaviour of different polyphenols with different fibres and their consequences on bioaccessibility and bioavailability, few studies directly compare adsorption behaviour and the mechanism underlying the interaction between catechin and a soluble and insoluble fibre.

The aim of this work was to compare the thermodynamics and kinetics of the adsorption of catechin onto cellulose with that of adsorption onto pectin. The isothermal adsorption of catechin onto cellulose and pectin was studied as a function of time, temperature and concentration of catechin. Various kinetic models (pseudo-first order, pseudo-second order and the Weber-Morris model) were used to evaluate the kinetics of the adsorption process. Both the Langmuir and Freundlich models were applied to derive the quantitative thermodynamic parameters describing the adsorption. Fluorescence microscopy was used to visualize the adsorption of catechin onto cellulose and pectin.