Structure-property relationships in vegetable cell wall suspensions

Plant cell wall suspensions are widely present in daily food, such as soups, dressings and sauces. Cell walls of edible plants are made up of an intricate biopolymer network of mainly cellulose microfibrils, pectins, and hemicelluloses. Foodsnbsp;as soups, ketchup, etc are made up of cell wall components. Modern processing methods alter the chemical and physical nature of the cell wall which in turn affect the properties of the end product. There is a need in the industry to build a fundamental toolbox for controlling the physical properties of products. {\color{red}To reach this goal}, the relationship between the properties and microstructure of cell wall biopolymers has to be established. This work aims to build a qualitative scientific foundation of how various biopolymers present in cell wall suspensions affect the bulk properties. In this quest, novel processing methods are used which engineer the biopolymers to deliver needed functionalities. Carrots and tomatoes were used as model systems in this study. Enzymes were used tonbsp;specific classes of biopolymers. By correlating the changesnbsp;in the properties to the specific biopolymer, the functionalnbsp;of that particular biopolymer was hypothesized. The cell walls of carrot are known to benbsp;resistant to disruption by heatingnbsp;shear. In addition to standard treatments, enzymesnbsp;added to hydrolyze specific classes of biopolymers and disrupt the cell wall. Incubation with pure cellulases, pectinases or hemicellulases resulted in suspensions with a different physical structure. The effect ofnbsp;enzymes was measured on the rheology over an 8 hour period and the microstructure was investigated by cryo-scanning electron microscopy. A homogenization step was applied after the enzyme incubation, resulting in distinctly different cell wall microstructures based on the type and extent of the pre-treatments. It was observed that there is an increase in storage modulus after homogenization due to the increase in particle size, hence the volume fraction of particles. Particle size and {\color{red}particle-serum} interactions were pinpointed as the most important contributors to the bulk rheological properties of thesenbsp;wall suspensions. nbsp;{\color{red}particle-serum} interactions {\color{red}to the physical properties} is crucial for tomato suspensions. Ion exchange resins were used tonbsp;and investigate these interactions. Two types of resinsnbsp;used, a hydrogen form cation exchange resin and an anion resin in the hydroxide form. The serum phase of tomato suspensionsnbsp;treated with either a cationic or an anionic resin to exchange various ionic compounds with hydrogennbsp;hydroxide ions respectively. The treated serum was then reconstituted to the tomato pulp and the suspension was re-suspended with shear. Effect of the resins was dependent on the concentration of resin used. The linear storage modulus varied with the different types of resin treatment. Samples treated with the anion exchange resin resulted in a higher modulus than the untreated tomato puree and the puree treated with the cation resin. The anion treated sample contained a network structure which was quite sensitive to pH. This was attributed tonbsp;range electrostatic interactions caused by protein-pectin interactions. Using Infrared spectroscopy conformational changes in the protein structure as a result of resin treatment were detected by analyzing the amide-I and amide-II regions. nbsp; In addition to particle sizenbsp;particle interactions, the role of particle hardness on rheological properties are yet to be established. Hence, an experimental methodnbsp;developed to study the effect of particle elasticity and electrostatic interactions on the rheological properties of cell suspensions. Enzymes were used to selectively depolymerize the pectin (backbone) and proteins in suspensions. Thenbsp;treatments affectednbsp;physical properties,nbsp;a hypothesis for the structure-function relationship of these biopolymers was formulated.nbsp;enzymatic treatments directly affected particle properties, resulting in loosened cell walls as visualized by cryo-SEM. The effect of the enzymatic treatment on the storage modulus was measured as a function of total solid content (below critical packing fraction). Furthermore, experiments were performed in the presence of varying concentrations of sodium chloride in order to changenbsp;Debye screening length. Such a method assisted in decoupling the electrostatic effects from particle elasticity. In addition, particle properties were measured directly by applying a compressive strain on the particles and measuring the normal force. By fitting the normal stressnbsp;with a Maxwell model, particle properties such asnbsp;scale of relaxationnbsp;elasticity were obtained. It is suggested that for carrot suspensions, pectins on thenbsp;walls could contribute to the particlenbsp;The pectins on carrot cell walls were responsible for electrostatic interactions between particles. Thus,nbsp;structure-property relationships in cell wall suspensions were hypothesized. nbsp;status: publishe